Mass Spectrometry and its Applications to Organic Chemistry

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NUNC COCNOSCO EX PARTE

TRENT UNIVERSITY LIBRARY

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MASS SPECTROMETRY

and its APPLICATIONS TO ORGANIC CHEMISTRY

MASS SPECTROMETRY and its

APPLICATIONS TO ORGANIC CHEMISTRY by

J. H. BEYNON Head of the Physics Section, I.C.I. Dyestuffs Division, Manchester (England)

ELSEVIER PUBLISHING COMPANY AMSTERDAM — LONDON — NEW YORK - PRINCETON i960

ELSEVIER PUBLISHING COMPANY 335 JAN VAN GALENSTRAAT, P.O. BOX 211, AMSTERDAM

AMERICAN ELSEVIER PUBLISHING COMPANY, INC. 52 VANDERBILT AVENUE, NEW YORK, N.Y. 10017

ELSEVIER PUBLISHING COMPANY LIMITED 12 B, RIPPLESIDE COMMERCIAL ESTATE RIPPLEROAD, BARKING, ESSEX

FIRST PUBLISHED: 1960 FIRST REPRINT: 1964 SECOND REPRINT 1967

LIBRARY OF CONGRESS CATALOG CARD NUMBER 60-8701

WITH 185 ILLUSTRATIONS AND 11 TABLES

ALL RIGHTS RESERVED THIS BOOK OR ANY PART THEREOF MAY NOT BE REPRODUCED IN ANY FORM, INCLUDING PHOTOSTATIC OR MICROFILM FORM, WITHOUT WRITTEN PERMISSION FROM THE PUBLISHERS

PRINTED IN THE NETHERLANDS

“I believe there is no branch of science where promise of great discoveries is more hopeful than those which will result by researches which involve the application of physical principles and physical measurements to chemical phenomena,” Sir J. j. Thomson, in a Discussion on “Isotopes”. Proc. Roy. Soc., 99 (1921) 87.

PREFACE The writing of this book has been prompted by the wide interest which has recently been aroused in the application of mass spectrometry to problems in organic chemistry. The book is intended to give a broad survey of the various facets of organic chemistry to which the technique has been applied, but empiresizes the applications in the field of qualitative identification of organic compounds in which the author’s experience of the subject has largely been obtained. The design of instruments for the various applications and their suitability in terms of the mass dispersion which they produce and the speed and accuracy with which measurements of mass and abundance of ions and of the energy necessary to form various ionic species can be made, are also discussed. Knowledge in these fields is essential whatever one’s application of the technique and although it has been included primarily to provide the necessary physical background to use of the technique in chemistry, it is hoped that by its inclusion the book will prove of interest to those whose interests lie in other fields. The field covered is so wide that all aspects cannot be treated in detail in a single book. It is hoped that this difficulty has been partly circumvented by including a large number of literature references so that the reader particularly interested in any topic can supplement the information given here. There have been many previous books and reviews dealing with various aspects of mass spectroscopy [57, 87, 124, 125, 127, 180, 203, 297, 482, 485, 488, 538, 542, 554, 575, 604, 720, 909, 910, 912, 921, 928, 931, 998, 1 oo2, 1011, x065, 1073, 1121, 1320, 1321, 1344, 1407, 1449, I5IO, i697, 1701, 1712, 1742, 1744, 1838, 2003, 2007, 2046, 2065/2082, 2098, 2106, 2107] and use has been made of all of them in compiling this monograph. Thanks are due to all who have given permission to make use of published diagrams: to A.E.I. Ltd., for Figs. 5 and 132; Physical Review for Figs. 16, 174, 178 and 179; Journal of Scientific Instruments for Fig. 57 ; Analytical Chemistry for Figs. 76 and 168; Review of Scientific Instruments for Figs. 78, 79, 92, 99 and 100; Journal of Chemical Physics for Figs. 37, 125, 126, 180, 181, 182 and 183; Journal of the Optical Society of America for Fig. 38; Zeitschrift fur Naturforschungfor Figs. 40 and 41; Applied Spectroscopy for Figs. 30, 102 and 139; U. S. Academy of Science for Fig. 112; National Bureau of Standards for Figs. 13, 28 and 29. The author would like to acknowledge here the help and encouragement he has received from many sources; from the Management of Imperial Chemical Industries Ltd. (Dyestuffs Division), especially in making the Library available at all times, from his colleagues by their critical discussion of the manuscript and from his wife in the preparation of the figures.

Manchester May, 1960

J. H. B.

CONTENTS PREFACE

.

INTRODUCTION.

.

.

CHAPTER 1. INSTRUMENTS.

4

1.1

4

Focussing. .. The parabola spectrograph. .. Velocity focussing . .. Direction focussing. The sector magnetic field .. The velocity selector. Double focussing. Methods of improving performance. The trochoidal-path mass spectrometer. Time-of-flight mass spectrometers using a magnetic field ...... Linear path time-of-flight mass spectrometers. Miscellaneous instruments ... . Instruments for isotope separation...

15 16 19 23 26 27

CHAPTER 2. THE MEASUREMENT OF MASS.

28

The physical and chemical mass scales. Accurate methods of mass measurement. .. Errors in the determination of mass by mass spectroscopy. Technique with a general purpose instrument and the attainable accu¬ racy .. The determination of mass number: magnetic field measurement . . Packing fractions of the isotopes .. Mass doublets and mass differences. .. Definitions of resolving power. Mass measurement in analysis..

28 30 33

49 51 54

CHAPTER 3. THE MEASUREMENT OF IONIC ABUNDANCE ...

58

1.2

1.3 1.4 1.5 1.6

1.7 1.8

1.9 4 1.10

1.11 1.12

1.13

2.1 2.2

2.3 2.4 2.5 2.6 J 2.7 2.8 2.9

3.1 ^3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10

4 5 5 8 11 11

38 43 48

Introduction .. 58 Possible errors in the measurement of relative abundance.\5$ Stable isotopes as tracers. 70 Determination of deuterium in organic compounds ........ 71 Determination of lsO in organic compounds . .......... 77 Determination of 15N in organic compounds. 79 Determination of 13C in organic compounds.\8f Determination of the isotopic abundances of other elements .... 81 Detection of small changes in abundance ratio.. . 83 Variations in the natural abundance ratios of the elements. 88

X

CONTENTS

3.11 The detection of rare isotopes .. 3.12 Isotopic dilution.

95 98

CHAPTER 4. SOURCES OF POSITIVE IONS.103 4.1 4.2 4.3 4.4 4.5 4.6 4.7

Introduction.103 The electron-bombardment ion source.103 The surface ionization source.Ill The vacuum spark ion source.115 The photo-ionization source.117 The field-emission source.120 Other types of ion source.123

CHAPTER 5. SAMPLE HANDLING.124 5.1 5.2 5.3 5.4

5.5 5.6 5.7 5.8 5.9 5.10 5.11 5.12 \) 5.13

The flow conditions in the mass spectrometer.124 Thermal transpiration ..126 The construction of leaks . . ..126 The vacuum system.132 (i) Materials of construction. 133 (ii) Demountable connections and vacuum valves.138 The choice of method of sample introduction.144 Estimation of sample volatility.145 The introduction of gases and volatile liquid samples.147 The introduction of less volatile samples.161 Other analytical information obtainable by mass spectrometry ... 172 Special problems concerned with mixtures. 174 The examination of small amounts of sample.178 Sample system for general use.184 Examination and separation of samples by auxiliary techniques ... 186 (i) Gas-liquid chromatography.187 (ii) Zone melting.191

CHAPTER 6. THE RECORDING OF POSITIVE ION BEAMS 6.1 6.2 6.3

6.4 6.5 6.6 6.7 6.8 6.9

....

195

Introduction.195 Photographic detection.195 Electrical detectors. 197 (i) Single collectors.197 (ii) Multiple collectors. 201 Signal-to-noise ratio in simple collecting systems. ......... 203 Multiplier detectors.206 Some uses of multiplier detectors. ..214 Fluctuations in gain of a multiplier.217 The recorder . ..219 The recording of derivatives.234

-CHAPTER 7. TYPES OF IONS IN MASS SPECTRA.238 7.1 7.2 J 7.3 '7.4 J1.5 ^7.6

Introduction.238 Total ionization.239 “Parent” or molecular ions.240 Fragment ions. 242 Meta-stable ions.251 Re-arrangement ions.262

CONTENTS

7.7 7.8 7.9 7.10

XI

Ions formed hy intermolecular processes.275 Multiply-charged ions.282 Ions formed with kinetic energy.284 Negative ions.286

CHAPTER 8. QUALITATIVE ANALYSIS BY MASS SPECTROMETER

291

8.1 8.2 8.3 8.4

Introduction. 291 Compounds to which the method can be applied.293 The compilation of a table of mass and abundance values.294 The determination of molecular formulae of organic substances by mass measurement. 302 8.5 The determination of molecular formulae of organic substances by isotopic abundance measurement. ..305 8.6 Errors caused by interference from other ions. 306 8.7 Methods of distinguishing “parent”ions.307 8.8 The uses of exact formulae in analysis.312 8.9 The listing of structural formulae from an exact molecular formula. . 313 8.10 Analysis of mixtures.319 8.11 The determination of molecular weight by effusion. 323 CHAPTER 9. CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA.325

•"9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 9.10 9.11 9.12 9.13 9.14 9.15 9.16 9.17 9.18 9.19

Aliphatic hydrocarbons. 325 Aikylbenzenes ..340 Monohydric alcohols and phenols.345 Ketones. 354 Aldehydes. 361 Ethers. 362 Carboxylic acids. . . 371 Esters. 375 Amines and other saturated nitrogen-containing compounds .... 387 Amides .. 396 Indoles, pyrroles, quinolines and pyridines.397 Nitriles. 404 Nitro-compounds and nitrites.406 Nitroso-derivatives and nitrosamines.409 Sulphur compounds. 410 Halogenated compounds ..413 Silicon compounds ..421 Other compounds . ..422 General remarks.423

CHAPTER 10. OTHER APPLICATIONS OF MASS SPECTROMETRY 424 10.1

Mass spectrometry in the petroleum industry: quantitative analysis . 424 (i) Introduction . .. 4-*i (ii) Simple mixtures. (iii) Multi-component mixtures.426 (iv) Limitations to the attainable accuracy.. 52 (v) Variation of cracking pattern.428 (vi) Interference of one sample with another.. 43i

10.2.

Identification of reaction products.432

XII

CONTENTS

10.3 The study of the interactions of ions with matter.440 10.4 The separation of isotopes.443 (i) The calutron.443 (ii) Separation by diffusion.443 (iii) Separation by distillation.444 (iv) Separation by chemical exchange.445 (v) Separation by electrolysis.446 (vi) Other separation methods.446 10.5 The uses in physics of separated isotopes.446 10.6 Isotopic age determination.448 10.7 Determination of the mechanisms and rates of reactions: isotope effects.453 10.8 The measurement of ionization potentials and bond strengths .... 459 10.9 The measurement of latent heats of vaporization and sublimation . . 474 10.10 Leak detection.479 10.11 High vacuum problems.482 10.12 Miscellaneous applications.483 APPENDIXES.486 Appendix 1. Masses and isotopic abundance ratios for various combinations of carbon, hydrogen, nitrogen and oxygen.486 Appendix 2. Nomograms for determination of the origin of meta-stable ions ..546 Appendix 3. Table of the masses and abundances of the naturally-occurring isotopes. 554 Appendix 4. International atomic weights (1955) ........... 570 Appendix 5. Possible peaks in the mass spectra of fluorocarbons and their composition.572 Appendix 6. The mass spectrum of Fluorolube residues (above mass 69) . 578 Appendix 7. Some'common mass doublets.582 REFERENCES.585 SUBJECT INDEX.631

INTRODUCTION A mass spectrometer is an instrument which produces a beam of ions from a substance being investigated, sorts these ions into a spectrum according to their mass to charge ratios, and records the relative abundance of each species of ion present. Almost all mass spectrometers measure only positively-charged ions, but there is no reason why studies of negatively-charged ions cannot be carried out with a mass spectrometer and such are occasionally performed. Mass spectrometers can therefore be used for the measurement of ionic mass to charge ratio, for the determination of ionic abundance, and for the study of the ion¬ ization process. The forty years in which the subject of positive ion analysis has grown have seen its applications widen continuously; new applications have fathered new designs of instrument, instrumental improvements have in their turn extended the range of application of the technique, and the expansion of scope and diversity of equipment is still continuing. At first, the technique developed along two main lines, one concerned primarily with the relative abundances of ion species, the other with their accurate mass determination, and the respective instruments used for these techniques became known as mass spectrometers and mass spectrographs. In mass spectrometry, the ion currents are detected electrically and the signal is usually amplified electronically before being recorded. In mass spectrographs, on the other hand, the ion beam is generally detected and recorded photographically. In the early days of the sub¬ ject, the sensitivity obtainable with photographic detection of an energetic ion beam was greater than that which could be achieved electrically. Mainly for this reason photographic detection became synonymous with very accurate mass measurement, for which only weak ion beams were employed. The photographic plate has other advantages as a detector, but it is not suited for the measurement of abundance, so that a machine capable of measuring both mass and abundance to a high degree of accuracy waited upon the improvement of electrical detectors. Recent advances in this respect have so increased the sensitivity of the mass spectrometer that the counting of single positive ions can now be accomplished. Thus, the mass spectrometrist has been able to incorporate refinements which would previously have been impossible because of the loss of beam intensity which they entailed, and to draw on the experience of the mass spectrographers for many of the improvements. For this reason, it is necessary when considering the development of the mass spectrometer to study at the same time the mass spectrograph. At the present time, the mass spectrometer can be used for almost any field of positive ion analysis, though photographic detection is still widely employed. Indeed a commercial mass spectrograph has recently been described for the elemental analysis of splids. The general field, which includes both types of instrument will be referred to as mass spectroscopy. The first, and probably the greatest achievement which can be claimed by the field of positive ion analysis was the discovery of the existence of stable isotopes, and the consequent realisation that the chemical properties of an element are

2

INTRODUCTION

determined by its atomic number and not its atomic weight. The steps associated with this discovery can be traced in the first papers on the technique. The first crude mass spectra were produced by Wien [2179] and Thomson, both of whom used the same principle for separating a beam of positively charged ions into its component masses. In the more refined experiment of Thomson (which he performed in 1910), a collimated beam of positive ions passed through a combined electrostatic and magnetic field. These fields were parallel to one another and perpendicular to the direction in which the ions entered them. The ions were therefore deflected in two orthogonal directions and the positions at which they emerged from the fields were recorded on a photographic plate. If the angles of deflection are small such an arrangement produces a series of parabolic curves on the photographic plate. Each curve corresponds to ions of a particular mass to charge ratio and the lengths of the curves are a measure of the energy spread in the incident beam. Later, in an attempt to estimate the relative abundances of the various ion species present, Thomson replaced his photographic plate by a metal sheet in which was cut a parabolic slit. By varying the magnetic field he was able to scan through a spectrum and measure the current corresponding to the various ion species. He must, therefore, be credited with the invention of the mass spectrometer as well as the mass spectrograph. The most noteworthy observation made by Thomson was carried out with his parabola spectrograph. He observed in a spectrum of rare gases, in addition to lines due to helium (mass 4), neon (mass 20) and argon (mass 40) a line corresponding to a mass to charge ratio of 22 which he could not attribute to any known gas [2022]. The existence of different mass forms of the same element had been suspected when it was found that many pairs of radioactive materials could not be separated by chemical means. It was further suspected that these pairs would be spectro¬ scopically indistinguishable. The name “isotopes” was suggested by Soddy [1905] for these differently radioactive forms of the same chemical species since they would be classified in the same place in the Periodic Table. It was thought that different mass forms or isotopes of the stable elements also existed, and that the unidentified ion observed by Thomson Was, in fact, a heavy isotope of neon. Although it was not until 1919 that Aston was able to demonstrate conclusively the existence of two isotopic forms of neon, the theory of the existence of isotopes following closely on Rutherford’s theory of the nuclear atom [1752] was already exerting an important influence in the formulation of theories to explain nuclear structure. Soddy [1906] appreciated that isotopes would show almost identical physical properties “save only as regards the relatively few physical properties which depend upon atomic mass directly”, but not that there could be small differences in for example the equilibrium constants and rates of chemical reactions involving molecules containing isotopic forms of the ele¬ ments. He predicted that these isotopic forms of stable elements would be found to be wide-spread, and that abundant isotopic species could be expected in the light elements magnesium (atomic weight 24.3) and chlorine (atomic weight 35.5) since their atomic weights differ so markedly from whole numbers. The mass spectrometer by means of which Aston was able to demonstrate the existence of stable isotopes of many of the elements [71] used a different arrangement of electric and magnetic fields to that used by Thomson. Aston was able to measure the masses of the various isotopes to an accuracy of the order 0.1% but his photographic recording was unsuited to the accurate determination of the relative abundances of these isotopes. Nevertheless, by 1924, Aston [73] had

INTRODUCTION

3

determined the isotopic species in about 50 stable elements, and his figures for the masses and abundances when used to calculate the chemical atomic weights gave values which were in good agreement with the chemically determined values. At about the same time that Aston built his first mass spectrograph, Dempster [455] also built an instrument for the study of positive ion beams. Although of simpler design and incapable of the accurate mass measurement which Aston’s instrument had achieved, it was better suited for measuring the relative abun¬ dances of the ion species since it measured the ion currents electrically. Using an electron-bombardment type of source, it was suitable for the study of the products of ionization and dissociation produced by electrons in gases and vapours. Thus, the earliest machines were capable of all three types of measurement which can be performed in mass spectroscopy — the measurement of the relative masses of the ions, determination of their relative abundances and the study of the ionization process. Nevertheless, the full potentialities of the methods of mass spectrometry were not realised in these early machines, and it was only after another twenty years’ development that the first commercial instrument was marketed. Today a wide variety of instruments is available and the steady improvement in performance and hence in application which has taken place in the past, as yet shows no sign of slowing.

CHAPTER 1

INSTRUMENTS

1.1

FOCUSSING

A most important feature in the design of a mass spectroscope, and one which it is convenient to use for distinguishing between different types of machine, is the method used to focus the ion beam. Focussing improves the degree of separation between adjacent masses, increases the intensity of the beam to be measured, and thus makes measurements of the strength and position of the beam more precise. The range of problems to which any particular mass spectro¬ scope is suited is controlled to a large extent by the efficacy of the focussing method. Types of focussing which can be used to concentrate a beam of ions all of the same mass are direction focussing in which the ion beam is focussed for a range of different initial directions when all ions in the beam are moving at the same speed, velocity focussing in which the ion beam is focussed when it contains ions travelling with a range of different speeds provided that they are all moving in the same initial direction, and double focussing in which the ion beam of varying initial speed and direction is brought to a focus. With rare exceptions, the focussing devices used in mass spectroscopy focus the ion beams in only a single plane and ate thus equivalent to cylindrical lenses. Instruments have been described which employ all these methods of focussing to first and higher order. Indeed methods are known for producing “perfect” double focussing and instruments using such systems have been constructed. Another important method of focussing a beam of ions is used in “time-of-flight” mass spectrometers which are described later. In this method all the ions of a particu¬ lar mass to charge ratio arrive at the collector at the same time, and can be distinguished from ions of other mass to charge ratios which arrive at the same collector at a different time. 1.2

THE PARABOLA SPECTROGRAPH

The parabola mass spectrograph of Thomson [2021] does not give a focussed ion beam and thus suffers from the disadvantages of low resolving power and sensitivity. In this instrument the ion beam is passed through parallel electric and magnetic fields. The locus of ions of a particular mass after deflection will be a parabola, the position of any ion on this parabola being a measure of its momentum. Few machines using parallel magnetic and electrostatic fields have been constructed or proposed, but they have been found useful for special applications since by their use it is possible to obtain two-fold dispersion according to mass and momentum [891] as mentioned above. The main justifi¬ cation for using the conventional parabola machine would now seem to be that

1.3

VELOCITY FOCUSSING

5

it provides a simple method of studying ion source characteristics and the steps in the dissociation of molecular ions [572, 603, 854, 856, 857, 1805, 2198] (see p. 282) though in the past it has also been used for ionization potential work [1891-1893] and the observation of negative ions [1032].

1.3

VELOCITY FOCUSSING

Aston’s mass spectrographs [71, 77, 85], as well as that of Costa [398], employed successive electrostatic and magnetic fields. Aston succeeded in obtaining velocity focussing [74] of his ion beams with the arrangement shown schemat¬ ically in Fig. 1. This arrangement of fields provides no direction focussing so that the intensity and resolution, though better than in the parabola spectrograph, ire not as high as in double focussing machines.

Fig. 1. Schematic diagram of Aston’s mass spectrograph.

There is no direction focussing such as would be obtained, to give the optical analogy, by a system of lenses, and an analogy can be drawn between Aston’s arrangement and achromatic prisms. Here, with a beam homogeneous as to mass but containing a spread in energy the dispersion produced by the electrostatic field leads to a velocity spectrum. This dispersion is cancelled by the magnetic field. The lines of focus of ions of different mass to charge ratio lie on a plane. A photographic plate can thus be used to record the entire mass spectrum. Later mass spectroscopes based on Aston’s design [1473, 1990] and other designs with velocity focussing [1760] have been constructed.

1.4

DIRECTION FOCUSSING

At about the same time as Aston was developing his first mass spectrograph, Dempster [455] was building his first mass spectrometer. Dempster based his method of focussing the individual mass beams on the method which had been discovered by Classen [348, 349] and used by him with an electron beam. Dempster’s apparatus is shown in schematic form in Fig. 2. The ion beam which is accelerated through the voltage V enters a uniform magnetic field at rightangles to its direction of motion. Ions of mass m and charge e enter the magnetic field with a velocity v given by

eV = Imv2

(1)

and traverse the magnetic field H in a circular orbit of radius R according to the equation

mv2 Hev =

R

(2)

6

INSTRUMENTS

1

Eliminating v from these two equations gives the well known formula for the Dempster instrument H2R2

(3)

m/e IV

In contrast to Aston’s mass spectrograph, Dempster’s mass spectrometer gave direction focussing of the ion beam but no velocity focussing. Whereas Aston has used his deflecting fields as “prisms”, Dempster used the magnetic field as a “lens”. Fig. 3 illustrates the focussing action of a homogeneous magnetic field on a beam of ions all of the same mass and energy, and shows how the ions, which move in the same radius irrespective of their direction, come to a first order focus after suffering 180° deflection. Focussing occurs mainly in the plane perpendicular to the magnetic field; as mentioned above, almost all the “lenses” which have been used in positive ion analysis are effectively cylindrical (but see p. 16). Since there is no velocity focussing, it is necessary to use an ion source which provides a sensibly mono-energetic beam, but the construction of such sources presents no great difficulty, and the entrance and exit slits can be of moderate width to give a beam strong enough for electrical detection and a resolving power sufficient for isotopic abundance and similar work. The accuracy of mass measurement which can be achieved with a machine of this type is, however, much lower than Aston attained. Nevertheless, for isotopic abundance studies or in other work where it is only necessary to measure masses of the order 100 or so to an accuracy sufficient to distinguish between peaks one mass number apart, the performance is adequate. A large number of machines using this method of obtaining direction focussing of an ion beam have since been constructed, and include commercial instruments designed for analytical work. For this reason it is worth considering Dempster’s design in a little more detail. Equation (2) written in a different form shows that R = mv/He so that all ions entering the magnetic field and having the same charge and momentum will follow a path of the same radius of curvature no matter what their mass, whilst ions of a different momentum move in a path of different radius. Thus it is

1.4

DIRECTION FOCUSSING

7

clear that this form of analyser produces an ion momentum spectrum which is also a mass spectrum when all ions enter the field with identical energy, so that a definite velocity is associated with each mass. This fact was pointed out by Aston [80] who objected, for this reason to the use of the term “mass spectro¬ graph to describe Dempster s instrument. Indeed, such instruments are some¬ times referred to as “momentum spectrometers”. Since they use electrical record¬ ing and are thus suitable for relative abundance measurements, they are also

Fig. 3. Illustration of the focussing action of a magnetic field. A mono-energetic beam of ions of tnass M is shoivn, the ions moving in paths of radius R. The ion beam originating at a point and having a half¬ angle a comes to a first-order focus (beam width «2R) after traversing 180°, and all ions pass exactly through the point of origin on each revolution.

sometimes referred to as “abundance spectrometers”. If, on the other hand, all the charged particles examined are of the same mass, a spectrometer with a 180° magnetic sector can be used to study the range of energies of the particles and the equipment becomes an energy spectrometer [1412]. From equation (3) one derives the relationship 2dR

dm

dV

2d hi

—R~

m

V

AT

(4)

which shows the effect of small changes in the various parameters on the radius of curvature of the ion paths. The width of the image of a line source is given by d = R • hoi— 1103, 1242, 1243, 1913]. It has been shown that third order focussing is possible by a sector with sharply defined boundaries [454] and Walton [2115] has designed a special drawing instrument which traces out the appropriate exit boundary required to give focussing of particles which have entered the magnetic field across an entrance boundary of arbitrarily chosen shape. An important focussing

16

INSTRUMENTS

1

effect in the direction of the magnetic field is produced by a magnetic sector [399, 876, 877, 1216]. This focussing is produced by the fringing field for particles which do not enter normal to the pole-face boundary and are not in the median plane, and systems are known for which stigmatic focussing can be achieved with sector magnetic fields [43, 314, 418, 879, 1796]; a stigmatic image gives the advantage of increased image intensity, which is an important consideration in high resolution instruments. Axial focussing in the magnetic field can also be achieved by appropriately shaping the pole faces [1070, 1184, 1224, 1735, 1736, 1848, 1849, 1851-1853]. Ewald and Liebl [606] have calculated the path followed by ions in passing through a toroidal condenser and used such a condenser in place of the more usual cylindrical condenser in a Mattauch-Herzog instrument to obtain stigmatic focussing [607, 609, 611, 1767]. Non-uniform magnetic sector fields have been studied by a number of workers [132]. If the field strength at radius r varies as r~n where n is slightly less than unity, the mass dispersion and resolving power can become much larger than in the case of a uniform field [27, 531, 1985] and values of resolving power of several thousand can be attained in a single-focussing instrument. It has been shown [154] that shaping the magnetic field in the radial direction to a special formula can be used to improve the focussing properties of a 180° spectrometer enabling the use of much more highly divergent ion beams [1123, 1208].

1.9

THE TROCHOIDAL-PATH MASS SPECTROMETER

A few methods of attaining perfect double focus are known which have been investigated by the construction of appropriate instruments. The simplest case of perfect double focussing is that in which the ions move in a plane perpen¬ dicular to a homogeneous magnetic field. If the ions originate at a point, they will all, when travelling in circular paths of radii appropriate to their masses and speeds, pass through this point after each revolution. This behaviour is illustrated in Fig. 3. Unfortunately, it therefore follows that this point of perfect focus does not depend on the mass or speed of the ions.

Fig. 11. Prolate (b > a) and oblate (b < aj trochoidal paths followed by ions moving in crossed electric and magnetic fields in the plane perpendicular to the magnetic field. The paths are the loci of points distant b from the centre of the circles of radius a as the circles roll along the line shown.

A slightly more complicated method of attaining perfect focus, and one which produces a spatial mass dispersion, was first used by Bleakney and Hippie [225, 918] who injected a beam of positive ions into crossed homogeneous magnetic

1

the TROCHOIDAL'PATH MASS SPECTROMETER

17

and electrostatic fields. It was already well known that the path of an ion moving under the influence of such a combination of fields and in a plane perpendicular to the magnetic field is a trochoid. This is the locus of a point on the radius of a circle as the circle rolls on a fixed straight line. The shape of these curves is illustrated in Fig. 11. If b is the distance of the point describing the trochoid from the centre of the circle of radius a, and 6 is the angle subtended at the centre of the circle as shown in the figure, then the equations of the trochoid are

x = a6 — b sind; y = a — b

cos6

At distances lira = A apart, the curve will cross in the same direction a line parallel to the x-axis, along which the circle rolls. It can be shown that this distance is given by

where H represents the magnetic field strength and E the strength of the electric field. Since neither the initial velocity nor direction of motion of the ions appear in this expression it is evident that initial values of these parameters do not affect the focus. It is worth noting, too, that the expression for the resolving power, defined in the same way as for the Dempster type instrument is m/dm = A/dA where A is the distance between object and image points, as opposed to m/dm = R/ldR, so that a higher resolving power can be obtained for the same area of magnetic field. Bleakney and Hippie constructed two instruments, one of which used a trochoidal path in the form of a curtate cycloid (b < a) and the other a prolate cycloidal path (b > a). Although, as has been shown above, A (and hence a), is independent of the initial velocity and direction of the ions, b does depend on this and is given, for an ion which enters the fields with a velocity v0 at an angle (90 — 0, and the interference from the peak at (M •— 1) vanishes, the correct ratio M + 1

_ n(l — P)

M

P

is obtained. As R increases, the measured ratio will approach the value n—1

~L

1 —P

'

P~

which is less than half the correct ratio. In the case R — 1,(1 —P)/P = 0.1 and the measured ratio will be 0.875 of the ratio in the absence of interference. The corresponding ratios if the heavy atom is deuterium are n =2,

1:

[

(n—1).

3.5

1 —P P

'

P-j

+ Rj

r(n— l)(n —2) (1 — P)2

1

DETERMINATION OF

2

180

1«

n(l — P)i +

R

.

IN ORGANIC COMPOUNDS

The conversion of the lsO in an organic compound into a volatile form suitable for analysis by mass spectrometer involves the use of one of the many methods [42, 579] which have been proposed for the direct determination of oxygen. One important method is that of ter Meulen [1390] by which the oxyger contained in the organic compound is converted quantitatively into water by vaporizing in a stream of pure hydrogen, cracking or pyrolyzing the compound at high temperature and passing the products over a nickel catalyst at 350°C. Another important method is that of Schiitze [1806] whose procedure was improved by Unterzaucher [669, 2066]. In the Schiitze-Unterzaucher method, the sample is thermally decomposed in a stream of pure nitrogen and the products are led over carbon at a temperature of about 1000°C, to yield carbon monoxide. This gas is converted into carbon dioxide by the action of iodine pentoxide. Doering and Dorfman [501] have obtained good results with this method. It should be noted that the conversion of the carbon monoxide to carbon dioxide is unnecessary if a high resolving power mass spectrometer is available for the analysis. M/AM for a mixture of CO and N2 is 2300. If the ter Meulen method of analysis is used the water may be examined directly, as described for the determination of deuterium, or the measurements may be performed on carbon dioxide. To do this, the water is equilibrated with carbon dioxide [1403] by sealing the water and carbon dioxide in a glass tube and shaking the tube for several hours at room temperature as described by Cohn and Urey [368]. The % lsO in the water may be calculated knowing the amount of water and carbon dioxide equilibrated and the equilibrium constant for the exchange reaction, which has the value 2.094 at 0°C [2141]. A method involving exchange at 100°C between potassium carbonate and water has also been described [2048], measure¬ ments of the isotopic abundance of 180 being performed on carbon dioxide liberated from the potassium carbonate. Dostrovsky and Klein [519] have reduced the time necessary for equilibration of the water and carbon dioxide from several hours to a few minutes by carrying out the reaction on a heated

78

3

THE MEASUREMENT OF IONIC ABUNDANCE

platinum wire at about 1000°K placed in a gaseous mixture of the two components. At this temperature, the equilibrium constant K has the value 2.005 [H2i60] [C160180] ~ [H21sO] [C1«02] The equilibrium constant for the reaction C1602 + C1802 ^ 2C160180 has the value 4 at room temperature and there is no need to correct for deviations from this value as is the case in deuterium analysis. If Ra and R& are the ratios of 12C160160/12C160180 before and after equilibration of x moles of water and y moles of CO2 and F is the atom fraction of lsO in the water, then [5x9] _

1

y

KRb T i

/

2

2.

x \ 2Rb T 1

2Ra

\

1 /

Fractionation of the isotopes during preparation of the samples for mass spectrometric analysis is also much less important in this case than in the case of deuterium, and there is not the same need to drive all reactions to completion. Examination of carbon dioxide does, however, introduce the added complication that the effect of the 13C isotope on the observed ratios must be corrected. It is usually the abundance of the lsO isotope which is required in tracer work, and in such measurements a correction must be applied for the effect of 170. The peaks at masses 44, 45 and 46 in carbon dioxide have the composition 12C1602, (i2Ci60i70 + 13C1602), and (12C160180 + 13C160170 + 12C1702) respec¬ tively. Peaks in the mass spectrum at masses 28, 29 and 30, which are prominent when the sample is bombarded with electrons having energies of the order 50 electron volts, correspond to the isotopic CO+ ions. In theory, these ions could also be used to obtain a measure of the isotopic abundance ratios, but the measurements are, in practice, complicated by the fact that most mass spectrom¬ eters show a prominent background peak at mass 28 corresponding to the presence of CO, and allowance must also be made for the fact that the relative probabilities of formation of 12CieO+ and 12ClsO+ from 12C160180 are in the ratios 0.485 and 0.515 respectively to that for the formation of 12ClsO+ from 12C1602. The ratio of 12C160180/12C160160 can be determined by measure¬ ments of the relative abundances of the peaks at masses 44, 45 and 46 [699]. If it is assumed that the natural abundance of 170 is 0.0004, and that the ionization efficiency is independent of the isotopic composition, then since the composi¬ tions of the peaks at the respective mass numbers are: (44) = 12C160160 (45) = i2ci60170 + i3C160160 (46) = i?C160180 + 13C160170 4- 12C170170 Then,

12C160180

(46) _ i2ci70170 — i3ci6Q170

12C160160

(44)

(46) (44)

(0.0004)2

2(0.0004) (45)

4(0.0004)2 (44) (44)

12C160180

(46)

12C160160

(44)

0.0008

(45) (44)

3(0.0004)2

and no assumption needs to be made about the abundance of 13C. Carbon dioxide can sometimes be produced directly from the sample; for example, methods of decomposing calcium carbonate have been discussed by

3.6

DETERMINATION OF

15N

IN ORGANIC COMPOUNDS

79

McCrea [1348] who has found 100% phosphoric acid to be a suitable reagent to use. Other suitable gases for the determination of lsO are nitrous oxide and oxygen, and these are often used in special cases, where they are more readily prepared from the sample than carbon dioxide. Nitrous oxide can be used when isotopic analysis of nitrates is necessary [699], while oxygen is often used for studies involving atmospheric oxygen, and is often prepared from water samples by electrolysis [169]. The use of oxygen gas has the two minor disadvantages that corrections are necessary in the event of an air leak during the measurements and that the life of the mass spectrometer filament is shortened when it is run in an oxygen atmosphere. For inorganic oxygenated compounds special methods have often to be employed. For example, silica can be reacted with carbon at 2000°C in a vacuum furnace. Carbon monoxide is evolved and can be used to determine the isotopic abundance ratios of the oxygen combined in silica [102, 1808].

3.6

DETERMINATION OF

15N

IN ORGANIC COMPOUNDS

A very suitable gas for the assay of 15N is nitrogen itself, and, except in special circumstances such as those mentioned above [699], it is always used for isotopic abundance determinations. The most generally applicable method for the preparation of nitrogen gas from nitrogen-containing organic chemicals is the classical method of Dumas. In this the compound is heated under carbon dioxide in the presence of copper oxide. Nitrogen and oxides of nitrogen are produced, and the oxides of nitrogen are reduced to nitrogen by reaction with heated copper. The main error in this method arises from the fact that the nitrogen is usually collected over 40% potassium hydroxide solution, which serves to remove carbon dioxide, and any acid or halogen vapours which may be present. Nitrogen is slightly soluble in this solution and the heavy isotope tends to be diluted by dissolved nitrogen unless the potassium hydroxide can be degassed before use. In other methods, ammonia is liberated from the organic compound, and then oxidized to nitrogen in a second reaction [1343]. Such methods have been discussed by Rittenberg [1702]. One such method for the production of ammonia involves the Kjeldahl procedure in which the sample, which is decom¬ posed with hydriodic acid, is digested with sulphuric acid in the presence of a catalyst to convert the nitrogen into ammonium sulphate. Ammonia is liberated by the addition of excess caustic soda, followed by distillation in a stream of air. The method is not universally applicable, but can be used with amino-, nitro-, nitroso- and azo-compounds and with hydrazones and oximes. The method fails with certain heterocyclic nitrogen compounds [1963]. Diazo ketones yield nitrogen directly when treated with hydriodic acid. Other methods of converting organic nitrogen to ammonia are listed by Rittenberg [1702]. Amides and amidine derivatives can be hydrolysed with alkali, while a-amino acids yield ammonia on treatment with triketohydrindene hydrate. With this latter reagent the isotopic constitution of the nitrogen in the amino group can be directly determined. The conversion of ammonia to nitrogen can be performed by the action of sodium hypobromite. Details of the preparation of the reactants and of a suitable apparatus in which the conversion can be performed in vacuo, are described by Rittenberg. The fraction of 15N atoms present in nitrogen is given by [14N15N] + 2[15N2] 2{[14N2] + [14N15N] + [15N2]}

80

THE MEASUREMENT OF IONIC ABUNDANCE

3

and the peaks at masses 28, 29 and 30 corresponding to the three molecular types present must be measured in the general case when nothing is known about the equilibration of the nitrogen. The equilibrium constant of the reaction 14N2 + 15N2 ^ 2 14N15N, given by [14N15N]2 K =-

- -

[14N2] [15N2]

has the value 4.000 at room temperature. Knowing the gas to have reached such an equilibrium, the fraction of 15N atoms present can be determined without reference to the peak at mass 30. If R is the ratio of the abundance of the ion of mass 28 to that of mass 29, then the fraction of 15N atoms present is 1/(2R + 1). (See equation (21) p. 72.) The main errors in the measurement of the relative abundances of the nitrogen isotopes arise from the presence of background peaks in the mass spectrometer and impurities in the nitrogen samples. The main background peak to be expected at mass 28 is due to CO+ and this must be allowed for in the measure¬ ments. If it is assumed that the background peak remains unchanged in height when the sample is inserted into the mass spectrometer this will usually limit the accuracy when the height of this peak is greater than about 1% that of the nitrogen. With sufficiently high resolution, the 12CieO+ peak and 14N2+ peaks can be resolved, and so can 13C160+ and 14N15N+ (M/AM = 5895 in the latter case). Another cause, of error is due to the application of a correction for any atmospheric nitrogen which has leaked into the sample from the air. The amount of such gas is determined either by measurement of the height of the associated argon peak at m/e 40 or of the oxygen peak at m/e 32. Because of its greater intensity, the peak at m/e 32 has generally been used to calculate the contribution from atmospheric nitrogen. Capindale and Tomlin [324] have pointed out that use of the oxygen peak implicitly assumes that all the oxygen in the sample comes from the air, and have been able to show by measuring the ratio of mass 40 to mass 32 that this is not so when sodium hypobromite is used to oxidise ammonia to nitrogen. Oxygen is evolved from sodium hypobromite, the amount present depending on the age and temperature of storage of the solution. They further showed that the peak at mass 30 was affected by the presence of NO+ ions due to the presence of nitrous oxide in the nitrogen obtained from the hypobromite oxidation [358]. The nitrous oxide which is present to the extent of 1.5-3%, was identified from the ratios of the isotopic peaks at masses 44, 45 and 46. These peaks had previously been assumed by some workers to be due to traces of carbon dioxide. It is always preferable to eliminate contaminants wherever possible, and to apply corrections only for substances which cannot be separated from the sample. Freedom from nitrous oxide (and also carbon dioxide) can be ensured by passing the sample through a liquid nitrogen trap. Evolution of oxygen from hypobromite can be prevented by the addition of 0.1% potassium iodide to this reagent [1859], enabling corrections for the presence of air based on the mass 32 peak to be made. As mentioned in the section dealing with lsO, nitrous oxide can be produced directly from nitrates, and is a suitable gas for measurement in the mass spectrom¬ eter provided that too large a background due to carbon dioxide is not present. The ratio of 15N/14N can be obtained from the peaks at 45 and 44 after correction for the presence of ions of composition 14N2,70 at mass 45.

3.7

DETERMINATION OF

3.7

13C

DETERMINATION OF

IN ORGANIC COMPOUNDS

13C

81

IN ORGANIC COMPOUNDS

A suitable material for the assay of 13C is carbon dioxide, and it can readily be prepared from almost all organic compounds by the Pregl method [1628] by burning them in a stream of oxygen, or, if this produces too violent a combustion, in air. The oxygen or air is first passed through a heated tube containing an oxidizing agent, then through “Carbosorb” to remove any impurities in the gas. The tube in which the sample is burned contains a universal filling which promotes the oxidation but retains such substances as the halogens, oxides of nitrogen and sulphur. The apparatus and procedure have been described in detail [1963]. Apparatus for the combustion of volatile organic samples has also been described [104]. For mass spectrometric measurements the carbon dioxide can be absorbed by a solution of barium hydroxide [1566], and this material taken to the instru¬ ment. Carbon dioxide can be re-generated from the barium carbonate by the addition of hydrochloric acid in sodium chloride solution. This procedure can be carried out in vacuo, using a “two-legged” tube apparatus similar to that for the production of nitrogen from ammonia. Wet oxidation can also be used to prepare carbon dioxide from many samples. The Van Slyke-Folch mixture is generally used and consists of chromium trioxide, potassium iodate, phosphoric acid and fuming sulphuric acid. The method [313] is of particular application to physiological specimens, and has the advantages, in cases where it can be used, of simplicity and of being capable of being carried out in vacuo. If preferred, the generated carbon dioxide can be condensed into a sample tube which may be sealed off and stored for mass spectrometric analysis [2146]. The method of sampling such material with the mass spectrometer is described in Chapter 5. It is not always necessary to oxidise the whole sample in order to convert the 13C into carbon dioxide. For example, if the 13C is contained in a carboxylic acid group, it is usually possible to decarboxylate by boiling in quinoline with copper oxide or copper chromite as catalyst [2147]. This has the advantage that the 13C when added as a tracer is not diluted by the carbon in the rest of the molecule, leading to greater sensitivity of detection of changes in the 13C abundance. There is no special difficulty in computing the abundance of 13C from the mass spectrum. The amount present is determined as in the case of the measurement of 15N in nitrous oxide, from the ratios of the peaks at masses 45 and 44; mass 46 need not be considered. The only correction which it is necessary to apply is for the contribution of (12C160170)+ ions to the 45 peak. It should be noted that when it is only required to compare the 13C content of samples of carbon dioxide or the 15N content of nitrous oxide samples which all contain the same amount of the isotopes of oxygen, there is no need to apply a correction for (12C160170)+ ions, and the differences in the ratios of 45/44 give directly the differences in 13C/12C ratios in the case of carbon dioxide, and the 15N/14N ratio in the case of nitrous oxide. 3.8

DETERMINATION OF THE ISOTOPIC ABUNDANCES OF OTHER ELEMENTS

The isotopic abundances of all the elements as they occur in nature are given in Appendix 3. Not all the elements can be converted into a suitable gaseous form for analysis in an electron-bombardment ion source and a variety of ion sources has been used for isotopic abundance determinations. Of the elements other than carbon, hydrogen, nitrogen and oxygen, the most often encountered in organic chemical work are sulphur and the halogens.

82

THE MEASUREMENT OF IONIC ABUNDANCE

3

Sulphur is usually measured as sulphur dioxide, [1652] and this can be produced from organic material by burning in a stream of pure oxygen. Hydrogen sulphide is usually converted to sulphur dioxide by precipitation as lead sulphide, and this is combusted in the usual way [1654]. Many inorganic compounds can also give sulphur dioxide when burned in oxygen, but special treatment is required for sulphate minerals [1653] which must first be converted to elemental sulphur or to a combustible sulphide. Sulphur dioxide (boiling point —10.0°C) can be separated from carbon dioxide (sublimation point —78.2°C) which is also produced when organic compounds are burned in oxygen, by vacuum distillation at low temperature, and water is removed with phosphorus pentoxide. Both parent ions and the fragment ions SO+ have been used in abundance measure¬ ment with no noticeable difference in results [2005]. There are a large number of volatile halogenated compounds which have been

Fig. 25. The mass spectrum of sulphur hexafluoride. Fluorine is anisotopic and the various positions in the spectrum at which the sulphur isotope ratios can be measured would make this a suitable substance for an investigation of the relative ease of fragmentation of fluorine atoms from the various sulphur isotopes.

used for abundance measurements on chlorine and bromine. The gases CI2 [252], Br2 [232, 2186] and HC1 [1489] have been used in measurements of the isotopic abundance ratios but have the disadvantage of giving large “memory” effects. Sometimes halogenated compounds are used, because of their volatility, to determine the abundances of the other elements in the compounds. The isotopic abundance ratios of titanium [945] have been measured in this way. Fluorinated compounds, which are more volatile than the other halogenated compounds, are often used and have the further advantage that fluorine has no heavy isotopes to interfere in the measurements. The mass spectrum of sulphur hexafluoride shown in Fig. 25 [470a] would be suitable for the study of the relative ease of fragmentation of fluorine atoms from the various sulphur isotopes. Organic compounds containing several halogen atoms are useful substances for comparative measurements. Many such compounds produce extremely weak “parent” ions, but give peaks corresponding to fragments. Carbon tetrachloride for example, gives prominent peaks at CCl3+, those at masses 117 and 119 being almost equal in intensity (117/119 = 1.022) and thus ideal for detecting variations between samples. Ions containing single bromine atoms also give a pair of almost equal isotope peaks. Inorganic compounds such as silver bromide [319] and sodium bromide [2167] have also been used in abundance measurements. Use of such materials in an electron-bombardment

3.9

DETECTION OF SMALL CHANGES IN ABUNDANCE RATIO

83

source involves the distillation of the sample into the source at high temperature, with the possibility of fractionation. Many of the elements cannot be analysed with an electron-bombardment source, due to the absence of any compounds of suitable volatility. Other types of ion source are dealt with in Chapter 4 and the elements which have been examined in the various sources are listed there. The method used for the examination of any particular element can be found by referring to the literature quoted in the list of masses and abundances in Appendix 3. Some of these methods present advantages in particular cases and have sometimes been used even when volatile compounds are available. For example, surface ionization sources have sometimes been used when only small amounts of sample are available for analysis. 3.9

DETECTION OF SMALL CHANGES IN ABUNDANCE RATIO

High sensitivity to changes in abundance ratio between samples is often of much greater importance in chemical work than a high absolute accuracy of measurement. Discriminations due to diiferences in mass or initial kinetic energy can often be ignored if they are known to remain constant with time. If it is required to measure small changes in abundance ratio of the isotopes of an element, it is first important to choose a molecule containing as large a number of the atoms of the element to be measured as possible. It can readily be shown that, provided the compound chosen does not introduce interference through the presence of the isotopes of other elements, the sensitivity of detection of changes is always increased by increasing the number of atoms. Consider an element consisting of two isotopes of masses M and (M + 1). Let the relative abundances of the isotopes be a and (1 — a) where a $> (1 — a). Then if the relative abundance ratio changes to (a + da) : (1 •— a — da), the ratio, R, of the peaks at (M + 1) and M will change by dR i

1 — a =-

1 —a— a

a

+

da

da a(a

da

+

(23) da)

If a molecule, which gives an ion in its mass spectrum containing N atoms of this element is examined, and the other elements in the molecule are considered anisotopic for the sake of simplicity, a series of isotopic peaks containing 0, 1, 2 ... N atoms of (M + 1) will be obtained. The probability of forming an ion containing N atoms of mass M and none of mass (M -fi 1) is, for the original abundance ratio, aN, and the probability of forming an ion containing (N — 1) atoms of mass M and one of mass (M + 1) is NaN 1 (1 — a). The change in ratio of these two peaks when the abundance ratio changes by the above amount will now be da dR jv

= N

a(a

\

N (5Ri

(24)

-f (5a) /

Furthermore, since (1 — a))a is small, the ratio of the peaks will have been brought closer to unity and thus be easier to measure on an amplifier of limited range, or in a mass spectrometer in which, due to interference from scattered ions, the small peak lies in the skirt of the large peak. If (1 a)ja is large a similar improvement in measuring accuracy is obtained, but then the peaks which must be measured are those corresponding to the two heaviest ions in the spectrum. If a and (1 — a) are comparable and N large, the relative abundance may be

84

THE MEASUREMENT OF IONIC ABUNDANCE

3

estimated from any pair of a number of peaks. The ratio of the (r + l)th to the rih peak will be given by N—r a

(25)

r+ 1

and the two peaks to be measured can be chosen to be of almost equal intensity. The relative abundance ratio of the isotopes of carbon has been estimated in this way using the parent ion from a fused-ring hydrocarbon. In this case there is only slight interference from the rare deuterium isotope. The method is also particularly suitable for use with chlorine and bromine, the number of atoms in the ion being limited only by the highest mass which can be measured in the instrument. Detection of small differences in isotopic abundance ratio between samples is facilitated by the use of one of the samples as a standard of reference. If the abundance ratios of this sample are measured before and after the measurement of any other sample, estimates both of the random error in successive measure¬ ments and of the magnitude of any slow drift in the readings given by the equipment can be obtained. Another method of increasing the sensitivity of the equipment to isotopic abundance ratio changes is to use a double collector system, in which the isotopic peaks concerned are collected simultaneously on separate collecting electrodes. Such a method was first suggested by Aston [78], and was used by Straus [i960] for measurement of the relative abundances of the nickel isotopes. Measurements of the abundance ratio were made directly by means of a null method. One of the ion collectors was fixed, and the other movable by means of a micrometer screw acting through a bellows. The system could be used over a range of isotopic mass ratios; the separation during the measurements on nickel was set either to two mass numbers when measuring the even-numbered isotopes or to three mass numbers when 61Ni was measured relative to 64Ni. Use of double collection enabled Straus to use a spark source in his measurements, the rapid variations in intensity being immaterial with ratio recording. The method was extended to the plotting of complete mass spectra using a spark source by Gorman, Jones, and Hippie [776] in a mass spectrometer in which the ratio of the intensity of any particular ion was measured to the total ion intensity as recorded by a monitoring electrode situated at the entrance to the magnetic analyser; a somewhat similar system has been used with a surface ionization source by Stevens and Inghram [1934]. There have been many examples of the use of double collection to extend the application to systems in which the output fluctuates with time, either because of fluctuations in the source or because of the low signal to noise ratio associated with weak ion beams. The system also improves the accuracy with which isotopic abundance ratios can be compared even when larger ion beams can be used. Most of the instruments have been constructed for measurements with a specific compound [2118] or, if for wider application, generally use a system in which one of the ion beams to be measured passes through a slit in the first collector before striking the second. The other isotope is recorded on the first collector, which, however is so wide that any other mass peaks differing by a few mass units are also recorded. Particular care must therefore be exercised to ensure that the samples used with such a system are free from impurities. A very successful design of double¬ collecting mass spectrometer has been described by Nier, Ney and Inghram [1502]. The use of a double collector has advantages even with ion sources such as the

3.9

DETECTION OF SMALL CHANGES IN ABUNDANCE RATIO

85

electron-bombardment type [167] which are very much steadier in operation than hot-spark sources. It is usual, with a double collector system, to record continuously the ratio of two peaks so that simultaneous measurement of the peaks is effectively achieved, and the errors introduced into ratio measurements by fluctuations of total ion beam intensity virtually eliminated. Using this method, Nier, Ney and Inghram were able to compare the isotopic abundance latios in two samples which were very nearly alike. The abundance ratios were of the order 100 and it was found possible to compare these ratios with an accuracy of the order 0.05%.

Fig. 26. Relay switching arrangement for the accurate measurement of the ratio of two peaks using a single collector. The two peaks to be measured are chosen by Vi and V2 and the signals backed off by Ei and E2. V3 corresponds to a blank region of the spectrum so that the zero of the amplifier can be continuously checked.

An accuracy of measurement almost equal to this was achieved by Schutten, Boerboom, v. d. Hauw and Monterie [1804] using a single collector. They employed a relay-switching arrangement of the form shown schematically in Fig. 26. With the switch in the position shown, the accelerating voltage Vi was applied to the mass spectrometer and one of the isotopic ion species was collect¬ ed. The voltage produced at the amplifier output could be balanced by a variable voltage Ei. Similarly, the output voltage corresponding to the second ion species (with accelerating voltage V2) was balanced by a second variable voltage E2. Accelerating voltage V3 corresponded to a blank part of the mass spectrum, and was used to check the zero of the amplifier. When the recorder showed the same output voltage on all three positions, the ratio Ei to E2 gave the isotopic abun¬ dance ratio. A ratio of the order 100 could be measured to an accuracy of about 0.1%. The period of a complete sequence of operation of the switches was adjusted between 1 and 10 seconds. The effect of a double collector was obtained by Taylor [1989] by using a circuit which scanned alternately the tops of two peaks and coupled in a precision potentiometer when the larger was being scanned. The divider was adjusted by hand until the peaks (displayed on a potcntiometric recorder) appeared equal in height. By backing off the recorder, the gain could be increased and the peaks kept on scale, and in this way a change in ratio of 1% could be expanded into a pen movement of more than 20 mm. Switched systems of the above type [613, 614] can be used on any pair of peaks in a mass spectrum. Just as a double-collecting system reduces the effects of variations in total ion beam intensity so the longer term effects on the performance of the instrument, which chiefly affect the reproducibility of measurement of the abundances of fragment ions in mass spectra over periods of months, can be minimized by the use of a double sample inlet system. Variations in abundance ratios due to such causes as changing ion source temperature have been discussed above. Since these variations occur very slowly their effects can be minimized by rapid

86

THE MEASUREMENT OF IONIC ABUNDANCE

3

change-over between the two samples which are to be compared. This can best be carried out by the use of two identical sample holders fitted with identical leaks through which the samples can be introduced alternately into the ionization chamber.

Fig. 27. Diaphragm valve to give short pump-out time so that samples to be compared may be examined with only a short intervening time interval.

Fig. 28. Arrangement by which alternate samples may be introduced in rapid succession to the ionization chamber. The device is positioned between the leaks and the ionization chamber.

One such system by which this may be accomplished uses valves manufactured by Metropolitan-Vickers Electrical Company Ltd., one of which is shown in schematic form in Fig. 27. When the valve is operated, the diaphragm is pressed directly on to the end of the tube holding the leak, leaving essentially only the volume of the leak itself to be pumped away. Another device by means of which the sample in the ionization chamber can rapidly be changed is shown in Fig. 28

3.9

DETECTION OF SMALL CHANGES IN ABUNDANCE RATIO

87

and was used by Epstein [590] in the work described in the next section. The positions of dumb-bell-shaped pieces of glass attached to discs of mild steel can be controlled electromagnetically so as to connect the sample flowing via a leak from one reservoir to the ionization chamber while the second sample is connect¬ ed to the pumps, and vice-versa. It is possible with such a system, fitted with taps by means of which the leaks may be closed, to change the sample in the ionization chamber in a few seconds, the pump-out time of the tubing between leak and ionization chamber being very low. One such system is described later. It is of particular use for comparing the spectra of isomers or for detecting the presence of an impurity which gives peaks only at masses which also arise from the major component present as well as for detecting very small changes in isotopic abundance ratio. The use of a double-inlet system with a double collector and ratio recorder for the detection of very small changes in isotopic abundance ratio has been described [590, 1356]. The papers emphasize the fact that the stability of the circuits is of great im¬ portance in this work and that the ion beams should be as intense as possible so as to raise the signal strength as far above the background noise variations as possible. They also show the advantage of an automatic recorder in this work in that the mean value on the final record can readily be determined in the presence of background noise. Changes in the mean value as successive samples are introduced of the order of the noise amplitude can be measured. Results for the variation of lsO and 13C abundance from a standard value, reproducible to the order 0.01% of the usual amount present, were obtained, and are discussed more fully in the next section. As an example of the accuracy attainable in isotopic abundance work, even when double collection is not availab e, consider the results given below for samples of carbon dioxide obtained in this laboratory by combustion of Penicillin-G which had been enriched in 13C, of the N-ethyl-hexamethylene-imine derivative of the same penicillin, and of un-enriched Penicillin-G. Since Penicillin-G contains 16 carbon atoms and its N-ethyl-hexamethylene-imine derivative 24, the ratio of the enrichments of the carbon dioxide obtained from these compounds should be 3 : 2 and provides a check on the accuracy of the results. The results were obtained by the measurement of the peak ratios for the un-enriched sample before and after every alternate measurement. The mass 44 peak height was adjusted to be equal within 5% for all measurements so as to minimize tne effects of any non-linearity in the amplifier, and the sample system was flushed out with each sample before any measurements on that sample were carried out. The ratio 44 : 45 was determined ten times at each sample introduction, and each sample was introduced three times (with interposed standard samples) to check that successive results lay within the calculated standard deviation. In three samples of un-enriched penicillin, the ratio of 12C : 13C was found to be 98.927 : 1.073; 98.931 : 1.069; and 98.931 : 1.069. A single enriched sample of penicillin converted to the N-ethyl-hexamethylene-imine salt gave a 13C en¬ richment of (0.058 ± 0.003)% or when converted to the sodium salt an enrich¬ ment of (0.092 ± 0.003)%. The ratio of the enrichments, is found to be (1.59 ± 0.10) : 1. It is estimated that with the precautions taken, a change of 0.3% in 13C content could be detected at the natural abundance level. The work describ¬ ed above using double collection and double inlet techniques has given results with a reproducibility 30 times better than this. An idea of the dilution factor which can be tolerated in isotopic abundance work can be obtained in the following way. Suppose that ordinary carbon dioxide con-

THE MEASUREMENT OF IONIC ABUNDANCE

88

3

taining carbon of composition 1.069% 13C and 98.931% 12C is used to dilute a carbon dioxide sample whose carbon consists of 2.069% 13C and 97.931 % 12C. Then a dilution with 332 times its volume will give a final carbon of composition 1.072% 13C and 98.928% 12C. This change can just be detected using a single inlet and single collection system. With the sensitivity attainable by the use of double inlet and double collection systems, a change in 13C content could just be detected in the above example with a dilution factor of 10,000. The effect on the final ratio of 12C/13C of various dilution factors and various degrees of enrichment of the tracer material as well as a change in the minimum observable abundance ratio difference can be obtained by reference to equation (32). 3.10

VARIATIONS IN THE NATURAL ABUNDANCE RATIOS OF THE ELEMENTS

Isotopes were discovered during studies of the radioactive elements, and were thought to be chemically identical although they were different in mass and in their radioactive properties. It was realized at an early date that the relative abundances of the isotopes of an element forming part of a radioactive decay scheme could differ from those for the same element when it is not found in a radioactive mineral. Richards and Lembert [1694] were the first to show that the atomic weight of lead varied in nature; “common lead” was found to have an atomic weight of 207.15, while lead samples from various radioactive minerals gave atomic weights as low as 206.40. Since these measurements, several similar variations have been found in elements associated with radioactive minerals, and as is mentioned below measurements of the relative abundance of a particular isotope, coupled with a detailed knowledge of the radioactive decay processes leading to its formation can be used in mineral age determinations. However, isotopic abundance variations can occur for processes which involve only stable isotopes because of small differences in physical and chemical properties which do exist between isotopes of the same element. Differences in vapour pressure between isotopes were shown to exist by Keesom and van Dijk [1087]; they succeeded in separating a sample of neon by distillation at low temperature into two fractions which differed in their chemical atomic weights by 0.09 units. Later it was shown [2067] that the equilibrium constants of reactions involving the hydrogen isotopes such as: H2 + 2DC1 ^ D2 + 2HC1 differed appreciably from unity, and soon afterwards that smaller but still observable differences in chemical properties existed for isotopes of other light elements [2068]. Urey [2069] has shown how it is possible to calculate the equilibrium constants for various reactions involving isotopes and has listed data for exchange reactions involving the isotopes of hydrogen, lithium, boron, carbon, nitrogen, oxygen, chlorine, bromine and iodine. Many of the reactions discussed can lead to fractionation of the isotopes in nature. For example, the calculated equilibrium constant (confirmed by experiment [1082]) for the reaction fC02 (gas) + H2180 (liquid) ^ ^C1802 (gas) + H2O (liquid)

(26)

showed that at 0°C there will be 4i% more lsO in the oxygen of the carbon dioxide than in the water. Carbon dioxide is generally made or found in contact with water, and will therefore in general have a high lsO content. Fractionation of the naturally occurring isotopes of the light elements can also be shown to occur due to the differences in those properties which are related to the kinetics of reactions [ioi]; there is a very slight difference in the activation energies of chemical reactions involving the various isotopes. As an example, the fact that

3.10

VARIATIONS IN THE NATURAL ABUNDANCE RATIOS OF THE ELEMENTS

89

algae have been found to contain 3% less 13C than the C02 in the solution in which they grow has been suggested by Urey [2070] as evidence that reactions are taking place whose rate constants are such as to concentrate the 12C in the algae. Such isotopic enrichments of one compound relative to another are of obvious importance in suggesting methods for the separation of the isotopes of the elements, and this is discussed later. The variations are also important in placing a limit on the accuracy with which chemical atomic weights can be stated without a knowledge of the isotopic abundances in the sample of the element used. These in turn will depend on the previous history of the sample. Often a fairly accurate estimate of the isotopic abundance ratios in a sample of any particular element can be made from a knowledge of the source from which the sample was obtained. In this connection, it is interesting that Thode [2004] examined a boron sample of unknown origin and found it to have a X1B/10B ratio similar to that of Turkish borax. The sample was later found to originate from Turkey. Nier [1496] found the 13C/12C ratio in a sample of sodium carbonate to be such as to lead him to suppose that the sample had been prepared from a limestone. This was found to be true on investigation of the sources of the sample. The significant work that has been done on the fractionation in nature of isotopes whose present relative abundance cannot have been affected by produc¬ tion or disintegration of radioactive isotopes has been summarized by Ingerson [999]. The elements which he discusses include hydrogen, helium, boron, carbon, nitrogen, oxygen, neon, silicon, sulphur, chlorine, potassium, argon, iron, copper, gallium, germanium, bromine, rubidium and uranium. The heaviest element for which there is decisive evidence of natural fractionation of the isotopes is germanium for which a variation of 0.7% in the isotopic abundance ratio has been found [782], but much larger variations are known in the relative abundances of the lighter elements. As the precision with which small changes in abundance ratio can be measured is increased, variations in the natural abundance of still heavier elements can confidently be expected to be discovered. The variations need to be determined as accurately as possible, since they affect nuclear physics calculations, chemical atomic weights, and the sciences of geology and biology in theories of the origin and condition of formation of naturally occurring materials. Chemical methods of following variations in isotopic abundance ratio by accurate measurement of atomic weight are not sufficiently sensitive to the very small changes involved, and only in the case of boron [274] has a chemical method been able to demonstrate a variation in natural abundance for an element involving only stable isotopes. The method of density determinations on water samples has been widely used in measurements on the abundances of hydrogen and oxygen isotopes. This method was used, for example, in confirming the equilibrium constant for the reaction given on the previous page [2139], but its use often involves the preparation of special samples. For example, in the work referred to, deuterium free water was employed. Natural variations in oxygen isotopic abundances were first demonstrated by Dole [505] by means of density measurements on water, thus confirming the prediction of Urey and Grieff [2068] that such variations would occur. As already mentioned, even when lsO was discovered, a reliable estimate of its abundance could not immediately be made due to the shortcomings of mass spectrography. The rarity of the heavy isotopes of nitrogen and carbon meant that it was not possible to deduce their presence by comparison of Aston’s isotopic masses with the chemical atomic weights, and these isotopes too were overlooked by the mass spectroscopists of the time. Under such circumstances it can be seen

90

THE MEASUREMENT OF IONIC ABUNDANCE

3

that there was little hope of discerning small changes in isotopic abundance ratios, and it was not until the techniques of mass spectrometry were improved and more widely employed that the full extent of natural variations in isotopic ratios became apparent. Perhaps the most widely studied element has been oxygen. This has been due both to its great importance in supporting life, its importance as the standard of the chemical atomic weight scale, its wide occurrence in combination with other elements and the fact that the seas provide a huge reservoir of oxygen in which local exchange processes can take place at an almost constant lsO level. Many of the lsO differences which have been observed can be explained when the ultimate origin of the sample can be traced to this supply. The lsO content of atmospheric oxygen is remarkably constant, samples taken at ground level from world-wide locations and at altitudes up to 51.6 km showing variations of only ±0.025% in 180/160 ratio [506]. This ratio is generally about 3% greater and that of ocean water about %% greater than that in fresh water. Changes in the lsO content, and also in the deuterium content of water occur between Polar and other ocean waterVnd between sea and fresh water for the following reasons. The freezing of water results in ice in which the lsO is enriched and the deuterium depleted [1171, 1996]. Alterations in the density of Polar ocean water in the vicinity of large masses of ice can thus be expected and these have been observed. The evaporation of water causes the heavy isotopes of both oxygen and hydrogen to be concentrated in the residue. Hence, fresh water which is formed by the evaporation and condensation of sea water should contain less lsO and D than sea water [413, 592]. Determinations of deuterium concentrations have been made for a wide variety of ocean waters. Values between 0.153 and 0.156% were found. For fresh waters, it has been observed that for a small country such as Britain, where the bulk of the rainfall is only an initial condensed fraction of the water vapour carried inland by the prevailing wind from the Atlantic, the deuterium concentration is about 0.0152% [347], close to that for ocean water. For a large land mass such as the United States, however, where a much larger proportion of the water vapour carried inland is condensed “en route”, values as low as 0.0133% have been measured [698]. A similar fractionation of the isotopes of oxygen was observed in the same series of measurements, and can provide a check on the figures, since a plot of the hydrogen isotope ratio against that for oxygen should give a straight line whose slope is the ratio of the vapour pressures of H2O/HDO to H20/H2180. Epstein and Mayeda [591] found that the lsO content of surface sea waters varied by about 6% and that the lowest values were obtained, as expected, from water diluted with melt water from the ice fields. In spite of the fact that the isotopic composition of oxygen is not homogeneous throughout the oceans, the accuracy with which lsO concen¬ trations may be determined has been developed until differences much smaller than these can be measured. The increased sensitivity has been applied to a study of oceanic paleotemperatures and the results obtained illustrate the importance of very accurate determinations of natural abundance variations. The accuracy achieved has been such that changes in lsO content such as would change the atomic weight of oxygen by only 0.0000007 units can be detected. Paleotempera¬ tures may be deduced from the temperature coefficients of the equilibrium constants of chemical reactions [580, 1000, 1998, 2070]. Calculations of the equilibrium constant for the reaction H2180 (liquid) ± -JCO3— (solution) ^ H2O (liquid) ± j,C180:j— (solution)

(27)

3.10

VARIATIONS IN THE NATURAL ABUNDANCE RATIOS OF THE ELEMENTS

91

have shown that the amount of lsO in calcium carbonate crystallised from water at, say, 0°C will be greater than that at higher temperatures. The temperature coefficient has been measured experimentally by allowing aquatic animals to deposit their calcarious shells in thermostatted tanks containing water of known isotopic composition and assuming that all such animals lay down shells in isotopic equilibrium with the surrounding water. Carbon dioxide is then generated from the crushed shells and its isotopes measured. If Ri represents the measured ratio of C0180/C02 in such a sample and R this ratio in a standard sample of carbon dioxide, one may define a quantity [A-C6H4] + [(CO)R]+

(66)

is given by the observed abundance of the ion (CO)R+ relative to the abundance of the parent ion. This gives a value for K. Use of the amount of parent ion still available for decomposition along the above path as a calibrating factor makes the measured value of K independent of the effect of fragmentation along other paths and a good measure of the lability of the particular bond considered. A plot of log (Ki'Ko) for 13 different m- and p- substituted acetophenones against the o value for the substituent (varying over the range —0.6 to +0.8) gave a fairly good approximation to a straight line. The value of Q found from the slope of the line (0.9) was similar to that found for substituted benzoic acids. Thus it would seem that quantitative values can be assigned to the influence of substituents on the stability of bonds in particular systems (e.g. Hammett’s a for aromatic systems) and to the tendency of a bond in such a system to be influenced by such substituents (e.g. the o-value for aromatic systems).

7.5

META-STABLE IONS

Some of the ions formed in the ionization chamber by electron bombardment of the sample vapour are meta-stable [923]. They are sufficiently stable to be withdrawn in large numbers from the ionization chamber, but their half-life is only of the order of a microsecond and many of them will dissociate during their passage towards the collector. Some of these ions (of original mass mi) will, of course, reach the collector without decomposition. Others will decom¬ pose (to give an ion of, say, mass m2) before leaving the ion chamber and thus peaks corresponding to both the initial and final masses of the meta-stable transition will be observed in the mass spectrum. The presence of meta-stable ions in a mass spectrum is made evident by the appearance in the spectrum of small diffuse peaks, generally in positions corresponding to non-integral masses, the intensity of which vary linearly with sample pressure. Such peaks are usually loosely referred to as meta-stable peaks and arise in

252

7

TYPES OF IONS IN MASS SPECTRA

a sector type instrument in the following manner. Suppose that after being formed in the ionizing region with zero kinetic energy the ion of mass mi falls through a potential difference of Vi before decomposing into the ion of mass m2. Let this new ion traverse the remainder of the accelerating voltage (V — Vi) and enter*the field-free region leading to the magnetic analyser. If the dissociation occurs with a very small release of internal energy and the dissociated neutral fragment moves with almost the same velocity as the ion of mass m2, the latter will enter the magnetic analyser with a velocity v given by s

im2v2 =

eVi + e(V — Vi)

(67)

The ion will traverse the magnetic field H with a radius of curvature R given by m.2V He

f2vy \ H2e /

r m22

1

l mi

(

(mi — m2)(V —

i

(68)

m2V

This is also the radius of curvature with which a normal ion of mass m* traverses the magnetic field, where (69)

Thus the meta-stable transition gives rise to a peak at a position corresponding to m* on the mass scale where m2

2

(mi _ m2)(V _ Vj)

m* = mi

(70) m2V

In general, dissociation can occur anywhere between ion source and collector and there may be a release of internal energy during the transition, so that conditions are more complex than indicated by the above equations and many of the ions are not recorded. A very small internal energy release will discriminate strongly against ions formed at large values of (V — Vi) and prevent their passing through the entrance slit of the spectrometer at the end of the accelerating region. Due to the directional focussing properties of the sector magnetic field, the decomposing ions most likely to be recorded are those which undergo the transition in the vicinity of the entrance slit, so that for the observed peak

mi

Examples of meta-stable peaks at various masses are shown in the mass spectrum of heptadecane in Fig. 113. The peak at mass 29.5 arises, for example by the transition (C4H9)+ (CaTL)"1" + CTL. It should be noted that although there is evidence to suggest that the neutral fragment in the above reaction is a methane molecule, and it has in fact been included as such in the equation the mass spectrum gives no information about the arrangement of the atoms forming any neutral fragments. Such evidence as has been assembled concerning the energetics of fragmentation reactions does, however, suggest that in all cases a group of neutral atoms given off in a single transition does so as an entity. Careful mass measurement shows the maximum intensity of the observed

7.5

META-STABLE IONS

253

meta-stable peak from the above transition to occur at 0.06 a.m.u. higher than the value given by the simple equation

m

*

7712

,

,

Vi

= - and corresponds to a value — = mi

0.995

in equation (70)

Many other examples of meta-stable peaks occur in various spectra throughout the book. See, for example, Fig. 103.

Fig.

113.

Meta-stable peaks in the spectrum of a heptadecane/air spectrum. Sixteen diffuse peaks due to meta-stable transitions can be seen in the spectrum.

The meta-stable peak shows a “tail” which is more pronounced on the high than on the low mass side. The process by which any meta-stable peak is formed can usually be determined by a process of trial and error, remembering that the apparent mass is smaller than either of the masses of the ions giving rise to the peak, and that the peaks at masses mi and m2 will be much larger than the meta¬ stable peak, in view of the low efficiency of collection of ions which have under¬ gone transition during acceleration and analysis. A nomogram of values of m* corresponding to various values of mi and m2 and which can be used for rapid determination of the origin of any meta-stable peak is included as Appendix 2. The above discussion has been restricted to single-focussing sector-type mass spectrometers. Hippie [926] has shown that the equations still apply in the case of 180° machines although the efficiency of collection of the final ions will be lower by a factor of about 3 than in a 600 sector instrument. The increased

254

TYPES OF IONS IN MASS SPECTRA

7

intensity of the meta-stable peaks in say, a 60° sector instrument is one of the advantages offered by such an instrument over a 180° instrument for qualitative identification. Meta-stable peaks will also occur in mass spectra obtained on double-focussing mass spectrometers, the ions most likely to be collected being once more those which undergo decomposition in the vicinity of the slit between the electrostatic and magnetic analysers. Analytical mass spectrometers are usually operated with the exit slit very slightly wider than the inlet slit. Since the magnification of the magnetic “lens” is almost invariably unity, such an exit slit is wide enough to be able to pass almost all the ions corresponding to any particular mass to charge ratio and there is negligible increase in peak height if the exit slit is widened. The same is not true when one is dealing with meta-stable ions and the ions which reach the collector slit spread over a greater width in the plane of this slit than stable ions. Increasing the exit slit width over that normally used will thus lead to an increase in the heights of meta-stable peaks relative to other peaks in the spectrum. Conversely, as the instrument slits are reduced in width to obtain high resolution, the meta-stable peaks become smaller relative to the other peaks in the mass spectrum and finally undetectable. In quantitative analysis of mixtures of known components, meta-stable peaks sometimes interfere with the measurement of the exact height of normal peaks occurring near them on the mass scale. In such cases the mass spectrometer is modified, as explained below, so as to suppress the meta-stable peaks. In qualitative identification on the other hand, the meta-stable peaks can be used to give valuable information about the arrangement of atoms in a molecule deduced from the constitution of the fragments formed, and are, therefore, carefully tabulated in such spectra. Meta-stable peaks Qan be recognized in many ways. As mentioned above, they are more diffuse in appearance than other peaks in a spectrum, and their height varies relative to other peaks if the exit slit width is changed or a derivative of the mass spectrum taken which, as is explained in Chapter 2, is equivalent to closing the slits. The heights of the meta-stable peaks can also be changed relative to other peaks in the spectrum by varying the potential of the ion repeller electrode located within the ionization chamber. Changing this potential changes the time which ions spend in the ionization chamber region (a considerable part of their total lifetime before collection), and since the detected meta-stable peak corresponds to ions which have decomposed in a particular region of their path the number decomposing as a function of time can be obtained. Hippie, Fox and Condon [925] found that the peak in the spectrum of n-butane at m/e 30.5 corresponding to the transition (C4Hio)+ -* (C3H8)+ + CH2

increased in strength by a factor of 6 as the ion repeller potential relative to the ionization chamber was varied from 1 to 5 volts in a sector instrument. The results are shown in Fig. 114. This figure also shows the variation of the metastable peaks at 39.2 and 31.9 formed respectively by the reactions (C3H7)+ -> (CsHsf + Ha and (C4Hio)+ -> (C3H7)+ + CH3

In the same figure is shown a typical variation in the strength of a “normal” peak. The large change between 1 and 2 volts is due to changes in the focussing conditions caused by changing the repeller voltage. Although only crude measure¬ ments of peak strength variation are necessary to identify a peak as meta-stable, the method can be refined to yield values for the half-life of an ion which under-

7*5

META-STABLE IONS

255

goes meta-stable decomposition if the total field within the ion chamber is corrected for the effect of penetration into it of part of the main accelerating field [926]. A double-focussing mass spectrometer is more versatile than a single¬ focussing machine for such measurements, since the focussing is not so much affected by changes in repeller voltage. The time between formation of an ion and its reaching a particular point on its path can also be affected by varying

Fig.

114.

Variation of the heights of meta-stable and fragment peaks as a function of ion-repeller voltage.

the accelerating voltage during successive spectra when the spectra are scanned by varying the magnetic field. This method could be used to supplement information obtained by repeller voltage variation. Another method of detecting meta-stable ions and one which can be used to give further information about their origin involves measurement of the kinetic energies of the ions. A normal ion will arrive at the collector with an energy equal to eV where V is the accelerating voltage. If the collector electrode is at such a potential that it repels the approaching ions the kinetic energy at the moment of collection will be lessened and, if the potential of the collector is only a few volts more negative than the ion chamber, ions which for any reason have lost kinetic energy during their passage through the instrument will not be recorded. When an ion of mass mi which has traversed the entire accelerating field decomposes to give an ion of mass m2 the kinetic energy is shared between the fragments in the ratio of their masses. The positive ion of mass m2 will possess only m2/mi of the initial kinetic energy and will thus be stopped by a repelling potential of m2 V/mi. Ions which decompose before traversing the entire accelerating field will not lose quite so much energy. The repelling potential can be used either to suppress all meta-stable ions or to obtain exact information on the kinetic energy possessed by the ions and hence on the ratio mi/m2. If this ratio is K, then mi = K2m* and m2 = Km*. It is thus possible to determine mi and m2 uniquely in the event that values for these masses cannot be selected from Appendix 2 due to the fa9t that m* is known with insufficient accuracy. Such a measurement could be used for example to resolve the problem of the origin

256

TYPES OF IONS IN MASS SPECTRA

7

of a meta-stable peak at about mass 38 in the mass spectrum of propylene. Socony Vacuum Laboratories attribute this [45] to the reaction (C3H4)+ -> (C3H3)+ + H, whilst Bloom and co-workers [239] point out that an alternative explanation would be (C3He)+ (C3H4)+ + H2. Values of m22/mi for these two reactions differ by only 0.064 a.m.u. One of the difficulties in working with meta-stable peaks lies in their low intensity. It has been estimated by Bloom, Mohler, Lengel and Wise [237] that if the strongest peak in a mass spectrum is assumed to be of unit intensity, then the intensity of m* divided by the product of the intensities of mi and m2 is usually of the order 0.01. They estimate that for the transition to be observable in their equipment both mi and m2 would need to be at least of intensity 0.05. Despite the variety of ways in which meta-stable ions can be recognized it is still all too easy to overlook the presence of such an ion especially when the meta¬ stable peak is very weak or is superimposed on an intense peak in the mass spectrum. Thus, even in the case of n-butane which is one of the most widelyused compounds in mass spectrometry several new transitions were recently reported [1741] and even this study was not exhaustive, since no tabulation was made of diffuse peaks which were present below mass 12. This region of the spectrum is all too often ignored when spectra are plotted, but the wealth of information often available in this region is well illustrated by Fig. 115. DISSOCIATIONS

IN

ACETYLENE.

Fig. 115. Mass spectrum of acetylene below mass 12 (see p. 280).

Meta-stable peaks very close to whole number masses arise for loss of mass 1 or 2 in the transition. Only 6 transitions involving mass 1 are listed by Bloom [239] in a study of the mass spectra of 170 hydrocarbons although adjacent intense mass peaks are extremely common in mass spectra. A special search for such ions could be undertaken making use of the favourable properties of the

7.5

META-STABLE IONS

257

cycloidal mass spectrometer. In this instrument, meta-stable ions which break up in the analysing region give rise to “smearing” of the mass peaks. Considering the cycloidal motion as circular motion in a moving co-ordinate system, it can be shown that the effect of a meta-stable transition is to reduce the time of flight of the ion and produce a linear displacement. If the apparent mass of the ion m* is plotted against the angle which has been traversed at the moment of decomposition a curve of the form shown in Fig. 116 is obtained. The curve

Fig. ii6. Curves showing the apparent mass at which an ion which undergoes fragmentation along its path in a cycloidal mass spectrometer will be recorded. The two curves are for ions which have lost 50 /0 and 33\% of their initial mass in the fragmentation.

shows the conditions for a prolate trochoid (see Chapter 1) for which a — 1, b = 7i and the ion of initial mass m loses a neutral fragment Am and becomes an ion of mass (m — Am). The cycloidal motion has been considered as circular motion in a moving co-ordinate system, the angle plotted as abscissa being the angle traversed in the circular motion before fragmentation occurs. The apparent mass starts for 6 = 0 at (m — Am) since the motion is rigorously independent of injection voltage and ends at m for 0 = 360 corresponding to no break-up having occurred during the traversing of the cycloidal path. Over the rest of the path, however, the curve will be somewhat dependent on the velocity of injection (i.e. on b/a) and on the percentage of the initial mass lost in the transition. When

258

TYPES OF IONS IN MASS SPECTRA

7

33^% of the initial mass is lost, the apparent mass extends upwards to a maximum value of about (m + 0.5 Atn) but this limit becomes greater as Am increases as can be seen from the figure. As in all mass spectrometers, the efficiency of collection of ions which undergo a meta-stable transition will be low, but it can be shown that the cycloidal mass spectrometer has the property that ions cannot reach the detector if „ „ m(2 A m — m) cos2 d A+ + A+ has been detected, and peaks have also been observed at twice the atomic weight of mercury in the mass spectrum of this element. Such peaks which are formed as a result of a collision process and not by spontaneous disintegration are treated in a following section. The mechanism of formation of a peak is not, however, deducible by mass measurement and the peaks formed obey the same equation as though the mass to charge ratio had been doubled in a meta-stable transition. Using the nomenclature already employed m2

2

m* =-= 4mi mi

so that the meta-stable peak occurs at twice the atomic weight of argon. An organic compound in which a similar effect has been observed is dichloromethane. The presence of the chlorine with its distinctive isotopic pattern can be seen in the meta-stable peaks, but the isotopes are now apparently separated by a mass of 4 instead of 2 mass units, and the peaks occur with heights in the ratio 9 : 6 : 1 at masses 168, 172 and 176. Occasionally, meta-stable peaks corresponding to formation of an ion of a particular formula in a transition, and also to break-up of an ion of this empirical formula in another meta-stable transition can be observed in the same spectrum. For example, in the spectrum of anthraquinone (C14H8O2) of molecular weight 208, meta-stable peaks at masses 155.8 and 128.3 correspond to the transitions 208+ -> 180+ + 28 and 180+ -> 152+ + 28 respectively. The atomic compo¬ sition of the neutral fragments lost in these transitions can be obtained by accurate mass measurements performed on the positive ions produced in the transitions. These measurements show that the formulae of the ions of masses 180 and 152 are respectively C13H8O and C12H8. Thus each loss of mass 28 corresponds to loss of CO. Presumably, the carbon atoms lost are those attached to the ketonic oxygens, and 2-stage fragmentation would seem necessary to preserve the final ion as a single entity. This mode of break-down of the parent anthraquinone ion is quite unexpected, and the detailed process is only made

260

7

TYPES OF IONS IN MASS SPECTRA

clear by the presence of the meta-stable ions. The relevant ion reactions taking place are thought to be

fluorenone

o-diphenylene

Further confirmation of this reaction chain is obtained from the mass spectrum of fluorenone which shows marked similarity to that of anthraquinone and suggests that most of the fragment ions below mass 180 may be formed through the fluorenone intermediate. The relevant spectra are shown plotted in Fig. 118.

ANTHRAQUINONE

‘| 1 40

^

)■—Y*- “ 60

80

IOO

I 180

-1—^-1— 1—r 140

160

180

800

880

Fig. ii 8. Mass spectra of fluorenone and anthraquinone showing the similarity in the cracking patterns.

The meta-stable peaks are also important in giving information on the number of stages involved in arriving at a particular ion formula by fragmentation of the parent ion. For example, the ion of mass 91 in the spectrum of t-butylbenzene has been shown by Rylander and Meyerson [1755] to be formed by the process [C6H5-C(CH3)3]+ -*[C«H5(C3H6)]+ + CH3 -> (C7H7R + C2H4 + CH3 A meta-stable peak at mass 69.6 confirms the latter stage of this reaction.

7.5

META-STABLE IONS

261

_ Meta-stable peaks can sometimes be used to infer the presence of ions not visible in the mass spectrum. For example, in the mass spectrum of symmetrical trioxane (M.W. 90) a very large peak occurs at mass 89 and any parent ions which may be present are obscured by the heavy isotope of this ion species. A meta-stable peak at mass 88 shows that the transition 90+

89+ -f H

is occurring and would suggest mass 90 as a possible parent mass in the event that this were the spectrum of an unknown compound. Evidence of a different kind is obtained from the mass spectrum of ethyl chloride at a nominal electron energy of 9.2 eV [375]. The ionization potential of this molecule is 11.2 eV and it was thought that ions might be formed in, for example, a pair-production process. The most prominent ion in the spectrum is at mass 28 and a meta-stable peak at mass 12.4 which also occurs shows that this ion is formed by the process (C2H5CD+ -> (C2H4)+ + HC1 Thus, although they have not been detected, positively charged parent ions are being formed, probably by a double-collision process, which shows that other explanations such as ion-pair production do not explain the whole peak height at mass 28. No parent ions could be detected at masses 64 and 66, perhaps due to incorrect setting up. The information obtainable by the listing of the meta-stable peaks in a mass spectrum is useful in analytical work in two ways. Firstly by revealing that a neutral fragment of a particular mass is eliminated in a single transition it enables groups present in the molecule to be deduced. For example in the mass spectrum of a particular hydrocarbon which was being studied in an attempt to identify it, the transition p+ ->(p —

69)+ 4- 69

was observed. Loss of C5H9 in this way immediately suggests that the parent ion includes a weakly-bonded group of this formula, probably cyclopentyl. The second kind of information enables one to deduce that a prominent peak is not formed in a single transition from the parent, but is formed by elimination of two groups in consecutive stages. One example is afforded by the peak at mass 152 in the spectrum of anthraquinone, which if this had been an unknown compound would have suggested the ready loss of mass 56 from the parent ion. It is only the meta-stable peak which shows that this loss occurs in two stages. Numerous examples of the use of meta-stable peaks in analysis are given in Chapter 9. Meta-stable transitions involving doubly-charged ions are seldom observed in the mass spectra of organic molecules. However, the mass spectrum of anthra¬ quinone also provides an example of such a transition. A group of peaks around mass 90 and separated by half mass units is due to doubly-charged ions, the most prominent of which corresponds to 180++ or doubly-charged fluorenone. A meta-stable peak at mass 64.2 corresponds to a transition in which an ion of mass to charge ratio 90 breaks to give an ion of mass to charge ratio 76. Actually, the ion at mass to charge ratio 76 in a doubly-charged ion of mass

262

TYPES OF IONS IN MASS SPECTRA

7

152 thought to be due to doubly-charged o-diphenylene. The transition is thus due to 180++

-*■

152++ -f- 28 or (fluorenone)++ -»■ (o-diphenylene)++ + CO

The apparent mass of a meta-stable ion gives no information concerning the reason for the transition, and depends only on the initial and final ionic mass to charge ratios. The mass spectrometer has been widely used in the study of collision-induced ionic dissociations since it has been found that almost any decomposition which is stoichiometrically possible can be produced by collision processes. Ions produced in this way are discussed below. 7.6

RE-ARRANGEMENT IONS

Many compounds when examined in a mass spectrometer give fragments, either charged or uncharged whose origin cannot be described by the simple assumption of the cleavage of bonds in the parent ion, and which are, in fact, due to atomic re-arrangement at the moment of fragmentation. If a peak occurs at an unexpected mass, it is necessary to be sure that it does not arise from the presence of an impurity or that it is not formed by an intermolecular re¬ action. In some cases, such a peak may be so large (even forming the base peak of the spectrum) that it must be due to the major component present. The molecular weight of the compound responsible for the peak may be confirmed by measuring its rate of decay as the sample effuses away; the fact that it is formed in an intramolecular process may be proved by showing that its relative abundance does not vary with the sample pressure, and it can also be shown in many cases that a high electron energy is necessary for the formation of the peak, and the peak may disappear below about 20 eV energy. In most spectra, the changes produced by increasing the electron energy above about 25 eV are small. Even when the conditions under which large quantities of energy are introduced to molecules are changed, for example when molecules are bombarded with high energy particles in radiolysis experiments, some strik¬ ing similarities with mass spectra have been found [300, 1349], even extending to the re-arranged products [1483]. Re-arrangement peaks which are not sensitive to small changes in bombardment conditions can be useful in identificational work; peaks at certain masses formed by a re-arrangement process are charac¬ teristic of certain chemical groups, for example at mass 19 for alcohols. Atomic re-arrangement accompanying fragmentation is known to occur to a certain extent in almost all spectra of molecules containing two or more carbon atoms. Especially common are re-arrangements which involve migration of hydrogen atoms [1205, 1363]. This is to be expected on account of the extreme lightness of hydrogen. In some cases such as the formation of an ion of mass 43 in the mass spectrum of neohexane [(CHsjsC'CEMCEh], the fact that re-arrangement is taking place is obvious. The ion formed can only be of formula (C3H7)"1" and its formation must therefore involve the migration of a hydrogen atom as well as the rupture of two C-C bonds. The same remarks apply to the formation of ions of mass 29 (C2H5)+ in the mass spectrum of isobutane [(CER^CTI]. In the case of unsaturated hydrocarbons, re-arrangements are even more common. Indeed the spectra of various isomers of the same molecular formula are often almost identical and it seems as though the structure of the molecule is lost at the moment of ionization and that randomization of the bonds has occurred. The spectra of the isomeric pentenes are shown in Fig. 119 and show the great similarity of the spectra especially for the branched isomers. A similar

7.6

RE-ARRANGEMENT IONS

263

effect has been observed for ten isomers of formula C5H8 [1416]. Field and Franklin [637] point out that the activation energies for re-arrangement processes in ions are usually quite small as compared to those in neutral molecules. These small activation energies permit a high degree of lability of the atoms and bonds 100 &o

2-METHYL-2-BUTENE

60 40 2.0 .

20

30

I.

SO

60

70

so

60

70

50

60

70

50

60

70

100 80

3-METHYL-1-BUTENE

60 40

20 20

30

40

100 80

2-METHYL- I - BUTENE

60 40 20

100 80

2-PENTENE

60 40

20 20

30

100 80

I - PENTENE

60 40 20

20

30

40

Fig. 119. The mass spectra of the isomeric pentenes. The spectra of all three of the methyl butenes are similar one to another and to that of 2-pentene; i-pentene has a more characteristic mass spectrum.

in a molecular-ion. The rate at which the re-arrangement reaction proceeds will depend on the energy and entropy of the activated intermediate state in the process. In hydrocarbons, there is usually little energy difference between a number of atomic arrangements which can be produced, so that a number of competing reactions occur giving a number of different re-arranged fragments. Such re-arrangements are termed “random” re-arrangements by McLafferty [1361] but this term does not imply that all bonds in the molecule lose their

264

TYPES OF IONS IN MASS SPECTRA

7

identity simultaneously followed by a random re-grouping of the atoms; the energy required for this to happen would be much larger than that available, and the term merely reflects the large number of different processes which can occur. But not all re-arrangements are easy to detect, and some may involve only subtle changes. Re-arrangements involving the neutral fragments formed during breakdown of the parent ion are more difficult to detect than those which involve the charged fragment. Two neutral fragments may combine to give a stable molecule and careful measurements of the energetics of the reactions taking place or a study of the meta-stable peaks in the mass spectrum are necessary in order to establish the state of aggregation of the neutral fragments, and thus to determine whether such re-arrangements are occurring. In spite of the diffi¬ culties, re-arrangements involving the neutral fragments have been known for many years [452]. Similar measurements will also give information about the structure of the ionized fragments and it must be remembered that although an ion may be of the correct mass to be formed by simple bond rupture, re-arrange¬ ment of the atoms within the ion may yet be occurring. Although the number of carbon and hydrogen atoms contained in the ions of a particular mass can easily be determined, it is not possible to state which of the hydrogen atoms in the original molecule have gone into a particular fragment unless isotopically labelled compounds are used. Work which has been performed with such com¬ pounds has shown that re-arrangements of hydrogen atoms do occur, even in cases where such re-arrangement involves an interchange of atoms between the separating fragments. For example, in l:l:l-trideutero-ethane [1784] ions of (CFf2D)+ and (CHD2)+ are formed. Also (CH2D)+ ions are observed in the spectra of 2-deutero paraffins [1947], and other work using deuteriated hydrocarbons [290, 387, 388, 1352, 1947] has demonstrated the very wide occurrence of hydrogen re-arrangement. McFadden and Wahrhaftig [1352] showed from the examination of compounds such as CaDv'CFR that ions such as (C2D4H)"1" must be formed by hydrogen exchange between the 1-3 or 1-4 carbons and that hydrogen re-arrangement is not restricted to adjacent carbon atoms. A similar observation may be made in unsaturated hydrocarbon molecules without the aid of deuterium labelling, for ions (C2H4)+ are observed in the spectrum of 2-butyne [1205] and (C2H5)+ and (C3He)+ in the spectrum of 2:3-pentadiene [1296]. Meyerson and Rylander [1396] have studied the formation of the ions of mass (p — CH3) in the spectrum of p-xylene. A meta-stable peak at mass 78.4 shows that the reaction 106+ -> 91+ + 15 or

(C8Hio)+ -*

(C7H7)+ + CHs

can occur in a single stage, and it seems reasonable to suppose that the methyl group lost is one of the side-chains present. This assumption can easily be checked by the examination of a-deuteriated xylenes. For a,a'-dideuteroxylene, for example, one would expect the ion (C?H6D)+ to be formed by loss of one of the CFBD groups. In fact, peaks occur at masses 91, 92 and 93 in the ratio 1 : 31 : 7 showing that hydrogen exchange has occurred between the side-chain and the ring before or during dissociation. Results showing exchange of hydrogen between the side-chains and the ring are also obtained when a-deuteroxylene and o-deuteroxylene are examined, and similar evidence is obtained from the spectra of a-, o-, m- and p-deuterotoluene [1756]. The peaks at masses 91 and 92 in the spectra of the latter three compounds show that the ions (CvFUD^ and (C 7 FI 7)41 are formed in the ratio of 9 : 1 and thus that 10% of the atoms lost in forming these peaks are deuterium which must come from the ring, although only a

7.6

RE-ARRANGEMENT IONS

265

peak at mass 92 formed by loss of H from the methyl group would be expected. However, contrary to what one might expect, the spectrum of a-deuterotoluene shows only slightly greater loss of deuterium relative to hydrogen than do the spectra of the ring deuteriated isomers. Exchange does not, however, occur for atoms on the carbon ft to the ring. In the spectrum of ^-deuteroethylbenzene, for example, the peak at mass 91 corresponding to loss of (CH2D) is not ac¬ companied by a peak due to loss of (CH3) and the corresponding ions formed by loss of the methyl group from a-, o-, m- and p-deuteroethylbenzenes all retain the deuterium atom. Detailed examination of the results for the deuteriated toluenes show that loss of any of the hydrogen atoms from this molecule is equally likely to occur. This suggests that re-arrangement of the carbon skeleton of the molecular ion has occurred whereby all the carbons have become indis¬ tinguishable, presumably by expansion of the ring to give a tropylium ion, since the carbons would not be indistinguishable in a benzyl ion. Re-arrangement processes such as this involving atoms heavier than hydrogen are discussed below. Perhalogen compounds often give “random” re-arrangements similar to those found with hydrocarbons [1363]. For example, the mass spectrum of CBrF2CF:CFCBrF2 shows] a significant (C3Br2F3)+ ion. Even in saturated com¬ pounds such as l-fluoro-3-bromobutane the spectrum of which is shown in Fig. 120 re-arrangements often make it impossible to confirm the structure by mass spectrometry; in this case, it was possible to confirm the structural formula by nuclear magnetic resonance.

Fig.

120.

Mass spectrum of i-fluoro-3-bromobutane.

The spectra of many hydrocarbons which have been labelled with 13C have been reported. For example, Stevenson [1948] reports the mass spectra of the isomeric propanes and butanes labelled with a single atom of 13C. He observed that in the dissociation of propane-l-13C and propane-2-13C the relative intensities of certain Ci and C2 ion fragments were indistinguishable so that in the states of the parent ions (C3Hs)+ which dissociate into these fragments, the carbon

266

TYPES OF IONS IN MASS SPECTRA

7

atoms must become essentially indistinguishable. For isobutane, too, the mass spectra of the isomeric compounds labelled with a single atom of 13C are indis¬ tinguishable for these Ci and C2 fragments. The configuration of the carbon atoms within hydrocarbon ions can be determined in many cases by energetic considerations [1757] without the use of labelled compounds. For example, Stevenson [1949] has deduced from the appearance potentials of the alkyl ions (C2H5)+, (C3H7)+, (C4FI9)4' and (CsHu)"1" in the mass spectra of the n-C5 to n-Cs paraffins that the propyl, butyl and amyl ions have the secondary structure, but that the accompanying free radical is sometimes of the primary structure, some¬ times the secondary. Wallenstein and his co-workers [2112] have also considered the structure of the (C3Hv)+ ions which are abundant in so many hydrocarbon spectra, and put forward a suggested structure to account for the stability possessed by this ion in the form in which it occurs in mass spectra. It now seems that the structure assumed by such ions may be a cyclopropane ring co-ordinated to a proton. Similar cationated cyclopropane rings have been postulated by Rylander and Meyerson [1755] in order to explain the breakdown of tert-butylbenzene-a-13C. They found that re-arrangement ions containing 7 carbon atoms were formed. About ^ of these ions, contained a 13C atom, about f did not. A puzzling feature of the reaction was that although C9 ions were present there were no Cs ions the presence of which would be expected if the parent ion lost 3 successive methyl groups. A meta-stable peak at mass 69.6 in the spectrum of tert-butylbenzene shows that the process 119+ -> 91+ + 28 occurs. Assuming that the (119)+ ion is formed by simple cleavage of a methyl group from the tert-butyl side chain, the isotopic distribution actually found in the C7 ions will be obtained if the phenyldimethylcarbinyl ion so formed has re¬ arranged before fragmentation so that the three side-chain carbons have become indistinguishable, i.e. symmetric with respect to the phenyl group. Rylander and Meyerson therefore propose that a cyclopropane ring co-ordinated with a phenyl cation occurs as an intermediate in the fragmentation of tert-butylbenzene. They also point out that a methyl cyclopropane cation would explain the fact that centrally 13C labelled neopentane gives 100% labelled C3 and C4 fragments, but C2 fragments only 50% labelled [1051, 1207], and give a molecular orbital formulation for a protonated cyclopropane ring. Re-arrangement of the carbon skeleton may sometimes occur, which could involve the opening of a benzene ring. Thus meta-stable peaks at masses 59.4 and 75.1 in the spectrum of ethylbenzene [1394] show that the processes 105+ -> 79+ + 26

i.e. (p — 1)+ -> (C6H7)+ + C2H2

and 79+ -* 77+ +

2 i.e. (C6H7)+

-> (C6H5)+ + H2

are occurring and that this process rather than cleavage of the ethyl side-chain accounts for some at least of the formation of the ions (C6bl5)+. Support for the tropylium structure suggested to occur in the fragmentation of toluene ions [1756] has been obtained from the mass spectrum of toluene-a-13C [1395]. Almost exactly f-th of the (C5H5)"1" ions contain the 13C atom supporting the view that all seven carbon atoms are equivalent. The tropylium ion would be expected to be formed by the loss of a single hydrogen atom from the parent ion of cycloheptatriene. The mass spectrum of this compound is remarkably similar [1395] to that of toluene adding further weight to the hypothesis that a tropylium ion is a fragment ion in both spectra. The tropylium ion was long

7.6

RE-ARRANGEMENT IONS

267

ago predicted to be a stable cation [985] and is probably at least as stable as the benzyl ion. The mass spectra of five deuteriated styrenes have been reported by Quinn and Mohler [1642]. They find evidence of a major fragmentation mode in which the deuterium distribution within the styrene appears to become randomized. They conclude that this is evidence in favour of the re-arrangement of the parent ion into a cyclo-octatetraene structure and cite as further evidence of this re¬ arrangement the close similarity between the spectra of cyclo-octatetraene and styrene. In some fragmentation processes, the movement of the atoms within the ionized fragment may be small, and the only change in the ion as fragmentation occurs may be the modification of the electron paths within it. As an example of such behaviour, one can instance the probable formation of an olefinic ion when a neutral water fragment is eliminated from the parent ion of a primary alcohol. The formation of extra bonds is most common in ionized fragments which contain oxygen or nitrogen atoms and this can often lead to subsequent atomic re-arrangement. The positive charge in the ion is likely to be located on the oxygen or nitrogen atom since this involves removal of a loosely-bound + + lone-pair electron. O and N have the capacity of increased valency over neutral + O and N. The O ion is iso-electronic with N, and the bond directions in this ion, will tend to be different from those in a neutral oxygen atom and may add drive to a re-arrangement which is not sterically favoured in a neutral molecule. Bonds may sometimes be formed which are sterically favoured at the moment of ionization; in other cases, bonds may develop which are favoured after re¬ orientation of the groups on the oxygen atom. The influence of a sterically favoured cyclic intermediate in leading to a re-arrangement peak is illustrated by the formation of a 13% re-arrangement peak at mass 60 in the spectrum of isocrotonic acid [831]

OH

whilst this peak is not appreciable in the spectrum of crotonic acid [1359]

H,C

X'

OH

Although it is difficult to predict with certainty all cases in which extra bonds will be formed, re-arrangements can sometimes be explained in terms of such bond formation. The most profound effects resulting from formation of an extra bond arise when the bond formed completes a ring system within the ion. Further fragmentation of the ion can then give rise to fragments apparently formed by a re-shuffling of the order of the atoms. Consider for example a molecule of formula A-B-C-D. Suppose that this can undergo ring formation in its ionized state. Then one might obtain the reaction sequence +

[A — B — C — D]+

-> AD+ + BC

268

7

TYPES OF IONS IN MASS SPECTRA

Such a re-arrangement has been termed by McLafFerty [1361] a “replacement” re-arrangement since the result is to replace part of the ion by part of the neutral fragment which would otherwise be lost. In other words, the result is the same as though the reaction were [A — B — C— D]^->D+ + A — B — C ->■ AD+ + BC

NITROBENZENE

ill,- .i.Ji

0

40

lull, 60

80

100

-,-i-T

f

lu—Y-1—u

SO

40

—, 60

llll

,

140

160

140

160

- DINITROBENZENE

(1

0

ISO

,I

r. 1—,Jj

80

100

T

ISO

o - NITROANILINE

M

iili

L

— i i.—«>*—,— r1-11—h—“h—-“r—n——“f-r—i—h-r—i—h-*t—h—-‘l11 A—J ,1-T1— , O SO 40 60 80 100 ISO 140 160

m - NITROANILINE

i-i—^ |!" SO

A

A

60

i 80

. b 100

■ ISO

-

i 140

160

- NITROANILINE

1-r

0

160

Fig.

121.

Mass spectra of aromatic nitro compounds.

When a cyclic intermediate ion is sterically possible, replacement re-arrangements often occur. The resultant peaks are often large especially if the re-arrangement reaction leads to particularly stable products. Re-arrangements in non-hydro¬ carbons have been widely reported. Many of them are discussed in Chapter 9 where the use of such ions in the identification of organic compounds is mention¬ ed. The introduction of functional groups into the molecule usually has the

7.6

RE-ARRANGEMENT IONS

269

effect of making one reaction path more favourable than the others, so that it is usual for the number of re-arrangement peaks to be reduced relative to those in hydrocarbon spectra, but for their abundance to be greater. Re-arrangements characterized by this greater specificity have been called by McLafferty [1363] “specific” re-arrangements. Examples of these leading to large peaks, sometimes the base peaks of the spectra, occur for example in carboxylic acids [831] and esters [70]; they can also occur in hydrocarbons when a particularly stable ion results from the re-arrangement [1938]. In many cases, the energy required to obtain the re-arrangement ion is comparatively low; the formation of the extra bond increases the stability of the intermediate and means that more energy is available for further reactions. Even reactions for which no obviously favourable intermediate exists sometimes occur with high probability and considerations such as the bond strengths in the original ion seem often of low importance compared with the stability of the fragments which can be formed. An early example of such behaviour [1305] was the formation of CO+ and H2O"1" in the spectrum of formic acid, (CO + H2O) being one of the most stable states of aggregation of the atoms which make up the formic acid molecule. Many examples are now known in which loss of CO from the ionized parent occurs. Usually the CO occurs as the neutral fragment; its ionization potential is higher than that of organic radicals, so a more stable state of aggregation is reached if the charge does not lie in the CO. Thus the reaction: M+ -> (M — 28)+ + CO, is usually observed. Loss of CO is known from ketones (anthraquinone can lose two CO molecules), ethers (diphenyl ether), esters and phenols. In a number of cases, loss of CO occurs from the parent ion, and this mode of breakdown is observed for such compounds as phenol and naphthol, diphenyl ether and a variety of quinones [195]. In other cases, loss of the stable CO molecule can occur as a later step in a fragmentation sequence. For example in the spectrum of o-nitroaniline, shown in Fig. 121 (see p. 268) the sequence

138+ —> 108+ + 30 V

80+ + 28 can be shown to occur by the presence in the spectra of meta-stable peaks corresponding to these processes. Accurate mass measurements on the peaks concerned show that the processes can be written [C6H6N2C>2]+^ [CeHsNOR + NO [CsHeNR + CO

Since the state of aggregation can only be inferred from the formulae of the fragment ions, no attempt has been made to write detailed structural formulae for the ions, except to indicate ring closure as a likely means of increasing the stability. Thus, the formula of the last ion species could probably be better written in the form

with the positive charge lying on the quadrivalent N.

270

TYPES OF IONS IN MASS SPECTRA

7

The fragmentation process detailed above shows, in the first stage, the elimi¬ nation of the stable nitric oxide neutral molecule, the nitrogroup behaving in this re-arrangement in the Way one might expect from a nitrite group. Another way in which the molecule can break to form stable fragments involves the elimination of NO2 and this does not involve re-arrangement. Loss of the oddelectron neutral fragments NO or NO2 from the parent positive ion of a nitro compound will lead to an even-electron fragment ion. In the case of fragments which still contain a single nitrogen atom (as in the particular compound being discussed), this will be an even-mass ion. The fact that even electron ions predominate in mass spectra was suggested [421] to be due to the fact that such ions contain no unpaired electrons, and are thus more stable than odd electron ions. When, however, only carbon and oxygen atoms are lost in the neutral fragment, as in the elimination of CO, the odd-electron parent ion leads to an odd-electron fragment. In many cases, therefore, a neutral radical is eliminated instead of a neutral molecule. Thus, loss of the atoms CHO often occurs. In diphenyl ether for example this fragment accompanies the loss of CO, an ion of benz-tropylium possibly being formed as the charged fragment. In some cases, however, the CHO can be shown to be lost in two stages as H + CO. This can be proved when a meta-stable ion connecting the initial and final ionic masses can be seen. It may occur widely, since many of the ion species will have life-times short compared with 10~6 seconds and will not give observable meta¬ stable peaks. An example of the 2-stage process is afforded by the spectrum of phenyl styryl ketone which breaks with loss of H followed by CO. This is a compound in which one might expect the C-H bonds to be unlikely to break, but the (p — 1)+ peak is of height 100%. (The parent peak height is 86%.) Possibly it is one of the ring hydrogens which is eliminated so readily so that the ion can gain stability by ring formation. Thus one might imagine the sequence

Many other cases are known in which stable neutral fragments have been formed in a re-arrangement process. Indeed, the fragment ion from o-nitroaniline formed by loss of NO2 will itself re-arrange so as to eliminate HCN, a fragment also formed from aniline itself. In this respect aromatic amines can be compared with phenols if one considers the divalent NH to be equivalent to an oxygen atom, and loss of HCN which is the equivalent of loss of CO is indeed very common. In the case of phenol, it is supposed [190] that the intermediate form of the molecular ion has the structure

7.6

RE-ARRANGEMENT IONS

271

Phenol is known to behave in some chemical reactions as though it had such a structure, and tracer work using hydroxy deuteriated phenol carried out by Momigny [1426] has shown, by the presence in the spectrum of ions such as C3D+ that migration of the deuterium atom to the ring can occur to a consider¬ able extent and similar migration of hydrogen may occur in anilines. Another example of loss of neutral HCN is in the spectra of alkyl indoles [191]. The elimination of H2O is also common in mass spectra. This mode of fragmentation in alcohols has already been mentioned. A further example occurs in ether spectra, for example in di-n-octyl ether [1360]. The relative abundance of a re-arranged ion depends on the relative activation energy and stability of products of the re-arrangement reaction compared to other possible reaction paths. For ion species which can be formed at low electron energies, it should be possible to formulate intermediate reaction states which can be formed with the expenditure of very little energy. This has been done in many instances. Three of these, which involve loss of CO, are detailed below. In anthraquinone, the most loosely-bound electron, which is therefore removed preferentially during formation of the molecular ion from electrons of low energy (say 10-15 eV) is one of the lone-pair electrons on the oxygen atom. + Since O has the capacity of trivalency, as this is developed the electron pair in one of the C-C bonds adjacent to the carbonyl group will become uncoupled to provide an orbital which can give a zz-bonding effect with the lone-pair oxygen orbital. Initially, the direction of this bond orbital is not parallel to the orbital available on the oxygen, so that a complete zz-bonding effect will not result. Nevertheless to a first approximation the situation can be represented by

The positive charge on the oxygen will induce a polarity in the adjacent bonds, and this will be a maximum in the remaining C—C bond. If we consider the carbon hybridization, then the possibility of increasing the strengths of the carbonyl bonds depends upon changing, at least partially, the trigonal hybridiza¬ tion for the digonal form, as this allows stronger a and n bonds in the carbonyl group. The bond next to the carbonyl group is then “bent” and weakened, but this is compensated by increase in the CO bonding. Moreover, the bent bond will be more readily polarized than the original stronger bond, and the electron pair may be regarded as almost entirely in the digonal orbital. This state of affairs corresponds exactly with the usual view of the electron distribution in neutral CO, lone pair electrons being accommodated at the carbon end of the molecule. The formula

♦

272

'*1

TYPES OF IONS IN MASS SPECTRA

7

represents an intermediate state in which the hybridization of the carbon is intermediate between trigonal and digonal and the orbitals are directed somewhat as shown. In this state, the carbonyl group is so weakly attached to the remainder of the molecule that the small amount of excess internal energy produced at ionization will be sufficient to give amplitudes of vibration in the weak bonds beyond the dissociation limit. When this occurs, we are left with

to -y/3 times the equilibrium bond distance. To reach the ground state of the fluorenone ion one of the oxygen lone-pair electrons must be added to the oneelectron bond. This will be associated with release of vibrational energy since the bond will close up to the normal co-valent distance. Some of the vibrational energy available will be transferred, in intermediate states not mentioned above, into kinetic energy of the ejected carbon monoxide. The fluorenone ion can itself break in an analogous fashion to eliminate a further neutral carbon monoxide to give a highlyexcited o-diphenylene ion as illustrated by

A more complicated re-arrangement process is that by which the neutral CO fragment is eliminated from diphenyl ether. This process has been postulated to take the form summarized by the reaction sequence

7.6

RE'ARRANGEMENT IONS

273

In the first stage, the dotted line merely emphasizes that the distance between the two carbon atoms connected by it is less than the C-C bonding distance if the rings are coplanar, and greater than this distance if the rings are at right angles. Steric repulsion of non-bonded atoms will cause the rings to rotate out of the plane; the stabilization due to resonance when the rings are coplanar will cause an approach to coplanarity and this suggests that re-arrangement can take advantage of the fact that the distance shown dotted will, in fact, be very close to a C-C bond distance. Rotation of the chain in the third stage leads to the fourth stage with a one-electron bond within the dotted region, and various resonance forms with the one-electron bond between various pairs of carbon atoms will stabilize the structure. An even more abundant fragment ion in the mass spectrum of diphenyl ether is that in which CHO is lost from the parent. Loss of a further hydrogen atom from the structure postulated above would lead to the even-electron benz-tropylium ion, which would be expected to be much more stable. The sequence of reactions which has been used to explain the presence of the abundant (C2H70)+ ion in the mass spectrum of dimethyl acetal supposes the loss of CO to occur from the fragment ion (p — Cbb)"1- and a meta-stable peak confirms that such a reaction does take place. Presumably the methyl group which is lost from the parent ion is that which is attached to the central carbon atom (see Chapter 9). The next stages in the process are postulated to be

cv

/0

hOi

'CHi

The hydrogen, due to its small size, can readily form a hydrogen bond as shown in stage 3, and rotation of the carbonyl part of the molecule with elongation of this weak hydrogen bond simultaneously facilitates electron pairing with the other oxygen atom. The ion thus has the dimethyl hydronium structure. In many of the cases in which a large re-arrangement peak occurs, it is not easy to visualize the intermediate structures, and often the most which can be done is to speculate about the possible final structure achieved. Often, the fact that some sterically favourable more stable atomic arrangement has been achieved can be inferred from the fact that a particular peak of known formula which could be formed by a simple bond-cleavage process is much bigger than would be expected. For example the spectra of the monobasic straight-chain aliphatic acids consist mainly of the peaks [ • (CH2)» ' COOH]+. A spectrum of any particular long-chain acid would be expected to show a gradual diminution in the strengths of these peaks as n increased. In fact the ion of mass 129 [ • (CH2)6COOH]+ is always stronger than its neighbours in the homologous series at masses 115 and 143 (see Chapter 9). In fused ring aromatic hydrocarbons the ion (CiiHv)+ of mass 139 is often prominent. This peak can be formed, too, in such substances as 2-methoxyanthraquinone (C15H10O3) where it is of height 21% of the base peak. Its formation in this case involves the loss of C4H3O3 from the parent ion. A large peak associated with the formula (CnH7)++ is usually also found in these spectra. The only compound of formula CiiHs m Beilstein’s formula index is CH3-CeC-C=C-C6H5 and (CnH7)+ might be

274

TYPES OF IONS IN MASS SPECTRA

related in structure to this.

7

One could also postulate a structure such as

to satisfy the formula (CnH7)+, but the mode of formation of the ion species actually responsible will not be easily found. The ion (CnHv)+ is also prominent in monohydroxyanthraquinones. Here it corresponds to loss of (CO + CO + CHO) from the parent ion, the hydrox o-biphenylene ion left after loss of two molecules of carbon monoxide behaving in the same way as phenol in losing a CHO fragment. McLafferty [1363] has reviewed the knowledge of re-arrangement reactions and discussed them, in terms of fragment stability, including the steric influence on the fragments formed. He discusses particularly which of the hydrogen atoms are likely to take place in re-arrangements; thus it is usually not a hydrogen attached to a carbon adjacent (a) to the bond broken which re-arranges, but a hydrogen on a /5-carbon. For example the re-arrangement peak at mass 60 due to formation of an ion (C2H4O2)"1" which occurs widely in aliphatic acids, e.g. butyric acid where it is the base peak of the spectrum, does not occur in pro¬ pionic acid which has no /5-hydrogens. Migration of a /5-hydrogen atom allows increase in stability by formation of a double bond in the neutral fragment. Thus [R • CH2 ' CH2 • X]+ -> R • CH : CH2 + [HX]+. Use of deuteriumlabelled compounds enables the hydrogen atoms taking part in re-arrangements to be identified (see Chapter 10). For example, the spectra of 2-phenylethanol and 2-phenylpropanol deuteriated in the hydroxyl group have shown [751] that in the major re-arrangement ions, which correspond to cleavage of the bond connecting the -CH2-OH group to the remainder of the molecule with transfer of a hydrogen atom to the charged fragment (containing the ring), it is mainly the hydroxyl hydrogen which re-arranges and not the hydrogens on the a-carbon atom. The spectra of deuteriated secondary butyl acetates [1362] have been used to show that in the re-arrangement ion of formula (G2H502)+ and mass 61, which occurs in the sec-butyl acetate spectrum and which involves migration of two hydrogen atoms, the y-and (5-carbon atoms supply 80% of the re-arranged hydrogens even though the /5-hydrogen atom is in the labile tertiary position. The formation of this mass 61 ion is a further example of a “double” re¬ arrangement (i.e. involving two hydrogen atoms). Other examples of such ions are the hydronium ion (H3O)"1" formed, for example, from isopropanol, and the ammonium ion (NH4)+ formed in the spectra of many nitrogen-containing com¬ pounds. The isopropyl group in a molecule often leads to double hydrogen re-arrangement, presumably because of the fact that the neutral fragment formed can assume the stable allyl structure. Another group which also seems to add drive to such processes is the cyclohexyl group, though in this case the reason is not obvious. Cyclohexyl acetic acid, and also cyclohexyl acetate give, for example, large peaks at mass 61. Occasionally, ions such as hydronium, am¬ monium or homologues of these are formed even though they involve transfer of more than two hydrogens. For example the (NH4)+ peak is 11% of the base peak in the mass spectrum of trimethylhydrazine. Examples of the re-arran-

7.7

IONS FORMED BY INTERMOLECULAR PROCESS

275

gement of more than 2 hydrogen atoms occur in the formation of a mass 61 ion in the spectrum of cyclohexane-carboxylic acid [1363] (3 hydrogens) and the loss of CeHn from the parent ion of triethyl phosphate [1359] (4 hydrogens). In other cases, the reason for the occurrence of a re-arrangement ion is difficult to visualize. For example, an oxonium type ion (CH3OH2) gives a large peak (38 /o) at mass 33 in the spectrum of ethylene glycol, even though the hydrogens available for transfer would not appear to be in sterically favourable positions, and the ion of mass 31 which would be formed by simple bond rupture in the parent ion would appear to be very stable as shown by the wide occurrence of a large peak of this mass number in the spectra of oxygen-containing molecules. The mass spectra of deuteriated isobutanol samples have shown that the CH3OH2 ions receive the re-arranged hydrogen atoms from the methyl groups [574]. Other re-arrangement processes which involve the migration of groups such as methyl [648] or skeletal re-arrangement [1071] are also known.

7.7

IONS FORMED BY INTERMOLECULAR PROCESSES

Most of the ions in a mass spectrum are formed by unimolecular processes, and over a wide range of sample pressures, their abundance will be directly proportional to the pressure inside the ionization chamber. However, peaks (usually of low intensity) have often been observed, whose intensities vary with pressure more rapidly than peaks formed in the above fashion. These must arise in a collision process involving two or more molecules [1951]. Some of the peaks which depend on pressure in this way are sharp, others are diffuse, suggest¬ ing that they are due to ions formed by a reaction which proceeds while the ion is travelling towards the collector, in similar fashion to the meta-stable ion reaction. For the sharp peaks, the collision process occurs within the ionization chamber. Little has been written about the study of the peaks in the spectra of organic compounds formed by collisions within the ionization chamber, and this is partly due to the extreme difficulty of observing many of them. A fragment ion formed by an ion-molecule collision within the ionization chamber will appear at the same point in the spectrum as a similar ion formed by a unimole¬ cular decomposition process, and the number of ions formed in the latter process is likely to be relatively very large. Ions formed by a collision process sometimes occur at a mass to charge ratio greater than that of the parent ion, and are then most easily observed since they usually suffer no interference from other ions. They can be distinguished from impurity peaks by their pressure dependence. Because of their comparative rarity of occurrence they are useful in qualitative analysis in suggesting the presence of certain groups. Attachment of an extra chemical group to a parent or fragment ion occurs most readily in compounds containing an oxygen or nitrogen atom. The most easily removed electron in an oxygenated system is likely to be one of the non-bonding lone-pair electrons. When this is removed, hybridization of the electron orbitals results in the development of some bonding character in the third orbital, so that the “triva+ lent” O can form an extra bond. A similar increase in valency can occur when nitrogen-containing compounds are ionized. The most commonly-formed ions of mass greater than the parent occur at one mass unit greater than the molecular weight. In many compounds, having large parent peaks in their spectra, the peak formed by inter-molecular action will be superimposed on the heavy isotope

276

TYPES OF IONS IN MASS SPECTRA

7

peak and can only be separated from this by the use of high resolution techniques. Probably for this reason, peaks formed by collision processes within the ioniza¬ tion chamber are most often reported in compounds showing very small or no parent peaks in their spectra. For example, the spectrum of valeronitrile gives peaks of the order 0.1% of the base peak intensity at masses 84 ip + 1) and 82 ip — 1) at an ion chamber pressure of the order 5 X 10 5 mm mercury. The absence of parent ions implies that it is not this ion species which leads to the peak at (p + 1) by collision with a neutral molecule. Another possibility for the formation of these ions, that they form part of the fragmentation pattern of dimeric molecules existing in the gas phase is ruled out by the fact that their intensity varies as the square of the gas pressure. At a similar pressure, amines give similar peaks and a peak of height about 0.05% appears at mass 117 (p + 1)

_o_

Fig.

122.

Peaks of mass greater than the parent ion in the mass spectrum of n-butyl acetate. Peaks can he seen corresponding to (p + HJ+ at mass 117 and at(p + CHsCO^+ at mass 159.

in the spectrum of hexamethylenediamine. Pressure-dependent peaks at a mass (p + 1) are very common in ester spectra. The relevant part of the spectrum of n-butyl acetate at a typical sample pressure is shown in Fig. 122. The height of the peak at mass 117 relative to that at 116 as a function of the sample pressure is plotted in Fig. 123. The graph shows that part of the 117 peak is formed in an inter-molecular process. An interesting peak formed in a similar fashion is that at mass 159. Accurate mass measurements show that this peak is due to the ion (C8ldi503)+ formed by addition of (CH3CO)"1" to the parent molecule. The ions (CH3CO)+ are particularly abundant in the mass spectra of acetates, and this suggests that the reaction which gives rise to the ion at mass 159 is formed by the combination of a charged acetyl fragment and a neutral ester molecule. The low stability and therefore short life-time of the ester parent ion would also make it appear probable that this is the reaction which occurs.' Similar ions can be detected in many other ester spectra. Peaks corresponding to the attachment of abundant fragment ions to neutral molecules are also

7.7

IONS FORMED BY INTERMOLECULAR PROCESS

277

i A^m°n *n t^ie sPectra °f nitriles. Adiponitrile, for example, of molecular weight 7,^e'a pressure-dependent peak at mass 149 corresponding to the addition or (CH3CN)+ to the adiponitrile molecule. The re-arrangement ion (CH3CN)+ orms the base peak in most nitrile spectra. Adiponitrile also gives a pressuredependent peak at mass 109; the heaviest fragment ion formed by a unimolecular process occurs at mass 107. In some cases, the cross-section for these ion-molecule reactions increases as the degree of excitation of the ion is reduced by lowering the energy of the bombarding electrons. For example, a plot of the ratio of

HEIGHT OF

MASS

117 PEAK

Fig. 123. Variation of the height of the peak at mass 117 relative to that at mass 87 in the spectrum of n-butyl acetate as the sample pressure is changed.

(p + 1)+ to p+ is given in Fig. 124 for a single pressure of methyl isobutyl ketone over the range of energies of the bombarding electrons from below 10 eV to 50 eV. It can be seen that the peak at (p + 1) is enhanced relative to the parent peak at the lower electron energies. There is a sharp peak in the ratio of (p + 1)+ to p+ showing some of the (p + 1)+ ions to be formed in a resonance capture process. The ratio of peak heights is higher than that to be expected merely from the abundance of the naturally occurring isotopes over the whole range of electron energies, showing that some of the ions of mass (p + 1) are always form¬ ed by a bi-molecular process.

278

TYPES OF IONS IN MASS SPECTRA

7

Tal’roze and Lyubimova [1980] report a number of cases in which a peak corresponding to (p + H)+ is found where p represents the molecular weight. For example, mass 43 in propylene, 57 in isobutene and 17 in methane [26]. The peaks are of intensity about 0.05% that of the parent peaks in contradistinction to the peaks mentioned above, which are often larger than the parent peaks, and are all formed by the process M+ + M -> (M + 1)+ + (M -— 1) where M represents the mass of the parent molecule [635, 1375. 1786, 1950]. This is shown by the fact that, for example, the appearance potential of (CH4)+and (CH5)+

Fig. 124. The ratio of the height of the peak at mass fp4 1) to that of the parent ion in the spectrum of methyl isobutyl ketone as a function of the energy of the bombarding electrons. The curve was obtained at a single sample pressure.

is the same [1192]. No (C2H5)+ ions are produced from ethylene, and no paraffinic hydrocarbon other than methane exhibits the phenomenon. An early example of a peak at (p + H) was found in hydrogen gas itself [1895] and another wellknown example of such a peak is that at mass 19 in the spectrum of water [1194, 1303]. Stevenson has given a detailed description of a quantitative study of such reactions in a mass spectrometer [1953] and their treatment by the methods of the modern kinetic theory [753]. Similar work has been carried out by Field and Lampe [640, 641]. Peaks at masses considerably higher than the parent mass have been found by Field, Franklin and Lampe in the spectra of methane, ethylene and acetylene [638, 639]. For example in the spectrum of acetylene, peaks were obtained at masses 51, 50, 49, 38 and 37. Probable reactions for the formation of these ions from acetylene and other compounds were discussed by these authors in detail and the rate constants for the various reactions were found and used to determine reaction cross-sections. Another possibility in a collision between a molecule and an ion is that

7.7

IONS FORMED BY INTERMOLECULAR PROCESS

279

charge-transfer, perhaps accompanied by dissociation, may occur or that an association reaction may occur [1370]. Peaks have been observed in the spectra of the rare gases corresponding to a mass to charge ratio of twice the atomic weight. A plot of the intensity of these ions against bombarding-electron energy has given in some investigations a sharp maximum intensity [977, 1007], suggest¬ ing that they are in fact diatomic molecular ions and confirmation of this fact is obtained by the observation that, in a mixture of 36A and 40A, ions of mass 72, 76 and 80 are formed. On the other hand, Norton [1523] has observed ions at twice the atomic weight in mercury and at mass 80 in argon, the intensity of which vary in the same way as the A++ ion as the energy of the bombarding electrons is changed. He, therefore, attributes the formation of the peak at mass 80 to a charge exchange mechanism occurring after acceleration according to the reaction A++ + A -5- A+ + A+

Confirmation that this kind of reaction can occur is obtained from the obser¬ vation by Norton of a peak at mass to charge ratio 56 in the mass spectrum of nitrogen. It has been mentioned on p. 259 that in the spectrum of dichloromethane peaks appear at masses 168, 172 and 176 in the correct isotopic abundance ratio for the isotopes of chlorine and they confirm the second of these reaction paths. Evidently both kinds of reaction are possible under suitable conditions. Collision processes involving electron stripping are also known and the process I+ -> I++ -> I+++ has been described [2165]. Ions of Cl3+ and Br3+ have been reported in the mass spectra of chlorine and bromine [1388] but it is not known whether these ions are formed directly from triatomic molecules or by a collision process. N3+ ions have been noted in the mass spectrum of nitrogen gas by Luhr [1280], by Dreeskamp [521] and by Junk and Svec [1072]. A study by Saporoschenko [1764] of the spectrum of nitrogen gas using a specially designed instrument in which the ion source could operate in the range of pressures 10-3 to 6 X 10-1 mm Hg has shown the ions N3+ and N4+ as well as N+ and N2+. The appearance poten¬ tial of N4+ was the same as that of N2+ within experimental error. Ions formed by collision processes in gas mixtures have been widely studied [J375]- Hydrogen, when mixed with oxygen [994, 996, 1523], can give the ions (OH)+, (H20)+, (HsO)+, (H02)+, (H202)+, (H402)+, 03+ and 04+, and in admixture with carbon dioxide and water (HC02)+ ions. In the latter case the hydrogen atom comes from the hydrogen and not from the water as can be proved by the formation of (DC02)+ when deuterium is used. Hydrogen and the rare gases will also form compound ions in admixture with mercury [1521]. Ions such as AH+ and KrH+ have also been reported [806, 1950]. The formation of NO+ in mixtures of oxygen and nitrogen is thought [1626] to occur by the process CA + N2 -^NO+ + N

Other collision processes in which charge inversion can occur are also knownThese are discussed below in the section which deals with negative ions. A specially-designed mass spectrometer has been used by Wells and Melton [2151] to study ion-molecule collision processes occurring outside the ionization chamber. A differential pumping system was employed whereby the pressures within the ionization chamber and the analyser tube could be controlled inde¬ pendently. A similar system has also been used by Bainbridge and Jordan [114]-

280

TYPES OF IONS IN MASS SPECTRA

7

Thus ions of one gas can be produced, and by introducing a variety of other gases into the analyser tube, collision-induced reactions between a particular ion species and a number of gases can be studied. It has been mentioned in the section on meta-stable ions that the mass measurements can tell nothing about the reason for the transition, but will only give information on the initial and final mass of the ion involved; the equation for a sector instrument, m* = m22/mi (see p. 252) holds whether the transition is induced by a collision or occurs spontaneously and peaks produced by meta-stable transitions can be distinguished from those which are collision induced by the fact that the relative abundances of the latter are pressure dependent. Secondary effects due to collisions have been known since the earliest days of mass spectroscopy [72] and some of the early work in this field was done by Friedlander [696] and Mattauch [1330]. A compound which has been widely studied [72, 114, 697] is carbon monoxide. The diffuse peak corresponding to the transition C0+-*C+ + 0 (m* = 5.145) was observed by Aston [72] and the value of m* was determined very accurately by Bainbridge and Jordan [114] who proved that it must arise from the process given. Melton and Wells [1383] observed the following dissociation reactions to occur in carbon monoxide CO+ + M

-> C+ + O + M

(m*

=

5.14)

-> C + 0+ + M

(m*

=

9.14)

(m*

= 10.28)

(m*

=

CO++ + M -> C+ + O + M+ H- CO+ + M+

5.6)

where M represents any neutral molecule. They were able to show that the dissociation reactions for the CO+ and CO++ ions are different in that the CO+ dissociates predominantly by conversion of kinetic energy into vibrational energy, whereas CO++ dissociates by changing electronic excitational energy into vibrational energy. The first conclusion is based on appearance potential data for the mass 5.14 peak, and is similar to the conclusion reached for the dissociation process involving singly-charged acetylene [1382] ions. Kuprianov [1182] has, however, shown that the process in both these compounds is not completely independent of the electronic excitational energy. As the complexity of the molecular species studied increases, the number of reactions which can be detected increases rapidly. Fig. 115 shows part of the spectrum of acetylene obtained by Melton, Bretscher and Baldock [1382] which illustrates this point. The figure shows the mass spectrum occurring below mass 12 with a pressure of 10-5 mm. It shows part of the mass 12 peak and seven other peaks. These seven peaks, the abundances of which increase with pressure, are due to the transitions CH+ -> C+ + H (11.08) CH2+ -> C+ + 2H

(10.29)

c2h2+-> ch2++ c

(7.54)

c2h2+-» CH+ + CH

(6.50)

C2+

-> C+ + C

(6.00)

C2H+ -* C+ + CH

(5.76)

C2H2+-> C+ + ch2

(5.50)

7.7

281

IONS FORMED BY INTERMOLECULAR PROCESS

The mass spectrum of acetylene also contains a meta-stable peak at 23.15 due to the process C2H+ -> C2"1" + H, but the relative abundance does not change with pressure, showing that it must be due to a spontaneous disintegration. The relative abundances of the peaks at 6.00 and at 11.08 increase with pressure, indicating that they are formed at least partly by an intermolecular process. The extrapolated relative abundance to zero pressure is not zero, however, and this means that the peaks are partly formed in a unimolecular reaction. Part of the peak at mass 6.00 is due to C++. The narrowness of this peak makes it obvious that it is a fragment ion formed within the ion chamber. The other peak does not occur at an integral mass number and is a diffuse peak. It is, in fact, partly due to a meta-stable transition. The three processes given above which involve the parent ion species can also be brought about by electron impact, and the relative cross-sections for the reactions by the two processes have been compared. Measurements are complicated by the very low relative abundance of some of the peaks which have to be studied. For example, in the above work the peak at mass 10.29 was of relative abundance 0.00009. TABLE 2 DISSOCIATION TRANSITIONS IN METHANE

\ m2 m, \ 16 15 14 13

15

14

13

12

14.06

12.25 13.06

10.56 11.26 12.07

9.00 9.60 10.28 11.07

A similar study of the collision induced dissociations in methane has been carried out by Melton and Rosenstock [1381]. Table 2 gives the values m* at which diffuse peaks would be expected due to transitions

mi+ -> ma++ (mi—m2) Peaks were observed at all expected masses except 12.07 and this peak would be obscured by strong peaks at 12 and 12.25. The relative abundances of all these peaks were shown to be directly proportional to pressure, showing them to be due to single-collision processes. The transition CH2+ -> CH+ + H has been observed by Henglein [854]. Others of the peaks listed in Table 2 have been observed by Mattauch and Lichtblau [1330]. There have been several other investigations of secondary processes involving methane ions [4, 1152, 2100]. In one of these [2100] using a mixture of CH4 and CD4 in a study of (CHs)+ ion formation, it was concluded that the activated complex leading to the formation of this peak in methane could better be written (CPU ■ CH4)+ than (C2H8)+. The collision-induced dissociation H2+ -* H+ + H leading to a peak at mass \ has also been reported [2056]. Collision induced transitions have been studied in n-butane [2026] and isobutane [1741] and peaks produced in this way have been distinguished from those formed in meta-stable transitions.

282

TYPES OF IONS IN MASS SPECTRA

7

The dissociation of any particular ion species in collision with gas molecules can be studied in a special instrument containing two magnetic sectors in tandem. The first of these is used to .select the ion species which is to be fired into the gas contained in the second sector. Such an apparatus has been used by Fedorenko [629] who has observed dissociation of l2+ ions into I+ ions. In another method which has been used by Melton and Ropp [1387], the energy of the bombarding electrons in a single-focussing mass spectrometer is reduced to a low value to minimize the number of fragment ions produced so that a study of the parent ion behaviour on collision can be made. With formic acid, for example, and a 12.5 eV electron beam, the abundance of the primary fragment ions is less than 1 % of that for the parent ions. Melton and Ropp examined the cross-sections for production of (CH02)+, (CHO)+, (FtaO)"1", (CO)+ and (OH)+ions from formic acid in collisions with molecules of hydrogen, deuterium, krypton, nitrogen, argon and helium, and corresponding ions produced from (HCOOD)+ and (DCOOH)+. Reduction of the ionizing voltage has the disadvantages that the sensitivity of the system is reduced and the method is also limited in the species of ion which can be studied. Fedorenko’s method is of more general applicability but uses complex apparatus. The parabola spectrograph is well suited to the study of ion-molecule collisions; the high pressure spectra obtained with such an instrument show a series of “beads” on the parabolic lines. The mass of an ion is given by the parabola on which it falls, and its energy by its position on the parabola. Calculation of the energies of the ions which go to make up the “beads” enables the nature of the processes by which they have been formed to be deduced [572, 603, 854, 8$6, 857, 1805]. Since the relative abundance of peaks formed by secondary processes increases with the pressure, they will be prominent in any mass spectrometer which requires to be operated at relatively high pressure. Thus, in a mass spectrometer in which the ions are produced by field-emission, collision induced dissociations have been observed [1014]. If a polyatomic ion beam is fired through a solid film of organic material a few hundred angstroms thick placed at a focal point, complete dissociation into the atomic constituents will occur, and White, Rourke and Sheffield [2165] suggest that the mass spectrum of the emergent beam will then give a method of distinguishing the elements which comprise a particular peak in a mass spectrum. As examples of the behaviour of polyatomic beams they instance the complete dissociation of (CO2)"1" and (H20)+ in traversing the film. However, equal quantities of O- and 0+ are formed making the method non-quantit^tive, and the large variable energy loss and spreading of the resultant beams make the atomic ion currents difficult to detect so that the method is as yet of no use in qualitative analysis.

7.8

MULTIPLY-CHARGED IONS

Multiply-charged monatomic ions are well known. The intensities and ap¬ pearance potentials of ions Hg+, Hg2+, Idg3+, Hg4+ and Hg5+ were measured, for example, by Bleakney [220, 221]. Here, however, we shall be concerned only with doubly-charged polyatomic organic ions. Until the work of Conrad [392] it was not realized that doubly-charged ions of polyatomic fragments could exist, but it is now known that the removal of two electrons from a molecule without fragmentation can occur, especially in the case of compounds of a strong ring

7.8

MULTIPLY-CHARGED IONS

283

structure, and also that doubly-charged parent ions can undergo decomposition reactions in some of which the double charge can remain on the one particle, m others of which the charges may be shared between the separating fragments. Doubly-charged ions are very readily observed in the mass spectra of compounds which contain a single nitrogen atom. In such compounds, the peaks due to doubly-charged ions often occur at non-integral values of mass to charge ratio (half-masses) corresponding to the fact that the ions are of odd mass, and the peaks are thus easily recognized as due to ions carrying two positive charges. In the spectra of compounds which contain no nitrogen, the doubly-charged ions of greatest abundance are often of even mass number and are thus liable to be confused with singly-charged ions of half the mass. In some cases, the doublycharged ions formed differ slightly in mass to charge ratio from the singly-charged ions of the same nominal mass to charge ratio and can then be separated from them in an instrument of sufficiently high resolution. This is illustrated in Fig. 22 Chapter 2 where, for example, at mass 95 peaks of composition (C7Hn)+ and (Ci5Hio)++ can be seen. The most stable ions are usually those which contain no unpaired electrons. For this reason singly-charged fragment ions are usually formed most readily from the odd-electron parent by loss of an odd-electron radical. In a similar way, doubly-charged ions would tend to lose two such radicals and thus to be of even mass for compounds containing even numbers (including zero) of odd-electron nitrogen atoms. Triply-charged ions are very much less common, but where these are formed they correspond to loss of three neutral radicals from a triply-charged parent. For example, in the spectrum of octamethyl-trisiloxane [484] [(CLL^Si ■ O • Si(CH2)2 • O ■ Si(CH3)3] a triplycharged ion at mass to charge ratio 632js corresponds to the ion (p — 3CHs)+++. Such ions are very readily identified by the masses at which they appear and by the fact that the peaks due to isotopes are separated by x/3 mass number. Except in special circumstances the intensity of each peak due to a doublycharged ion will be less than 5% of that of the base peak of the spectrum. Much larger peaks can occur in compounds containing nitrogen or oxygen atoms. For example, peaks of height greater than 10% of the base peak due to p++ occur in the spectra of alkyl indoles [191]. This reflects both the strong ring structure of this compound and the localization of one of the positive charges on the hetero¬ atom making it easier to remove a second electron without fragmentation. Large doubly-charged peaks such as this are generally accompanied by a series of doubly-charged ions corresponding to the removal of successive hydrogen atoms. This series usually extends further than the series of corresponding singly-charg¬ ed ions, and sometimes continues until most of the hydrogen atoms have been removed. To continue the example of the alkyl indole spectra, 2-methylindole gives apeak corresponding to (p — 6H)++. Another example of loss of a number of hydrogen atoms occurs in the spectrum of tetrahydrofuran where the peak due to (C4H20)++ [(£> — 6H)++] is of intensity 0.14% that of the base peak. As well as these smaller doubly-charged peaks which have no parallel among the singlycharged ions, there exist many large peaks due to doubly-charged ions corre¬ sponding to a fragmentation process which is improbable for the singly-charged parent ion [855]. For example, the base peak in the spectrum of maleic anhydride is that corresponding to (p — CO2D; there is a peak of 1 % of this in height corre¬ sponding to (p — 0)+. The peaks (p — CC>2)++ and (p — 0)++ are, however, of respective heights 6% and 7%. The large intensity of the (C4bl202)++ ion suggests that this is due to the localization of the charges on the oxygen atoms leading to a stable ion of structure 0=C-CH=CH-C=0.

284

TYPES OF IONS IN MASS SPECTRA

7

Multiply-charged ions of the elements are produced abundantly in the high frequency spark source generally used with the Mattauch instrument for solids analysis (see Chapter 4), and ions carrying n charges occur with an abundance of about (0.2)» of that of the singly-charged ions [416]. In spectra containing such ions, many kinds of charge transfer processes can be observed [416, 827]. Suppose for example that an ion M(m+) changes to the ion M(w+) in a collision. Then if the charge transfer takes place between the electrostatic and magnetic analysers a sharp line will appear on the photographic plate at a mass nM/m2; if it occurs within the electrostatic analyser, a continuum extending downwards in mass from nM/m2 is seen, and if it occurs within the magnetic analyser a continuum extending upwards from M/m.

7.9

IONS FORMED WITH KINETIC ENERGY

It has been known for many years [221] that fragment ions may be formed with excess kinetic energy. In the simplest case, that of the formation of atomic ions from diatomic molecules, detailed studies of the energy distribution have been carried out [815]. In this case, the ions having kinetic energy are formed by Franck-Condon transitions to points on the molecule-ion potential energy curve above the dissociation asymptote, and if the shape of the potential energy curve is known, the distribution of kinetic energy to be expected can be calculated. For curves of small slope in the transition region the range of kinetic energy will be small and vice-versa. The simple considerations which apply to diatomic molecules do not extend to large organic molecules. As explained in the section on fragment ions, due to crossings of the potential energy surfaces, excess energy can be converted into vibrational energy and may not appear as kinetic energy of separation of the fragments. Nevertheless, a number of cases are known of the formation of ions with kinetic energy from large molecules. A rule put forward by Stevenson [1946] states that when a molecule can dissociate by two processes R1R2 ->■ [RiR2]+ “*■ Ri+ -f- R2 x [RiR2*]+ -*• Ri + R2+ the fragments (Ri+ + R2) will be produced in their lowest states or without kinetic energy only when the ionization potential of Ri is less than that of R2. Thus, bond dissociation energies (see Chapter 10) can be calculated only for the process giving the ion of lower ionization potential. This rule has been found to hold in almost all cases investigated but no satisfactory theoretical explanation for it has been put forward. Ions formed with kinetic energy can lead to errors in the interpretation of bond-strength measurements and also to discriminations in the slit systems of conventional instruments [177, 2131]. The methods of detecting such ions can be divided into retardation methods [674, 815, 927, 1075a, 1076], deflection methods [177, 1017, 1920] and methods involving study of peak shapes [813, 1187, 1421]. It is also possible to deduce that ions are being formed with initial kinetic energy by a study of the shapes of ionization efficiency curves [1454] as discussed in Chapter 10. A retarding potential can be applied to the ion beam by arranging that the potentials of the ionization chamber and analyser region can be varied indepen¬ dently of the collector system (which is maintained at earth potential) or by employing a so-called meta-stable suppressor electrode in front of the analyser

7.9

IONS FORMED WITH KINETIC ENERGY

285

exit slit. In both cases, the ions are formed, accelerated and mass-analysed then retarded by an amount sufficient to stop all ions having less than a certain energy. By increasing the retarding potential, ions which have lost energy by collisions or meta-stable transitions will first be prevented from reaching the collector, followed by those which have emerged from the ion chamber without appreciable kinetic energy, and finally ions with increasing amounts of kinetic energy in the direction of acceleration will be successively prevented from reaching the collector. A plot of retarding potential against ion current thus gives an energy analysis for any ion. In the experiments of Berry [177], the ion beam emerging from the source slit could be deflected in the direction of the length of the slit by a suitably positioned pair of electrodes. The effect of a velocity component in the direction of the length of the slit is to cause the beam to fan out so that the collector slit usually takes a sample of the original beam, the sample consisting of those ions entering the analyser with very small velocity components in the relevant direction. The deflecting electrodes enable the beam to be scanned across the collector in the direction of its length so that each part of the original beam can be sampled and an estimate of the kinetic energy distribution obtained.

Fig. 125. The kinetic energy of some ions in the mass spectrum of propane.

Another method of studying the kinetic energy distribution which has been used by Inghram and Stanton [1017, 1920] uses a 90 electrostatic analyser insulated from and at high voltage with respect to the magnetic analyser. A typi¬ cal record of the kinetic energy distribution of some ions in the mass spectrum of propane is shown in Fig. 125. As mentioned in Chapter 1, the single-focussing mass spectrometer with a sector magnetic field usually used for the analysis of organic compounds produces not a mass spectrum but a momentum spectrum of the ions which enter the magnetic analyser. An estimate of the kinetic energy with which any ion species is formed can thus be obtained from the difference between the accelerating voltage at which the peak due to this ion is recorded and the voltage at which a peak due to ions of this mass would be calculated to appear had they emerged from the ionizing region without appreciable kinetic energy [813, 2105]. For the same reason, in a single focussing instrument of this kind if a particular ion species is formed with a range of kinetic energies, this wi be reflected in an increased peak width [i35°]> the shape of the peak being sen-

286

TYPES OF IONS IN MASS SPECTRA

7

sitive to the energy distribution. If ions are formed by two processes, in one of which they have effectively zero kinetic energy and in the other an energy con¬ centrated in a narrow range, the peak at a particular mass number may split into a doublet [925, 1421]. The separation of the components of such a doublet will depend on the fraction of the momentum of the ion which arises from its kinetic energy of fragmentation and will thus be greatest at low ion accelerating voltage. Values as low as 100 eV have been used in order to enhance this separation. Minor differences in shape between two peaks suspected to be due to kinetic energy of fragmentation possessed by one of the ion species can be detected using the technique of rapid scanning. The two peaks can be displayed in rapid succession on a cathode-ray tube screen and by superposing the images very mi¬ nor differences in shape can be made apparent [1187]. In diatomic molecules it is to be expected that as the energy of the bombarding electrons is increased, fragment ions with kinetic energy will be formed. In some processes, ions cannot apparently be formed except with kinetic energy; for example in the formation of C+ from CO by electron impact [177, 813, 815] it is found that few if any of these ions are formed without kinetic energy. Berry [177] finds the energy range of the ions to be from 0.65 eV to more than 2.5 eV. Values of total kinetic energy up to 4.6 eV have been found [1421] in the formation of (CH3)+ ions from hydrocarbon molecules, and such high values can cause serious errors in the calculation of bond strengths from appearance poten¬ tial measurements and can also lead to errors in mass measurement of the ions when perfect velocity focussing does not obtain in the instrument being used. Hippie has reported [919] that using a cycloidal-focussing mass spectrometer the abundance of H+ ions in the spectrum of ethane was measured as ten times larger than in a single-focussing instrument. If, however, the appearance potential of ions formed with a particular kinetic energy is measured, information as to the process by which the ions are being formed is obtained [1920]. In some cases it is found that the appearance potential of an ion of mass p/2 is the same as that for a doubly-charged parent ion. If such an ion possesses kinetic energy it must, however, be a singly-charged fragment ion formed from the doubly-charged parent. Ions of other mass-to-charge ratio are often observed which have the same appearance potential as the p++ ion and the postulate that they also are formed from the doubly-charged parent receives support if it appears that their measured kinetic energy corresponds to the calculated Coulomb repulsion energy between the two charges in the p++ ion. Energetic measurements carried out on large molecules may however give erroneous results if energy is removed as internal vibrational energy in the separating fragments.

7.10

NEGATIVE IONS

The mass spectrometer can, of course, be used to study negatively-charged ions [1674], but far fewer studies of such ions have been made [1675] than of positive ions. Under the usual conditions of operation of a mass spectrometer, negatively-charged parent ions are seldom formed and fragment ions usually occur with initial kinetic energy. Measurements are therefore very difficult to carry out, and the peak heights and variety of ionic species formed are usually far smaller than for the positively-charged ions. An exception is the spectrum of perchloryl fluoride [491] in which the most abundant positively or negatively-

7.10

NEGATIVE IONS

287

charged ion is C103 . Negative ions may be formed either by an electron capture process AB + e -> AB- -> A + Bor by ion-pair production, in which process a positive ion is simultaneously formed AB + e -> A+ + B- + e Negative ions can be observed by reversing the fields in a mass spectrometer, but the appearance potential of a negative ion is more difficult to determine absolutely than that of a positive ion. There is no spectroscopic standard which can be used as an internal reference as in the use of argon in positive ion measure¬ ments and since many of the errors are changed when the fields are reversed, since this changes the conditions within the ion source, values obtained for positive ions cannot be used as references. The smaller beam intensities which are usually obtained for negative ions also add to the difficulties of measurement. The retarding potential difference method of Fox, Hickam, Kjeldaas and Grove [675] eliminates all difficulties concerned with the energy of the bombarding electrons and has given results on positive ions without the use of standard references which agree to within 0.1 eV with spectroscopic values. Use of this technique with negative ions can be expected to give comparable accuracy. The shape of the ionization efficiency curve for the production of negative ions by an electron capture process is very different from that for formation of a positive ion by the process X l e a X+ + 2e. In the latter case the process can occur at any energy above the threshold; any excess energy is removed as kinetic energy of the electrons. In the electron capture process on the other hand the energy of the electron must lie within a very narrow range for capture to occur since there are no electrons to carry away the excess energy and the ion is there¬ fore formed in a resonance process. Very careful measurements have been made by Hickam and Fox [889] using the retarding potential difference method with a pulse operated electron beam of mono-energetic electrons of known energy on the resonance capture peak SF6- in the spectrum of sulphur hexafluoride. This compound is of particular importance in having the highest known dielectric strength for a gas. This property is thought to be associated with the ease with which electrons can be picked up [890] before they acquire sufficient energy to initiate breakdown. The results of Hickam and Fox showed that the capture process occurs at less than 0.1 eV energy and over an energy range not greater than 0.05 eV. This value for SF6 is now used as a standard to calibrate the voltage scale for determinations on other negative ions [711]. If, however, the energy difference between the appearance potential of the standard and the measured ion is large (10 eV say) errors may still arise due to the fact that space charge conditions within an ion chamber may be very different for the two ion species. Resonance capture leads to a very narrow efficiency curve even in cases in which dissociation of the parent ion occurs as for example in the formation of C + Ofrom carbon monoxide [1310], in the formation of H“ and H from H2 [1108, 1801], U and I from I2 [211, 712], O- and NO- in N2O and NO [713, 1139> 1749]> negative ions X- of the halogens from compounds HX [678, 710, 803] or in the formation of SF5“ + F from SF6 [407, 889]. The complete process can be written SF6 + e -> SF6~ -> SF5“ + F + K.E. where

K.E.

represents the kinetic energy of the fragments formed. Assuming

288

7

TYPES OF IONS IN MASS SPECTRA

that the fragments are not formed in an excited state, the kinetic energy will be determined by the difference in energy between the electron affinity and the dissociation energy, and will be divided between the fragments inversely as their masses. If measurements of the kinetic energy of the charged fragment are carried out the value obtained can therefore be used to calculate the total kinetic energy. This value combined with the appearance potential of the ion enables a value of the electron affinity of the corresponding uncharged fragment to be obtained.

Fig. 126. Comparison of the efficiency of production of SF5 electron energy.

and SFe

ions

from SFe as a function of

A comparison of the efficiency curves for SFe- and SF5- obtained by Hickam and Fox [889] showed that the relative heights of the curves depended critically on the range of energies in the bombarding electron beam, and the ratio changed from the value of about 25:1 for a sensibly mono-energetic electron beam to 1: 1 for a conventional electron source and larger electron current [9] due to the greater spread of electron beam energy in the latter case. The curve can never become narrower than the range of energies in the electron beam and in all cases in which effectively mono-energetic electrons are not used comparatively great spreading of the curve must be expected due to the spread in electron energies. As the temperature is increased, the ratio SF6~/SF5_ decreases and at 280° has become about 3:5. The ionization efficiency curve for pair-production is spread over a much wider range of electron energies since this is not a resonance process. The observed appearance potential of the ions A(B“) will be given by the equation A(B-) = D( A — B) — E.A.(B) + 1(A) +

K.E.

+

Ee

where D(A — B) is the dissociation energy of AB, E.A.(B) is the electron affinity of B, 1(A) the ionization potential of A, K.E. the kinetic energy of the fragments and Ee the total vibrational and excitational energy of the fragments.

7.10

NEGATIVE IONS

289

On the assumption that the decomposition products are un-excited, measure¬ ment of the ion appearance potential and kinetic energy of the fragments enables a value for the electron affinity of the neutral particle B corresponding to the negative ion to be obtained. The largest value so far measured is that for Cl (3.7-4.0 eV) [829, 1479, 1732]. Other values of interest in organic chemical work are F (3.6 eV) [106], Br (3.4 eV) [232], I (3.1 eV) [292], C (1.8 eV) [1872], O (1.6-2.2 eV) [815, 1277], H (0.9 eV) [1276, 1303], CH (0.9-1.5 eV) [1872] and SO (1.8 eV) [1478]. The effect of chlorine in reducing the intensity of parent positive ions in mass spectra is discussed in Chapter 9. More confusing in identificational work are ions formed in halogenated compounds by pair-pro¬ duction, since as a result of the large electron affinity of the halogens, positive (and negative) ions can be formed at values of electron energy below the appear¬ ance potential for the parent positive ion. In such cases, reduction of electron energy does not lead to outstanding parent peaks but to large peaks such as (p •— Br)+, etc., and sometimes, especially for chlorine, to peaks (p •— HC1)+ formed by pair production. Examples are given above on p. 261. Other interesting negative ions are the Br3- and CI3- ions reported by Melton, Ropp and Rudolph [1388] in the spectrum of chlorine gas. They were unable to determine whether any of these ions resulted from ionization of neutral Br3 and CI3, the stabilities of which have been predicted. Ions CN-have been detected in the spectrum of mixtures of formic acid and nitrogen gas [1385] but the mechanism of formation is not clear. Negative ions can sometimes be formed as a result of collisions between a positive ion and a neutral molecule. For example, Henglein [857] has reported the formation of (C2H)- and C2- ions from (C2H2)+. Melton [1386, 1729] has shown that O-, (OH)- and (HCOO)- ions can be formed from positively-charged molecular ions of formic acid during collisions with krypton atoms. Transformation of He+ to He- [548] and of H+ to H- [653] in collisions with neutral gases has also been described [548]. Melton has also shown that charge inversion can occur when negatively-charged ions collide with neutral molecules; the ion (HCOO)- from formic acid can, by collisions with helium atoms, form the ions (CO)+, (HCO)+, (COO)+ and (HCOO)+. The state of charge of the other separating particles in the above reactions are not known. Other examples of charge permutation in collisions of negative halogen ions with neutral molecules have been given by Dukel’skii and Fedorenko [546, 547] who also found that the transfer of electrons from donors such as Sb-, Sb2- and Sb3- to acceptor atoms such as Ag was possible if the electron affinity of the acceptor was not less than that of the donor. Dukel skii and Sokolov [549] have produced negative ions containing Si, Ge, Sn and Pb; Fogel et al. [651, 652] have studied the cross-sections of collisions involving positive ions of carbon and oxygen with rare gas and other molecules in which the reac¬ tion A+ + B -> A- + B++ is observed where A represents carbon or oxygen. Negative ions of bromine have been observed by Kuprianov and Petapov [ 1180] formed from ions in the spectrum of dibromomethane by the capture of two electrons in passing through the exit slit. Other examples of the formation of negative ions by secondary processes have been discussed [1463, 1769]- Collisions between negative ions and neutral gas atoms resulting in the loss of one and two electrons by the ions have also been studied [306, 654]. The dissociation of negative molecular ions upon collision with neutral gas atoms has been studied in a mass spectrometer [545]*, the spectrum of negative ions sputtered from a metal surface under bombardment by positive ions has been studied [972, 1408, 1609]. Negative ions of iron, cobalt and nickel have

290

TYPES OF IONS IN MASS SPECTRA

7

been produced by electron bombardment of the vapours of their chlorides [550]. The action of photons on O- ions has been discussed by Branscomb et al. [265, 266] who have determined experimentally the cross-section and threshold law for the reaction O- +

hv

-» O -f-

e~

Negatively charged meta-stable ions were observed by Donnally and Carr [511] in the spectrum of C2H5PO2CI2. They list seven meta-stable transitions, and meas¬ ured their half-lives using the method of Hippie, Fox and Condon [925]. One reaction sequence observed was [C2H5PO2CI2]- -HPO2CI]- + C2H5C1 (half-life 1.9 microsecond) [P02C1]~ -> Cl-

+ P02

(half-life 4.8 microsecond)

The mass spectrum of n-octadecyl alcohol has been obtained by the attach¬ ment of slow electrons (about 1 eV) to the vapour of this compound. Peaks in the region (p — 5) to (p — 1) were seen and also a corresponding series due to peaks (p -—H7O) to (p ■—H2O). Some of these peaks are presumably formed from thermal decomposition products of the alcohol. Spectra of paraffins and of BiCh (which gives peaks due to BiCb- and BiCl-) were also obtained [62].

CHAPTER 8

QUALITATIVE ANALYSIS BY MASS SPECTROMETER 8.1

INTRODUCTION

The use of mass spectra as a means of analysing chemical substances was realized by Thomson, but difficulties of instrumentation and manipulation at first deterred chemists from applying the technique to their problems, and it was not until much later that the first application to organic chemistry was made. The products and processes of ionization by low energy electrons in many monatomic, diatomic and polyatomic molecules were discussed by Smyth [1895]. The spectra of propane and butane were given by Stewart and Olson 954 who considered many possible explanations for the large number of different kinds of ion which they observed such as thermal degradation, multiple ionization and meta-stable ion formation, and came to the conclusion that there seemed “to be only one alternative, that the ionizing electrons decompose the hydrocarbons on impact”. Eisenhut and Conrad [572], using a parabola spectro¬ graph, investigated the disintegration and combination of molecules of methane, ethane, ethylene and acetylene in the discharge tube. They found that in all cases there was a systematic breaking up of the initial molecule into all the possible simpler atomic combinations, down to ions of the elements. They also observed combination ions, some of which corresponded to ions of hydrocarbons having no existence in the free state. In another paper, Conrad [392] described his observation of doubly charged ions of the following atomic combinations: CH, CH3, C2, C2H, C2H2, C2H3, C2H4, CO, O2, C2O and CO3. Before this work it had been thought that only elements could possibly give multiply-charged ions in their mass spectra. Though it was not possible to measure the relative abundances of the various ion species formed, this work pointed the way to chemical analysis. Taylor [1990], another of the early workers in the field, who produced some of the first complete mass spectra of complex molecules, used a modified Aston-type instrument in contrast to most other workers. He was able to bombard his compounds with electrons of any desired energy, and plotted the ion intensities corresponding to the various mass numbers over a range of energies of the bombarding electrons from 30 to 120 eV. He measured the ion currents with a sensitive electrometer amplifier, and so was able to obtain reasonably accurate measurements of the relative abundances of the various ion species which he observed. The first industrial use of mass spectrometry as an analytical technique was in the petroleum industry. The mass spectrometer was used to determine quantitatively the amounts of various compounds present in volatile samples. The object was not to identify any unknown compounds; all compounds present in the samples had already been identified in other ways, and the amounts of

[i

],

292

QUALITATIVE ANALYSIS BY MASS SPECTROMETER

8

most of them could be estimated fairly accurately. Rather, the usefulness of the mass spectrometer lay in the fact that it increased both the speed and accuracy of the analysis of such mixtures, and the method proved more generally useful than any other. Here we shall be concerned with the use of the mass spectrometer for quali¬ tative analysis. In such work the instrument is used in conjunction with other techniques to identify unknown substances. A substance is considered to be identified only when its structural formula can be stated, and the analyses, which are mainly restricted to organic compounds, differ in their objective from in¬ organic analyses where a substance is usually identified when its elemental composition can be stated. Determination of the elemental composition of an organic compound as exemplified by its molecular formula is, however, a neces¬ sary preliminary to identification. The mass spectrometer can be used to obtain three different kinds of infor¬ mation about any positive ion. The mass to charge ratio of the ion can be measured relative to that of ions of known mass to charge ratio; the abundance of the ion can be measured relative to that of other ions in the spectrum and detailed information on the mode of formation of the ion from the sample material can also be obtained. All three types of measurement are important in the determination of the structural formulae of unknown organic compounds. Mass measurement can be used to determine the number and identity of the various atoms making up an ion, but it will give no information on the arran¬ gement of these atoms within the ion. It leads to a molecular formula rather than a structural formula. Abundance measurements of ions different only in the isotopic content of one or more of their constituent atoms also give information about the molecular formula. These measurements are therefore used in com¬ bination to give this data. The amount of knowledge obtainable is a function of the accuracy of measurement. Suppose, for example, that one could measure mass to an accuracy such that a particular ion could be stated to be of mass to charge ratio 16 rather than 15 or 17. One could immediately limit the number of possible empirical formulae of the ion to those such as 0+, NH2+, CH4+, ND+, CH2D+, 13CH3+, 15NH+, 13CHD, S++, CH40++, NH20++, Ti+++, Zn++++ and so on. In organic chemical work one can usually ignore the possibility of occurrence of atoms such as titanium and zinc which are mentioned above, and restrict oneself to consideration of carbon, hydrogen, nitrogen, oxygen, sulphur, the halogens and a few other elements. Polyatomic ions having more than two charges are seldom encountered and possible empirical formulae to be considered are limited by this fact. Further information can be obtained by increasing the accuracy of mass measurement. Since the nuclear packing fraction is different for every isotopic species, it is possible with a sufficiently accurate measurement to distinguish such small differences in mass as to arrive at a unique formula by mass measurement alone. This has been briefly mentioned in Chapter 2. For example, referring to Appendix I, it can be seen that at a mass to charge ratio of 28 the possible atomic combinations of carbon, hydrogen, nitrogen and oxygen in ascending order of mass are, (omitting combinations containing heavy isotopes), CO, N2, CH2N and C2H4. The respective mass differences between adjacent combinations are 11.3, 12.6 and 12.6 mMU respectively. Assuming that there is no systematic error in the mass measurements, one can be 99% certain that a measured mass will lie within 5 mMU of the true value when the standard deviation of a mass measurement is 2 mMU (i.e. rCH3—CHa—C=CH2~| + CH3

J

L

^ C2H5 +

CH3

H2O

J

r—C=CH2-| + L

ch3

-I

The formulae are not meant to be accurate representations of the steric arrange¬ ments in the fragments formed, neither is it suggested that the two stages of the fragmentation process necessarily occur in this order. In 3-methyl-1-butanol on the other hand the C3 ions in the spectrum would be expected to come from the isopropyl group in this molecule and to contain masses 43, 42 and 41. The lack of similarity between the spectra of the branched primary alcohols and the corresponding olefines suggests that in these cases the olefinic parent ion does not enjoy a separate existence. Some other points of interest in alcohol spectra are concerned with re-arrangement ions in which hydrogen addition to the positively-charged fragment has occurred. The presence of some of these ion species can be used as evidence of the presence of oxygen. For example mass 19 (H30+) frequently occurs in the spectra of alcohols, and so does mass 33, the methyl homologue of this ion. It is interesting that the largest abundance of this ion (7% of base peak) occurs in the spectrum of isopropanol, (CH3)2 • CH • OH, and the largest abundance of the ion of mass 33 (51% of base peak) in the spectrum of 2-methyl-l-propanol, (CHs)2 • CH • CH2 • OH. In each case, fragmentation has taken place at the isopropyl group with transfer of two hydrogens. In each case the remaining neutral fragment could be the allyl radical, and the stability of this rather than of the charged fragment probably explains the re-arrangement. The next homo¬ logue in this series of alcohols (3-methyl-l-butanol) does not give an ion at mass 47, though it does give an ion at mass 19 which may be a decomposition product of the mass 47. The only other monohydric alcohol known to give a prominent peak at mass 33 is n-butanol (8% of base peak). Here, too, re-arrangement to give a neutral allyl radical might be possible. Ions which contain oxygen atoms and also more hydrogen than would be required to satisfy all valency requirements in a neutral molecule are very easily recognized. Thus the ion of mass 19 has been referred to in the above as having the composition (H30)+; it would be unthinkable that its formula should be (CHv)+. In other cases, however, the composition of a re-arrangement ion cannot be stated on sight. For example, a re-arrangement peak occurs in the mass spectrum of 2-ethyl-l-butanol at mass 43. This could be of composition (C3H?)+ or (C2H30)+. Theories as to the mode of formation of such ions are hampered by this uncertainty as to their atomic constitution, which can, however, be readily resolved by accurate mass measurement in a double-focussing mass spectrometer. In the above case, the composition can be shown to be (C3H?)+, and suggests that a two-stage breakdown involving loss of the oxygen containing fragment in one stage must be postulated to explain the peak. A detailed mech¬ anism for the formation of such peaks can only be given by the study of isotopically enriched materials so that the actual atoms from the original mole¬ cule which go to make up the re-arrangement peak can be distinguished. Spectra of several deuteriated aliphatic alcohols have been published [388, 574,701, 1428].

352

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

The spectra of the alcohol derivatives of cycloparaffins show many of the expected characteristics of ring compounds such as larger parent ions and a tendency for fragmentation of side-chains at the C-C bond linking the side-chain to the ring. The compounds form a special class of secondary alcohols because cleavage of a single C-C bond (3 to the oxygen atom will not cause the loss of an alkyl group. The tendency for fragmentation with loss of H2O is, however, unaffected by the presence of the ring and a peak formed in this way is prominent in all such compounds, as also is the peak at mass (p — 33). The peak at mass 31 can still be used to confirm the presence of oxygen. Aromatic hydroxy compounds give still larger parent peaks. In phenol itself, the parent ion forms the base peak of the spectrum. The peak at (p — 1) is small. Apparently the tendency of cleavage at the O-H bond [3 to the ring is opposed by the oxygen. Prominent peaks due to ring cleavage can be seen at masses 65 and 66. These correspond to loss of CO and CHO as can be proved by mass measurement. The composition of the peaks at masses 65 and 66 could also be inferred by the study of hydroxy-deuteriated phenol and of thiophenol respecti¬ vely which have been plotted by Momigny [1426]. These show respectively peaks at 65, 66, 67 and 65, 66. The mass spectrum of aniline also shows peaks of comparable intensity at masses 65 and 66 due to loss of the corresponding nitrogen compounds HCN and H2CN. Fragment ions analogous to these are also formed in the spectra of naphthols [190]. Cresols give a large parent peak and an even larger (p ■— 1) peak, as would be expected by analogy with toluene. In this case, loss of CHO occurs readily but not loss of CO from the parent molecule. A re-arrangement ion at mass 77 indicates the aromatic nature of the nucleus. Spectra such as 2-phenylpropanol and 2-phenylethanol have similar peaks to those of aromatic hydrocarbons. In both the cases given [751] fragmen¬ tation occurs at the bond (3 to the ring and the oxygen atom, with loss of the alcoholic side-chain to give ions of masses 91 and 105 respectively. Fragmentation with loss of methyl from 2-phenylpropanol corresponding to the alternative fragmentation j3 to the ring is of low probability. Re-arrangement ions are formed at masses 92 and 106. Study of the spectra of the corresponding deuteriated compounds in which the deuterium atom is incorporated as the -OD group [751] has shown that it is largely the hydroxyl hydrogen which is transfer¬ red to give these peaks. In general, aromatic and naphthenic hydroxy deri¬ vatives are fairly easy to identify, partly because of the fact that the parent ion is unlikely to be overlooked. Aliphatic alcohols are more difficult to recognize and the sort of difficulties of recognition encountered are illustrated in the fol¬ lowing. Consider the spectrum of 3-methyl-l-butanol shown in Fig. 142, and let us assume this to be an unknown compound. The peaks at masses 19, 31 and 45 suggest the compound to contain oxygen. The heaviest ion in the spectrum (excluding heavy isotopes and the very small peak at mass 87 which may be due to an impurity), is of mass 70. The group of peaks immediately below this of masses 57, 56 and 55 differ from mass 70 by 13, 14 and 15 units respectively. This suggests that the group is not formed by fragmentation of the peak at mass 70 but by fragmentation of a heavier ion and than all the peaks observed, including mass 70, may be fragmentation products. All four peaks can be shown to contain carbon and hydrogen only so have all lost oxygen in their fragmen¬ tation. The peak at mass 87 (OH + 70) is now seen to be important, but has an odd mass and thus is itself a fragment. The pattern should now be recognizable as that of a compound of molecular weight 88 which loses mass 1, 18, 31, 32

9.3

MONOHYDRIC ALCOHOLS AND PHENOLS

353

and 33 but not 15 or 29. It is thus a primary alcohol, unbranched on the /1-carbon. Its formula is therefore (C3H?)CH2 • CH2 • OH. Meta-stable peaks at 43.2 and 25.2 are formed by the reactions 70+ ->55+ + 15 and 70+ -> 42+ + 28 Loss of CH3 and C2H4 from the ion of mass 70 coupled with the relatively promi¬ nent meta-stable ion at mass 39.1 suggesting that much of the 41 intensity is due to the meta-stable transition 43+ -> 41+ -I- 2 and not to direct loss of C2H5 from mass 70 would suggest that the C3H7 group in the above structure had the isopropyl structure. Even the identification of this relatively small molecule is difficult because of the absence of a parent peak and the complex decompositions of the parent ions. Similar difficulties will be seen later to arise in the examination of the mass spectra of unknown amines and it is necessary to use all means at one’s disposal to obtain the molecular weight. A method which is sometimes useful for both oxygen- and nitrogen- containing molecules, and which is referred to again later, uses ions which have been formed by inter-molecular reactions. In many classes of compounds, including the alcohols, a peak is often observed at mass (p -fi 1) the intensity of which increases with sample pressure (indicating its origin in an inter-molecular reaction) and which also increases relative to other peaks in the mass spectrum as the ion-repeller potential within the ionization chamber is reduced. If it appears that the parent ion may be too small to be observed, one will generally introduce as large an amount of sample as practicable in order to increase the sensitivity of detection and one should also search for a peak which depends on pressure or ion draw-out potential since, if such a peak is found its mass can be assumed to be one greater than the molecular weight. The difficulties are, of course, greatly increased when one examines a simple mixture of such compounds, or a mixture of unsaturated hydrocarbons or ethers containing one or more alcohols, and it becomes necessary to separate the mixture into its constituents as far as possible before examining it. The potential usefulness of trimethylsilyl ether derivatives ((CH3)3Si-OR where ROH represents the original alcohol) in separation procedures has been demonstrated [1210]. They are easy to form, are resistant to oxidation and thermally stable, readily distilled and the original alcohol can easily be regenerated from them. Similar trimethyl¬ silyl derivatives of primary amines can also be made. It is often preferable, however, not to regenerate the parent alcohol or amine but to obtain the mass spectrum of the derivative itself and to use this spectrum, which is free of interference from hydrocarbons for identifying the original constituent. Because of the absence of interference between trimethylsilyl ethers and hydrocarbons, these derivatives can be used for the rapid, direct analysis of alcohols in hydro¬ carbon solution [1209]. The derivatives have the further advantage from the mass spectrometric point of view that their volatility is greater than that of either the alcohol or hydrocarbon containing the same number of carbon atoms. The spectrum of a trimethylsilyl ether derived from a C10 alcohol can be measured without difficulty using a sample handling system at room temperature. The spectra of trimethylsilyl derivatives are discussed below. Twenty-six such spectra have been given by Sharkey, Friedel and Langer [1822]. The parent peak

354

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

is often small in the spectra of such derivatives and thus may be overlooked. However, the determination of the molecular weight is still easy to perform because of the characteristic fragmentation pattern of the trimethylsilyl group. This breaks to give a large peak due to loss of one of the methyl groups from the highly-branched silicon atom and this is always the largest peak in this heavy ion group and may be the base peak of the spectrum. Thus, if one knows that one is dealing with alcohols and has prepared the trimethylsilyl derivative, the (p — 15) peak can easily be identified and used (by addition of 15) to obtain the molecular weight of the original alcohol. Use of the derivative therefore over¬ comes the main difficulty in the analysis of alcohols, namely the determination of the molecular weight. The mass spectra of the derivatives of primary straightchain and primary branched alcohols show characteristic peaks (containing the (CH3)3Si- end of the molecule) at masses 73, 89 and 103 and an intense series of re-arrangement ions at masses 45, 61 and 75. Heavier ions (other than (p — 15)) are very small. Primary straight-chain derivatives (from C4 upwards) are best identified from the ratio of mass 75 to mass 73 which varies by only 5% for all such compounds which have been examined, and this ratio is also characteristic for primary branched and secondary derivatives. Secondary derivatives of formula (CH3)3Si—O-HC^

show an intense peak corresponding to loss

TR2 of the larger alkyl group on the functional carbon and this mode of fragmentation gives rise to a peak heavier than mass 103 which is useful for identificational purposes. In general, however, more useful information can usually be obtained from the original alcohol spectrum regarding the structural details; the trimethyl¬ silyl ether’s importance lies in the molecular weight determination.

9.4

KETONES

Ketones show a tendency for fragmentation at the C-C bonds adjacent to the >C = 0 group, the charge tending to remain with the oxygen-containing fragment. The spectra of ketones are usually more easily identified than those of alcohols, mainly because the intensity of the parent ion is much greater in ketones, so much so that in most cases it is unlikely to be overlooked. Sharkey, Shultz and Friedel [1820] have listed the partial mass spectra of 35 aliphatic ketones and 7 cyclic and aromatic ketones. Only for 2-methyl-5-undecanone and 7-tridecanone is no parent given, but the sensitivity at which these two spectra are listed is such that no peak below 0.04% of the base peak would be apparent. The next least intense parent ion is that of 4-decanone which is 2.6% of the base-peak intensity. The aliphatic ketones are isobaric with the paraffinic hydrocarbons, but the presence of oxygen in the ketones can be distinguished by unusually prominent peaks at masses 31, 45, 59 or 73. These peaks may be quite small relative to the base peak in the spectrum; in the spectrum of acetone, for example, mass 31 is of height 0.5% of the base peak. It is, however, too large to be an isotopic peak being larger than mass 30 and over 10% the intensity of mass 29. Such a peak gives conclusive evidence for the presence of oxygen. If a high resolution double-focussing instrument is available, the identification of ketones becomes easier. It is then apparent that most of the heavy ions in the spectrum contain the oxygen atom. The main fragmentation pattern consisting of loss of Ri or R2 where the formula of the ketone is Ri(CO)R2 becomes easy to distinguish. Consider for example the mass spectrum of 3-octanone. The out-

9.4

KETONES

355

standing peaks in each group as one proceeds through the spectrum are 27, 29; 39, 41, 43> 55, 57; 71, 72; 85; 99; 114 (weak) and 128. The peaks given in italics are the largest in each group. Apart from masses 72 and 114 these are the peaks one would expect from a paraffin. 72 and 114 might be extra parent ions. Smaller peaks at 31, 45, 59 and 73 indicate, however, that oxygen is present and the pattern of prominent peaks obtained then suggests a ketone, since this class of compounds tends to give peaks isobaric with hydrocarbons. (The fragmentation of aldehydes, which are also isobaric with the paraffins is discussed later.) The peak at mass 99 (p — 29) is larger than that at 85 or 114 and this preferential frag¬ mentation suggests Ri to be an ethyl group. The ketone is thus C2H5 . CO • C5H11 (from its molecular weight). Mass 57 is the base peak of the spectrum and this would be suspected to correspond to loss of the heavier substituent from the carbonyl group and give further confirmation of this semi-structural formula. When fragmentation occurs at a bond /3 to the >0=C group, re-arrangement of a hydrogen atom is likely to occur. This behaviour is also seen in the spectra of aldehydes, and is discussed in greater detail below. The peak at mass 72 is due to such a re-arrangement to form the ketone ion (C2H5 • CO • Cbb)*. Formation of this peak therefore suggests the formula to be C2H5 • CO • CH2 • C4H9 (i.e. that the carbon atom adjacent to the functional group is unbranched), but further information on the arrangement of the longer side-chain is very difficult to obtain. A high resolution mass spectrum would show that the ions 57, 71, 85, 99 and 114 all contain a single oxygen atom and hence immediately suggest a ketonic structure because of the tendency of the carbonyl group to retain the positive charge during fragmentation. The atomic constitution of prominent ions can, occasionally, be deduced from measurements of isotopic abundance ratios. For example, in the spectrum of 2-hexanone, the intensity of mass 44 is only 2.05% that of mass 43, proving that mass 43 is almost entirely due to (CH3*CO)+andnotto (C3Hv)+. In most cases, however, the constitution of such ions cannot be deduced without an accurate mass measurement or without sufficient resolution to enable such ions as (C3Hv)+ and (C2H30)+ to be resolved. Elucidation of the atomic composition of fragment ions is necessary in order to understand the mode of fragmentation of a ketone of known composition. Information of this kind must be available in order to enable one to deduce the structure of an unknown compound from its spectrum. As was mentioned in connection with alcohol spectra, the presence of re-arrangement peaks and peaks formed by multiple fragmentation of C-C bonds makes interpretation of spectra very difficult unless this information as to the formulae of the fragments is available. Ketones give a variety of re-arrangement peaks in their spectra, mainly to form lower molecular weight ketones as in the case of the ethyl hexyl ketone given above. The re-arrangement peaks are often very intense. For example, in the spectrum of 2-undecanone, they form 27.5% of the total positive ion current. Since they occur at masses which correspond to the parent peaks of low mole¬ cular weight ketones or at masses one greater than this, their presence can cause considerable analytical difficulty and it is essential that they be recognized as re-arrangement ions. It is common practice in analytical mass spectrometry to scan each spectrum at a low ionizing electron voltage, so as to simplify the spectra by leaving only parent ions or those formed from the parent ion by processes which are energetically favoured. Such processes do not usually include hy¬ drogen re-arrangement but in the case of the ketones the re-arrangement to lower molecular weight ketones can occur at low electron energy, the stability of both fragments formed in the re-arrangement being high. Fig. 144 shows a

356

9

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

rapid scan of the mass spectrum of methyl n-butyl ketone using 50 eV electrons and then scans obtained by reducing the electron bombardment energy until the intensity of the parent ion is reduced by a factor of about 50. The re-arrangement ion at mass 58 is clearly distinguishable. Once it is realized that an unknown

IOeV

20 eV

ju-LJ

15 eV SPE.CTRA OF MCTHYL , n -BUTYL KFTON2

Fig.

144.

The mass spectrum of methyl n-butyl ketone at bombarding electron energies of 50 eV to The re-arrangement peak at mass 58 can be seen to persist even at the lowest energies.

10

eV.

compound is a ketone and that peaks at masses 58, 72, 86, etc. and 31, 45, 59, 73, etc. could be from re-arrangement ions, the intensities of the peaks at these masses can be used to deduce molecular structure. Mass 58 (the molecular weight of acetone) occurs in all ketones of formula CH3 • CO • CH2 • R where R is C2H5 or heavier, but an intense re-arrangement peak also occurs at this mass for many ketones in which the methyl group is replaced by n-propyl, isopropyl or a heavier group. Ethyl ketones give their most prominent re¬ arrangement peak at a mass of 72, 14 units higher than the methyl ketones, and propyl and isopropyl ketones give a large re-arrangement peak at mass 86 as well as the peak at mass 58 which often occurs in their spectra. Ketones which con¬ tain an isopropyl side-chain can readily be distinguished by the fact that the peak in their mass spectra at mass 59 is always more intense than that at mass 58 and they are the only ketones for which this is so. The mass 59 peak is larger than the mass 58 peak even in cases where both these re-arrangement ions are very weak as in the spectrum of methyl isopropyl ketone. Some methyl ketones give a peak at mass 45 larger than that at mass 44. The peak at mass 43 is large in

9.4

KETONES

357

these spectra, since it corresponds to fragmentation at the carbonyl group with loss of the larger side-chain, and when allowance is made for the contribution of the heavy isotope of this ion to the peak at mass 44, it is found that the peak at mass 45 is more intense than the peak from ions of composition (C2H40)+ in all methyl ketones, and also in some others. The re-arrangement to lower ketones is interesting in that this fragmentation is /? to the -CO- group and not a to it as for the main peaks in the spectra. This type of cleavage with transfer of a single hydrogen is usually more likely than /? cleavage without hydrogen transfer. Thus, isopropyl ketones lose CH2 rather than CH3. Sometimes, as with methyl sec-butyl ketone loss of CH3 by a-bond cleavage is a likely process, but there is also a peak (0.2% of base peak) corresponding to loss of CH presumably by /3-bond cleavage in the longer alkyl group with re-arrangement of two hydrogen atoms. Cleavage of the other //-bond in the secondary butyl group with the more usual re-arrangement of a single hydrogen atom gives rise to a much larger peak (15%). The reason why the re-arrangement peak should be so large in this particular compound is not clear. Other re-arrangement peaks such as those formed by loss of C5H9 from methyl Ji-nonyl ketone and by loss of C2H3 from isopropyl n-butyl ketone are also unexpected, although they are of low intensity. An alternative method of identifying a ketone [1854] which may be used to supplement the information obtained when complete identification by direct mass spectrometric examination does not prove possible is by catalytic reduction of the ketone to the corresponding alkane, followed by mass spectrometric examination of the alkane. Characterization of the carbon chain configuration of the alkane will often lead to the identification of the original ketone. When ketones which contain a ring structure are considered problems of identification increase. Once again peaks of nominal mass the same as those encountered in spectra of hydrocarbons of corresponding molecular weight are observed. A system of empirical rules cannot be devised until the processes occurring in simple model compounds are clearly understood. Few accurate mass measure¬ ments have, as yet, been carried out on such compounds, and it is not known, for example, whether the peaks at (p — 28)+ and (p — 29)+ in the spectrum of methyl cyclohexyl ketone are formed by loss of C2H4 and C2H5 from the parent ion, or loss of CO and CHO respectively. It is known that aliphatic ketones can undergo fragmentation in a re-arrangement process involving loss of CO, acetylacetone being an example [193]. It has already been mentioned (see p. 270) that anthraquinone and such diverse compounds as phenol and diphe¬ nyl ether can give large peaks corresponding to loss of neutral carbon monoxide. It might therefore be expected that cyclic ketones would readily undergo such a process. It has been found, however, that in many cases the dominant fragmen¬ tation process leading to the base peak (p — 28)+ is, in fact, due to elimination of C2H4 in the same way as from cycloparaffins. The mass spectra of cyclopentanone, cyclohexanone and cycloheptanone are given in Table 8. These are the first ketones whose detailed mass spectra have been published and clearly illustrate the tendency to lose C2H4 on fragmentation.

358

9

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA TABLE 8 MASS SPECTRA OF CYCLIC KETONES

Ions containing the isotopes 13C and lsO are only mentioned in the cases where they make the major contribution to the peak. All ions carry a single positive charge except where indicated. Mass/charge

12 13 14 15 24 25 25.5 26 27 28 } 29 } 30} )

31

1

32 32.5 33 33.5 34 35 37 38 39

40 } 4! } 42 } 43 }

44 } 45 46 49 50 51 52

53

}

54 }

55

}

56 }

Ion

c CH ch2 ch3 c2 C2H c4h3++ C2H2 c2h3 CO c2h4 CHO c2h5 ch2o 12C13CH5 C5H0++ ch3o c2h7 ch4o c5h5++ C5H(i++ c5h7++ C5H8++ c5h10++ c3h c3h2 c3h3 c2o c3h4 c2ho c3h5 c2h2o c3h6 c2h3o c3h7 c2h4o c3h8 c2h5o c2h6o c4h C4H2 c4h3 c4h4 c3ho C4Hs C3H20 c4h6 c3h3o C4H7 c3h4o c4h8

Cyclopentanone

0.13 0.67 1.0 0.04 0.31 0.22 4.0 11.0 1.1 33.0 0.45 3.9 0.09 0.14 0.05 0.03

0.54 1.0 0.65 0.11 1.6 1.5 30.3 11.1 11.3 2.1 2.1 0.13

0.07 (and 12C213CH7)

0.22 0.72 0.64 0.25 0.22 0.89 0.13 0.99 94.4 5.6 12.7 3.5

Cyclohexanone

0.03 0.44 1.2 0.01 0.10

Cycloheptanone

0.08 0.67 2.3 0.13

2.0 6.1 1.9 5.4 0.86 4.9 0.11 0.11 0.37 0.36

3.4 23.1 5.0 16.7 1.3 10.6 0.25 0.24

0.26 0.11 0.25 0.05 0.25 0.07 0.53 1.3 12.3

0.08 0.04 0.08

3.4 0.16 33.5 18.9 38.2 9.2

2.4 (and 12C213CH6)

0.33

0.21 0.50 1.5 19.6 0.04 5.1 1.0 56.4 12.2 28.8 8.0 9.7

2.0 0.07

0.71 0.25

0.23 0.01 0.14 0.75 0.95 0.38 0.19 1.6 0.16 7.1 86.7 13.3 7.9 4.6

0.33 0.17 1.1 1.5 0.71 0.21 3.4 0.17 3.4 74.5 25.5 7.0 31.5

9.4

KETONES

Mass/charge

}

57

58 } 59 60 61 62 63 64 65 66 67 68

}

69

\

70 71 72 77 78 79 80 81 82 83

84 }

-1| 86 91 93 94 } 95 96 97 }

Ion

Cycloheptanone

5.6

3 0

0.87 0.18

0.34

C3H60 12C313CH9 C3H70 6J3H80 CJ5H (J5H3 C5H3 C5H4 CJ5H5 05H.6 05H7 C4H40 CsHs C4H5O C5H9

0.11

0.40

C4HeO C5H10 C4H7O 12C413CHi0 U4H8O c6h5 c6h6 C5H30 C6H7 c6h8 C5H50 06H9 C5H60 CeHio C5H7O O6H11 (JsHsO C6Hl2 (JsHgO 12c413ch8o CsH7180 CsHgiso

C5H10O C7H7 C7H9 CeHfiO C7H10 c6h7o c6h8o c6h9o C7H13 CgHioO 12C613CHi3

99

,2C513CHioO C6Hio180 C7H10O C7HnO C7H12O 12C613C H12O C7Hi2lsO

111 112 113 114

Cyclohexanone

C3H5O c4h9

98 } 100 110

Cyclopentanone

359

0.06 0.03 0.10

0.20 0.22

0.15 0.03 0.03 0.26 0.18 1.6

0.60 1.2

0.65 0.18 0.18 0.03

1.1 21.1

5.2 14.8 3.4 1.9 0.19 0.34 0.14 0.18 0.08 1.1 3.4 0.04 0.53

0.07 1.1

22.8

1.8 6.0 0.12 (and 12C513CHio) 0.57 0.08

1.5 0.03

2.1 (and 1 2C3i3CH8) 2 5 0 03 0 13 0.04 0.08 0.17 0.08 0.08 0.59 0.46 0.44 0.41 70.7 12.4 5.0 (and 12C413CH8) 9.5 1.6 1.2 0.09 0.17 0.38 0.17

2.1 0.42 0.17 0.75 0.08 0.29 10.0 1.2 20.3 0.59 1.3 1.1 0.04

0.07 0.13 0.13 0.21 0.11 0.03 0.11 1.7 32.2

2.4 0.03 0.04 4.0 0.08 2.0 0.04

2.5 0.07 0.04 0.59 27.3 2.5 0.05

360

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

The fragmentation pattern of anthraquinone has been mentioned in con¬ nection with the re-arrangement reactions which it undergoes with loss of neutral CO fragments, forming presumably ions of fluorenone and o-diphenylene (see p. 271). Substituted anthraquinones show the same tendency; for example monohydroxyanthraquinone parent ions tend to break with loss of a neutral CO molecule, followed by a second CO followed by either a third CO or a CHO fragment (c/. behaviour of phenols). The ion (p — C303H)+ is of mass 139 and this ion seems to be exceptionally stable and to exist abundantly in the doublycharged state giving a peak at mass to charge ratio 69J of the order 5% of base peak. Such an ion is often seen in the mass spectra of fused ring hydrocarbons, but its structural formula is not known. Dihydroxyanthraquinones give a corresponding ion of mass 155 as well as a peak at mass 156 corresponding to (p ■— C303)+ and this latter ion can lose a fourth CO to give a peak at mass 128. Associated peaks at masses 127 and 126 are smaller than the 128 peak. This is different from the behaviour of monohydroxyanthraquinones which break to give a larger mass 139 than 140 and of dihydroxyanthraquinones which give a peak at mass 155 larger than at 156 due presumably to the stability of the mass 139 nucleus. Other peaks in the mass spectra of hydroxyanthraquinones corre¬ spond to loss of part or all of the hydroxy side-chain. Compounds in which hydroxy groups are substituted in the 2, 3, 6 or 7-positions around the rings tend to lose -OH in fragmentation but those substituted in the 1, 4, 5 or 8-positions giving an internal hydrogen-bond with the keto-group show an increased ten¬ dency to loss of the oxygen atom only, but a reduced tendency to combined loss of either O or OH. The behaviour of the hydroxy compounds is paralleled by that of the corresponding amino derivatives. Loss of CO from the parent ion is common in many related quinones and other ketones. It occurs, for example, in naphthoquinone, benzoquinone, phenanthrenequinone and benzanthrone. Benzanthrone also gives rise to a prominent peak at mass 200 due to loss of H2CO from the parent ion. The loss of this fragment leaving, presumably, a stable ion, is difficult to explain unless re-arrangement of the carbon atoms making up the ring structure is postulated. Loss of CO from benzanthrone by a similar process to that in anthraquinone would give rise to the ion

of formula C16H10 which has a fully conjugated system of double bonds. In order to obtain a stable CieHs structure it is necessary either to increase the number of rings to give some such structure as

or to include a triple bond in the structure. The large parent peaks which occur in all these ring compounds make deter-

9.5

ALDEHYDES

361

mination of the empirical formula straightforward. Many of the compounds can be identified by their distinctive colours.

9 .5

ALDEHYDES

These compounds are isomeric with ketones but the fragmentation patterns of the two groups of compounds are quite different. The mass spectra of twenty aliphatic aldehydes have been reported by Gilpin and McLafferty [750]. These include straight-chain aldehydes and also aldehydes branched on the carbon atom adjacent to the functional group. In all cases, the height of the parent peak is appreciable and in almost all cases there is also a peak at mass (p — 1) of comparable intensity. Even for tetradecaldehyde the parent peak is 0.5% of the base peak and so is unlikely to be overlooked. Once again, the presence of oxygen in the molecule is apparent from the presence of peaks at masses 31, 45, etc. It is not, however, possible to distinguish in all cases which ions contain oxygen and which do not without either high resolution coupled with accurate mass meas¬ urement or the use of isotopically labelled compounds as described by Gilpin and McLafferty. This latter method involves the preparation of a compound labelled with lsO in order to decide the constitution of the ions in any single spectrum and so cannot be used as widely as the first method. It has however been used to show that the ions of mass 29 largely contain oxygen in the case of propanal [(CHO)+ ion] but not butanal [(C2Hs)+ ion] or heavier aldehydes. Ions formed by fragmentation a or (3 to the -CHO group are more readily recognized in the case of aldehydes than are the corresponding ions formed by fragmentation a or /3 to the -CO- group in ketones since the aldehyde group is at the end of the molecule and thus can be observed without an appended alkyl chain. The ion formed by a-cleavage in aldehydes is the (CHO)+ ion, but as already mentioned, this mode of fragmentation becomes unlikely for aldehydes heavier than propanal. Much more likely for heavier aldehydes is /5-bond cleavage with transfer of a single hydrogen atom to form a peak at mass 44 composed presumably of acetaldehyde ions. This behaviour is qualitatively the same as in ketones where a similar re-arrangement of a hydrogen atom can accompany /Lbond cleavage. However, in aldehydes the process is much more probable and a large peak at mass 44 together with peaks at mass 31 or 45 indicating the presence of oxygen (and thus distinguishing the mass 44 peak from that often formed in amines) is strong evidence for an aldehyde structure. When the aldehyde is substituted in the a-position with a methyl or an ethyl group, the corresponding large peak (usually at least 50% of the height of the base peak of the spectrum) will be displaced to mass 58 or 72. Many of the other lower-mass peaks in the spectra of aldehydes are those which one would expect from the breakdown of a hydro¬ carbon chain. Other peaks which are useful for structural diagnosis occur at the high-mass end of the spectrum and illustrate the importance of noting the masses both of the positive ions and of the neutral fragments formed. The most likely mode of breakdown of the parent ion of an aliphatic aldehyde involves loss of mass 1 (mentioned above) and also masses 18, 28 and 44. Alcohols can also break with loss of 18, but the complementary loss of mass 28 (CO) and of 44 (CH3CHO) is quite characteristic of aldehydes. In some cases the (p — 44) peak can be one of the strongest in the spectrum. The loss of CO is one of many examples of the loss of this stable molecule from oxygen-containing compounds. The mass spectrum of benzaldehyde is shown in Fig. 145. This shows the large parent peak characteristic of aromatic compounds and also the large (p — 1)

362

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

peak associated with the presence of the aldehyde group. The absence of a large peak at mass 91 distinguishes the spectrum from that of the isobaric xylenes and ethylbenzene. At lower masses, peaks characteristic of the fragmentation of the aromatic nucleus can be seen. A small peak at mass 29 is due to ions of com¬ position (CHO)+. BENZALDEHYDE

30

40

50

60

?o

70

80

90

100

Fig. 145. The mass spectrum of benzaldehyde.

9.6

ETHERS

Ethers are isomeric with alcohols containing the same degree of unsaturation. Thus the aliphatic ethers form a homologous series in which the molecular weights of the member compounds are the same as those of the homologous series of aliphatic alcohols. There are marked similarities between the spectra of compounds in the two series, and to some extent the alcohols can be looked upon as members of the ether series for which one of the hydrocarbon substituents on the oxygen atom is replaced by a hydrogen atom. The presence of the oxygen atom is again readily perceived from the strengths of the peaks at masses 31, 45, 59, 73, 87, etc. The mass spectra of 25 saturated aliphatic ethers have been given by McLafferty [1360]. The parent ion intensity is small, and since if this ion is overlooked identification becomes more difficult, search should be made in case of difficulty for an ion the relative abundance of which is pressure dependent and varies with ion-repeller potential. Such an ion, if found, will be of mass (p + 1) and enables the nominal molecular weight to be obtained. If the ethers have the

9.6

ETHERS

363

formula Ri • O • R2 where the mass of Ri > the mass of R2, the following facts emerge from a study of their mass spectra. Fragmentation of the molecular ion at the C-C bond /5 to the oxygen atom (i.e. between the a and P carbons) is often a probable process. The fragmentation tends to occur preferentially in the longer chain. If R2 is a methyl group and Ri is unbranched on the a-carbon, then mass 45 corresponding to P cleavage is the base peak of the spectrum. In dimethyl ether, a C-H bond p to the oxygen atom is broken to give the same ion. When Ri contains, say, a methyl group substituted on the a-carbon atom, as in methyl isopropyl or methyl sec-butyl ethers the base peak is at mass 59 and similarly for methyl tert-butyl ether the base peak at mass 73 is still formed by fragmentation at the /5-bond. Similar behaviour is evident in the spectra of ethyl and propyl ethers giving a closely analogous form of fragmentation to that exhibited by the alcohols. Fragmentation by cleavage of the C-C bond a to the functional group can also occur, and appears to be especially favoured in the case of symmetrical ethers. This process is analogous to loss of -OH from alcohols. Fragmentation at this bond is sometimes accompanied by hydrogen migration to give a peak corresponding to (Ri — 1)+ and (Ri—2)+ in an analogous fashion to the loss of H2O and H3O from some alcohols. Generally speaking, however, it is the Ri+ ion which is formed and ions such as (Ri — CH2)+, (Ri —- CH3K, (Ri — CH4)+ are usually weak (i.e. ions analogous to those formed by loss of 31, 32, 33 in alcohols are not prominent) though in the spectra of long-chain ethers, e.g. di-n-decyl ether there is a peak at mass 112 corresponding to the ion (Ri — C2H5) which closely parallels the loss of mass 46 in long-chain alcohols. Fragmentation to give an alkyl radical Ri+ is reflected in the pattern of the peaks at the lower mass numbers. This is much more similar to the paraffinic than to the olefinic pattern and gives the best method of distinguishing between alcohols and ethers. The peaks at masses 31, 45, 59, etc. are often formed in a re-arrangement pro¬ cess. It appears that these ions are formed by a cleavage of both the a- and /5bond on either side of the functional group with simultaneous re-arrangement of a hydrogen atom from a /5-carbon atom by the process [1360]

I

Ri—-C—CH2O -CH2—ch2—r4

I

1

R3

McLafferty has deduced that a /5-hydrogen takes part in the re-arrangement from the fact that such a peak does not appear in methyl ethers. In the above formula, R2 and R3 can be alkyl groups or hydrogen atoms, but the re-arrangement is favoured by substitution at this point in the molecule. This may be cited as an example of fragmentation at a highly branched position as was noted in the spectra of hydrocarbons. An interesting series of re-arrangement peaks occurs for the ethers with long straight alkyl chains. This series of peaks occurs at the masses (p —18), (p—32), (p—46), etc. Although the individual peaks are small (of the order 0.5% of the base-peak intensity) in the mass spectrum of di-n-decyl ether they are the most intense ions in that region of the spectrum. The fact that this series of ions does not contain oxygen can be proved by accurate mass measurement. As an example of the identification of an unknown ether, consider the mass spectrum of ethyl sec-butyl ether. This has a molecular weight of 102, two masses greater than a saturated hydrocarbon. It gives a 1% parent ion, 0.4% at mass 101

364

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

(by cleavage of a C-H bond /5 to the oxygen) the other outstanding peaks in the spectrum being at 87 (4%), 73(51%), 59 (19%), 57 (10%), 45 (100%), 43 (10%), 41 (12%), 31 (8%), 29 (34%) and 27 (26%). The peaks at masses 31, 45 and 59 suggest an oxygen atom. Assuming that mass 102 is the parent ion, the other peaks are all formed by loss of reasonable mass fragments so there is no evidence that the parent has been overlooked or that one is dealing with an alcohol. Assuming only a single oxygen to be present (this can be confirmed by mass measurement) the compound must be an ether of formula C6H14O (no rings or double-bonds are possible with this formula). The base peak at mass 45 contains the oxygen atom and is probably formed by /5-bond cleavage suggesting an • O • C2H5 group, and a formula C4H9 • O • C2H5. This is confirmed by the ion at mass 57 ((C4Hg)+ formed by a-bond cleavage). The oxygen-containing ion at mass 73 is unusually prominent, more so than one might expect by a-bond cleavage to give the smaller radical. It would thus be suggested that the longer radical was branched at the a-carbon thus enhancing the a-/5-bond cleavage with hydrogen re-arrangement mentioned above. The structural formula follows. Occasionally, the empirical rules of breakdown whilst predicting correctly in which C-C bond fragmentation of the parent ion is most likely to occur do not predict correctly the number of hydrogen atoms in the fragment ion. An out¬ standing case is provided by the mass spectrum of di-n-butyl ether. Fragmen¬ tation at the bond /5 to the oxygen atom occurs but an ion of mass 41 is formed with high intensity, presumably by further decay of the intermediate ion of mass 43. A special study has been made of the mass spectra of a special class of di¬ ethers, the acetals, by Friedel and Sharkey [692]. The general formula of the acetals is RiCH(OR2)2 and Friedel and Sharkey investigated a number of formals, acetals and propionals (for which Ri is H, CH3 and CH3CF12 respectively). It can be seen that the two oxygen atoms in the various possible molecules of this formula are separated by a single carbon atom to which are attached a hydrogen atom and the group Ri. The characteristic feature of the spectra is the extremely low intensity of the parent ion and the high intensity of the ions of masses (p — 1) and (p — Ri) respectively which correspond to fragmentation of the substituents on the central carbon atom. The height of the peak at the parent ion mass is so small that it can be accounted for almost entirely by the isotopic abundance to be expected from the large peak at a mass one below. This is true even for the smallest molecule viz. dimethyoxymethane. Other large peaks in the spectra occur at mass (p — OR2). In some respects the presence of oxygen in a molecule makes identification more easy; in the case of saturated compounds such as aliphatic mono-ethers prominent peaks two masses greater than can arise from hydrocarbon fragments pin-point those ions which contain the oxygen atom, and make its exact location an easily-established fact. The acetal-type compounds are an example of the confusion which can, however, arise when hydrogen trans¬ fer is suspected to occur or when even more complicated re-arrangements can take place. Decomposition on the mass spectrometer filaments can give unstable patterns and lead to still further difficulties [1220]. One must be particularly cautious in one’s interpretation of any peaks from a molecule known to contain several oxygen atoms. Once again, the careful tabulation of all meta-stable peaks sometimes indicates the path by which a particular ion has been formed and this knowledge enables one to deduce the ionic formula. On the other hand, in many cases, no meta-stable transition leading to a particular re-arrangement peak can be observed, but it is often possible to obtain the fragment ion formula

9.6

ETHERS

365

by means of an accurate mass measurement. On some occasions it is possible, by studying the mass spectra of a homologous series, to deduce from the spectra of the smaller molecules the fragmentation pattern which has occurred to give peaks in the spectra of the larger molecules. The acetal-type compounds give an interesting series of peaks at masses 19, 33, 47, 61 and 75. The corresponding hydrocarbon radical to the last of these ions would have the mass 71 and one might therefore deduce that mass 75 contains both oxygen atoms in all cases. But, proceeding downwards along the homologous series of ions, it is obvious that mass 19 can contain only a single oxygen and has, in fact, the formula H3O. But how is one to decide the composition of heavier ions on an arbitrary basis 1 The peak at mass 47 (of abundance 17%) in the spectrum of CH3 • CH(0 • C2Hs)2 and at the same mass in the spectrum of CH3 • CH2 * CH(0 • CbHs^ (63%) might be explained as (H2 + O • C2H5), but the peak of abundance 21% in the spectrum of CH3 • CH2 • CH(0 • CH3)2 is difficult to understand on either formula. The spectra of two formals (dimethyl and diethyl) and also of dimethyl acetal have been obtained in these laboratories under conditions of high re¬ solution, and the identity of all the ions in these spectra have been determined by accurate mass measurement. Let us consider in detail the spectrum of dimethyl acetal. This compound has a molecular weight of 90 and as in all other compounds in which two ether oxygens are attached to a single carbon, no parent ion is observed. The peaks correspond¬ ing to loss of fragments from the parent ion by breaking bonds attached to this carbon atom, viz. (p — 1)+, (p —- 15)+ and (p — OCH3)+ at masses 89, 75 and 59 are all strong, the last-mentioned being the base peak of the spectrum, and the atomic compositions of these ions could readily have been surmised without mass measurement. Equally obvious are the compositions of the peaks at masses 15 and 31 which could correspond to fragmentation at the same bonds, but with the charge remaining on the smaller fragment. Prominent ions whose atomic composition is not immediately obvious occur at masses 27, 28, 29 and at mass 43. Before considering the mass measurements let us see whether one can deduce the formula of these ions using the information available from meta-stable transitions. The following appear in the spectrum: 89+ -> 75+ + 14 (m.s. at 63.1) 75+ -> 47+ + 28 (m.s. at 29.4) 45+ ->31++ 14 (m.s. at 21.5) 43+ -» 27+ + 16 (m.s. at 17.0) 59+ ->31+4" 28 (m.s. at 16.3) 59+

'-*■

29+ + 30 (m.s. at 14.3)

The formula of the ion of mass 75 in the first of these transitions is clear; its mass is only 15 less than that of the parent ion, so that its formula must be (C3H7C>2)+ and this transition can be re-written (C4H902)+

-> (Q)H702)+ -|- CH2

The meta-stable peak is interesting in that it shows that some, at least, of the ions at mass 75 do not arise by a simple fragmentation involving loss of a methyl

366

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

group from the parent ion. Two other transitions lead to the formation of an ion of mass 31, and if one deduced that the composition of this ion was always (CHsO)+ and that of mass 59 was always (CsHvO)"1" one could write two other reactions as (C3H70)+ -» (CH30)+ + C2H4 and (C2H5CO+ -► (CH30)+ + CH2 The first of these reactions shows that part at least of the mass 31 peak is formed by a two-stage process and not directly from the parent ion. The second reaction begins from a re-arrangement ion and shows that mass 31 ions can be formed in even more round-about ways. Such arguments as the above assume, of course, that ions such as those at mass 31 are all of a single formula, but the variety of reactions leading to product ions of this mass shows this to be a dangerous basis for argument. In the examples given above, the formulae quoted have all been confirmed by mass measurement. Mass measurement also enables us to write the reactions corresponding to two other transitions as (C3H702)+ -> (C2H70)+ + CO and (C2H30)+ -> (C2H3)+ + O since all the ions concerned in these reactions are singlets. The remaining transition can, however, be written either as (C3H70)+ -> (CHO)+ + C2H6

or—* (C2H5)+ + CH20

since the ion at mass 29 is a doublet. Measurement shows that in the series of masses 19, 33, 47, 61 mass 61 is anomalous in containing two oxygen atoms; the other

Fig.

. Photograph of a molecular rnodel of CH3 • O • CHfCH3).

146

9.6

ETHERS

367

ions are homologues of H30+. The method by which some, at least, of the ions at mass 47 are formed, illustrated by one of the meta-stable transitions above is particularly interesting since it involves loss of neutral CO from a fragment which can itself be formed from the parent ion by cleavage of a single C-C bond. Loss of carbon monoxide is common in fragmentation of oxygenated organic compounds and has been mentioned in the general discussion of meta-stable transitions. The acetals provide a striking example of the occurrence of this mode of fragmentation in non-ringed compounds. Presumably the intermediate ion must assume a ringed structure before CO can be emitted without loss of other atoms. Fig. 146 shows a photograph of a molecular model of the fragment CH3 • O • CH(CH3) from which it can be seen that formation of a ring structure is sterically possible. The ring could be a 3 or 4 membered heterocyclic one. Such a reaction sequence might explain the formation of an ion of mass 61 in the spectrum of diethyl acetal but a different explanation would be necessary in this case for the formation of the ion of mass 47. The mechanism of formation of the very prominent ion at mass 47 in the spectrum of diethyl propional (64% of base peak) is also not clear, but another ion at mass 75 in this spectrum might be formed in the same way as the ion of mass 47 in the dimethyl acetal spectrum. We are still a long way from understanding spectra of this sort and though an understanding will be brought nearer when a large number of acetal spectra have been obtained under conditions of high resolution, it would seem that only a more detailed understanding of the structural configuration and energy distri¬ bution in pdsitive ions will explain why, for example, the mass 47 peak should be small (1% of base peak) in the spectrum of CH2(0 • C3H?)2 but large in the spectra of CH3CH(0 • C2Hs)2 and CH3CH2CH(0 • CH3)2. Large peaks at masses 47, 61, 75 due to ions of formulae (C2H?0)+, (CsfRO)4" and (C4HnO)+ can, however, be taken as strong evidence of the presence of two atoms of oxygen in an unknown compound. Lack of a parent ion, as shown by accurate mass measurement of the heaviest ion in the spectrum may be due to the presence of Ri the group -O-C-O- where Ri and R2 may be alkyl groups or hydrogen atoms.

I r2 The heaviest ions will usually correspond to loss of Ri and R2 respectively from the parent ion and these will generally be prominent ions in contra-distinction to other compounds which show no parent ions such as long-chain alcohols or amines. The rule that if a compound contains two ether oxygens separated by only a single carbon atom it will exhibit no parent peak also extends to cyclic polyethers. We have measured under conditions of high resolution, the mass spectra of l:3-dioxolane, l:3-dioxane, 4-methyl-l:3-dioxane and symmetrical trioxane, all of which come into the above category, and have compared the spectra with that of l:4-dioxane in which the oxygen atoms are separated by two carbon atoms. The spectra are shown in Table 9.

368

9

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

TABLE 9 THE MASS SPECTRA OF CYCLIC POLY-ETHERS

Ions containing heavy isotopes are only mentioned when they make a major contribution to a peak. All ions carry a single positive charge unless otherwise indicated Mass

Formula

c

12 13 14 15

CH

id}

17

}

18

ch2 ch3 O ch4 HO HaO 13CH4

h3o

19 20

}

22

24 25 26 27

} 29} 28

)

30

J ) 31} J 32

33} 35.5 36 36.5 37 38 39

} 41} 42} 40

43} ) 441

J }

45

461

J 47}

C2O++ CO2++ C2H4O++ C2 C2H C2H2 C2H3 CO C2H4 CHO C2H5 CH170 CHaO CaHe CHlsO CH3O 13CCHo CH4O

1:3-Dioxolane

1 :3-Dioxane

0.11 0.35 1.98 13.10 0.25 1.73 4-35 21.08 0.02 3.83 0.04 0.06

0.05 0.06 0.40 0.84 0.13 0.06 1.52 7.72

0.02 0.13 0.96 6.57 2.91 0.77 25.68 0.01 1.65 0.05 5.67 0.50

ch318o CH5O C3H302++ C3H402++

++

C3Hs02 c3h C3H2

c3h3 c2o C3H4 C2HO

c3h5 C2H20

c3h6 c2h3o

C2H40 C3H8

cho2 C2H50 CH202

c2h6o i3cch5o CH302

c2h7o

0.09 0.88 4.60 0.15 0.15 0.32 1.61

Sym-trioXane

1:4-Dioxane

0.54 0.49 1.12 0.46 0.36 0.06 0.35 1.53

0.06 0.22 1.28 10.92 0.19 0.23 0.82 3.90

1.47 0.08 0.47 0.16 3.10 8.45 2.22 45.89 20.51 14.62

0.15 3.11 16.83 0.67 11.85 15.60 18.30

2.15 9.87

2.29 10.50

45.41 0.16 0.81 0.08 0.10

45.16

0.11 0.22

0.38 0.97 4.49

0.67 0.08

25.14 87.78 70.00 100 6.00 0.21 0.36

0.09 0.11 4.68 8.82 2.55 97.45 13.18 13.74 3.34 10.72 15.96 0.14 0.44 0.04 0.04

0.05 0.77 0.16 0.01 0.03 0.03 0.20 0.04 2.38 0.04 14.88

C3H7

co2

6-Methyl1 :3-dio5cane

1.28 45.16 0.26 18.94 0.21 0.47

0.02 0.35 0.32 0.30 0.98 0.14 1.28 0.89 0.14 1.95 0.06

2.61 0.12 17.36 2.58 30.38 86.80 13.20 1.29 15.13 12.26 0.12 36.60 0.03 0.82

0.18 0.08 5.61 0.14

0.16 0.21 0.01 0.16 0.12 0.46 1.28 0.05 10.44 0.02 0.35 4.29 4.50

0.10 0.09

0.03 0.10 0.04

0.21

0.01

9.6

ETHERS

Mass

50 51 52 53X /

54 55X / I

56

1 D573f 58 1

59

J

60| 61 \

62 68

69 70 71 \

ns

a) l

73jr ia\

/4/

75 83 84 85\

Formula

c4h2 C4H3 C4H4 c3ho C4H5 c4h6 (J3H30 c4h7 C2CJ2 C3H40 c4h8 C3H50 C4H9 c3h6o U2H302 13cc2h6o C3H70 (J2H402 13cc2h7o C2H5O2 c3h7i8o c2h602 C4H40 C4H50 C4HeO C3H3O2 c4h7o C3H402 c4h8o C3H502 C4H90 C3H602 C4H10O C3H51800 C4H302 C4H402 C4H502 C5H90

86

C4H602

87 88

C4H702 C4H802 13CC3H802 c3h5o3 c4h718oo C4Hs1800 13cc2h5o3

1

89 j on1 t 90j r 101 102

C5H902

13cc4h9o2

1 : 3-Dioxolane

1:3-Dioxane

369

4-Methyl1 :3-dioxane

S^m-trioxane

0.21 0.23 0.09 0.06 1.06 0.70 1.17 62.46

0.02

0.02 0.02

11.46

0.20 0.03

16.20 0.76

0.12 2.90 18.77 0.23 14.49 1.61

1:4-Dioxane

0.01 0.01 0.01 0.01 0.01 0.19 0.05 0.01 0.03 0.05 7.47 0.02 32.25 0.29 1.00

0.24

11.01 0.10 0.38 0.08 0.02

1.17 0.42

0.13

36.67

0.13

0.81 0.32 0.26

0.09 0.07 0.02

3.87

0.01

54.84

0.01 0.03

0.28 0.28 100

0.05 2.73

5.14

0.14 0.15

0.54 0.01 0.09 0.01 0.06 0.05 0.04 0.06

100 4.78

0.08 2.68 35.73 1.30

15.25 0.67 27.92

0.40 0.02

0.06 1.04 51.61 2.99

The first four compounds all give a large peak at (p ■— 1) corresponding to loss of a hydrogen atom presumably from the carbon situated between the oxygen atoms. For this reason, it is difficult to distinguish under low resolving power whether a small amount of parent ion is present, because of interference from the isotopes of ions of formula (p — 1). It is sometimes possible to detect the presence of a peak at a mass one above this intense peak and to infer that it is due to the parent ion, by plotting the ratio of the peak heights at the two mass

370

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

numbers as a function of the energy of the bombarding electrons. This has been done in Fig. 147 for the ions at masses 88 and 87 in the spectrum of 2-methyl--, l:3-dioxolane. It can be seen that this ratio increases as the electron energy is reduced, suggesting that the parent peak is at mass 88. In one spectrum, that of

Fig.

.

147

Plot of the ratio of peak heights at masses 8/ and 88 in the mass spectrum of 2-methyl-1: 3dioxolane as a function of bombarding electron energy.

symmetrical trioxane (M.W. =90), there is the expected large peak at mass 89. A meta-stable peak at mass 88 is formed by the reaction 90+ -> 89+ + 1 and indicates the presence of a peak at mass 90, which one would again infer to be the molecular ion. The peak at mass (p — 15) in the spectrum of 2-methyl1:3-dioxolane is the base peak of the spectrum, but is smaller than the peak at (p — 1) in the spectrum of 4-methyl-l:3-dioxane. This provides further evidence that it is the groups attached to the carbon between the two oxygen atoms which are lost very readily from the parent ion. Interesting peaks are those correspond¬ ing to the loss of neutral CHO from the parent ion which occur in some of the spectra in preference to loss of CH2O. (CHO)+ ions are prominent in all spec¬ tra; only in the spectrum of symmetrical trioxane are these accompanied by strong peaks due to (CO)+ and (CH20)+ though all spectra give an ion corre¬ sponding to (CH30)+ at mass 31. Loss of a single carbon or oxygen from the

9.7

CARBOXYLIC ACIDS

371

ring is always of low probability so that loss of CH3 is indicative of the presence of a side-chain. Another point of interest in the spectra is the large (CH3)+ ion in 1:3dioxolane and the 2-methyl derivative of this compound. The only other com¬ pound to show a similarly large mass 15 ion is l:4-dioxane. The mechanism of formation of these peaks is not clear. It can be seen that many of the ions in these mass spectra are formed by re-arrangement of hydrogen atoms and this makes structural diagnosis from a low resolving power mass spectrum difficult since in such a spectrum one relies to a certain extent on the increased mass of oxygen containing peaks over the corresponding hydrocarbon ion as a means of determining the position of the oxygen atoms within the molecule. None of the spectra show re-arrangements involving atoms of carbon or oxygen and for this reason accurate mass measurement of all peaks in the spectrum often makes location of the oxygen atoms a simple matter. For example, in the spectrum of symmetrical trioxane the ion at mass 16 is a doublet (CH4+ and 0+) and shows oxygen to be present; the group of ions at low mass between masses 29 and 35 have the compositions (CHO)+, (CbbO)4-, (CH30)+, (CH40)+ and (CH50)+. These include a number of ions which have been formed with hydrogen re¬ arrangement. The fact that they all include a single carbon and a single oxygen atom and that no ions containing C2 or O2 are present strongly suggests that carbons and oxygens are arranged alternately in a chain and indicates the presence of more than one oxygen. Confirmatory evidence of this is provided by heavier ion's in the spectra which do not include any “C3O” or “CO3” type ions, as well as by the absence of a parent ion, which would be expected behaviour of such a structure. In a similar fashion, the presence of a large peak at (GafT)-1" in the spectrum of 4-methyl-l:3-dioxane is a key to the structure of this compound. The spectrum of l:4-dioxane in which the two oxygens are separated by two carbon atoms is entirely different from the other spectra in Table 9. It behaves like a normal ring compound giving a large parent peak and shows little tendency to lose hydrogen. A similar difference is observed between straightchain compounds which include two oxygens separated by two carbons, and the acetals. For example, cellosolve (2-ethoxy-ethanol) of molecular weight 90 gives a parent ion of intensity 0.3% that of the base peak of the spectrum which is much larger than the ion at mass 89. The spectrum shows many of the charac¬ teristics of an alcohol such as a peak at mass (p — 18) and a peak at mass 19 corresponding to HsO+. This compound breaks very readily at the bond /3 to the two oxygens to give a large peak at mass 59. The mass spectra of 6 cyclic mono-ethers have been obtained under conditions of high resolution [190]. Their spectra are somewhat different from those of aliphatic ethers in that the oxygen atom can readily be lost with formation of only one other fragment. For example peaks at (p— CffO)+ are a feature of the spectra. A few cases are known in which stable neutral oxygen-containing fragments can be lost in ethers in which the oxygen is not part of a ring system. For example, di-n-octyl ether gives a peak (p ■— HgO)* [1360] and the peak at ip - CO)+ in the spectrum of diphenyl ether has been discussed at length in Chapter 7. 9.7

CARBOXYLIC ACIDS

A series of ten aliphatic acids were purified by Happ and Stewart and their mass spectra obtained [831]. The presence of oxygen is easy to deduce; the

372

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

molecular weights of a series of carboxylic acids are two greater than the molecular weights of the corresponding hydrocarbons and the parent peaks are generally large enough not to be overlooked, so that for acids with an alkyl group attached to the carboxy group it is obvious that the compound being studied is not a hydrocarbon. Large peaks at masses 31, 45, 59, etc. also indicate the presence of oxygen and the characteristic breakdown pattern and rearrange¬ ment peaks associated with the presence of the carboxylic acid group generally make the identification of acids a relatively straightforward matter. It should be remembered that many of the lower members of the monobasic acids possess pungent smells; it is a simple matter to distinguish, say, butyric and valeric acids from this property alone. The dibasic acids do not show this property but since such acids are generally thermally decomposed if heated to temperatures at which their vapour pressures are sufficient to plot a mass spectrum, such com¬ pounds are usually converted into their methyl or ethyl esters before examination by mass spectrometry or by gas-liquid chromatography [1643]. Many of these esters can also be distinguished by smell alone. The main peaks in the mass spectra which are used for structural diagnosis besides the parent peak are formed by fragmentation of the - (CO)OH group or by re-arrangement processes. Usually the peaks corresponding to loss of -OH (often accompanied by peaks due to loss of H2O and H3O) and -(CO)OH from the parent ion are the most prominent in these regions of the spectra. The peak at mass 45 due to (CO2H)4" is invariably stronger than the peaks at masses 31 and 59 and is sometimes accompanied by peaks at 46 and 44. Prominent peaks occur at mass 17 and at the associated masses 16 and 18. The most characteristic re-arrangement ion occurs at mass 60, the molecular weight of acetic acid and

aiJTYRftMIOE.

n - BUTYRIC

100.

100

90

90

60

80

70

70

60

60

50

50

40

40

30

30

20

20

10

10

ACID

.

Hi 10

20

30

40

50

60

70

80

90

10

20

30

40

50

60

70

60

90

Fig. 148. The mass spectra of n-butyramide and n'butyric acid. In both cases, the base peak is a rearrange¬ ment, in the one spectrum to the parent mass of acetamide and in the other to that of acetic acid.

this may form the base peak of the spectrum as, for example, in the spectrum of n-butyric acid (Fig. 148) and lauric acid (Fig. 108, p. 232). The formula of the mass 60 ion in butyric acid was deduced by Happ and Stewart [831] by comparing its spectrum with that of 13C carboxyl labelled butyric acid. They conclude that the carboxyl carbon is always retained in this re-arrangement ion. This re-

9.7

CARBOXYLIC ACIDS

373

arrangement is another example of the fragmentation at a bond p to a -CO- group with re-arrangement of a single hydrogen, and has also been mentioned in the spectra of ketones and aldehydes. Peaks corresponding to re-arrangement to other molecular acid ions such as 74 and 88 are also common but these peaks are generally smaller than the mass 60 peak. Such ions also occur in the spectra of some esters and the mass spectrum of an acid can best be distinguished from that of an ester by reference to the loss of -OH from the parent ion and by the prominent peaks in the 16, 17, 18 region of the spectrum. In some of the spectra re-arrangement peaks occur at masses 47, 61 etc. These ions have been observed in many other spectra of oxygenated compounds.

Fig.

14Q.

The mass spectra of stearic acid and methyl stearate.

The long-chain fatty acid spectra also show the re-arrangement ion at mass 60. In the spectrum of stearic acid (C17H35COOH, see Fig. 149) given by McLafferty [1361] this peak has a height of almost 80% of the base peak. The loss of -OH and -(CO)OH is much less likely in this long-chain compound and the peaks corresponding to this mode of fragmentation would usually not be observed. The height of the parent peak of the fatty acids increases in size relative to the base peak from valeric to stearic acid, falling again for still longer-chain acids; in the case of stearic acid a parent peak of 6.8% is obtained. This anomalously large parent peak compensates for the absence of {p •— 17) and (p — 45) peaks and enables the presence of two oxygen atoms in the parent compound to be deduced with ease by accurate mass and isotopic abundance measurements. The spectra of some aromatic carboxylic acids have been given by Gohlke and McLafferty [760]. The presence of the aromatic nucleus leads to a much in¬ creased parent peak. This peak, together with those formed from the parent ion by loss of - OH and - (CO)OH are usually the strongest in the spectra. When

374

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

there are hydrogens available on the a-carbon in a position o to the carboxylic acid group, hydrogen-bonding to the hydroxyl group can occur and the stable fragment H2O rather than OH is lost from the parent ion. A re-arrangement peak at a mass (p •— 44) sometimes accompanies that formed by loss of the carboxyl group. This must not be confused with the thermal degradation product of the acid which will also give a peak at this mass number. The peak at (p — 44) is particularly large in the spectra of the isomeric toluic acids. For o-, m- and pisomers it is respectively 8, 17 and 15% of the height of the base peak. Another interesting re-arrangement ion is that due to loss of neutral CO from the parent ion. Although the abundance of the ions so formed is small (AT to 3% of base peak) the peak is another example of fragmentation controlled by the stability of the CO fragment. The peak at mass (p — 1) is always small even when methyl groups are substituted on the ring. This would suggest that the driving force in formation of, say, (p ■— 1) in toluene is due to the stability of the ion formed (C?H8)+ and not primarily to the weakness of the C-H bond ft to the ring. On the other hand fragmentation of a C-C bond ft to the ring (with loss of CH3)

PHTHftLIC MMHIDRIDt

30

Fig.

40 150.

SO

60

70

&0

90

100

110

120

130

MO

150

The mass spectra of maleic anhydride and phthalic anhydride.

leads to the base peak formation in the case of ethyl or isopropyl ring substituted benzoic acid. Generally speaking it is a simple matter to identify an aromatic acid as such and to deduce the lengths (but not the positions on the ring) of any alkyl side-chains. Dibasic aromatic acids are generally thermally too unstable for their spectra to be obtained and must be esterified before examination. Study of

9.8

ESTERS

375

the decarboxylation products can also be used as a means of helping to distin¬ guish the various acids. Gohlke and McLafferty [760] point out that if a dibasic aromatic acid has the two carboxyl groups ortho to one another on the ring, the thermal degradation product will be the anhydride and this can easily be iden¬ tified and used to deduce the structure of the acid. Phthalic acid can be identified in this way. The spectrum of phthalic anhydride is shown in Fig. 150. It is easy to identify since the parent ion loses a mass of 44 and also 72. This pair of peaks, formed by loss of CO2 and C2O3, can only arise from an acid anhydride. It is interesting to compare this spectrum with that of maleic anhydride which is also shown in Fig. 150 and which also loses CO2 to give the base peak of the spectrum. In this case, too, loss of mass 28 (CO) is extremely unlikely. This is surprising when one considers the number of re-arrangement reactions in which neutral CO is eliminated, an indication of the stability of this neutral fragment. However, the predominant loss of CO2 is an example of the way in which fragmentation is concentrated in a small number of processes, and one would not expect to see peaks corresponding to a large number of separate reaction paths even though all would be predicted by empirical rules. The ease of removal of one of the non¬ bonding electrons on an oxygen atom and the high stability of the ion so formed is illustrated in an interesting fashion when one examines the doubly-charged ions in the maleic anhydride spectrum. The peak (p — C02)++ is very small (2%), although the (p — C02)+ ions which contain only one positive charge for each oxygen atom form the base peak of the spectrum. (p — 0)++ has an abundance of 3% and illustrates how a single charge can be accommodated on each re¬ maining oxygen. Once again, however, loss of CO from the doubly-charged parent ion is of negligible probability, even though loss of this fragment would leave two oxygen atoms on which the charge could be accommodated. 9.8

ESTERS

The aliphatic esters form a class of simple, volatile oxygenated compounds about which there has been little published information relating spectra [mo] and structure. The compilation of spectra by the American Petroleum Institute [45] contains, however, a number of spectra of such esters. The parent peaks from these compounds are small and it is perhaps best to begin by studying simple esters of aromatic acids since in this case the probability of fragmentation of one of the hydrocarbon groups in the molecule is much reduced making interpretation easier, and the parent peaks are increased in height which also simplifies the problem of identifying the ester. Esterification of aromatic dibasic acids is usual as a preliminary to their examination by mass spectrometry. It is usual to form either the methyl or the ethyl esters in this work and since these esters behave in a different fashion from those in which the alcohol chain is longer we shall at first confine ourselves to the shorter chains. Information on aromatic esters has been published by Gohlke and McLafferty [76°1- They give data on the methyl esters of eleven acids, viz• benzoic, o-, m-, p-toluic, 0-, m-, pbenzenedicarboxylic, a toluenedicarboxylic, a tricarboxylic, a tetracarboxylic and a naphthoic acid. In all cases, a clear pattern of breakdown is obtained. The parent peak is apparent in all cases. It is of height 80% of the base peak for the naphthoic ester, 50% for the esters of the other monobasic acids, 20% for the dicarboxylic acids, 5% for the tricarboxylic and 2% for the tetracarboxylic. The base peak in all cases corresponds to loss of —OCH3, and the parent ion decreases as the number of ways in which this ion can be formed becomes larger.

376

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

The peak corresponding to fragmentation at the other side of the CO group to give the (p — 59) peak is of about the same intensity as the parent ion except when two ester groups are ortho to one another. This provides a method of distinguishing phthalic acid from iso- and terephthalic acids; its methyl ester gives a peak of only 6% of the base peak at (p — 59) = 135 which is about a quarter of the value for dimethyl isophthalate and dimethyl terephthalate. The spectrum of dimethyl terephthalate is shown in Fig. 151 and also shown here is

100 90

80

DIMETHYL

TEREPHTHALATE

70

SO SO 40 30 30

10

Fig. isi. The mass spectrum of dimethyl terephthalate and also of an ester mixture.

the spectrum of an ester mixture. It can be seen that this mixture contains one of the isomeric dimethyl phthalates. The extra peaks at masses 150 and 136 would not be expected to lead to a fragment at mass 135 and are in fact due to impurities. The height of the peak at mass 135 relative to that at mass 194 suggests that the ester of molecular weight 194 which is present is not dimethyl phthalate (it is in fact dimethyl isophthalate). Detailed examination of the spectrum of dimethyl terephthalate shows several other less probable modes of breakdown. Thus, loss of CH3 leads to a peak of 3.5%, loss of C2H5O (i.e. OCH3 + CH3 — H) gives a re-arrangement peak of abundance 0.35% at mass 149, loss of both OCH3 groups gives a peak at mass 132 of abundance 0.55%. Other peaks formed by

9.8

ESTERS

377

multiple-bond cleavage are at masses 120 (5.85%) and 104 (6.63%). These correspond to loss of (COOCH3 T~ CH3) and (COOCH3 T OCH3) respectively from the parent ion. Of comparable probability, however, is the simultaneous loss of a hydrogen atom from the ring to give the ions of mass 119 (4.12%) and 103 (11.98%). It appears that such ions, corresponding to the fragmentation of 3 bonds, only occur when the whole side-chain is lost in one of the fragmenta¬ tions, and suggests that the hydrogen may separate as part of the same neutral fragment. The driving force may be the fact that loss of the extra hydrogen leads to an odd mass ion, i.e. one with no unpaired electrons. For this reason the hydrogen is not lost when only one bond is broken to give mass 135. It is also interesting that no doubly-charged ions are seen in this spectrum. The peaks at masses 15 and 31 corresponding to (CTh)"1" and (OCH3)+ are respectively of height only 4.23% and 0.74%. New fragmentation processes become apparent for aromatic esters in which the methyl groups are replaced by ethyl or heavier aliphatic alkyl side-chains. These processes all result in the formation of re-arrangement peaks. For ethyl esters, loss of C2H5 remains unlikely, but loss of C2H4 is sometimes probable giving a peak of 33% in ethyl benzoate. For still heavier groups the pattern of fragmentation changes again. Let us denote the alkyl group by R. Then if R is heavier than C2H5, fragmentation of the O-R bond can be accompanied by the transfer of two hydrogen atoms to the oxygenated fragment, the charge remaining on this fragment. In all cases in which the carboxyl groups are directly attached to the arotnatic nucleus this mode of fragmentation is more likely than that involving transfer of only a single hydrogen, though this, too, can be quite probable. In the case of n-butyl benzoate the peak at mass 123 formed in this way is the second largest in the spectrum, being 70% of the height of the base peak. This re-arrangement also occurs for the esters of poly-carboxylic acids. The peak corresponding to the ion R+ tends to increase as R becomes larger. Esters of dicarboxylic acids all show a re-arrangement peak at mass 149. This has been mentioned in the case of dimethyl terephthalate and is due to loss of OR together with R, a single hydrogen returning to the charged fragment. Though this peak was very weak for dimethyl terephthalate it is likely to be the base peak of the spectrum for heavy esters (ethyl and upwards) of phthalic acid. The wide variety of phthalates which give this peak suggest that the ion responsible is of special stability. One path along which this ion can be formed is made clear by study of meta-stable peaks. For example, in the mass spectrum of di-n-butyl phthalate, a meta-stable peak occurs at mass 108. This arises from the transition 205+ -» 149+ + 56

or (p — OC4Hg)+ -*■ (p — OC4H9 — C4H8L + C4H8

As well as showing that the ion can be formed in a two-step process, the tran¬ sition shows that the re-arrangement takes place in the second step when C4H8 is lost, and suggests that the hydrogen atom transferred is located on the terminal oxygen atom to form a carboxylic acid group. Now 0+ is trivalent, since removal of one of the lone pair electrons of an oxygen atom to form the ion enables hybridization of the remaining orbitals to occur. Thus, if the charge in the ion is located on the hydroxyl oxygen, a ring can be formed to the carbonyl group in the ortho position, which also contains an unpaired electron. Thus, presumably the ion of mass 149 has a similar structure to phthalic anhydride, the positive charge being localized on one or othet of the oxygen atoms, thereby enabling the charged atom to accommodate a hydrogen atom to complete its electron pairing.

378

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

Interesting rearrangement ions are also observed in the spectra of the esters of co-phenyl fatty acids. The expected pattern of fragmentation is generally observed, but superimposed on this pattern are re-arrangement ions in those esters in which ring formation is sterically possible. For example in the spectrum

of the ester

of molecular weight 178, as well

as the expected peak at mass 133 due to loss of O • C2H5 there is another

peak at mass 131. It is suggested that this is the ion

analogous

to the 149 peak in the spectrum of esters of phthalic acid and that the ion is formed because of its high resonance stability and the fact that it contains no unpaired electrons. In other cases, however, ions of even mass number are form¬ ed. When this occurs it is assumed that a molecular neutral fragment is broken off and it is the stability of this rather than of the fragment ion which controls the process. Consider, for example the ester

of molecular weight 292. The parent ion loses OC2H5 to give an ion of mass 247 but not OCH3 or OC3H7, which if this were an unknown compound would suggest that we were dealing with an ethyl ester. It also loses (C2H5O + H) to form an ion of mass 246 and (COOC2H5 4~ H) to form an ion of mass 218. It is suggested that in each case the extra hydrogen comes from the aromatic ring and there is formation of a second ring in the fragment ion. In the first case, C2H5O would be lost from the short chain (similar behaviour to the example above) but C2H5 • O • CO would be lost from the longer chain to give an ion

Although we do not know enough about the factors influencing the formation of these re-arrangement ions to predict exactly the masses of the fragments which will be formed in any particular instance, such re-arrangements as do occur give very useful clues as to the lengths of the carbon chains connecting the aromatic nucleus to the COOR portion of the molecule. Esters of this kind in which the carbon chains are very short e.g. C2H5 • O • CO • C6H4 • CH2 • CO • O • C2H5 break in a very similar fashion to the terephthalic acid esters. For example, this ester gives ions at masses (p — 28) (together with a meta-stable peak at mass 183 corresponding to this transition) (p — OC2H5), which probably mainly comes from the chain without CH2, {p — CO • O • C2H5) (giving a meta-stable peak at

9.8

ESTERS

379

112.5), which probably comes mainly from the longer chain (since the bond broken is /? to the ring as well as a to the CO group) and peaks at 134 and 135 which are [p — (CO • O ■ C2H5 + C2H5)]+ and [p — (CO • O • C2H5 + C2H4)]+ respectively. Other esters, the spectra of which are comparatively easy to interpret, are the symmetrical esters formed from the dibasic aliphatic acids.

Fig.

.

152

The mass spectrum of diethyl succinate.

These acids must be esterified before they can be examined in the mass spectrom¬ eter, so these esters form an important group. The mass spectrum of diethyl succinate is shown in Fig. 152. Let us examine this spectrum and see what can be deduced from it. The parent peak is at mass 174 and is of height 4% that of the base peak which is at mass 101 and is formed from the parent ion by loss of CO • O • C2H5. Almost as strong is the peak at mass 129 due to the ion (p — OC2Hs)+. The small peak at mass 146 of 1% due to ions of formula (p — C2H4)+ would be expected from such a compound, but we should not expect a peak at mass 147 where a 3.6% peak appears nor at 160 (p — 14)+ where in fact a peak of height 0.4% appears. The peak at 147 is a re-arrangement, but from the peak at

380

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

(p — 14)+ one can immediately surmise that the ester is impure and contains either a lower homologue (diethyl malonate) or some methyl ethyl succinate which would be present if the ester had been made from succinic acid using ethanol denatured with methanol. Such a compound would break to give a peak at mass 129 of identical composition to that from diethyl succinate by loss of OCH3, and also a peak at mass 115 by loss of OC2H5. The malonate would also give a peak at this mass number. Without a reference spectrum of diethyl malonate, the decision as to which ester is present could be decided from the peak at mass 59 (CH3 • O • C0)+, the presence of a peak here indicating the methyl ethyl ester to be responsible for the impurity parent ion at mass 160. The peak at mass 31 (CH3 • 0)+ would not be used to distinguish the presence of a methyl ester since this peak is so readily formed by re-arrangement processes. The spectrum emphasizes the extra difficulties in interpretation which can arise due to careless use of denatured alcohol. In general it is always best to esterify to the methyl ester when attempting to identify a poly-basic acid. Methyl esters break with the minimum of re-arrangements; ethyl esters on the other hand can some¬ times lead to difficulties due to their loss of C2H4 which can cause confusion with loss of CO. It should be noted that diethyl succinate gives only a very small peak at mass 87 corresponding to the breaking of the parent ion exactly in half and a peak at this mass provides good evidence of the presence of homologues

Z rO X

Fig. 153. Illustration of the resolution attainable on the M.S.8 mass spectrometer currently in use in these laboratories. M/AM for the doublet (CsHrO-^CCzHeO) is 13,500.

in a sample of diethyl succinate. An elegant method of detecting homologues in dibasic acids depends on the use of high resolution. Suppose, for example, that one wished to analyse for a small amount of succinic in adipic acid. If the di¬ methyl ester was prepared, dimethyl adipate would give a large peak at mass 115 due to the ion [CH3 • O • CO • (CH2)4]+ formed by loss of CO • O • CH3

9.8

ESTERS

381

from the parent ion which would interfere with the large succinic acid peak at this mass number due to [CH3 • O ■ CO • (CH2)2CO]+ formed, by loss of OCH3 from its parent ion. The other succinate peaks, with the exception of the small parent peak would all show gross interference from adipate ions. The two peaks at mass 115 differ in composition, however, by C2H4 minus CO and thus can be separated under high resolution. The mass difference is 36.4 mMU so that M/AM is less than 3200 which means that a complete separation can be made on a double-focussing mass spectrometer of modest performance. An example of the separation of C2H4 and CO at mass 28 which can be achieved with a double-focussing instrument with a 6 inch radius magnetic sector is shown in Fig. 153. Esters of the long-chain fatty acids give mass spectra very re¬ miniscent of the spectra of the acids themselves. The spectra of stearic acid and methyl stearate are shown in Fig. 149. The spectra have been plotted in such a way that the respective mass scales have been displaced by 14 mass units so that the positions of the parent peaks coincide. It can be seen that the prominent parent peak associated with the fatty acids also occurs in the ester spectrum and that the majority of the large peaks in the ester spectrum have the formula [(CH2)» • CO • O ■ CH3]+ and hence would be difficult to distinguish from the isobaric series of peaks of composition [(CTkV+i • CO • OH]+ from the monobasic acid. Probably the best means of distinguishing the straight-chain fatty acid from the ester mass spectrometrically lies in the fact that the acid gives a prominent re-arrangement peak at mass 60, the ester a corresponding peak at mass 74 and the presence of a peak (though not necessarily very large) at (p — OR)+ in the ester spectrum. In the example given this peak is at mass (p — 31). However, if the mass spectrum can restrict the possibilities to either an acid or an ester, chemical methods are easily employed to determine the nature of the unknown substance. A series of spectra of the methyl esters of the long-chain fatty acids up to methyl n-hexacosanoate have been given by Asselineau, Ryhage and Stenhagen [70]. The spectra again show the prominent peaks corresponding to fragments con¬ taining an intact methoxycarbonyl group, both the series at masses 73, 87, 101, 115, etc. corresponding to a single C-C bond rupture in the parent ion and also the series at masses 74, 88, 102, 116, etc. corresponding to fragmentation of the same C-C bonds but with hydrogen re-arrangement. The positions and nature of side-chains on the long-chain can often be determined from study of these peaks [70]. The largest of the re-arrangement peaks (which is often the base peak of the spectrum) always corresponds to fragmentation at the C-C bond /3 to the car¬ bonyl group. Thus, if the carbon atom next to the carbonyl is not substituted, the largest re-arrangement peak will be at mass 74, corresponding to the fragment (CH3 • O • CO • CH2 + H)+, whereas if this carbon atom has substituents such as CH3, C2H5 (or (CHs^), etc. attached to it, the major re-arrangement peak will be at masses 88, 102, etc. For example, the mass spectrum of methyl 2-methyl2-ethyleicosonate, CH3 ' O ■ CO • (CH3)C(C2H5) ' (CH2)i7CH3, has its base peak at mass 116. The spectra all contain a series of these re-arrangement peaks, at masses smaller and larger than the peak discussed above. These other re¬ arrangement peaks are, however, always smaller than the corresponding peak formed without hydrogen re-arrangement. The positions of other substituents on the long-chain can often be detected by study of the relative intensities of the peaks [CH3 • O • CO ■ (CH2)W]+. For example, the ester which gives a peak at mass 115 (0.3%) much smaller than that at mass 101 (50%) or at mass 129 (6%) probably contains the grouping [CH3 ‘ O • CO • CFKCH3) • CH2 ' CH(CH3)], since such a compound can only give mass 115 by re-arrangement.

382

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

The relatively high volatility of esters enables their mass spectra to be obtained even in the molecular weight range above 300. An example of the use of these spectra to obtain evidence as to the structure of organic compounds is afforded by the comparison of the spectra of the methyl esters of the isomeric dextropimarie, isodextropimaric and cryptopimaric acids (C19H29COOH) [288]. Such a method of detecting substituents has also been mentioned with regard to hydro¬ carbons, but is more difficult to apply in the case of esters since the size of the homologous series of peaks 73, 87, 101, 115, etc., does not decrease smoothly even for straight-chain esters. For the ester CH3 - O • CO(CH2)« ‘ CH3 where n > 6 the heights of the homologous series decrease to mass 115, but the peak at mass 129 > 115 and that at mass 143 larger still. At higher masses the inten¬ sity falls again. The reason for the large mass 129 and 143 peaks is not clear. When one considers some of the esters with substituted side chains, e.g. methyl 4:5-dimethyltricosanoate and methyl 2:4-dimethylheneisocanoate, it appears that ring formation leading to increased ion stability could be an important factor in leading to large peaks. The first of these esters breaks preferentially to give the ion [CH3 • O ■ CO • CH2 • CH(CH3) ■ CH2 • CH(CH3)]+ and the second to give [CH3 ■ O • CO • CH(CH3) ■ CH2 • CH(CH3)]+. Both these ions correspond to fragmentation at a branched carbon, where the bond strength will be expected to be weak, and the ions are of sterically favourable structure to assume the forms

,CH,

ch^0ANch-

\CH-CHS/ 3

CH

respectively. Such a mechanism does not seem to operate in the case of the mass 129 and 143 peaks of the straight-chain esters, but it is interesting that these ions contain the necessary number of carbon, hydrogen and oxygen atoms to form two fused rings of possible formulae

v^n*.

•CH,

u

CH, /

CHt

\

CH 'OL

and

.CH

ZHt 'O

CH* \

XH,

XH, / CH,

respectively. Considerable re-arrangement would have to occur to form such ions, but the manner in which, for example, dimethyl acetal can be shown to re-arrange during fragmentation (see above) indicates that such a final form may not be impossible. When still longer-chain methyl esters are considered, it is found [1754] that there appears to be a periodic variation in the intensity of the peaks due to the methoxycarbonyl fragments [CH30(C0)(CH2)«]+. It has already been explained that the peak at mass 143 for which n = 6 is outstanding; other outstanding peaks in the spectrum of methyl n-hexacosanoate occur at

9.8

ESTERS

383

masses 199, 255. 311 and 367 corresponding to n — 10, 14, 18, 22. The reason for this periodicity is not known but it seems to suggest a special stability for each of these ion species over its neighbours. A good illustration of the way in which mass spectra can be used to give structural information is given by the deduction of the structure of the methyl ester of mycocerosic acid [70] (of molecular weight 494) as CH3 • O • CO ■ CH (CH3) • CH2 • CH(CH3) ' CH2 ' CH(CH3) ' (042)22 ' CH3. The strong peak at mass 88 in the spectrum showed the presence of a methyl group in the 2-position, the weak peaks at masses 115 and 157 relative to the peaks at masses 101, 129, 143 and 171 fixed the positions of the two other methyl groups in the 4 and 6 positions respectively. The molecular weight was readily obtained from the large parent peak. As in the case of the mass spectra of the fatty acids themselves, the spectra of their methyl esters show a minimum parent ion relative abundance correspond¬ ing to the ester of the C5 acid (methyl valerate), and the abundance of the parent ion increases for heavier methyl esters of the normal chain carboxylic acids. If the abundance of the parent ion is plotted in divisions per unit pressure there appears to be a maximum corresponding to the ester of the C17 acid [1754]. The esters of the short-chain fatty acids of formula R • CO • O • R' are characterized by relatively small parent peaks as the length of the alkyl radical R', exceeds 3 or 4 carbon atoms. Increasing the chain-length of the alcohol end of the molecule does not have the effect of increasing the parent peak intensity as does lengthening of the acid chain. The difficulty of determining the molecular weight when the parent peak is very small can sometimes be overcome in a similar fashion to that already mentioned in connection with alcohol spectra. As the pressure within the ionization chamber is increased a peak becomes apparent at a mass of (p + 1). This peak can be shown to be formed in an intermolecular reaction since its intensity changes with the pressure relative to other peaks in the mass spectrum and also with ion repeller voltage, the peak being larger the longer the time the ions spend within the ion chamber itself (i.e. the peak decreases as the repeller voltage is made several volts positive with respect to the walls of the ion chamber). A similar type of peak is sometimes formed at still higher mass, and this is useful in giving structural information. Esters R • CO • O • R' tend to give a peak corresponding to (p + RCO)+ formed by reaction of an (RCO)+ ion with a neutral molecule. For example, in a typical mass spectrum of n-propyl acetate (M.W. = 102) peaks at masses 102, 103 and 145 were of height 0.12%, 0.21% and 0.04% the base-peak intensity, the values for the mass 103 and 145 peaks varying with sample pressure. Detection of two peaks of this kind separated by mass 28, 42, 56 etc. in the spectrum of an un¬ known is strong evidence that the compound being examined is an ester. Further, the actual mass difference between the two peaks identifies the mass of the acid end of the molecule. In the example given above, RCO = 43 since the mass difference = 42, so that the alkyl group R has a mass of 15 and we are therefore dealing with an acetate. Again since the parent mass of the ester is established, the mass of the alcohol end of the ester can also be deduced as 43 or C3H?. The importance of the peaks in structural diagnosis is obvious; if the presence of one such peak in a spectrum is suspected it is usually worthwhile to increase the pressure and confirm this fact and also to make a special search for any similar peaks. However, even if such peaks are not evident, structural information is abundantly available in the remainder of the mass spectrum. The mass spectra of 31 esters have been given by Sharkey, Shultz and Friedel [1823] and correlations

384

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

TABLE 10 THE MASS SPECTRA OF n-AMYL ACETATE AND ISOPROPYL PROPANOATE Mass

14 15 25 26 27 28} 29} 30 31 32 39 40 41 42} 43} 44} 45} “} 56} 57} 58 59 60 61 68 69 70 71 72 73} 74 75 76 85 86 87 88 97 98 99 100 101 102 103 114 115 116 129

Empirical formula

CH2 ch3 c2h C2H2 c2h3 CO C2H4 COH c2h5 c2h6 CH3O

o2

C3H3 C3H4 c3h5 C2H20 c3h6 C2H30 C3H7 C2H40 CgH8 cho2 C2H5O C3H3O c4h7 C3H40 C4H8 C3H5O C4H9 c3h6o C3H70 CgHsO C2H502 C5H8 C5H9 C5H10 C5H11 C5H12 C3H502 C4H90 C3Hg02 C3H702 C3H802 C5H90 C5H10O c4h7o2 C4H802 C5H502 CsH602 C5H7O2 CsH802 C5H902 Cf,Hio02 CeHn02 C6H12O2 C7H13O2

n-Amyl acetate

Isopropyl propanoate

%

%

2.8

6.5

1.1 0.54 0.54

0.26 0.90 0.39 4.5 1.1 12.9 2.2 19.4 92.2 7.8 2.4 0.75 18.3 1.4 1.6 3.7 1.0 0.13 23.4 1.7 6.5 46.3 4.0 0.90 11.9 0.52 0.26 0.54 0.52 1.4 0.26 0.34 0.31 0.13 0.13 0.64 0.13 0.62 0.26

*

0.19 1.7 0.06 0.95 10.1 0.54 2.8 0.68 28.1 0.54 1.5 0.27 2.4 0.54 10.0 0.68 4.3 20.7 41.6 0.54 1.1 0.27 3.1 0.8 0.41 1.1 3.8 100 3.2 10.0 0.40 0.95

0.95 0.95 5.1 33.5 1.2

3.5 0.18

11.9 0.62 0.10 0.28 0.67 0.53 0.10

9

9.8

ESTERS

385

of spectra and molecular structure are clearly evident. The major peaks are usually formed by fragmentation at bonds adjacent to the carbonyl group to give, for the ester R ■ CO • O ■ R' the unusual variety of peaks R+, (RCO)+ (COOR')+ or (OR )+. The peaks R+ and (RCO)+ will give peaks at the same mass numbers as the corresponding hydrocarbons, and it is necessary to be able to distinguish the presence of the keto group. By far the most satisfying way of doing this is with a high resolution spectrometer and examples of high resolution spectra of esters are given in Table 10. However, it is possible to deduce the presence of the keto part of the molecule in other ways. The fact that oxygen is present in the molecule when an unknown compound which is actually an ester is being examined, will be patently obvious. Peaks at masses such as 31, 45, 59, etc., 46, 60, 74, etc., 47, 61, 75, etc. will appear in all cases. The peak (RCO)+ will be at least of height 75% of the base peak (it will often be the base peak) for acetates, propanoates and butanoates and will be a major peak for the esters of higher acids. However, the peaks at masses 43 and 57 in acetates and propanoates are not accompanied by large peaks two mass numbers lower as in the case of hydrocarbons. The ratios of 43 : 41 and 57 : 55 are at least 4 : 1 and often much larger. Very large peaks at masses 71, 85, 99, etc. are not usual in hydrocarbons and again suggest the (RCO)+ formula. A corresponding peak at mass 29 is not characteristic of formates which tend, instead to give the re-arrangement ion at mass 31. (Methyl formate can give an ion of this mass without re-arrangement.) The ion R+ is often difficult to distinguish but an olefinic re-arrangement ion (R' — 1)+ is’often formed when fragmentation of this end of the molecule occurs and this can often be detected. It is particularly in evidence when R' is heavier than propyl. For example, there is an outstanding peak (17% of base peakj at mass 98 in the spectrum of heptyl propanoate and at mass 56 in the spectrum of butyl acetate. This is a further example of a re-arrangement ion being of more use in an identification than a more common species formed by simple bond cleavage. Another useful peak formed by re-arrangement corresponds to the remaining part of the molecule when the above olefinic fragment is removed or to a mass one higher than this. For example, the spectrum of heptyl propanoate (M.W. = 172) referred to above gives an obvious oxygen-containing re-arrange¬ ment ion at mass 75 (mass 75 + mass 98 = 173). Similarly, butyl acetate gives a peak at mass 61. These unusual ions which correspond to (R • CO • O + 2H)+ can be used to confirm the molecular weight in conjunction with the (R' minus H)+ ion. Oxygenated fragment ions of formula (O • R')+ are not prominent in heavy esters and are usually only strong in small methyl, ethyl and propyl esters. An even mass number oxygen-containing re-arrangement ion is sometimes ob¬ served and its strength increases with the size of the radical R. It consists of the ion(CH3 + CO • OR')+ and is particularly prominent at mass 74 in the spectra of higher methyl esters. Corresponding ions at 88 and 102 often give outstanding peaks in the spectra of ethyl and propyl esters too, especially when R becomes large. Esters are isomeric with the carboxylic acids and isobaric with ethers and alcohols. It will be instructive to consider the spectrum of an unknown (actually an ester) to see whether its chemical class and structure can be deduced. Such a spectrum is shown plotted under low resolution conditions in Fig. 154. The largest peaks in the spectrum all occur at odd mass numbers and this, together with the absence of a number of half-masses at low mass numbers suggests that the compound being studied contains no nitrogen. Prominent ions at the higher masses occur at masses 101 and 115. These cannot be hydrocarbon ions; alkyl radicals in this region of the spectrum (corresponding to (CnH2»+i)+ for which

386

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

n = 7 and

8 respectively) would be of masses 99 and 113. Thus the compound would appear to contain oxygen and to be an alcohol, ether or ester and this is confirmed by the presence of peaks at masses 45 and 73, 74, 75. The molecular weight would appear to be 130 and the peaks at 115 and 101 to be due to loss of CH3 and C2H5 respectively from the parent ion. The smallness of the parent peak (coupled with the fact that if the amount of sample were to be greatly increased a

SEC

BUTYL

PROPANOATE

Fig. 154. The mass spectrum of sec-butyl propanoate.

pressure dependent peak would appear at mass 131) suggests an alcohol or ester. There are some features such as the peak at mass 75 and the absence of (p — 18) and of a strong 31, 45, 59, etc. which suggest that the compound is not an alcohol, so one would attempt to explain the spectrum as that of an ester. Assuming the formula to be RiCO • O • R2, (Ri + R2) = C7H16. One would expect prominent peaks corresponding to Ri+, (RiCO)+, and the peaks at masses 29 and 57 (and the absence of strong peaks at 15 and 43 or 43 and 71) suggest that Ri = C2H5. Thus the ester is C2H5 • CO • O • C4H9. Now one would not expect ready loss of CH3 and C2H5 but not C3H7 unless the butyl group contained a methyl and an ethyl branch. The peak at mass 86 (p— C3H8)"1" also suggests that two radicals have been lost to give this even-mass peak. Thus one would postulate a sec-butyl configuration. The peaks at masses 74 and 75 are seen to be re-arrangement peaks and would be expected to occur when the radical R2 is lost. The gap from the 75 region to the 57 region confirms that an oxygen atom is lost in this stage of fragmentation. Thus the entire spectrum supports the supposition that the compound is sec-butyl propanoate. The identification is rendered much easier if the spectrum is plotted under high resolution conditions and the composition of each peak determined from its accurate mass. The presence of two oxygen atoms is then immediately obvious and the number of oxygen atoms in the various fragments makes the position of the oxygen atoms within the molecule much easier to establish. An example of the identification of co-butenyl benzoate from its mass spectrum is given in the literature [186]. The position of the double-bond in the side-chain is apparent from the fact that the parent ion does not lose one or two carbon atoms to give fragment ions but loses (C3Hs)+. This can be recognized as an example of frag-

9.9

AMINES AND OTHER SATURATED

NITROGEN-CONTAINING COMPOUNDS

387

mentation /> to. a double bond and shows that the double bond is located in the terminal position of the chain. When an oxygen atom in an ester is replaced by a sulphur, corresponding bonds are broken to give the mass spectrum. The parent ion intensity is usually reduced by the presence of sulphur and frag¬ mentation at ^>C=0 bonds, or bonds between ^>C = S and — S— groups is more likely. These features are illustrated in Fig. 155. 100 90 m-ETHYL

80

THIOCARBOXY

ETHYL

C0-0CaHs

{ft

BENZOATE

^CSOCaHs

70 60 50 40 30

ZO

10 100 90

2

CO O C Hs DIETHYL

80 '

ISOPHTHALATE

ft

^co-o-c2hs

70 60 50 40 30 ZO

10

100 90

COS-CjHs DIETHYLDITHIOL

80

ISOPHTHALATE

ft

^C0SC2H5

70 60

50 40 30

20 10

70

80

90

100

110

IZO

130

140

150

160

170

180

190

200

210

220

230 240 250 260

Fig. 155. Illustration of the effect on the mass spectrum of replacement of oxygen by sulphur in esters.

9.9

AMINES AND OTHER SATURATED NITROGEN-CONTAINING COMPOUNDS

The similar fragmentation patterns produced by the presence of oxygen and nitrogen atoms in organic compounds is well illustrated by the similarities in the

388

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

spectra of amines and of ethers and alcohols, though the amine spectra tend to be the more complex because of the fact that three side-chains are possible. Just as in the case of the aliphatic ethers, one can put forward the rule that a very prob¬ able mode of fragmentation is by rupture of a C-C bond /? to the nitrogen atom. The rule holds for all primary amines whose spectra have been plotted. All such amines give a base peak at mass 30 provided that the carbon atom next to the nitrogen is not substituted. This peak can therefore be used to suggest that an unknown compound is an amine. It is often found, however, from other than primary amines unbranched on the a-carbon and the presence of a large mass 30 peak cannot, therefore, be taken as conclusive evidence for any particular type of amine. A compilation of amine spectra by Collin [370] showed that for the 13 amines which he examined in only one case (dimethylamine) was the mass 30 peak not the largest in this region of the spectrum. Mass 30 is also smaller than 28 and 27 in the spectrum of isopropylamine. The mechanism by which this re-arrangement ion at mass 30 is formed can sometimes be deduced from a study of meta-stable transitions. For example, in the mass spectrum of diethylamine, meta-stable peaks at masses 46.1 and 15.5 arise respectively from the transitions: 73+ -> 58+ + 15;

[(CHs • CH2)2 • NH]+ -* [C3H8N]+ + CH3

58+ -> 30+ + 28;

[C3H8N]+ -> [CH4N]+ + C2H4

The first of these transitions is an example of fragmentation by /1-bond cleavage, the second shows how the fragment ion formed can give rise to the peak at mass 30. In secondary and tertiary amines, fragmentation at /1-bonds is again a prob¬ able process.' Generally, cleavage of the longer chain is preferred. Such amines are likely to give base peaks at masses greater than 30. For example, di-n-amylamine gives its base peak at mass 100 due to /1-bond cleavage, and triethylamine at mass 86 from a similar kind of bond cleavage. This type of fragmentation does not always lead to base-peak formation although the ions formed in this way are always prominent. For example in di-isoamylamine the peak at mass 100 is only 40% of the base-peak intensity. A base peak at mass 44 or 58 usually arises from primary amines in which the a-carbon is branched with a methyl or with an ethyl or dimethyl group. In such cases fragmentation of the /1-bond is assisted by the fact that this bond is connected in the carbon chain at a highly-branched position. For secondary and tertiary amines, branched on the a-carbon atom, however, the above mode of fragmentation becomes less likely as the lengths of the side-chains increase, and prominent re-arrangement ions are often formed by double fragmentation at both an a- and a /1-bond on either side of the functional group with re-arrangement of a single hydrogen atom. For example, di-isopropyl¬ amine and di-sec-butyl, amyl and hexylamines all give a base peak at mass 44 by this process. This process can also lead to peaks at masses such as 58 and 72 when the substituent on the a-carbon atoms is heavier than methyl. This mode of fragmentation can also occur when the a-carbon is unbranched and in such cases produces a peak at mass 30. This has been illustrated above in the case of diethylamine. Similar meta-stable transitions occur in the spectra of many other amines corresponding to a-bond cleavage with transfer of a hydrogen atom in a fragment ion formed from the parent ion by /1-bond cleavage. Thus, in di-npropylamine 72+ (P — C2H5) -»• 30+ 4- CsHe (m.s. at 12.5) and in di-n-butylamine 86+ (p — C3H7) -> 30+ -|- C4H8 (m.s. at 10.4)

9.9

AMINES AND OTHER SATURATED NITROGEN-CONTAINING COMPOUNDS

389

Fragmentation at the bond a to the functional group only can occur to give a hydrocarbon ion. This is very similar behaviour to the ethers and can also be compared with loss of-OH in alcohols. In some cases, confusion as to the atomic constitution of the peaks can arise. For example di-isopropylamine gives a hydrocarbon peak at mass 43 corresponding to a-cleavage and a nitrogen-con¬ taining re-arrangement peak at mass 44 from «-/3-cleavage. Only by accurate mass measurement is it possible to obtain the composition of the various ions.

Fig. 156. The mass spectrum of hexamethylenediamine.

An interesting method of obtaining some idea of the molecular weight of an amine makes use of the observation by Collin [370] that in primary amines, the abundance of the peak at mass 28 relative to the base peak at mass 30 decreases steadily as the molecular weight increases, being of abundance 84% in methylamine, 14% in n-propylamine and 6% is isoamylamine. Very characteristic of the presence of nitrogen is the appearance of a peak in the mass spectrum at mass 18. This is the ion (NHi)+ and it appears in almost all amine spectra. It is analogous to the (H30)+ ion which occurs in many alcohol spectra, but the appearance of higher homologues of this ion is much less likely in nitrogen-containing than in oxygen-containing compounds. The reasons for the appearance of these ions can often be seen to be similar in amines and alcohols. For example, it was postulated that the reason for the high intensity of the (H30)+ ion in isopropyl alcohol was that a highly stable neutral fragment (allyl) could be formed at the same time. The (NH4)+ ion is particularly prominent in the spectrum of isopropylamine where this same neutral fragment is presumably

390

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

formed. The (NH4)"1" peak in isopropylamine is the second largest peak in the spectrum, being almost 20% of the intensity of the base peak at mass 44. The only other ion species likely to give rise to a peak at mass 18 is (H20)+ which may arise due to the presence of water in the sample being examined. The mass difference NH4-H2O is so large that such a doublet can be partially resolved in most analytical instruments; it is usually sufficient, however, to measure the ratio of the heights of the peaks at masses 18 and 17 in order to be certain that the peak is not due to accidental contamination of the sample with water. The ratio is generally much greater for amines than for water. The formation of (NH4)"1" is not peculiar to primary amines and is extremely common in secondary and tertiary amine spectra, occurring for example in the spectra of di- and also tri-methyl and ethylamines and in cyclohexylamine. It does not occur in methylamine. A large variety of other re-arrangement ions occur in the spectra of amines. Sometimes the re-arrangement is to a lower molecular weight amine and peaks at masses 31, 45, 59 etc. can arise in this way. These peaks are usually weak, however, are generally accompanied by larger peaks at masses 30, 44, 58 and are distinguished in these ways from the similar series of peaks which can be given by oxygenated compounds. Not many diamine and triamine spectra have been plotted in these laboratories. Such compounds are difficult to identify because of the very low intensity of their parent ions. The spectrum of hexamethylenediamine is shown plotted in Fig. 156. In cases in which the parent ion is obviously weak, it is usual to intro¬ duce as much sample as possible into the mass spectrometer so as to increase the intensity of the spectrum. A danger in such cases is that inter-molecular reactions can occur, and, in fact in the case of hexamethylenediamine, a peak occurs at mass 117 i.e. (p + 1). Though this peak is so small that its accurate mass and intensity variation with sample pressure have not, so far, been measured it is thought that it may correspond to attachment of a proton to a hexamethylenediamine molecule. It can be shown that it does not correspond to the presence of co-aminohexanol as an impurity since the parent ion intensity in this compound is also extremely weak. In the case of amines containing a ring, such as cyclohexylamine, the parent ion intensity is much greater and this peak is unlikely to be overlooked. The presence of the nitrogen can readily be detected by mass and abundance meas¬ urements. Such amines are isomeric with the cyclic imines which also give large parent peaks, and the possibility of this type of structure must not be overlooked when an identification is being undertaken. The mass spectra of hexamethyleneimine, cyclohexylamine and N-methylcyclopentylamine are shown in Fig. 157. The three spectra show very great differences. That of methylcyclopentylamine is characterized by a very large peak at mass 30, corresponding to fragmentation at the cyclopentane ring. All other peaks in the spectrum are small and the presence of the ring would be suspected only by the fact that the parent ion is slightly stronger than would be expected for a straight-chain, and that the molecular formula indicates a single ring or double bond. The spectrum of cyclohexylamine is strikingly different. In this case, fragmentation at the ring is unlikely and suggests that the reason why the corresponding fragment in methylcyclopentyl¬ amine is large is that the ion formed in the latter case is the very stable ion of mass 30. When fragmentation of a single bond cannot produce the mass 30 ion, the mass spectrum is not dominated by a single fragmentation mode. The strength of the parent ion is increased, relative to the other peaks even though the chain-

y.V

AMINES AND OTHER SATURATED NITROGEN-CONTAINING COMPOUNDS

391

length may be greater. For example, in the spectrum of N-n-butylcyclopentylamine (see Fig. 158) the presence of a ring is clearly evident from the strength of the parent ion, the presence of the nitrogen atom is clear from the re-arrangement peak at mass 30, and the most likely mode of fragmentation is, as expected, at the

Fig. 157. The mass spectra of cyclohexylamine, methylcyclopentylamine and hexamethyleneimine.

bond /? to the nitrogen atom with loss of C3H7. In order to lose other than mass 1 or 15 from the parent ion, at least two bonds must be broken in the case of N-methylcyclopentylamine. It is interesting that the most abundant ion formed is at mass 56, which can be shown by accurate mass measurement to be of formula C3H6N, and to be formed by loss of a C3H7 fragment from the parent ion. The parent ion is a little more prominent than for methylcyclopentylamine, and

392

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

the characteristic mass 30 amine peak is formed quite strongly by re-arrangement. The spectrum of hexamethyleneimine reflects much more strongly the ringstructure of this compound, and the parent peak is very much more intense. The considerable number of large peaks of comparable strength also suggests strongly

Fig. 158. The mass spectrum of N-n-butylcyclopentylamine.

the presence of a saturated rather than a resonance-stabilized ring. N-alkyl substituted hexamethyleneimines also show interesting spectra. The spectrum of N-ethylhexamethyleneimine is shown in Fig. 159. This is characterized by a prominent parent peak and the most likely fragmentation is by breaking the bond /? to the nitrogen atom, though some a-bond cleavage also occurs. The large parent ion and /9-bond fragmentation are reminiscent of the behaviour of alkylbenzenes and provide an exception to the rule that saturated rings break at a bond adjacent to the ring. The overriding tendency is to break /? to the nitrogen atom. A similar mode of fragmentation can be seen in the spectrum of N-coaminobutylhexamethyleneimine (Fig. 159). This behaviour provides a means of determining in the case of these heterocyclic saturated compounds whether an alkyl side-chain is attached to the nitrogen atom or not. Just as a saturated ring such as cyclohexyl exerts stabilizing influence on the parent ions, as has been illustrated by the example of cyclohexylamine, so the effect of including the nitrogen within the ring as in N-alkyl-hexamethyleneimines also has a very marked effect on the parent peak height. A similar effect is noted in a large number of other nitrogen-containing compounds in which the nitrogen is included in a saturated ring.

9.9

AMINES AND OTHER SATURATED NITROGEN-CONTAINING COMPOUNDS

393

Consider the mass spectrum of hexamethylenediamine shown in Fig. 156 and let us attempt its identification from its spectrum. This spectrum is characterized, as would be expected, by a very large peak at mass 30, which indicates that the compound is a primary amine. The peak at mass 28 is about 7% of the height of the peak at mass 30 and suggests (as mentioned above) that the carbon number of this compound is of the order 5. The second largest peak in the spectrum is (NH4)+ and also indicates an amine. Peaks at 31,45, 59, 73,87,101 would thus be suspected to be re-arrangement ions to a series of saturated amines and suggest

N - w- AMINO&UTYLHEXAMETHYLtNEIMINE.

20

40

60

4

-

SO

120

160

ISO

Fig. 159. The mass spectra of N-ethylhexamethyleneimine and N-co-aminobutylhexamethyleneimine.

that we are dealing with a paraffinic chain. The peaks at masses 72, 86 and 100 presumably correspond to direct fragmentation without re-arrangement, the heaviest fragment corresponding to [C6H12 • NIT]"1-. There is no parent ion in the spectrum, and it could only be deduced that the compound was a diamine of molecular weight 116 by noting the three peaks at 99, 100, 101 and deducing that these were due to loss of NH, NH2 and NH3 from the parent ion. It might, however be argued that this fragmentation to three peaks might also be expected in certain cases from compounds with a single amino group. It can be seen that although a good deal of useful information has been obtained the most that can be stated with certainty is that we are dealing with a paraffinic primary amine containing at least six carbon atoms. The outstanding peak at mass 56 is difficult to explain, and it can be seen that a correct and full interpretation of the spectrum would rest on a very tenuous line of argument. It must be remembered, too, that the spectrum shown is of a highly purified material, and if this were not so, small impurity peaks would be expected to add considerably to the difficulties of interpretation. It should be noted that the absence of a detectable parent peak in such diamines parallels the behaviour of the corresponding diols. The mass spectrum is changed in a striking fashion when one of the nitrogens is included in a saturated ring. The spectrum of N-(ca-aminohexyl)-pyrrolidine /CH2—ch2 NH2 • (CH2)6 • N< | \ch2—ch2

394

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

shows a parent ion of about 0.3% of the base peak, and fragment ions correspond¬ ing to loss of 16(NH2), 30(NH2 • CH2), 44, and so on. The parent ion intensity increase when a nitrogen is included in the ring is illustrated, too, by the case of N-((o-aminobutyl)-hexamethyleneimine which is shown in Fig. 159. As menti¬ oned above the tendency is to break /? to one of the nitrogen atoms, the charge tending to remain on the fragment including a nitrogen atom.

N - acetylmorphoune

0

20

40

60

80

100

ISO

14-0

Fig. 160. The mass spectrum of morpholine and some N'Substituted derivatives.

Similar behaviour is exhibited by compounds which include an oxygen or other atom in the heterocyclic ring together with the nitrogen atom. The mass spectra of morpholine and some of its Id-substituted derivatives are shown in Fig. 160. The tendency to break /? to the nitrogen atom is apparent in all the N-substituted compounds. In the spectrum of N-acetylmorpholine the tendency to break at the ring (which is also a to the carbonyl group) is increased relative to the tendency for cleavage /? to the nitrogen at the bond on the other side of the carbonyl. To obtain any detailed information on the mode of fragmentation of these heterocyclic rings, it is necessary to carry out accurate mass measure¬ ments so as to determine the atomic composition of the fragments, under conditions of high resolution to ensure that isobaric pairs can be distinguished. The behaviour of substituted piperidines is of course similar to that of the pyrrolidines and hexamethyleneimine derivatives which have already been

9.9

AMINES AND OTHER SATURATED NITROGEN-CONTAINING COMPOUNDS

395

described The fragmentation of the saturated ring itself leads to a number of large peaks of comparable intensity which are of similar distribution in the 3-classes of compound. It is usually not possible to deduce the complete formula rom the mass spectrum. One would generally be able to state the total number of carbons in the ring plus those substituted on positions other than the nitrogen atom, but not to separate the two effects, especially when the substituents were methyl or ethyl.

ja-n- DOOECXL

ANILINE.

Fig. 161. The spectra of two dodecylanilines. The fragmentation pattern given by the isomer with the straight-chain substituent (lower spectrum) should be contrasted with that of the upper spectrum which is, as can be seen from the fact that i, 2, 3, 4, 5, 6, 7, 8, 9, 10 or 11 carbons can be lost, a mixture of highly branched isomers, fragmentation occurring readily at the branches.

Aromatic amines behave in many respects like the corresponding hydrocarbon. In amines which include a long alkyl side-chain on the aromatic ring, such as p-n-dodecylaniline, the pattern of the fragmentation is dominated at the high masses by the breaking of the alkyl chain. The parent peak is large, as would be predicted from the presence of the aromatic nucleus, and the base peak of the spectrum corresponds to fragmentation of the C-C bond /? to the ring. The spec¬ trum is illustrated in Fig. 161. The spectrum of a mixture of highly branched

.396

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

isomers is also shown in this figure. When no such alkyl chains are present the interest lies in the fragmentation of the ring itself and in the various ion reactions concerned with fragmentation of N-C bonds. Aniline itself readily loses HCN and H2CN to give hydrocarbon ions of masses 66 and 65. Similar behaviour is observed in the case of phenol, which, as has been mentioned above, readily loses CO and HCO to give ions of the same masses. Particularly interesting are the spectra of N-di-substituted anilines, toluidines and related compounds, in that hydrogen re-arrangement often accompanies fragmentation of the groups attached to the nitrogen atom. 9.10

AMIDES

The mass spectra of 35 aliphatic amides have been tabulated by Gilpin [752]. These include primary, secondary and tertiary amides, and interesting compar¬ isons can be made between their spectra and those of carboxylic acids and esters. The spectrum of n-butyramide is compared with that of n-butyric acid in Fig. 148. Qualitatively, the spectra can be seen to be similar; they each give a parent peak of a few % intensity, a somewhat smaller peak at (p — 1), a large peak due to loss of CH3 and the base peak at a mass corresponding to the loss of neutral C2H4 in a re-arrangement reaction. In the case of the amide, the ion simultaneously formed in the latter reaction when the C-C bond /? to the carbonyl is broken has the mass 59 (acetamide) and in the case of the acid the mass 60 (acetic acid). Each spectrum contains a prominent (C4H?)+ ion and there are several other points of similarity. The peak at mass 59 forms the base peak in the spectra of all primary straightchain amides with the exception of propionamide, suggesting that the hydrogen atom taking part in the re-arrangement reaction is that shown below R—CH—CH2-. CH2—CO—NH2 I

*

For long-chain amides a large peak also occurs at a mass 72 corresponding to fragmentation y to the carbonyl group, and this is accompanied by a re-arrangement peak at mass 73. This behaviour is paralleled by the corresponding acids which give prominent peaks at masses 73 and 74. The spectra of the secondary amides can be compared with those of esters assuming the -NH- group in the one case to parallel the -O- atom in the other. In the compound Ri • CO ■ NH • R2, fragmentation of the R2 chain at the NH group can occur with re-arrangement of a hydrogen to give (Ri • CO ■ NH2)+ or (Ri • CO • NH3)+ ions. If the Ri chain is long (> 3 carbons) the re-arrangement described above for primary amines can occur with loss of neutral (Ri minus CH). A large peak at mass 30 in some secondary amide spectra is formed by a re-arrangement process similar to that which can occur with many amines. In the mass spectra of tertiary amides large re-arrangement peaks can also occur due to processes similar to those given above. For example the amide R2 r2-~i + Ri • CO • N will give peaks CH3 • CO • N and [R3NHCH2]+, the R3 Rs. latter corresponding to the mass 30 peak in secondary amide spectra. The parent peaks are usually easily distinguished in amide spectra and are much larger than those in ester or amine molecules of comparable size. The presence of the nitrogen atom which would be suspected by the number of even-

9.11

INDOLES, PYRROLES,

QUINOLINES AND PYRIDINES

397

mass fragment peaks would be confirmed by the odd-mass parent peak, which latter peak would be seen to end a chain increasing by successive mass 14 inter¬ vals with a mass 15 (CH3) end group. The comparatively large parent peak would suggest an amide; rules for deciding the structure of an amide from its mass spectrum have been given by Gilpin [752].

9.11

INDOLES, PYRROLES, QUINOLINES AND PYRIDINES

The mass spectra of a series of eleven alkyl indoles have been obtained in these laboratories [191]. The spectra obtained are given in Table 11. Several conclusions which are of use in structural diagnosis or of interest in throwing light on the mode of fragmentation of this type of compound emerge from a study of these spectra. All the compounds studied give a re-arrangement peak at mass 103. In the case of 2-methyl- and of 3-methylindole, a meta-stable ion at mass 81.6 indicates that the re-arrangement ion arises by the reaction (p — 1)+ -> 103+ + 27 Further meta-stable peaks at masses 57.5 and at 33.8 show that ions of mass 103 can take part in the reaction 103+ -> 77+ + 26 and that ions of mass 77 can undergo the transition 77+ ->51+ + 26 The last two reactions are typical of those found in conjugated hydrocarbon structures and lend weight to the theory that the nitrogen atom is lost in the initial reaction involving the parent ion. The mass 103 ion may exist in the styryl form. The series of peaks present in all spectra at masses 77, 63, 52, 51, 50 and 39 are characteristic of benzene derivatives and provide an illustration of the way in which an aromatic ring can be detected by the pattern of the peaks at low mass numbers, even when it is heavily substituted. Many of the most prominent ions in the spectra, however, include the nitrogen atom. In all spectra, the parent peak is large, reflecting the conjugated ring structure. All indoles with methyl substituents also have a large peak at mass (p — 1) corresponding to fragmen¬ tation at a bond /3 to a double bond or nitrogen atom. Those indoles with longer chain alkyl substituents also show this tendency for /3-bond fragmentation. For example, butyl substituted indoles readily lose C3H7 from the parent ion. Loss of a methyl group is very unlikely from the parent ion of a monomethylindole, but is quite probable for di- and multi-methyl substitution. This is similar be¬ haviour to alkylbenzenes. One may postulate that just as in the case of the alkylbenzenes, the ion formed by loss of methyl from dimethylindoles or of hydrogen from monomethylindoles is in an extremely stable steric configuration, and that re-arrangement may occur to form this stable ion. Consider, for example, 2:3-dimethylindole. The reactions occurring may be

to give a highly resonance-stabilized ion.

398

9

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

TABLE II MASS SPECTRA OF ALKYLINDOLES Mass to Charge

2-Methyl'

3-Methyl-

indole

indole

Ratio

1:3Dimethyl-

1:7Di methyl-

indole

indole

2:3indole

.40 2.56 .86 .01

1.19 5.74 8.91 1.32

.34 1.64 1.40 2.51

.75 2.57 2.81 6.4713

.12 .06 6.20 .61 2.49 3.31 .19 .30

.09 .40 2.72 .35 .56 1.26 .06 .42

.07 .36 2.94 .36 .75 .76 .30 .45

.06 .36 2.32 .35 1.06 1.05 .18 .36

.05 .34 3.76 .42 .91 .91 .09 .07

.11 .81 2.94 1.00 2.93 1.87 1.24 2.75

.03 .10 1.69 .20 1.69 .25 .13 .06

.07 .23 1.66 .28 6.3513 .84 2.3113 1.03

3.13 .01 8.20 .05 3.18 .01 1.39 .37 1.12 .15 .04 .44 .93 .44 4.44 .58 .29 .04 .17 1.56

1.32

1.00

1.31

1.05

.38

.37

.20

6.18 .02 1.61 .01 .69 .10

2.92 .02 1.40

3.78 .03 .97 .01 .72 .36 .29 1.21 .02 .18 .33 .11 .67 .09 .006 .003 .04 .39

.95

1.34

.50

.43

.61 .01 .22

.87 .40 .64 .86

.34 .10 .52 .12

.42 .32 2.57 1.01

.01 .11 .45 .15 3.03 .31 .05 .02 .08 .60

2.85 .05 1.48 .01 • .77 .16 .27 .05 .01 .34 .30 .11 .31 .28 .08 .02 .06 .56

.34 .01 .04 .06 .10 .03 .08 .08 .09

.24 .07 .04 .11 .02 .02

4.871? .03 .25 .04 .04

.01 .11

2.47 .03 1.02 .38 6.37 .47 .24

2.05 .01 .68 .09 2.06 .06 .20

2.01 .01 .67 .13' 2.13 .07 .16

.67

.67

.44 .03 1.37 .11 .51

.21 .04 .70 .04 .11

.06

.12 .06 .08 .04 .08 1.35 .43 9.90 1.99 4.73

.36 2.44 2.05 .05

.28 1.28 1.20 .08

.59 4.08 2.77 .14

37 38 39 40 41 42 43 44

.25 .85 4.01 .03 1.44 1.16 .55 .14

.21 .72 3.51 .32 .23 .07 .01 .12

.09 .36 2.52 .27 .64 1.50 .07 .12

50 50.5 51 51.5 52 52.5 53 54 55 56 56.5 57 57.5 58 58.5 59 59.5 60 61 62 62.5 63 63.5 64 64.5 65 65.5 66 66.5 67 68 69 69.5 70 70.5 71 71.5 72 72.5

2.12 .04 6.63 .35 2.80 .01 .69 .27 .15 .33 .02 .39

3.11 .07 10.71 .45 3.90 .22 .79 .28

1.44

.01 .24 1.35 3.89 .63 2.75 10.59 15.63 12.13 1.11 .04 .004

.02

.01

.03 .002 .003 .002

.02 .02 .01 .02

.04 .09 .58

.67 .07 1.16 .07 .33

6.40 .04 2.32 .34 4.94 .31 .75

.07 .06 .02 .02 .03 .42 .16 2.24 1.98 3.60

.29 .08 .09 .06 .14 2.43 .76 16.98 4.41 1.87

2.27

indole

.27 1.47 1.18 .11

.41 2.66 2.11 .25

.005 .25 1.86 .01 4.78 .39 2.65 7.55 10.14 8.72 .89 .03 .04 .01 .03

butyl

indole

.28 1.53 2.15 .11

26 27 28 29

.04 .06 .006

1: 3-Di ri-

.22 1.46 1.01 .004

.18

.04 .03 .01

indole

1-n-Butyl3-methyl-

.li

1.04

.12 .30 .12 .83 .09

indole

methyl-

.49

.91

.02 .03 .04

indole

1 :2 :3 :5 :7Penta-

.66

.03

.39 .09 .05 .04

1:2:3-

.15

.29

1.03

indole

2:6-

Dimethyl- Tri methyl-

.24

15

3.41

2:5-

Di methyl- Dimethyl-

.25 .02 1.30 .31 7.29 1.61 6.35

.06 1.07 .33 8.33 1.28 3.46

.58 .16 .11 .06 .01 .19 .36 .18 . 3.05 .49 .07 .12 .08 .53

2.53 .09 1.07 .40 2.36 1.11 .55 .05 .11 .05 .05 .05 .50 .33 2.79 3.29 .33

.35

.13 .02 .04 .38 .01 .24 .05 .44 .11 .15

.01 .50 .08 .31

.06 .04 .04

.10 .04 .11 .43 .13 .14

.03 .06 .07 .30 .23 3.23

.94 .31 1.401? .01 .68 .03 1.921? .12 .18 .04

INDOLES, PYRROLES,

Mass to

2-Methyl-

3-Methyl -

pharge

indole

indole

Ratio

73 73.5 74 75 76 76.5 77 77.5 78 78.5 79 79.5 80 80.5 81 I 82

1:7Dimethyl-

2:3-

2:5-

2:6-

1:2:3-

Dimethyl-

Dimethyl-

Dimethyl-

Dimethyl-

rrimethyl-

indole

indole

indole

indole

indole

indole

.46

.54

3.61

.02

.02

.94 2.14 2.97

.42 1.08 1.24

.47 .03 .55 1.27 1.45

1:3-

.87 1.94 2.34

2.23 3.78 4.78

1.04 2.16 2.67

1.56 .16 1.33 3.63 4.78

18.59

28.27

8.64

20.08

12.48

7.00

5.78

2.11

3.32

1.37

.67

1.69

1.46

1.50

.04

.15

.19

.07

.08

.04

,ii

399

QUINOLINES AND PYRIDINES

1 : 2 :3 : 5 :7 Pentamethylindole

O on

9.11

l-n-Butyl-

1:3-Di-n-

3-methyl-

butyl-

indole

indole

.19

.58 2.42 2.62 .29 8.10

.09 .13 .37 .05 4.10

.36 .02 .16 .66 1.04 .02 6.27

1.00 3.32 1.85 .66 .41

1.05 .02 .19 .01 .03

.18 .31 .50 .01 3.06 .03 .55 .03 .38 .12 .18

.34 .09

.03 .03

.98 .66

.04

.20

.28

.17

.07

.06

.03

.05 .08

.06

.02 .02

.11

.22

.07

.06

.06 .04

2.22 8.05 2.83 11.98 1.38 .07 .05 .04

.29 .49 .53 3.99

.20

.40

.19 .45 .40 1.48 .83 .24

.41 .38 1.70 .65 1.98

.33 .70 .87 4.22 1.33 6.14

.04 .23 .26 1.75 .74 2.26

1.32 .26 .26 1.66 .70 2.33

.11 .33 .40 2.62 1.05 3.83

.55 .11 .08 .78 .86 4.10

.03 .08 .13 .94 .58 2.43

.12 .51 .05 .63 .40 1.36

98 * 99

.09

.21

.14

.09

.17

101 102

2.01

.17 .89 2.09 3.14 .76 .09

.06 .08 .18 2.35 5.30 4.93 .80 .18

.05

moo

.08 .04 .03 .99 2.46 3.50 1.48 .17

1.83 1.21 .49 1.16 .54 1.22

.02 .21 .45 1.39 2.65 4.88 .95 .15

.29 .27 .06 .19 1.94 3.02 .71 .60

.28 .54 9.75 2.78 2.80 .60

.42 .75 13.84 4.96 3.62 .99

.11 .12 5.77 1.92 1.19 .62

.07 .19 6.11 2.16 2.41 .51

.33 .19 3.48 1.38 1.66 .55

.12 .40 1.72 1.14 14.64 1.98 .16 .03

.34 2.12 6.12 3.67 2.89 1.09 .28 .10

.37 3.18 6.08 4.30 3.17 3.01 .80 .10

.07 1.16 3.34 2.33 14.95 .25 .63 .08

.49 1.41 8.48 3.10 26.99 3.24 .33 .37

.14 1.21 1.11 6.13 20.89 53.75 6.57 .39 .06

.33 .61 1.52 1.89 1.59 1.91 .88 3.50 .45

.04 .17 .26 1.67 6.91 100.00 21.60 1.68 .08

.22 .25 1.04 1.97 8.69 18.78 2.58 .34 .21

87

88 89 90 91

103 ' 104 (105 113

H4 115 1116 117 1118

S 126 1 127 ; 128 129 i 130 : 131 132 133 ! 139 1 140 141 142 143 144 145 146 147

.02

2.66

.12

5.24 15.47 2.65 .19

.10

.17 .13 2.59 7.94 16.35 2.71

.10

.07 .24 .29 .24 .24 .03

.03

.12 .31

.15 2.78 4.52 1.96

4.14 .59

.10 .15

.12 .20

100.00 100.00 85.36P 69.12P 9.94 .40

.01 .03 .03

5.91 .31

.02 .10 .08

.11 .12 1.82 6.15 5.59 1.13 .16

.70 .05 .31

.08 .28 .42 3.49 8.34 15.61 3.83 .62

.04 .25 .28 1.80 1.04 1.55

.10 1.71 5.64 7.27 1.33 .04

.30 .43 7.21 2.25 2.43 .70

15.47 5.20

4.66 1.22

2.91 3.50 .82

.25 .43 8.45 2.59 2.51 .59

.11

.21

.12

.11

1.40 6.33 1.71 51.76 6.94 .39

.53 1.98 1.24 16.06 1.89

1.39 6.57 4.36 3.37 .52

.02 .03

.56

1.01

.85 5.56 4.80 20.08 6.48 .51 .06

.29 .57

8.22

.10 .05

.07 .07 .03 .62 .70 .48 .58 .73 1.31 1.16 .62 6.40 6.14 4.58 3.22 15.08 14.23 11.07 12.81 10.64 100.00 100.00 100.00 93.02 100.00 71.08P 100.00P 82.28P 91.66P 93.58P 10.32 8.32 7.70 13.27 7.86 .33 .31 .30 .84 .39 .23 .33 1.87 4.08 9.71

400

9

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

TABLE 11 (continued) Mass to Charge Ratio

1:31:7-, 2-Methyl- 3-MethylDimethyl- Dimetnylindole indole indole indole

153 154 155 156 157 158 159 160 161

.31 .45 .26 .63 .10

167 168 169 170 171 172 173 174 175

0.12 0.16

.08 .43

2:32:5Dimethyl- Dimethylindole indole

.02 .08 .08

2:61:2:3Dimethyl- Trimethylindole indole

.02 .04 .05

.18 1.14 .81 5.46 5.57 100.00 94.99P 15.10 .70

.01 .06 .06

182 183 184 185 186 187 188 189

1:2:3:5:7Pentamethylindole

1-n-Butyl3-methylindole

1 :3-Di-nbutylindole

.49 3.63 1.80 7.54 6.33 3.84 .68 .09 .04

.03 .56 .23 1.56 .59 1.20 .25 .05 .02

.23 1.21 .61 2.93 1.00 2.72 1.08 .18 .04

.82 3.04 1.18 8.90 8.79 66.47 12.37 1.10 .07

.19 .20 .06 .43 .23 1.65 .23 .03

.42 .64 .25 .69 .28 .67 .28 .19 .03

.46 2.09 1.89 3.01 74.28 100.00P 11.76 .79

.03 .01 .13 .10 1.05 .46 .22 100.00 2.77 12.00 41.5 IP 5.80 .97 .43

212 213 214 215

.23 .05 .27 .27

227 228 229 230 231

.22 .51 21.98P 3.35 .27

Metastable peaks

33.8 57.5 81.6

33.9 57.6 81.6

33.8 81.6

33.8

I ? = Peaks probably enhanced by the presence of impurities P = Parent ion

Evidence in favour of this proposed structure is given by the fact that the peak at mass (p ■—15) is weak in 1:3-dimethylindole and in monomethylindoles (which would be expected sinceTor the above reactions to occur methyls are necessary in both the 2- and 3-positions). The peak at mass (p — 15) is also comparatively weak in 2:5- and 2:6-dimethylindoles but is strong in l:2:3-trimethyl- as well as 2:3-dimethylindole. The above facts would also agree with a

9.11

INDOLES, PYRROLES, QUINOLINES AND PYRIDINES

401

re-arrangement in which the final ion had an increased number of carbon atoms in the heterocyclic ring to give the structure quinolinium

in an analogous manner to the formation of the tropylium ion from alkylbenzenes. Such a structure with its system of conjugation would be expected to be very stable. Other stable structures can be postulated to be formed in the break¬ down of dimethyl substituted indoles in which one of the substituents is attached to the nitrogen atom. Thus, for l:7-dimethylindole one can postulate the reaction

The quinolinium structure might also, of course, be formed in such cases. Another characteristic of the indole spectra plotted is the appearance of a peak at mass 115 in all alkylindoles other than the monomethylindoles of height about 10% that of the base peak. Associated with this peak are two others of about ljz its height at masses 116 and 117. The mechanism of formation of these ions is not clear but from the point of view of recognizing the spectrum of an unknown compound as being that of an indole, these peaks (and also the peak at mass 103 which has been mentioned above) are extremely useful. In common with most nitrogen containing compounds, the indoles show a large number of doubly-charged ions in their spectra. It is thought that due to the even atomic weight and odd valency of nitrogen the most stable singly-charged ions containing a single nitrogen atom are likely to be those of even mass, since there will be no un-paired electrons in such ions. Similarly the stable doublycharged ions are likely to be those of odd mass and will thus appear to have a mass differing by 0.5 from a whole number. Such ions are, therefore, always clearly visible since they suffer no interference from singly-charged ions. In hydrocarbon spectra, on the other hand, the most stable doubly-charged ions will be of even mass and thus are more likely to suffer interference from singlycharged ions or be wrongly identified as ions carrying only a single positive charge. The intensity of some of the doubly-charged ions in the indole spectra is large, and peaks greater than 10% of the height of the base peak are not un¬ common. It is thought that perhaps the localization of charge on the nitrogen atom facilitates the removal of a second electron without cleavage of any bonds in the ion. The largest peaks correspond to doubly-charged parent ions, but the intensity of these peaks does not fall as rapidly with loss of hydrogen atoms as do the singly-charged ions. Peaks such as (p — 6H)++ are often observed. Sometimes peaks due to doubly-charged ions are larger than the corresponding singly-charged ion peaks. In l:7-dimethylindole the peak at 58.5 is of height

402

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

4.4% whilst that at 117 is 3.5%. Although the most intense singly-charged ion in this mass region is of mass 115 (15.5%) the corresponding doubly-charged ion is only of intensity 0.9%. Most of the meta-stable transitions in hydrocarbon spectra which involve only fragment ions consist in an odd mass ion giving off a neutral fragment of even mass, and though this is not invariably the case, the majority of the meta-stable transitions are of this kind. In the paraffins for example loss of neutral ethylene in a meta-stable transition is common. In indoles, an example of the effect of nitrogen is seen in the transition which leads to the formation of mass 103 in methylindoles, and which has been mentioned above. This is (p — 1)+ -> 103+ + 27

or

C9H8N+

C8H7+ + HCN

In this case a stable even-mass number nitrogen-containing ion has given off the odd mass number nitrogen-containing molecule HCN to give the stable odd-mass number hydrocarbon ion which has been surmised to be a styryl ion.

10

2,6 - DIMETHYL INDOLE.

O

10

20

30

40

50

60

70

80

90

100

110

120

V

O

130

140

150

[Fig. 162. The mass spectrum of 2:6-dimethylindole.

A large number of “half-masses” is readily detected in a mass spectrum due to the unusual closeness of the peak pattern. The mass spectrum of 2:6-dimethylindole is shown plotted in Fig. 162 and illustrates that this is so. The appearance of these half-masses would immediately suggest to a practised observer the presence of nitrogen in the compound being studied. The presence of intense doubly-charged ions corresponding to the masses of the heaviest ions in the spectra strongly suggests these to be parent ions. The fact that the parent ions

9.11

403

INDOLES, PYRROLES, QUINOLINES AND PYRIDINES

are so strong would suggest a conjugated ring structure. The empirical formula would show that R (the number of rings and double bonds) was 6 and would immediately suggest an indole structure (as explained below). One would imme¬ diately study the intensity of peaks such as 103, 115, (p — 1), (p — 15), (p — 29), etc., so as to obtain information on the number and lengths of the substituent chains. The number of rings 4- double bonds (R) in a benzene ring is 4 and this number is unchanged in the corresponding six-membered ring which contains a nitrogen atom (pyridine). The molecular weights of the homologous series of pyridines are one greater than those of the corresponding benzene derivative, but this effect due to the presence of the nitrogen causes no confusion since the presence of a nitrogen atom leads to an odd molecular weight and thus is easily recognizable. 4Vhen two such rings are fused to form a quinoline nucleus the number R changes to 7 (and this is of course, also the number for naphthalenes, pyridopyridines, benztriazines and all similar combinations of 2 fused sixmembered rings with a conjugated system of double bonds). The R-number for the pyrrole ring is 3 (and again this number applies to other five membered rings such as cyclopentadiene, pyrazole, triazole and so on). When fused to an R = 4 ring, an R = 3 ring leads to a compound for which R = 6 (such as indole). Thus

2 - METHYL- 5- ETHYL pyridine

30

40

50

60

70

60

90

100

IIO

120

PYRIDINE

30

40

50

60

70

60

Fig. 163. The mass spectra of pyridine and some alkyl derivatives.

whenever a spectrum appears to show a large parent peak indicating a ring structure and accurate mass measurements indicate a single nitrogen atom one would consider for R = 7, alkylquinolines and aminonaphthalenes as well as various possibilities involving unsaturated side-chains. Similarly, the other R-values mentioned above would suggest the other ring combinations listed.

404

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

Thus, it is often possible to eliminate several possible basic ring structures immediately merely on the basis of the “R” number. All the various combinations of rings which have been mentioned obey the general rules about breaking side-chains at bonds to the ring. The increased stability of the pyridine nucleus over the pyrrole nucleus is also reflected in the greater abundance of the fragment ions in the latter case. The heights of the “half-mass” peaks corresponding to doubly-charged ions are generally greater the more stable the nucleus, and the variety of doubly-charged ion species also increases with the stability of the ring. The spectrum of pyridine and also two isomeric alkylpyridines, 2-propylpyridine and 2-methyl-5-ethylpyridine are shown in Fig. 163. It can be seen that fragmentation of the ring is more likely to occur with pyridine than with benzene. The main mode of fragmentation is with loss of mass 27 which can be shown by accurate mass measurement to be mainly HCN. The differences in the other two spectra clearly point to the identity of the side-chains. In n-propylpyridine fragmentation to the ring gives the base peak with loss of C2H5 from the parent ion. In the methylethylpyridine both a- and ^-bond cleavage occur showing the multiple substitution. The shorter sidechains lead, in this case to a more abundant parent ion. Pyridines give a characteristic fragmentation pattern at low mass numbers in the same way as do benzenes. Such patterns can be detected even in cases in which the pyridine nucleus is highly substituted. For example, even the mass spectrum of Tchitchibabine’s base

shown in Fig. 164 contains features which one would associate with a pyridine.

Fig. 164. The mass spectrum of Tchitchibabine’s base.

9.12

NITRILES

An aliphatic nitrile can generally be classified as such from its mass spectrum whichcontains peaks characteristic of the class, but it is often difficult to identify the compound completely due to the absence of a parent ion in many cases. The

9.12

405

NITRILES

spectra of several straight-chain alkylnitriles have been given by McLafferty [1358]. The base peak is often formed by cleavage of the bond /3 to the functional group with re-arrangement of a single hydrogen atom. This reaction may be written H

1 ) 1 R - CH - CH2 > CH2

CN

(CH3CN)+ + R - CH = CH?

>

the final products being presumably an acetonitrile ion and a neutral olefine fragment. The ion so formed (of mass 41) forms the base peak of the spectra of all such nitriles (except propionitrile) up to C10H21CN; for longer chains than this the spectra become more and more closely similar to the spectra of the cor¬ responding paraffin. Propionitrile cannot form a neutral olefine by fragmentation in this way and in this particular compound the peak at mass 41 is only of the order of 1% of the base peak. The peak at mass 41 and its homologues at masses 55, 69, 83, etc. must not be confused with those due to (C3Hs)+, (C4H7)+ etc. which occur in olefine spectra. When the alkyl chain is not too long (below, say 6 carbon atoms) the mass 41 peak is much larger relative to its odd mass neighbours than in hydrocarbon spectra. Also the series dies away much more rapidly with increasing mass than in the hydrocarbon case. This is illustrated in Fig. 165 which shows the mass spectrum of capronitrile. It can be seen that the

Fig. 165. The mass spectrum of capronitrile.

Fig. 166. The mass spectrum of adiponitrile.

homologous series which persists to the higher masses is formed by the ions [(CH2)«CN]+ which are produced by simple cleavage without hydrogen re¬ arrangement and these ions remain evident up to (p — 1)+ in nitriles containing 15 and more carbon atoms. These ions of even mass number indicate clearly the presence of the nitrogen atom. A characteristic of nitriles is the ease with which ions formed in inter-molecular processes can be observed. The most readily observed is the ion of mass (p + 1) and this can be seen in the spectrum of capronitrile referred to above and also that of adiponitrile which is shown in Fig. 166. These ions occur to a measurable extent even when samples are intro-

406

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

duced at the usual working pressures, but can become as large as 1% of the base peak when the sample pressure is increased 5 to 10 times. The twin peaks at (p + 1) and (p — 1) terminating a series [(CH2)«CN]+, the relative abundance of the (p + 1) peak varying with sample pressure enable the molecular weight of a nitrile to be located. Confirmation of the molecular weight can be obtained by seeking a still heavier pressure-dependent peak at a mass (p + 41) formed by a process in which a (CH3CN)4 ion attaches itself to a neutral molecule. The abundance of this peak is usually lower than that of the (p + 1) peak by a factor of at least 10. In aromatic systems there is usually little difficulty in detecting the presence of a -CN group which always behaves as an entity. The parent peak is usually observable in such compounds. As an example of the breakdown of such a com¬ pound consider say, an ester of cyanobenzoic acid. This would fragment in the way one would expect for an aromatic ester giving strong peaks at masses p+, (p — OR)+, (p — COOR)+, the latter peak corresponding to the ion (CeHsCN)4". 9.13

NITRO-COMPOUNDS AND NITRITES

An interesting re-arrangement occurs in many compounds in which a nitro group is attached to an aromatic ring. The effect is seen most clearly in the mass spectrum of nitrobenzene itself which is shown in Fig. 121, p. 268. This compound gives a very prominent parent ion and a base peak at mass 77 which, if it were required to identify the compound from its spectrum, would provide clear evidence of the presence of a phenyl derivative. The molecular weight shows that the compound is of formula (phenyl + 46) and using only C, H, N and O, the substituent would have to include an odd number of nitrogens (to give the odd molecular weight) and nitrobenzene would be the first formula to come to mind. Peaks at 50,51 and 65 are due to fragmentation of the ring, the peak at mass 30 is a re-arrangement ion due to (NO)+, that at mass 107 is due to (p — 0)+ and we are left with the rc-arrangement ion at mass 93, [(p — NO)+] and the group at 74, 75, 76 similar to the ions in benzene itself formed by loss of hydrogens from the phenyl portion of the molecule. The mass spectrum of benzene itself gives peaks at masses 50, 51 and 52 of the same order of intensity as the peaks at masses 50 and 51 in the nitrobenzene spectrum. The peak at mass 65 does not occur in the spectrum of benzene so would not be expected from further frag¬ mentation of the ion of mass 77. It is interesting to speculate about the probable mode of fragmentation of the ion of mass 93, and a likely reaction for it to undergo would seem to be the loss of a neutral CO fragment, because of the stability of this fragment coupled with the fact no further re-arrangement is necessary for this reaction to proceed. This would, of course, lead to the ion of mass 65. The loss of mass 30 from many aromatic nitro-compounds provides a very satisfactory means of detecting the presence of this group. The mass spectra of the three isomeric nitroanilines are also very interesting, and provide an example of three isomers which can readily be distinguished by mass spectro¬ metry. They are shown in Fig. 121. The o-nitroaniline is the only compound which breaks with loss of -OF! (proved by mass measurement), the other com¬ pounds losing mass 16 by loss of a single oxygen in the expected fashion from the nitro-group. o-Nitroaniline also gives smaller peaks due to loss of O and H2O from the parent ion. Loss of-OH from the ortho-isomer is not difficult to explain in terms of internal hydrogen bonding. Loss of the entire -NO2 group from the parent ion is again a very likely mode of breakdown, but the relative size of the

9.13

NITROCOMPOUNDS AND NITRITES

407

peaks so formed at mass 92 can be used to distinguish between the isomers. The relative size of this peak varies in the order ortho-, meta-, para-, the ortho-isomer giving the largest peak. The parent peak intensity is large in the ortho- and meta¬ isomers but is smaller in the para-isomer. The ratio of the height of the peak at (P 00) to the parent peak is much smaller in p-nitroaniline than in the other two cases; even when the absolute heights of the (p — 30) peaks are considered, the peak is largest in the para-compound. These characteristics provide for easy analysis of mixtures of the isomers. A prominent peak in all spectra is at mass 80. This is the analogue of the mass 65 ion in nitrobenzene and is probably formed by loss of CO from the re-arran¬ gement ion (p — NO)+ at mass 108. The secondary breakdown of the ion at mass 92 (p — NCb) gives an interesting example of fragmentation of a nitrogen-containing ion into a neutral nitrogenous molecule and an odd mass number ion. By a re-arrangement reaction this ion undergoes a meta-stable transition (giving a meta-stable peak at mass 46) as shown by the equation 92+ -* 65+ + 27, i.e.

(C6H6N)+ -> (C5H5)+ + HCN

This provides a further example of the loss of HCN which is common in aroma¬ tic amines and has also been observed to occur in indoles. The spectrum of p-dinitrobenzene is different in pattern from that of nitro¬ benzene. Loss of a single oxygen atom and of an entire nitro-group occurs, but the re-arrangement ion formed by loss of NO is almost completely absent in this case. However, an ion occurs at mass 92 formed by loss of one complete nitro-group together with NO from the opposite end of the aromatic nucleus. The spectrum is shown in Fig. 121. Other small peaks in this spectrum at high masses are at mass 152 ([p — OJ+), and at mass 106 ([p — NO2 — 0]+). There is a strong peak at mass 64 which may be formed by loss of CO from the (p —- NO2 -—- NO)+ ion at mass 92 in a similar manner to the formation of mass 65 in nitro¬ benzene from the (p ■— NO)+ ion. A prominent ion at mass 76 corresponding to loss of both NO2 groups is not unexpected, but the spectrum also includes promi¬ nent ions at masses 75, 74 and 63 the mass 75 ion being the base peak of the spectrum. The mechanism by which these ions are formed is not clear. The second largest peak in the spectrum is at mass 30, and a peak at this mass number has previously been noted to occur in all amine spectra when it has the com¬ position (CNH4)+. In the present instance, the ion has the composition (NO)+ and could readily be distinguished from the amine ion by mass measurement. One would be unlikely, however, to confuse an amine spectrum with that of a nitro-compound because of the characteristic breakdown pattern of the nitrogroup which always gives a large peak at (p — 46)+. It is interesting to compare the mass spectra of the alkyl nitrites with those of the nitro-paraffins. The mass spectra of eleven alkyl nitrites up to the amyl nitrites have been given by D’Or and Collin [515] and Collin has also determined the mass spectra of nitromethane, -ethane and both nitropropanes [372]. A variety of nitro-compounds including dinitropropane and a nitropentane have been examined by Boschan and Smith [249]. Except for nitromethane which gives a strong parent peak and large peaks at masses 46 and 30 corresponding to (N02)+ and (NO)+ the main peaks in the mass spectra of the nitro-paraffins come from the hydrocarbon end of the molecules. Peaks due to (C2Hs)+ and (C2Ha)+ are very large in nitroethane; (C3H7)+, (C3Hs)+, (C3H3)+ and (C2H3)+ stand out in the nitropropane spectrum. (C-iEH)4 is a large in 1-nitrobutane. The size of the

408

CORRELATIONS OF MOLECULAR SYRUCTURE AND MASS SPECTRA

9

hydrocarbon portion of the molecule is thus immediately obvious. The presence of the nitro-group is evidenced by a peak (NO)+ of about 15% at mass 30 and a peak (N02)+ of about 3% at mass 46. No ions formed by loss of small alkyl fragments from the parent ion are seen. The two isomeric nitropropanes could not be distinguished without reference spectra of each pure compound. The alkyl nitrites, on the other hand are readily identified from their mass spectra. The spectra contain many more ions which include part or all of the nitrite group. As well as a large peak at mass 30 [(NO)+] which forms the base peak of the spectrum in most cases and always at least 40% of the base peak intensity, there is a large peak corresponding to the ion • CH2 • O • NO+ at mass 60 in all nitrites in which there is no substituent group on the carbon atom situated next to the nitrite. Fragmentation at this bond /? to the oxygen atom is the behaviour that one would expect by analogy with the spectra of ethers and

10

IS

20

25

30

35

40

45

50

55

60

65

70

75

80

85

TERT - BUTYL NITRITE

JJ 10

15

20

25

30

35

40

L

45

50

55

60

6S

70

75

80

85

Fig. 167. The mass spectra of the four butyl nitrites.

alcohols. Similariy, the lack of an ion at mass 46 corresponding to cleavage of the bond a to this oxygen is not unexpected and the absence of a peak of more than 1 % at this mass number (except for the anomalous methyl nitrite) gives another means of distinguishing nitrites from nitro-compounds. This tendency to break bonds to the oxygen atom enables the various isomeric nitrites to be distin-

9.14

NITROSO DERIVATIVES AND NITROSAMINES

409

guished. The spectra of the four butyl nitrites are shown in Fig. 167. Let us consider these spectra as being of unknown compounds and attempt to identify them. All show hydrocarbon peaks at masses 57, 43, 41, 29 and 27 indicating that they are C4 compounds and contain at least nine hydrogen atoms. They all give large peaks at mass 30 (suggesting an amine, nitrite or nitrocompound) but do not give peaks at masses 18, 44, 58, etc., which might be expected from an amine. It would also, of course, be possible to determine the accurate mass of the peak at the nominal mass of 30 and show that it is due to (NO)+ ions. The smallness of the peak at mass 46 suggests nitrites rather than nitrocompounds. This is confirmed-in the first two spectra which have a large mass 60 peak showing that they contain the grouping CH2 • O • NO and in the other spectra by the heavier ions which obviously are not hydrocarbons. Since we have decided that all spectra are those of C4 nitrocompounds or nitrites, their molecular weights will be 103 (C4H9NO2). The third spectrum shows loss of CH3 from this parent ion (small peak at mass 88) and also loss of C2H5 (large peak at mass 74). CH3 This suggests its formula to be q ^ CH-O-NO whilst the large peak at mass 88 and absence of a peak at mass 74 in the other spectrum suggests this to be the tertiary butyl derivative. Both are obviously nitrites rather than nitro-paraffins because these heavy ions contain the NO2 group. Boschan and Smith [249] report the spectra of ten nitrate esters. In all cases a peak containing the complete nitrate group together with part of the hydro¬ carbon portion of the molecule is obtained. This peak is generally formed by fragmentation fi to the nitrate group with loss of the heaviest radical (R) attached in this position. For example, n-propyl nitrate gives a peak (• CFTONCb)"1" at mass 76, and 2-octyl nitrate a peak (Cf43CFI0N02)+ at mass 90. Such a peak containing the 3 oxygen atoms is convincing evidence for a nitrate, and is always accompanied by a large (NCb)4" peak at mass 46. For alkyl nitrates, the parent peak is too small to be detected, and the molecular weights of these compounds can be decided by mass spectrometry only by study of the hydrocarbon fragments which include the ion R+ formed by fragmentation (i to the nitro-group. Nitrates in which the parent ion is stabilized by the presence of an aromatic ring, for example benzyl nitrate, do however give a parent peak and thus can be identified more readily. 9.14

NITROSO DERIVATIVES AND NITROS AMINES

The spectra of nitrosomethane and nitrosobenzene have been given by Collin [373]. Nitrosobenzene gives a large parent peak and a base peak at mass 77 corresponding to loss of the nitroso group. It is thus readily identified from its mass spectrum. It is interesting that the ion of mass 51 from fragmentation of the benzene ring is much more abundant than in the spectrum of benzene itself. Nitrosomethane gives, as might be expected, peaks corresponding to (CH3)-, (NO)+ and (CH3NO)+ but in addition gives peaks at masses 90 (11%), 75 (0.4%), 60 (5%), 57 (1%), 47 (1%) and 46 (6.5%) which arise from the dimer of nitrosomethane. Although at ordinary temperatures, nitrosomethane exists entirely as the dimer, it is interesting that it can also exist in this form in appre¬ ciable concentration in the vapour phase at 10“4 mm mercury. This is the only case published which has shown from mass spectra the presence of dimer in the vapour phase.

410

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

A series of compounds (R)2 N—N=0 has been studied by Collin [373], in which R had the composition methyl, ethyl, n-propyl, isopropyl and n-butyl. The spectra show some interesting similarities with those of the secondary amines, but the general form of the spectra is very different. The height of the parent peaks is much greater than in either amines or nitrites, this being partially a result of the strength of the N-NO bond. Even for di-n-butylnitrosamine, the parent peak (mass 158) is 14% of the height of the base peak. The presence of nitrogen can be detected by re-arrangement peaks at mass 18 due to (NhU)"1" similar to those formed in the spectra of secondary amines and by the presence of a peak at mass 30. Diethylnitrosamine gives a large peak at mass 44 and the propylnitrosamines a peak at mass 58 which also contain nitrogen. Large peaks (often the base peak) occur in all spectra due to R+. The composition of many of the other peaks in the mass spectra is uncertain and could only be decided by accurate mass measurement. For example, all the spectra (except that of the dimethyl derivative) show a prominent peak at a mass corresponding to loss of mass 17 from the parent ion; di-n-butylnitrosamine shows a large peak at mass 84 and so on.

9.15

SULPHUR COMPOUNDS

The presence of a sulphur atom in an organic molecule can readily be recog¬ nized from the abundant 34S isotope, which gives a clearly recognizable peak at two mass numbers above the parent mass (see Chapter 8) and often also at 2 masses above abundant fragment ions. Due to its large mass defect, the presence of sulphur can be readily deduced from accurate mass measurements. Sulphur compounds occur in homologous series the numbers of which have molecular weights 4 mass units higher than the corresponding hydrocarbon having the same number of rings and double bonds in its structure, and are often recogniz¬ able from this fact. Thus, methyl mercaptan has the molecular weight of 48, the only other volatile organic compounds which also have this molecular weight being methylene glycol and methyl hydroperoxide. The comparatively large atomic weight of sulphur generally means that a large mass interval of value unusual in C, H, N, O compounds will occur between groups of peaks some¬ where in the spectra and give further evidence of the presence of this element. The similar chemical properties of oxygen and sulphur are reflected in the similar fragmentation schemes followed by their compounds of corresponding structure. Re-arrangement peaks analogous to those in oxygenated compounds occur and can be used to identify the groups present in some cases. The ion hRS4* is, for example, often observed. The simple volatile compounds of sulphur such as SO2 and CS2 are easily recognizable and identifiable simply by measurements on parent ions. There is not a great deal of published information on some classes of sulphur compound, but in other cases, the information available, though not as abundant as for hydrocarbons is adequate to demonstrate that the general rules of breakdown which have been developed above also apply to these compounds. (i) Thiophenes Correlations of mass spectra and molecular structure for thiophene and 26 alkyl derivatives have been carried out by Kinney and Cook [394, 1114]. In respect of fragmentation of alkyl side-chains, the derivatives behave very similarly

9.15

SULPHUR COMPOUNDS

411

to the alkylbenzenes. In all singly-substituted compounds the strongest peak in the mass spectrum is formed by cleavage of a bond /? to the ring; methylthiophenes lose a hydrogen atom, ethyl, isopropyl and tert-butyl derivatives lose CH3) and so on. If the side-chain is branched, or if there is more than one sidechain, the most probable fragment ion is that formed by breaking the /3-bond which leads to loss of the greatest number of carbons; sec-butyl- and methylpropylthiophene both lose C2H5 to form the strongest peak in their spectra. The only exception to this rule has been found for 2:4-dimethyl- and for 2:3:5trimethylthiophene, in which as in benzenes substituted with several methyl groups, an a-bond breaks with loss of mass 15 from the parent ion. Thiophene spectra are complicated by fragmentation of the ring which occurs most readily by breaking a double-bond and the bond on the opposite side of the sulphur atom so as to remove one ring carbon and the sulphur. This fragmentation occurs most readily when the 2- or 5-positions are methyl substituted, in which case the ion (SC2H3)+ of mass 59 is formed. This offers one method of deter¬ mining the positions of substituents on the ring. Re-arrangement peaks and other peaks not formed by cleavage of a single bond in the parent ion can again be used to give information concerning substituents. It is found that a re¬ arrangement peak at mass 85 is most abundant for mono-substituted derivatives when the carbon adjacent to the ring is tertiary (up to 15% of the most abundant ion in the spectrum) less so when this atom is secondary and is small (less than 4% of the base peak) for all other forms of substitution. The 85 peak is larger in thiophene spectra than either the 84 or 83 peaks except when a single alkyl group with unsubstituted a-carbon atom is attached to the ring, in which case the peak at mass 84 is larger than that at 85. In the case of benzene derivatives, too, the ion corresponding to the complete aromatic ring (78) is larger than 79 for a single side-chain in which the a-carbon atom is not substituted. In other cases, however, it is unusual for 79 or 78 to be as large as 77. The high probability of forming the 85 ion in thiophenes is thought to result from the attraction of hydrogen atoms in the side-chains to an ionized sulphur atom. If the positive charge is mainly located on this atom, corresponding to removal of one of the lone-pair electrons, it will exhibit some tri-valent character, in a some¬ what similar manner to an ionized oxygen atom in an organic molecule. An¬ other rearrangement peak which is useful for identificational purposes is that at (p — 31)+. Alkyl substituents would normally be expected to lose mass 29, but in isopropyl and tert-butyl derivatives, the peak at (p — 31)+ is always larger than G> —29)+. As an example of the identification of a thiophene consider a spectrum which gives a large peak (about 30% the base-peak intensity) at a mass of 140. This is thought to be a parent ion and can be seen from the strength of the isotope at mass 142 to contain a sulphur atom. Accurate mass measurement shows the formula to be C8H12S showing it to contain a total of 3 rings and double bonds. It could be an alkylthiophene with 4 carbons in side-chains and this seems the most likely explanation; no alternative formulae containing, say, unsaturated chains or multi-ring systems are given in Beilstein’s formula index. The base peak is at mass (p — CH3) suggesting the substituents to be either tert-butyl, isopropyl and methyl or two ethyl groups. There are no other large peaks in the spectrum. Absence of a prominent peak at 85, a (p — 31) very small and less than (p — 29) and only small peaks at masses 41 and 43 rule out a tert-butyl structure, and also a methylisopropyl derivative and point to a diethylthiophene.

412

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

(ii) Mercaptans The mercaptans are usually more easily identified from their mass spectra than the corresponding alcohols. This is because the parent peaks are much larger so that the molecular weight can be determined readily. The mass spectra of 25 mercaptans have been given by Levy and Stahl [1237]. For the straight-chain primary mercaptans, the parent peak lies between 4 and 100% of the base peak for carbon number up to C13, and the distinctive isotopic pattern of sulphur makes the presence of this element easily detectable. Fragmentation of C-C bonds P to the sulphur atom occurs to give the (CFtaS)"1" ion of mass 47 and also y to this atom to give the homologous ion of mass 61 in primary mercaptans. Unfortunately secondary and tertiary mercaptans also give a re-arrangement ion at mass 47 and this peak may be as large as 50% of the height of the base peak. The peak at mass 61 is weak in secondary and tertiary mercaptans, but the peak formed by fragmentation P to the sulphur atom to lose the longest alkyl chain is large. Thus, the mercaptan C2H5 • CH(SH)C2H5 gives a large peak at mass 75. The ratio of the heights of the peaks at masses 47 and 61 remains remarkably constant at a value of about 2 for all primary straight-chain mercaptans and can be used to recognize such compounds. Loss of H2S is a probable fragmentation mode in primary mercaptans to give a peak at (p — 34)+. This gives the base peak in the C4 mercaptan and the intensity falls steadily to about 5% of the base peak in the C13 mercaptan. Other peaks corresponding to loss of masses 46 and 58 also occur, but in straight-chain primary mercaptans the peak at (p — 58)+ is generally at least ten times larger than that at (p — 46)+. In secondary and tertiary mercaptans loss of SH occurs in preference to loss of SH2. As an example of the identification of a thiol, consider the spectrum [1237] in which prominent peaks occur at masses 47 (11%), 56 (3%), 70 (3%), 71 (62%), 75 (27%) and 104 (21%). The molecular weight of 104 (4 masses higher than the corresponding paraffin) and the isotopic ratios would give the formula C5H11S, and this could be confirmed, if desired, by an accurate mass measurement. The formula shows that no rings or double bonds occur, so that we are dealing with a thiol or a thio-ether. The spectra of thio-ethers are discussed below. The absence of a peak at mass 61 shows that we are not dealing with a primary straight-chain thiol, the peak at mass 71 {p — 33) suggests a secondary or tertiary thiol rather than a thio-ether and the peak at mass 75 (p — 29) suggests an ethyl group to be attached to the branched carbon. The formula is thus C2H5 • C • (SH) (C2H6) and

CH3 is thus either C2H5-C-SH or (C2H5)2-CH-SH CH3 (It is actually that of the former compound.) (iii) Thio-ethers The mass spectra of 30 thio-ethers of formula R-S—R' have been given by Levy and Stahl [1237]. The fragmentation modes are similar to those of ethers, but the parent peak intensities are much larger and this, together with the large 34S isotope makes identification easier, by the fact that the molecular formula is always easily obtained. The parent ion for (n-CvHis^S is, for example, 20% of the base-peak intensity. Thio-ethers can be distinguished from the isomeric mercaptans by the absence of peaks at masses (p — 33) or (p — 34). Fragmen¬ tation occurs P to the sulphur atom in the larger alkyl group to give the ion (R-S-CH2)+ and also a to this atom to give the ion (R')+. Double fragmen-

9.16

HALOGENATED COMPOUNDS

413

tation involving cleavage of bonds a and on either side of the sulphur atom accompanied by re-arrangement of a single hydrogen atom to give the ion +

R3

/

•S•C-

+

H

\ R4

enables information to be obtained regarding the size of groups R3 and R4 which may be attached to the a-carbon. A series of re-arrangement peaks also occurs of formulae C71FI2M+2S and CraH2W+3S the most intense always being at masses (RSH)+ and (RSH2)+ where R is the smaller radical attached to the sulphur atom. These ions which occur in the series 47, 49; 62, 63; 76, 77, etc., are similar to the oxygenated re-arrangement ions in ethers, but occur at a mass 16 higher. Peaks at masses such as these give further evidence of the presence of sulphur.

9.16

HALOGENATED COMPOUNDS

The unusual isotopic patterns associated with chlorine and bromine have already been mentioned in Chapter 8, and enable the number of these atoms in any molecular ion or fragment ion to be determined with ease. An excellent example of this has been given by McLafferty [1359] and part of the spectrum of octachloro-l:3:5-hexatriene given by him, is reproduced as Fig. 168. It is possible to distinguish (with the aid of Fig. 128, p.299) ions which contain numbers of

Fig. 168. The mass spectrum of a molecule containing eight chlorine atoms which shows how the formulae of the ions responsible for each group of peaks can be deduced from considerations of isotopic abundance ratios.

chlorine atoms between 1 and 8 in the spectrum of this compound. It should be noted that, making use of the isotopic abundances, one can distinguish between ions such as (CstT)4- and (CeFR)4"1-. Another interesting group of ions which contains two species is that centred at about mass 143 which consists of the species (C6C16)++ and (CeCh)4-, alternate peaks corresponding to the respective ion species. Similar use can be made of the abundances of the bromine isotopes in determining ionic formulae. The other two members of the halogens, fluorine and iodine are, however, anisotopic and their presence in a molecule can some¬ times be inferred by the paucity of isotopic peaks. More often, however, the presence of these atoms can be deduced from the unusual peaks observed in the spectra. Fully fluorinated saturated hydrocarbons all give a base peak at mass 69 corresponding to the ion (CF3)+ and outstanding peaks at masses 119, 169, 219,

414

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

etc., are often observed. The ions responsible are heavier by CF2 groups, of mass 50. Other prominent ions often occur at such masses as 131 and 181 corresponding to C3F5 and C4F7. At masses below 300 the ionic formulae of fragment ions in the spectrum of a fluorocarbon can be determined uniquely, simply by measuring the mass of the ion to the nearest whole number. This is illustrated in Appendix 5 in which are listed the masses and formulae of various ions formed from carbon and fluorine only, of mass up to 1000. All formulae

SPECTRUM

OF HIGH

$

§

Fig. 169. The mass spectrum of a high molecular weight mixture of fluorocarbons. The spectrum was plotted on a pen recorder in a time of under seven minutes.

listed contain both carbon and fluorine; the ratio of the number of atoms of carbon is limited in the table to ten times the number of fluorine atoms and the number of fluorine atoms is limited to that which would satisfy all valency requirements of the carbon. The list has been compiled because of the widespread use of fluorinated compounds for the superposition of reference peaks on a mass spectrum either for the purpose of carrying out an accurate mass measurement or for calibrating the whole mass scale. Fluorocarbons have the advantage for this work that they form a class of comparatively votalile com¬ pounds the members of which can be of very high molecular weight. It can be taken as a rough general rule that the substitution of hydrogen atoms by fluorine does not affect the boiling point of compounds containing only these atoms and since the ratio of the mass of CnF2n+2 to that of C«H2m+2 3.57 as n becomes large, one can expect to be able to introduce fluorocarbons of molecular weight greater than 2000 using conventional heated inlet systems. Some very heavy ions were reported by Bradt and Mohler [260] in a study of fluorinated poly¬ phenyls. For example, they mention an ion (C66F44l)+ of mass 1755. Fluoro¬ carbons are inert and are not strongly adsorbed so that they can be quickly pumped away despite their high molecular weight. Part of the mass spectrum of a high molecular weight fluorocarbon mixture is shown in Fig. 169 and the

9.16

HALOGENATED COMPOUNDS

415

masses of outstanding peaks have been marked. This spectrum provides another example of the way in which a general impression of the main features of a mass spectrum can be obtained at a high recording speed. The recorder used here had a response time of 1 second for full-scale deflection, and the complete spectrum from mass 81 to mass 1279 was plotted in a time of 63/4 minutes. The use of a halogenated compound for mass calibration purposes has the further advantage that there is usually little overlap between its spectrum and that of an organic compound which contains no halogens. As a result, molecules containing atoms of chlorine, bromine and iodine are often used for calibration purposes. In most cases, the particular compound used is determined by what is readily available. We regularly use the anaesthetic “Fluothane” of formula CF3 • CFICIBr as a reference. A fully fluorinated compound available commercially in Britain (from L. Lights Co.) is heptacosafluorotributylamine (C4Fg)3N of molecular weight 671. This can be introduced via a gas handling system at room temperature. It does not give a parent ion, but gives its heaviest peak at mass 614 corresponding to (p — F3)+. It should be emphazised that one does not necessarily require a highly pure compound for use as a reference and that even impurity peaks can be used provided that their atomic composition is known. It is as well, however, to maintain a large stock of any compound which is often used for calibration purposes and for which the composition of many of the peaks has been determined since the impurity content can be expected to change in future consignments. The list of formulae and their masses given in Appendix 5 cannot be used if atoms other than C and F are present. Nitrogen is, in fact, one of the least desirable elements to be included, because, as can be seen in Fig. 169, mass peaks in successive groups in a fluorocarbon can differ by mass 14 (F2 minus C2) and the presence of nitrogen can lead to confusion. A compound of iodine useful in mass spectrometry is mercuric iodide. It gives prominent parent peaks and the pattern of the mercury isotopes of masses 196, 198, 199, 200, 201, 202 and 204 is reproduced between masses 450 and 458 and is easily recognized. The pattern can also be used to give a check on the resolution of an analytical in¬ strument in the high mass region. The mass spectra of a series of fluorocarbons have been obtained by Mohler and his co-workers [1414, 1420]. The compounds studied included perfluoroparaffins, olefines, a perfluoro-diene and acetylene, cyclic compounds, tricyclic C8F12 and perfluoro-1-methyldecahydronaphthalene. As mentioned above, the CF3 peak is the largest in all perfluoro-paraffin spectra, sometimes accounting for more than half the ions in the spectrum. This peak can also be expected to be large in the spectrum of any compound which contains a methyl group. For example it is the base peak in the spectra of perfluoro-l-methyldecahydronaphthalene and perfluoro-methylcyclohexane, and is large in the spectra of all perfluoro-olefines (except C2F4). It is readily formed by re-arrangement, however, being of intensity 70% in the spectrum of perfluoro-cyclohexane. For identificational work, too much reliance cannot be placed on the presence of a large peak at this mass number. This is, however, a mass number which would suggest that oxte was dealing with a fluorocarbon and so is useful in this respect. Another prominent ion in many spectra is (CF)+. This is generally larger than 50% of the base peak in the case of unsaturated perfluoro-hydrocarbons. The parent peaks are much smaller in fluorocarbon spectra than in the spectra of the corresponding hydrocarbon and this is also true of other fully halogenated compounds, but the peak corresponding to loss of a single halogen atom is always prominent in the spectra of small fluorocarbon molecules.

416

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

A double-bond increases the intensity of the parent ion much more than the formation of a ring; a triple bond increases the intensity still further. The parent peak is undetectable in the majority of perfluoro-paraffins and is less than 1% of the base peak in perfluoro-cycloparaffins, being vanishingly small for cycloC6F12. This is suggested by Field and Franklin [636] to be due to the fact that for this molecule the most probable ionization process involves loss of an F~ ion, which would also account for the peak at (p — F)+ if this were formed by this ion-pair production process. A similar phenomenon could occur in many of the other cases. The intensities of the fragment ions do not show a maximum in the C3 region as do those of the hydrocarbons, but the outstanding peaks within each group are similar to those in hydrocarbon spectra, those ions with an odd number mass predominating. In other respects the spectra are very different from those of the corresponding hydrocarbons. For example, the (C3F?)+ ion is weak in iso-C5Fi2 and iso-CeFu relative to the n-fluorocarbons, which suggests no special stability for the (iso-C3Fv)+ ions. Doubly-charged ions are observed in many of the spectra. Common examples are (CF3)++, (CF2)++, (CsFa)"1""1" and (C3F3)++. All are easily identified from Appendix 5. Meta-stable transitions be¬ tween two ions both of which give large peaks in the spectra are sometimes seen. These often involve loss of CF2. Impurity peaks due to incomplete halogenation are easy to detect in the spectra of compounds such as those mentioned above. Another type of impurity peak is sometimes observed. In perfluoro compounds it appears at mass 85 and is due to the reaction of any trace of FIF or F2 present, with glass to form silicon tetrafluoride. By far the largest peak in the mass spectrum of SiF4 is at mass 85 from the ion (SiF3)+. Silicon tetrachloride can also give impurity peaks in, for example, carbon tetrachloride. The mass spectra of carbon tetrachloride and hexachloroethane have been compared with the compounds CCI2H2, CCI3H, CCI2H—CCI2H and C2CI5H by Bernstein, Semeluk and Arends [176]. In every case, a strong peak is formed by loss of a single chlorine atom. The parent peak heights are respectively 78%, 2%, 10% and 1% for the last four compounds mentioned above, and zero for the fully chlorinated compounds. Loss of hydrogen is always much less likely than loss of chlorine. An interesting parallel exists between the spectra of CF2=CF2 and CC12=CC12. For the latter compound the parent peak is the largest in the spectrum illustrating the effect of a double bond in enhancing the parent peak, but again, loss of Cl is a very likely process. Chlorinated compounds give larger parent ions than the corresponding fluorinated compounds and this trend continues as one proceeds to the still less electro-negative bromine and iodine atoms. Halogenated compounds containing only a single halogen atom have been studied by McLafferty [1357], Irsa [1029], Momigny [1427] and D’Or [517]. The spectra of five such compounds have also been given by Taylor, Brown, Young and Headington [1994]. Similarities in the changes in the spectra of many com¬ pounds on addition of a halogen atom enable McLafferty to predict qualitatively the spectra of many mono-halogenated compounds for which the non-halogenated spectrum is known. In general, chlorine, bromine and iodine have quali¬ tatively similar effects on the spectra, but the effect of fluorine is often markedly different. For alkyl compounds containing more than six carbon atoms, the spectra are very similar to those of the corresponding hydrocarbons. For smaller molecules, the parent peaks are always detectable and enable molecular formulae to be obtained, but parent ions generally form a smaller proportion of the total

9.16

HALOGENATED COMPOUNDS

417

ions present than in the corresponding hydrocarbons. Formation of a fragment ion by loss of the halogen atom from the parent ion is usually a probable process and the chance of such a process increases as one goes from Cl -> Br -» I. Loss of the halogen atom is often accompanied by loss of one or two hydrogen atoms to give peaks at (p — HX) and (p — H2X) respectively where X represents the halogen atom. The ratios of peak heights at masses (p — X), (p — HX) and (p H2X) often show regularities as the halogen atom is changed. For example these peak heights in the compounds CH3-CH(CH3)X are (relative to the base peaks in the spectra) 100 : 5 : 20 for Cl, 100 : 5 : 47 for Br and 100 : 4 : 45 for I. In the n-butyl halides on the other hand the figures are respectively 7 : 100 : 7 for Cl, 100 : 13 : 5 for Br and 100 : 1 : 5 for I, and in the ethyl halides [375] 93 : 100 : 97 for Cl, 100 : 18 : 79 for Br and 100 : 10 : 94 for I. The differences become even more pronounced as the energy of the bombarding electrons is progressively reduced from the value of 50 eV for which the above results are quoted to a nominal value of just over 9 eV. At this latter voltage, the parent ion of ethyl chloride has become undetectable, but the ratios of the peaks mentioned above are now 31 : 100 : 0 for this chlorinated compound. A meta-stable peak at mass 12.4 is the only other peak in the spectrum and corresponds to the process C2H5CI+ -> C2H4+ + HC1 showing that the C2H4+ ions are formed in a one-stage fragmentation process and that parent ions must still be present. The persistence of (C2H5)+ ions in the chloride, bromide and iodide even at the lowest electron-bombarding energies is due to the formation of this ion by the process C2H5X + e -> C2H5+ + X~ +

e

and it can be shown that this positive ion can, indeed, be formed at a potential well below the ionization potential of the parent molecule. This example illustrates the difficulty widely encountered in the examination of halogenated derivatives that an ion species formed by a process of ion pair production can become large compared to other ions in the mass spectrum as the electron-bombarding energy is reduced. Identification of the parent ions in a mixture spectrum by progressive reduction of the electron voltage is not possible in such cases. An ion of mass smaller than the molecular weight by the mass of one halogen atom generally is the last to disappear. Such ions can, of course, be recognized as due to fragments of a complete molecule by accurate mass meas¬ urement since they give an odd mass for an even number of nitrogen atoms and vice'versa. More difficult to identify correctly are those halogenated compounds which give no ions heavier than the parent mass less two halogen atoms. In such a case, such as tetrafluorohexachlorobutane (Fig. 170) the heaviest ion could be mistaken for a molecular ion, the relatively prominent peak at this mass number being attributed to the effect of a double bond. An example of a molecule which gives as the heaviest ion in its spectrum the species (p —- F3)+ is perfluorotributylamine which is mentioned above. Similar groups of peaks to those at masses (p — X), (p — HX) and (p — H2X) occur at masses (p — CH2X), (p — CH3X) and (p — CH4X) and also at (p — C2H4X), (p —- C2H5X) and (p — C2H6X) in the spectra of mono-halogenated paraffins. Loss of these fragments from halogenated paraffins leads, of course, to the hydrocarbon ions C»H2«+i, C„H2w and CwH2w-i. One might expect such ions (especially those of odd mass number) to be prominent when n = 3, 4 or 5 but it is interesting that in com-

418

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

pounds containing bromine and iodine the ion (CmH2w-i)+ is by far the largest for compounds containing 3, 4, 5 or 6 carbons breaking to lose (CHmX) where m = 2, 3 or 4. This is often but not invariably so for chlorine compounds. The peak (p — C2H4X)+ is, however, always larger than (p — C2HeX)+ when X = Br or I and usually so when X = Cl. The peak at (p — ChUX)"1" is particularly prominent in the mono-halogenated cyclohexanes, but is weak in the corre¬ sponding cyclopentane derivatives.

Fig. i/o. The heaviest ions in the mass spectrum of C4F4CU correspond to loss of two chlorine atoms from the molecular ion, and could be mistaken for molecular ions of a compound containing a ring or double bond.

A large number of partial spectra of halogenated aromatic compounds have also beeAgiven by McLafferty. They provide interesting examples of the way in which the empirical rules developed for hydrocarbons still largely apply when halogen atoms are present. If the halogen atom is directly on the ring, the peak at (p — X) is prominent but if the halogen forms part of a side-chain, the bond /? to the ring will show a high probability of breaking. Thus, the peak (p — CH2X) will be very large in all compounds in which a -CH2-CH2-X group is attached to the aromatic nucleus. If more than one side-chain occurs, one can also expect the C—C bonds /3 to the ring to break in any chain, irrespective of whether it contains the halogen atom. Again, for polymethyl substitution, one can expect loss of -CH3 to occur in the same way as it does in the absence of halogen. Nevertheless loss of halogen tends to be prominent and in favourable cases,

9.16

HAIOGENATED COMPOUNDS

419

CeH5-CH(CH3)Br where the C-Br bond is 0 to the ring the peak at U? — Br) is the base peak of the spectrum, whilst the peak (p — CH3)+ formed by the competitive fragmentation of the C-C bond 0 to the ring is insignificant. As in the case of alkyl halides, loss of X, CH2X and so on may be accompanied by peaks one and two masses smaller in mass. Such peaks tend to be prominent in compounds in which our experience might suggest more than one bond as likely to break. For example, the peak at (p — CH4C1)+ is particularly large K 40%) in the spectra of the isopropyl-chlorobenzenes. Here loss of CH3 would be

Fig. 171. The mass spectrum of an impure sample of 2-nitro-4:6-dichlorophenol. Impurity peaks can be recognized in this spectrum.

expected to give a large peak (it is in fact the base peak) and loss of Cl would also be expected to be a probable mode of breakdown (the peak at (p — Cl)+ is, in fact about 85% in the ortho-isomer, but only 8% in the para-disubstituted ben¬ zene). Now loss of both CH3 and Cl from a parent ion would lead to a fragment ion of even mass. The increased stability of the ion formed by loss of a further hydrogen, together with the added stability which would be obtained if this atom combined with either CH3 or Cl to form a molecule, increases the prob¬ ability of loss of CH4CI. In a similar fashion C2H5 • CH(CH3) • C6H4 • Cl gives a large peak at (p — C2HeCl)+, whilst in 2:5-dimethylchlorobenzene the (p — H2C1)+ peak is prominent. Such peaks are also observed in aromatic halogenated compounds containing other groups, for example in the spectrum of 2-nitro-4:6-dichlorophenol which breaks with loss of (HNO2CI). Let us con¬ sider the partial spectrum of impure 2-nitro-4:6-dichlorophenol shown in Fig. 171 and attempt to identify it from this spectrum. The molecular formula of

420

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

such a compound cannot be completely deduced from the mass spectrum due to the fact that the isomers give such similar spectra, but all the information necessary to deduce which groups are present can be quickly obtained. The pattern of peaks at the parent region shows the presence of 2 chlorine atoms (9:6:1 ratio), and the odd molecular weight shows that nitrogen is present. Loss of 17, 30 and 46 to give strong fragment ions indicates a nitro-group (2 chlorines are still present in each fragment) and the fact that 17 rather than 16 is the smallest unit lost suggests hydrogen bonding to a group attached to an

10

MULTIPLY HEIGHTS OF ALL PEAKS BY 10

11111 60

100

120

140

—i—r

160

160

200

r 220

i

i

i ■~r i

i

240

260

260

i

r*1— 300

Fig. 172. The mass spectrum of hexadecyl bromide. The spectrum at high mass numbers has been plotted at high sensitivity to enable the small peaks to be seen.

adjacent carbon. The formula is thus (NO2CI2 + 91), i.e. there is tri-substitution on a nucleus of mass 94 (phenol). Loss of 17 (without loss of 16) indicates that the nitro-group and hydroxy-group are adjacent. The prominent peaks at masses 162, 164, 166 which contain 2 chlorine atoms are not formed from the same compound. Loss of mass 45 without intermediate fragments between this and the parent would be extremely unlikely and it would seem probable that these peaks are the parent ions of another compound differing in that the nitro-group is replaced by a hydrogen atom. Aromatic halogenated compounds are generally extremely easy to identify (except for the relative positioning of substituents on the ring). The large parent peak makes elucidation of the molecular formula by accurate mass and abundance measurements an easy matter and one is seldom confused by re-arrangement peaks. The spectrum of another halogenated compound is shown in Fig. 172. The pattern at low mass numbers with prominent (Csbb)"1" (base peak) and (C4Hg)+ peaks suggest a paraffinic structure. At higher masses, pairs of peaks differing in

9.17

SILICON COMPOUNDS

421

mass by 2 units and of almost equal intensity suggest a single bromine atom. These pairs of peaks extend in homologous series up to (Ci6H33Br)+ with a gap at (Ci5H3iBr)+. (The heights of peaks in this mass region are of the order 0.01% and thus too small to appear on the chart.) This behaviour suggests Ci6H33Br to be the molecular formula; loss of CH3 would not be expected from a long-chain compound unless it was substituted with methyl side-chains, and the smallness of the parent peak is to be expected for a halogenated compound. Even if the sensitivity of the equipment used were too low to pinpoint the molecular weight in this way it could be found by seeking a prominent heavy ion which does not contain the bromine atom, since the (p ■— Br)+ peak would be expected to be large. The peaks at mass 225 and 224 due to (p —- Br)+ and (p — HBr)+ respec¬ tively give the mass of the hydrocarbon part of the molecule and lead to the identification of the compound giving the spectrum as a hexadecyl bromide. It is interesting to observe that the homologous series of peaks (CreH2WBr)+ reaches its maximum height with the (C^gBr)* ions.

9.17

SILICON COMPOUNDS

The mass spectra of trimethylsilyl ethers [1822] have already been mentioned in connection with the analysis of alcohols. The spectra of these compounds are interesting in that they give very large re-arrangement peaks. One series of such peaks at masses 31, 45 and 59 haverhe formulae (SiH3)+, (SiCHs)+and (SiC2H7)+. They are formed from the Si(CH3)3 end of the molecule by elimination of methyl groups with replacement of a single hydrogen atom for every methyl lost. A series also occurs at masses 47, 61 and 75 formed by the addition of the oxygen atom to the above ions. Similar ions to those of the first series have been found in the spectra of methylsiloxanes [484] and in the spectrum of silicon tetramethyl [483]. A series of very interesting re-arrangement peaks occurs in the spectra of trimethylsilyl aromatic ethers [1822]. The mass spectrum of the ether C6H5 • O • Si(CH3)3, for example contains peaks in its mass spectrum due to loss of masses, 31 and 33 from the parent ion. By means of accurate mass measurements, it can be shown that these peaks are due to the ions (CvH70Si)+, and (CsHgSi)"1-. Ions at masses 43, 44, 45, 46 and 47 are due to (SiCH3)+, (SiCH4)+, the doublet (SiOH)+ and (SiCH5)+, (SiCHe)+, and (SiOH3)+. Other silicon compounds which have been studied by mass spectrometry include the methylchlorosilanes [2205], and other alkyl- and aryl-chlorosilanes [1907] including phenyltrichlorosilane, phenylmethyldichlorosilane and com¬ pounds of the type R3SiCl, RgSiCB and RSiCl3 where R is CH3 or C2H5. In compounds of the type R2SiCl2, cleavage of an R—Si bond is twice as probable as cleavage of an Si-Cl bond; in R~Si-Cl3 compounds cleavage of Si-Cl is some¬ what more likely but in R3SiCl compounds fragmentation with loss of R predo¬ minates. Silicon compounds can usually be readily identified from the abundance of the heavy isotopes of 29Si and 30Si both of which occur with a probability of several percent. It has already been mentioned that a peak at mass 85 due to (SiF3)+ ions is often seen when fluorinated compounds being examined react with the glass walls of the sample container; its identity can be established from the isotopes at masses 86 and 87. The widespread use of silicones in vacuum work and in gas-liquid chromatography means that silicon-containing peaks often appear in the background and are often difficult to remove. Such peaks can

422

CORRELATIONS OF MOLECULAR STRUCTURE AND MASS SPECTRA

9

be used as mass references provided that their composition is known. Particularly common is a peak at mass 207 due to (CH3)2

I

(CH3)2Si

1/

Sid

\Q/

xch3__

together with higher members of the series formed by addition of [-0-Si(CH3)2-] groups at masses 281, 355, and so on. 9.18

OTHER COMPOUNDS

The references given in the preceding sections are not comprehensive, and no attempt has been made to discuss the wealth of information contained in the list of mass spectra published by the American Petroleum Institute [45]. This is an essential document for all who are engaged in obtaining mass spectra of organic compounds and it has been assumed that the reader is familiar with its contents. However, there is no discussion of the spectra in this compilation, and the litera¬ ture references given above and in Chapters 5 and 10 (when the interest is in the method of sample introduction or the use made of the mass spectrum) will be found useful in that they generally include such a discussion, or some other information such as the changes which occur as the bombarding energy is reduced. Details of the spectra of simple hydrocarbon molecules such as methane [503] propane and propene [1940], the butenes [516, 1258], dialkyl-cyclopentanes [1468], octanes [238], nonanes [1417],cis^ and trans-decahydronapththalene [1415] and other groups of molecules are given as the primary content in some papers, but other spectra are more difficult to find when they are contained in a paper concerned with a bond-dissociation energy [1785] (this particular paper includes information on toluene, ethylbenzene and bibenzyl) or the study of diffusion pump fluids [936] (this paper includes information on hexadecyl substituted naphthalene and di-n-octyl phthalate). Other spectra of simple molecules con¬ taining only C, H and O atoms which have been given include cyclic hydro¬ carbons and ethers [937], alcohols [707], acetone and acetic acid [1075], carbon monoxide [1856], hydrogen peroxide [1106] and measurements on the unstable molecules ozone [866], glyoxal, methylglyoxal and diacetyl [1671] have been given. The mass spectra of ketene monomer and dimer have been considered in relation to dimer structure [1256], the spectra of methyl methacrylate, methyl a-hydroxy-isobutyrate and methacrylic acid [58] have been obtained. The fragmentation processes occurring in the mass spectra of /?-propio-, y-butyro-, y-valero-, y-crotono-, ^-angelica- and a-angelica-lactones have been discussed [702]. Nitrogen-containing compounds whose mass spectra have been discussed include hydrogen cyanide and cyanogen [1945], ethylamine [376], nitrobenzene and the chloronitrobenzenes [1429], nitrogen peroxide, nitric oxide and nitrous oxide [380, 690], hydrazoic acid (HN3) and methyl azide (CH3N3) [681], hydrazine and methylhydrazines [192, 493], and the very unstable s-triazine (C3H3N3) [1071], triazene (N3H3), tetrazene (N4H4) [664] and di-imide (N2H2) [663], nitramines and cyanamides [249], nitric acid [693] and nitrate and nitrite esters [249]. Many spectra of halogenated compounds have been given. These include the chlorinated [510, 1181] and fluorinated [1272] methanes and some bromine

9.19

GENERAL REMARKS

423

derivatives [804], n-propyl and text-butyl chlorides [1939]) some chloroethylenes [61], hexafluorobenzene [490] and trifluoromethyl halides [489]. A variety of halogen fluorides including IF5, BrFs, BrF3 and CIF3 have been studied [1030]; the mass spectra of cyanogen chloride [1945], sulphuryl fluoride [1675], oxygen difluoride (OF2) [492] and perchloryl fluoride (O3CIF) [491] are also known. Spectra of fluoropicrin and tetrafluorodinitroethane have been reported [249]. Some of the volatile compounds of silicon have been mentioned above. Other elements, too, form volatile compounds which have been studied mass spectrometrically. The hydrides of silicon [1638] and also selenium, phosphorus and germanium [1477] as well as several boranes [1089] including diborane [472, 1160, 1304, 1518], pentaborane [474], hexaborane [740], an octaborane thought to be BgHi2 [18x7] and nona- and decaboranes [1520] have been given. Alkyl derivatives whose spectra have been reported include tetramethyl carbon, silicon, germanium, tin and lead [483], trimethyl boron [2036], and higher alkyl deriv¬ atives of mercury [481] and lead [1641]. The mass spectrum of dimethylphosphinoborine trimer has already been mentioned in Chapter 7 with regard to the very prominent re-arrangement peaks which it exhibits [648]. Some alkylborons and also acetyl and halogenated derivatives of this element have been recorded [1218]. Boron trifluoride [1563] and trichloride [1311] have been studied in other investigations. Other halogenated compounds volatile enough for examination by mass spectrometry include those of phosphorus [1094, 1095], arsenic [805], titanium [945] and uranium [749]. Organic phosphates [1640] and chlorophosphates [264] give mass spectra in conventional sample systems and so do carbonyls (Fe, W, and Ni have been given) [119], cyclopentadienides (manganese, magnesium and sodium have been given), and the spectra of biscyclopentadienyl iron, cobalt, nickel, vanadium, chromium, rhenium and ruthenium have also been recorded [704, 2184]. Many inorganic compounds are stable to heat and can be introduced into a mass spectrometer. Most such compounds which have been examined are discussed in Chapter 10 in a descrip¬ tion of the investigations into their latent heats or state of aggregation in the vapour phase. The presence of an infrequently encountered element in a sample submitted for identification will usually be suspected from the unusual masses and mass differences between major peaks in the mass spectrum or by the unusual isotopic abundances which are apparent. If the presence of, say, silicon is shown by the isotopes, subtraction of the mass of the appropriate number of silicon atoms from the accurate mass of the peak will enable the formula of the remaining part of the ion to be obtained from Appendix I.

9.19

GENERAL REMARKS

Information concerning the structures of quite complicated organic molecules [ 1753] can be seen to be obtainable by mass spectrometry. In preliminary examina¬ tions of unknown compounds such information as the molecular weight will be valuable and can be obtained readily by this technique [1864]. Many examples of peaks characteristic of the presence of certain groups within a molecule [59] have been given in the preceding pages, and as one’s experience in the application of mass spectrometry grows it becomes easier to recognize such peaks and to interpret them correctly. One’s memory can, of course, be supplemented in this work by the filing of the accumulated spectral data on cards [1366, 2206] similar to those used for example in infra-red spectroscopy [1168] but the total useful information on each spectrum is so large that no completely suitable system for filing mass spectra has yet been devised.

CHAPTER 10

OTHER APPLICATIONS OF MASS SPECTROMETRY 10.1

MASS SPECTROMETRY IN THE PETROLEUM INDUSTRY: QUANTITATIVE ANALYSIS

(i) Introduction The quantitative analysis of mixtures has been mainly performed on hydro¬ carbons. At the present time, many hundreds of mass spectrometers are in use in the petroleum industry for the study of a wide range of materials from some which are volatile enough to be gases at ordinary temperatures, to those such as lubricating oils and waxes which can only be examined at high temperatures. Crude oil, the raw material of the petroleum industry, contains such atoms as nitrogen, oxygen and sulphur in addition to those of carbon and hydrogen, but it is with complex hydrocarbon mixtures which form so large a part of the products of this industry that we shall be mainly concerned here. It has already been mentioned that the first major industrial application of mass spectrometry was to the quantitative analysis of volatile mixtures of hydrocarbons. It is mainly due to the success achieved in this field [278, 2204] that the rapid growth of the number of machines used in industry took place. The first samples analysed contained hydrocarbons with up to 4 carbon atoms. The constituents of these mixtures were well established, and were all available in a fairly pure state. Methods of analysis were available which were long and tedious, but which gave analyses of satisfactory accuracy. The mass spectrometer enabled this accuracy to be maintained and the speed of analysis to be greatly increased. The mass spectrometer has also found wide use within refineries [946] where it can be used in continuous analysers [1212] and in process control [420]. (ii) Simple mixtures In this type of analysis, it is not necessary to correlate structure and mass spectrum, but the method does depend on the fact that the mass spectrum of any organic chemical is unique and analyses are generally easier to perform, and the results more accurate when the spectra of the individual constituents of the mixture to be analysed exhibit gross differences. Before analyses can be performed, it is necessary that the spectra of pure samples of all the components to be analysed be available and that the relative sensitivities of detection of each com¬ ponent be known. Spectra are usually tabulated after normalization which makes the most intense peak in any spectrum (the so-called “base peak”) of height 100 units. The height of a peak at any mass number is then referred to as the “crack¬ ing pattern” at this mass number. The sensitivity is usually either given in terms of the height of the most prominent peak per unit pressure, or the relative intensity of this peak to that of the corresponding peak in a standard compound.

10.1

MASS SPECTROMETRY IN THE PETROLEUM INDUSTRY

425

For-volatile materials, n-butane is often used as the standard of reference and at higher masses hexadecane. Two methods of analysis are used [1059] and are best illustrated by an example. Suppose, first that a simple two-component mixture is to be analysed, and that major peaks characteristic of each component occur in the mixture spectrum. That is to say, the height of these peaks is due entirely to the presence of one component and contains no contribution from the pres¬ ence of the other. In the first method of analysis, the height of each peak is measured and divided by the appropriate sensitivity factor to give its partial pressure. The sum of the partial pressures determined in this way should equal

Fig. 173. The mass spectra of n-butane and isobutane.

the total sample pressure. A discrepancy would indicate either the presence of an extra component or a change in the sensitivity of detection. In the second method, the mixture pressure need not be known and the ratios of the amounts of the components present is determined from the peak heights and sensitivities of each component relative to n-butane. No check on sensitivity changes or the presence of unsuspected components is possible when this final pressure measurement is dispensed with. The determination of the relative partial pressures (or molar amounts) of the components present depends for its accuracy on several assumptions. These are: (a) That the mass spectrum of any component is unaffected by the presence of

426

OTHER APPLICATIONS OF MASS SPECTROMETRY

10

another component, and that when the spectral peaks superimpose at any mass number that the intensities due to the various components are linearly additive. (b) That the cracking patterns and sensitivities of the various components have not changed since the pure components were examined. (c) That the ion beam intensity for any component is proportional to the partial pressure of that component in the sample handling system. It should be noted that only two peaks need to be measured for the analysis of a two-component mixture so that there is usually a wide choice as to which peaks to use in the determination. Often, however, it is not possible to choose a suitable pair of peaks unaffected by interference from the presence of the other component. Consider the choice of peaks when analysing a mixture of n-butane and isobutane. The spectra of these two substances are shown plotted in dia¬ grammatic form in Fig. 173. The peaks chosen should be intense peaks, so as to keep the sensitivity as high as possible, and to reduce the error from the possible presence of a background peak or small impurity peak at the masses to be measur¬ ed. The ratio of the peaks chosen should be sensitive to small changes in the ratio of the two components, and masses chosen should not be those likely to give a variable cracking pattern. Cracking pattern variations are discussed below. Regarding the other points it can be seen that mass 58 in the spectrum of n-butane and 57 in the spectrum of isobutane would be a suitable pair of peaks for the analysis since they are relatively strong and their ratio would be sensitive to small compositional changes. Peaks at masses 50 and 51 would, on the other hand, be completely unsuitable, since they are weak peaks, and their ratio, being almost identical in each spectrum, would be insensitive to the amount of either component present. (iii) Multi'Component mixtures In multi-component mixtures, the choice of peaks on which to base the analysis becomes more difficult, although for most of the mixtures of light hydro¬ carbons the analytical procedure is well established. The system of analysis used may be generalized for such mixtures, for if the mixture peak height at a mass m is given by Pm, we have that Pm = HiCmjSjpj where Cmj is the cracking pattern of the jth component at mass m, Sj the sensitivity of detection of the jth component referred to the base peak of this component and p] the partial pressure of the jth component in the sample. To find the values of pj in an n-component mixture from the known values of Cmj and Sj, it is necessary to choose n peaks for measurements of Pm. It can be appreciated that the effort necessary to solve the set of simultaneous equations appropriate to any mixture may far outweigh that involved in plotting the spectrum, and thus that careful choice of the masses to be measured with a view to simplifying the calculation as far as possible as well as maintaining the accuracy of the final answer is well worth while. It should be noted that for every component which gives a usable peak at a mass number to which none of the other components contribute, the number of linear simul¬ taneous equations to be solved is reduced by one. Such a component can, in other words be determined from this single measurement no matter how many other components are present. Choosing the peaks so that the number of inter¬ fering components at each mass number is minimized is generally found to be helpful both in decreasing the effort involved in the inversion of the relevant matrix and in increasing the accuracy. Often, the only interference at a particular mass number may arise from peaks containing heavy isotopes. A considerable

10.1

MASS SPECTROMETRY IN THE PETROLEUM INDUSTRY

427

saving in calculating time can often be achieved by correcting for the presence of these heavy isotopes before beginning the calculation. An example of the analysis of a hydrocarbon mixture by mass spectrometry was first given by Hoover and Washburn [974, 975] and was soon followed by further papers [267, 2129, 2130] which demonstrated the order of accuracy attainable and that the method was not restricted to hydrocarbons. The accuracy depends, of course, on the differences in the mass spectra of the components which make up the sample, and will thus be least for those isomers for which the mass spectra are very similar. It is sometimes possible to analyse such isomers as a group more accurately than as individual isomers. For example, the butene content of a mixture can be determined more accurately than the respective amounts of 1- and 2-butene present. Fortunately, not all isomers are as difficult to distinguish and it is generally true that all isomers of saturated hydrocarbons can readily be distinguished in mixtures of light hydrocarbons. Even with the techniques available in 1943, Hoover and Washburn were able to analyse a 9-component mixture of C5 and C6 hydrocarbons in 4j man hours, which in¬ cluded only about £ hour of instrument time. An alternative method of analysis of this sample by refractive index measurements on a large number of small fractions from a 100-plate fractionating column required a fractionating time of 240 hours. It is now common to analyse even more complicated mixtures con¬ taining 25 or more constituents. Instrument time is continually being reduced both by more rapid change-over of samples, and by programming the mass spectrometer so that it measures only those peaks needed in the subsequent analysis. More and more powerful calculating machines are helping to keep pace with the increasing rate at which data is being produced. Co-operative analyses of standard samples containing Ci to C4 hydrocarbons have been carried out by groups of laboratories both in the United States [1837] and in Europe [231] and have helped to establish the attainable accuracy in such analyses. Comparable accuracy was achieved in both tests, and was independent of whether 180° or 90° sector instrument ? were employed. The results of one of these co-operative analyses was compared with chemical analysis of the same sample by volumetric methods. It was concluded [1836] that the mass spectrom¬ eter gave, in general, a better reproducibility than that obtained from the chemical methods. For components present to more than 10 mole % it was found that the mean of all the mass spectrometric determinations of abundance lay within 0.1 to 0.2 mole % of the true value, the standard deviation of in¬ dividual determinations being about 0.4 mole %. The above comparison was carried out on a sample simple enough to be analysed by the chemical technique. As more complicated samples are considered the advantages of mass spectro¬ metric analysis increase. In particular gas mixtures [1137] (for example, those containing carbon dioxide) the errors obtained in mass spectrometric analysis are greater and chemical methods may be preferable for such components. The attainable accuracies are illustrated for the case of a sample of carburetted water gas [1839, 1840, 2094]. , When the techniques of distillation, elution chromatography, liquid thermal diffusion and other physical analytical methods such as infra-red and ultra-violet absorption analysis are combined with mass spectrometry, an extremely powerful combination results. Using a combination of techniques Melpo er, Brown, Young and Headington [1378] studied the composition of a naphtha produced by fluid catalytic cracking. They present analytical data for 152 hydro¬ carbons and groups of hydrocarbons including 20 individual olefines, the vast

428

OTHER APPLICATIONS OF MASS SPECTROMETRY

10

majority of the components containing 8 or fewer carbon atoms. Another technique which is becoming very prominent in the analysis of mixtures of compounds all of known type or formula is gas chromatography. The com¬ bination of this and other techniques with mass spectrometry is discussed in Chapter 5. (iv) Limitations to the attainable accuracy The accuracy achieved in an analysis may sometimes be limited by other than mass spectrometric factors. For example, in the analysis of gases many workers use a mercury manometer of only a few mm diameter as the basis of their pressure measurement and expand a known volume of gas at the measured pressure into a reservoir for examination. If, say, the manometer pressure is read to ±0.1 mm, then the pressure used must be at least 10 cm to achieve an accuracy in this stage of ± 0.1%. Other sample inlet systems are arranged so that a known volume of gas at a predetermined pressure can be introduced into the equipment whenever desired. By this method equimolar fractions of any substance can be measured into the sample system. The amount of sample introduced is, however, affected by the ambient temperature, and an error of almost j% for each °C change in this temperature can arise from this cause. Again, such factors as the linearity of the recording system are important when accurate measurements are to be performed. When all such sources of error have been eliminated there still remain errors characteristic of the mass spectrom¬ eter and these are considered below. (v) Variation of cracking pattern The mass spectrometer is unique among spectroscopic instruments in that the information which it gives about a molecule is obtained not at the moment of interaction of the molecule with the energy supplied to it but at a comparatively long time of the order 10-5 second later. Generally, the energy is transferred to the molecule via a bombarding electron. An electron energy of the order 10-15 eV is sufficient to ionize any organic molecule, but energies of this magnitude are seldom used in quantitative analysis unless extremely complicated mixtures are being examined, as is explained below. As the electron energy is increased above these values, the total probability of ion formation increases steadily and the sensitivity of the instrument is thus enhanced. At the same time, the number of possible reactions which the ionized parent molecule can undergo increases, and the cracking pattern changes markedly as more peaks appear in the mass spectrum and the relative heights of those already present alter. These changes in cracking pattern continue to a certain extent for all higher electron energies but have become small by the time energies of 50-100 eV are reached. Since the sensitivity is quite high at these values and since the large number of peaks produced by electrons of this energy in the spectrum of a hydrocarbon offers wide choice in peaks which can be used for analytical purposes, it has become usual to work with electrons of about this energy. In particular applications there are often advantages in using much lower electron energies. The differences between the mass spectra of isomers are often enhanced at low electron energies, making analysis of mixtures such as cis- and trans-1:2-dialkyl derivatives of cyclopentane [1432] possible. In the analysis of complex mixtures, interference between compounds is minimized and the analysis often simplified by the use of low electron energies [634, 967, 1285]. In cases where it is possible to adjust the electron energy to be larger than the

10.1

MASS SPECTROMETRY IN THE PETROLEUM INDUSTRY

429

ionization potential but smaller than the first appearance potential of a fragment ion, each peak in the spectrum would represent the presence of isomers of a given compound type and molecular weight, and an analysis could be carried out if calibration spectra were available without complicated matrix calculations necessitated by fragmentation interference effects. The method has been used to analyse mixtures of isotopically enriched paraffins [1944], to analyse olefines and aromatics in the presence of paraffins, taking advantage of the fact that the ionization potentials of paraffins are about a volt higher than those of unsaturated hydrocarbons [634], and to analyse complex mixtures of unsaturated, high molecular weight compounds [1285]. Any changes in conditions which alter the effective electron energy, such as changes in work function of the filament, or formation of insulating layers in the source or which change the time of flight, such as changes in the potentials within the ionization chamber, will produce changes in the cracking patterns. Some mass peaks will be more affected than others by such changes, since many of the ion species are formed by multi-stage processes and their intensities will depend more critically on the energy of the bombarding electrons than those of other ions. Not all ions formed in the ion chamber have an equal chance of collection and changes in source potentials which cause the position of the electron beam within the ionization chamber to change can cause changes in relative peak intensities in the spectrum. The instrument also discriminates strongly against ions formed, with initial kinetic energy. Thus any change affecting this energy will also react strongly on the intensities of these particular ions. A most im¬ portant cause of changes in ion kinetic energies is change of source temperature. This temperature determines that of the gas being analysed and its effect on mass spectra was first measured by Fox and Hippie [673] and later by Stevenson [1943] and by Reese, Dibeler and Mohler [1673]. The specific intensities of all ions decrease with increasing temperature [1983] and that of the “parent” or molecular ion decreases more rapidly than do those of fragment ions. Due to changes in source temperature, the number of molecules in the ion chamber will vary as if molecular flow is assumed through the mass spectrometer. The discrimination losses will increase as T£ due to the changes of kinetic energy concomitant with temperature changes. Stevenson states that the ion current i at any temperature is related to that (i0) at a standard temperature by the relationship i = i0

T-i(l—aTi)

(83)

where a depends on the source design and operating conditions. This equation represents the changes in intensity which have been observed with simple molecules such as those of the rare gases. In more complicated molecules, the increase in vibrational energy due to an increase in temperature results in dissociation of a larger fraction of the molecular ions initially formed in the ion chamber before the ions can reach the collector so that molecular ions form a smaller proportion of all ions in the spectrum. The largest effects so far reported have been found in the mass spectra of branched paraffins. The parent ion of 2:2:3-trimethylpentane is five times as large at 175°C as at 225°C. Temperature changes of this order may occur over a period of several months due to aging of the filament, and changes of the order 20°C can occur in a few hours if the surface condition of the filament is changed by the introduction of, say, an oxygen-containing compound so that a different filament heating wattage is necessary to maintain the electron emission. It may, sometimes, be necessary to run with the tube continuously baked and such change in ambient temperature

430

OTHER APPLICATIONS OF MASS SPECTROMETRY

10

will markedly alter the source temperature. It is usual, therefore, in quantitative analytical work to thermostat the ion chamber and a heater is provided within the vacuum system for this purpose in commercial machines. Eberhardt and Osberghaus [561] carried out a study of the effect of ionization chamber temperature [1981] using a system which could be heated up to 1000°C. With it, they investigated the spectra of the three pentanes, the five hexanes and n-heptane. The walls of the ionization chamber were of gold. Their results show the distribution of the various fragment ions in the mass spectra as functions of temperature. In other work which points the way to the study of the effects of surface contamination as well of surface reactions [1544], the walls were covered with an Al-Si cracking catalyst and it was possible to measure the catalysed methane formation as a function of temperature. The time of flight of an ion is the time which elapses between ionization and determination of the cracking pattern corresponding to this ion. In sector mass spectrometers using magnetic scanning, all ions receive the same energy of acceleration and the time taken by heavy ions to reach the collector is longer than that for light ions and both these times depend on the accelerating voltage used to record a particular spectrum. With voltage scanning, the differences in time of flight between heavy and light ions are further increased. A considerable percentage of the total time of flight of an ion is spent within the ionization chamber, where the ion is under the action of weak fields and is moving relatively slowly. The potential of the ion repeller electrode within the ionization chamber can thus control the time of flight to a large extent, especially when it is slightly negative with respect to the walls of the ionization chamber and the field due to the “repeller” cancels the stray field which penetrates from the main accelerating field into the ionization chamber. Under such conditions, the effect of the ribbon of electrons in the ion chamber is to produce a potential “well” which can cause the positive ions to remain within the ionization chamber for a period longer than the remainder of their time of flight. It can be seen that any change which increases the times of flight of the various ions, will cause the cracking pattern to change since most of the ions in mass spectra of organic molecules are formed by multi-stage ion reactions and the spectrum is not independent of time. As well as the more obvious causes given above for variation of the time of flight, one must include such factors as the formation of insulating layers on electrodes in the ion chamber, which, by changing the position of say, the elec¬ tron beam, and hence the region in which ions are formed may also affect the time of flight. With careful reproduction of the operating conditions used by other labora¬ tories it is possible to reproduce their cracking patterns quite closely, especially when the same commercial model mass spectrometer is used. This enables fairly accurate analyses to be carried out for compounds which are not available in a pure condition in one’s own laboratory but whose spectra have been tabulated, for example, in the American Petroleum Institute Catalogue of Mass Spectral Data [45]. Significant differences in cracking patterns will, however, result if any of the operating conditions are modified. The discriminations attendant upon the scanning of a mass spectrum by variation of the accelerating voltage would differ so markedly from those in a magnetically-scanned mass spectrum as to vitiate comparisons of spectra produced in these two ways. Spectra of the same compounds obtained with 60°, 90° and 180° spectrometers have been compared by Caldecourt [312]. Major differences are found between spectra obtained on the three instruments. Compared with the 180° spectrometer, the 90° instru-

10.1

MASS SPECTROMETRY IN THE PETROLEUM INDUSTRY

431

ment was operated with about one tenth the magnetic field in the source region, less than one tenth of the ion repeller voltage and (due to the use of a metastable ion suppressor) with a much smaller ion energy in the collector region. In the 180 ° spectrometers, a fixed magnetic field was used and the ion accelerating voltage was varied between 400 and 3800 volts, whilst in the case of the 90° instruments, magnetic scanning was used, together with a fixed accelerating voltage of 450 volts. At low masses the 180° spectrometer showed much the greater sensitivity (higher accelerating voltage). Also, since the 90° instrument was adjusted to give maximum sensitivity in the mass 100 region, and the mag' netic field in the ion source is not varied when scanning a spectrum the mass discrimination will be particularly marked at low masses. The comparison curves showing the relative cracking patterns for the different spectrometers show variations depending on the type of compound being compared. Changing such parameters as repeller voltage and the energy of arrival of ions at the col¬ lector also changes the cracking pattern curves and certain types of compound, for example halogenated compounds, are particularly sensitive to such changes. Caldecourt concludes that it is not possible to use spectra obtained on one type of instrument for quantitative analysis using a different type of instrument, except possibly over a limited mass range. The peak height versus ion repeller voltage curves for our double-focussing mass spectrometer are shown in Fig. 33 (p. 105). These curves, which are typical of the Nier type source show, for small sam¬ ple sizes, a sharp maximum at a low value of ion repeller voltage and a subsidiary maximum at about +4 volts (with an accelerating voltage of 4 kV). As the amount of sample is increased, the height of the first maximum “saturates” but the second maximum increases linearly with amount of sample to much higher sample pressure. Thus, if too much sample is introduced, even of a single component, non-linearity may result. The fact that this non-linearity does not arise in the recording system can be shown by introducing small amounts of a second component in quantities proportional to that of the main component. (vi) Interference of one sample with another Under certain conditions of operation of a mass spectrometer it is observed that the sensitivity obtained for a particular compound is changed by the intro¬ duction of a second substance [1988]. Usually, the presence of the second material reduces the sensitivity but it sometimes happens that the sensitivity is increased by the interference. In the majority of cases, the relative intensities of the peaks in the affected mass spectrum do not change and the intensity of all peaks is depressed to the same degree. If the cracking pattern itself is changed in small details this generally means that the operating pressure in the source is too great and that these changes are being caused by collision between molecules or ions of the materials examined. Such effects can easily be overcome by reduction of the pressure. The effect is widely known in the analysis of hydrocarbon mixtures but can occur with any pair of compounds. It has been found [979] that addition of a component can even change the measured values of the isotopic abundance ratios of the main component and that the effect is not one of beam spreading due to the increased pressure. Barnard [123] considered some of the reasons for interference and showed curves of the effect of the admission of isobutane into a source containing krypton. Admission of the butane changed the filament temperature, total emission from the filament and the temperature of the source and caused an 8% fall in the intensity of the krypton beam over a period of 30 minutes. Such interference effects can be minimized by thorough pre-conditioning

432

OTHER APPLICATIONS OF MASS SPECTROMETRY

10

of the filament. This treatment is carried out by operating the filament for several hours in a high pressure of an appropriate compound and is essential for the attainment of stable operating conditions. It had been known for many years that the admission of large pressures of unsaturated hydrocarbons such as butene and acetylene into the mass spectrometer could lead to a desirable increase in stability of operation and manufacturers of commercial instruments issued detailed instructions as to the way in which such treatment should be carried out when a new filament was inserted, since it was well known that the benefits could be nullified by over-conditioning. A detailed explanation of the changes taking place in the chemical composition of the tungsten filament during the condition¬ ing process was first given by Sharkey [1821] and is discussed in Chapter 4. The use of rhenium as an electron emitter (see Chapter 4) overcomes the need for filament conditioning. Other causes of interference between samples leading to non-additivity are surface and space charges in the ionizing region. These have been discussed by Brubaker [283]. In order that the potentials and gradients in the ionizing region shall be relatively independent of such charges, the electron current must be kept small and the applied gradients must be large. Unfortunately, this leads to an increased energy spread in the ion beam emerging from the ionization chamber causing a falling off in sensitivity and since a single-focussing magnetic sector instrument produces a momentum spectrum, the mass peaks will be broadened and the highest resolution of which the instrument is capable at very small applied fields will not be attained. 10.2

IDENTIFICATION OF REACTION PRODUCTS

The advantages of mass spectrometric qualitative analysis are generally greatly increased when one is studying the products of a reaction between starting materials of known composition. Consider, for example, the reaction of cyclopentanone with n-butylamine in the vapour phase at 300-350°C both catalysed and uncatalysed. This and similar reactions have been comprehensively in¬ vestigated as part of a study of the mode of thermal decomposition of nylon 6:6 polymer [566]. No attempt will be made here to report fully the chemistry of these reactions for an account of which the original paper should be consulted. We shall consider only the identification of one or two of the products by mass spectrometry. The formula of cyclopentanone is CsHsO and its nominal mole¬ cular weight is 84. n-Butylamine has the formula C4H11N and the molecular weight of 73. Many of the reaction products could be identified without complete separation from the product mixtures and due to the fact that the formulae of the starting materials are known, identification is often possible by a study of the parent peaks only. It has already been mentioned that mass spectrometry often enables the accurate molecular formula of an unknown (or of each constituent of a mixture of unknowns) to be determined. The results can be compared with the C : N : H : O ratios obtained by conventional chemical means to ascertain whether all components present have been detected. Alternatively, one can sometimes study a single molecular species in a complicated, unanalysed mixture. Consider one component often present in the reaction products which gives a large peak at mass 150 thought to be due to a molecular ion. Even without a more accurate mass measurement the compound can be identified in the following way. This compound cannot be a product of the reaction of two molecules of butylamine, since the molecular weight of such a compound could

10.2

IDENTIFICATION OF REACTION PRODUCTS

433

not be greater than 2 X 73 = 146. Neither could it be formed from the reaction of a single molecule of cyclopentanone with one of butylamine (total mass = 157) since one would not lose seven hydrogens in a reaction and in any case if the product is of even molecular weight it must contain an even number of nitrogen atoms. The compound could be formed from two molecules of cyclopentanone (total mass = 168) with elimination of mass 18. It is known that the dehydration of cyclopentanone vapour over activated alumina at elevated temperatures produces 2-cyclopentylidenecyclopentanone

of molecular weight 150. This compound would be expected to give a large parent peak in its mass spectrum and thus is suggested as the compound being examined. In a similar fashion by an extension of the above argument, a peak at mass 198 which has the correct mass for a product of the reaction of three molecules of cyclopentanone with elimination of three molecules of water would be identified as the cyclic trimer formed by an extension of the reaction which gave 2-cyclopentylidenecyclopentanone. This trimer has the formula

By similar lines of argument a compound of molecular weight 139 would be suggested (since it contains a single nitrogen) to be a reaction product formed from one molecule of cyclopentanone and one of butylamine (total mass = 157) with loss of water. Such a compound would be N-cyclopentylidene-n-butylamine

N-OVChVCHi-CH,

A parent ion at mass 140 can be suggested to be due to the reaction 84 + 73 140 + 17 where the mass 17 would have to contain the nitrogen atom (even molecular weight product) and thus would be ammonia. The acidic hydrogen in the 2-position of cyclopentanone would be most likely to react in this fashion and the suggested product would be 2-n-butylcyclopentanone

434

OTHER APPLICATIONS OF MASS SPECTROMETRY

10

An interesting fraction of the reaction products gave a mass spectrum with two outstanding peaks at the high mass numbers 201 and 196. Both of these ions could be parent ions; in any event two ions differing by mass 5 would not be expected in the spectrum of a single compound. The ion of mass 201 might arise in the following ways. (Cyclopentanone is represented merely by its molecular weight of 84 and butylamine by 73.) 3 X 73

-* 201 + 18

2 X 73 + 1 X 84 ->201 + 29 2 X 84 + 1 X 73

201 + 40

The reaction 3 X 84 201 + 51 is not possible since the odd molecular weight of the product shows it to contain nitrogen. The odd molecular weight shows that if the first reaction occurs, then the mass 18 given off must be of formula CHe so as to retain in “201” an odd number of nitrogens. By arguments of this sort, the reactions can be re-written 3(C4HhN)

C11H27N3

+CHe

(84)

2(C4HhN) + (C5H80) -> C12H27NO + CH3N

(85)

2(C5H80) + (C4HnN) h- C11H23NO2 + C3H4

(86)

or

-> C12H27NO + CaO

(87)

or

-> Ci4Hi9N

(88)

+ H802

Similarly the reaction by which the compound “196” is formed might be 3(C5H80)

C11H16O3

+ c4h8

(89)

or

Ci2H2o02

+ C3H40

(90)

4" C202

(91)

+ CH3NO

(92)

or 2(C5H80) + (C4H11N) or

-> Ci3H240 Ci3H240

-»• Ci2H2o02 + c2h7n

2(C4HnN) + (C5H80) -> Ci3H240

+ NaHe

(93) (94)

In this case the reaction involving three butylamine molecules is not possible. An obvious way of reducing the number of possible reactions is to measure as accurately as possible the masses of the product compounds. This was done (in a six-inch radius single focussing instrument) using a mercury isotope as a reference mass, and the figures 201.22 and 196.25 were obtained. It was estimated that the true masses lay within ± 0.01 a.m.u. of these calculated values with 95% probability. Possible formulae for the “201” compound are then (from Appen¬ dix I) C14H19N, C9H19N3O2 and C10H19NO3. For the “196” compound the formulae C13H24O, C12H24N2 and C10H20N4 are possible. Combining the mass measurements with the evidence listed in the series of reactions given above, it seems that the two compounds present must be of formula C14H19N and C13H24O respectively. By the rules given in Chapter 8, the first of these compounds must contain a total of 6 rings and double bonds and the second compound a total of 2. Considering first the reactions leading to “mass 196” of the correct formula,

10.2

IDENTIFICATION OF REACTION PRODUCTS

435

those given in equations (91) and (92) would both seem at first sight unlikely, since they would both involve cleavage of one or two cyclopentanone rings which would need to be followed later by ring or double-bond formation to lead to the correct formula. The reaction (94) on the other hand could result in the elimination of two molecules of ammonia, and by an extension of the argument used above, the resulting compound could be of structural formula

o

i.e.

2:5-di-n-butylcyclopentanone. This formula was confirmed by chemical evidence. The reactions leading to the compound of molecular weight 201 is clearly that shown in (88) involving 2 molecules of cyclopentanone and one of butylamine. If one assumes as a first hypothesis that the cyclopentanone rings are not broken in the course of the reaction, a further 4 rings and double bonds must be included in the final formula. Now this is precisely the number contained in an aromatic ring, and the infra-red spectrum of the mixture showed a band due to conjugated C=C or C=N. The ultra-violet spectrum too showed the con¬ jugated system to be present. The complete argument used in the identification of C14H19N. has been given [566, 770] and by no means rests completely upon the mass spectrometric evidence. It is instructive, however, to see what can be inferred from the facts already stated and it is pertinent to remark that through¬ out the investigations of the reaction products, the compound presently being discussed was always referred to by the empirical formula C14H19N which had been determined by mass spectrometry, and this was also true of many of the other compounds isolated. This knowledge of the exact empirical formula constantly influenced the thought concerning the compound to be identified. Equation (88) above indicates that the total fragments eliminated during the reaction are of formula H8O2. Now the reactions which eliminate the oxygen atom usually do so in the form of H2O, giving, for example, 2-cyclopentylidenecyclopentanone or N-cyclopentylidene-n-butylamine as discussed above. Thus it might be expected that two molecules of cyclopentanone and one of butylamine could react together to give

nc„h9

(where C4H9 = n-butyl), with elimination of H4O2. This would lead to a molecule of molecular weight 205 (C14H23N). To obtain the correct structure which gives a large parent peak and to give the desired total number of rings and double bonds in a conjugated system it is necessary to eliminate a further 4 hydrogen atoms. An obvious ring formation would be to the compound C14H21N of formula

436

OTHER APPLICATIONS OF MASS SPECTROMETRY

10

with elimination of 2 hydrogens and the last two hydrogens eliminated would give a pyridine nucleus and a formula

It is interesting that the pattern of peaks obtained at low mass numbers is typical of a pyridine ring even though the ring is so highly substituted. This is mentioned again in Chapter 9. In the above example the mass spectrometer is being used to analyse the products of a reaction carried out in a separate apparatus and though there are many examples in the literature of the use of the instrument in this way [214, 773], in some of which, the taking of samples at known time intervals for analysis enables the progress of the reaction to be followed in detail [2205], many reactions can be carried out in the sample-handling system of the mass spectrometer itself [342] and a continual study of the mass spectrum of the vapour can be made, or a continuous record kept of the height of any peak characteristic of one of the components present. In connection with the work described above on the ther¬ mal degradation of nylon, a sample of polymer was placed in a glass tube connected to the gas-handling system of the equipment and this tube immersed in a salt bath at 300°C. Cyclopentanone was one of the major volatile compounds evolved. Degradation of the polymer under vacuum does not give the same products as are obtained during degradation in air, but the rapid removal of some of the volatile compounds evolved enables these to be identified before they have a chance to react further. It is difficult to identify cyclopentanone in the reaction products of nylon degraded under atmospheric pressure because it is so reactive, so that the mass spectrometric method is important in showing that this highly reactive substance is, in fact, formed. Identification of products such as this is helpful in giving an understanding of the route by which the ultimate products of the reaction are produced, and rapid separation of the primary products is often used in studies of pyrolysis products [1294, 2111]. Several early investigations of the degradation of a polymer by mass spectro¬ metry were concerned with polystyrene. Cast films of the polymer were exposed to heat and/or ultra-violet radiation followed by analysis of the small quantities of volatile material produced [3]. The high sensitivity of the mass spectrometer made it possible to investigate the degradation reactions under conditions more comparable to service conditions than in previous studies. There are several other examples in the literature of studies of thermal decomposition products by mass spectrometry, involving various polymers [1295, 1961, 2111], metal acetylacetonates [2095], azoethane [345], diborane [263] and hydrazoic acid [680, 682]. Nitrogen, hydrogen and ammonia are the only products of the last of these reactions [680] and no intermediates can be observed. In an investigation of hydrocarbons at high temperature [561] the gold walls of the ionization chamber could be heated to 1000°C. The walls could also be covered with an Al-Si cracking catalyst to obtain the cracked products as a function of temperature. Analyses by mass spectrometry of the products formed in the nitric oxideinhibited decomposition of n-pentane have been made [428] for a wide variety of temperatures, pressures and reaction percentages. The large number of products

10.2

IDENTIFICATION OF REACTION PRODUCTS

437

which can be analysed and the speed with which analyses can be carried out makes the mass spectrometer a valuable tool for the detailed study of the mechanism and kinetics of such reactions. Another example of the use of the mass spectrometer to study reaction prod¬ ucts is in the radiolysis of cyclohexane [802]. The liquid undergoing radiolysis was contained in a closed circulating loop with a bleed for removal of gas and to take samples at various times. Zemany [2210] has used the mass spectrometer to analyse the gases formed in the irradiation of polythene with high-energy electrons, the method being ideal for studying the complex mixture of volatile substances formed. It has been pointed out by Achhammer [2] that although the mass spectrom¬ eter is extremely useful in identification of reaction products, the conventional methods of sample handling are of limited value for observing products as reactive as, say, HC1. In order to observe such materials, to obtain information about species formed at an early stage in a reaction or to study very rapid reactions [218, 1226, 1549] special sample handling methods are necessary. An example of the study of highly reactive short-lived species is afforded by the work of Eltenton [576] who was the first to show that free radicals could be studied by means of the mass spectrometer. Among the special techniques which are sometimes employed with free radicals must be included special techniques of preparing and mixing the reactants which could also be used to study the formation of other transient species. As an example, the technique of mixing atomic hydrogen and molecular oxygen in the detection of HO2 radicals has been described [658]. A number of studies of reactions leading to the formation of free radicals [148, 1264, 1269] have been carried out by mass spectrometry [53, 90, 148, 170, 289, 378, 577, 578, 624-628, 657-659, 661, 662, 853, 922, 1019, 1020, 1034, 1035, 1048, 1217, 1229, 1263, 1265-1267, 1269, 1270, 1351, 1544, 1657, 1708, 1709, 2051, 2052]. Many of these have involved the identification of the free radicals, measurement of their ionization potentials or their rates of reaction. Many of the measurements, such as the determination of ionization potentials, can be per¬ formed in the presence of molecular species with which free radicals are in¬ evitably contaminated since the ionization potentials of free radicals are gener¬ ally lower than those of molecules and the energy of the bombarding electrons can be chosen to be sufficient to ionize the free radicals but to be below the ionization potential of all molecules present. Free radicals can be detected in the presence of neutral molecules even when higher electron energies are used. It is preferable to use energies of the order 50 eV for the quantitative determination of free radicals, since at these energies the sensitivity is greatly increased and is largely independent of small changes in electron-bombarding energy or of contact potential changes. The problem is, in fact, not different from the analysis of any other mixture of compounds. The peak at mass Ri+ due to the molecule R1R2 can be estimated by a subsidiary determination of the mass spectrum of pure R1R2, and subtraction of this amount from the total peak at mass Ri will give the peak height due to Ri+ ions from Ri free radicals. It is usual in the analysis of mixtures to introduce samples of each component at known pressures in order to obtain a quantitative measure of the amount of each in the mixture. A calibration with pure free radicals is not possible and a rather more complicated calibration as described by Lossing and Tickner [1261] must be used instead. To obtain the sensitivity factor for methyl radicals, known quantities of mercury dimethyl mixed with a carrier gas were passed through a furnace and decomposed

438

OTHER APPLICATIONS OF MASS SPECTROMETRY

10

to give methane, ethane and methyl radicals. The partial pressures of methane and ethane could be found by ordinary methods of analysis and the partial pressure of methyl radicals could then be found from the assumption of a 100% carbon balance. The ratio of the sensitivity of detection of methyl radicals to that for detection of methane was found to be 0.47 ± 0.07 using 50 eV bom¬ barding electrons. The main experimental difficulty in studying free radicals is often the intro¬ duction of the radicals from the sample vessel in which they are formed to the ionization chamber along a low-pressure practically collision-free path so that they cannot react with other species in the vapour phase or on the walls. This involves the siting of the leak (which should be of the “pin-hole” variety so as to minimize the possibility of collisions within the leak) as near as possible to the ionization chamber [658, 1709]. Such techniques are also necessary in studies of other transient species such as those in reaction-mechanism and kinetic work [1227] where continuous sampling of the reaction mixture is necessary [1262]. In other cases, as when measuring the ionization potentials of radicals, the radicals can be produced actually inside the ion chamber. For example, Waldron [2105] describes a source which contains in addition to the conventional tungsten filament for producing the beam of bombarding electrons, an auxiliary platinum filament placed in the path of the incoming sample. When this second filament is maintained at a temperature of 1000°C and mercury dimethyl introduced, a local methyl radical concentration can be obtained. Even when the free radicals are produced in the ion chamber itself, the sensitivity of detection may be small, especially if the probability K of a radical undergoing a reaction on striking the wall is high. In this case, methods must be employed to prevent collisions with the ion chamber walls, for example, by firing a molecular beam through the ion chamber in a direction perpendicular both to the electron beam and the direction of extraction of ions. The wall of the ionization chamber which faces the in¬ coming molecular beam is removed and the beam passes straight into a wide pumping line [1711]. When a sample is introduced into a Nier-type electron bombardment source in the conventional manner, it is possible to calculate the mean number of collisions made by every particle on the walls before being pumped out [1228]. A typical value is about 50, and it is clear that free radicals which can be obtained in good yield inside a Nier-type ion chamber cannot be very sensitive to collisions on the surfaces [616]. Other similar studies of reactions on filaments at low pressure have been described [217, 1230, 1231]. The methods can be applied generally to the study of the products of reactions occurring during the flow of vapour at low pressures past a heated ribbon. They can also be used to give information about the surface on which reaction is occurring and to study phenomena such as poisoning [615]. The analysis of the transient species in flames has also been undertaken by mass spectrometry and has given some valuable results which can be interpreted to support conclusions reached from spectroscopic data. Eltenton’s [577] pioneering study with low pressure flames of methane and propane indicated the presence of some intermediate oxidation products such as HCHO, HCO and CH3O but it was found that these had markedly negative temperature coefficients, the HCHO and HCO falling to much smaller concentrations when additional heat was supplied to the flame. Acetylene and CH3 radicals were also detected, and the CH3, unlike formaldehyde became much stronger in the hotter flames. The study of flames places special requirements on the sample-handling system [444] and it is necessary to remove the gas sample from the reaction zone as rapidly as

10.2

IDENTIFICATION OF REACTION PRODUCTS

439

possible and by a collision-free path. A generally useful apparatus for this work has been described by Foner and Hudson [657] in which gases from the reaction zone stream through an orifice as a sonic jet. A second collimating slit selects the central portion of the gas stream and a third slit provides additional collimation. The gas jet is chopped by a mechanical chopper and by the use of an A.C. amplifier with phase-sensitive detector the D.C. background signal is elim¬ inated. Vanreusel and Delfosse [2074] have also described a 3-chamber system in which ions from flames at 50 mm Hg pressure in a combustion chamber pass through revolving discs into a mass spectrometer at 10-5 mm Hg. To determine the composition profile within a flame [1629], very fine probes made of fused quartz can be traversed through the flame, although by this method only neutral compounds will be obtained. To study the kinetics of the gas phase reactions which are taking place, very fast scanning is necessary [1227]. The fact that flames are electrically conducting has been known for many years but the origin of this conductivity has only recently been investigated by Knewstubb and Sugden [1134, 1135] by directing a flame burning at atmospheric pressure against a thin window in which is a small hole about 0.002 inch dia¬ meter. About 0.25 cc/sec from the flame is thus sampled into a continuously evacuated chamber. Using a series of ion lenses it has been found possible to direct the ions in the sample into a mass spectrometer and analyse them. Con¬ centrations as low as 107 ions/cc are detectable. A feature common to all simple ions found is an apparent facility of attachment of one, two or three water molecules to the original ion. For example, in hydrogen flames, the main ion in the gases above the reaction zone is H3O"1". When strontium chloride solution is sprayed into the flame peaks due to Sr+, (SrOH)+ and (SrOH + nH20)+ up to n = 3 can be detected. In hydrocarbon flames, the ions are mainly hydro¬ carbon molecules and radicals formed by very rapid polymerization of the fuel in the pre-heating zone of the flame; the peaks are grouped around mass numbers corresponding to (CH)m+ where m lies between 3 and 10. Other work on the analysis by mass spectrometry of ions extracted from lower pressure flames has been carried out by Deckers and Van Tiggelen [443, 445, 446]. They found that there seems to be a tendency for H+ to be attached to polar molecules such as H2O, NH3, and HCN; NHO can be formed as soon as the flame is propagated in a mixture where nitrogen is a part of the fuel or part of the oxidiser molecule. Once too great an amount of NO is present in the burned gases, most of the ions are NO+, probably formed by charge transfer, due to the low ionization potential of NO. When the vapour from a mixture of strontium and potassium carbonate is heated to 1000-1900°K, the complex ion (SrOK)+ is formed [1970]; ions of this type can be detected by mass spectrometry using techniques similar to those described above for flames. Other reactions studied have involved the unstable “active nitrogen” [53, 1048, 1657]. When nitrogen is subjected to an electric discharge at low pressures, a peach-coloured glow is emitted, and when the discharge is switched off, a golden-yellow afterglow due to active nitrogen re¬ mains for several seconds. It has now been established that the main reactive component in active nitrogen is the nitrogen atom in its ground state. The first study of the phenomenon by mass spectrometry was by Jackson and Schiff [1034, 1035] who carried out a measurement of the appearance potential of N+ from the active species. The value obtained was close to the ionization potential of the nitrogen atom in its ground state. A second appearance potential at 16.1 eV has been suggested to be due to ground state molecules containing about 8 eV

440

OTHER APPLICATIONS OF MASS SPECTROMETRY

10

of vibrational energy [1084]. The kinetics of the afterglow was studied using a mass spectrometer (in combination with a photomultiplier) by Berkowitz, Chupka and Kistiakowsky [170], and by Herron, Franklin, Bradt and Dibeler [868]. Addition of NO to the afterglow was found to result primarily in the production of N2O. The products of reaction of active nitrogen and acetonitrile have been studied by mass spectrometry [668]; a detailed study of the kinetics of the reactions of nitrogen atoms in the afterglow with oxygen and each of the oxides of nitrogen has been given [1127] using the principle of a “stirred reactor” and a mass spectrometer for the determination of steady-state concentrations. The reactions of hydrogen, carbon monoxide and ammonia with nitrogen atoms [1129] are too slow to be detectable in the above system even at 250°C. In the presence of ammonia the intensity of the afterglow is reduced due to transfer of electronic energy between excited nitrogen molecules and ammonia molecules. This is followed by dissociation of the ammonia. There have been other investigations of species formed in gases which have been subjected to an electrical discharge [142, 1217, 1555-1558, 1782]. The formation of molecular ions of very short lifetime in discharges through the rare gases has been detected by the use of radio-frequency mass spectrometers as probes [1446]. Studies involving oxygen passed through an ozonizer [801] have shown peaks due to C>3+ and C>4+ in the mass spectrum, and oxygen subjected to microwave and A.C. glow discharge has also been examined [661, 867]. A field of reaction kinetics in which mass spectrometry promises to be of assistance is the study of photosensitized reactions. Examples of this type of investigation have been the mercury photosensitized decomposition of acetal¬ dehyde and acetone [626, 1270] and of Ci to C4 olefines [378, 1267]. Kinetic studies can, of course, be carried out by mass spectrometer for reactions involv¬ ing unstable species such as free radicals just as for reactions between stable compounds. Examples of such work are contained in the references given above. For instance we can take the measurement of rate ratios for the various reactions which can occur in mixtures of methyl and ethyl radicals [853] and the study of the reactions of methyl radicals with water on quartz and glass surfaces [90]. Other unstable reaction products examined by mass spectrometry are those formed in flash photochemical reactions. For example, the decomposition of ketene and nitrogen peroxide have been studied in this way [1128], using a timeof-flight mass spectrometer to produce a complete mass spectrum every 50 microseconds of the product continuously sampled from the reaction vessel via a pinhole leak. The products of flash photolysis could be examined in apparatus of the type already described for studies involving free radicals provided that their life-times were greater than about 10~4 sec [578]. The kinetics of the reaction of ozone with olefines have been investigated using a mass spectrometer [657] suitable for the study of very fast reactions. This work [1761] is being carried out as part of a programme to determine the exact mechanisms of oxidant formation and the roles played by pollutants in smog.

10.3

THE STUDY OF THE INTERACTIONS OF IONS WITH MATTER

The mass spectrometer can be used to isolate a particular ion species and to control its kinetic energy. Some of the reactions which ions undergo have already been discussed from the point of view of dissociations induced by collision or in other ways. Many other features of the interaction of such ions with matter can be studied [745] and using a second mass analyser to investigate the reacted

10.3

THE STUDY OF THE INTERACTIONS OF IONS WITH MATTER

441

beam in the manner described by Lindholm [1247] and by Fedorenko [629] very detailed information can be obtained. In this method, the bombarding ions are generated in a conventional ion source, accelerated, and separated by a magnetic field. The chosen ion species is passed through a slit in the “collector assembly” into the collision chamber. Product ions can be withdrawn from this “ionization chamber” into a second mass spectrometer and their mass spectrum obtained. In the charge-exchange process by which the product ion is formed, the bom¬ barding ion is neutralized. If the energy of recombination of the bombarding ion and the electron is close to the appearance potential for a particular ion in the bombarded gas, it is found experimentally that the cross-section for formation of this ion will be large. If, on the other hand, the recombination energy is more than about 0.5 eV removed from the appearance potential, the cross-section will be relatively small. This fact can be used to deduce appearance potentials for various ions. For example if He+ is used to bombard nitrogen gas, it is found that the cross-section for production of N+ is large. Thus the appearance potential of N+ from nitrogen must be close to that of FIe+ from helium (24.6 eV). The cross-section for production of N+ increases as the energy of the bombarding ions is reduced and this suggests that the appearance potential of N+ is in fact lower than 24.6 eV. (The value obtained by electron bombardment is 24.3 eV.) Bombardment of a target gas by a succession of positive ion species having a range of recombination energies [1248, 1250] sometimes enables the higher ionization potentials to be studied. As the recombination energy is increased past the first ionization potential, the cross-section passes through a maximum value, and falls off as the recombination energy is increased as it becomes less likely that the excess energy can be carried away as vibrational energy. Further increase of the recombination energy often, however, leads to a large cross-section again corresponding to excitation to a higher ionization level. Thus the crosssection for formation of CC>2+ by bombardment of carbon dioxide with F+ is large [1250]. The recombination energy for F+ is 17.4 eV and the second ioni¬ zation potential of CO2 is 17.3 eV. The reasons for the observed values of crosssection are not always easy to understand, and somewhat tenuous arguments based on the bombardment of carbon monoxide have led to a value of 136 kcal/ mole for the latent heat of sublimation of carbon [1249] in disagreement with the more convincing value of 170 kcal/mole (see p. 476). Using such an apparatus Fedorenko has for example investigated such pro¬ cesses as electron stripping, electron capture and dissociation during passage of the ion beam through a gas at low pressure and determined the cross-sections for these various processes as a function of ion energy. Using two additional dia¬ phragms to collimate the beam of ions scattered through an angle 0, Fedorenko [630] was able to study the scattering in terms of the masses of the scattered and scattering particles, and the ion energy. His studies included cases in which no change of mass to charge ratio occurred, those in which a mass change took place (e.g. N2+ -> N+) and those involving a change in charge (e.g. Ba+ -> Ba+++ in krypton). The latter process became more probable as the particles approached closer together. The collisions observed were inelastic due to interpenetration of the electronic shells. If a velocity focussing mass spectrometer is used to analyse the positive ions, the relative loss of energy for colliding particles scattered through a small angle can also be obtained [63 ij. The emission of secondary electrons when a beam of positive ions strikes a metal surface has already been discussed since exploitation of this phenomenon forms the basis of the multiplier ion-detector. Positive ions can also be sputtered

442

OTHER APPLICATIONS OF MASS SPECTROMETRY

10

out of a metal surface under the influence of an incident ion beam and the mass spectrometer offers the opportunity of carrying out an analysis of the masses and abundances of the sputtered ions, and also, if a subsidiary source of ionization is employed, of the neutral fragments driven out of the surface. This sputtering of a cathode is conceived as being produced by purely mechanical collisions between the impinging ions and surface atoms of the target. A difficulty in carrying out a successful experiment is in obtaining a clean surface for sputter¬ ing and, even when the background pressure of adsorbable impurities lies below 10~7 mm mercury, a background of hydrocarbon ions is observed [972]. In most of the work described [1868, 2078, 2079] only monatomic ions are reported as leaving the surface, but Honig [972] has succeeded in detecting polyatomic clusters positively- and negatively-charged as well as neutral. For example, from a germanium surface bombarded by rare gas ions he obtained the positivelycharged particles Ge+, Ge2+, GeH+, GeOH+, Ge20+, Na+, Al+, K+, Rb+ and Hg+. Honig considers the conditions involved in an ideal sputtering experiment. Some of the conditions are mutually exclusive, some are very difficult to realize experimentally, and although Honig’s work constitutes a considerable advance over many of the earlier studies, he still considers that it can only be thought of as a preliminary study. Similar experiments have been performed by Bradley [257] who investigated the positive ion emission from Mo, Ta and Pt under bom¬ bardment with rare gas ions. He chose these metals for their high melting points since a wide temperature range was thus accessible for study and the targets could be cleaned by “flashing” them at high temperature. Unflashed materials gave ions characteristic of surface impurities, freshly-cleaned materials showed atomic ions from impurities known from spectroscopic evidence to be present in the bulk of the target. Bradley suggests that some of the most fruitful applications of the technique will probably be in the realm of surface kinetics, for example in studies of corrosion or catalysis. Its value for solids analysis would be greatly enhanced if the sensitivity could be increased. For metallic targets, secondary ion emission is predominantly an atom by atom process and does not involve large aggregates of atoms. Bombardment of organic compounds in this manner has not been attempted and if the only ions to be emitted were atomic ions the analytical information obtainable would be of little interest. If, however, larger ionized fragments could be produced these might reflect the bond-strengths in the original molecule and give a useful spectrum. The advantage of such a method of producing a mass spectrum would be that it would extend the range of com¬ pounds which could be examined to embrace polymers and other large molecular species which cannot be obtained in the vapour phase for examination in con¬ ventional ways. Extra difficulties can be expected due to the build-up of charge on the bombarded solid and the deposition of insulating films on surrounding surfaces. Apparatus to carry out experiments is now being constructed in these laboratories. The mass spectrometer can be used [287] to monitor the energy of reflection of ions from metal surfaces, and the results which have been obtained are qualitatively in agreement with the classical model of elastic collisions between rigid spheres, leading to isotropic multiple back-scattering of the ions. The use of the mass spectrometer in related studies suggests itself for the study of the threshold at which sputtering occurs [1202] and to investigate in detail the phenomenon by which ions can be thermally desorbed from metal surfaces which have been subjected to ion bombardment [1222] even several days after bombardment has taken place [209].

10.4

THE SEPARATION OF ISOTOPES

443

Other studies with positive ions parallel those which have been carried out using electrons. Determination of proton affinities and bond-dissociation ener¬ gies [1982] can be performed by the method of ion impact, and it is possible to study appearance potentials and fine structure of ionization efficiency curves using positive ions as the bombarding agent [1411].

10.4

THE SEPARATION OF ISOTOPES

The physical and chemical processes which can result in fractionation of the isotopes of the elements as they occur in nature have already been mentioned briefly. Some of the observed natural fractionation is of biological origin, but this is, of course merely the result of physical or chemical processes operating in an organism. Below are listed some of the methods by which isotopic separa¬ tion can be effected in the laboratory. Some of the methods are those which give rise to natural isotopic abundance variations, some are suitable for large scale separation [153], some for obtaining high degrees of enrichment. (i) The calutron This mass spectrometric method of separating isotopes has already been described (see p. 203). It enables large enrichments to be achieved in a single stage and has the advantage of being an extremely versatile separator. It can be used to separate the isotopes of any element and, if an element has more than two naturally-occurring isotopes, it can be used to collect samples of those of intermediate mass as well as the lightest and heaviest isotopes. It is a particularly useful method of obtaining pure samples of the isotopes of the heavy elements, but is restricted to the separation of quantities of the order of a gramme and the capital cost of the equipment is high. Other methods of separating isotopes are of less general applicability and all depend on the very slight differences in the physical and chemical properties of isotopic substances which can produce slight concentration of an isotopic species [107]. When the separation process can be repeated a very large number of times, considerable concentration of an isotope can result. A variety of these multistage processes is discussed below. (ii) Separation by diffusion The rate of diffusion of a gas through an orifice is inversely proportional to the square root of its mass provided that the mean free path is large compared with the orifice dimensions. The fractionation effects produced in a sample reservoir when a mixture of gases flows into the ionization chamber through a “molecular leak” are discussed in Chapter 5. In the same way isotopic fraction¬ ation will occur, although the effect will be very small. To increase the “through¬ put” of a separator working on this principle, the gas is allowed to diffuse through a porous membrane containing a very large number of small holes. In a commer¬ cial separator consisting of a large number of stages in series, the vapour in the Nth stage which diffuses through the porous membrane and is thus enriched in the lighter isotope is compressed and fed to the (N + l)th stage. The fraction which has not diffused through the Nth membrane and which is enriched in the heavier isotope is returned to the (N — l)th stage where it is mixed with the lighter isotope concentrate from the (N — 2)th stage. Such a system is known as a “cascade” process and in an ideal arrangement, the two streams fed to a particular stage are arranged to have the same isotopic content. The method was

444

OTHER APPLICATIONS OF MASS SPECTROMETRY

10

first used by Hertz [869] for the separation of the neon isotopes on a small scale. Diffusion plants are more economic to operate the larger their size, and the method has been used for the separation of 235U in a plant extending over an area of several square kilometres. It is to be expected that after formation of the earth, a considerable portion of the lighter molecules escaped by diffusion, and that those molecules of any substance containing the lighter isotopes escaped preferentially. Some isotopic separation might sometimes occur naturally by diffusion of gases and liquids through porous regions of the earth’s crust [1814]. Diffusion processes might also give isotopic separation in another way. Under the influence of gravity, isotopic molecules will tend to separate, the heavier atoms tending to collect in the lower part of the system, since the isotopic atoms differ appreciably in mass but not in size. Turbulence in the atmosphere upsets the formation of such a gradient up to great heights, but natural isotopic fractionation of nitrogen under the influence of gravity has been detected. In the laboratory, separation of the isotopes in a gas can often be accomplished by setting up a temperature gradient [1807]. There is a tendency for one com¬ ponent to concentrate in the cold region and the other in the hot region [329, 588]. It is usual for the heavy component to concentrate at the cold end, but the tendency depends not only on the molecular weights, but also on the repulsive forces between the molecules and in some cases, the direction of separation re¬ verses as the temperature or concentration is changed. An elegant method of isotopic separation based on thermal diffusion was first used by Clusius and Dickel [355]. The isotopic mixture is introduced into the annular space between a long vertical wire or tube and a co-axial outer tube. If the inner tube is heated and the outer tube cooled, separation occurs for two reasons. Firstly thermal diffusion gives a higher concentration of one (usually the heavier) isotope on the cold wall and secondly, thermal convection gives a downward stream of cold gas at the outside and an upward flow of warm gas at the inside wall of the annular space. Consequently there is a tendency for one isotope to go into the downward stream. The process is analogous to the cascade process described above except that the discrete number of stages are merged into a continuous “countercurrent” flow. Very pure samples of many isotopes have been prepared by this method. It has the advantage that the hold-up of enriched materials is very small and the method is thus particularly suitable for the concentration of very rare isotopes. The power of the method is well-illustrated by a separation of 100 cc of 3He at concentrations between 50 and 80% by Bowring and Davies [251] using as starting material helium containing about 1 part in 106 of the light isotope. (iii) Separation by distillation The separation of substances of different vapour pressures by distillation is a familiar process and since the vapour pressures of isotopic substances show differences [359, 1050] isotopic separation by distillation is a possibility. It has already been mentioned that evaporation is responsible for the naturally-occur¬ ring differences in isotopic composition between sea and fresh water. A single stage of vaporization is usually insufficient to produce appreciable separation, and the process must be repeated many times. It is possible to obtain the effect of many “theoretical plates” in a single fractionating column in which an upwardflowing stream of vapour is brought into intimate contact with a downward¬ flowing stream of liquid. The separation is greatly enhanced in this counter-

10.4

THE SEPARATION OF ISOTOPES

445

current flow. In general, the differences of vapour pressure of isotopic substances, which are related to the zero-point energies of the molecules are only appreciable at low temperatures. If the molecular radii and the energy of the van der Waals attraction are the same for the light and for the heavy isotopic molecular species, the frequency of vibration of the molecule as a whole with respect to the position of equilibrium in the solid or liquid lattice will be higher for the light molecules. The higher vibration frequency involves a higher zero-point energy and con¬ sequently in the region of low temperature, a lower heat of vaporization and a higher vapour pressure for the lighter isotopic species. Distillation is normally only an attractive method of separation when a substance exists which can be distilled at a low temperature. For example, 20Ne and 22Ne have a vapour pressure ratio of 1.045 at 24.7°K and were fractionated at this temperature by Keesom and Flaantjes [1088]. It is not always true that the molecular species containing the heavy isotope is less volatile than that containing the light isotope. In the case of carbon tetrachloride, for example, molecules containing light carbon or heavy chlorine are the least volatile [105]. Some substances show exceptionally large vapour pressure differences at room temperature. In most cases, these substances are in associated form. One example is provided by the vapour pressure ratio of H2O to HDO which is 1.059 at 313 °K [1120] and separation by distillation is attractive in this case. It has also been employed for the enrichment on an appreciable scale of D (from hydrogen gas), 10B (from BF3), 13C (from, .CO) and lsO (from H2O). The isotopes of lithium have been frac¬ tionated by the molecular distillation of lithium metal [2043]. A 10-stage countercurrent molecular still for the fractionation of the mercury isotopes has been described by Brewer and Madorsky [268]. (iy) Separation by chemical exchange Chemical exchange has been mentioned as being responsible for many of the variations in isotopic abundance which are observed to occur in nature and the different rates of chemical reactions for the various isotopic species of an element may also produce variations. Suitable reactions are known by which isotopic enrichment can be produced in hydrogen, boron, lithium, carbon, nitrogen, oxygen, sulphur and some other elements. As with the two previous processes described, to obtain a large separation factor a counter-current system must be used. In the case of lithium exchange, two immiscible liquids of different density, lithium amalgam and an alcoholic solution of lithium chloride were used, but it is much more usual to obtain counter-current flow by passing a gas upwards through a liquid. A good example of the use of the method for producing highlyenriched isotopes is provided by the reaction often used for 15N concentration which has been used to provide this isotope in 99.8% purity [1914]. 15NO (gas) + H14NC>3 (aqueous)?^14NO (gas) + H15NC>3 (aqueous)

(95)

Such reactions are attractive in that room temperature operation is often practicable, the separation factors are often much larger than in other methods and the power costs are very small. The separation factor for the above reaction is 1.05 at 298°K. Nitric acid is run downwards through an exchange column against an upward flow of nitric oxide, but in order to reduce costs it is essential to be able to regenerate the materials. For this purpose, sulphur dioxide is intro¬ duced at the base of the column to regenerate nitric oxide and the gas issuing from the top of the column is mixed with air to form nitrogen peroxide which is dissolved to give nitric acid. The 15N which goes preferentially into the nitric

446

OTHER APPLICATIONS OF MASS SPECTROMETRY

10

acid concentrates at the bottom of the column. The cost of such a method of separation is generally determined by the cost of the chemicals used in the endconversions. Chemical exchange methods are suitable for use on a laboratory scale, but with quite modest apparatus it is usually possible to obtain a considerably greater quantity of enriched material than by thermal diffusion. The apparatus (including the end-conversion units) is, however, usually only applicable to a single chemical exchange process, and considerable chemical knowledge is usually required to set up the system. Differences in the rates of irreversible chemical reactions for different isotopes might also be used in a separation method. This has been considered by Bern¬ stein [175] who suggests how a cascade process could be used to amplify the effect. (v) Separation by electrolysis A small isotopic separation can be produced by electrolysis. This method has been used for the preparation of deuterium oxide from water, and is attractive when a cheap source of electricity is available [2049]. Electrolysis was the first method to produce a separated isotope on the ton scale, but its use has been mainly restricted to the separation of deuterium from hydrogen. If Ri is the ratio of deuterium to hydrogen in the gas liberated during the electrolysis of water, and R2 the corresponding ratio in water, then a, the separation factor which is defined as R2/R1, has the value 6, and it follows that it is easily possible to obtain a large increase in the deuterium concentration by electrolysing a large quantity of water down to a small volume. For other isotopes, values of a of about 1.01 are more usual, and the method is then unattractive. As was the case with the other methods of concentration which have been described, a counter-current flow technique can be used with advantage, and this avoids the wasting of water of high deuterium concentration. The gases evolved at the Nth stage are combined to form water and added to the (N -—■ l)th stage. The water vapour which is carried along with these gases from the Nth stage is condensed out and added to the (N + l)th stage. (vi) Other separation methods For a more complete account of isotope separation, reference should be made to books devoted to this topic [1126, 1144]. Other methods are described in reviews of the subject [1049] and include evaporation from a centrifuged liquid [64], ion exchange [757], counter-current electromigration [1132], chromato¬ graphy [758], streaming through a separation nozzle [146], photosensitization [207, 1591], non-stationary molecular flow [1861], counter-current gas centrifuging [799], use of a D.C. electric field [791], passage of a molecular beam through an ionizing region [i860], electrophoresis [248] and methods specific to certain elements [89]. In one of these, the so-called “heat flush” method which utilizes the peculiar superfluid properties of liquid 4He below the lambda point, which are not exhibited by 3He, a very rapid separation of the isotopes is possible, and in a single flush a concentration of 3He by an enrichment factor of 3 X 104 has been obtained [1908].

10.5

THE USES IN PHYSICS OF SEPARATED ISOTOPES

The use of these materials in isotopic dilution analyses has been described in Chapter 3, and their use as tracer materials is treated in the next section.

10.5

THE USES IN PHYSICS OF SEPARATED ISOTOPES

447

Separated isotopes find many uses in physics [34] and some of these will be mentioned briefly. In nuclear physics research, one use is in the identification of naturally-occurring radio-active isotopes, and an early example of such a problem concerns the identity of the isotope responsible for the natural radio¬ activity of potassium [2020]. The emission of /5-particles by such a light element was so surprising that it was at first suggested that it must be due to the wide¬ spread occurrence of element 87 (Fr) as an impurity in potassium [499]. Partial separation of the isotopes led to the conclusion [885] that the radio-activity was not due to 39K. It was not, however, until the complete separation of the isotopes by Smythe and Hemmendinger [1901] that the radio-activity was conclusively proved to arise from 40K and not 41K. A neat method of deciding which of several isotopes of a particular element is radio-active was first used by Dempster and Wilkins [2183]. The isotopes of samarium were collected in a mass spectro¬ graph on a photographic plate. An unexposed plate placed emulsion-to-emulsion against this plate and stored with it in the dark for several weeks when developed showed a line under the position of 148Sm. This separation of 148Sm enabled the abundance of this a-particle emitter to be established, and hence its half-life to be determined from its rate of decay. Even more detailed information concerning a decay scheme can be obtained by the use of the technique of isotopic dilution to analyse for the very small amount of a particular isotope present in a large quantity of another element. Hayden, Reynolds and Inghram [840] were able to obtain the thermal neutron cross-sections, half-lives and branching ratios of five of the europium isotopes in the first investigation of this kind. The principle of the method of determination of branching ratio can be made clear by consideration of an analogous but rather simpler investigation [1687] of 64Cu which is produced from stable 63Cu by a (n, y) reaction. The odd-odd nucleus 64Cu (atomic number 29, neutron number 35) decays by /3“ emission to 64Zn and by K-capture to 64Ni. Addition of ordinary Zn and Ni followed by mass spectrometric measurement enables the relative amounts of 64Zn and 64Ni to be found and hence the branching ratio of the decay scheme to be determined. For many measurements in nuclear physics of the properties of the various nuclides, separated samples are essential. Neutron bombardment of separated uranium isotopes [1498] was necessary to identify the fissionable nucleus, and for studying nuclear cross-sections or nuclear reactions [51] on, say, van de Graafl accelerators, targets of separated isotopes are necessary. Separated isotopes also find application in spectroscopy and in solid-state physics [1169]. The differences in the masses of isotopes give rise to vibrational and rotational isotope effects in molecular spectra. A great variety of interesting spectroscopic effects is caused by differences between the various isotopes of their values of nuclear spin, magnetic moment, and electric quadrupole moment. Investigations of these effects are usually very difficult and sometimes impossible unless highly enriched isotopic samples are available. The study of isotope shifts in the optical spectra of atoms [670, 1170, 1847] is yielding information on the charge distribution in the nuclei of different isotopes and hence on nuclear sizes, shapes and structure. Many of the bulk properties of solids depend on the atomic masses, and though the effects are small and difficult to determine, work has been carried out on electrical conductivity, melting point, specific volume, specific heat and thermo-electric force [1 '346]. In the field of superconductivity, it has been shown that the critical temperature is inversely proportional to the atomic mass [1130]. The lattice difference between 6LiF and 7LiF has been

448

OTHER APPLICATIONS OF MASS SPECTROMETRY

10

measured by X-ray diffraction and it has been found that 7LiF is the smaller by the factor 1.0002. Samples of separated isotopes have found application as light sources. They can be used to produce very highly monochromatic light and are thus suitable as wavelength standards and for the accurate measurement of length. The techniques of measurements with separated isotopes often differ drastic¬ ally from corresponding techniques when large amounts of material are available. The physical and chemical properties of a new artificially-produced isotope are usually carried out on extremely minute quantities of a material which has a very short half-life. In work involving the stable nuclei the time factor is not a limi¬ tation and the absence of any radiation hazard simplifies the handling of materials. Nevertheless, the small scale on which most separated isotopes have hitherto been available has often limited the information obtainable in a measurement.

10.6

ISOTOPIC AGE DETERMINATION

The absolute time scale of geologic history can be constructed only by the use of various isotopic clocks based on the radio-active decay of relatively long-lived isotopes. Soon after radio-activity was discovered, it was recognized that such a method existed in principle, and in the last fifteen years, the experimental work in the field of geochronometry has produced accurate and sensitive methods of measurement, and several decay schemes are now widely used in age deter¬ minations. The age of the earth deduced from these measurements is of the order of 4.5 X 109 years and the age of the oldest rocks about 3.5 X 109 years. The most generally used methods in dating studies are the various methods based on lead, the rubidium-strontium method, and the potassium-argon method, and these have been reviewed by Kulp and others [15, 1138, 1174, 1664, 1830, 2085, 2092]. The principle behind the technique can best be explained by reference to the lead methods of age determination. The three naturally occurring radio¬ active series all result in the ultimate formation of a stable isotope of lead. Thus, 238U decays to give 206Pb, 235U to give 207Pb and 232Th to give 208Pb. The effective half-lives of all these decay schemes are known. It is generally assumed that before the earth solidified, the elements which make up the earth’s crust were all thoroughly mixed together. During solidi¬ fication, which occurred in a negligible time relative to the age of the earth, local concentrations of particular elements were formed, but it is generally assumed that no appreciable isotopic fractionation of the heavier elements accompanied the solidification process. Uranium and thorium occur widely distributed in minute quantities but occasionally are found in higher local concentrations in radio-active rocks. They are always accompanied by radiogenic lead consisting of the isotopes of masses 206, 207 and 208 as mentioned above. In addition they may be associated with “primaeval” lead which has not been entirely formed in a radio-active decay process. The presence of this primaeval lead would be extremely difficult to detect and measure were it not for the fact that it contains the stable isotope 204Pb which is not radiogenic. The amount of radiogenic lead formed by radio-active decay in any of the three natural radio¬ active series can readily be plotted as a function of time, since in less than a million years after deposition of a radio-active mineral the three decay series are essentially in equilibrium, and the ratio of the amount of this stable endproduct lead formed to the amount of any of the members of the series with which it is in secular equilibrium can, in theory, be used to determine the age of the mineral.

10.6

ISOTOPIC AGE DETERMINATION

449

The isotopes 206Pb, 207Pb and 208Pb have been produced by radio-active decay since the earth was formed. Before solidification of the earth’s crust these iso¬ topes would have been intimately mixed with the isotopes of primordial lead. The lead mixture, which we have called “primaeval lead”, would be solidified and thereafter, if separated from uranium and thorium, would remain invariable in isotopic composition, whereas any of this lead which solidified in a radio¬ active mineral, would continue to increase in 206Pb, 207Pb and 208Pb content. The age of any radio-active mineral may be found by the methods given below [4i, 152, 308, 332, 381-383, 426, 564, 620, 723, 734, 778, 846, 959-96i, 981, 982, 1027, 1028, 1099, 1153, 1161, 1172, 1173, 1175, 1213, 1214, 1223, 1313, 1323, 1347, 1457, 1493, 1495, 1499, 1564, 1574, 1575, 1604, 1749, 1750, 1803, 1844, 1921-1923, 1955, 2055, 2083, 2089, 2090, 2093, 2200, 2211]; the time between formation of the earth and solidification of this mineral can be calculated if the isotopic constitution of primordial lead and of primaeval lead are known as well as the terrestrial abundances of uranium, thorium and lead. Let na;(206Pb), na;(238U), etc., represent the number of atoms of radiogenic 206Pb, 238U, etc., present at time x ago all relative to 204Pb. Let A(238U), etc., represent the decay constants of 238U, etc. Then,

n,(238U) .

Similarly and

: no(238U).eA‘

(96)

no(206Pb) = ru(238U) — n0(238U)

(97)

.-. no(206Pb) = no(238U) [eA(aaaU)‘ — 1]

(98)

no(207Pb) = no(235U) [eA R'

\ /O i II

1

\ \

\R'

/

O

II

180 — C

HOR

R'

+ R18OH

The question as to which carbon atom was attacked in the reaction was answered by carrying out the reaction with n-amyl acetate in water enriched with lsO [i6ri]. The n-amyl alcohol formed was not enriched in lsO, proving that the first scheme represents the reaction occurring. The reverse reaction of esterifi¬ cation has also been studied. Using 180-labelled methanol to esterify benzoic acid, Roberts and Urey [1703] showed that the reaction occurring was CeHsCOOH + CH318OH -* C6H5C0180CH3 + HaO An experiment using heavy nitrogen has thrown some light on the decompo¬ sition of phenylhydrazine when distilled at atmospheric pressure in the absence of oxygen [357], which occurs according to the equation 2C6H5NHNH2 -> C6H5NH2 + C6H6 + N2 + NH3 Decomposition of the labelled molecule C6HsNH15NH2 gave ammonia containing only heavy nitrogen, nitrogen gas containing 1 atom of 15N (indicating that only one of the two atoms originated from the unsubstituted end of the hydrazine) and aniline which was not enriched in 15N. The scheme which suggests itself is thus that one molecule of phenylhydrazine gives up its nitrogen and hydrogenates its own phenyl residue to benzene as well as another phenylhydrazine molecule to aniline and ammonia. Tracers have been widely employed in the study of re-arrangement reactions and can give information about whether the reaction occurs by an inter- or an intra-molecular process as well as other details of the mechanism and rate. In some cases it would not be possible to detect even that a re-arrangement was taking place without the use of tracers. For example, when propane-l-13C is held over aluminium bromide at room temperature [150] it isomerizes to propane-2-13C until, at equilibrium, the number of molecules with 13C in an end position is twice that with 13C in the centre as would be expected since the introduction of 13C into the molecule changes the partition function only slightly. The reaction can be followed very elegantly by mass spectrometry. The fact that no propane molecules are formed in an inter-molecular process is evident from the fact that no increase in the peak at mass 46 corresponding to propane containing 2 atoms of 13C is observed during the isomerization. The rate of isomerization can be followed by the rate of growth of the peak at mass 30 relative to that at mass 29. Symmetrical 13C-containing propane gives the peak of mass 30 by simple bond cleavage [(C13CH5)+], whilst propane containing 13C in an end position gives both masses 29 and 30. The results must be corrected for the different probabilities of fragmentation of C-C and C-13C bonds in the enriched molecule (see below). These are 7% greater and 12% less respectively than the probability of breaking a C-C bond in ordinary propane,

456

OTHER APPLICATIONS OF MASS SPECTROMETRY

10

In some cases, it is necessary to label a molecule in two different ways in order to elucidate the mechanism of a reaction. For example, Doering, Taylor and Schoenewaldt [500] investigated the mechanism of the alkali-catalysed conversion of phenylglyoxal to mandelic acid. By using phenylglyoxal labelled with 13C on the ketone carbon, it was possible to prove that there was no re-arrangement of the carbon skeleton during the conversion. The failure of solvent deuterium to become attached to the alcohol carbon atom of mandelic acid prepared from phenylglyoxal in heavy water has allowed exclusion of a proton-removal mecha¬ nism and substantiated a mechanism involving intra-molecular transfer of hydrogen. It has been mentioned above how the mass spectrum of a labelled compound can be used to deduce the position of the labelled atom within the compound. In the case of the mandelic acid, however, the position of the 13C atom was deduced by a special technique of oxidation. The permanganate oxi¬ dation of mandelic acid gives benzoic acid and carbon dioxide, both of which can be analysed by mass spectrometry. C6H6C*H(OH)COOH +

o2

-> C6H5C*OOH +

co2

+ HaO

Complete oxidation to carbon dioxide and water would have led to mixing of the carbon atoms. Techniques such as this are also useful in, for example, iso¬ topic dilution work. If the carboxylic acid carbon had been labelled, isolation of CO2 by permanganate oxidation would have kept the specific activity at as high a level as possible, and given maximum sensitivity for the detection of small changes. A reasonably comprehensive list of tracer applications (using both radio¬ active and stable isotopes) for the study of organic reactions and covering the period up to 1954 has been given by Burr [301]. Similar applications of tracers to biology and medicine have been reviewed, for example, by Arnstein and Bentley [68]. Many of these applications are to extremely complicated systems and are often directed to the discovery of the starting material from which a particular product is formed. For example in an experiment to decide the source of the toxic oxides of nitrogen in silage making [1594] it was found that the percentage of 15NO in the gas above the silage in closed bottles was about the same whether the 15N was added as Na15N03 or as 15N-labelled amino acids, and the formation of this gas was attributed to reaction between nitrous acid and o-amino nitrogfen. The difficulty of obtaining an exact homologue of an atom is not completely overcome by the use of tracers. There are appreciable “isotope effects” by which isotopes may be distinguished apart from their difference in weight. A molecule can be optically active when the only cause of asymmetry is the replacement of a hydrogen atom by a deuterium atom [28, 573]. The differences between iso¬ topes are not apparent in the electronic configuration or the energy levels of the molecules which contain them; the bulk chemical behaviour of the molecules is not affected. If, however, a bond connecting the isotope to the molecule is broken during a chemical reaction, it will be found that the rate of the reaction [202] will be appreciably different for the several isotopes involved, especially when the breaking of the relevant bond is the rate-determining step in the reaction. The factors which contribute to the generally lower reactivity of bonds to deuterium compared to the corresponding bonds to hydrogen are the differences in free energy, the effect of the difference in mass on the velocity of passage over the potential-energy barrier and the possibility for non-classical penetration of

10.7

DETERMINATION OF THE MECHANISMS AND RATES OF REACTIONS

457

the energy barrier. The major factor which contributes to the free-energy difference is the difference in zero-point energy between a bond to deuterium or to hydrogen. The difference in energy is of the order 1.2-1.5 kcal/mole, the deuteriated compound having the lower energy and thus being the more stable. It can be shown [200, 2170] that this difference has the consequence of leading to different rates for reactions involving breaking of bonds to hydrogen or deute¬ rium. It can also be shown that differences in the equilibria reached in isotopic exchange reactions will be observed. Processes such as these are partly responsi¬ ble for naturally-occurring differences in the isotopic abundance ratios of the lighter elements. These isotope effects will be larger the larger the mass ratio of the isotopes; C-D bonds break 3 to 10 times more slowly than C—H bonds; 13C-C bonds break 3 to 4% more slowly than C-C bonds whilst 15N-N bonds break about 6% more slowly than N-N bonds. An example of a practical measurement of an isotope effect in chemical kinetic work is the study of the decarboxylation of malonic acid by Bigeleisen and Friedman [201] and by Lindsay, Burns and Thode [1251]. They found a value of hl2k2 of 1.037 at 137 °C for the reactions ,COOH ch2/

CO2 + CH3COOH

and

\:ooh 13COOH ch2/

k2

13C O2 + CH3COOH

\x>oh The differences are very much easier to detect in the case of the hydrogen isotopes. The rate of disappearance of the double-bond in 1-butene on nickel is four times as rapid in the presence of H2 as in the presence of D2 [r 995]. The difference in energy between C-H and C-T bonds is even greater than for C-H and C-D bonds, and makes the differences in rates of bond breaking amount to extreme selectivity. The isotopic rate effect is large enough to be useful only for the light elements such as C, H, N and O; if its effect is incautiously neglected it can by the same reasoning be large enough to be misleading. The isotope effect in the rupture of C—C bonds in propane by pyrolysis has been compared with the probability of rupture of a C-C bond in the mass spectrum. The probabilities of rupture of C-C bonds in propane-l-13C are 8% and about 20% greater than the rupture of 13C-C bonds in pyrolysis [1942] and mass spectrometry [150] respectively. For diatomic molecules, it would be expected that those containing the lighter isotopes would be the less stable due to their greater zero-point energies. This reflects itself in the fact that the ratio of parent ions to fragment ions is greater for molecules containing heavy isotopes. For tritium the parent to frag¬ ment ion ratio is almost twice that in deuterium and almost four times the value for hydrogen. Various measurements of the spectra of FI2, FID, D2, FIT and T2 have been made [134, 477, 687, 1772, 1941] and compared with values predicted from the zero-point energy and potential energy curves. Satisfactory agreement is found bearing in mind the possibility of discrimination effects at these low masses. Other diatomic molecules which have been studied with regard to the isotope effects include carbon monoxide'[477], oxygen [1778] and nitrogen [475]* In cases in which the shape of the potential energy curve of the molecular ion is not known, the isotope effects can be estimated by empirical methods [1773] and

458

OTHER APPLICATIONS OF MASS SPECTROMETRY

10

these can be extended [1779] to apply to polyatomic molecules such as carbon dioxide [477, 1480, 1775-1778, 1927] with good results. A number of deuteriated hydrocarbons [703] both saturated and unsaturated have also been studied by mass spectrometry. The saturated hydrocarbons in¬ clude deuteromethanes [478, 594, 1423, 2024], ethanes {476, 650], propanes [387, 388, 966, 1728, 1784, 2059] and butanes [1947], and cycloparaffins [1469] and the unsaturated hydrocarbons include ethylenes [290, 451, 486], acetylenes [379], benzenes and naphthalenes [1419]. The probabilities of loss of H and D atoms from a molecule can be given in several ways. For example, one could give for the molecule XD (where X represents, say, a hydrocarbon radical) the ratio of the probability of loss of D to the probability of loss of an individual H. Alternatively the probability of loss of D could be compared with the probability of loss of an individual H in the undeuteriated molecule. Another important ratio is that of the probabilities of loss of an individual H in the deuteriated and undeuteriated molecules. These ratios generally remain fairly constant as the ionizing electron voltage is increased above about 20 eV. Below this electron energy, the ratio of the probability of loss of deuterium relative to loss of hydrogen increases with decreasing voltage in saturated hydrocarbons. For CH3D, for example [594], at 50 eV loss of D is 0.38 as probable as loss of any one of the H atoms and becomes 0.7 as probable at 15 eV. It is generally true that loss of a single hydro¬ gen atom is from 10 to 80% more probable from a partially deuteriated aliphatic molecule than from the corresponding compound of carbon and hydrogen only. The probability of rupture of C-C bonds is also affected by the incorporation of deuterium into a molecule and the probability of a peak at, say, (p — CD3) from CD3CD2CD3 is nearly 20% greater than the corresponding peak formed by loss of a methyl radical from propane [387]. The differences in probabilities for removal of an FI or a D atom become much smaller in aromatic systems; in the case of benzene and naphthalene for example, it is possible to compute the mass spectra of the monodeutero derivatives on the basis of an equal a priori proba¬ bility of removing a single FI or D atom. The spectra of deuteriated compounds containing oxygen [273, 298, 388, 574, 691, 701, 707, 75T r 106, 1353, 1362, 1426, 1428, 1669], halogen [388, 479, 1431, 1728], and other [476, 1160, 1476, 1518, 1818] atoms also show interesting iso¬ tope effects, but insufficient work has yet been carried out to enable gener¬ alizations to be made. Work with 13C-enriched compounds has included carbon dioxide [1927] the isomeric propanes and butanes [1948], methylcyclopentane [1952] and camphene [708]; work with lsO has included carbon dioxide [1480, 1776] and lactones [702] and sulphur dioxide enriched in 34S has also been examined [2117]. It is to be expected [1947] that the effect of isotopic substitution on total ion intensity will be small, and this is confirmed by the various measurements which have been made of this quantity. Ionization potentials are little affected by isotopic substitution [1260]; no change is found in the ionization potentials of acetylene and ethylene when all their hydrogen atoms are replaced by deuterium. The methanes, on the other hand, show a detectable increase in ionization potential as successive hydrogen atoms are replaced by deuterium, amounting to 0.18 eV between CD4 and CH4 [1165]. This difference is too great to be accounted for entirely in terms of differences in zero-point energies. A similar result is obtained when the hydrogen atoms in ammonia are successively replaced by deuterium, the difference in ionization potential between ND3 and NFI3 being 0.22 eV. Various means have been suggested for analysing mixtures which differ only

10.8

MEASUREMENT OF IONIZATION POTENTIALS AND BOND STRENGTHS

459

in their isotopic contents. In theory it is possible to separate many of the peaks in the spectra of such mixtures by using a sufficiently high resolution. The mass difference between H2 and D is unfortunately only 1.55 mMU, and thus a resolving power of 646 per mass number is required to separate isobaric ions. In the mass spectra of the methanes, for example, the value of M/AM between (CH4)+ and (CH3D)+ at mass 16 is 10,365. The mass difference CH minus 13C is 4.48 mMU so that a resolution of 223.4 per mass number is required to separate doublets differing in this way. Although the appearance potentials of the parent ions lie close together, the value for ions such as (CFkDA from, say monodeuteromethane will lie comparatively much further away from the ionization potential (of the order 1 eV) [1944] and it is possible to analyse for parent ions without interference from fragments of other compounds present by working at an appropriate low value of bombarding-electron energy.

10.8

THE MEASUREMENT OF IONIZATION POTENTIALS AND BOND STRENGTHS

The first ionization potential of a molecule is defined as the energy difference between the ground vibrational levels of the lowest electronic states of the molecule and molecular ion. It is sometimes referred to as the adiabatic ioni¬ zation potential. The measured value of the minimum energy necessary to produce the molecular ion by a particular process may not equal the adiabatic ionization potential. The adiabatic ionization potential can generally be deduced spectroscopically by extrapolation of the positions of vibrational bands, but it may be impossible to produce the molecular ion in its ground state by, for example, electron bombardment and in such cases the measured value will exceed the spectroscopic value. The conditions under which ionization by electron impact takes place are described by the Franck-Condon principle according to which the positions of the relatively massive atomic nuclei within the molecule do not change while the electronic transition is taking place. If the potential energy curves representing the molecule before the transition and the molecular ion after the transition are plotted the points on these curves rep¬ resenting the conditions before and after ionization will correspond to the same nuclear configuration and will thus lie on a line parallel to the energy axis (see Fig. 177). Such a transition is referred to as a vertical transition, and the ionization potential measured in this way as the vertical ionization potential. This is the minimum energy necessary to remove an electron from the normal molecule without change of nuclear configuration. As pointed out by Morrison and Nicholson [1451] the spectroscopic ionization potential will not always give the adiabatic value. The position of maximum absorption in each band in the Rydberg series will correspond to vertical transitions through the FranckCondon region, and only in those cases where vibration lines are observed and the (0, 0) transition is identified positively by analysis of the vibrational struc¬ ture or by comparison with the spectra of isotopic molecules can the ionization limit found be labelled adiabatic with certainty. One can distinguish four different consequences of an electronic transition depending on the shapes of the po¬ tential energy curves of the initial and final states. These are illustrated in the curves shown in Fig. 177. In the first case, the equilibrium internuclear distance is effectively the same in the molecule and in the molecular ion (curve a), and under these conditions the probability of a transition between the ground vibra¬ tional states, given by the “overlap” integral, will be large. Under this condition, the vertical ionization potential will also be the adiabatic ionization potential. In

460

OTHER APPLICATIONS OF MASS SPECTROMETRY

10

the second case (not shown in the figure), the overlap of the wave functions for the lowest vibrational states is smaller and although there is still a probability that the quantum number will remain unchanged, an appreciable overlap now occurs between the wave functions of the lowest vibrational state of the mo¬ lecule and a higher state of the ion. Although the transition always results in a stable molecular ion, some of the ions will possess vibrational excitation. In

Fig. 177• Some methods of formation of (AB)+ and A+ ions from the molecule AB and the resultant transition probabilities.

the third case (shown as curve b), a certain proportion of the ions formed will be stable although vibrationally excited; since the region in which the final state of the ion must lie includes the continuum of [energy levels above the dissociation asymptote, a certain proportion of the transitions will lead to fragmentation. Since in the curve shown the equilibrium nuclear distance is different for the molecule and molecular ion the chance of obtaining an adiabatic transition is negligible. The width of the Franck-Condon region is generally less than 0.2 and in this case the “vertical” transition value gives only an upper limit for the adiabatic value. Nevertheless the probability of the adiabatic transition is still finite, and this suggests that in some cases the measured value of an ionization potential may depend on the sensitivity of the detecting apparatus used. Increasing the sensitivity is, in fact, equivalent to broadening the Franck-Condon region. The shape of the ionization efficiency curves will, as

A

10.8

MEASUREMENT OF IONIZATION POTENTIALS AND BOND STRENGTHS

461

discussed below, indicate when condition b obtains in an actual case. The fourth condition (curve c) illustrates a transition to a repulsive upper energy state. Here, the final state always lies in the continuum of energy levels; fragmentation ac¬ companies all such transitions and the excess energy of the fragments formed is given by the height of the transition region above the dissociation asymptote. N

Fig. 178. Ionization efficiency in mercury vapour as a function of the energy of the bombarding electrons with mass to charge ratio analysis of the ions formed.

Fig. 179. Ionization efficiency in mercury vapour as a function of the energy of the bombarding electrons without analysis of the ions formed.

For this reason, appearance potential values for fragments, as well as appearance potentials for molecular ions, which are obtained by methods involving vertical transitions will also lead to values which must be considered as upper limits of the “adiabatic” or “true” value. Ionization efficiency curves showing the number of ions formed as a function

462

OTHER APPLICATIONS OF MASS SPECTROMETRY

10

of bombarding-electron energy have been obtained for a wide variety of molecu¬ lar species [1193, 1322, 1548, 2042]. Curves obtained for mercury with and without mass analysis of the products formed are shown in Figs. 178 and 179. These were obtained by Bleakney [220]. The curve for Hg+ shows a maximum at about 50 eV followed by a slow fall in intensity to about 40% of the maximum value at 400 eV energy. Curves for the respective multiply-charged ions show maxima at progressively increasing values of electron energy. The curve shape of Fig. 179 is also typical of that obtained for large organic molecules. In these cases the majority of the ions formed may not be molecular ions and fragment ions may predominate. The proportion of multiply-charged ions is smaller than for monatomic gases and reflects the increased probability of fragmentation at high electron energies. The maximum efficiency of ionization usually occurs in the region of 50-100 eV [1987] which is one reason for the use of values of this order in chemical analysis. Many investigators have studied the shapes of the ionization efficiency curves for particular ion species in the vicinity of the ionization potential. The general form of the function appears to be of an initial curved region extending over about 1 eV followed by an approximately straight portion extending over, perhaps, 10 eV. In other words it would appear that usually the ionization crosssection is linearly related to electron energy for energies which exceed the ionization potential by more than about a volt. For the most accurate measure¬ ments it is necessary to be certain that there is only one process of ionization possible over the range of energies which is being studied. If more than one process contributes to the ion formation it is difficult to deduce from the shape of the curve obtained that two processes are occurring because of the fact that the onset of a new process is not a discontinuous effect. Indeed when the curve of Fig. 178 for the positive charges produced in mercury vapour is compared with that of Fig. 179 the onset of the various multiple-ionizations cannot be distin¬ guished and the only effect observed is a difference in the curvature of the two graphs. Early results showed a considerable variation in the amount of curvature for fragment ion efficiency curves [1147] and also that the amount of curvature for the graphs of doubly-charged molecular ions was much greater than that for the corresponding singly-charged ions [1936]. Several attempts have been made to obtain the shape of the ionization efficiency curve immediately above the threshold energy of ionization [131, 727, 2119, 2120, 2180, 2193]. Wannier [2119] deduces for the case of ionization by electron bombardment resulting in the formation of a parent ion and two electrons that the ion intensity should vary as the 1.1th power of the excess voltage above the ionization threshold. For multiple ionization (n-fold) the intensity should vary somewhat more rapidly than the nth power of the excess voltage [2120]. Instrumental effects can also give rise to curvature in the ionization efficiency graph, especially due to the spread in the energies of the bombarding electrons. The distribution in energy of the electrons emitted from a hot filament has been shown experimentally to be essentially Maxwellian [965, 1524], and the effect of this spread in energies upon the shape of the ionization curve has been the subject of detailed study [656, 965, 1710, 1936]. The most accurate measurements have been those in which the electron energy spread has been eliminated. The ionization probability curves for singly-charged ions obtained with essentially mono-energetic electrons show in the case of helium [888] that over a range of 8 eV the ion current varies linearly with the excess electron energy. The results are sufficiently accurate to suggest that the power of 1.1 suggested by Wannier should be 1.0. For doubly-

10.8

MEASUREMENT OF IONIZATION POTENTIALS AND BOND STRENGTHS

463

charged ions of the rare gases, the fact that several processes contribute to the observed ion current makes the power law difficult to obtain, though Hickam and his co-workers conclude that there is no evidence for other than a linear relationship. On the other hand, Clarke [346] using an electron energy selector has obtained in the case of Xe++ a curve which obeys the square law relationship predicted by Wannier [2120] over a range of 1.0 volt above the threshold, i.e. up to the onset of the first excited state of the ion. There are many sources of error which can enter into the measurement of an ionization potential by mass spectrometry. These have been discussed by Waldron and Wood [2104] and are briefly enumerated below. The occurrence of a potential gradient within the ionization chamber will give to the electrons an additional increment of energy and the ion repeller voltage is kept as small as possible for this reason, but provided the voltage of the repeller plate is kept constant this effect should not lead to an error in measuring the difference between two ionization potentials. The field within the ionization chamber is partly due to penetration of the ion accelerating field through the slit via which the ions emerge. Thus if the field within the ion chamber is to be the same for the examination of two ions of different mass, magnetic scanning must be used. The slit leading from the ionization chamber to the electron trap is usually relatively wide and the voltage on the trap can produce a considerable field penetration extending to the ionizing region. It is therefore usual to work with the trap, at the same potential as the ionization chamber. There is also the effect of the magnetic field in which the ionizing region is immersed. This will give rise to discriminations in that ions of different mass which are observed will originate in different regions of the ionization chamber and might thus receive different accelerations as they travel along the weak fields within the ionization chamber. This effect will be negligible if all ions are formed in a region in which the lines of equipotential are parallel to the electron beam. If the temperature of the filament which produces the electron beam increases, so does the spread in electron energy. This means that more electrons of a higher energy are available and the ionization probability curve therefore extends to a lower appearance potential. It is therefore desirable that the filament tem¬ perature should be held constant during the measurement of a pair of ionization efficiency curves rather than that the emission current should be controlled since this will tend to vary the filament temperature as the electron accelerating voltage is changed. The effect will be greatest at the lowest electron energies when the emission from the filament is space-charge limited. Contact potentials within the ionization chamber may vary when sample is introduced and for this reason it is usual to use an “internal” standard when measuring an ionization potential, i.e. to introduce a second substance of known ionization potential whose ionization efficiency curve can be compared with that of the substance under investigation while they are both in the system and are thus under identical contact potential conditions. The effective contact potential of the surface of a tungsten filament can vary depending upon its surface condition, and it is there¬ fore necessary to ensure that the filament is properly conditioned as dis¬ cussed earlier in this Chapter before measurements are commenced. Even when all variations due to the above causes have been eliminated, there still remains the problem of choosing the value of the desired potential from the curve. The various methods which have been used are critically discussed by Nicholson [1485]. He distinguishes nine different methods and these are listed below.

464

OTHER APPLICATIONS OF MASS SPECTROMETRY

10

One of the early methods used [1884] consisted in extrapolating the linear portion of the ionization probability curve to zero current and using the extra¬ polated value as the ionization potential. This method assumes that a linear relationship exists between ion current and electron energy down to the onset of ionization and that the entire “tail” of the curve is due to the energy spread in the electron beam. The method is now rarely used. In certain cases when a well-defined straight region exists, it can give quite accurate results and in these circumstances has the advantage of being less subjective than some of the other methods. In many cases, however, there is no such well defined linear region and under such circumstances the method is completely unusable. Its main dis¬ advantage can be summarized as due to the fact that it makes use of the currentvoltage relationship comparatively far above the ionization potential where the ions mav not be in their lowest energy states. The results obtained are generally higher than those from other methods especially when the appearance potentials of fragment ions formed by complex reaction paths are studied. An example of an occasion in which this method could not be used is given by the case of the appearance potential of H+ from CH4 compared with the appearance potential of He+. It was found [1872] that the upward break of the He+ curve was more abrupt than that for H+; the He+ curve was nearly a straight line, but the H+ curve showed considerable upward curvature. There are also objections to the use of the electron energy corresponding to the minimum detectable current [1891] as a measure of ionization potential because of the effect of electron energy spread in making the curve asymptotic to the energy axis so that the point detected in this way will depend on the sample pressure, the number of bombarding electrons and the sensitivity of the amplifier employed. When a rare gas such as neon is introduced at the same time as the sample, differences in the electron energy corresponding to the minimum detectable current in each case can be used to obtain the ionization potential. In this case seme of the objections to this method are overcome but there remains the objection that the shapes of the curves may not be the same even very near to the potentials at which the currents vanish. The use of the above two methods is discussed by Mariner and Bleakney [1306] who give a number of examples of their use in early measurements.

>

a-oo-

a)

£

I 90

1'70 l—-----._._.-....,0 I a 3 4 S 6 20

Fig. 180. Failure of the extrapolated difference method of estimating the first ionization potential when used at low currents (< io-12 amp). Krypton is being compared with argon.

An extension of the “vanishing current” method due to Warren [2128] has become known as the “extrapolated difference” method of determining appear¬ ance potentials. In this method, which uses an internal standard, the ion-current scales for the two graphs are adjusted so as to make the straight-line portions parallel and the difference in voltage between corresponding points on the curves is extrapolated to zero. The method is open to the same objections as the “vanish¬ ing-current” method but has the advantage of rendering differences in the shapes of the two curves clearly visible. It is, in fact important that the curves for the

1U.O

MEASUREMENTS OF IONIZATION POTENTIALS AND BOND STRENGTHS

465

compound studied and the internal reference should be closely similar in shape before accurate results can be achieved by any of these methods. Nicholson [1485] considers the measurement of the ionization potential difference between argon and krypton. He shows that the extrapolated potential difference method of kX arren leads to a value of 1.74 eV (the correct value is 1.759 eV) if the differences between 6 and 2 X 10-12 amp are extrapolated to zero, but that below 10~12 amp the difference curves sharply upwards as shown by Fig. 180. The effect is actually due to a small probability in the particular mass spectrometer used that ions of krypton can be formed after collision with two electrons [1433]. This process can be recognized since the dependence of ion current on electron current will follow a square-law relationship. Nevertheless it serves to illustrate how differences in the shape of the curves at very low values of ion current can lead to errors. Such changes can also occur, of course, from the presence of energy levels of the positive ion very near to the lowest state in which it can be formed. Several other methods which also depend for their success on the shapes of the two curves compared being closely similar are classified by Nicholson as the “logarithmic methods”. The first of these which is due to Honig and Wannier [965], whilst not attempting to eliminate the electron energy spread, accounts for it in an analytical expression which indicates that for about 1 eV below the ionization potential the curve will be approximately exponential. A semi-log plot of the ionization efficiency will give a curve of slope 2/(3/cT) at the ionization potential. Once again all curves are implicitly assumed to be of similar shape. Other methods which are similar to this are due to Lossing, Tickner and Bryce [1260] and to Dibeler and Reese [487]. All the methods discussed give values which are usually reliable to 0.1 eV for ions which give curves similar in form to those of rare gas ions. If special apparatus is used to give an essentially mono-energetic electron beam, the curves become easier to interpret since any fine structure which is smoothed out when there is a spread in electron energies is more easily observed. If, as suggested by Nicholson [1485] all errors arise from the fact that ionization efficiency curves do not have the same shape and differences in shape are due to different contri¬ butions of upper energy levels of the positive ion to the ionization efficiency curve, accuracy will be increased by any method which can detect fine structure in the curves. The early work of Nottingham [1524] using electrons homogeneous in energy made apparent the fine structure in the ionization efficiency curve of mercury near the ionization potential; it has also been shown that if photo¬ electrons rather than thermal electrons are used to produce ionization, fine structure is sometimes visible [1631, 1969], although the apparatus used did not include mass analysis. More recently Clarke [346] has used a 127° electrostatic velocity selector to obtain a beam of nearly monoenergetic electrons and has shown that as the spread in electron energy is reduced the curves do, in most ca¬ ses, become segmented straight lines with little curvature as the ionization potential is approached. In favourable cases he was able to obtain results accurate to 0.02 eV independent of the sensitivity of detection over a range of sensitivity of more than 10 to 1. A quite different approach to the problem of obtaining the effect of a monoenergetic electron beam has been made by Fox, Hickam, Kjeldaas and Grove [675, 677] with their “retarding potential difference” method. An electrode containing a slit is positioned between the filament and ionization chamber and its potential made negative with respect to the filament. Electrons from the

466

OTHER APPLICATIONS OF MASS SPECTROMETRY

10

lower portion of the energy distribution cannot pass through the slit in this electrode, but those which do are accelerated towards the ionization chamber, which they reach with an energy dependent only on the potential of this assembly relative to the filament and not on the interposed retarding potential. The electrons which enter the ionization chamber have a distribution in energy with a sharply-defined lower limit corresponding to those electrons which only just had sufficient energy to pass through the retarding electrode. If now the negative voltage on the retarding electrode is increased by a small amount, extra electrons contained in a narrow energy band corresponding to this voltage change will be prevented from entering the ion chamber. The reduction in ion current caused by the loss of these extra electrons (which are essentially mono-energetic) from the beam can thus be measured. The ionization efficiency curves obtained in this way are much more linear than those obtained from an inhomogeneous electron beam, but they still exhibit a very slight “tailing” at the lowest values of positive ion current, and this is attributed to the fact that the beam is rendered slightly inhomogeneous in energy by potential gradients in the ionization cham¬ ber as already discussed. This is overcome in a novel fashion by the use of a pulse technique. The ion repeller is adjusted to zero potential relative to the ionization chamber walls at the same time as a pulse of electrons is admitted. Thus ioni¬ zation takes place in a field-free region (except for field penetration from electrodes located outside the ion chamber). The electron beam is then turned off and the ion repeller switched to its operating potential so that ions leave the chamber and can be recorded. The repetition rate of this sequence of events is in the region 25-200 lcc/sec. Other precautions observed by Fox and his colleagues include operation with low magnetic field strength in the source region and the gold plating of critical electrodes to minimize contact potential effects which would give errors in the absolute measurement of the relevant voltages. Ionization potentials determined by the retarding potential difference technique give results much closer to those calculated by spectroscopic methods than do the methods which do not employ “mono-energetic” electrons. This is especially so for molecules such as benzene [633, 965, 1450, 1451] in which there are excited states of the ion close to its ground state [676, 1452]. Nevertheless, there are still some difficulties in the results obtained by this method which have not been resolved [1485], and the effective energy spread of 0.1 eV for the ionizing electrons is not adequate for all molecules studied. It has nevertheless proved possible to assign many of the segments of the curves to particular ionization processes. In particular it has been shown that auto-ionization [1835] occurs widely in these ionization processes. It is possible for an atom, by the excitation of two electrons, to contain more than sufficient energy to ionize it by the re¬ moval of one of the electrons. Such an atom, excited to a discrete energy level above the first ionization potential and into a region accompanied by a con¬ tinuum characterized by the same quantum numbers and parity, can undergo a radiationless transition into the state where it exists as an ion and an electron. The effect is sometimes referred to as the Auger effect by analogy with the effect obtained with X-rays. Morrison [1453] has emphasized that one of the difficulties in comparing the values for various energies obtained by mass spectrometry and by other methods is due to the failure to define exactly which energies are being measured; in order to interpret the ionization efficiency curves completely so that they can be made to give the probabilities for electronic transitions from the ground state

10.8

MEASUREMENTS OF IONIZATION POTENTIALS AND BOND STRENGTHS

467

of the molecule to the various excited states of the ion, it is necessary to make certain assumptions about the ideal shape of the ionization efficiency curves for various ionization processes. Morrison shows that if the probabilities for elec¬ tronic transitions can be obtained, these can be used not only to obtain ioni¬ zation potentials but also to obtain information about the shapes of the potential energy functions corresponding to the various ionic states. The threshold laws for ionization assumed by Morrison are those proposed by Wigner [2180] and Wannier [2120], namely that for single ionization by electron impact the prob¬ ability of ionization increases with the first power of the energy excess above the ionization potential and that for double ionization the probability increases with the second power of this energy excess. With these assumptions, Morrison proceeds to argue that the only discontinuity in the probability curve for single ionization occurs at the ionization potential and thus the second derivative of this curve will show a maximum at this value. Further, if the separate probability values for transitions to various excited ionic states are assumed to be additive, the fact that a transition to one of these states is taking place will be shown by a separate maximum in the second derivative of the probability curve. The maxima will not be infinitely sharp but the areas under them can be shown to be propor¬ tional to the transition probabilities for the processes concerned: the maxima will be further broadened due to the thermal excitation of the bombarded molecules, and the energy spread in the bombarding-electron beam. Such peaks are much more conveniently studied than are the direct ionization probability curves, especially since a definite significance can be attached to their shapes. These second derivatives do not actually contain any more information than is con¬ tained in the original ionization efficiency curves; the information is merely presented in a more easily studied fashion. The advantages of this method of presentation are illustrated in the examples below, but it is necessary to empha¬ size that the method is not an alternative to the “monochromatic” methods described above. Rather it is complementary to them and the most accurate results are obtained when the two methods are combined. Similar considerations apply to the third derivative of the ionization probability curve for production of doubly-charged ions by electron impact. In this case, the first derivative yields a curve of the same form as the ionization efficiency curve itself for single ionization. Morrison pointed out that many of the diffi¬ culties in interpretation of ionization efficiency curves have arisen in the past because of the use of electrons as the ionizing particles. If photons were used instead, many of the instrumental difficulties due to the charge carried by the electron which leads to changes in its kinetic energy in stray fields would imme¬ diately be overcome, contact potential difficulties would be eliminated so that the energy of the beam would be known accurately and the problem of producing a beam of radiation much more uniform in energy would be made easier. But equally as important, the threshold law for ionization with photons which has been given by Geltman [727] is of very suitable shape for an experimental study; the probability of ionization changes discontinuously at the critical energy from zero to a value which remains steady for more energetic beams. The first deriva¬ tive of the ionization efficiency curve leads to the peaks from which the prob¬ abilities of the electronic transitions can be derived. A simple example of the transition probability curves to be expected in the case of a hypothetical diatomic molecule is given by Fig. 177. ^Fhere the equilib¬ rium internuclear spacing for the ionized state of the molecule is close to that of the ground state, the relative electronic transition probabilities will be as at a,

468

OTHER APPLICATIONS OF MASS SPECTROMETRY

10

there will be a high probability associated with the lowest energy process and the adiabatic ionization potential will be measurable. Where the equilibrium internuclear spacing in the ion differs from that in the molecule (case b) the probability will rise as the energy is increased from zero to a maximum value. Under these conditions, the adiabatic ionization potential cannot be measured. In case b, the probability of formation of the molecular ion will show a cut-off value due to transitions above the dissociation limit: the fragment ion A+ will then appear with the probability shown corresponding to transitions to the continuum of energy states above the dissociation limit. The third type of transition probability shown at c corresponds to intersection of the Franck-Condon region with an upper state potential curve entirely above the dissociation limit. In this case the probability rises from zero to a maximum before falling again to zero. No transitions to give parent ions are observed in this case. The interpretation of the results obtained is much more complicated in the case of polyatomic molecules. When one weak bond joins two relatively stronglybound groups together it is sometimes possible to gain a close description of the dissociation of this bond in terms of two-dimensional potential energy curves but, in general, the states of a polyatomic molecule can be described only in terms of multidimensional potential energy surfaces. d'tl)

Fig. 181. The first derivatives of the ionization efficiency curves for the production of molecular ions and fCH2NH2j+ ions from ethylamine using photons monochromatic to 6 A. The curves, which give the tran¬ sition probabilities to the upper energy states should be compared with those of Fig. 177.

Some results obtained for polyatomic molecules [992, 993] are shown in Figs. 181 and 182. Fig. 181 shows the first derivative of the ionization efficiency curves obtained from ethylamine using photons monochromatic to about 6 A (i.e. about 0.05 eV at 10 eV). The curves obtained for the parent ion and also for the fragment ion (CFl2NH2)+ are very similar in shape to those shown in case c (Fig. 177) especialy when allowance is made for the smearing out of the curves in the practical case due to the spread in energy of the ionizing beam. A vertical ionization potential of 9.5 eV can be measured [371] but it is evident from the shape of the curve at lower energies that the adiabatic ionization potential cannot be measured and it is only possible to give an upper limit of about 8.75 eV for this quantity. The dissociation limit for production of the (CH2NH2)+ ion can be seen to be about 9.6 eV. Fig. 182 shows the first derivative curves for acetaldehyde, n-propanol and nitric oxide obtained by photon impact. The curves have been constructed by measuring slopes over energy intervals of 0.03 eV, and the fine structure shown is quite reproducible. This alone is not proof that the structure is caused by vibrational energy levels; such “structure”

10.8

MEASUREMENT OF IONIZATION POTENTIALS AND BOND STRENGTHS

469

may also arise due to imperfections in the micrometer drive for the grating or from the line structure in the light source usually used (capillary discharge in a mixture of hydrogen and deuterium). These effects have been allowed for in obtaining the spectra of Figs. 181 and 182. cl'Cl)

Fig. 182. The first derivatives of the ionization efficiency curves for the formation by photons of molecular ions in acetaldehyde, n-propanol and nitric oxide.

Other things being equal, the more easily ionized an electron (the less strongly bound) the less strongly bonding it is in a chemical sense, and attempts have been made to define the concept of bond order in terms of ionization potentials [2113]. It has hitherto been considered that ionization potentials obtained by mass spectrometry have not been as accurate or as reliable as those obtained spectroscopically in view of the difficulties of interpretation. The work described above with monochromatic ionizing beams should increase the usefulness of the mass spectrometric method, especially in cases such as the study of free radicals and other transient species where spectroscopic information is not available. Ionization potentials can be used to confirm values predicted for the removal of electrons from particular molecular orbitals [816, 1460, 1461] and to provide data for the calculation of electronic wave functions [817]. Appearance potentials of fragment ions can be used in conjunction with molecular ionization potentials and calorimetric data to estimate various thermo¬ dynamic quantities such as latent heats and bond strengths [1435]. The variety

470

10

OTHER APPLICATIONS OF MASS SPECTROMETRY

of ion species which can be studied by the mass spectrometer makes the methods of wide applicability, though the absolute accuracy attainable could usefully be increased in many cases. An example of the estimation of a latent heat of subli¬ mation is given below. This also gives a further illustration of the value of the second differential of the ionization efficiency curves obtained by electron bombardment in giving accurate information about the processes occurring. The example is provided by the determination of the latent heat of sublimation of carbon by a study of the fragmentation of methane on electron impact [1454]. The use of the mass spectrometer to measure this quantity by the direct eva¬ poration of carbon is described in the following section and gives unequivocally the value of 171 kcal/mole for the latent heat. Until the work described, how¬ ever, the electron impact method appeared to favour one of the other values (136 kcal/mole) possible on spectroscopic evidence. The electron impact method involves the measurement of the appearance potential of C+ from CH4. The measurement of the critical potential by conventional methods is of doubtful value due to the long region of initial curvature in the ionization efficiency curve and the measurements are also made more difficult by the relatively low abundance of formation of the ion C+ in the mass spectrum. The nett result of the possible reactions leading to formation of the C+ ion can be written CH4 -> C+ + 4H + e or

-* C+ + 2H + H2 + e

or

_> C+ + 2H2 + e

depending on the final state of aggregation of the fragments. Writing A(C+) as the appearance potential of the C+ ion, D(CH3 — H), D(CH2 — H), etc., as the bond dissociation energies for separation of a hydrogen atom from CH4, CH3, etc., and 1(C) as the ionization potential of carbon, one can write for the above three reactions the energy relationships A(C+) > 1(C) + D(CH3 — H) + D(CH2 — H) + D(CH — H) + D(C — H) or

> 1(C) + D(CH3 — H) + D(CH2 — H) + D(CH — H; + D(C — H) — D(H — H)

or

> 1(C) + D(CH3 — H) + D(CH2 — H) + D(CH — H) + D(C — H) — 2D(H — H)

From these one can obtain by measuring A(C+) and subtracting from it 1(C) = 11.2 eV an upper limit for the sum of the dissociation energy involved in the appearance of C+ ions. The energy obtained will only be a limiting value since it will include any kinetic or excitational energy of the fragments. From calorimetric measurements the heats of the following reactions are known. The values given have been converted into electron volts using the relationship 1 eV = 23.05 kcal/mole. C(s) + 2H2((7)

-*■

CH^);

H2(g) ~■> 2H(9);

AH = —0.80 eV AH = 4.48 eV

••• C(,) + 4H(ff) -> CH4(,)J

AH = —L(C) — 9.76 eV

(105)

C(,) + 2H (g) + H2((,) -»• CH4(S>;

AH = —L(C) — 5.28eV

(106)

+ 2H2(?) -> CH4(p);

AH = —L(C) — 0.79 eV

(107)

C(g)

10.8

MEASUREMENTS OF IONIZATION POTENTIALS AND BOND STRENGTHS

471

The values of L(C) in eV corresponding to 171 and 136 kcal/mole are respectively 7.42 and 5.90 eV. Measurements of A(C+) by Smith [1872] and by McDowell and Warren [1350] gave values of 26.7 ± 0.7 eV and 26.2 ± 0.2 eV, giving for the upper limit of the dissociation energy, values of 15.5 and 15.0 eV respectively. Attempts to measure the kinetic energy of the fragment C+ ions gave negative results and it was concluded that the amount of such energy could be neglected. Thus (105) above would seem to give the state of aggregation which results in the fragmentation process, since the latent heat of sublimation of carbon is then given by L(C) > or

>

5.7

eV

5.2

eV

which favour the value of 5.90 eV since the excess energy is thought to be small. The second derivative of the ionization probability as a function of electron energy has been given by Morrison and Stanton [1454] both for C+ ions from methane and for Ne+ ions from neon introduced at the same time and is shown in Fig. 183. The electrons used had “thermal” energy spread, otherwise as d"I

Fig. 183. The second derivatives of the probabilities of ionization, as functions of electron energy, of ions from methane and Ne+ ions.

C+

mentioned above even more accurate information could have been obtained. The effect of this spread in energies is shown by the threshold “tail” of the Ne+ peak. The curve obtained is close to that expected for a Maxwellian distribution of electron energies from a filament at about 2500°K. The threshold “tail” of the C+ ion is very much longer and is of the form which would be expected if the Franck-Condon region intersected a part of the potential energy surface for a completely repulsive state of (Cbb)"1", the vertical ionization potential of this state being 26.7 eV, but this level lying about 2.5 to 3 eV above the dissociation limit so that the decomposition will be accompanied by an excess energy of this order of magnitude. The derivative curve gives an extremely sensitive method of detecting this excess energy. Careful energy analysis of the C+ ions from methane shows that they possess an average kinetic energy of about 0.15 eV. The fact that the C+ ions possess any kinetic energy at all seems to rule out any possibility of a completely symmetrical mode of dissociation such as (105) and hence the dissociation occurring is probably (106) and/or (107). The low intensity part of the curve between 19.5 and 24 eV may be due to (107) but it is not possible to be sure of this or that there is not a small peak above 27 eV due to reaction (105) without using a monochromatic electron beam. It is not possible from the measurement of the C+ kinetic energy to estimate the total kinetic energy of all the separating frag-

472

OTHER APPLICATIONS OF MASS SPECTROMETRY

10

merits. If it is assumed, however, that the “adiabatic” appearance potential is < 24 eV and this value is inserted in the expressions, only for (106) does a reasonable value (7.5 eV) emerge suggesting this to be the reaction occurring and the value of the latent heat of sublimation of carbon to be 7.42 eV (171 kcal/mole) in agreement with the value obtained from vapour pressure measurements. Electron impact measurements on CF4, CCI4 and CBr4 by Reed and Sneddon [1670] lead to the value 7.4 eV for the latent heat indicating the ions to be formed without kinetic energy in these cases. The mass spectrometer can also be used to obtain estimates of the strengths of chemical bonds [524-526, 1154, 1246, 1669, 1982]. To continue with the example of methane, the calorimetric determination of the heat of the reaction C(p) + 4H(0) -* CH4(0) gives in the manner already outlined above the value of AH = •—396 kcal/mole. Thus, one may write the average C-H bond strength for removal of successive hydrogens from methane as 99 kcal/mole. Much more detailed information can sometimes be obtained by mass spectro¬ metry, since measurements can be made on products corresponding to removal of successive hydrogen atoms. The first mass spectrometric measurement of the dissociation energy D(CH3 — H) w&s made by Stevenson [1937]. If one considers the molecule R1R2 then one can write A(Ri+) > D(Ri — Ra) + I(Ri)

(108)

where A(Ri+) represents the appearance potential of the fragment ion Ri+, J(Ri) is the ionization potential of Ri and D(Ri — R2) is the dissociation energy or strength of the bond connecting Ri and R2. The inequality reflects the fact that, as in the case of the C+ ion# mentioned above, the ion may be formed with excess vibrational and kinetic ynergy. The reaction can be written in the alter¬ native fashion R1R2 ->Ri++R2;

AH4 C7H1.0N2O3 C7H12N3O2

.0870 .1109 .1347 .0996 .1235 .1473

170 7.499 7.873 8.248 8.230 8.604 8.979

.1994 .0578 .0817 .1055 .1405 .1643 .1882 .2120 .0466 .0704 .0943 .1181 .1769 .2007 .2246 .0830 .1068 .1307 .2133 .2372

6.469 10.602 9.047 7.506 8.860 7.314 9.930 8.416 8.212 9.380

18.38 11.35 13.71 17.02 13.82 17.26 13.23 16.05 16.26 15.18

10.454 8.729 7.018 10.976 9.279 7.595 5.926 6.737 11.553 9.882 8.226 6.584 4.956 9.069 7.460 5.865 10.539 8.911 7.296 5.695

7.16 9.00 11.73 7.48 9.26 11.80 15.76 15.18 7.74 9.43 11.78 15.29 21.07 11.67 14.69 19.32 9.54 11.70 14.80 19.62 9.58 11.52 14.19 18.09 11.56 14.30 18.35 11.34 13.70 17.00 13.81 17.23 13.22 16.03 16.24 15.16 14.22

11.414 9.818 8.237 6.669 9.648 8.061 6.488 10.621 9.067 7.527 8.880 7.334 9.951 8.438 8.233 9.402 10.650

10.466 8.741 7.031 10.990 9.293 7.610

7.17 9.01 11.73 7.49 9.26 11.80

MASSES AND ISOTOPIC ABUNDANCE RATIOS

C7H14N40 C8H2N40 C8H10O4 C8H12N03 C8H14N202 CgHieNgO C8H18N4 C9H2N2O2 C9H4N3O C9H6N4 C9H14O3 C9H16NO2 C9H18N2O C9H20N3 C10H2O3 C10H4NO2 C10H6N2O CioHgN3 C10H18O2 C10H20NO C10H22N2 C11H6O2 CnHsNO C11H10N2 C11H22O C11H24N C12H10O C12H12N C12H26 C13H14 C14H2

C6H7N2O4 C6H9N3O3 C6H11N4O2 C7H9NO4 C7H11N2O3 C7H13N3O2 c7h15n4o C8HN302

c8h3n4o C8H1104 C8H13N03 C8H15N202 C8H17N30 C8H19N4 C9HN03 C9H3N202 C9H5N30 c9h7n4 C9H1503 C9H17N02 C9H19N20 C9H21N3 C10H3O3 C10H5NO2

.1711 .0772 .1122 .1360 .1599 .1837 .2076 .0660 .0898 .1137 .1486 .1725 .1963 .2201 .0547 .0786 .1024 .1263 .1851 .2089 .2327 .0912 .1150 .1388 .2215 .2453 .1276

9.353 10.242 8.961 9.336 9.710

.1514 .2579 .1640 .0701

13.541 13.383 14.272 15.160

.0952 .1190 .1429 .1078 .1316 .1554 .1793 .0615 .0854 .1203 .1442 .1680 .1919 .2157 .0503 .0741 .0980 .1218 .1568 .1806 .2045 .2283 .0629 .0867

10.084 10.459 10.598 10.973 11.347 10.067 10.441 10.815 11.190 10.955 11.330 11.704 12.078 11.172 11.547 11.921 12.061 12.435 12.810 12.278 12.652 13.166

171 7.515 7.889 8.264 8.246 8.620 8.995 9.369 9.883 10.258 8.977 9.352 9.726 10.100 10.475 10.240 10.614 10.989 11.363 10.083 10.457 10.831 11.206 10.971 11.346

5.941 6.753 11.567 9.897 8.242 6.600 4.973 9.086 7.478 5.883 10.555 8.927 7.313 5.713 11.431 9.836 8.255 6.689 9.666 8.080 6.507 10.640 9.087 7.547 8.899 7.354 9.972 8.460 8.255 9.425 10.675

15.74 15.17 7.75 9.43 11.78 15.28 21.03 11.66 14.67 19.29

10.478

7.17 9.01 11.73 7.49 9.26 11.80 15.73 11.76 15.15 7.75 9.43 11.78 15.27 20.99 9.55 11.66 14.66 19.25

8.754 7.044 11.003 9.306 7.624 5.956 8.405 6.770 11.581 9.912 8.257 6.616 4.990 10.724 9.103 7.495 5.902 10.571 8.944 7.331 5.731 11.449 9.855

9.54 11.70 14.79 19.59 9.58 11.52 14.18 18.06 11.56 14.29 18.32 11.34 13.68 16.97 13.80 17.20 13.20 16.01 16.21 15.14 14.20

9.54 11.69 14.78 19.55 9.58 11.51

C10H7N2O C10H9N3 C10H19O2 C10H21NO C10H23N2 C11H7O2 C11H9NO C11H11N2 C11H23O C11H25N C12H11O C12H13N C13HN C13H15 C14H3

C6H8N2O4 C6H10N3O3 C6H12N4O2 C7H10NO4 C7H12N2O3 C7H14N3O2 c7h,6n4o C8H2N302 C8H4N40 C8H1204 C8H14N03 C8H16N202 CsHigNsO C8H20N4 C9H2NO3 C9H4N2O2 C9H6N3O C9H8N4 C9H16O3 C9H18NO2 C9H20N2O C9H22N3 C10H4O3 C10H6NO2 C10H8N2O C10H10N3 C10H20O2 C10H22NO C10H24N2 C11H8O2 CnHioNO C11H12N2 C11H24O C12H12O C12H14N c13h2n

.1106

11.720

.1344 .1932 .2170 .2409 .0993 .1231 .1470 .2296 .2535 .1357 .1596 .0657 .1721 .0783

12.094 11.188 11.563 11.937 12.077 12.451 12.826 12.294 12.668 13.182 13.557 14.445 14-288 15.176

.1033 .1272 .1510 .1159 .1397 .1636 .1874 .0697 .0935 .1285 .1523 .1762 .2000 .2239 .0584 .0823 .1061 .1300 .1649 .1888 .2126

C14H4

.2364 .0710 .0949 .1187 .1425 .2013 .2252 .2490 .1074 .1313 .1551 .2378 .1439 .1677 .0738 .1803 .0864

C6H9N2O4

.1115

Ci3Hi4 C9H11NO3 C9H13N2O2 C9H15N3O C9H17N4 C10HN2O2 C10H3N3O

12.54 14.79 14.97 14.05

C10H5N4 C10H13O3 C10H15NO2 C10H17N2O

7.23 7.60

C10H19N3 C11HO3 C11H3NO2 C11H5N2O

.0984 .1223 .0872 .1110 .1349 .1587 .0998 .1236 .1474 .1713 .1951 .0774 .1012 .1362 .1600 .1839 .2077 .0661 .0900 .1138 .1626 .1964 .2203 .0787 .1025 .1264 .2090 .2329 .1151 .1390 .2455 .1516

.0827 .1066 .1304 .0953 .1192 .1430 .1668 .0729 .1079 .1317 .1556 .1794 .2033 .0617 .0855 .1094 .1443 .1682 .1920 .2158 .0504 .0743 .0981

8.922 9.296 9.279 9.653 10.027 10.402 10.010 10.384 10.759 11.133 11.507 12.021 12.396 11.115 11.490 11.864 12.238 12.378 12.753 13.127 12.221 12.595 12.970 13.109 13.484 13.858 13.326 13.701 14.215 14.589 14.432 15.321

9.564 7.893 11.849 10.191 8.548 6.918 12.503 10.873 9.257 7.654 6.066 8.625 7.070 11.608 10.019 8.444 6.883 11.021 9.479 7.952 10.834 9.287 7.753 11.902 10.387 8.887 10.183 8.677 11.349 9.876 9.655 10.918

9.33 11.78 7.83 9.47 11.73 15.04 8.01 9.55 11.62 14.55 18.97 13.94 17.53 9.58 11.47 14.05 17.78 11.23 13.45 16.51 11.28 13.56 16.73 11.01 12.98 15.59 13.09 15.79 12.53 14.77 14.95 14.03

11.262 9.578 7.907 11.864 10.207 8.564 6.935 7.842 12.519 10.889 9.274 7.672 6.085 10.212

7.60 9.33 11.78 7.83 9.47 11.73 15.02 14.42 8.01 9.55 11.62 14.53

181 8.564 8.938 9.312 9.295 9.669 10.043 10.418 11.306 10.026 10.400 10.775 11.149 11.523 11.663 12.037 12.412 11.131 11.506 11.880 12.254 12.020

8.644 7.090 11.625 10.037 8.463 6.903 12.596

12.394 12.769

11.041 9.500

18.94 11.42 13.93 17.51 9.58 11.46 14.04 17.75 9.54 11.23 13.44

515

MASSES AND ISOTOPIC ABUNDANCE RATIOS

C11H7N3 C11H1702 C11H19NO C11H21N2 C12H502 C12H7NO C12H9N2 C12H21O C12H23N C13H9O C13H11N C13H25 C14H13 C15H

C7H6N2O4 C7H8N3O3 C7H10N4O2 c8h8no4 C8H10N2O3 C8H12N302 C8H14N40 C9H2N40 C9H10O4 C9H12N03 C9H14N202 C9H16N30 C9H18N4 C10H2N2O2 C10H4N3O C10H6N4 C10Hi4O3 C10H16NO2 C10H18N2O C10H20N3 C11H203 C11H4N02 C11H6N20 C11H8N3 C11H1802 C11H20NO C11H22N2 C12H602 C12H8NO C12H10N2 C12H220 C12H24N Ci3HioO C13H12N C13H26 C14H14 C15H2

C7H7N2O4 C7H9N3O3

.1219 .1807 .2046 .2284 .0869 .1107 .1345 .2172 .2410 .1233 .1471 .2536 .1597 .0658

.0909 .1147 .1386 .1035 .1273 .1511 .1750 .0811 .1160 .1399 .1637 .1876 .2114 .0698 .0937 .1175 .1525 .1763 .2002 .2240 .0586 .0824 .1063 .1301 .1889 .2127 .2366 .0950 .1188 .1427 .2253 .2492 .1314 .1553 .2617 .1678 .0740

.0990 .1229

13.143 12.237 12.611 12.986 13.125 13.500 13.874 13.342 13.717 14.231 14.605 14.448 15.337 16.225 182 8.580 8.954 9.328 9.311 9.685 10.059 10.434 11.322 10.042 10.416 10.791 11.165 11.539 11.679 12.053 12.428 11.147 11.522 11.896 12.270 12.036 12.410 12.785 13.159 12.253 12.627 13.002 13.141 13.516 13.890 13.358 13.733 14.247 14.621 14.464 15.353 16.241 183 8.596 8.970

7.973 10.854 9.307 7.774 11.923 10.409 8.909 10.205 8.699 11.372 9.899 9.678 10.943 12.287

11.276 9.592 7.922 11.879 10.222 8.580 6.952 7.860 12.535 10.906 9.291 7.690 6.103 10.231 8.663 7.110 11.643 10.056 8.482 6.922 12.615 11.061 9.520 7.994 10.873 9.327 7.795 11.944 10.431 8.932 10.226 8.721 11.394 9.923 9.701 10.967 12.133

11.290 9.606

16.49 11.27 13.55 16.70 11.01 12.97 15.57 13.07 15.77 12.51 14.75 14.93 14.02 13.21

7.61 9.34 11.78 7.84 9.48 11.72 15.01 14.41 8.01 9.55 11.61 14.52 18.91 11.42 13.91 17.48 9.57 11.46 14.03 17.73 9.54 11.22 13.43 16.46 11.27 13.54 16.68

C7H11N4O2 C8H9NO4 CsHnNaOa C8H13N3O2 c8h15n4o C9HN3O2 C9H3N4O C9H11O4 C9H13NO3 C9H15N2O2 C9H17N3O C9H19N4 C10HNO3 C10H3N2O2 C10H5N3O C10H7N4 C10H15O3 C10H17NO2 C10H19N2O C10H21N3 C11H3O3 C11H5NO2 C11H7N2O C11H9N3 C11H19O2 C11H21NO C11H23N2 C12H7O2 C12H9NO C12H11N2 C12H23O C12H25N CiaHnO C13H13N C13H27 C14HN C14H15 C15H3

12.96 15.55 13.06 15.75 12.50 14.73 14.91 14.00 13.19

C7H8N2O4 C7H10N3O3 C7H12N4O2 C8H10NO4 C8H12N2O3 C8H14N3O2 C8H16N4O C9H2N3O2 C9H4N4O C9H12O4 C9H14NO3 C9H16N2O2 C9H18N3O

7.61 9.34

C9H20N4 C10H2NO3 C10H4N2O2 C10H6N3O

11.00

.1467 .1116 .1354 .1593 .1831 .0654 .0892 .1242 .1480 .1719 .1957 .2196 .0541 .0780 .1018 .1257 .1606 .1845 .2083 .2321 .0667 .0906 .1144 .1382 .1970 .2209 .2447 .1031 .1270 .1508 .2335 .2573 .1396 .1634 .2699 .0695 .1760 .0821

.1072 .1310 .1548 .1198 .1436 .1674 .1913 .0735 .0974 .1323 .1562 .1800 .2039 .2277 .0623 .0861 .1100

9.344 9.327 9.701 10.075 10.450

7.937 11.894 10.238 8.596 6.968 9.473 7.878 12.551 10.923 9.308 7.708 6.121 11.831 10.250 8.683 7.129 11.661

11.77 7.84 9.48 11.72 15.00 11.57 14.39 8.01 9.55 11.61 14.51 18.88 9.57 11.41 13.90 17.45 9.57 11.45 14.01 17.70

10.964 11.338 10.058 10.432 10.807 11.181 11.555 11.321 11.695 12.069 12.444 11.163 11.538 11.912 12.286 12.052 12.426 12.801 13.175 12.269 12.643 13.018 13.157 13.532 13.906 13.374 13.749 14.263 14.637 14.480 15.526 15.369 16.257

12.634 11.080 9.541 8.015 10.893 9.348 7.816 11.965 10.452 8.954 10.248 8.743 11.417 9.946 9.724 11.228 10.992 12.339

9.54 11.22 13.42 16.44 11.26 13.53 16.66

184 8.612 8.986 9.360 9.343 9.717 10.091 10.466 10.980 11.354 10.074 10.448 10.823 11.197 11.571 11.337 11.711 12.085

11.303 9.621 7.952 11.909 10.253 8.612 6.985 9.490 7.896 12.567 10.939 9.326 7.726 6.140 11.849 10.268 8.702

7.62

10.074 8.501 6.942

11.00 12.95 15.53 13.05 15.73 12.49 14.72 14.89 13.83 13.98 13.18

9.34 11.77 7.85 9.48 11.72 14.98 11.57 14.38 8.02 9.55 11.61 14.49 18.85 9.57 11.41 13.89

516 C10H8N4 C10H16O3 C10H18NO2 C10H20N2O C10H22N3 C11H403 CuH6N02 C11H8N2O C11H10N3 C11H20O2 C11H22NO C11H24N2 C12H8O2 C12H10NO C12H12N2 C12H24O c12h26n C13H12C) Ci3H14N C13H28 C14H2N C14H16 C15H4

C7H9N204 C7H11N303 C7H13N402 c8hn4o2 C8HhN04 C8H13N2O3 C8H15N3O2 C8H17N4O C9HN2O3 C9H3N3O2 C9HeN40 C9H13O4 C9Hi5N03 C9Hl7N202 C9H19N3O C9H2lN4 C10HO4 C10H3NO3 C10H5N2O2 C10H7N3O C10H9N4 C10H17O3 C10H19NO2 C10H21N2O C10H23N3 C11H6O3 C11H7NO2 ChH9N20

CxiHnNa C11H21O2 C11H23NO C11H25N2

APPENDIX

.1338 .1688 .1926 .2164 .2403 .0749 .0987 .1225 .1464 .2052 .2290 .2529 .1113 .1351 .1590 .2416 .2654 .1477 .1716 .2780 .0777 .1841 .0902

.1153 .1392 .1630 .0691 .1279 .1517 .1756 .1994 .0578 .0817 .1055 .1405 .1643 .1882 .2120 .2358 .0466 .0704 .0943 .1181 .1419 .1769 .2007 .2246 .2484 .0830 .1068 .1307 .1545 .2133 .2372 .2610

12.460 11.179 11.554 11.928 12.302 12.068 12.442 12.817 13.191 12.285 12.659 13.034 13.173 13.548 13.922 13.390 13.765 14.279 14.653 14.496 15.542 15.384 16.273 185 8.628 9.002 9.376 10.265 9.359 9.733 10.107 10.482 10.622 10.996 11.370 10.090 10.464 10.839 11.213 11.587 10.978 11.353 11.727 12.101 12.476 11.195 11.570 11.944 12.318 12.084 12.458 12.833 13.207 12.301 12.675 13.050

7.149 11.679 10.092 8.520 6.962 12.654 11.100 9.561 8.036 10.913 9.368 7.837 11.986 10.474 8.976 10.269 8.765 11.440 9.970 9.747 11.253 11.017 12.365

11.317 9.635 7.967 8.782 11.924 10.269 8.628 7.002 11.115 9.508 7.914 12.583 10.956 9.343 7.744 6.158 13.461 11.867 10.287 8.721 7.169 11.697 10.111 8.539 6.981 12.673 11.120 9.582 8.057 10.932 9.388 7.857

17.43 9.57 11.45 14.00 17.67 9.54 11.21 13.41 16.42 11.26 13.51 16.63 10.99 12.94 15.51 13.04 15.70 12.48 14.70 14.87 13.81 13.96 13.16

7.62 9.34 11.77 11.69 7.85 9.48 11.71 14.97 9.56 11.57 14.37 8.02 9.55 11.60 14.48 18.81 8.16 9.57 11.40 13.88 17.40 9.57 11.44 13.99 17.65 9.54 11.20 13.39 16.39 11.25 13.50 16.61

1

C12H9O2 CmHuNO C12H13N2 C12H25O C12H27N C13HN2 Ci3H130 C13H] 5N C14HO C14H3N C44HX7

C15H5

C7H10N2O4 C7H12N3O3 C7H14N4O2 C8H2N4O2 C8H12NO4 C8H14N2O3 C8H16N3O2 C8Hi8N40 C9H2N2O3 C9H4N3O2 C9H6N4O C9H14O4 C9H16NO3 C9H18N2O2 C9H20N3O C9H22N4 C10H2O4 C10H4NO3 C10H6N2O2 CxoH8N30 C10H10N4 C10H16O3 C10H20NO2 C10H22N2O C10H24N3 ChH(,03 C11H8NO2 C11H10N2O C11H12N3 C11H22O2 C11H24NO C11H2SN2 C12H10O2 C12H12NO C12H14N2 C12H26O C13H2N2 C13Hi40 CxsHieN C14H2O C14H4N C14H18 Ci5H6

.1194 .1433 .1671 .2498 .2736 .0732 .1559 .1797 .0620 .0858 .1923 .0984

.1235 .1473 .1711 .0772 .1360 .1599 .1837 .2076 .0660 .0898 .1137 .1486 .1725 .1963 .2201 .2440 .0547 .0786 .1024 .1263 .1501 .1851 .2089 .2327 .2566 .0912 .1150 .1388 .1627 .2215 .2453 .2692 .1276 .1514 .1753 .2579 .0814 .1640 .1878 .0701 .0939 .2004 .1065

13.189 13.564 13.938 13.406 13.781 14.827 14.295 14.669 15.183 15.558 15.400 16.289 186 8.644 9.018 9.392 10.281 9.375 9.749 10.123 10.498 10.638 11.012 11.386 10.106 10.480 10.855 11.229 11.603 10.994 11.369 11.743 12.117 12.492 11.211 11.586 11.960 12.334 12.100 12.474 12.849 13.223 12.317 12.691 13.066 13.205 13.580 13.954 13.422 14.843 14.311 14.685 15.199 15.574 15.416 16.305

10.98 12.92 15.49 13.03 15.68 14.51 12.47 14.68

12.007 10.496 8.998 10.290 8.787 10.218 11.463 9.993 12.714 11.278 11.041 12.391

11.94 13.80 13.95 13.15

11.331 9.650 7.982 8.798 11.939 10.285 8.645 7.018 11.132 9.525 7.933 12.600 10.973 9.360 7.762 6.177 13.479 11.885 10.306 8.741 7.189 11.715 10.129 8.558 7.001 12.692 11.140 9.602 8.078 10.952 9.408 7.878 12.028 10.517 9.021 10.313 10.242 11.486 10.016 12.739 11.303 11.066 12.417

7.63 9.35 11.77 11.69 7.85 9.48 11.71 14.96 9.56 11.56 14.35 8.02 9.55 11.60 14.47 18.78 8.16 9.57 11.39 13.86 17.38 9.57 11.44 13.98 17.62 9.53 11.20 13.38 16.37 11.25 13.49 16.59 10.98 12.91 15.47 13.02 14.49 12.46 14.66 11.93 13.78 13.93 13.13

517

MASSES AND ISOTOPIC ABUNDANCE RATIOS

C7H11N204 C7H13N303 C7H15N402 c8hn3o3 c8h3n4o2 C8Hi3N04 CgHisNaOs CBH17N3O2 C8H19N4O C9HNO4 c9h3n2o3 C9H5N302 c9h7n4o

CgHi504 C9Hi7N03 C9Hi9N202 C9H2iN30 c9h23n4 CioH304 Ci0H5NO3 CioH7N202 C10H9N3O C10H11N4 C10H19O3 CioH2iN02 CioH23N20 CioH25N3 ChH703 C11H9NO2 CiiHhN20 C11H13N3 ChH2302 C11H25NO Ci2HN3 Ci2Hh02 C12H13NO c12h15n2 Ci3HNO C13H3N2 C13H15O c13h17n Ci4H30 C14H5N Ci4Hl9 C15H7

C7Hi2N204 C7Hi4N303 C7H18N4O2

C8H2N303 C8H4N402 C8Hi4N04 C8Hi6N203 C8Hi8N302 C8H2oN40

.1316 .1554 .1793 .0615 .0854 .1442 .1680 .1919 .2157 .0503 .0741 .0980 .1218 .1568 .1806 .2045 .2283 .2521 .0629 .0867 .1106" .1344 .1582 1193 2 .2170 .2409 .2647 .0993 .1231 .1470 .1708 .2296 .2535 .0769 .1357 .1596

187 8.660 9.034 9.408 9.922 10.297 9.391 9.765 10.139 10.514 10.279 10.654 11.028 11.402 10.122 10.496 10.871 11.245 11.619 11.010 11.385 11.759 12.133 12.508 11.227 11.602 11.976 12.350 12.116 12.490 12.865 13.239 12.333 12.707 14.128 13.221 13.596 13.970

.1834 .0657 .0895 .1721 .1960 .0783 .1021 .2086 .1147

14.484 14.859 14.327 14.701 15.216 15.590 15.432 16.321

.1397 .1636

188 8.676 9.050

.1874 .0697 .0935 .1523 .1762

.2000 .2239

9.424 9.938 10.313 9.407 9.781 10.155 10.530

11.345 9.664 7.997 10.448 8.814 11.954 10.300 8.661 7.035 12.769 11.149 9.543 7.951 12.616 10.990 9.378 7.780 6.196 13.496 11.904 10.325 8.760 7.209 11.733 10.148 8.577 7.021 12.712 11.160 9.623 8.099 10.972 9.427 9.257 12.049 10.539 9.043 11.728 10.265 11.509 10.040 12.763 11.327 11.090 12.443

11.359 9.678 8.012 10.464 8.831 11.969 10.316 8.677 7.052

7.63 9.35 11.76 9.50 11.68 7.86 9.48 11.71 14.95 8.05 9.56 11.56 14.34 8.02 9.55 11.59 14.45 18.75 8.16 9.56 11.39 13.85 17.35 9.57 11.43 13.96 17.59 9.53 11.19 13.37 16.35 11.24 13.48 15.26 10.97 12.90 15.45 12.35 14.48 12.45 14.64 11.92 13.76 13.92 13.12

7.64 9.35 11.76 9.50 11.68 7.86 9.48 11.70 14.93

C9H2NO4 c9h4n2o3

C9HeN302 c9h8n4o CgHi604 C9Hi8N03 C9H2oN202 c9h22n3o c9h24n4 CioH404 CioH6N03 CioH8N202 CioHioN30 CioHi2N4 CioH2o03 CioH22N02 CioH24N20 ChH803 C11H10NO2 CnHi2N20 CuHi4N3 ChH2402 Ci2H2N3 Ci2Hi202 Ci2Hi4NO Ci2Hi6N2 Ci3H2NO Ci3H4N2 C13H16O c13h18n Ci4H40 c14h6n Ci4H2o Ci5H8

C7Hi3N204

C?Hi5N303 C7Hi?N402 CsHN204 c8h3n3o3 C8HsN402 C8Hi5N04

C8Hi7N203 C8Hi9N302 C8H2iN40 C9H3N04 C9H5N203

C9H7N302 c9h9n4o

C9Hi704 C9Hi9N03 C9H2iN202 CgH23N30

CioHs04 C10H7NO3 CioH9N202

.0584 .0823 .1061 .1300 .1649 .1888 .2126 .2364 .2603 .0710 .0949 .1187 .1425 .1664 .2013 .2252 .2490 .1074 .1313 .1551 .1790 .2378 .0851 .1439 .1677 .1915 .0738 .0977 .1803 .2041 .0864 .1102 .2167 .1228

.1479 .1717 .1956 .0540 .0778 .1017 .1605 .1843 .2082 .2320 .0666 .0904 .1143 .1381 .1731 .1969 .2207 .2446 .0792 .1030 .1268

10.295 10.670 11.044 11.418 10.138 10.512 10.887 11.261 11.635 11.026 11.401 11.775 12.149 12.524 11.243 11.618 11.992 12.132 12.506 12.881 13.255 12.349 14.144 13.237 13.612 13.986 14.500 14.875 14.343 14.717 15.232 15.606 15.448 16.337 189 8.692 9.066 9.440 9.580 9.954 10.329 9.423 9.797 10.171 10.546 10.311 10.686 11.060 11.434 10.154 10.528 10.903 11.277 11.042 11.417 11.791

12.786 11.166 9.561 7.969 12.632 11.007 9.395 7.798 6.214 13.514 11.922 10.344 8.779 7.229 11.751 10.167 8.597 12.731 11.180 9.643 8.121 10.991 9.280 12.071 10.561 9.065 11.751 10.289 11.532 10.063 12.787 11.352 11.115 12.469

12.44 14.63 11.91 13.75 13.90 13.10

11.373 9.693 8.027 12.126 10.480 8.847 11.984 10.331 8.693 7.069 12.802 11.183 9.578 7.987 12.648 11.023 9.412 7.816 13.532 11.940 10.362

7.64 9.35 11.76 7.90 9.50 11.67 7.86 9.48 11.70 14.92 8.05 9.56 11.55 14.32 8.03 9.55 11.58 14.43 8.16 9.56 11.38

8.05 9.56 11.55 14.33 8.03 9.55 11.59 14.44 18.72 8.16 9.56 11.38 13.84 17.32 9.57 11.43 13.95 9.53 11.19 13.36 16.32 11.24 15.24 10.97 12.89 15.43 12.34 14.46

518

APPENDIX

C10H11N3O C10H13N4 C10H21O3 C10H23NO2 C11HN4 C11H903 C11H11N02 C11H13N20 C11H15N3 C12HN20 Ci2H3N3 C12H1302 Ci2Hi5NO Ci2H17N2 C13HO2 C13H3NO C13H5N2 C13H17O C13H19N C14H5O C14H7N C14H21 C15H9

.1507 .1745 .2095 .2333 .0806 .1156 .1394 .1633 .1871 .0694 .0932 .1520 .1759 .1997 .0581 .0820 .1058

C7H14N2O4 C7Hl6N303 C7H18N402 C8H2N204 C8H4N303 C8H6N402 C8H16N04 C8H18N203 C8H20N3O2 C8H22N40 C9H4N04 C9H6N203 C9H8N302 C9H10N4O CgHisOj

.1560 .1799 .2037 .0621 .0860 .1098 .1686 .1925 .2163 .2401 .0747 .0986 .1224 .1462 .1812 .2050 .2289 .0873 .1112 .1350 .1588 .1827 .2176 .0888 .1237 .1476

C9H20NO3 C9H22N202 C10H6O4 C10H8NO3 C10H10N2O2 C10H12N3O C10HX4N4 C10H22O3 C11H2N4 C11H10O3 C11H12N02 C11H14N20 C11H16N3 C12H2N20 C12H4N3 C12H1402 C12H16NO

.1884 .2123 .0945 .1184 .2249 .1310

.1714 .1953 .0775 .1014 .1602 .1840

12.165 12.540 11.259 11.634 13.428 12.148 12.522 12.897 13.271 13.785 14.160 13.253 13.628 14.002 14.142 14.516 14.891 14.359 14.733 15.248 15.622 15.464 16.353 190 8.708 9.082 9.456 9.596 9.970 10.345 9.439 9.813 10.187 10.562 10.327 10.702 11.076 11.450 10.170 10.544 10.919 11.058 11.433 11.807 12.181 12.556 11.275 13.444 12.164 12.538 12.913 13.287 13.801 14.176 13.269 13.644

8.799 7.249 11.769 10.185 8.345 12.750 11.200 9.664 8.142 10.791 9.302 12.092 10.582 9.088 13.250 11.775 10.313 11.554 10.087 12.812 11.377 11.140 12.495

11.387 9.708 8.042 12.141 10.496 8.864 11.999 10.347 8.709 7.086 12.819 11.200 9.596 8.006 12.665 11.040 9.430 13.549 11.958 10.281 8.818 7.269 11.787 8.366 12.770 11.220 9.685 8.163 10.813 9.325 12.113 10.604

13.83 17.30 9.57 11.42 16.09 9.53 11.18 13.35 16.30 12.77 15.22 10.96 12.88 15.41 10.67 12.33 14.44 12.43 14.61 11.90 13.73 13.88 13.09

7.65 9.36 11.76 7.90 9.50 11.67 7.87 9.48 11.70 14.91 8.06 9.56 11.54 14.30 8.03 9.55 11.58 8.16 9.56 11.37 13.81 17.27 9.57 16.07 9.53 11.18 13.33 16.28 12.76 15.20 10.95 12.87

1

C12H18N2 C13H2O2 C13H4NO C13H6N2 Cj3Hi80 C13H20N CmHgO Ci4H8N C14H22 C15H10

C7H15N2O4 C7H17N3O3 C7Hi9N402 C8H3N204 C8H5N303 C8H7N402 c8Hl7N04 C8Hl9N2C>3 C8H21N302 C9H5N04 C9H7N203 C9H9N302 C9H11N40 C9H1904 C9H21N03 C10H7O4 C10H9NO3 C10H11N2O2 C10H13N3O C10H15N4 CnHN30 C11H3N4 C11H11O3 C11H13NO2 C11H15N2O C11H17N3 C12HNO2 C12H3N2O C12H5N3

.2078 .0663 .0901 .1139 .1966 .2204 .1027 .1265 .2330 .1391

.1642 .1880 .2119 .0703 .0941 .1180 .1768 .2006 .2244 .0829 .1067 .1306

C14H23 C15H11

.1544 .1894 .2132 .0955 .1193 .1431 .1670 .1908 .0731 .0969 .1319 .1557 .1796 .2034 .0618 .0857 .1095 .1683 .1921 .2160 .0744 .0982 .1221 .2047 .2286 .1108 .1347 .2412 .1473

C7H16N2O4 C7H18N3O3

.1723 .1962

Ci2Hi502

C12H17NO C12H19N2 C13H3O2 C13H5NO

Cl3H7N2 C13H190 C13H21N C14H70 Ci4H9N

14.018 14.158 14.532 14.907 14.375 14.749 15.264 15.638 15.480 16.369

9.110 13.273 11.798 10.337 11.577 10.111 12.836 11.402 11.165 12.521

15.39 10.67 12.32 14.42 12.42 14.59 11.89 13.72 13.87 13.07

191 8.724 9.098 9.472 9.612 9.986 10.361 9.455 9.829 10.203 10.343 10.718 11.092 11.466 10.186 10.560 11.074 11.449 11.823 12.197 12.572 13.086 13.460 12.180 12.554 12.929 13.303 13.443 13.817 14.192 13.285 13.660 14.034 14.174 14.548 14.923 14.391 14.765 15.280 15.654 15.496 16.385

11.400 9.722 8.058 12.157 10.512 8.881 12.014 10.363 8.726 12.835 11.217 9.614 8.024 12.681 11.057 13.567 11.977 10.400 8.838 7.289 9.903 8.388 12.789 11.240 9.705 8.184 12.337 10.835 9.348 12.134 10.626 9.132 13.295 11.821 10.361 11.600 10.134 12.860 11.427 11.189 12.548

7.65 9.36 11.76 7.91 9.50 11.67 7.87 9.49 11.69 8.06 9.56 11.54 14.29 8.03 9.55 8.16 9.56 11.37 13.80 17.25 13.21 16.05 9.52 11.17 13.32 16.25 10.90 12.75 15.18 10.95 12.86 15.37 10.66 12.31 14.40

11.414 9.737

7.66 9.36

192 8.740 9.114

12.41 14.57 11.88 13.70 13.85 13.06

519

MASSES AND ISOTOPIC ABUNDANCE RATIOS

C7H20N4O2 C8H4N204 C8H6N303 C8H8N402 C8H18N04 C8H20N2O3 c9h6no4 C9H8N203 C9H10N3O2 C9H12N40 C9H20O4 CioH804 C10H10NO3 C10H12N2O2 C10H14N3O C10H16N4 C1XH2N3O C11H4N4 C11H12O3 C11H14NO2 C11H16N2O C11H18N3 C12H2NO2 C12H4N2O C12H6N3 C12HX6O2 CxaHxsNO C12H20N2 C13H4O2 CxaHeNO CxsHsNa C13H20O Cx3H22N Ci4H80 C14H10N C14H24 C15H12

C7H17N2O4 C7H19N3O3 C8H5N2O4 C8H7N3O3 C8H9N4O2 C8H19NO4 C9H7NO4 C9H9N2O3 C9H11N3O2 C9H13N4O C10HN4O C10H9O4 C10H11NO3 C10H13N2O2 C10H15N3O C10H17N4 C1XHN2O2 C11H3N3O

.2200 .0784 .1023 .1261 .1849 .2086 .0910 .1149 .1387 .1625 .1975 .1036 .1274 .1513 .1751 .1990 .0812 .1051 .1400 .1639 .1877 .2145 .0700 .0938 .'1176 .1764 .2003 .2241 .0826 .1064 .1302 .2129 .2367 .1190 .1428 .2493 .1554

.1805 .2043 .0866 .1104 .1343 .1931 .0992 .1230 .1468 .1707 .0768 .1117 .1356 .1594 .1833 .2071 .0655 .0894

9.488 9.628 10.002 10.377 9.471 9.845 10.359 10.734 11.108 11.482 10.202 11.090 11.465 11.839 12.213 12.588 13.102 13.476 12.196 12.570 12.945 13.319 13.459 13.833 14.208 13.301 13.676 14.050 14.190 14.564 14.939 14.407 14.781 15.296 15.670 15.512 16.401 193 8.756 9.130 9.644 10.018 10.393 9.487 10.375 10.750 11.124 11.498 12.387 11.106 11.481 11.855 12.229 12.604 12.744 13.118

8.073 12.172 10.528 8.897 12.029 10.379 12.852 11.235 9.631 8.042 12.697 13.585 11.995 10.419 8.857 7.310 9.924 8.409 12.809 11.260 9.726 8.206 12.359 10.857 9.370 12.155 10.648 9.155 13.318 11.844 10.385 11.624 10.158 12.885 11.452 11.214 12.574

11.428 9.751 12.188 10.544 8.914 12.044 12.868 11.252 9.649 8.061 9.064 13.603 12.013 10.438 8.877 7.330 11.473 9.945

11.75 7.91 9.50 11.66 7.87 9.49 8.06 9.55 11.53 14.28 8.04 8.16 9.56 11.36 13.79 17.22 13.20 16.03 9.52 11.16 13.31 16.23 10.89 12.74 15.16 10.94 12.84 15.35 10.66 12.30 14.39 12.39 14.55 11.87 13.68 13.83 13.04

7.66 9.36 7.91 9.50 11.66 7.88 8.06 9.55 11.53 14.26 13.67 8.16 9.56 11.36 13.78 17.20 11.11

13.19

CxiH5N4 C11H13O3 C11H15NO2 CxxHi?N20 C11H19N3 C12HO3 C12H3NO2 C12H5N2O C12H7N3 C12H17O2 C12H19NO C12H21N2 C13H5O2 C13H7NO C13H9N2 C13H21O C13H23N C14H9O C14H11N

.1132 .1482 .1720 .1958 .2197 .0543 .0781 .1020 .1258 .1846

C14H25 C15H13 CifiH

.2574 .1635 .0696

C7H18N2O4 C8H6N2O4 C8H8N3O3 C8H10N4O2 C9H8NO4 C9H10N2O3 C9H12N3O2 C9H14N4O C10H2N4O

.1886 .0947 .1186 .1424 .1073 .1311 .1550 .1788 .0849 .1199 .1437 .1676 .1914 .2153 .0737 .0975

C10H10O4 C10H12NO3 C10H14N2O2 C10H16N3O C10H18N4 C11H2N2O2 C11H4N3O CuH6N4 C11H14O3 C11H16NO2 CxiHxsNaO C11H20N3 C12H2O3 C12H4NO2 C12H6N2O C12H8N3 C12H18O2 C12H20NO C12H22N2 C13H6O2 Ci3H8NO C13H10N2 C13H22O C13H24N

.2084 .2323 .0907 .1145 .1384 .2210 .2449 .1271 .1510

.1214 .1563 .1802 .2040 .2278 .0624 .0863 .1101 .1339 .1927 .2166 .2404 .0988 .1227 .1465 .2292 .2530

13.492 12.212 12.586 12.961 13.335 13.100 13.475 13.849 14.224 13.317 13.692 14.066 14.206 14.580 14.955 14.423 14.797 15.312 15.686 15.528 16.417 17.306 194 8.772 9.660 10.034 10.409 10.391 10.766 11.140 11.514 12.403 11.122 11.497 11.871 12.245 12.620 12.760 13.134 13.508 12.228 12.602 12.977 13.351 13.116 13.491 13.865 14.240 13.333 13.708 14.082 14.222 14.596 14.971 14.439 14.813

8.431 12.828 11.280 9.747 8.227 13.895 12.380 10.880 9.393 12.177 10.670 9.177 13.341 11.868 10.409 11.647 10.181 12.909 11.478 11.239 12.600 14.040

16.00 9.52 11.16 13.30 16.21 9.43 10.88 12.73 15.14 10.94 12.83 15.33 10.65 12.29 14.37 12.38 14.53 11.86 13.67 13.82 13.03 12.33

11.442 12.203 10.560 8.930 12.885 11.269 9.667 8.079 9.083 13.620 12.032 10.457 8.896 7.350 11.493 9.966 8.453 12.848 11.301 9.767 8.248 13.916 12.402 10.902 9.416 12.198 10.692 9.200

7.67 7.92 9.50 11.66 8.06 9.55 11.52 14.25 13.65 8.17 9.56 11.35 13.76 17.17 11.10 13.18 15.98 9.52 11.15 13.29 16.19 9.43 10.88 12.72 15.12 10.93 12.82 15.31 10.64 12.28 14.35 12.37 14.52

13.364 11.891 10.432 11.670 10.205

520 C14H10O C14H12N C14H26 C15H14 C16H2

C8H7N204 C8H9N303 C8H11N402 C9H9N04 C9H11N203 C9H13N302 C9H15N4C) C10HN3O2 C10H3N4O C10H11O4 C10H13NO3 C10H15N2O2 C10H17N3O C10H19N4 C11HN03 C11H3N202 C11H5N30 C11H7N4 C11H1503 C11H17N02 C11H19N20 C11H21N3 C12H303 C12H0NO2 C12H7N20 C12H9N3 C12H1902 C12H21NO C12H23N2 C13H702 C13H9NO C13H11N2 C13H230 C13H25N C14H110 Ci4H13N C14H27 C15HN C16H15 CieHs

C8H8N204 C8H10N3O3 C8H12N4O2 C9H10NO4 C9H] 2N2O3 C9H14N3O2 C9HieN40 C10H2N3O2

APPENDIX

.1353 .1591 .2656 .1717 .0778

15.328 15.702 15.544 16.433 17.322

.1029 .1267 .1505 .1155 .1393 .1631 .1870 .0692 .0931 .1280 .1519 .1757 .1996 .2234 .0580 .0818 .1057 .1295 .1645 .1883 .2121 .2360 .0706 .0944 .1182 .1421 .2009 .2247 .2486 .1070 .1308 .1547 .2373 .2611 .1434 .1673 .2737 .0734 .1798 .0859

195 9.676 10.050 10.425 10.407 10.782 11.156 11.530 12.045 12.419 11.138 11.513 11.887 12.261 12.636 12.401 12.776 13.150 13.524 12.244 12.618 12.993 13.367 13.132 13.507 13.881 14.256 13.349 13.724 14.098 14.238 14.612 14.987 14.455 14.829 15.344 15.718 15.560 16.606 16.449 17.338

.1110 .1349 .1587 .1236 .1474 .1713 .1951 .0774

196 9.692 10.066 10.441 10.423 10.798 11.172 11.546 12.061

12.934 11.503 11.264 12.626 14-068

11.85 13.65 13.80 13.02 12.31

12.219 10.576 8.947 12.902 11.286 9.685 8.097 10.657 9.103 13.638 12.050 10.476 8.916 7.370 13.054 11.514 9.987 8.474 12.867 11.321 9.788 8.270 13.937 12.423 10.924 9.439 12.219 10.714 9.222 13.386 11.914 10.456 11.693 10.229 12.958 11.528 11.289

7.92 9.50 11.65 8.07 9.55 11.52 14.24 11.30

12.906 12.653 14.095

13.64 8.17 9.55 11.35 13.75 17.15 9.50 11.10 13.17 15.96 9.52 11.15 13.27 16.16 9.42 10.87 12.71 15.10 10.93 12.81 15.29 10.64 12.27 14.33 12.36 14.50 11.84 13.64 13.78 12.87 13.00 12.30

12.234 10.592 8.964 12.918 11.308 9.703 8.116 10.677

7.92 9.50 11.65 8.07 9.55 11.51 14.23 11.30

1 .1012 .1362 .1600 .1839 .2077 .2315 .0661 .0900 .1138 .1376 .1726

12.435

C12H10N3 Ci2H2o02 C12H22NO C12H24N2 C13H8O2 C13H10NO C13H12N2 C13H24O C13H26N C14H12O Ci4H14N C14H28 Ci5H2N C15H16 C16H4

.1964 .2203 .2441 .0787 .1025 .1264 .1502 .2090 .2329 .2567 .1151 .1390 .1628 .2455 .2693 .1516 .1754 .2819 .0815 .1880 .0941

12.634 13.009 13.383 13.148 13.523 13.897 14.272 13.365 13.740 14.114 14.254 14.628 15.003 14.471 14.845 15.360 15.734 15.576 16.622 16.465 17.354

C8H9N204 CsHnNsOs C8H13N4O2 CflHN402 C9H11NO4 C9H13N2O3 C9H15N3O2 CeHi7N40 C10HN2O3 C10H3N3O2 C10H5N4O C10H13O4 CioHjsNOa C10H17N2O2 C10H19N3O C10H21N4 C11HO4 CnH3N03 CnH5N202 C11H7N3O C11H9N4 C11H17O3 C11H19NO2

.1192 .1430 .1668 .0729 .1317 .1556 .1794 .2033 .0617 .0855 .1094 .1443 .1682 .1920 .2158 .2397 .0504 .0743 .8981 .1219 .1458 .1807 .2046

197 9.708 10.082 10.457 11.345 10.439 10.814 11.188 11.562 11.702 12.077 12.451 11.170 11.545 11.919 12.294 12.668 12.059 12.433 12.808 13.182 13.556 12.276 12.650

C10H4N4O C10H12O4 C10H14NO3 C10H16N2O2 C10H18N3O C10H20N4 C11H2NO3 C11H4N2O2 CnHeNsO CiiH8N4 C11H16O3 ChH18N02 C11H20N2O CiiH22N3 C12H4O3 C12H6NO2 C12H8N2O

11.154 11.529 11.903 12.277 12.652 12.417 12.792 13.166 13.540 12.260

9.123 13.656 12.069 10.495 8.936 7.390 13.074 11.534 10.008 8.496 12.887 11.341 9.809 8.291 13.958 12.445 10.946 9.461 12.241 10.736 9.245 13.409 11.938 10.480 11.716 10.253 12.983 11.553 11.314 12.932 12.679 14.123

12.250 10.608 8.980 9.891 12.935 11.321 9.721 8.134 12.263 10.696 9.143 13.674 12.087 10.514 8.955 7.411 14.648 13.094 11.554 10.029 8.517 12.907 11.361

13.63 8.17 9.55 11.34 13.74 17.12 9.50 11.09 13.16 15.94 9.51 11.14 13.26 16.14 9.42 10.87 12.70 15.09 10.92 12.80 15.27 10.63 12.25 14 32 12.35 14.48 11.83 13.62 13.94 12.85 12.99 12.29

7.92 9.50 11.64 11.48 8.07 9.55 11.51 14.21 9.54 11.29 13.62 8.17 9.55 11.34 13.73 17.09 8.23 9.50 11.09 13.14 15.92 9.51 11.13

521

MASSES AND ISOTOPIC ABUNDANCE RATIOS

C11H21N20 C11H23N3 C12H503 C12H7N02 C12H9N20 C12H11N3 C12H2102 C12H23NO C12H25N2 C13H902 C13H11NO C13H13N2 C13H250 C13H27N C14HN2 C14H130 C14Hi5N C14H29 C15HO C15H3N C15H17 CieHs

.2284 .2523 .0869 .1107 .1345 .1584 .2172 .2410 .2649 .1233 .1472 .1710 .2536 .2774 .0771 .1597 .1835 .2900 .0658 .0896 .1961 .1022

13.025 13.399 13.164 13.539 13.913 14.288 13.381 13.756 14.130 i 1.270 14.644 15.019 14.487 14.861 15.907 15.376 15.750 15.592 16.264 16.638 16.481 17.370

9.830 8.312 13.979 12.467 10.968 9.484 12.262 10.758 9.268 13.432 11.961 10.504 11.739 10.276 11.820 13.008 11.578 11.339 14.355 12.959 12.705 14.151

13.25 16.12 9.42 10.86 12.69 15.07 10.91 12.79 15.25 10.62 12.24 14.30 12.34 14.46 13.46 11.82 13.60 13.75 11.33 12.84 12.97 12.28

C8H10N2O4 C8H12N3O3 C8H14N4O2 C9H2N4O2 C9H12NO4 C9H14N2O3 C9K16N3O2 C9H18N4O C10H2N2O3 C10H4N3O2 C10H6N4O C10H14O4 C10H16NO3 C10H18N2O2 C10H20N3O C10H22N4 C11H2O4 C11H4NO3 C11H6N2O2 CnHsNsO C11H10N4 C11H18O3 C11H20NO2 C11H22N2O C11H24N3 C12H6O3 C12H8NO2 C12H10N2O C12H12N3 C12H22O2 C12H24NO C12H26N2 C13H10O2

-.1273 .1511 .1750 .0811 .1399 .1637 .1876 .2114 .0698 .0937 .1175 .1525 .1763 .2002 .2240 .2478 .0586 .0824 .1063 .1301 .1539 .1889 .2127 .2366 .2604 .0950 .1188 .1427 .1665 .2253 .2492 .2730 .1314

198 9.724 10.098 10.473 11.361 10.455 10.830 11.204 11.578 11.718 12.093 12.467 11.186 11.561 11.935 12.310 12.684 12.075 12.449 12.824 13.198 13.572 12.292 12.666 13.041 13.415 13.180 13.555 13.929 14.304 13.397 13.772 14.146 14.286

12.265 10.624 8.997 9.909 12.952 11.338 9.738 8.153 12.282 10.715 9.163 13.692 12.105 10.533 8.975 7.431 14.667 13.114 11.575 10.050 8.539 12.926 11.381 9.851 8.334 14.000 12.488 10.991 9.507 12.284 10.780 9.290 13.455

7.93 9.50 11.64 11.47 8.07 9.55 11.50 14.20 9.54 11.29 13.61 8.17 9.55 11.33 13.72 17.07 8.23 9.49 11.08 13.13 15.89 9.51 11.13 13.24 16.10 9.41 10.85 12.67 15.05 10.91 12.78 15.23 10.62

C13H12NO C13H14N2 Ci3H2eO C13H28N C14H2N2, C14H14O CwHieN C14H30 C15H2O C15H4N C15H18 Ci6H6

.1553 .1791 .2617 .2856 .0852 .1678 .1917 .2982 .0740 .0978 .2043 .1104

14.660 15.035 14.503 14.877 15.923 15.392 15.766 15.608 16.280 16.654 16.497 17.386

11.985 10.528 11.762 10.300 11.846 13.032 11.603 11.364 14.381 12.985 12.732 14.179

12.23 14.28 12.33 14.44 13.44 11.81 13.59 13.74 11.32 12.83 12.96 12.26

C8H11N2O4 C8H13N3O3 C8H15N4O2 C9HN3O3 C9H3N4O2 C9H13NO4 C9H15N2O3 C9H17N3O2 C9H19N4O C10HNO4 C10H3N2O3 C10H5N3O2 C10H7N4O C10H15O4 C10H17NO3 C10H19N2O2 C10H21N3O C10H23N4 C11H3O4 C11H5NO3 C11H7N2O2 C11H9N3O C11H11N4 C11H19O3 C11H21NO2 C11H23N2O C11H25N3 C12H7O3 C12H9NO2 C12H11N2O C12H13N3 C12H23O2 C12H25NO C12H27N2 C13HN3 C13H11O2 C13H13NO C13H15N2 C13H27O C13H29N C14HNO C14H3N2 C14H15O

.1354 .1593 .1831 .0654 .0892 .1480 .1719 .1957 .2196 .0541 .0780 .1018 .1257 .1606 .1845 .2083 .2321 .2560 .0667 .0906 .1144 .1382 .1621 .1970 .2209 .2447 .2686 .1031 .1270 .1508 .1747 .2335 .2573 .2811 .0808 .1396 .1634 .1872 .2699 .2937 .0695 .0934 .1760

199 9.740 10.114 10.489 11.003 11.377 10.471 10.846 11.220 11.594 11.360 11.734 12.109 12.483 11.202 11.577 11.951 12.326 12.700 12.091 12.465 12.840 13.214 13.588 12.308 12.682 13.057 13.431 13.197 13.571 13.945 14.320 13.413 13.788 14.162 15.208 14.302 14.676 15.051 14.519 14.893 15.565 15.939 15.408

12.281 10.640 9.014 11.520 9.927 12.968 11.355 9.756 8.171 13.880 12.300 10.735 9.183 13.710 12.124 10.055 8.995 7.451 14.686 13.134 11.595 10.071 8.561 12.946 11.402 9.872 8.355 14.021 12.510 11.013 9.530 12.305 10.802 9.313 10.784 13.478 12.008 10.553 11.785 10.324 13.293 11.871 13.057

7.93 9.51 11.64 9.55 11.46 8.07 9.55 11.50 14.19 8.18 9.54 11.28 13.59 8.17 9.55 11.33 13.70 17.04 8.23 9.49 11.07 13.12 15.87 9.51 11.12 13.23 16.08 9.41 10.85 12.66 15.03 10.90 12.76 15.21 14.10 10.61 12.22 14-26 12.32 14.43 11.71 13.43 11.80

522 C14H17N C15H30 C15H5N C15H19 CioHv

APPENDIX 1 .1998 .0821 .1059 .2124 .1185

15.782 16.296 16.670 16.513 17.402

C15H20 C10H8

.1141 .2206 .1267

200 9.756 10.130 10.505 11.019 11.393 10.487 10.862 11.236 11.610 11.376 11.750 12.125 12.499 11.218 11.593 11.967 12.342 12.716 12.107 12.481 12.856 13.230 13.604 12.324 12.698 13.073 13.447 13.213 13.587 13.961 13.336 13.429 13.804 14.178 15.224 14.318 14.692 15.067 14.535 15.581 15.955 15.424 15.798 16.312 16.686 16.529 17.418

C8H13N2O4

.1517

201 9.772

C8H12N2O4 C8H14N3O3 C8H16N4O2 C9H2N3O3 C9H4N4O2 C9H14NO4 C9H16N2O3 C9H]8N3C>2 C9H20N4O C10H2NO4 C10H4N2O3 C10H6N3O2 C10H8N4O C10H16O4 C10H18NO3 C10H20N2O2 C10H22N3O C10H24N4 CnH404 CnHeNOs CnHsN202 C11H10N3O C11H12N4 C11H20O3 C11H22NO2 C11H24N2O C11H26N3 C12H8O3 C12H10NO2 C12H12N2O C12H14N3 C12H24O2 C12H26NO C12H28N2 C13H2N3 C13H12O2 C13H14NO Ci3HiaN2 C13H28O C14H2NO C14H4N2 C14H10O C14H18N C15H4O CisHeN

.1436 .1674 .1913 .0735 .0974 .1562 .1800 .2039 .2277 .0623 .0861 .1100 .1338 .1688 .1926 .2164 .2403 .2641 .0749 .0987 .1225 .1464 .1702 .2052 .2290 .2529 .2767 .1113 .1351 .1590 .1828 .2416 .2654 .2893 .0889 .1477 .1716 .1954 .2780 .0777 .1015 .1841 .2080 .0902

11.629 14.407 13.Q12 12.758 14.207

13.57 11.31 12.81

12.296 10.656 9.031 11.538 9.945 12.985 11.373

7.93 9.51 11.63 9.55 11.46 8.08 9.55 11.50 14.18 8.19 9.54 11.28 13.58 8.17 9.55 11.32 13.69 17.02 8.23 9.49 11.07 13.11 15.85 9.51 11.12 13.22 16.05 9.42

9.774 8.190 13.898 12.319 10.754 9.203 13.728 12.142 10.571 9.014 7.471 14.706 13.154 11.616 10.092 8.583 12.966 11.422 9.892 8.377 14.042 12.532 11.035 9.553 12.326 10.824 9.336 10.808 13.500 12.032 10.577 11.809 13.318 11.897 13.081 11.654 14.433 13.039 12.785 14.234

12.312

12.94 12.25

10.84 12.65 15.01 10.90 12.75 15.19 14.09 10.61 12.21 14.25 12.31 11.70 13.41 11.79 13.56 11.30 12.80 12.93 12.24

7.94

C8H15N3O3 C8Hl7N4C>2 C9HN2O4 C9H3N3O3 C9H5N4O2 C9H15NO4 C9H17N2O3

.1756 .1994 .0578 .0817 .1055 .1643 .1882 .2120 .2358

C9H19N3O2 C9H21N4O C10H3NO4 C10H5N2O3 C10H7N3O2 C10H9N4O C10H17O4 C10H19NO3 C10H21N2O2 C10H23N3O C10H25N4 C11H5O4 C11H7NO3 C11H9N2O2 C11H11N3O C11H13N4 C11H21O3 C11H23NO2 C11H25N2O C11H27N3 C12HN4 C12H9O3 C12H11NO2 C12H13N2O C12H15N3 C12H25O2 C12H27NO C13HN2O C13H3N3 C13H13O2 C13H15NO C13H17N2 C14HO2 C14H3NO C14H5N2 C14H17O C14H19N C15H5O C15H7N C15H2I C16H9

.1194 .1433 .1671 .1910 .2498 .2736 .0732 .0971 .1559 .1797 .2035 .0620 .0858 .1096 .1923 .2161 .0984 .1222 .2287 .1348

C8H14N2O4 C8H16N3O3 C8H18N4O2 C9H2N2O4 C9H4N3O3 C9H6N4O2 C9H16NO4

.1599 .1837 .2076 .0660 .0898 .1137 .1725

.0704 .0943 .1181 .1419 .1769 .2007 .2246 .2484 .2723 .0830 .1068 .1307 .1545 .1784 .2133 .2372 .2610 .2848 .0845

10.146 10.521 10.661 11.035 11.409 10.503 10.878 11.252 11.626 11.392 11.766 12.141 12.515 11.234 11.609 11.983 12.358 12.732 12.123 12.497 12.872 13.246 13.620 12.340 12.714 13.089 13.463 14.509 13.229 13.603 13.977 14.352 13.445 13.820 14.866 15.240 14.334 14.708 15.083 15.223 15.597 15.971 15.440

10.673 9.047 13.161 11.555 9.964 13.002 11.390 9.792 8.209 13.916 12.338 10.773 9.223 13.745 12.161 10.591 9.034 7.492 14.725

15.814 16.328 16.702 16.545 17.434

13.174 11.637 10.113 8.604 12.985 11.442 9.913 8.398 9.796 14.063 12.553 11.057 9.576 12.348 10.846 12.281 10.832 13.523 12.055 10.601 14.778 13.343 11.922 13.106 11.679 14.459 13.066 12.811 14.262

202 9.788 10.162 10.537 10.677 11.051 11.425 10.519

12.327 10.689 9.064 13.178 11.573 9.982 13.019

9.51 11.63 8.10 9.55 11.45 8.08 9.55 11.49 14.16 8.19 9.54 11.27 13.57 8.17 9.55 11.31 13.68 17.00 8.23 9.49 11.06 13.10 15.83 9.50

11.11 13.20 16.03 14.81 9.41 10.84 12.64 14.99 10.89 12.74 12.11 14.07 10.60 12.20 14.23 10.30 11.69 13.40 11.78 13.54 11.29 12.78 12.92 12.22

7.94 9.51 11.63 8.10 9.55 11.45 8.08

523

MASSES AND ISOTOPIC ABUNDANCE RATIOS

C9H18N203 C9H20N3O2 C9H22N40 C10H4NO4 C10H6N2O3 C10H8N3O2 C10H10N4O C10H18O4 C10H20NO3 C10H22N2O2 C10H24N3O C10H26N4 C11H604

CnHsNOs C11H10N2O2 C11H12N3O C11H14N4 C11H22O3 C11H24NO2 C11H26N2O C12H2N4 C12H10O3 C12H12NO2 C12H14N2O C12H16N3 C12H26O2 C13H2N2O C13H4N3 C13H14O2 CiaHieNO C13H18N2 C14H2O2 C14H4NO C14H6N2 C14H18O C14H20N CisHeO Ci5H8N C15H22 CieHio

.1963 .2201 .2440 .0786 .1024 .1263 .1501 .1851 .2089 .2327 .2566 .2804 .0912 .1150 .1388 .1627 .1865 .2215 .2453 .2692 .0926 .1276 .1514 .1753 .1991 .2579 .0814 .1052 .1640 .1878 .2117 .0701 .0940 .1178 .2002 .2243 .1065 .1304 .2369 .1430

10.894 11.268 11.642 11.408 11.782 12.157 12.531 11.250 11.625 11.999 12.374 12.748 12.139 12.513 12.888 13.262 13.636 12.356 12.730 13.105 14.525 13.245 13.619 13.993 14.368 13.461 14.882 15.256 14.350 14.724 15.099 15.239 15.613 15.987 15.456 15.830 16.344 16.718 16.561 17.450

11.407 9.810 8.227 13.935 12.357 10.793 9.243 13.763 12.180 10.610 9.054 7.512 14.744 13.194 11.657 10.135 8.626 13.005 11.463 9.934 9.819 14.084 12.575 11.080 9.599 12.369 12.305 10.857 13.546 12.079 10.625 14.803 13.368 11.948 13.131 11.704 14.485 13.092 12.837 14.290

9.55 11.49 14.15 8.19 9.54 11.26 13.56 8.17 9.54 11.31 13.67 16.97 8.23 9.48 11.06 13.09 15.81 9.50

11.11 13.19 14.79 9.40 10.83 12.63 14.97 10.88 12.09 14.05 10.59 12.19 14.21 10.29 11.68 13.38 11.77 13.53 11.28 12.77 12.90 12.21

203 C8H15N2O4 C8H17N3O3 C8H19N4O2 C9H3N2O4 C9H5N3O3 C9H7N4O2 C9H17NO4 C9H19N2O3 C9H21N3O2 C9H23N4O C10H5NO4 C10H7N2O3 C10H9N3O2 C10H11N4O C10H19O4

.1680 .1919 .2157 .0741 .0980 .1218 .1806 .2045 .2283 .2521 .0867 .1106 .1344 .1582 .1932

9.804 10.178 10.553 10.693 11.067 11.441 10.535 10.910 11.284 11.658 11.424 11.798 12.173 12.547 11.266

12.343 10.705 9.081 13.196 11.591 10.000 13.036 11.425 9.828 8.246 13.953 12.376 10.812 9.263 13.781

7.94 9.51 11.62 8.10 9.55 11.44 8.08 9.55 11.48 14.14 8.19 9.53 11.26 13.55 8.18

C10H21NO3 C10H23N2O2 C10H25N3O C11H7O4 C11H9NO3 C11H11N2O2 C11H13N3O C11H15N4 C11H23O3 C11H25NO2 C12HN3O C12H3N4 C12H11O3 C12H13NO2 C12H15N2O C12H17N3 C13HNO2 C13H3N2O C13H5N3 C13H15O2 C13H17NO C13H19N2 C14H3O2 C14H5NO C14H7N2 C14H19O C14H21N C15H7O C15H9N C15H23 Ci6Hn

C8H16N2O4 C8Hi8N303 C8H20N4O2 C9H4N2O4 C9H6N3O3 C9H8N4O2 C9H18NO4 C9H20N2O3 C9H22N3O2 C9H24N4O C10H6NO4 C10H8N2O3 C10H10N3O2 C10H12N4O OioH2(j04 C10H22NO3 C10H24N2O2 C11H804 C11H10NO3 C11H12N202 C11H14N30 C11H16N4 C11H2403 C12H2N30

.2170 .2409 .2647 .0993 .1231 .1470 .1708 .1947 .2296 .2535 .0769 .1008 .1357 .1596 .1834 .2072 .0657 .0895 .1133 .1721 .1960 .2198 .0783 .1021 .1259 .2084 .2324 .1147 .1385 .2450 .1511

11.641 12.015 12.390 12.155 12.529 12.904 13.278 13.652 12.372 12.746 14.167 14.541 13.261 13.635 14.009 14.384 14.523 14.898 15.272 14.366 14.740 15.115 15.255 15.629 16.003 15.472 15.846 16.360 16.734 16.577 17.466

204 9.820 10.194 10.569 10.709 11.083 11.457 10.551 10.926 11.300 .2364 .2603 11.674 .0949 11.440 .1762 .2000 .2239 .0823 .1061 .1300 .1888 .2126

.1187 .1425 .1664 .2013 .2252 .2490 .1074 .1313 .1551 .1790 .2028 .2378 .0851

11.814 12.189 12.563 11.282 11.657 12.031 12.171 12.545 12.920 13.294 13.668 12.388 14.183

12.198 10.629 9.074 14.764 13.214 11.678 10.156 8.648 13.025 11.483 11.317 9.842 14.105 12.597 11.102 9.622 13.790 12.328 10.881 13.569 12.102 10.649 14.827 13.393 11.973 13.156 11.729 14.'512 13.119 12.864 14.318

9.54 11.30 13.65 8.23 9.48 11.05 13.07 15.79 9.50 11.10 12.52 14.77 9.40 10.82 12.62 14.95 10.53 12.09

12.359 10.721 9.098 13.213 11.609 10.018 13.052 11.442 9.846 8.264 13.971 12.394 10.832 9.283 13.799 12.217 10.648 14.783 13.234 11.698 10.177 8.670 13.045. 11.340

7.95 9.51 11.62 8.10 9.55 11.44 8.08 9.55 11.48 14.13 8.19 9.53 11.25 13.53 8.18

14.04 10.59 12.18 14.19 10.29 11.67 13.37 11.76 13.51 11.27 12.76 12.89 12.20

9.54 11.30 8.23 9.48 11.05 13.06 15.77 9.50 12.51

524 C12H4N4 C12H1203 C12H14N02 C12H16N20 C12H18N3 C13H2N02 C13H4N20 C13H6N3 C13H1602 CisHigNO C13H20N2 C14H4O2 Ci4H6NO Ci4H8N2 C14H20O C14H22N Ci5H80 C15H10N C15H24 C16H12

C8H17N2O4 C8H19N3O3 C8H21N4O2 C9HSN2O4 C9H7N3O3 C9H9N4O2 C9H19NO4 C9H21N2O3, C9H23N3O2 C10H7NO4 C10H9N2O3 C10H11N3O2 C10H13N4O C10H21O4 C10H23NO3 C11HN40 C11H904 CiiHhN03 C11H13N2O2 C11H15N3O C11H17N4 C12HN2O2 C12H3N3O C12H5N4 C12H13O3 C12H15NO2 C12H17N2O C12H19N3 C13HO3 C13H3NO2 c13h5n2o C13H7N3 C13H1702 C13H19NO C13H21N2

APPENDIX 1

.1089 .1439 .1677 .1915 .2154 .0738 .0977 .1215 .1903 .2041 .2280 .0864 .1102 .1341 .2165 .2406 .1228 .1467 .2531 .1592

.1843 .2082 .2320 .0904 .1143 .1381 .1969 .2207 .2446 .1030 .1268 .1507 .1745 .2095 .2333 .0806 .1156 .1394 .1633 .1871 .2109 .0694 .0932 .1171 .1520 .1759 .1997 .2235 .0581 .0820 .1058 .1296 .1884 .2123 .2361

14.557 13.277 13.651 14.025 14.400 14.539 14.914 15.288 14.382 14.756 15.131 15.271 15.645 16.019 15.488 15.862 16.376 16.750 16.593 17.482 205 9.836 10.210 10.585 10.725 11.099 11.473 10.567 10.942 11.316 11.456 11.830 12.205 12.579 11.298 11.673 13.467 12.187 12.561 12.936 13.310 13.684 13.824 14.199 14.573 13.293 13.667 14.041 14.416 14-181 14.555 14.930 15.304 14.398 14.772 15.147

9.866 14.127 12.619 11.125 9.645 13.813 12.352 10.906 13.592 12.126 10.673 14.852 13.418 11.999 13.180 11.755 14.538 13.146 12.890 14.346

12.373 10.738 9.115 13.230 11.626 10.037 13.069 11.459 9.865 13.990 12.413 10.851 9.303 13.818 12.235 10.402 14.803 13.254 11.719 10.198 8.692 12.850 11.362 9.889 14.148 12.641 11.147 9.668 15.310 13.836 12.376 10.930 13.615 12.149 10.697

14.76 9.40 10.82 12.61 14.93 10.53 12.07 14.02 10.58 12.17 14.18 10.28 11.66 13.35 11.75 13.49 11.26 12.74 12.87 12.19

7.95 9.51 11.61 8.11 9.55 11.43 8.09 9.55 11.47 8.19 9.53 11.25 13.52 8.18 9.54 12.95 8.23 9.48 11.04 13.05 15.74 10.76 12.50 14.74 9.40 10.81 12.60 14.91 9.26 10.52 12.06 14.00 10.58 12.16 14-16

C14H5O2 C14H7NO C14H9N2 C14H21O C14H23N C15H9O CisHnN C15H25 C16H13 C17H

C8H18N2O4 C8H20N3O3 C8H22N4O2 C9H6N2O4 C9H8N3O3 C9H10N4O2 C9H20NO4 C9H22N2O3 C10H8NO4 C10H10N2O3 C10H12N3O2 C10H14N4O

.0945 .1184 .1422 .2247 .2487 .1310 .1548 .2613 .1674 .0735

.1925 .2163 .2401 .0986 .1224 .1462 .2050 .2289 .1112 .1350 .1588 .1827 .2176 .0888 .1237 .1476

C10H22O4 C11H2N4O C11H10O4 C11H12NO3 C11H14N2O2 CuHieNsO C11H18N4 C12H2N2O2 C12H4N3O C12H6N4 C12H14O3 C12H16NO2 C12H18N2O C12H20N3 C13H2O3 C13H4NO2 C13H6N2O C13H8N3 C13H18O2 C13H20NO C13H22N2 C14H6O2 Ci4H8NO C14H10N2 C14H22O C14H24N C15H10O C15H12N

.1014 .1252 .1602 .1840 .2078 .2317 .0663 .0901 .1139 .1378 .1966 .2204 .2443 .1027 .1265 .1504 .2330 .2568 .1391 .1630

C15H26 C16H14 C17H2

.2694 .1755 .0816

C8H19N2O4

.2006

.1714 .1953 .2191 .0775

15.287 15.661 16.035 15.504 15.878 16.392 16.766 16.609 17.498 18.386 206 9.852 10.226 10.601 10.741 11.115 11.489 10.583 10.958 11.472 11.846 12.221 12.595 11.314 13.483 12.203 12.577 12.952 13.326 13.700 13.840 14.215 14.589 13.309 13.683 14.057 14.432 14.197 14.571 14.946 15.320 14.414 14-788 15.163 15.303 15.677 16.051 15.520 15.894 16.408 16.782 16.625

14.876 13.443 12.025 13.205 11.780 14.564 13.173 12.917 14.374 15.910

12.390 10.754 9.132 13.247 11.644 10.055 13.086 11.477 14-008 12.432 10.871 9.323 13.836 10.424 14.822 13.274 11.740 10.220 8.714 12.872 11.385 9.912 14.169 12.662 11.170 9.691 15.333 13.859 12.400 10.954 13.638 12.173 10.722 14.901 13.468 12.050 13.230 11.805 14.590 13.199

17.514 18.402

12.944 14.402 15.940

207 9.868

12.406

10.28 11.65 13.34 11.74 13.48 11.26 12.73 12.86 12.17 11.56

7.95 9.51 11.61 8.11 9.55 11.43 8.09 9.55 8.19 9.53 11.24 13.51 8.18 12.93 8.23 9.48 11.03 13.04 15.72 10.75 12.49 14.72 9.39 10.81 12.59 14.89 9.26 10.51 12.05 13.99 10.57 12.15 14.14 10.27 11.64 13.32 11.73 13.46 11.25 12.71 12.84 12.16 11.54

7.95

525

MASSES AND ISOTOPIC ABUNDANCE RATIOS

C8H21N303 C9H7N204 C9H9N303 C9H11N402 C9H21N04 C10H9NO4 C10H11N2O3 C10H13N3O2 C11H15N40 C11HN302 C11H3N40 C11H1104 C11H13N03 C11H15N2O2 C11H17N3O C11H19N4 C12HNO3 C12H3N2O2 C12H5N3O C12H7N4 C12H15O3 C12H17NO2 C12H19N2O C12H21N3 C13H3O3 C13H5NO2 C13H7N2O C13H9N3 C13H19O2 C13H21NO C13H23N2 C14H7O2 C14H9NO C14H11N2 C14H23O C14H25N CisHnO Ci6Hi3N C15H27 CieHN C16H15 c17h3

C8H20N2O4 C9H8N2O4 C9H10N3O3 C9Hl2N402 C10H10NO4 C10H12N2O3 C10H14N3O2 C10H16N4O C11H2N3O2 C11H4N4O C11H12O4 C11H14NO3 C11H16N2O2

.2244 .1067 .1306 .1544 .2132 .1193 .1431 .1670 .1908 .0731 .0970 .1319 .1557 .1796 .2034 .2272 .0618 .0857 .1095 .1333 .1683 .1921 .2160 .2398

10.242 10.757 11.131 11.505 10.599 11.488 11.862 12.237 12.611 13.125 13.500 12.219 12.593 12.968 13.342 13.716 13.482 13.856 14.231 14.605 13.325 13.699 14.073 14.448 14.213 14.587 14.962 15.336 14.430

-.0744 .0982 .1221 .1459 .2047 .2286 .2524 .1108 .1347 .1585 .2412 .2650 .1473 .1711 .2776 .0772 .1837 .0898

16.641 17.687 17.530 18.418

.2088 .1149 .1387 .1625

208 9.884 10.773 11.147 11.521

.1274 .1513 .1751 .1990 .0812 .1051 .1400 .1639 .1877

14.804 15.179 15.319 15.693 16.067 15.536 15.910 16.424 16.798

11.504 11.878 12.253 12.627 13.141 13.516 12.235 12.609 12.984

10.770 13.264 11.662 10.073 13.103 14.026 12.451 10.890 9.343 11.959 10.445 14.842 13.294 11.761 10.241 8.736 14.394 12.894 11.408 9.936 14.191 12.684 11.192 9.714 15.356 13.883 12.424 10.979 13.662 12.197 10.746 14.925 13.493 12.076 13.255 11.831 14.616 13.226 12.970 14.700 14-430 15.969

12.422 13.281 11.680 10.092 14.045 12.470 10.910 9.364 11.980 10.467 14.861 13.314 11.781

9.51 8.11 9.54 11.42 8.09 8.19 9.53 11.24 13.50 10.97 12.92 8.23 9.47 11.03 13.03 15.70 9.37 10.75 12.47 14.70 9.39 10.80 12.57 14.87 9.26 10.51 12.04 13.97 10.56 12.14 14.13 10.26 11.63 13.30 11.72 13.45 11.24 12.70 12.83 12.03 12.15 11.53

7.96 8.11 9.54 11.42 8.19 9.53 11.23 13.49 10.97 12.91 8.23 9.47 11.02

CnHigNsO C11H20N4 C12H2NO3 C12H4N2O2 C12H6N3O C12H8N4 C12H16O3 C12H18NO2 C12H20N2O C12H22N3 C13H4O3 Ci3HfiNC>2 C13H8N2O C13H10N3 C13H20O2 C13H22NO C13H24N2 C14H8O2 Ci4HioNO C14H12N2 C14H24O C14H26N C15H12O C15H14N C15H28 Ci6H2N Ci6Hig

c17h4

C9H9N204 C9H11N303 C9H13N402 C1CHN402 C10HI1NO4 C10H13N2O3 C10H15N3O2 C10H17N4O C11HN203 C11H3N302 C11H5N40 C11H1304 C11H15N03 C11H17N202 C11H19N3C) C11H21N4 C12H04 .Ci2HeN03 C12H5N2O2 C12H7N3O C12H9N4 Ci2Hi703 C12H19NO2 C12H21N2O C12H23N3 C13H5O3 Ci3H7NO?

.2115 .2354 .0700 .0938 .1176 .1415 .1764 .2003 .2241 .2480 .0826 .1064 .1302 .1541 .2129 .2367 .2606 .1190 .1428 .1667 .2493 .2731 .1554 .1792 .2857 .0853 .1918 .0979

.1230 .1468 .1707 .0768 .1356 .1594 .1833 .2071 .0655 .0894 .1132 .1482 .1720 .1958 .2197 .2435 .0543 .0781 .1020 .1258 .1496 .1846 .2084 .2323 .2561 .0907 .1145

13.358 13.732 13.498 13.872 14.247 14.621 13.341 13.715 14.089 14.464 14.229 14.603 14.978 15.352 14.446 14.820 15.195 15.335 15.709 16.083 15.552 15.926 16.440 16.814 16.657 17.703 17.546 18.434 209 10.789 11.163 11.537 12.426 11.520 11.894 12.269 12.643 12.783 13.157 13.532 12.251 12.625 13.000 13.374 13.748 13.140 13.514 13.888 14.263 14.637 13.357 13.731 14.105 14-480 14.245 14.619

10.262 8.757 14.416 12.916 11.431 9.959 14.212 12.706 11.215 9.737 15.378 13.906 12.448 11.004 13.685 12.220 10.770 14.950 13.519 12.102 13.280 11.856 14.643 13.253 12.997 14.728 14.458 15.998

13.299 11.697 10.110 11.117 14.063 12.489 10.929 9.384 13.527 12.001 10.488 14.881 13.334 11.802 10.284 8.779 15.951 14.438 12.938 11.453 9.982 14.233 12.728 11.237 9.760 15.401 13.930

13.02 15.68 9.36 10.74 12.46 14.68 9.39 10.79 12.56 14.85 9.25 10.50 12.03 13.95 10.56 12.13 14.11 10.26 11.62 13.29 11.71 13.43 11.23 12.69 12.82 12.02 12.14 11.52

8.11 9.54 11.41 11.18 8.19 9.52 11.23 13.47 9.45 10.96 12.90 8.23 9.47 11.02 13.00 15.66 8.24 9.36 10.73 12.45 14.66 9.38 10.79 12.55 14.84 9.25 10.49

526

C13H9N20 C13H11N3 C13H2102 C13H23NO C13H25N2 C14H9O2 C14H11NO C14H13N2 C14H25O C14H27N C15HN2 c15h13o C15H15N C15H29 CigHO C16H3N CigHi7 C17H5

C9H10N2O4 C9H12N303 C9H14N402 C10H2N4O2 C10H12NO4 C10H14N2O3 C10H16N3O2 C10H18N4O C11H2N203 C11H4N302 C11HGN40 C11H1404 C11H16N03 C1XH18N202 C11H20N3O C11H22N4 C12H204

C12H4N03 C12H6N202 C12H8N30 C12H10N4 C12H1803 C12H20NO2 C12H22N20 C12H24N3 C13H6O3 CisHgNOa C13H10N2O C13H12N3 C13H22O2 C13H24NO C13H26N2 C14H10O2 C14H12NO C14H14N2 C14H2GO C14H28N

APPENDIX 1

.1384 .1622 .2210 .2449 .2687 .1271 .1510 .1748 .2574 .2813 .0809 .1635 .1874 .2939 .0696 .0935 .2000 .1061

.1311 .1550 .1788 .0849 .1437 .1676 .1914 .2153 .0737 .0975 .1214 .1563 .1802 .2040 .2278 .2517 .0624 .0863 .1101 .1339 .1578 .1927 .2166 .2404 .2643 .0988 .1227 .1465 .1704 .2292 .2530 .2768 .1353 .1591 .1829 .2656 .2894

14.994 15.368 14.462 14.836 15.211 15.351 15.725 16.099 15.568 15.942 16.988 16.456 16.830 16.673 17.345 17.719 17.562 18.450 210 10.805 11.179 11.553 12.442 11.536 11.910 12.285 12.659 12.799 13.173 13.548 12.267 12.641 13.016 13.390 13.764 13.156 13.530 13.904 14.279 14.653 13.373 13.747 14.121 14.496 14.261 14.635 15.010 15.384 14.478 14.852 15.227 15.367 15.741 16.115 15.584 15.958

12.472 11.028 13.708 12.244 10.795 14.974 13.544 12.127 13.305 11.882 13.539 14.669 13.280 13.023 16.113 14.757 14.486 16.028

13.316 11.715 10.129 11.137 14.081 12.508 10.949 9.404 13.548 12.022 10.510 14.900 13.355 11.823 10.305 8.801 15.972 14.459 12.961 11.476 10.006 14.255 12.750 11.260 9.783 15.424 13.953 12.496 11.053 13.731 12.268 10.819 14.999 13.569 12.153 13.329 11.907

12.02 13.94 10.55 12.12 14.09 10.25 11.61 13.28 11.70 13.42 12.55 11.22 12.67 12.80 10.76 12.01 12.12 11.51

8.11 9.54 11.41 11.17 8.19 9.52 11.22 13.46 9.45 10.96 12.89 8.23 9.47 11.01 12.99 15.64 8.24 9.36 10.73 12.44 14.64 9.38 10.78 12.54 14.82 9.25 10.49 12.01 13.92 10.54 12.11 14.07 10.25 11.60 13.26 11.69 13.40

C15H2N2 C15H14O C15Hi6N C15H30 CioHaO Ci6H4N C16H18 C17H6

C9H11N2O4 C9H13N3O3 C9H15N4O2 C10HN3O3 C10H3N4O2 C10H13NO4 C10H15N2O3 C10H17N3O2 C10H19N4O C11HNO4 C11H3N2O3 C11H5N3O2 Q1H7N4O C11H15O4 C11H17NO3 C11H19N2O2 C11H21N3O C11H23N4 C12H3O4 C12H5NO3 C12H7N2O2 C12H9N3O C12H11N4 C12H19O3 C12H21NO2 C12H23N2O C12H25N3 C13H7O3 C13H9NO2 C13H11N2O C13H13N3 C13H23O2 C13H25NO C13H27N2 C14HN3 C14H11O2 C14H13NO C14H15N2 C14H27O C14H29N C15HNO C15H3N2 C15H15O C15H17N C15H31 CieHsO Ci6H5N

.0890 .1717 .1955 .3020 .0778 .1016 .2081 .1142

.1393 .1631 .1870 .0692 .0931 .1519 .1757 .1996 .2234 .0580 .0818 .1057 .1295 .1645 .1883 .2121 .2360 .2598 .0706 .0944 .1182 .1421 .1659 .2009 .2247 .2486 .2724 .1070 .1308 .1547 .1785 .2373 .2611 .2850 .0846 .1434 .1673 .1911 .2737 .2976 .0734 .0972 .1798 .2037 .3102 .0859 .1098

17.004 16.472 16.846 16.689 17.361 17.735 17.578 18.466 211 10.821 11.195 11.569 12.084 12.458 11.552 11.926 12.301 12.675 12.440 12.815 13.189 13.564 12.283 12.657 13.032 13.406 13.780 13.172 13.546 13.920 14.295 14.669 13.389 13.763 14.137 14.512 14.277 14.651 15.026 15.400 14.494 14.868 15.243 16.289 15.383 15.757 16.131 15.600 15.974 16.646 17.020 16.488 16.863 16.705 17.377 17.751

13.566 14.695 13.307 13.050 16.140 14.785 14.514 16.057

13.333 11.733 10.147 12.709 11.157 14.100 12.527 10.969 9.424 15.108 13.568 12.043 10.532 14.920 13.375 11.844 10.327 8.823 15.993 14-481 12.983 11.499 16.029 14.276 12.772 11.282 9.807 15.447 13.976 12.520 11.077 13.754 12.292 10.843 12.427 15.023 13.594 12.179 13.354 11.933 14.975 13.594 14.722 13.334 13.077 16.168 14.814

12.53 11.21 12.66 12.79 10.76 12.00 12.11 11.50

8.12 9.54 11.40 9.51 11.17 8.19 9.52 11.21 13.45 8.23 9.45 10.95 12.88 8.23 9.46

11.00 12.98 15.62 8.24 9.35 10.72 12.43 14.63 9.38 10.78 12.53 14.80 9.24 10.48 12.00 13.90 10.54 12.10 14.06 13.11 10.24 11.59 13.24 11.68 13.39 11.12 12.52 11.20 12.65 12.77 10.75 11.98

MASSES AND ISOTOPIC ABUNDANCE RATIOS

C16H19 c17h7

.2163 .1224

17.594 18.482

14.542 16.087

12.10 11.49

212 C9H12N204 C9HX4N303 C9H16N402 C10H2N3O3 C10H4N4O2 C10H14NO4 C10H16N2O3 C10H18N3O2 C10H20N4O C11H2N04 C11H4N203 C11H6N302 cxxhsn4o C11H1604 C11H18N03 C11H20N2O2 CHH22N30 C11H24N4 C12H404 C12H6N03 Cl2H8N202, C12H10N3O C12H12N4 C12H20O3 C12H22N02 C12H24N20 C12H26N3 C13H803 Ci3H10NO2 C13H12N2O C13H14N3 C13H24O2 C13H26NO C13H28N2 C14H2N3 C14H12O2 C14H14NO C14H16N2 C14H28O C14H30N C15H2NO C15H4N2 CisHleO CisHigN C15H32 C16H4O CieHeN C16H20 c17h8

C9H13N2O4 C9H16N3O3

.1474 .1713 .1951 .0774 .1012 .1600 .1839 .2077 .2315 .0661 .0900 .1138 .1376 .1726 .1964 .2203 .2441 .2680 .0787 .1025 .1264 .1502 .1741 .2090 .2329 .2567 .2805 .1151 .1390 .1628 .1867 .2455 .2693 .2931 .0928 .1516 .1754 .1992 .2819 .3057 .0815 .1053 .1880 .2118 .3183 .0941 .1179 .2244 .1305

.1556 .1794

10.837 11.211 11.585 12.100 12.474 11.568 11.942 12.317 12.691 12.456 12.831 13.205 13.580 12.299 12.673 13.048 13.422 13.796 13.188 13.562 13.936 14.311 14.685 13.405 13.779 14.153 14.528 14.293 14.667 15.042 15.416 14.510 14.884 15.259 16.305 15.399 15.773 16.147 15.616 15.990 16.662 17.036 16.504 16.879 16.721 17.393 17.767 17.610 18.498 213 10.853 11.227

13.350 11.751 10.166 12.729 11.176 14.118 12.544 10.988 9.445 15.128 13.589 12.064 10.553 14.940 13.395 11.865 10.348 8.846 16.014 14.503 13.005 11.522 10.053 14.297 12.794 11.305 9.830 15.470 14.000 12.544 11.102 13.777 12.315 10.867 12.453 15.048 13.619 12.205 13.379 11.958 15.002 13.621 14.748 13.361 13.104 16.196 14-842 14.571 16.117

13.368 11.769

8.12 9.54 11.40 9.51 11.16 8.19 9.52 11.21 13.44 8.23 9.44 10.95 12.87 8.23 9.46 11.00 12.97 15.60 8.24 9.35 10.72 12.42 14.61 9.38 10.77 12.52 14.78 9.24 10.48 11.99 13.89 10.53 12.09 14.04 13.09 10.23 11.58 13.23 11.67 13.37 11.11 12.51 11.19 12.63 12.76 10.74 11.97 12.09 11.48

8.12 9.54

C9Hi7N402 C10HN2O4 C10H3N3O2 C10H5N4O2 C10H15NO4 CioHi7N203 C10H19N3O2 C10H21N4O C11H3NO4 C11H5N2O3 ChH7N302 C11H9N4O ChHi704 CiiHi9NC>3 C11H21N2O2 C11H23N3O C11H25N4 C12H5O4 Ci2H7N03 Ci2H9N202 C12H11N30 Cj2Hi3N4 C12H2103 C12H23N02 C12H25N20 Cl2H27N3 Ci3HN4 C13H903 C13H11N02 C13H13N20 C13H15N3 C13H2502 Cx3H27NO C13H29N2 C14HN2O C14H3N3 Cx4Hi302 C14H15NO Cx4Hi7N2 Cx4H290 C14H31N C15HO2 C15H3NO C15H5N2 Cx5Hx70 C15H19N CieHsO Cx(jJ47N C16H21 Cx7H9

C9H14N2O4 C9HifiN303 C9Hi8N402 C10H2N2O4 C10H4N3O3

.2033 .0617 .0855 .1094 .1682 .1920 .2158 .2397 .0743 .0981 .1219 .1458 .1807 .2046 .2284 .2523 .2761 .0869 .1107 .1345 .1584 .1822 .2172 .2410 .2649 .2887 .0883 .1233 .1471 .1710 .1948 .2536 .2774 .3013 .0771 .1009 .1597 .1835 .2074 .2900 .3139 .0658 .0896 .1135 .1961 .2200 .1022 .1261 .2325 .1387

.1637 .1876 .2114 .0698 .0937

527

11.601 11.741 12.116 12.490 11.584 11.958 12.333 12.707 12.472 12.847 13.221 13.596 12.315 12.689 13.064 13.438 13.812 13.204 13.578 13.952 14.327 14.701 13.421 13.795 14.169 14.544 15.590 14.309 14.683 15.058 15.432 14.526 14.900 15.275 15.946 16.321 15.415 15.789 16.163 15.632 16.006 16.303 16.678 17.052 16.520 16.895 17.409 17.783 17.626 18.514 214 10.869 11.243 11.617 11.757 12.132

10.184 14.313 12.748 11.196 14.137 12.565 11.008 9.465 15.147 13.609 12.085 10.575 14.959 13.415 11.885 10.370 8.868 16.035 14.524 13.028 11.545 10.076 14.319 12.816 11.328 9.853 11.364 15.493 14.023 12.568 11.12713.800 12.339 10.892 13.887 12.479 15.072 13.645 12.231 13.404 11.984 16.423 15.029 13.648 14.775 13.388 16.224 14.870 14.599 16.146

13.385 11.787 10.203 14.332 12.767

11.39 8.20 9.50 11.16 8.19 9.52 11.20 13.43 8.23 9.44 10.94 12.86 8.23 9.46 10.99 12.96 15.57 8.23 9.35 10.71 12.41 14.59 9.37 10.76 12.51 14.76 13.72 9.24 10.^7 11.98 13.87 10.53 12.08 14.02 11.48 13.08 10.23 11.57 13.21 11.66 13.36 9.93 11.10 12.49 11.18 12.62 10.73 11.96 12.07 11.47

8.12 9.54 11.39 8.20 9.50

528

C10H6N4O2 cloHlfiNo4 C10H18N2O3 C10H20N3O2 C10H22N4O C1JH4NO4 C11HBN2O3 C11H8N3O2 C11H10N4O C11H18O4 C11H20NO3 C11H22N2O2 C11H24N3O C11H20N4 C12H6O4 C12H8NO3 Cl2HloN202 C12H12N3O C12H14N4 C12H22O3 C12H24NO2 C12H26N2O

APPENDIX 1

.1175 .1763 .2002 .2240 .2478 .0824 .1063 .1301 .1539 .1889 .2127 .2366

12.506 11.600 11.974 12.349 12.723 12.488 12.863 13.237 13.612 12.331 12.705 13.080 13.454 13.828 13.220

11.216 14.155 12.584 11.028 9.485 15.167 13.630 12.106 10.597 14.979 13.436 11.906 10.391 8.890 16.056 14.546 13.050 11.568 10.100 14.340 12.838 11.350 9.876 11.389 15.516 14.047 12.592 11.151

C16H22 C17H10

.2604 .2843 .0950 .1188 .1427 .1665 .1904 .2253 .2492 .2730 .2968 .0965 .1314 .1553 .1791 .2029 .2617 .2856 .3094 .0852 .1090 .1678 .1917 .2155 .2982 .0740 .0978 .1216 .2043 .2281 .1104 .1342 .2407 .1468

13.594 13.968 14.343 14.717 13.437 13.811 14.185 14.560 15.606 14.325 14.699 15.074 15.448 14.542 14.916 15.291 15.962 16.337 15.431 15.805 16.179 15.648 16.319 16.694 17.068 16.536 16.911 17.425 17.799 17.642 18.530

13.824 12.363 10.916 13.913 12.505 15.097 13.670 12.256 13.429 16.449 15.055 13.675 14.801 13.415 16.252 14.899 14.627 16.176

C9H15N2O4 C9H17N3O3 C9H19N4O2 C10H3N2O4 C10H5N3O3 C10H7N4O2 C10H17NO4 C10H19N2O3 C10H21N3O2

.1719 .1957 .2196 .0780 .1018 .1257 .1845 .2083 .2321

215 10.885 11.259 11.633 11.773 12.148 12.522 11.616 11.990 12.365

13.403 11.805 10.221 14.351 12.787 11.236 14.174 12.604 11.048

C12H28N3 C13H2N4 C13H10O3 C13H12NO2 C13H14N2O C13H16N3 C13H26O2 C13H28NO C13H30N2 C14H2N2O C14H4N3 C14H14O2 Ci4Hi6NO C14H18N2 C14H30O C15H2O2 C15H4NO c15h6n2 C15H180 C15H20N CieHgO C16H8N

11.15 8.19 9.51 11.20 13.41 8.23 9.44 10.93 12.85 8.23 9.46 10.99 12.95 15.56 8.23 9.35 10.70 12.40 14.57 9.37 10.76 12.50 14.74 13.70 9.23 10.46 11.97 13.85 10.52 12.07 14-01 11.47 13.06 10.22 11.56 13.20 11.65 9.92 11.09 12.48 11.17 12.61 10.72 11.95 12.06 11.46

8.12 9.54 11.38 8.20 9.50 11.14 8.20 9.51 11.19

C10H23N4O CnHflN04 C11H7N2O3 C11H9N3O2 C11H11N4O C11H19O4 C11H21NO3 C11H23N2O2 C11H25N3O C11H27N4 C12H7O4 C12H9NO3 G12H11N2O2 C12H13N3O C12H15N4 C12H23O3 C12H25NO2 C12H27N2O C12H29N3 C13HN3O C13H3N4 C13H11O3 C13H13NO2 C13H15N2O C13H17N3 C13H27O2 C13H29NO C14HNO2 C14H3N2O C14H5N3 C14H15O2 C14H17NO C14H19N2 c15h3o2 C15H5NO C15H7N2 C15H190 C15H21N CieHbO c16h9n C16H23 C17H11

C9Hi6N204 C9H18N3O3 C9H20N4O2 C10H4N2O4 C10H6N3O3 C10H8N4O2 C10H18NO4 C10H20N2O3 C10H22N3O2 C10H24N4O C11H6NO4 C11H8N2O3 C11H10N3O2

.2560 .0906 .1144 .1382 .1621 .1970 .2209 .2447 .2686 .2924 .1031 .1270 .1508 .1747 .1985 .2335 .2573 .2811 .3050 .0808 .1046 .1396 .1634 .1872 .2111 .2699 .2937 .0695 .0934 .1172 .1760 .1998 .2237 .0821 .1059 .1298 .2124 .2363 .1185 .1424 .2488 .1549

.1800 .2039 .2277 .0861 .1100 .1338 .1926 .2164 .2403 .2641 .0987 .1225 .1464

12.739 12.504 12.879 13.253 13.628 12.347 12.721 13.096 13.470

9.506 15.187 13.650 12.128 10.619 14.999 13.456

14.341 14.715 15.090 15.464 14-558 14.932 15.604 15.978 16.353 15.447 15.821 16.195 16.335 16.710 17.084 16.552 16.927 17.441 17.815 17.658 18.546

11.927 10.413 8.912 16.077 14.568 13.072 11.591 10.123 14.362 12.860 11.373 9.900 12.848 11.414 15.538 14.070 12.616 11.176 13.847 12.387 15.359 13.938 12.531 15.122 13.695 12.282 16.476 15.082 13.703 14.827 13.442 16.279 14.927 14.655 16.206

216 10.901 11.275 11.649 11.789 12.164 12.538 11.632 12.006 12.381 12.755 12.520 12.895 13.269

13.420 11.823 10.240 14.370 12.806 11.256 14.193 12.623 11.067 9.526 15.207 13.671 12.149

13.844 13.236 13.610 13.984 14.359 14.733 13.453 13.827 14.201 14.576 15.247 15.622

13.40 8.23 9.44 10.93 12.83 8.23 9.45 10.98 12.94 15.53 8.23 9.34 10.70 12.39 14.55 9.37 10.75 12.49 14.72 11.87 13.69 9.23 10.46 11.96 13.84 10.51 12.05 10.16 11.46 13.05 10.21 11.55 13.19 9.91 11.08 12.47 11.16 12.59 10.71 11.93 12.05 11.44

8.12 9.54 11.38 8.20 9.50 11.14 8.20 9.51 11.19 13.39 8.23 9.43 10.92

529

MASSES AND ISOTOPIC ABUNDANCE RATIOS

C11H12N40 C11H20O4 C11H22N03 CX1H24N202 C11H26N3C) C11H28N4 C12H804 C12H10NO3 CX2H12N202 C12H14N3C) C12H16N4 C12H2403 C12H26N02 C12H28N20 C13H2N30 C13H4N4 C13H1203 C13H14N02 C13H16N20 C13H18N3 Cl3H2802

C14H2NO2 C14H4N2O Ci4HfiN3

C14H1602 C14H18NO C14H20N2 C15H402 Ci5H6NO C15H8N2 C15H20O C15H22N CxeHgO C16H10N C16H24 C17H12

C9H17N2O4 C9H19N3O3 C9H21N4O2 C10H5N2O4 C10H7N3O3 C10H9N4O2 C10H19NO4 C10H21N2O3 C10H23N3O2 C10H25N4O C11H7NO4 C11H9N2O3 C11H11N3O2 C11H13N4O C11H21O4 C11H23NO3 C11H25N2O2 C11H27N3O C12HN4O

.1702 .2052 .2290 .2529 .2767 .3005 .1113 .1351 .1590 .1828 .2066 .2416 .2654 .2893 .0889 .1128 .1477 .1716 .1954 .2192 .2780 .0777 .1015 .1253 .1841 .2080 .2318 .0902 .1141 .1379 .2206 .2444 .1267 .1505 .2570 .1631

.1882 .2120 .2358 .0943 .1181 .1419 .2007 .2246 .2484 .2723 .1068 .1307 .1545 .1784 .2133 .2372 .2610 .2848 .0845

13.644 12.363 12.737 13.112 13.486 13.860 13.252 13.626 14.000 14.375 14.749 13.469 13.843 14.217 15.263 15.638 14.357 14.731 15.106 15.480 14.574 15.620 15.994 16.369 15.463 15.837 16.211 16.351 16.726 17.100 16.568 16.943 17.457 17.831 17.674 18.562 217 10.917 11.291 11.665 11.805 12.180 12.554 11.648 12.022 12.397 12.771 12.536 12.911 13.285 13.660 12.379 12.753 13.128 13.502 14.548

10.641 15.019 13.476 11.948 10.434 8.934 16.098 14.590 13.095 11.614 10.147 14.383 12.883 11.396 12.872 11.439 15.561

12.82 8.23 9.45 10.97 12.93 15.51 8.23 9.34 10.69 12.38

14.684 16.235

14.54 9.36 10.75 12.48 11.86 13.67 9.23 10.45 11.95 13.82 10.51 10.15 11.45 13.04 10.21 11.54 13.17 9.91 11.07 12.45 11.15 12.58 10.71 11.92 12.04 11.43

13.437 11.841 10.259 14.389 12.826 11.277 14.211 12.642 11.087 9.546 15.227 13.692 12.170 10.662 15.038 13.497 11.969 10.456 11.857

8.12 9.54 11.37 8.20 9.50 11.13 8.20 9.51 11.18 13.38 8.23 9.43 10.92 12.81 8.23 9.45 10.97 12.91 12.27

14.094 12.640 11.201 13.870 15.384 13.964 12.558 15.147 13.720 12.308 16.502 15.109 13.730 14.854 13.469 16.307 14.956

Cl2H(l04 C12H11N03 C12H13N202 C12H15N30 C12H17N4 CX2H2503 C12H27N02 C13HN202 C13H3N30 C13H5N4 C13H13O3 C13H15NO2 Cl3H]7N20 C13H19N3 C14HO3 C14H3NO2 C14H5N2O C14H7N3 C14H17O2 C14H19NO C14H21N2 C15H5O2 C15H7NO C15H9N2 C15H21O C15H23N CxeHgO CxeHxiN C16H25 C17H13 CigH

C9H18N2O4 C9H20N3O3 C9H22N4O2 C10H6N2O4 C10H8N3O3 C10H10N4O2 C10H20NO4 C10H22N2O3 C10H24N3O2 C10H26N4O C11H8NO4 C11H10N2O3 C11H12N3O2 C11H14N4O C11H22O4 C11H24NO3 C11H26N2O2 C12H2N4O C12H10O4 C12H12NO3 C12H14N2O2 C12H16N3O C12H18N4 C12H26O3

.1194 .1433 .1671 .1910 .2148 .2498 .2736 .0732 .0971 .1209 .1559 .1797 .2035 .2274 .0620 .0858 .1096 .1335 .1923 .2161 .2400 .0984 .1222 .1461 .2287 .2525 .1348 .1586 .2651 .1712 .0773

13.268 13.642 14.016 14.391 14.765 13.485 13.859 14.905 15.279 15.654 14.373 14.747 15.122 15.496 15.262 15.636 16.010 16.335 15.479 15.583 16.227 16.367 16.742 17.116

16.120 14.611

8.23

16.584 16.959 17.473 17.847 17.690 18.578 19.467

12.334 16.528 15.136 13.757 14.880 13.496 16.335 14.984 14.712 16.265 17.897

9.34 10.69 12.37 14.52 9.36 10.74 10.39 11.85 13.65 9.22 10.45 11.94 13.80 9.06 10.15 11.45 13.02 10.20 11.53 13.16 9.90 11.06 12.44 11.15 12.57 10.70 11.91 12.02 11.42 10.88

218 10.933 11.307 11.681 11.821 12.196 12.570 11.664 12.038 12.413 12.787 .2804 .1150 12.552 .1388 12.927 .1627 13.301 13.676 .1865 .2215 12.395 .2453 12.769 .2692 13.144 .0926 14.564 .1276 13.284 13.658 .1514 14.032 .1753 14.407 .1991 .2229 14.781 .2579 13.501

13.455 11.859 10.277 14.408 12.845 11.297 14.230 12.661 11.107 9.567 15.247 13.712 12.191 10.684 15.058 13.517 11.990 11.881 16.141 14.633 13.139 11.660 10.194 14.427

8.13 9.53 11.36 8.20 9.49 11.13 8.20 9.51 11.18 13.37 8.23 9.43 10.91 12.80 8.23 9.45 10.96 12.26 8.23 9.33 10.68 12.36 14.50 9.36

.1963 .2201 .2440 .1024 .1263 .1501 .2089 .2327 .2566

13.117 11.637 10.170 14.405 12.905 14.345 12.897 11.464 15.584 14.117 12.664 11.226 16.843 15.409 13.989 12.584 15.171 13.745

530

C13H2N202 C13H4N30 C13H6N4 C13H1403 C13H16N02 C13H18N20 C13H20N3 C14H203 C14H4N02 C14H6N20 C14H8N3 C14H1802 C14H20NO C14H22N2 C15H602 CisHsNO

Ci5H10N2 C15H22O

Ci5H24N C16H10O Ci6Hi2N Cif,H26 C17H14 C18H2

APPENDIX 1

.0814 .1052 .1290 .1640 .1878 .2117 .2355 .0701 .0939 .1178 .1416 .2004 .2243 .2481 .1065 .1304 .1542 .2369 .2607 .1430 .1668 .2733 .1794 .0855

14.921 15.295 15.670 14.389 14.763 15.138 15.512 15.278 15.652 16.026 16.401 15.495 15.869 16.243 16.383 16.758 17.132 16.600 16.975 17.489 17.863 17.706 18.594 19.483

14.368 12.921 11.48-9 15.607 14.141 12.689 11.250 16.867 15.434 14.015 12.610 15.196 13.771 12.360 16.554 15.162 13.785 14.907 13.523 16.363 15.013 14.740 16.295 17.928

10.38 11.84 13.64 9.22 10.44 11.93 13.79 9.06 10.14 11.43 13.01 10.20 11.52 13.14 9.90 11.05 12.43 11.14 12.55 10.69 11.90 12.01 11.41 10.87

219 C9H19N2O4 C9H21N3O3 C9H23N402

Ch>H?N204 C10H9N3O3 C10H11N4O2

.2045 .2283 .2521 .1106

.1344 .1582 CioH2iN04 .2170 C10H23N2O3 .2409 CioH25N302 .2647 .1231 C11H9N04 CnHuN203 .1470 CnHi3N302 .1708 C11H15N4O .1947 .2296 C11H23O4 .2535 C11H25NO3 .0769 C12HN3O2 C12H3N4O .1008 .1357 c12h„o4 Ci2Hi3N03 .1596 Ci2Hi5N202 .1834 Ci2Hi7N30 .2072 .2311 C12H19N4 .0657 C13HNO3 C13H3N2O2 .0895 C13H5N3O .1133 .1372 C13H7N4 .1721 C13H15O3 .1960 C13H17NO2 Ci3Hi9N20 .2198 C13H21N3 .2437 .0783 C14H3O3

10.949 11.323 11.697 11.837 12.212 12.586 11.680 12.054 12.429 12.568 12.943 13.317 13.692 12.411 12.785 14.206 14.580 13.300 13.674 14.048 14.423 14.797 14.563 14.937 15.311 15.686 14.405 14.779 15.154 15.528 15.294

13.472 11.877 10.296 14.426 12.865 11.317 14.248 12.680 11.127 15.268 13.733 12.213 10.706 15.078 13.538 13.377 11.904 16.162 14.655 13.162 11.683 10.218 15.851 14.391 12.946 11.514 15.630 14.165 12.713 11.275 16.892

8.13 9.53 11.36 8.21 9.49 11.12 8.20 9.51 11.17 8.23 9.42 10.90 12.79 8.23 9.44 10.62 12.25 8.23 9.33 10.67 12.35 14.48 9.19 10.38 11.83 13.62 9.22 10.43 11.92 13.77 9.05

C14H5NO2 C14H7N2O C14H9N3 C44H19O2 C14H21NO C14H23N2 C15H7O2 C15H9NO C15H11N2 C15H23O C15H25N CieHnO CieHisN

CigH27 C17HN C17Hl5 C18H3

.1021 .1259 .1498 .2086 .2324 .2563 .1147 .1385 .1624 .2450 .2688 .1511 .1749 .2814 .0810 .1875 .0936

15.668 16.042 16.417 15.511 15.885 16.259 16.399 16.774 17.148 16.616 16.991 17.505 17.879 17.722 18.768 18.610 19.499

15.459 14.041 12.636 15.221 13.796 12.386 16.580 15.189 13.812 14.934 13.551 16.391 15.042 14.768 16.611 16.324 17.959

10.14 11.43 12.99 10.19 11.51 13.13 9.89 11.04 12.42 11.13 12.54 10.68 11.89 12.00 11.30 11.40 10.86

13.490 11.895 10.315 14.445 12.884 11.337 14.267 12.700 15.288

8.13 9.53 11.36 8.21 9.49 11.12 8.20 9.50 8.23 9.42 10.90 12.78 8.23 10.61

220 C9H20N2O4 C9H22N303 C9H24N402 C10H8N2O4 C10H10N3O3 C10H12N4O2 CioH22NC>4 C10H24N2O3 C11H10NO4 C11H12N2O3 C11H14N3O2 C11H16N4O C11H24O4 Ci2H2N302 C12H4N4O C12H12O4 C12H14NO3 C12H1GN2O2 Ci2Hi8N30 C12H20N4 C13H2NO3 C13H4N2O2 C13H6N3O C13H8N4 C13H16O3 C13H18NO2 Ci3H2oN20 C13H22N3 C14H4O3 C14H6NO2 C14H8N2O C14H10N3 C14H20O2 C14H22NO C14H24N2

Cl5Hs02 C15H10NO C15H12N2

.2126 .2364 .2603 .1187 .1425 .1664 .2252 .2490 .1313 .1551 .1790 .2028 .2378 .0851 .1089 .1439 .1677 .1915 .2154 .2392 .0738 .0977 .1215 .1453 .1803 .2041 .2280 .2518 .0864 .1102 .1341 .1579 .2167 .2406 .2644 .1228 .1467 .1705

10.965 11.339 11.713 11.853 12.228 12.602 11.696 12.070 12.584 12.959 13.333 13.708 12.427 14.222 14.596 13.316 13.690 14.064 14.439 14.813 14.579 14.953 15.327 15.702 14.421 14.795 15.170 15.544 15.310 15.684 16.058 16.433 15.527 15.901 16.275 16.415 16.790 17.164

13.754 12.234 10.728 15.098 13.400 11.927 16.183 14.677 13.184 11.706 10.241 15.875 14.415 12.970 11.539 15.653 14.188 12.737 11.300 16.916 15.484 14.066 12.662 15.246 13.822 12.412 16.607 15.216 13.840

12.24 8.23 9.33 10.67 12.33 14.46 9.18 10.37 11.82 13.61 9.21 10.43 11.91 13.76 9.05 10.13 11.42 12.98 10.18 11.50 13.11 9.88 11.03 12.40

531

MASSES AND ISOTOPIC ABUNDANCE RATIOS

C15H240 QsHseN C16H12O C16H14N C16H28 C17H2N C17H16 C18H4

.2531 .2770 .1592 .1831 .2896 .0892 .1957 .1018

C9H21N2O4 C9H23N3O3 C10H9N2O4 C10H11N3O3 C10H13N4O2 C10H23NO4 C11HN4O2

.2207 .2446 .1268 .1507 .1745 .2333 .0806

C11H11NO4 .1394 C11H13N2O3 .1633 C11H15N3O2 .1871 C11H17N4O .2109 C12HN2O3 .0694 .0932 C12H3N3O2 C12H5N4O .1171 ..1520 C12H13O4 C12H15NO3 .1759 C12H17N2O2 .1997 C12H19N3O .2235 C12H21N4 .2474 .0581 C13HO4 .0820 C13H3NO3 .1058 C13H5N2O2 C13H7N3O .1296 .1535 C13H9N4 C13H17O3 .1884 .2123 C13H19NO2 .2361 C13H21N2O .2600 C13H23N3 .0945 C14H5O3 C14H7NO2 .1184 C14H9N2O .1422 .1661 C14H11N3 C14H21O2 .2249 C14H23NO .2487 .2725 C14H25N2 .1310 C15H9O2 C15H11NO .1548 .1786 C15H13N2 .2613 C15H25O .2851 C15H27N .0847 Ci6HN2 C16H13O .1674 .1912 CieHisN .2977 C16H29 .0735 C17HO .0973 C17H3N .2038 C17H17

16.632 17.007 17.521 17.895 17.738 18.784 18.626 19.515 221 10.981 11.355 11.869 12.244 12.618 11.712 13.507 12.600 12.975 13.349 13.724 13.863 14.238 14.612 13.332 13.706 14.080 14.455 14.829 14.220 14.595 14.969 15.343 15.718 14.437 14.811 15.186 15.560 15.326 15.700 16.074 16.449 15.543 15.917 16.291 16.431 16.806 17.180 16.648 17.023 18.069 17.537 17.911 17.754 18.425 18.800 18.642

14.960 13.578 16.419 15.070 14.797 16.641 16.354 17.990

11.12 12.52 10.67 11.87 11.99 11.29 11.39 10.85

13.507 11.913

8.13 9.53 8.21 9.49

14.464 12.904 11.357 14.286 12.459 15.308 13.775 12.255 10.750 14.909 13.423 11.951 16.205 14.699 13.207 11.729 10.265 17.371 15.898 14.439 12.995 11.564 15.677 14.212 12.761 11.325 16.941 15.509 14.092 12.689 15.271 13.847 12.438 16.633 15.243 13.867 14.987 13.605 15.375 16.447 15.099 14.825 17.987 16.672 16.384

11.11 8.20 10.84 8.23 9.42 10.89 12.77 9.30 10.61 12.23 8.23 9.32 10.66 12.32 14.45 8.19 9.18 10.37 11.81 13.59 9.21 10.42 11.90 13.74 9.05 10.12 11.41 12.96 10.18 11.50 13.10 9.88 11.03 12.39

11.11 12.51 11.75 10.66 11.86 11.98 10.24 11.28 11.38

CisHs

.1099

C9H22N2O4 C10H10N2O4

.2289 .1350 .1588 .1827 .0888 .1476

19.531

18.022

10.84

13.525 14.483 12.923 11.377 12.481 15.328 13.795 12.277 10.772 14.931 13.445

8.13 8.21 9.49 11.10 10.83 8.23 9.42 10.89 12.76 9.30 10.60 12.22 8.23 9.32 10.66 12.31 14.43 8.18 9.18 10.36 11.80 13.58 9.21 10.42 11.89 13.72 9.04 10.12 11.40 12.95 10.17 11.49 13.08 9.87 11.02 12.38 11.10 12.50

222

C10H12N3O3 C10H14N4O2 C11H2N4O2 C11H12NO4 C11H14N2O3 C11H16N3O2 C11H18N4O C12H2N2O3 C12H4N3O2 C12H6N4O C12H14O4 C12H16NO3 C12H18N2O2 C12H20N3O C12H22N4 C13H2O4 C13H4NO3 C13H6N2O2 C13H8N3O C13H10N4 C13H18O3 C13H20NO2 C13H22N2O C13H24N3 C14H6O3 C14H8NO2 C14H10N2O C14H12N3 C14H22O2 C14H24NO C14H26N2 C15H10O2 C15H12NO C15H14N2 C15H26O C15H28N C16H2N2 C16H14O C16H16N C16H30 C17H2O C17H4N 43i7H« Ci8H«

C10H11N2O4 C10H13N3O3 C10H15N4O2 C11HN3O3 C11H3N4O2 C11H13NO4

.1714 .1953 .2191 .0775 .1014 .1252 .1602 .1840 .2078 .2317 .2555 .0663 .0901 .1139 .1378 .1616 .1966 .2204 .2443 .2681 .1027 .1265 .1504 .1742 .2330 .2568 .2807 .1391 .1630 .1868 .2694 .2933 .0929 .1755 .1994 .3059 .0816 .1055 .2120 .1181

.1431 .1670 .1908 .0731 .0969 .1557

10.997 11.885 12.260 12.634 13.523 12.616 12.991 13.365 13.740 13.879 14.254 14.628 13.348 13.722 14.096 14.471 14.845 14.236 14.611 14.985 15.359 15.734 14.453 14.827 15.202 15.576 15.342 15.716 16.090 16.465 15.559 15.933 16.307 16.447 16.822 17.196 16.664 17.039 18.085 17.553 17.927 17.770 18.441 18.816 18.658 19.547 223 11.901 12.276 12.650 13.164 13.539 12.632

11.974 16.226 14.721 13.229 11.752 10.289 17.393 15.921 14.463 13.019 11.589 15.700 14.236 12.786 11.350 16.965 15.534 14.118 12.715 15.295 13.873 12.464 16.659 15.270 13.895 15.013 13.632 15.404 16.475 15.127 14.854 18.016 16.702 16.414 18.053

14.502 12.943 11.397 14.015 12.503 15.348

11.74 10.65 11.85 11.96 10.24 11.27 11.37 10.83

8.21 9.49 11.10 9.39 10.83 8.23

532

C11H15N203 C11H17N302 C11H19N40 c12hno4 C12H3N203 C12H5N302 C12H7N40 C12H1504 C12H17N03 C12H19N202 C12H21N30 Cl2H23N4 Ci3H304 C13H5N03 C13H7N202 C13H9N30 Ci3HhN4 C13H19O3 C13HaiN02 C13H23N2O C13H25N3 Cl4H703

C14H9N02 C14H11N20 Ci4Hi3N3 C14H2302 Ci4H25NO C1-1H27N2 C15HN3 C15H11O2 C15H13NO C15H15N2 C15H27O C15H29N CieHNO C16H3N2 C16H15O Ci6H17N C16H31 c17h3o C17H5N C17H19 C18H7

C10H12N2O4 CioHi4N303 C10H16N4O2 C11H2N3O3 CnH4N402 CuHi4N04 C11H16N2O3 C11H18N3O2 ChH2oN40

C12H2NO4 Ci2H4N20a Ci2HfiN302

APPENDIX 1

.1796 .2034 .2272 .0618 .0857 .1095 .1333 .1683 .192. .2160 .2398 .2637 .0744 .0982 .1221 .1459 .1698 .2047 .2286 .2524 .2762 .1108 .1347 .1585 .1824 .2412 .2650 .2888 .0885 .1473 .1711 .1949 .2776 .3014 .0772 .1010 .1837 .2075 .3140 .0898 .1136 .2201 .1262

.1513 .1751 1990 .0812 .1051 .1639 .1877 .2115 .2354 .0700 .0938 .1176

13.007 13.381 13.756 13.521 13.895 14.270 14.644 13.364 13.738 14.112 14.487 14.861 14.252 14.627 15.001 15.375 15.750 14-469 14.843 15.218 15.592 15.358 15.732 16.106 16.481 15.575 15.949 16.323 17.369 16.463 16.838 17.212 16.&80 17.055 17.726 18.101 17.569 17.943 17.786 18.457 18.832 18.674 19.563 224 11.917 12.292 12.666 13.180 13.555 12.648 13.023 13.397 13.772 13 537 13.911 14.286

13.816 12.298

9.41 10.88

10.794 16.452 14 953 13.468 11.997 16.247 14.743 13.252 11.775 10.313 17.416 15.945 14.487 13.044 11.614 15.723 14.259 12.810 11.375 16.990 15.559 14.144 12.741 15.320 13.898 12.490 14.187 16.685 15.297 13.922 15.040 13.659 16.774 ,5.433 16.503 15.156 14-882 18.046 16.732

12.74 8.22 9.29 10.60 12.21 8.23 9.32 10.65 12.30

16.444 18.084

14.41 8.181 9.17 10.35 11.79 13.56 9.20 10.41 11.88 13.71 9.04 10.11 11.39 12.94 10.17 11.48 13.07 12.24 9.87 11.01 12.36 11.09 12.49 10.57 11.73 10.65 11.84 11.95 10.23 11.26 11.36 10.82

14-521 12.963 11.418 14.036 12.524 15.368 13.837 12.319 10.816 16.474 4.975 13.491

8.21 9.48 11.09 9.39 10.82 8.23 9.41 10.88 12.73 8.22 9.29 10.59

Ci2H8N40 Ci2Hi604 C12H18NO3 Ci2H2oN202 Ci2H22N30 Ci2H24N4 Ci3H404 C13H6NO3 C13H8N2O2 C13H10N3O

Cl3Hl2N4 C13H20O3 C13H22N02 C13H24N20 Ci3H26N3 Ci4Hs03 CwHioNOa Ci4Hi2N20 Ci4Hi4N3 Ci4H2402 Ci4H26NO Ql4H28N2 Ci5H2N3 Ci5Hi202

CisHmNO C15H1CN2 Cl5H280

C15H30N c16h2no Ci6H4N2 CieHieO CieHigN Cl6H32 Ci?H40 Ci7H6N C17H20 C18H8

CioHi3N204 c10h15n3o3 CloHl7N402

ChHN204 CiiH3Ns03 CuH5N402

CnHi5N04 C11H17N2O8 CuHi9N302

ChH2iN40

Ci2H3N04 Ci2H5N203 Ci2H7N302

Ci2H»N40 Ci2Hi704 Ci2Hi9N03 Ci2H2iN202

Ci2H23N30

.1415 .1764 .2003 .2241 .2480 .2718 .0826 .1064 .1302 .1541 .1779 .2129 .2367 .2606 .2844 .1190 .1428 .1667 .1905 .2493 .2731 .2970 .0966 .1554 .1792 .2031 .2857 .3096 .0853 .1092 .1918 .2157 .3221 .0979 1218 .2282 .1344

.1594 .1833 .2071 .0655 .0894 .1132 .1720 .1958 .2197 .2435 .0781 1020 .1258 1496 .1846 .2084 .2323 .2561

14.660 13.380 13.754 14.128 14.503 14.877 14-268 14-643 15.017 15.391 15.766 14.485 14.860

12.021 16.269 14.765 13.275 11.798 10.336 17.439 15.968 14.511 13.068 11.640 15.746 14.283 12.834 11.400

12.20 8.22 9.32 10.64 12.29 14.39 8.18 9.17 10.35 11.78 13.54 9.20 10.40 11.87 13.69 9.04 10.11 11.37 12.92 10.16 11.47 13.05 12.23 9.86

15.234 15.608 15.374 15.748 16.122 16.497 15.591 15.965 16.339 17.385 16.479 16.854 17.228 16.696 17.071 17.742 18.117 17.585 17.959 17.802 18.473 18.848 18.690 19.579

17.014 15.585 14.169 12.768 15.345 13.924 12.516 14.215 16.712 15.324 13.950 15.067 13.687 16.802 15.462 16.532 15.185 14.910 18.075 16.762 16.474 18.116

11.24 11.35 10.81

225 11.933 12.308 12.682 12.822 13.196 13.571 12.664 13.039 13.413 13.788 13.553 13.927 14.302 14.676 13.396 13.770 14.144 14.519

14-541 12.982 11.437 15.582 14.057 12.546 15.389 13.858 12.341 10.838 16.495 14.998 13.514 12.044 16.290 14.787 13.297 11.822

8.21 9.48 11.09 8.23 9.39 10.82 8.23 9.41 10.87 12.72 8.22 9.29 10.58 12.19 8.22 9.31 10.64 12.28

11.00

12.35 11.08 12.47 10.56 11.72 10.64 11.83 11.94 10.22

533

MASSES AND ISOTOPIC ABUNDANCE RATIOS

C12H25N4 C13H504 C13H7N03 C13H9N202 C13H11N30 C13H13N4 C13H2103 C13H23N02 C13H25N20 C13H27N3 C14HN4 C14H903 C14H11N02 C14H13N20 C14H15N3 C14H2502 C14H27NO CX4H29N2 c15hn2o C15H3N3 C15H1302 C15H15NO C15H17N2 C15H290 C15H3iN Ci6H02 C16H3NO C16H5N2 C16H170 C16Hi9N C16H33 c17h5o C17H7N C17H21 C18H9

CioHi4N204 C10H16N3O3 CioHisN402

ChH2N204 C11H4N3O3 C11H6N4O2 CiiHi6N04 CnHigNaOa ChH2oN302

ChH22N40 Ci2H4N04 c12h6n2o3 C12H8N3O2 C12H10N4O C12Hi804 Ci2H2oN03 Cx2H22N202 Ci2H24N30 Ci2H26N4 Ci3He04

.2800' .0907 .1145 .1384 .1622 .1861 .2210 .2449 .2687 .2925 .0922 .1271 .1510 .1748 .1986 .2574 .2813 .3051 .0809 .1047 .1635 .1874 .2112 .2939 .3177 .0696 .0935 .1173 .2000 .2238 .3303 .1061 .1299 .2364 .1425

.1676 .1914 .2153 .0737 .0975 .1214 .1802 .2040 .2278 .2517 .0863 .1101 .1339 .1578 .1927 .2166 .2404 .2643 .2881 .0988

14.893 14.284 14.659 15.033 15.407 15.782 14.501 14.876 15.250 15.624 16.670 15.390 15.764 16.138 16.513 15.607 15.981 16.355 17.027 17.401 16.495 16.870 17.244 16.712 17.087 17.384 17.758 18.133 17.601 17.975 17.818 18.489 18.864 18.706 19.595 226 11.949 12.324 12.698 12.838 13.212 13.587 12.680 13.055 13.429 13.804 13.569 13.943 14.318 14.692 13.412 13.786 14.160 14.535 14.909 14.300

10.360 17.462 15.991 14.535 13.093 11.665 15.769 14.307 12.859 11.425 13.048 17.039 15.610 14.195 12.794 15.370 13.949 12.543 15.610 14-243 16.738 15.351 13.977 15.093 13.714 18.185 16.831 15.491 16.560 15.214 14.939 18.105 16.792 16.503 18.147

14.560 13.002 11.458 15.603 14.078 12.568 15.409 13.879 12.362 10.860 16.517 15.020 13.537 12.068 16.312 14.809 13.320 11.845 10.384 17.485

14.38 8.18 9.17 10.34 11.77 13.53 9.20 10.40 11.86 13.68 12.77 9.03 10.10 11.37 12.91 10.15 11.46 13.04 10.91 12.22 9.86 10.99 12.34 11.07 12.46 9.56 10.55 11.71 10.63 11.82 11.93 10.21 11.23 11.33 10.80

8.21 9.48 11.08 8.23 9.38 10.81 8.23 9.41 10.86 12.71 8.22 9.28 10.58 12.17 8.22 9.31 10.63 12.27 14.36 8.18

Ci3H8N03 .1227 Ci3HioN202 .1465 Cx3Hi2N30 .1704 C13H14N4 .1942 Ci3H2203 .2292 Ci3H24N02 .2530 Ci3H26N20 .2768 Ci3H28N3 .3007 Ci4H2N4 .1003 CxxHxoOs .1353 Ci4Hi2N02 .1591 Ci4Hi4N20 .1829 C14H16N3 .2068 Cx4H2602 .2656 Ci4H28NO .2894 Ci4H3oN2 .3133 Ci5H2N20 .0891 Ci5H4N3 .1129 Cl5Hl402 .1717 CxsHisNO .1955 CxsHxsNa .2194 C15H30O .3020 Ci5H32N .3259 Cx6H202 .0778 Cx6H4NO .1016 Cl6HeN2 .1255 C16H180 .2081 Ci6H2oN .2320 C16H34 .3384 CxtHbO .1142 C17H8N .1381 Cl7H22 .2445 .1506 CisHjO

CioHi5N204 C10H17N3O3 CioHi9N402 CiiH3N204 ChH5N303 C11H7N4O2 C11H17NO4 CnHi9N203 ChH2iN302 CiiH23N40 Ci2H5N04 Ci2H7N203 Ci2H9N302 Ci2HhN40 Cx2Hi904 Ci2H2iN03 Ci2H23N202 Ci2H25N30 Ci2H2?N4 Ci3H704 Ci3H9N03 Ci3HhN202

.1757 .1996 .2234 .0818 .1057 .1295 .1883 .2121 .2360 .2598 .0944 .1182 .1421 .1659 .2009 .2247 .2486 .2724 .2962 .1070 .1308 .1547

14.675 15.049 15.423 15.798 14.517 14.892 15.266 15.640 16.686 15.406 15.780 16.154 16.529 15.623 15.997 16.371 17.043 17.417 16.511 16.886 17.260 16.728 17.103 17.400

16.015 14.559 13.118 11.690 15.792 14.331 12.883 11.450 13.075 17.063 15.635 14.221 12.821 15.395 13.975 12.569 15.638 14.271 16.765 15.378 14.005 15.120 13.741 18.213 16.859 15.520 16.588 15.242 14.968

17.774 18.149 17.617 17.991 17.834 18.505 18.880 18.722 19.611

18.134 16.822 16.533 18.178

227 11.965 12.340 12.714 12.854 13.228 13,603 12.696 13.071 13.445 13.820 13.585 13.959 14.334 14.708 13.428 13.802 14.176 14.551 14.925 14.316 14.691 15.065

14.579 13.022 11.479 15.623 14.099 12.590 15.429 13.899 12.384 10.882 16.539 15.042 13.560 12.091 16.333 14.831 13.342 11.868 10.408 17.508 16.038 14.583

9.16 10.34 11.76 13.51 9.19 10.39 11.85 13.66 12.76 9.03 10.09 11.36 12.89 10.14 11.45 13.03 10.90 12.20 9.85 10.98 12.32 11.06 12.45 9.55 10.54 11.69 10.62 11.80 11.92 10.21 11.22 11.32 10.79

8.21 9.48 11.08 8.23 9.38 10.81 8.23 9.40 10.86 12.70 8.21 9.28 10.57 12.16 8.22 9.31 10.63 12.26 14.34 8.18 9.16 10.33

534

C13H13N30 C13H15N4 C13H2303 C13H25N02 C13H27N20 C13H29N3 C14HN30 C14H3N4 C14H1103 C14H13N02 C14H15N&O C14H17N3 C14H27O2 C14H29NO C14H31N2 C15HNO2 C15H3N2O C15H5N3 C15H15O2 C15H17NO C15H19N2 CX5H31O C15H33N C16H3O2 Ci6H5NO Ci6H7N2 C16H19O C16H21N C17H7O C17H9N C17H23 C18H11

C10H16N2O4 C10H18N3O3 C10H20N4O2 CX1H4N2O4 C11H6N3O3 C11H8N4O2 C11H18NO4 C11H20N2O3 C11H22N3O2 C11H24N4O C12H6NO4 C12H8N2O3 C12H10N3O2 C12H12N4O C12H20O4 C12H22NO3 C12H24N2O2 C12H26N3O C12H28N4 C13H8O4 C13H10NO3 C13H12N2O2 C13H14N3O

APPENDIX 1

.1785 .2023 .2373 .2611 .2850 .3088 .0846 .1085 .1434 .1673 .1911 .2149 .2737 .2976 .3214 .0734 .0972 .1210 .1798 .2037 .2275 .3102 .3340 .0859 .1098 .1336 .2163 .2401 .1224 .1462 .2527 .1588

15.439 15.814 14.533 14.908 15.282 15.656 16.328 16.702 15.422 15.796 16.170 16.545 15.639 16.013 16.387 16.685 17.059 17.433 16.527 16.902 17.276 16.744 17.119 17.416 17.790 18.165 17.633 18.007 18.521 18.896 18.738 19.627

228 .1839 11.981 12.356 .2077 .2315 12.730 .0900 12 870 .1138 13.244 .1376 13.619 12.712 .1964 .2203 13.087 13.461 .2441 .2680 13.836 .1025 13.601 13.975 .1264 .1502 14.350 .1741 14.724 .2090 13.444 .2329 13.818 14.192 .2567 .2805 14.567 .3044 14.941 .1151 14.332 .1390 14.707 .1628 15.081 15.455 .1867

13.142 11.715 15.816 14.355 12.908 11.475 14.495 13.102 17.088 15.660 14.247 12.847 15.420 14.001 12.595 17.045 15.665 14.298 16.791 15.405 14.032 15.147 13.769 18.241 16.888 15.549 16.616 15.271 18.164 16.852 16.563 18.210

14.598 13.041 11.499 15.644 14.121 12.611 15.450 13.920 12.405 10.904 16.560 15.065 13.583 12.115 16.355 14.853 13.365 11.891 10.432 17.530 16.062 14.607 13.167

11.75 13.50 9.19 10.39 11.84 13.64 11.27 12.75 9.02 10.09 11.35 12.88 10.14 11.44 13.01 9.79 10.89 12.19 9.84 10.97 12.31 11.05 12.43 9.55 10.53 11.68 10.61 11.79 10.20 11.21 11.31 10.78

8.21 9.48 11.07 8.23 9.38 10.80 8.23 9.40 10.85 12.69 8.21 9.28 10.57 12.15 8.22 9.30 10.62 12.25 14.32 8.18 9.16 10.33 11.74

C13H16N4 C13H24O3 C13H26NO2 C13H28N2O C13H30N3 C14H2N3O C14H4N4 C14H12O3 Ci4Hi4N02 C14H16N2O C14H18N3 C14H28O2 C14H30NO C14H32N2 Ci5H2N02 CisHUNaO C15H6N3 C15H16O2 CxsHxsNO C15H20N2 C15H32O C16H4O2 CxeHeNO C16H8N2 C16H20O C16H22N Cx7H80 C17H10N C17H24 C18H12

C10H17N2O4 C10H19N3O3 C10H21N4O2 C11H5N2O4 C11H7N3O3 C11H9N4O2 C11H19NO4 C11H21N2O3 C11H23N3O2 C11H25N4O C12H7NO4 C12H9N2O3 C12H11N3O2 C12H13N4O C12H21O4 C12H23NO3 C12H25N2O2 C12H27N3O C12H29N4 C13HN4O C13H9O4 C13H11NO3 C13H13N2O2 C13H15N3O C13H17N4

.2105 .2455 .2693 .2931 .3170 .0928 .1166 .1516 .1754 .1992 .2231 .2819 .3057 .3296 .0815 .1053 .1292 .1880 .2118 .2357 .3183 .0941 .1179 .1418 .2244 .2482 .1305 .1543 .2608 .1669

.1920 .2158 .2397 .0981 .1219 .1458 .2046 .2284 .2523 .2761 .1107 .1345 .1584 .1822 .2172 .2410 .2649 .2887 .3125 .0883 .1233 .1471 .1710 .1948 .2186

15.830 14.549 14.924 15.298 15.672 16.344 16.718 15.438 15.812 16.186 16.561 15.655 16.029 16.403 16.701 17.075 17.449 16.543 16.918 17.292 16.760 17.432 17.806 18.181 17.649 18.023 18.537 18.912 18.754 19.643 229 11.997 12.372 12.746 12.886 13.260 13.635 12.728 13.103 13.477 13.852 13.617 13.991 14.366 14.740 13.460 13.834 14.208 14.583 14.957 15.629 14.348 14.723 15.097 15.471 15.846

11.741 15.839 14.378 12.932 11.500 14.522 13.128 17.113 15.686 14.273 12.874 15.445 14.026 12.621 17.072 15.692 14.326 16.817 15.432 14.060 15.174 18.269 16.916 15.578 16.644 15.300 18.194 16.883 16.593 18.241

14.617 13.061 11.519 15.664 14.142 12.633 15.470 13.941 12.427 10.926 16.582 15.087 13.606 12.138 16.376 14.875 13.388 11.915 10.456 13.429 17.553 16.085 14.632 13.192 11.766

13.48 9.19 10.38 11.83 13.63 11.26 12.74 9.02 10.08 11.34 12.86 10.14 11.43 13.00 9.78 10.88 12.18 9.84 10.96 12.30 11.05 9.54 10.53 11.67 10.60 11.78 10.19 11.20 11.30 10.77

8.21 9.47 11.07 8.23 9.38 10.79 8.23 9.40 10.84 12.68 8.21 9.27 10.56 12.14 8.22 9.30 10.61 12.24 14.31 11.64 8.17 9.15 10.32 11.73 13.47

535

MASSES AND ISOTOPIC ABUNDANCE RATIOS

C13H2503 C13H27N02 C] 3H29N2O C13H31N3 C14HN202 C14H3N30 C14H5N4 C14H1303 C14H15N02 C14H17N20 C14H19N3 C14H2902 C14H31NO C15H03 C15H3N02 C15H5N20 C15H7N3 C15H1702 C15H19NO C15H21N2 C16H502 Ci6H7NO C16H9N2 C16H210 C1GH23N C17H9O C17H11N C17H25 C18H13 C19H

C10H18N2O4 C10H20N3O3 C10H22N4O2 C11H6N2O4 C11H8N3O3 C11H10N4O2 C11H20NO4 C11H22N2O3 C11H24N3O2 C11H26N4O C12H8NO4 C12H10N2O3 C12H12N3O2 C12H14N4O C12H22O4 C12H24NO3 C12H26N2O2 C12H28N3O C12H30N4 CX3H2N4O C13H10O4 C13H12NO3 CX3H14N2O2 C13H16N3O C13H18N4

.2536 .2774 .3013 .3251 .0771 .1009 .1247 .1597 .1835 .2074 .2312 .2900 .3139 .0658 .0896 .1135 .1373 .1961 .2200 .2438 .1022 .1261 .1499 .2325 .Z564 .1387 .1625 .2690 .1751 .0812

.2002 .2240 .2478 .1063 .1301 .1539 .2127 .2366 .2604 .2843 .1188 .1427 .1665 .1904 .2253 .2492 .2730 .2968 .3207 .0965 .1314 .1553 .1791 .2029 .2268

14.565 14.940 15.314 15.688 15.985 16.360 16.734 15.454 15.828 16.202 16.577 15.671 16.045 16.342 16.717 17.091 17.465 16.559 16.934 17.308 17.448 17.822 18.197 17.665 18.039 18.553 18.928 18.770 19.659 20.547 230 12.013 12.388 12.762 12.902 13.276 13.651 12.744 13.119 13.493 13.868 13.633 14.007 14.382 14.756 13.476 13.850 14.224 14.599 14.973 15.645 14.364 14.739 15.113 15.487 15.862

15.862 14.402 12.956 11.525 15.954 14.548 13.155 17.138 15.711 14.299 12.900 15.470 14.052 18.492 17.099 15.719 14.354 16.844 15.459 14.088 18.297 16.945 15.607 16.673 15.329 18.223 16.913 16.623 18.272 20.001

14.636 13.081 11.540 15.685 14.163 12.655 15.490 13.962 12.448 10.949 16.604 15.109 13.629 12.162 16.398 14.897 13.411 11.938 10.480 13.454 17.576 16.109 14.656 13.217 11.791

9.18 10.37 11.82 13.61 10.02 11.25 12.72 9.02 10.07 11.33 12.85 10.13 11.42 8.84 9.78 10.87 12.17 9.83 10.95 12.29 9.54 10.52 11.66 10.60 11.77 10.18 11.19 11.29 10.76 10.27

8.21 9.47 11.06 8.23 9.37 10.79 8.23 9.40 10.84 12.67 8.21 9.27 10.55 12.13 8.22 9.30 10.61 12.23 14.29 11.63 8.17 9.15 10.31 11.72 13.45

C13H26O3 C13H28NO2 C13H30N2O C14H2N2O2 C14H4N3O C14H6N4 C14H14O3 C14H16NO2 C14H18N2O C14H20N3 C14H30O2 C15H2O3 C15H4NO2 CxsHeNaO Cx5H8N3 C15H18O2 C15H20NO C15H22N2 C16H6O2 CxeHsNO C16HioN2 C16H22O C16H24N C17H10O C17H12N C17H26 C18H14 C19H2

C10H19N2O4 C10H21N3O3 C10H23N4O2 C11H7N2O4 C11H9N3O3 CnHiiN4C>2 C11H21NO4 C11H23N2O3 C11H25N3O2 C11H27N4O C12H9NO4 C12H11N2O3 C12H13N3O2 C12H15N4O C12H23O4 C12H25NO3 C12H27N2O2 C12H29N3O C13HN3O2 C13H3N4O C13H11O4 C13H13NO3 C13H15N2O2 C13H17N3O C13H19N4 C13H27O3 C13H29NO2

.2617 .2856 .3094 .0852 .1090 .1329 .1678 .1917 .2155 .2394 .2982 .0740 .0978 .1216 .1455 .2043 .2281 .2520 .1104 .1342 .1581 .2407 .2645 .1468 .1706 .2771 .1832 .0893

.2083 .2321 .2560 .1144 .1382 .1621 .2209 .2447 .2686 .2924 .1270 .1508 .1747 .1985 .2335 .2573 .2811 .3050 .0808 .1046 .1396 .1634 .1872 .2111 .2349 .2699 .2937

14.581 14.956 15.330 16.001 16.376 16.750 15.470 15.844 16.218 16.593 15.687 16.358 16.733 17.107 17.481 16.575 16.950 17.324 17.464 17.838 18.213 17.681 18.055 18.569 18.944 18.786 19.675 20.563 231 12.029 12.404 12.778 12.918 13.292 13.667 12.760 13.135 13.509 13.884 13.649 14.023 14.398 14.772 13.492 13.866 14.240 14.615 15.286 15.661 14.380 14.755 15.129 15.503 15.878 14.597 14.972

15.885 14.426 12.981 15.980

9.18 10.37 11.81 10.01

14.574 13.182 17.162 15.736

11.24 12.71 9.01 10.07 11.32 12.84 10.12 8.83 9.77 10.86 12.15 9.83 10.95 12.27 9.53 10.51 11.65 10.59 11.76 10.17 11.18 11.28 10.75 10.27

14.324 12.927 15.495 18.518 17.125 15.747 14.382 16.870 15.486 14.115 18.324 16.973 15.636 16.701 15.358 18.253 16.943 16.653 18.304 20.033

14.655 13.101 11.560 15.706 14.184 12.677 15.510 13.983 12.470 10.971 16.626 15.132 13.652 12.186 16.419 14.919 13.433 11.961 14.912 13.479 17.599 16.133 14.680 13.241 11.817 15.909 14.450

8.21 9.47 11.05 8.23 9.37 10.78 8.23 9.39 10.83 12.66 8.21 9.27 10.55 12.12 8.22 9.29 10.60 12.22 10.25 11.62 8.17 9.15 10.31 11.71 13.44 9.18 10.36

536 C14HN03 C14H3N202 C14H5N30 C14H7N4 C14H1503 C14H17N02 C14H19N20 C14H21N3 C15H303 C15H5N02 C15H7N20 C15H9N3 C15H1902 C15H21NO C15H23N2 C16H702 CieHgNO C16H11N2 C16H23O C1BH25N C17H11O C17H13N

APPENDIX

15.643 16.017 16.392 16.766 15.486 15.860 16.234 16.609

17.425 16.005 14.600 13.209 17.187 15.762 14.350 12.953 18.544 17.152

.2124 .2363 .2601 .1185 .1424 .1662 .2488 .2727 .1549 .1788 .2853 .0849 .1914 .0975

16.374 16.749 17.123 17.497 16.591 16.966 17.340 17.480 17.854 18.229 17.697 18.071 18.585 18.960 18.802 19.848 19.691 20.579

15.774 14.410 16.897 15.513 14.143 18.352 17.002 15.665 16.729 15.387 18.283 16.974 16.683 18.639 18.335 20.066

C10H20N2O4 .2164 C10H22N3O3 .2403 C10H24N4O2 .2641 .1225 CX1H8N2O4 CuHioNsOs .1464 C11H12N4O2 .1702 .2290 C11H22NO4 C11H24N2O3 .2529 C11H26N3O2 .2767 CnH28N40 .3005 C12H10NO4 ..1351 .1590 C12H12N2O C12H14N3O2 .1828 .2066 C12H16N4O .2416 C12H24O4 C12H26NO3 .2654 C12H28N2O2 .2893 .0889 C13H2N3O2 .1128 C13H4N4O .1477 C13H12O4 .1716 CX3H14NO3 C13H16N2O2 .1954 C13H18N3O .2192 .2431 C13H20N4 .2780 C13H28O3 .0777 C14H2NO3 .1015 C14H4N2O2 C14H6N3O .1253 .1492 C14H8N4

232 12.045 12.420 12.794 12.934 13.308 13.683 12.776 13.151 13.525 13.900 13.665 14.039 14.414 14.788 13.508 13.882 14.256 15.302 15.677 14.396 14.771 15.145 15.519 15.894 14.613 15.659 16.033 16.408 16.782

14.675 13.121 11.581 15.726 14.205 12.699 15.531 14.004 12.492 10.993 16.648 15.154 13.675 12.209 16.441 14.941 13.456 14.937 13.505 17.622 16.156 14.704 13.266 11.842 15.932 17.450 16.031 14.626 13.236

C17H27 CisHN CisHis C19H3

.0695 .0934 .1172 .1410 .1760 .1998 .2237 .2475 .0821 .1059 .1298 .1536

8.98 10.01 11.23 12.69 9.01 10.06 11.31 12.82 8.83 9.77 10.86 12.14 9.82 10.94 12.26 9.52 10.50 11.64 10.58 11.74 10.17 11.17 11.27 10.65 10.74 10.26

8.21 9.47 11.05 8.23 9.37 10.78 8.23 9.39 10.83 12.64 8.21 9.26 10.54 12.11 8.22 9.29 10.60 10.24 11.61 8.17 9.14 10.30 11.70 13.42 9.17 8.97 10.00 11.22 12.68

1

C14H16O3 C14H18NO2 C14H20N2O C14H22N3 C15H4O3 CxsHsNOa CxsHgNaO C15H10N3 C15H20O2 C15H22NO

.1841 .2080 .2318 .2557 .0902

C18H16 C19H4

.1141 .1379 .1618 .2206 .2444 .2682 .1267 .1505 .1743 .2570 .2808 .1631 .1869 .2934 .0930 .1995 .1056

C10H21N2O4 C10H23N3O3 C10H25N4O2 C11H9N2O4 C11H11N3O3 C11H13N4O2 C11H23NO4 C11H25N2O3 C11H27N3O2 C12HN4O2 C12H11NO4 C12H13N2O3 C12H15N3O2 C12H17N4O C12H25O4 C12H27NO3 C13HN2O3 C13H3N3O2 C13H5N4O C13H13O4 C13H15NO3 C13H17N2O2 C13H19N3O C13H21N4 C14HO4 Ci4H3N03 C14H5N2O2 C14H7N3O C14H9N4 C14H17O3 C14H19NO2 C14H21N2O C14H23N3

.2246 .2484 .2723 .1307 .1545 .1784 .2372 .2610 .2848 .0845 .1433 .1671 .1910 .2148 .2498 .2736 .0732 .0971 .1209 .1559 .1797 .2035 .2274 .2512 .0620 .0858 .1096 .1335 .1573 .1923 .2161 .2400 .2638

C15H24N2 Cx6H802 C16H10NO C16H12N2 C16H24O C16H26N C17H12O C17H14N C17H28 Ci8H2N

15.502 15.876 16.250 16.625 16.390 16.765 17.139 17.513 16.607 16.982 17.356 17.496 17.870 18.245 17.713 18.087 18.601 18.976 18.818 19.864 19.707 20.595 233 12.061 12.436 12.810 12.950 13.324 13.699 12.792 13.167 13.541 14.587 13.681 14.055 14.430 14.804 13.524 13.898 14.944 15.318 15.693 14.412 14.787 15.161 15.535 15.910 15.301 15.675 16.050 16.424 16.798 15.518 15.892 16.266 16.641

17.212 15.787 14.376 12.980 18.570 17.179 15.802 14.438 16.923 15.540 14.171 18.380 17.030 15.694 16.757 15.415 18.313 17.004 16.714 18.671 18.367 20.099

9.01 10.06 11.30 12.81 8.83 9.76 10.85 12.13 9.81 10.93 12.25 9.52 10.49 11.63

14.694 13.141 11.601 15.747 14.227 12.721 15.552 14.025 12.513 13.919 16.669 15.177 13.698 12.233 16.462 14.964 16.407 14.961 13.530 17.645 16.180 14.728 13.291 11.867 18.907 17.475 16.057 14.653 13.262 17.237 15.813 14.402 13.006

8.21 9.46 11.04 8.22 9.37 10.77 8.23 9.39 10.82 10.48 8.21 9.26 10.53 12.10 8.22 9.29 9.11 10.24 11.60 8.17 9.14 10.29 11.69 13.41 8.09 8.97 10.00 11.21 12.67 9.00 10.05 11.29 12.80

10.57 11.73 10.16 11.16 11.26 10.64 10.73 10.25

537

MASSES AND ISOTOPIC ABUNDANCE RATIOS

C15H503 C15H7N02 C15H9N20 C15H11N3 C15H2102 C15H23NO C15H25N2 C16H902 C16H11NO C16H13N2 C16H250 C16H2vN C17HN2 C17H13O C17H15N C17H29 CigHO CigHaN C18H17 C19H5

C10H22N2O4 CxoH24N303 C10H26N4O2 C11H10N2O4 C11H12N3O3 C11H14N4O2 C11H24NO4 C11H26N2O3 C12H2N4O2 C12H12NO4 C12H14N2O3 C12H16N3O2 C12H18N4O C12H26O4 C13H2N2O3 C13H4N3O2 Ci3H6N40 C13H14O4 C13H16NO3 C13H18N2O2 CX3H20N3O C13H22N4 C14H2O4 C14H4NO3 C14H6N2O2 CiiHsNaO C14H10N4 C14H18O3 C14H20NO2 C14H22N2O C14H24N3 C15H6O3 C15H8NO2 C15H10N2O C15H12N3

.0984 .1222 .1461 .1699 .2287 .2525 .2764 .1348 .1586 .1825 .2651 .2890 .0886 .1712 .1951 .3016 .0773 .1012 .2077 .1138

.2327 .2566 ."2804 .1388 .1627 .1865 .2453 .2692 .0926 .1514 .1753 .1991 .2229 .2579 .0814 .1052 .1290 .1640 .1878 .2117 .2355 .2594 .0701 .0939 .1178 .1416 .1655 .2004 .2243 .2481 .2719 .1065 .1304 .1542 .1781

16.406 16.781 17.155 17.529 16.623 16.998 17.372 17.512 17.886 18.261 17.729 18.103 19.149 18.617 18.992 18.834 19.506 19.880 19.723 20.611 234 12.077 12.452 12.826 12.966 13.340 13.715 12.808 13.183 14.603 13.697 14.071 14.446 14.820 13.540 14.960 15.334 15.709 14.428 14.803 15.177 15.551 15.926 15.317 15.691 16.066 16.440 16.814 15.534 15.908 16.282 16.657 16.422 16.797 17.171 17.545

18.597 17.206 15.829 14.466 16.950 15.567 14.199 18.408 17.059 15.724 16.786 15.444 17.327 18.342 17.034 16.744 19.978 18.703 18.398 20.132

1*4.713 13.160 11.622 15.768 14.248 12.742 15.572 14.046 13.942 16.691 15.199 13.721 12.257 16.484 16.431 14.986 13.555 17.668 16.204 14.753 13.316 11.893 18.932 17.500 16.082 14.679 13.289 17.261 15.838 14.428 13.033 18.623 17.233 15.856 14.494

8.82 9.75 10.84 12.12 9.81 10.92 12.24 9.51 10.49 11.61 10.56 11.72 11.05 10.15 11.15 11.25 9.76 10.63 10.72 10.24

8.21 9.46 11.04 8.22 9.36 10.76 8.23 9.39 10.47 8.21 9.26 10.53 12.09 8.21 9.11 10.23 11.59 8.17 9.14 10.29 11.68 13.39 8.09 8.97 9.99 11.20 12.65 9.00 10.04 11.29 12.78 8.82 9.75 10.83 12.11

CX5H22O2 C15H24NO C15H26N2 C1GH10O2 C16H12NO C16H14N2 C16H26O C16H28N C17H2N2 C17H14O C17Hi6N C17H30 C18H2O CxsHUN CisHis C19H6

C10H23N2O4 C10H25N3O3 C11H11N2O4 C11H13N3O3 C11H15N4O2 C11H25NO4 C12HN3O3 C12H3N4O2 Ci2Hi3N04 C12H15N2O3 C12H17N3O2 C12H19N4O C13HNO4 C13H3N2O3 C13H5N3O2 C13H7N4O C13H15O4 C13H17NO3 C13H19N2O2 C13H21N3O C13H23N4 C14H3O4 C14H5NO3 C14H7N2O2 C14H9N3O C14H11N4 C14H19O3 C14H21NO2 C14H23N2O Cl4lj25N3

C15H7O3 C15H9NO2 C15H11N2O C15H13N3 C15H23O2 C15H25NO C15H27N2 C16HN3 C16H11O2

.2369 .2607 .2845 .1430 .1668 .1906 .2733 .2971 .0967 .1794 .2032 .3097 .0855 .1093 .2158 .1219

.2409 .2647 .1470 .1708 .1947 .2535 .0769 .1008 .1596 .1834 .2072 .2311 .0657 .0895 .1133 .1372 .1721 .1960 .2198 .2437 .2675 .0783 .1021 .1259 .1498 .1736 .2086 .2324 .2563 .2801 .1147 .1385 .1624 .1862 .2450 .2688 .2927 .0923 .1511

16.639 17.014 17.388 17.528 17.902 18.277 17.745 18.119 19.165 18.633 19.008 18.850 19.522 19.896 19.739 20.627 235 12.093 12.468 12.982 13.356 13.731 12.824 14.245 14.619 13.713 14.087 14.462 14.836 14.602 14.976 15.350 15.725 14.444 14.819 15.193 15.567 15.942 15.333 15.707 16.082 16.456 16.830 15.550 15.924 16.298 16.673 16.438 16.813 17.187 17.561 16.655 17.030 17.404 18.450 17.544

16.977 15.595 14.226 18.436 17.088 15.753 16.814 15.473 17.358 18.372 17.065 16.774 20.009 18.735 18.430 20.165

9.80 10.91 12.22 9.51 10.48 11.60 10.55 11.71 11.04 10.14 11.14 11.24 9.76 10.62 10.71 10.23

14.733 13.180 15.788 14.269 12.764 15.592 15.437 13.966 16.713 15.222

8.21 9.46 8.22 9.36 10.76 8.23 9.23 10.47 8.21 9.25 10.52 12.08 8.15 9.10 10.23 11.58 8.16 9.13 10.28 11.67 13.38 8.09 8.96 9.98 11.19 12.64 9.00

13.744 12.280 17.913 16.455 15.010 13.580 17.692 16.227 14.777 13.341 11.918 18.956 17.525 16.108 14.705 13.316 17.286 15.863 14.455 13.060 18.649 17.260 15.884 14.422 17.003 15.622 14.254 16.064 18.464

10.04 11.27 12.77 8.81 9.74 10.82 12.18 9.80 10.90 12.21 11.48 9.50

538 Ci6Hi3NO C16H15N2 C16H27O C16H29N C17HNO C17H3N2 C17H15O C17H17N CX7H31 CisHaO C18H3N C18H19 C19H7

CX0H24N2O4 C1XH12N2O4 CxiHx4N303 CnHif,N402 C12H2N3O3 Cl2H4N402 C12H14NO4 C12H16N2O3 C12H18N3O2 C12H20N4O C13H2NO4 C13H4N2O3 C13HGN3O2 C13H8N4O C13H16O4 Cx3H18N03 C13H20N2O2 C13H22N3O C13H24N4 C14H4O4 Cx4HgN03 C14H8N2O2 C14H10N3O C14H12N4 C14H20O3 C14H22NO2 C14H24N2O C14H26N3 C15H8O3 C15H10NO2 C15H12N2O C15H14N3 C15H24O2 CxsHaeNO C15H28N2 C16H2N3 C16H12O2 Ci6Hi4NO C16H16N2 C16H28O CxeHsoN Cx7H2NO

APPENDIX 1 .1749 .1988 .2814 .3053 .0810 .1049 .1875 .2114 .3178 .0936 .1175 .2239 .1301

.2490 .1551 .1790 .2028 .0851 .1089 .1677 .1915 .2154 .2392 .0738 .0977 .1215 .1453 .1803 .2041 .2280 .2518 .2757 .0864 .1102 .1341 .1579 .1818 .2167 .2406 .2644 .2882 .1228 .1467 .1705 .1943 .2531 .2770 .3008 .1004 .1592 .1831 .2069 .2896 .3134 .0892

17.918 18.293 17.761 18.135 18.807 19.181 18.649 19.024 18.866 19.538 19.912 19.755 20.643 236 12.109 12.998 13.372 13.747 14.261 14-635 13.729 14.103 14.478 14.852 14.618 14.992 15.366 15.741 14.460 14.835 15.209 15.583 15.958 15.349 15.723 16.098 16.472 16.846 15.566 15.940 16.314 16.689 16.454 16.829 17.203 17.577 16.671 17.046 17.420 18.466 17.560 17.934 18.309 17.777 18.151 18.823

17.116 15.782 16.843. 15.502 18.690 17.389 18.402 17.095 16.804 20.040 18.767 18.462 20.198

14.752 15.809 14.291 12.786 15.460 13.989 16.735 15.244 13.767 12.304 17.936 16.479 15.035 13.605 17.715 16.251 14.801 13.366 11.944 18.981 17.550 16.134 14.732 13.343 17.311 15.889 14.481 13.086 18.676 17.287 15.911 14.550 17.030 15.649 14.282 16.094 18.493 17.145 15.811 16.871 15.531 18.720

10.47 11.59 10.55 11.70 10.06 11.03 10.13 11.13 11.23 9.75 10.61 10.70 10.22

8.21 8.22 9.36 10.75 9.22 10.46 8.20 9.25 10.52 12.07 8.15 9.10 10.22 11.57 8.16 9.13 10.28 11.66 13.36 8.09 8.96 9.98 11.18 12.63 8.99 10.03 11.27 12.75 8.81 9.74 10.81 12.08 9.79 10.89 12.20 11.47 9.50 10.46 11.58 10.54 11.69 10.06

C17H4N2 C17H16O C17H18N C17H32 C18H4O CisHeN C18H20 CigHs

C11H13N2O4 C11H15N3O3 C11H17N4O2

.1130 .1957 .2195 .3260 .1018 .1256 .2321 .1382

.1633 .1871 .2109 .0694 .0932 .1171 .1759 .1997 .2235

C12HN2O4 C12H3N3O3 C12H5N4O2 C12H15NO4 C12H17N2O3 C12H19N3O2 C12H21N4O .2474 .0820 C13H3NO4 .1058 C13H5N2O3 .1296 C13H7N3O2 C13H9N4O .1535 C13HJ7O4 .1884 .2123 C13H19NO3 C13H21N2O2 .2361 C13H23N3O .2600 .2838 C13H25N4 .0945 C14H5O4 C14H7NO3 .1184 .1422 C14H9N2O2 C14H11N3O .1661 .1899 C14H13N4 C14H21O3 .2249 C14H23NO2 .2487 C14H25N2O .2725 C14H27N3 .2964 .0960 C15HN4 .1310 C15H9O3 .1548 C15H11NO2 C15H13N2O .1786 C15H15N3 .2025 C15H25O2 .2613 C15H27NO .2851 .3090 C15H29N2 C16HN2O .0847 .1086 C16H3N3 C16H13O2 .1674 CxgHxsNO .1912 .2151 C16H17N2 C16H29O .2977 C16H31N .3215 .0735 C17HO2 C17H3NO .0973 .1212 C17H5N2 C17H17O .2038

19.197 18.665 19.040 18.882 19.554 19.928 19.771 20.659 237 13.014 13.388 13.763 13.902 19.277 14.651 13.745 14.119 14.494 14.868 14.634 15.008 15.382 15.757 14.476 14.851 15.225 15.599 15.974 15.365 15.739 16.114 16.488 16.862 15.582 15.956 16.330 16.705 17.751 16.470 16.845 17.219 17.593 16.687 17.062 17.436 18.108 18.482 17.576 17.950 18.325 17.793 18.167 18.464 18.839 19.213 18.681

17.419 18.432 17.126 16.834 20.072 18.799 18.493 20.231

15.830 14.312 12.808 16.968 15.483 14.013 16.757 15.267 13.790 12.328 17.960 16.503 15.059 13.630 17.738 16.275 14.826 13.391 11.969 19.005 17.575 16.160 14.758 13.370 17.336 15.914 14.507 13.113 14.850 18.702 17.313 15.939 14.579 17.057 15.676 14.310 17.450 16.123 18.521 17.174 15.841 16.899 15.560 20.064 18.750 17.450 18.462

11.02 10.13 11.12 11.22 9.74 10.60 10.69 10.21

8.22 9.35 10.75 8.19 9.22 10.46 8.20 9.25 10.51 12.06 8.15 9.09 10.21 11.56 8.16 9.13 10.27 11.65 13.35 8.09 8.96 9.97 11.17 12.61 8.99 10.03 11.26 12.74 11.95 8.81 9.73 10.80 12.07 9.78 10.88 12.18 10.38 11.46 9.49 10.45 11.37 10.53 11.68 9.20 10.05 11.01 10.12

539

MASSES AND ISOTOPIC ABUNDANCE RATIOS

C17H19N C17H33 C18H50 C18H7N C18H21 C19H9

C11H14N204 C11H16N303 C11H18N402 C12H2N204 C12H4N303 C12H6N402 C12H16NO4 C12H18N2O3 C12H20N3O2 C12H22N4O C13H4NO4 C13H6N2O3 C13H8N3O2 C13H10N4O

.2277 .3341 .1099 .1338 .2402 .1463

.1714 .1953 .2191 .0775 .1014 .1252 .1840 .2078 .2317 .2555 .0901 .1139 .1378 .1616 .1966

C13H18O4 C13H20NO3 .2204 C13H22N2O2 ,.2443 C13H24N3O .2681 .2919 C13H26N4 .1027 C14H6O4 .1265 CmHsNOs C14H10N2O2 .1504 C14Hi2NsO .1742 .1980 C14Hi4N4 .2330 C14H2203 .2568 C14H24NO2 C14H26N2O .2807 .3045 C14H28N3 .1042 C15H2N4 .1391 C15H10O3 .1630 C15H12NO2 .1868 C15H14N2O .2106 C15H16N3 C15H26O2 CisHasNO C15H30N2 C16H2N2O C16H4N3 C16H14O2 CisHieNO C16H18N2 C16H30O C16H32N C17H2O2 C17H4NO C17H6N2 C17H18O C17H20N C17H34

.2694 .2933 .3171 .0929 .1167 .1755 .1994 .2232 .3059 .3297 .0816 .1055 .1293 .2120 .2358 .3423

19.056 18.898 19.570 19.944 19.787 20.675 238 13.030 13.404 13.779 13.918 14.293 14.667 13.761 14.135 14.510 14.884 14-650 15.024 15.398 15.773 14.492 14.867 15.241 15.615 15.990 15.381 15.755 16.130 16.504 16.878 15.598 15.972 16.346 16.721 17.767 16.486 16.861 17.235 17.609 16.703 17.078 17.452 18.124 18.498 17.592 17.966 18.341 17.809 18.183 18.480 18.855 19.229 18.697 19.072 18.914

17.156

11.11

16.864 20.103 18.830 18.525

11.21

20.264

15.851 14.334 12.830 16.990 15.506 14.036 16.779 15.289 13.813 12.352 17.983 16.527 15.084 13.655 17.761 16.298 14.850 13.415 11.995 19.030 17.601 16.185 14.784 13.397 17.361 15.940 14.533 13.140 14.878 18.728 17.340 15.967 14.607 17.083 15.704 14.338 17.479 16.153 18.549 17.202 15.870 16.928 15.590 20.093 18.780 17.481 18.492 17.186 16.895

9.74 10.59 10.68 10.20

8.22 9.35 10.74 8.19 9.22 10.45 8.20 9.25 10.51 12.05 8.15 9.09 10.21 11.55 8.16 9.12 10.26 11.64 13.33 8.08 8.95 9.97 11.16 12.60 8.99 10.02 11.25 12.73 11.94 8.80 9.72 10.79 12.06 9.78 10.88 12.17 10.37 11.45 9.48 10.44 11.56 10.52 11.66 9.20 10.04

11.00 10.11 11.10 11.20

CisHeO CigHsN

.1181 .1419

C18H22 C19H10

.2484 .1545

C11H15N2O4 C11H17N3O3 C11H19N4O2 C12H3N2O4 C12H5N3O3 C12H7N4O2 C12H17NO4 C12H19N2O3 C12H21N3O2 C12H23N4O C13H5NO4 C13H7N2O3 C13H9N3O2 C13H11N4O

.1796

C13H19O4 C13H21NO3 C13H23N2O2 C13H25N3O C13H27N4 C14H7O4 C14H9NO3 C14H11N2O2 C14H13N3O C14H15N4 C14H23O3 C14H25NO2 C14H27N2O C14H29N3 c15hn3o C15H3N4 C15H1103 C15H13N02 C15H15N20 C15H17N3 C15H2702 C15H29NO C15H31N2 C16HN02 C16H3N20 C16H5N3 C16H1502 £i6Hi*NO C16H19N2 C16H3iO C16H33N C17H3O2 C17H5NO C17H7N2 C17H19O C17H21N C17H35

.2034 .2272 .0857 .1095 .1333 .1921 .2160 .2398 .2637 .0982 .1221 .1459 .1698 .2047 .2286 .2524 .2762 .3001 .1108 .1347 .1585 .1824 .2062 .2412 .2650 .2888 .3127 .0885 .1123 .1473 .1711 .1949 .2188 .2776 .3014 .3253 .0772 .1010 .1249 .1837 .2075 .2314 .3140 .3378 .0898 .1136 .1375 .2201 .2439 .3504

19.586 19.960 19.803 20.691 239 13.046 13.420 13.795 13.934 14.309 14.683 13.777 14.151 14.526 14.900 14.666 15.040 15.414 15.789 14-508 14.883 15.257 15.631 16.006 15.397 15.771 16.146 16.520 16.894 15.614 15.988 16.362 16.737 17.408 17.783 16.502 16.877 17.251 17.625 16.719 17.094 17.468 17.765 18.140 18.514 17.608 17.982 18.357 17.825 18.199 18.496 18.871 19.245 18.713 19.088 18.930

20.134 18.862 18.557 20.297

9.73 10.58 10.67 10.19

15.872 14.355 12.852 17.012 15.529 14.059 16.801 15.312 13.837 12.375 18.007 16.551 15.109 13.681

8.22 9.35 10.73 8.19 9.21

17.784 16.322 14.874 13.440 12.021 19.055 17.626 16.211 14.811 13.424 17.386 15.965 14.559 13.167 16.260 14-907 18.755 17.367 15.994 14.635 17.110 15.731 14.366 18.848 17.508 16.182 18.577 17.231 15.899 16.956 15.619 20.123 18.810 17.512 18.521 17.217 16.925

10.44 8.20 9.24 10.50 12.04 8.15 9.09 10.20 11.54 8.16 9.12 10.26 11.63 13.32 8.08 8.95 9.96 11.15 12.59 8.98 10.01 11.24 12.71 10.71 11.93 8.80 9.72 10.79 12.04 9.77 10.87 12.16 9.43 10.36 11.44 9.48 10.44 11.55 10.51 11.65 9.19 10.03 10.99 10.10 11.09 11.19

540 C18H70 C18H9N C18H23 CigHn

C11H16N2O4 CX1H18N3O3 C11H20N4O2 C12H4N2O4 C12H6N3O3 C12H8N4O2 C12H18NO4 C12H20N2O3 C12H22N3O2 CX2H24N4O C13H6NO4 C13H8N2O3 C13H10N3O2 C13H12N4O C13H20O4 C13H22NO3 C13H24N2O2 C13H26N3O C13H28N4 C14H3O4 C14H10NO3 C14H12N2O2 C14H14N3O Ci4Hi6N4 C14H24O3 C14H26NO2 C14H28N2O C14H30N3 C15H2N3O C15H4N4 C15H12O3 C15H14NO2 C15H16N2O C15H18N3 C15H28O2 C15H30NO C15H32N2 C16H2NO2 C16H4N2O C16H6N3 C16H16O2 CxeHisNO C16H20N2 C16H32O Ci6H34N C17H4O2 Ci7H6NO C17H8N2 C17H20O C17H22N C17H36

APPENDIX 1 .1262 .1500 .2565 .1626

.1877 .2115 .2354 .0938 .1176 .1415 .2003 .2241 .2480 .2718 .1064 .1302 .1541 .1779 .2129 .2367 .2606 .2844 .3082 .1190 .1428 .1667 .1905 .2143 .2493 .2731 .2970 .3208 .0966 .1204 .1554 .1792 .2031 .2269 .2857 .3096 .3334 .0853 .1092 .1330 .1918 .2157 .2395 .3221 .3460 .0979 .1218 .1456 .2282 .2521 .3586

19.602 19.976 19.819 20.707 240 13.062 13.436 13.811 13.950 14.325 14.699 13.793 14.167 14.542 14.916 14.682 15.056 15.430 15.805 14.524 14.899 15.273 15.647 16.022 15.413 15.787 16.162 16.536 16.910 15.630 16.004 16.378 16.753 17.424 17.799 16.518 16.893 17.267 17.641 16.735 17.110 17.484 17.781 18.156 18.530 17.624 17.998 18.373 17.841 18.215 18.512 18.887 19.261 18.729 19.104 18.946

20.166 18.894 18.588 20.331

15.893 14.377 12.875 17.034 15.552 14.083 16.823 15.335 13.860 12.399 18.030 16.575 15.133 13.706 17.807 16.346 14.899 13.466 12.046 19.079 17.651 16.237 14.837 13.451 17.411 15.991 14.585 13.193 16.288 14.935 18.781 17.394 16.022 14.663 17.137 15.758 14.394 18.877 17.537 16.212 18.605 17.260 15.929 16.985 15.648 20.152 18.840 17.542 18.551 17.248 16.955

9.72 10.57 10.66 10.19

8.22 9.35 10.73 8.19 9.21 10.44 8.20 9.24 10.49 12.03 8.14 9.08 10.20 11.53 8.16 9.12 10.25 11.62 13.30 8.08 8.94 9.95 11.15 12.57 8.98 10.01 11.23 12.70 10.70 11.92 8.80 9.71 10.78 12.03 9.77 10.86 12.15 9.42 10.35 11.43 9.47 10.43 11.53 10.50 11.64 9.19 10.03 10.98 10.10 11.08 11.17

Ci8HsO CxgHioN C18H24 C19H12

C11H17N2O4 C11H19N3O3 C11H21N4O2 C12H5N2O4 C12H7N3O3 C12H9N4O2 C12H19NO4 C]2H2]N203 C12H23N3O2 C12H25N4O C13H7NO4 C13H9N2O3 C13H11N3O2 C13H13N4O C13H21O4 C13H23NO3 C13H25N2O2 C13H27N3O C13H29N4 C14HN4O C14H9O4 C14H11NO3 C14H13N2O2 C14H15N3O C14H17N4 C14H25O3 C14H27NO2 C14H29N2O C14H31N3 C15HN2O2 C15H3N3O Ci5H5N4 C15H13O3 C15H15NO2 C15H17N2O C15H19N3 C15H29O2 C15H31NO C15H33N2 CxeHOs CieH3NC>2 C16H5N2O C16H7N3 C16H17O2 C16H19NO C16H21N2 C16H33O C16H35N C17H5O2 C17H7NO C17H9N2

.1344 .1582 .2647 .1708

.1958 .2197 .2435 .1020 .1258 .1496 .2084 .2323 .2561 .2800 .1145 .1384 .1622 .1861 .2210 .2449 .2687 .2925 .3164 .0922 .1271 .1510 .1748 .1986 .2225 .2574 .2813 .3051 .3290 .0809 .1047 .1286 .1635 .1874 .2112 .2351 .2939 .3177 .3415 .0696 .0935 .1173 .1412 .2000 .2238 .2476 .3303 .3541 .1061 .1299 .1538

19.618 19.992 19.835 20.723 241 13.078 13.452 13.827 13.966 14.341 14.715 13.809 14.183 14.558 14.932 14.698 15.072 15.446 15.821 14.540 14.915 15.289 15.663 16.038 16.709 15.429 15.803 16.178 16.552 16.926 15.646 16.020 16.394 16.769 17.066 17.440 17.815 16.534 16.909 17.283 17.657 16.751 17.126 17.500 17.423 17.797 18.172 18.546 17.640 18.014 18.389 17.857 18.231 18.528 18.903 19.277

20.197 18.926 18.620 20.364

15.914 14.398 12.897 17.057 15.575 14.106 16.845 15.357 13.883 12.423 18.054 16.599 15.158 13.731 17.831 16.370 14.923 13.491 12.072 15.118 19.104 17.676 16.263 14.864 13.478 17.436 16.017 14.611 13.220 17.682 16.316 14.963 18.807 17.421 16.049 14.691 17.164 15.786 14.422 20.258 18.905 17.566 16.242 18.633 17.289 15.958 17.013 15.677 20.182 18.871 17.573

9.71 10.56 10.66 10.18

8.22 9.34 10.72 8.19 9.21 10.43 8.20 9.24 10.49 12.02 8.14 9.08 10.19 11.52 8.15 9.11 10.25 11.61 13.29 11.05 8.08 8.94 9.95 11.14 12.56 8.97 10.00 11.22 12.69 9.65 10.69 11.91 8.79 9.71 10.77 12.02 9.76 10.85 12.13 8.60 9.41 10.35 11.42 9.47 10.42 11.52 10.50 11.62 9.18 10.02 10.97

MASSES AND ISOTOPIC ABUNDANCE RATIOS

C17H210 C17H23N C18H90 C18H11N C18H25 C19H13 C20H

.2364 .2602 .1425 .1663 .2728 .1789 .0850

18.745 19.120 19.634 20.008 19.851 20.739 21.628

18.581 17.278 20.228 18.958 18.652 20.397 22.221

242 .2040 13.094 15.934 C11H20N3O3 .2278 13.468 14.420 C11H22N402. .2517 13.843 12.919 .1101 C12H6N204 13.982 17.079 C12H8N303 .1339 15.598 14.357 C12H10N4O2 .1578 14.731 14.130 .2166 C12H20NO4 13.825 16.867 C12H22N203 .2404 14.199 15.380 C12H24N302 .2643 13.906 14.574 C12H26N40 .2881 14.948 12.447 Ci3H8N04 .1227 14.714 18.077 C13H1CN2O3 .1465 15.088 16.623 C13H12N3O2 .1704 15.462 15.183 C13H14N4O .1942 15.837 13.757 C13H22O4 .2292 14.556 17.854 .2530 14.931 C13H24NO3 16.394 15.305 C13H26N2O2 .2768 14.948 C13H28N3O .3007 15.679 13.516 C13H30N4 .3245 12.098 16.054 C14H2N4O .1003 16.725 15.145 C14H10O4 .1353 15.445 19.129 .1591 17.702 C14H12NO3 15.819 C14H14N2O2 .1829 16.289 16.194 C14H16N3O .2068 16.568 14.890 C14H18N4 .2306 16.942 13.505 .2656 15.662 17.461 C14H26O3 16.036 16.042 C14H28NO2 .2894 C14H30N2O .3133 16.410 14.638 16.785 C14H32N3 .3371 13.247 C15H2N2O2 .0891 17.082 17.709 C15H4N3O 17.456 16.343 .1129 17.831 14.992 C15H6N4 .1367 16.550 C15H14O3 .1717 18.834 .1955 16.925 17.448 C15H16NO2 C15H18N2O 17.299 16.077 .2194 C15H20N3 .2432 17.673 14.720 C15H30O2 .3020 17.190 16.767 C15H32NO 15.813 .3259 17.142 17.516 14.450 C15H34N2 .3497 .0778 17.439 20.286 C16H2O3 .1016 17.813 C16H4NO2 18.934 17.595 C16H6N2O .1255 18.188 .1493 18.562 CiaHsNs 16.271 .2081 17.656 18.662 CieHi802 .2320 18.030 C16H20NO 17.317 .2558 18.405 15.987 CieH2£N2 C16H34O 17.873 17.042 .3384 C17H6O2 .1142 18.544 20.212 C11H18N204

10.09 11.07 9.71 10.55 10.64 10.17 9.73

C17H8NO C17H10N2 C17H22O C17H24N C18H10O C18H12N C18H26 CigH]4 C2DH2

.1381 .1619 .2445 .2684 .1506 .1745 .2810 .1871 .0932

541

18.919 19.293 18.761 19.136 19.650 20.024 19.867 20.755 21.644

18.901

10.01 10.96 10.08 11.06 9.70

17.604 18.611 17.309 20.260 18.990 18.683 20.430 22.255

10.54 10.63 10.16 9.73

15.955

8.22

14.441 12.941 17.102 15.621 14.154 16.890 15.403 13.930 12.471 18.101 16.647 15.207 13.782 17.877 16.418 14.972 13.541 12.123

9.34 10.71 8.19 9.20 10.42 8.20 9.23 10.47 12.00 8.14 9.07 10.18 11.50 8.15 9.10 10.23 11.59 13.26 9.88 11.03 8.07 8.93 9.94 11.12 12.53 8.97 9.99 11.20 12.66 8.75 9.64 10.67 11.88 8.78 9.69 10.75 11.99 9.75 10.83 8.59 9.40 10.33 11.40 9.46 10.40

8.22 9.34 10.72 8.19 9.20 10.43 8.20 9.23 10.48 12.01 8.14 9.08 10.18 11.51 8.15 9.11 10.24 11.60 13.27 11.04 8.07 8.94 9.94 11.13 12.55 8.97 10.00 11.21 12.67 9.65 10.68 11.89 8.79 9.70 10.76 12.01 9.75 10.84 12.12 8.60 9.41 10.34 11.41 9.46 10.41 11.51 10.49 9.18

C11H19N2Q4 C11H21N3O3 C11H23N4O2 C12H7N2O4 C12H9N3O3 C12H11N4O2 C12H21NO4 C12H23N2O3 C12H25N3O2 C12H27N4O C13H9NO4 C13H11N2O3 C13H13N3O2 C13H15N4O C13H23O4 C13H25NO3 C13H27N2O2 C13H29N3O C13H31N4 C14HN3O2 C14H3N4O C14H11O4 C14H13NO3 C14H15N2O2 C14H17NSO C14H19N4 C14H27O3 C14H29NO2 C14H31N2O C14H33N3 c16hno3 C15H3N202 C15H5N30 C15H7N4 C15H1503 C15H17N02 C15H!*N20 C15H21N3 C15H3102 C15H33NO C16H303 C16H5N02 C16H7N20 C16H9N3 C16H1902 C16H21NO

.2121 .2360 .2598 .1182 .1421 .1659 .2247 .2486 .2724 .2962 .1308 .1547 .1785 .2023 .2373 .2611 .2850 3088 .3327 .0846 .1085 .1434 .1673 .1911 .2149 .2388 .2737 .2976 .3214 .3453 .0734 .0972 .1210 .1449 .1798 .2037 .2275 .2514 .3102 .3340 .0859 .1098 .1336 .1575 .2163 .2401

243 13.110 13.484 13.859 13.998 14.373 14.747 13.841 14.215 14-590 14.964 14.730 15.104 15.478 15.853 14.572 14.947 15.321 15.695 16.070 16.367 16.741 15.461 15.835 16.210 16.584 16.958 15.678 16.052 16.426 16.801 16.724 17.098 17.472 17.847 16.566 16.941 17.315 17.689 16.783 17.158 17.455 17.829 18.204 18.578 17.672 18.046

16.564 15.172 19.153 17.727 16.315 14.917 13.532 17.486 16.068 14.664 13.274 19.115 17.736 16.371 15.020 18.860 17.476 16.105 14.748 17.217 15.841 20.314 18.962 17.625 16.301 18.690 17.346

542

C16H23N2 C17H702 C17H9NO C17H11N2 C17H230 C17H25N C18H110 C18H13N C18H27 C19HN C19H15 C20H3

C11H20N2O4 C11H22N303 C11H24N402 C12H8N204 C12H10N3O3 C12H12N402 C12H22N04 C12H24N203 C12H26N302 C12H28N40 C13H10NO4 C13H12N203 C13H14N302 C13H16N40 C13H2404 C13H26N03 C13H28N202 C13H30N3O C13H32N4 C14H2N3O2 C14H4N4O C14H12O4 C14H14NO3 C14H16N2O2 C14H18N3O C14H20N4 C14H28O3 C14H3oN02 C14H32N2O

APPENDIX 1

.2639 .1224 .1462 .1700 .2527 .2765 .1588 .1826 .2891 .0887 .1952 .1013

.2203 .2441 .2680 .1264 .1502 .1741 .2329 .2567 .2805 .3044 .1390 .1628 .1867 .2105 .2455 .2693 .2931 .3170 .3408 .0928 .1166 .1516

C1GH10N3

.1754 .1992 .2231 .2469 .2819 .3057 .3296 .0815 .1053 .1292 .1530 .1880 .2118 .2357 .2595 .3183 .0941 .1179 .1418 .1656

C16H20O2

.2244

C15H2NO3 C15H4N2O2 Ci5H6N30 Ci5H8N4 C15H16O3 C15H18NO2 C15H20N2O C15H22N3 C15H32O2 C16H4O3 C16H6NO2 C16H8N2O

18.421 18.560 18.935 19.309 18.777 19.152 19.666 20.040 19.883 20.929 20.771 21.660 Z44 13.126 13.500 13.875 14.014 14.389 14-763 13.857 14.231 14.606 14.980 14.746 15.120 15.494 15.869 14.588 14.963 15.337 15.711 16.086 16.383 16.757 15.477 15.851 16.226 16.600 16.974 15.694 16.068 16.442 16.740 17.114 17.488 17.863 16.582 16.957 17.331 17.705 16.799 17.471 17.845 18.220 18.594 17.688

16.017 20.241 18.931 17.635 18.641 17.339 20.291 19.022 18.715 20.784 20.463 22.290

15.976 14.463 12.963 17.124 15.644 14.177 16.912 15.425 13.953 12.495

11.50 9.17 10.00 10.95 10.07 11.05 9.69 10.54 10.62 10.07 10.15 9.72

16.094 14.690 19.142 17.764 16.399 15.049 18.887 17.503 16.132 14.776

8.22 9.33 10.70 8.18 9.20 10.41 8.19 9.23 10.47 11.99 8.14 9.07 10.17 11.49 8.15 9.10 10.23 11.58 13.24 9.88 11.03 8.07 8.93 9.93 11.11 12.52 8.96 9.98 11.19 8.75 9.63 10.66 11.87 8.87 9.69 10.74 11.98

17.244 20.342 18.991 17.654 16.331 18.718

9.74 8.59 9.40 10.32 11.39 9.45

18.124 16.671 15.232 13.807 17.900 16.442 14.997 13.566 12.149 16.590 15.199 19.178 17.752 16.341 14.943 13.560 17.511

C16H22NO C16H24N2 C17H8O2 C17H10NO C17H12N2 C17H24O C17H26N Ci8H120 C18H14N Ci8H28 c19h2n C19H16 C20H4

CHH21N204 CI1H23N303 C11H25N402 C12H9N204 C12H11N303 C12H13N402 C12H23N04 C12H25N203 C12H27N302 C12H29N40 Cl3HN402 C13H11N04 C13H13N203 C13H15N302 C13H17N40 C13H2504 C13H27N03 C13H29N202 C13H31N30 C14HN203 C14H3N302 C14H5N40 C14H1304 C14H15N03 C14H17N202 C14H19N30 C14H21N4 C14H2903 C14H31N02 C15H04 C15H3N03 C15H5N202 C15H7N30 C15H9N4 C15H1703 C15H19N02 C15H21N20 C15H23N3 C16H503 C16H7N02 C16H9N20 C16H11N3

.2482 .2721 .1305 .1543 .1782 .2608 .2847 .1669 .1908 .2973 .0969 .2034 .1095

.2284 .2523 .2761 .1345 .1584 .1822 .2410 .2649 .2887 .3125 .0883 .1471 .1710 .1948 .2186 .2536 .2774 .3013 .3251 .0771 .1009 .1247 .1597 .1835 .2074 .2312 .2551 .2900 .3139 .0658 .0896 .1135 .1373 .1612 .1961 .2200 .2438 .2676 .1022 .1261 .1499 .1737

18.062 18.437 18.576 18.951 19.325 18.793 19.168 19.682 20.056 19.899 20.945 20.787 21.676 245 13.142 13.516 13.891 14.030 14.405 14.779 13.873 14.247 14.622 14.996 15.668 14.762 15.136 15.510 15.885 14.604 14.979 15.353 15.727 16.025 16.399 16.773 15.493 15.867 16.242 16.616 16.990 15.710 16.084 16.381 16.756 17.130 17.504 17.879 16.598 16.973 17.347 17.721 17.487 17.861 18.236 18.610

17.375 16.046 20.271 18.961 17.666 18.671 17.370 20.323 19.054 18.747 20.818 20.496 22.325

15.997 14-484 12.985 17.146 15.667 14.201 16.934 15.448 13.977 12.519 15.495 18.148 16.695 15.257 13.833 17.924 16.466 15.021 13.591 18.022 16.616 15.225 19.203 17.778 16.367 14.970 13.587 17.536 16.119 20.561 19.169 17.791 16.427 15.078 18.913 17.530 16.160 14.805 20.370 19.019 17.683 16.360

10.40 11.49 9.16 9.99 10.94 10.07 11.04 9.68 10.53 10.61 10.06 10.14 9.71

8.21 9.33 10.70 8.18 9.19 10.41 8.19 9.22 10.46 11.98 10.11 8.13 9.07 10.17 11.48 8.15 9.10 10.22 11.57 8.89 9.87 11.02 8.07 8.93 9.92 11.10 12.51 8.96 9.98 7.97 8.74 9.63 10.66 11.86 8.78 9.68 10.73 11.97 8.59 9.39 10.31 11.38

MASSES AND ISOTOPIC ABUNDANCE RATIOS

C16H2102 C16H23NO C16H25N2 C17H902 Ci7HuNO C17H13N2 C17H25O Ci7H27N Ci8HN2 C18H13O CisHisN C18H29 C19H17 C19HO C19H3N C20H5

.2325 .2564 .2802 .1387 .1625 .1863 .2690 .2928 .0924 .1751 .1989 .3054 .2115 .0812 .1050 .1176

17.704 18.078 18.453 18.592 18.967 19.341 18.809 19.184 20.230 19.698 20.072 19.915 20.803 20.586 20.961 21.692

18.746 17.404 16.076 20.301 18.992 17.697 18.702 17.401 19.397 20.354 19.087 18.779 20.530 22.086 20.851 22.359

9.44 10.39 11.48 9.16 9.99 10.93 10.06 11.03 10.43 9.68 10.52 10.61 10.13 9.32 10.05 9.70

C16H12N3 C16H22O2 C16H24NO C16H26N2 C17H10O2 C17H12NO C17H14N2 C17H26O C17H28N C18H2N2 C18H14O C18Hi6N C18H30 C19H2O C19H4N C19H18 C20H6

.1819 .2407 .2645 .2884 .1468 .1706 .1945 .2771 .3010 .1006 .1832 .2071 .3135 .0893 .1132 .2196 .1258

543 18.626 17.720 18.094 18.469 18.608 18.983 19.357 18.825 19.200 20.246 19.714 20.088 19.931 20.602 20.977 20.819 21.708

16.390 18.775 17.433 16.105 20.330 19.022 17.728 18.732 17.431 19.429 20.386 19.119 18.811 22.119 20.885 20.563 22.394

11.36 9.44 10.38 11.47 9.15 9.98 10.92 10.05 11.02 10.42 9.67 10.51 10.60 9.31 10.04 10.12 9.69

246

C11H22N2O4 C11H24N3O3 C11H26N4O2 C12H10N2O4 C12H12N3O3 C12H14N4O2

.2366

.2604 .2843 .1427 .1665 .1904 C12H24NO4 .2492 C12H26N2O3 .2730 C12H28N3O2 .2968 C12H30N4O .3207 C13H2N4O2 .0965 C13H12NO4 .1553 C13H14N2O3 .1791 C13H16N3O2 .2029 C13H18N4O .2268 C13H26O4 .2617 C13H28NO3 .2856 C13H30N2O2 .3094 C14H2N2O3 .0852 C14H4N3O2 .1090 C14H6N4O .1323 .1678 C14H14O4 C14H16NO3 .1917 C14H18N2O2 .2155 C14H20N3O .2394 C14H22N4 .2632 .2982 C14H30O3 .0740 C15H2O4 .0978 C15H4NO3 C15H6N2O2 .1216 CisHgNsO .1455 .1693 C15H10N4 .2043 C15H18O3 .2281 C15H20NO2 C15H22N2O .2520 .2758 C15H24N3 C16H6O3 .1104 .1342 C16H8NO2 C16H10N2O .1581

13.158 13.532 13.907 14.047 14-421 14.795 13.889 14.263 14.638 15.012 15.684 14.778 15.152 15.526 15.901 14.620 14.995 15.369 16.041 16.415 16.789 15.509 15.883 16.258 16.632 17.006 15.726 16.397 16.772 17:146 17.520 17.895 16.614 16.989 17.363 17.737 17.503 17.877 18.252

16.018 14.506 13.008 17-169 15.690 14.224 16.956 15.471 14.000 12.543 15.520 18.171 16.720 15.282 13.858 17.947 16.489 15.046 18.047 16.643 15.252 19.228 17.803 16.393 14.996 13.614 17.561 20.587 19.196 17.819 16.455 15.106 18.940 17.557 16.188 14.833 20.398 19.048 17.712

8.21 9.33 10.69 8.18 9.19 10.40 8.19 9.22 10.46 11.97 10.11 8.13 9.06 10.16 11.47 8.15 9.09 10.22 8.89 9.86 11.01 8.07 8.92 9.92 11.09 12.49 8.96 7.97 8.74 9.62 10.65 11.85 8.77 9.68 10.73 11.96 8.58 9.39 10.31

247

C11H23N2O4 .2447 C11H25N3O3 .2686 C11H27N4O2 .2924 C12H11N2O4 .1508 C12H13N3O3 .1747 C12H15N4O2 .1985 .2573 C12H25NO4 C12H27N2O3 .2811 C12H29N3O2 .3050 .0808 C13HN3O3 C13H3N4O2 .1046 C13H13NO4 .1634 C13H15N2O3 .1872 C13H17N3O2 .2111 C13H19N4O .2349 .2699 C13H27O4 C13H29NO3 .2937 .0695 C14HNO4 C14H3N2O3 .0934 C14H5N3O2 .1172 C14H7N4O .1410 .1760 C14H15O4 .1998 C14H17NO3 C14H19N2O2 .2237 C14H21N3O .2475 C14H23N4 .2714 .0821 C15H3O4 .1059 C15H5NO3 .1298 C15H7N2O2 .1536 C15H9N3O .1775 C15H11N4 C15H19O3 C15H21NO2 C15H23N2O C15H25N3 C16H7O3 C16H9NO2 C16H11N2O

.2124 .2363 .2601 .2839 .1185 .1424 .1662

13.174 13.548 13.923 14.063 14.437 14.811 13.905 14.279 14.654 15.325 15.700 14.794 15.168 15.542 15.917 14.636 15.011 15.682 16.057 46.431 16.805 15.525 15.899 16.274 16.648 17.022 16.413 16.788 17.162 17.536 17.911 16.630 17.005 17.379 17.753 17.519 17.893 18.268

16.039 14.528 13.030 17.191 15.713 14.248 16.978 15.494 14.023 16.977 15.545 18.195 16.744 15.307 13.884 17.971 16.513 19.491 18.073 16.669 15.279 19.253 17.828 16.419 15.023 13.641 20.613 19.223 17.846 16.483 15.135 18.967 17.584 16.216 14-861 20.426 19.076 17.741

8.21 9.33 10.69 8.18 9.19 10.40 8.19 9.22 10.45 9.03 10.10 8.13 9.06 10.15 11.46 8.14 9.09 8.05 8.88 9.86 11.00 8.06 8.92 9.91 11.08 12.48 7.96 8.73 9.62 10.64 11.83 8.77 9.67 10.72 11.95 8.58 9.38 10.30

544

C16H13NS C16H2302 C16H25NO CX6H27N2 C17HN3 Ci7Hn02 C17H13NO C17H15N2 C17H27O C17H29N c18hno C18H3N2 C18H150 C18H17N C18H31 c19h3o C19H5N C19H19 C20H7

C11H24N204 CnH2fiN303 CX1H28N4O2 Cl2Hx2N2C>4 C12H14N303 C12H16N402 C12H26N04 C12H28N203 C13H2N303 Cl3H4N402 C13H14N04 Cx3HxfiN203 C13H18N302 C13H20N4O C13H2804 Ct4H2N04 C14H4N203 C14H6N302 Ci4H8N40 C14H16O4 C14H18N03 C14H20N2O2 C14H22N30 C14H24N4 C15H404 CxsHeNOs C18H8N2O2 C15H10N3O C15H12N4 C15H20O3 C15H22NO2 C16H24N2O C15H28N3 Ci6H803 C16H10NO2 CieHi2N20

APPENDIX 1

.1900 .2488 .2727 .2965 .0961 .1549 .1788 .2026 .2853 .3091 .0849 .1087 .1914 .2152 .3217 .0975 .1213 .2278 .1339

.2529 .2767 .3005 .1590 .1828 .2066 .2654 .2893 .0889 .1128 .1716 .1954 .2192 .2431 .2780 .0777 .1015 .1253 .1492 .1841 .2080 .2318 .2557 .2795 .0902 .1141 .1379 .1618 .1856 .2206 .2444 .2682 .2921 .1267 .1505 .1743

18.642 17.736 18.110 18.485 19.531 18.624 18.999 19.373 18.841 19.216 19.887 20.262 19.730 20.104 19.947 20.619 20.993 20.835 21.724 248 13.190 13.564 13.939 14.079 14.453 14.827 13.921 14.295 15.341 15.716 14.810 15.184 15.558 15.933 14.652 15.698 16.073 16.447 16.821 15.541 15.915 16.290 16.664 17.038 16.429 16.804 17.178 17.552 17.927 16.646 17.021 17.395 17.769 17.535 17.909 18.284

16.420 18.803 17.462 16.135 18.058 20.360 19.052 17.759 18.762 17.462 20.722 19.461 20.417 19.151 18.843 22.152 20.918 20.596 22.429

11.35 9.43 10.37 11.46 10.82 9.15 9.97 10.91 10.04

16.061 14.549 13.052 17.214 15.736 14.272 17.001 15.517 17.001 15.571 18.219 16.768 15.332 13.909 17.994 19.516 18.099 16.695 15.306

8.21 9.32 10.68 8.18 9.19 10.39 8.19 9.21 9.02 10.09 8.13 9.06 10.15 11.46

19.277 17.854 16.445 15.050 13.668 20.639 19.249 17.873 16.511 15.164 18.993 17.611 16.244 14.890 20.454 19.105 17.770

11.00 9.60 10.41 9.66 10.50 10.59 9.31 10.04 10.12 9.69

8.14 8.04 8.88 9.85 10.99 8.06 8.91 9.91 11.07 12.47 7.96 8.73 9.61 10.63 11.82 8.76 9.67 10.71 11.93 8.57 9.37 10.29

C16H14N3 C16H24O2 C16H26NO C16H28N2 C17H2N3 C17H12O2 C17H14NO C17H16N2 C17H28O C17H30N C18H2NO C18H4N2 GisHxeO CxsHxgN C18H32 C19H4O Cx9H6N C19H20 C20H8

C11H25N2O4 C11H27N3O3 C12H13N2O4 C12H15N3O3 C12H17N4O2 C12H27NO4 C13HN2O4 C13H3N3O3 C13H5N4O2 C13H15NO4 C1SH17N2O3 C13H19N3O2 Cx3H2iN40 C14H3NO4 C14H5N2O3 C14H7N3O2 C14H9N4O C14H17O4 C14H19NO3 C14H21N2O2 C14H23N3O C14H25N4 C15H5O4 C15H7NO3 C15H9N2O2 C15H11N3O C15H13N4 C15H21O3 C15H23NO2 C15H25N2O C15H27N3 C10HN4 C16H9O3 C16H11NO2 C16H13N2O C16H15N3

.1982 .2570 .2808 .3047 .1043 .1631 .1869 .2108 .2934 .3172 .0930 .1169 .1995 .2234 .3298 .1056 .1295 .2359 .1420

.2610 .2848 .1671 .1910 .2148 .2736 .0732 .0971 .1209 .1797 .2035 .2274 .2512 .0858 .1096 .1335 .1573 .1923 .2161 .2400 .2638 .2876 .0984 .1222 .1461 .1699 .1937 .2287 .2525 .2764 .3002 .0998 .1348 .1586 .1825 .2063

18.658 17.752 18.126 18.501 19.547 18.640 19.015 19.389 18.857 19.232 19.903 20.278 19.746 20.120 19.963 20.635 214309 20.851 21.740 249 13.206 13.580 14-095 14-469 14.843 13.937 14.983 15.357 15.732 14.826 15.200 15.574 15.949 15.714 16.089 16.463 16.837 15.557 15.931 16.306 16.680 17.054 16.445 16.820 17.194 17.568 17.943 16.662 17.037 17.411 17.785 18.831 17.551 17.925 18.300 18.674

16.450 18.831 17.491 16.165 18.089 20.390 19.083 17.790 18.792 17.493 20.754 19.494 20.449 19.183 18.875 22.185 20.952 20.630 22.464

16.082 14.571 17.236 15.759 14.296 17.023 18.470 17.026 15.596 18.242 16.792 15.356 13.934 19.541 18.124 16.722 15.333 19.302 17.879 16.471 15.077 13.696 20.666 19.276 17.901 16.540 15.192 19.020 17.639 16.271 14.918 16.768 20.482 19.134 17.800 16.480

11.34 9.43 10.36 11.45 10.81 9.14 9.96 10.90 10.04 10.99 9.59 10.40 9.66 10.49 10.58 9.30 10.03 10.11 9.68

8.21 9.32 8.18 9.18 10.38 8.19 9.11 9.02 10.09 8.13 9.05 10.14 11.45 8.04 8.88 9.85 10.98 8.06 8.91 9.90 11.06 12.45 7.96 8.73 9.61 10.62 11.81 8.76 9.66 10.70 11.92 11.23 8.57 9.37 10.28 11.33

MASSES AND ISOTOPIC ABUNDANCE RATIOS

C16H2502 C16H27NO C16H29N2 C17HN20

.2651 .2890 .3128 .0886

C17H3N3 C17H1302 C17H15NO

.1124 .1712 .1951 .2189 .3016

C17H17N2 C17H290 C17H31N

C18H5N2 C18H170 C18H19N C18H33 C19H50 C19H7N

.3254 .0773 .1012 .1250 .2077 .2315 .3380 .1138 .1376

C19H21 C20H9

.2441 .1502

C11H26N204 C12H14N204

.2692 .1753 .1-991 .2229

C18H02 C18H3NO

C12H16N303 C12H18N402 C13H2N204 C13H4N303 C13H6N402 C13H16N04 C13H18N203 C13H20N3O2 C13H22N40 C14H4N04 C14H6N203 C14H8N302 C14H10N4O C14H1804

.0814 .1052 .1290 .1878 .2117 .2355 .2594 .0939 .1178 .1416 .1655 .2004

17.768 18.142 18.517 19.188 19.563 18.656 19.031 19.405 18.873 19.248 19.545 19.919 20.294 19.762 20.136 19.979 20.651 21.025 20.867 21.756 250 13.222 14.111 14.485 14.859 14.999 15.373 15.748 14.842 15.216 15.590 15.965 15.730 16.105 16.479 16.853 15.573

18.860 17.520 16.194 19.407 18.120 20.420 19.113 17.821 18.822

9.42 10.36 11.43 9.89 10.80

17.524 22.059 20.786 19.526 20.480 19.215 18.907 22.218 20.986 20.663 22.498

9.14 9.96 10.89 10.03 10.98 8.86 9.58 10.39 9.65 10.48 10.57 9.30 10.02 10.10 9.67

16.103 17.259 15.782 14.319 18.494 17.050 15.621 18.266 16.817 15.381 13.960 19.566 18.150 16.748 15.360 19.327

8.21 8.18 9.18 10.38 8.11 9.02 10.08 8.13 9.05 10.14 11.44 8.04 8.87 9.84 10.97 8.06

C14H20NO3 C14H22N2O2 C14H24N3O C14H26N4 C15H6O4 CisHsNOs C15H10N2O2 C15H12N3O C15H14N4 C15H22O3 C15H24NO2 C15H26N2O C15H28N3 Cl6H2N4 C16H10O3 C16H12NO2 C16H14N2O C16H16N3 C16H26O2 C16H28NO C16H30N2 C17H2N2O C17H4N3 C17H14O2 C17H16NO C17H18N2 C17H30O C17H32N C18H2O2 C18H4NO C18H6N2 CisHisO C18H2oN C18H34 C19H8O Ci9H8N C19H22 C20H10

.2243 .2481 .2719 .2958 .1065 .1304 .1542 .1781 .2019 .2369 .2607 .2845 .3084 .1080 .1430 .1668 .1906 .2145 .2733 .2971 .3210 .0967 .1206 .1794 .2032 .2271 .3097 .3335 .0855 .1093 .1332 .2158 .2396 .3461 .1219 .1457 .2522 .1583

545

15.947 16.322 16.696 17.070 16.461 16.836 17.210 17.584 17.959 16.678 17.053 17.427 17.801 18.847 17.567 17.941 18.316 18.690 17.784 18.158 18.533 19.204 19.579 18.672 19.047 19.421 18.889 19.264 19.561 19.935 20.310 19.778 20.152 19.995 20.667 21.041 20.883 21.772

17.905 16.497 15.103 13.723 20.692 19.303 17.928 16.568 15.221 19.047 17.666 16.299 14.947 16.798 20.510 19.162 17.829 16.510 18.888 17.549 16.224 19.438 18.152 20.450 19.144 17.852 18.852 17.554 22.090 20.817 19.559 20.512 19.247 18.939 22.251 21.019 20.696 „ 22.533

8.91 9.89 11.06 12.44 7.96 8.72 9.60 10.61 11.80 8.76 9.65 10.69 11.91 11.22 8.57 9.36 10.27 11.32 9.42 10.35 11.42 9.88 10.79 9.13 9.95 10.88 10.02 10.97 8.86 9.58 10.38 9.64 10.47 10.56 9.29 10.01 10.09 9.66

APPENDIX 2

NOMOGRAMS FOR DETERMINATION OF THE ORIGIN OF META-STABLE IONS It has been shown in Chapter 7 that if an ion of mass mi undergoes a transition mi+-> m2+ + (mi — m2)

(112)

in the vicinity of the entrance slit to the magnetic field, it will be recorded as an ion of mass m* given by 2

m2 m* = mi

(113)

Determination of m* by trial and error often involves several tiresome calculations and it is the purpose of the following nomograms to speed up the determination of mi and m2 corresponding to a particular value of m*. Equation (113) can be re-written log m* = 2 log m2 — log mi

(114)

or, log m2 = J {log m* -f log mi}

(115)

In other words, if values of m2, m* and mi are plotted on a logarithmic scale, the value of m2 satis¬ fying equation (115) can be found since its logarithm is the mean of the logarithms of m* and mi, and the inter-connected values of m*, mi and m2 will lie along straight lines if the nomograms are constructed in the form of 3 identical and equally spaced logarithmic scales. In the first nomogram, for example, suppose one draws a straight line connecting masses mi = 200 and m2 = 171. It passes through the point m* = 146.2, which is the value corresponding to 200+ -* 171+ + 29 A suitable instrument for use with the nomograms can easily be made by scribing a fine line along the centre of a strip of perspex or other transparent material, and laying this strip upon the nomogram. Each nomogram scale covers a maximum range of masses of a factor of 4. The scales on successive pages have been overlapped so that it is possible to calculate m* for any ion which does not lose more than 30% of its mass in the transition and for some ions which lose up to half their masses in the transition. In most of the known meta-stable transitions involving heavy ions, the neutral fragment lost represents a much smaller fragment of the initial mass than this, though cases are known where the neutral fragment is heavier than the final ion formed. Thus, for example, in the transition in carbon monoxide induced by collision with neutral molecules (M) and given by CO++ M ^C++ O + M

(116)

more than half the mass of carbon monoxide is lost as neutral oxygen. A less accurate nomogram is included onp.552and553 which covers a range of 10:1 in mass and can be used for such reactions. The tables can be used for masses differing from those shown by any factor. Thus the nomo¬ gram covering the range 50 to 12.5 could equally well be used for the range 500 to 125 and so on. The use of the nomograms is not restricted to spontaneous meta-stable ion disintegrations, but can be applied to any transition involving a change from mass to charge ratio mi to mass to charge ratio m2. It can thus be used, for example, in connection with collision induced dissociations an example of which has been given above.

ORIGIN OF META-STABLE IONS

m,

p—100

m2

547 m*

p—100

p—100

=—HO

-HO

=—no

=—120

=—120

i—120

E-130

E—130

E—130

;—14-0

E—140

E-140

I—150

E—150

I—150

|—160

E—160

=—160

|-170

|—170

|—170

|—180

|—180

|—180

|—190

|—190

|—190

|—200

|—200

|—200

|—210

|—2I0

|—210

I—220

|—220

|—220

|— 230

1—230

1—230

|—240

|—240

I—240

|—250

1—250

|—250

---260

E—260

E—260

=—280

=—280

E—280

E—300

=—300

=—300

|—320

|—320

|—320

|—340

=—340

|—340

|—360

|—360

|—360

|—380

|—380

|—380

=—400

1— 400

i—400

548

APPENDIX

2

m*

m,

p

- 50

50

F— 50

60

i

60

60

70

i

70

=— 70

80

80

90

=^-90

1^-100

Ip-lOO

IOO

^—110

!—no

^—-110

E—120

p—120

p—120

E—130

=—130

=—130

E—140

E—140

E—140

|—ISO

E—150

=—150

|—160

|—160

|—I60

|—170

|—170

|—170

|—180

|—180

|—180

|—190

190

|—190

200

^—200

—

p

—

-

—

—

=— ao

p - 90

i—200

--

- —

ORIGIN OF META-STABLE IONS

ma -25

=—30

=-35

=-40

45

=-

|—50

I—55

W

-—

60

i—65

El—70

|—75 —60

— 85 |—90 ^—95 ^-100

549

550

APPENDIX

m

2 ID*

,

E—O

I—is

^—-13

|—14

|—14

E-14

I—15

|—tS

|—!S

I—16

I—16

I—16

I—17

I—17

|—i17

|—18

I—16

I—18

I—19

=—19

I—19

|—20

|—20

|—2.1

1—21

|—22

|—22

I—23

|—23

I—Z4

I—84

I—25

|—25

|— 26

§—Z7

I—Z€>

F

f-28

-27 |— 28

f^29

[1-29

E

30

|— 32

|— 34

I—36 §—36 |-40

|— 42

*^30

1—32

I—34 |—36 |—36 |—40

I—42

|—44

|—44

|—46

|— 46

|— 4ft

|—46

i— 50

i—SO

ORIGIN OF META-STABLE IONS

551

APPENDIX 2

100

100

90

90

90

ao

80

80

70

70

70

so

60

60

50

50

50

40

30

20

10

ORIGIN OF META'STABLE IONS

553

APPENDIX 3

TABLE OF THE MASSES AND ABUNDANCES OF THE NATURALLY-OCCURRING ISOTOPES The table lists the name, chemical symbol, atomic number, isotopic mass number, accurate isotopic weight of all the naturally-occurring isotopes which are stable or whose half-lives are long enough for them to be of geologic significance. The relative abundances of occurrence of the isoto¬ pes of each element are also listed. The table does not include the radioactive elements technetium (element 43), promethium (61), polonium (84), astatine (85), radon (86), francium (87), radium (88), actinium (89), or protactinium (91) but does include the naturally radioactive isotopes of potassium (40K), rubidium (87Rb), indium (115In), lanthanum (138La), neodymium (150Nd), samarium (147Sm), lutetium (176Lu), rhenium (187Rh), thorium (232Th) and uranium (234U, 235U, 238U). The fact that an isotope listed is naturally radioactive is indicated by placing an asterisk after its isotopic mass number in the table. Literature references are given for both mass and isotopic abundance deter¬ minations. These references are listed at the end of the Table in chronological order, so that an idea of the date of a determination may be obtained from the magnitude of its associated reference number. The references are mainly to the period after 1935, except in a few instances where the results were widely used in later years or (in the case of Aston’s assertion that fluorine and phos¬ phorus are anisotopic) have remained unchanged to the present day. The mass references refer to a specific isotope, whereas the abundance references refer of course to all or several isotopes of the element. Some of the mass and abundance references are in the form of review articles (see, for example, refs. A32, A35, A70, A80, A105, A170, A171, A188, A250, A251, A270); some of the mass data in the literature are given only in the form of mass differences and no specific isotopic mass is deduced from it (see, for example, refs. A34, A81, A124, A144, A166, A173, A187, A203, A223, A252, A253, A257, A261).

Hydrogen

H

1

Mass

number

Atomic

number

Chemicc

Element

symbol

—

1

Mass

Refs.

1.0081451 ± 2 A28, A29, A44,

Relative Refs.

abundance %

99.9849-—99.9861

A5, A48, A160

A146, A147, A154, A162, A164, A186, A195, A212, A236, A240, A258, A269 2

2.0147406

±

6 A4, A28, A29, A44,

0.0151 —0.0139

A146, A154, A162, A164, A195, A212, A240, A269 Helium

He

2

3

3.0169860 ±50 A161, A264

4

4.0038739 ±26 A4, A28, A39.A135, A146, A154, A162, A195, A212, A248, A269

1.34X 10~4 (in atm.) A75, A108, A110 ^ 100

A237

MASSES AND ABUNDANCES OF NATURALLY-OCCURRING ISOTOPES

a u

•rs o B JD 1) c| o Li

^ S3 o 6 3 < G 3

555

M. and H. H. Gunthard, Rev. Sci. Instr., 28 (1957) 510. [47] 1633 Prior, A. C., J. Sci. Instr , 35 (1958) 382. [139] 1C14 Prout, W., Annals of Philosophy, 4 (1815) 321. [28] 1635 Prout, W., Annals of Philosophy, 5 (1816) 111. [28] Numbers in square brackets indicate pages on which references appear

618

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Raghavendra Rao, K. S., J. Kamatak Univ., 1 (1956) 143. [437, 439]

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Rankama, K., J. Qeol, 56 (1948) 199. [92] Rankama, K., Bull. Qeol. Soc. Am., 59 (1948) 389. [92]

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Numbers in square brackets indicate pages on which references appear

SUBJECT INDEX Aberration constant, 52. Aberrations (see also image), 8, 15, 18, 52. —, second order, 13. Absolute isotopic abundance measurements, 65. Abundance, see Relative abun¬ dance. — measurement, analytical in¬ formation from, 293. -, determination of molecu¬ lar formulae by, 305. -, possible errors in, 306. — sensitivity; 11, 96. — spectrometer, 7. Accuracy in analyses of hydro¬ carbons, 427, 428. — necessary in determination of molecular formulae by mass measurement, 302—305. Acetal, dimethyl—, spectrum of, 365. Acetals, spectra of, 364—366. Acetate, n-amyl-, high-resolu¬ tion spectrum, 384. Acetylene, meta-stable peaks in mass spectrum of, 256. Acetylenes, spectra of some iso¬ meric C10_, 339. Acids, see Carboxylic acids. Activation energies from ther¬ mal degradation, 479. Active nitrogen, 439. -, reactions of, 440. Adiabatic ionization potential, 459, 461. Adiponitrile, mass spectrum of, 405. Age determination, lead meth¬ ods, 449-451. -, miscellaneous methods, 453. —- — of minerals, 449—453. — -—■, potassium-argon meth¬ od, 452. -, rubidium-strontium meth¬ od, 452, 453.

—- of the earth, 452, Air pollution studies, 322. Alcohols and olefines, similari¬ ty of spectra, 340, 346. —, branched primary, 350. —, cycloalkyl, 352. —, meta-stable transitions in, 352, 353. —, 3-methyl-1-butanol, spec¬ trum of, 352. —, monohydric, spectra of, 345-354. —■, re-arrangements in, 347, 351, 352. —, spectra of, 340. —, straight-chain primary, spec¬ tra Ci-Cio, 349, 350. Aldehydes, spectra of, 361, 362. Aliphatic acids, re-arrangement peak at mass 60, 274. — hydrocarbons, correlations of spectra and structure, 325— 340. Alkanes, see Paraffins. Alkenes, see Olefines. Alkyl benzenes, doubly-charged ions in, 344, 345. -, identification scheme, 343. -, spectra of, 340—345. Alkynes, see Acetylenes. Alpert valves, 140. Aluminium reduction cells, 484. Amides and acids, similarity of spectra, 372. —, re-arrangements in, 396. —, spectra of, 396. Amines, aromatic, spectra of, 395. —, meta-stable peaks in, 388. —, methyl cyclopentylamine, spectrum of, 391. —, molecular weight estima¬ tion, 389. —, N-n-butyl cyclopentylamine, spectrum of, 392. —, N-ethylhexamethyleneimine, spectrum of, 393.

—, N-co-aminobutylhexamethyleneimine, spectrum of, 393. —, re-arrangement peak at mass 18, 389, 390. —-, spectra of, 387—396. —, spectrum of cyclohexylamine, 391. —, spectrum of hexamethylenediamine, 389. —, spectrum of hexamethyleneimine, 391. n-Amyl acetate, high-resolution spectrum, 384Analysis by mass measurement, 54. — of hydrocarbons, accuracy in 427,428. — of mixtures, 424-. Anhydrides, spectra of, 374, 375. Aniline, p-n-dodecyl-, spectrum of, 395. Anthraquinone, mechanism of loss of neutral CO, 271, 272. —, meta-stable ion in mass spec¬ trum of, 259, 260. Anthraquinones, spectra of, 360. Apparent mass of ions formed in collision-induced dissoci¬ ations, 280. Appearance potentials, see also ‘' Ionization potentials ’ ’. -, use in bond-strength mea¬ surement, 472, 473. Arc source, 27. Aromatic esters, spectra of, 375. Aston’s spectrograph, 2, 5. Atomic weights, variation of, 28, 29. Automatic recording of mass spectra, 220. Background spectrum, 67. Bainbridge and Jordan’s spec¬ trograph, 14Benzaldehyde, spectrum of, 362

632 Benzenes, alkyl substituted, see “Alkyl benzenes”. Binding energy, 48. Bismuth, magneto-resistive ef¬ fect in, 45. Blood analyses during anaesthetisation studies, 484. Bombardment by positive ions, 483. Bond strength in ions, 243. — strengths, isotope effects, 455-457. — —, measurement of, 459, 472,473. Bracketing, method of mass measuring, 33. “Branching ratio”, determina¬ tion of, 447. “Break-off” sample system, 17 9— 180. “Break-seal”, 151, 154. Bromo-hexadecane, spectrum of, 420, 421. Burette for liquid samples, 161. iso-Butane, spectrum of, 425. n-Butane, spectrum of, 425. Butane-, tetrafluorohexachlorospectrum of, 417, 418. 1-Butanol, 3-methyl-, spectrum of, 352. n-Butyl acetate, peaks of mass greater than the parent ion, 276. tert-Butylbenzene, re-arrangement in, 266. sec-Butyl propanoate, spectrum of, 386. Butyl ijitrites, isomeric, mass spectra of, 408. 13C-conversion of samples to carbon dioxide, 81. 13C-determination of in carbon dioxide, 81. 13C-enriched compounds, spec¬ tra of, 458. 13C, ionization of compounds containing, 458. Calutrons, 27, 203. Calutron, The, 443. Capillary leaks, 130. -, time-constant of, 174. Capronitrile, mass spectrum of, 405. Carbon as mass standard, 30. -— isotopes, natural abundance variations, 93.

SUBJECT INDEX

—, latent heat of sublimation, 470, 471. —, variations in isotopic abun¬ dance, 305. Carboxylic acids and amides, similarity of spectra, 372. -, aromatic, 373, 374. -, distinguishing from esters, 373. -— -—, re-arrangements in, 269, 372. -, spectra of, 371-375. Catalytic activity of nickel, 484. Cathode-ray oscillograph re¬ corder, see “Recorders”. Cathodes, study of gases in, 482. Cationated cyclopropane, 266. Charge distribution in ions, 243. — transfer, negative ions, 289. Chemical exchange, separation of isotopes by, 445. Chloro-derivatives of hydro¬ carbons, meta-stable peak, 417. -, spectra of, 416—421. Cholestane, spectrum of, 336— 337. Chromatograph, gas-liquid, used in sample-handling sys¬ tem, 190. Chromatography, gas-liquid, 183, 187. —, —, collection of samples from, 187—191. —, —, mass spectrometer as detector for, 190, 221. Chronotron, The 20, 36, 37. Cigarettes, extract of phenols from smoked, 321. Clausius-Clapeyron relation¬ ship, 145, 175. CO, loss of neutral from various ions, 269—273. Cold-trap, cleaning of, 68. Collection of ions, The Faraday cylinder, 197. Collector assembly, multiple, 201-202. -—, single, 200. —, multiple, 202. —, -—, for reducing effect of beam intensity changes, 202— 203. Collectors, for samples of sepa¬ rated isotopes, 203. Collision-induced reactions, 275-282.

Conditioning of filaments, 432. Conditions for molecular flow, 125. Cracking pattern, changes in, 428. -, stability of, using electron bombardment source, 110. Cresols, 352. Criteria for mass spectrum of organic compound, 293. “Cross-talk”, 53. Cut-offs, mercury, 149. Cycloalkanes, spectra of, 335. Cyclohexylamine, spectrum of, 391. Cycloidal mass spectrometer, 16-18. -, meta-stable ions in, 257-259. Cyclo-octatetraene ion, 267. Cycloparaffins, spectra of, 326. Cyclopentylamine, methyl-, spectrum of, 391. —, N-n-butyl-, spectrum of, 392. Cyclotron resonance, possible application to mass spectro¬ metry, 27. Decomposition probability, de¬ finition, 241. De-gassing by ion beam, 134. — of glass and metal, 133—134. — within ion chamber, 134. Dempster’s double-focussing spectrograph, 13. — spectrometer, 3,5. Derivatives of ionization effi¬ ciency curves, 467—471. Derivative spectra, 53, 54. -, for high resolution, 234— 236. Desorption of gases, 484. Detection of leaks, see “Leak detection.” Detectors, electrical, 197—203. Detector sensitivity, advantages for organic compounds, 216. Detectors, linearity of galvano¬ meters as, 69, 70. —, multiplier, see “Electron multipliers”. —, signal-to-noise ratio in, 203—. Detector, The fluorescent screen as an ion-, 195. —, The photographic plate as, 195-197. Deuteriated alcohols, 352.

SUBJECT INDEX

— compounds, spectra of, 458. — samples, conversion into volatile form, 71, 72. Deuterium, determination by density measurement, 71. —, — in hydrogen gas, 72—74. —, — in methane, 76. —, — in phosphine, 74. —, — in water, 74, 75. —, ionization of compounds containing, 458. —, natural . abundance varia¬ tions, 90. —, use of to determine rear rangement, 264Dibasic acids, need to esterify, 379. -, esters of, 375. Dichlorotoluene, partial spec¬ trum, 230, 231. Diethyl succinate, spectrum of, 379. Differentiation, effect on sensi¬ tivity, 236.— with respect to independent variable, effect of, 235. Diffusive flow, 124. — separation of isotopes, 443. Digitiser, multivibrator type, 228. Digitisers, unprogrammed, 227— 229. Dimethyl acetal, mechanism of re-arrangement ion formation, 273. — terephthalate, spectrum of, 376. Di-nitrobenzene, mass spectrum, 268. Diphenyl ether, loss of neutral CO, 269, 272. Direct connection of sample to ionization chamber, 180. Direct-entry method of sample introduction, 165. Dispersion, mass and momen¬ tum, 4. —, momentum, 7. Dissociation of ions on bom¬ bardment of solid films, 282. Distillation, separation of iso¬ topes by, 444Double bonds and rings, calcu¬ lation of number of, 313. — collection, 84Double-focussing mass spectro¬ meter, 309.

Double-inlet system, 86, 87, 184. Doublets(seealso “Multiplets”), 49. — from hydro-carbons, 56. — from organic chemicals, 50, 51. —, possible, from organic com¬ pounds, 303-305. — “unresolved”, 55. Doubly-charged ions, 238, 291. -in alkyl benzenes, 344, 345. -in anhydrides, 375. -in fluorocarbons, 416. -in indoles, 401. -, meta-stable transitions, 261, 262. -, use in distinguishing “parent” peak, 311. Drift tube, 24. Dynamic-condenser electrome¬ ter, 204. Dynodes, materials for, 208, 209.

Effective slit widths with dif¬ ferentiation of ion beam, 235, 236. Effusion through an orifice, 129. Electrical discharge, transient species in, 440. Electrolysis, isotope separation by, 446. Electron affinity, values of, 289. — bombardment ion source, 103-111. -, advantages of, 105. -, compounds exam¬ ined by, 104. -, discriminations in, 106. -, electrode arrange¬ ment in, 104. -, filament materials, 108-111. -, for producing A.C. ion beams, 107. -, long term stability of, 106. — capture, 287. — energy, use of reduced, 320, 428. — multiplier, 206—213. — -, efficiency of, 210, 211, 219.

633 -, electrode activation, 208209. -, electrode materials for, 208, 209. -, fatigue in, 215. -, gain as a function of ener¬ gy of bombarding ions, 211. -, gain as function of ionic mass, 211. -, gain of, 210. -, gain of, effect of mole¬ cular complexity of bombard¬ ing ions, 212. -, magnetically focussed, 209. -, poisoning of, 213. -, sensitivity of, 214. -, statistical fluctuations in gain, 217-219. -, time-constant of210, 214, 216. Energy and mass, equivalence of, 31. — spectrometer, 7. Enriched isotopes, spectra con¬ taining, 458. Equivalent orbital treatment of bond dissociation, 244Errors, (see also “Mass” measu¬ rement, errors in). —, in abundance measurements due to non-linearity of detec¬ tor, 69. —, sources of, in mass synchro¬ meter, 36. —,-, in sector spectro¬ meter, 37. Ester mixtures, analysis of, 376, 380. Esters, aromatic, spectra of, 375. —, dibasic acid, re-arrangement in, 378. —, distinguishing from car¬ boxylic acids, 373. —, isomeric, spectra of, under high resolution, 384. —, mass spectra of, 375—387—, meta-stable peaks in, 377. — of dibasic acids, analysis under high resolution, 381. — of fatty acids, identification of, 381. — of a)-phenyl fatty acids, re¬ arrangement in, 378. —, re-arrangement in, 269, 377, 381, 385. —, thio-, spectra of, 387.

634 Ether, Ethyl sec-butyl-, spec¬ trum of, 363, 364Ethers, cyclic poly-, spectra of, 368-371. —, spectra of, 362—370. Faraday cylinder, 197. Fast scanning, application of, 174. Field emission source, 120—123. -, uses in surface studies,

121, 122. Fields, crossed electric, 26. —, crossed electric and magnet¬ ic, 11, 16. —, non-uniform magnetic, 16. —, parallel electric and magnet¬ ic, 2. Field-shift bracket method of mass measurement, 34Fields, successive electric, 27. —, successive electric and mag¬ netic, 5. —, successive magnetic, 96, 97. Filaments, evaporation of sam¬ ples from heated, 477. —, materials suitable for, 108—

111. Filter, velocity, 11. First ionization potential, 459. Flames, mass exchange in, 485. —, study of transient species in, 438, 439. Flap valves, 140. Flash photolysis, examination of products of, 440. Flash-pyrolysis, study of invola¬ tile compounds by, 168—169. Flavours, identification of, 183— 184. Flow conditions in mass spec¬ trometer, 124. —through an orifice, 129. l-Fluoro-3-bromobutane, mass spectrum of, 265. Fluorenone, mechanism of loss of neutral CO, 272. Fluorine as mass standard, 30. Fluorocarbons, doubly-charged ions in, 416. — for mass calibration pur¬ poses, 414—, high molecular weight, mass spectrum of, 414. —, spectra of, 415. Focussing action of a magnetic field, 6, 8, 9.

SUBJECT INDEX

—, direction, 4, 5, 6, 8, 10. —, double, 4,11. —r, double, conditions for, 11. —, effect of shaping field boun¬ daries on, 15, 16. —, effect of space-charge, 27. —, high order, 15. —, perfect, 4, 16, 19. —, simultaneous for all masses, 12. —, stigmatic, 16. —, time-of-fiight, 4, 19. —, velocity, 4, 5. Formula determination by mass measurement, 459. Formulae, determination by mass measurement, 292, 302. Fractionation due to capillary leak, 130. Fragment ions, 242—251. Fragmentation at chain branches, 332—334. — processes in propane, 248. — rules for paraffins, 329, 330. Franck-Condon principle, 242, 284, 459, 460. Free radicals, production of in the ion chamber, 438. -, study of, 437—438. Frequency of bond fracture, 243. Fringing fields, effects of, 15. Furnace, use of an ion source, 167. Fused rings, molecular weights of, 322. Fused-ring systems, molecular weights of, 3 16.

Getters, study of gases liberated from, 482. Glass, as a vacuum material, 133-137. — to metal seals, 135-137, 144. “Graded” glass seals, 136—137. Graphite as lubricant for taps, 153. Greaseless glass vacuum taps, 155.

Halogenated compounds, mass spectra of, 413-421. Flalogens, intermolecular reac¬ tions in, 279. —, isotopic abundances of molecules containing, 297— 300. —, see fluoro-, chloro-, bromo-, perchloro-, and perfluoro-. Heated sample systems, 162—165. Heats of reaction, determination of, 476, 477. Helium, diffusion through glass of, 482. Hexadecyl bromide, spectrum of, 420,421. Hexamethylenediamine, spec¬ trum of, 389. Hexamethyleneimine, spectrum of, 391-393. —, N-ethyl-, spectrum of, 393. —, N-fo-aminobutyl-, spectrum of, 393. High-energy radiations, mass spectra from, 238. High molecular weight samples, study of, 167. Gallium-covered sinters, sample High-speed recording using introduction through, 162, multiplier, 216. 181. High temperature examination Galvanometer as recorder, 69, of samples, 476—478. 70, 220, 226. — vacuum research, use of mass Galvanometers, linearity of, 69. spectrometer, 482. Gas flow in mass spectrometer, Homologues, detection of esterby high resolution, 381. 124. Gas-liquid chromatography, see Housekeeper seal, 142. “Chromatography”. Hydrocarbons, peaks at (p+1), Gas samples, introducing at 278. —, re-arrangement in, 269. fixed pressure, 149. -, preparation of mixtures, Hydrogen isotopes (see “Deu¬ 150. terium”). — ■—, storage vessels for, 152. -, transfer through sintered Identification of unknowns discs, 160. from mass spectra, 176—177Geochemistry, application of Image broadening (see also aber¬ mass spectrometry to, 172. rations), 7, 8.

SUBJECT INDEX

Impurities, detection of, 319. Indole,

2,6-dimethyl-,

mass

spectrum of, 402.

635

-, discovery of oxygen, car¬ bon, nitrogen, hydrogen, 59. Isotope separation, 443-446.

Indoles, alkyl, mass spectra of, 397.

-by chemical exchange, 445 -by diffusion, 443, 444.

—, alkyl substituted, multiplycharged ions in, 283.

—- — by electrolysis, 446.

—, meta-stable peaks in, 397, 402. Inorganic compounds, evapora¬ tion of, 167.

446.

Interference

radioactive,

431,

432. reactions,

ions

formed by, 275-282.

tion of, halogens, 82.

tion, 449-451. Leak detection, 479-483.

nection to ionization cham¬

-, natural variations, 88—95.

--, use of a “probe gas”, 480.

ber, 165—167.

-of complex

tane spectra,. 245, Ionization

239.

Ionization-efficiency curves,461. -, derivatives of, 467—471. -, for negative ions,

287,

288. -,

480. molecules,

calculation of, 295—297. of

multiply-charged

ions,

-, shape at threshold, 462, 467.

polyatomic

molecules containing a num¬ ber of isotopes, 301—302.

459, 461. -, error due to spread in electron energy, 463.

of, 294.

463.

93.

measurement

of,

459—

Leaks, capillary, 130. —, construction of, 126.

-, — in lead, 85.

—, made from porous materials,

— potential,

various

methods

of estimating, 464, 465.

—, “pin-hole”, 128.

-, — in oxygen, 89, 90, 91.

—, “pin-hole’ ’ for study of free

-, — in sulphur, 93, 94-

radicals, 438.

determination”.

Ion-molecule

— -—,

inorganic

132.

compounds,

100, 101. ultimate

275—

sensitivity

in,

— labelling, uses of, 453—459.

282, 405, 440-443. Ketone, identification by reduc¬

Ion sources, characteristics of, 5. -, various, 103—123. acid,

systems, 161—167. Logarithmic and linear scales, comparison

of spectra

on,

233. Low-energy electrons, use of,

Ions, negative, 5.

Iso-crotonic

spectrometer, 26. Liquids, introduction via heated -—, volatile, 147—161.

98.

reactions,

—, variable-flow, 124, 127, 131— Linear accelerator used as mass

— —, accuracy attainable, 100.

-,

-, vertical, 459.

127, 128.

-, — in nitrogen, 94, 95.

-, elements studied by, 99.

— potentials, measurements, 5.

—, positioning of, 147, 174.

-, — in hydrogen, 93

— dilution, 98-102.

474.

—, diaphragm, 129. —, molecular flow, 129.

— age determination, see ‘ ‘Age

-, errors in measurement of,

481. — detector, omegatron used as as a, 20.

—- abundance table, compilation

-, — in light elements, 95.

Ionization potential, adiabatic,

-, use of helium, 481. -, use of mass spectrometer,

-, variation in carbon, 92,

461, 462.

-,

phur, 82.

— abundances

cross-sections,

of,

surement of, 474Lead method of age determina¬

—-—, technique of “bagging”,

Ion intensities at (p—15) in oc¬

determination

— heats of vaporization, mea¬

sul¬

Involatile samples, direct con¬

-,

Latent heat of sublimation of carbon, 470-472.

—, separated, uses of, 446-448.

temperatures from, 90, 91.

-, peaks due to, 66, 353.

284-286.

spectra of, 246-250.

-, determination of paleo-

-in the rare gases, 279.

-, of C+ ions from CH4,

Large molecules, theory of mass

—, rare, detection of 95—98.

Isotopic abundance, determina¬

Intermolecular

formed

identifi¬

cation of, 447.

97.

of samples,

ions

Knudsen effusion cell, 476.

-, limits for abundance of,

— solids, analysis of, 181.

energy,

with, 284-286.

-of formation, study of,

-, miscellaneous methods, Isotopes,

—, re-arrangements in, 397.

spectra of, 354. Kinetic

471.

-, instruments for, 37.

—, doubly-charged ions in, 401.

—, use of high resolution in

re-arrange¬

ment in, 267.

Ketones, aliphatic, spectra of, 354-357. —, cyclic, spectra of, 358, 359.

Isomers, spectra of, 325.

—, loss of neutral CO, 269, 270.

Isotope effects, 447, 455-459.

—, methyl n-butyl ketone, low

--in bond reactivity, 456. Isotopes, discovery of, 1, 2.

428.

tion to alkane, 357.

electron energy spectra, 356. —, re-arrangements in, 355—357.

Magnetic

analyser,

two-stage,

96. — field, focussing action of, 6, 8,9. -measurement, 43. Magneto-resistive effect in bis¬ muth, 45.

636

SUBJECT INDEX

Magnet pole face, shaping of, 15.

— standard, carbon as, 30.

Maleic anhydride, spectrum of,

-, fluorine as, 30.

Mica, study of gases liberated from, 482.

—i standard of, 28—30.

283, 374, 375. Mass and energy, equivalence of, 31.

Micro-burette for liquid sam¬

— standard, oxygen as, 28.

ples, 161.

— standards, secondary, 32.

Micro-furnace for flash-pyroly¬

— differences, 49.

— table, compilation of, 294.

— discrimination of multipliers,

Mattauch

— marking,

superposition

measurement,

of

accuracy

necessary in organic chemis¬

analytical

information

magneto-resistive

effect in bismuth, 45, 46.

nuclear

magnetic

resonance, 47. by

of,

Mercury-covered

sinters, sam¬

introduction

through,

transmuta¬

tions, 31, 32.

—, production of standard gas,

connections,

age, 236. — of ion beam, 204.

154. for introduction

of

Molecular

distillation,

— flow, 124. — formulae by mass measure¬

133-137.

ment, 302—305.

— to glass seals, 135—137, 144.

— ions, 238.

Meta-stable ions, 251—262.

-, see “Parent ions”.

-, apparent mass of, 252.

— sieves, 184, 194, 484.

-, errors due to asymmet¬

-, negatively-charged,

— weight,

in

hydrocarbons, 417.

-, — due to photographic

— peak in trioxane, 370.

-, — due to stray potentials, 35. -, — due to visual obser¬ vation, 36. -, — in, 32, 38—40. -, field-shift bracket meth¬ -, for mass number deter¬ mination, 43—47. from

magnetic

field

strength, 43—45. in

single-focussing

in¬

struments, 41—43. --, with C.R.O. display, 40. — scales, 28. -,

conversion factor, 29. ■—spectra,

basic

Morpholine, N-acetyl, spectrum

-in indoles, 397, 402.

of, 394.

-, intensity of, 256.

—, N-ethyl-, spectrum of, 394.

-, strength as a function of

—, spectrum of, 394.

repeller voltage, 254, 255.

—,

from, 292.

a>-amino-N-propyl-,

spec¬

trum of, 394.

— transitions, apparent mass in

Multiplets, (see “Doublets”) 33.

cycloidal

Multiplicative

mass

spectrometer,

chain

process,

statistical theory of, 217.

257. -in alcohols, 352, 353.

Multiplier detector, 37.

-in paraffins, 331, 332.

— detectors, see “Electron mul¬

255.

tipliers”. Multiply-charged ions, 282—284.

— -—, peak pattern in cycloidal

anthraquinone,

-, separation from singlyfor¬

mation of mass 139 ions, 273. 2-Methyl-1:3-dioxolane,

-of the elements, produc¬ tion in spark source, 284.

-, peaks due to, 252. Methoxy

information

-from high-energy parti¬

effect

charged

under

high

resolu¬

tion, 283. Multi-range recording, 226.

of reducing electron energy, 312.

cles, 239. --of

-in esters, 377.

mass spectrometer, 258.

chemical and physical,

275—

-in amine spectra, 388.

-, measure of mass lost in,

-, possible errors in, 306.

reactions,

282, 440-443.

“Mono-energetic” electrons, methods of Simulation, 465— 466.

-— transition in trioxane, 312.

od, 34.

by

Momentum spectrometer, 7.

— peaks in acetals, 356, 366.

plate, 35.

— —,

Molecule-ion

chloro-substituted

-, — due to impurities, 34-

determination

rate of effusion, 323, 324.

— ions, tests for, 253—255. — peak

34.

-,

290.

— ion suppressor, 200.

-, — due to gas scattering,

separa¬

tion of samples by, 170, 176.

liquids, 159.

-, by rotating coil, 44. ric aberrations, 35.

analysis

of, 229. 150.

— gasket seals, 141—142.

nuclear

319-323, 424-432.

Modulation of accelerating volt¬

Mercury cut-offs, 149.

Metal, as a vacuum material,

scopy, 30, 31. by

spectra

—, multi-component,

— orifice

-, by micro-wave spectro¬

-,

mass

412.

Mercury-sealed

by

ples, 158. Mixtures, analysis of, 174—178,

157.

from, 292.

-,

Mercaptans,

Micro-manometers, 148. Micro-pipettes for liquid sam¬

67, 68, 74, 75.

ple

try, 33, 51, 55. -, accurate, 30.

-,

sis, 169.

spec¬

Memory effects, “reduction” of

mass scale on spectrum, 43.

-,

Herzog’s

trograph, 12.

212. — dispersion, 7.

—

and

miscellaneous

pounds, list of, 422, 423.

com¬

Methyl stearate, 373.

spectrum of,

15N, Conversion of ammonia to nitrogen, 79.

SUBJECT INDEX —, Conversion of samples to nitrogen gas, 79. —,

correction

abundance

-—,-—in nitrous oxide, 79. Odours, identification of, 183.

to

for

637

measured

lsO,

background

equilibration

between

water and carbon dioxide, 77,

air, 80.

78.

—, determination of in nitrogen

ty of spectra, 340, 346.

Naphthols, 352.

to

intermolecular

Pellet press for powder samples, 169. Pentanols, spectra of isomeric, 348.

—, spectra of, 338, 339.

Pentenes, mass spectra of the

Negative ions, 5, 286—290.

Omegatron, The, 19, 36, 37.

-, charge permutation, 289.

—, used as a leak detector, 20.

— ion spectrum of n-octadecyl

—, used as a pressure gauge, 20,

alcohol, 290.

due

reactions, 66.

Pen recorders, 220.

Olefines and alcohols, similari¬

gas, 80.

Peaks

isomeric, 263. Perchloro 1,3,5-hexatriene, mass

482.

spectrum of, 413. Perchloryl

fluoride,

negative

— ions, reactions of, 289.

O-ring, seals, 138-140.

ions in mass spectrum of, 286.

— peaks, origin of, 199.

Outgassing of metals, study of,

“Perfect” tracer, 99.

Neutron irradiation of uranium, 485. Nier

482.

and Roberts’s double-fo¬

cussing spectrometer, 14, 15. Nitriles, mass spectra of, 404— 406.

Petroleum analysis, 323. Phenol, loss of neutral CO, 269,

effect

on

270.

molecular

weight, 317.

409.

279.

butyl,

mass

variations, 90. v/

o-,

for

ion

Photo-ionization

m-

and

mass spectra, 268.

“(P + 1)” peaks, 275.

268. Nitro-compounds,

aromatic,

-, use of lithium fluoride,

Parabola mass spectrograph for

Photo-multipliers as detectors,

tion of, 90.

279.

study of ion-molecule reac¬

— mass rule, 307, 309.

— spectrograph, 2, 4.

“Noise” voltage in A.C. ampli¬ fiers, 204—206. — — in D.C. amplifiers, 204— 206.

of, 440.

and structure, 326—335. —, spectra of, Ci to Cio, 328,

intensity,

effect

carbon dioxide, 77.

in,

Phthalic

anhydride,

spectrum

of, 374, 375. Polarization of resistors, 69.

of Poly-ethers, cyclic, spectra of,

branching on, 330.

368-371.

-— ions, effect of source temper¬

Polymer degradation, study of, 436.

ature on, 429, 430. -, features leading to weak-,

Polymers,

study

of

thermal

degradation 167, 170.

311. lsO, conversion of samples to

re-arrangement

Pipettes for liquid samples, 158.

329. “Parent” ion, 238, 240-242. ion

Phthalates, 377.

—, long-chain, spectra of, 327.

—

207. Photosensitized reactions, study

Paraffins, correlations of spectra

— isotopes, natural abundance variations, 94.

118, 119.

tions, 282.

Nitrogen, “active”, 439, 440. —, intermolecular reactions in,

119, 120. determina¬

409. tra of, 409.

of -, elimination of window,

ions by, 287, 288.

Nitrosamines, mass spectra of, Nitroso derivatives, mass spec¬

118.

formation

Paleotemperatures,

269. —, mass spectra of, 406-409.

117—

-, electrode arrangement in,

Packing fractions, 48. Pair-production,

loss of neutral NO and CO,

source,

120.

p-,

Nitro-benzene, mass spectrum,

detection,

195,207, 213. Photographic detection, 195.

spectra of, 408. Nitro-anilines,

Phenols, spectra of, 352—354* Phosphors

— isotopes, natural abundance isomeric

—, 2-nitro-4:6-dichloro-, spec¬ trum of, 419.

—, intermolecular reactions in,

Nitrites, mass spectra of, 406— the

alkaline-earth metal ions on,

— atom,

the parent ion, 277.

peaks

in spectrum of, 415.

483. Oxygen as mass standard, 28.

—, peaks of mass greater than

—,

Perfluorotributylamine,

Oxide cathodes, deposition of

-, methods of recognizing,

Potassium-argon method of age determination, 452.

307-313.

—, --water, 77.

-, stability of, 240, 241.

Pressure dependent peaks, 277.

Octanes, isomeric, spectra of,

Pattern

—, gauges for use in sample¬

325, 330-334. 3—Octanone, spectrum of, 355. lsO, determination of in carbon dioxide, 78.

recognition,

impor¬

tance in qualitative analysis, 231. Peak height as a function of repeller voltage, 431.

handling system, 148. Process monitoring, 485. Propane, kinetic energy of ions in mass spectrum of, 285.

638

SUBJECT INDEX

Propanoate,

iso-propyl,

high-

— in esters of a)-phenyl fatty

resolution spectrum, 384.

acids, 378.

Rhenium filaments, 110. Rings and double bonds, calcu¬ lation of number of, 313.

—, sec-butyl, spectrum of, 386.

— in indoles, 397.

iso-Propyl

— in ketones, 355—357.

“R” number, 313, 403.

— in phthalates, 377.

-, benzenes, naphthalenes,

propanoate,

high-

resolution spectrum, 384. Pyridine, 2-methyl-5-ethyl-, mass spectrum of, 403. —, 2-n-propyl-, mass spectrum of, 403.

— in silicon compounds, 421.

Rocket studies, 483.

— in thiophenes, 411.

Rotating electrostatic fields, use

graph, 221.

Rubber O-ring seals,

— for high resolution, 229, 230. Quantitative analysis by mass

—,

—, galvanometer, 220, 226.

Quasi-equilibrium theory of

—, logarithmic, 226, 233.

250.

pen,

self-balancing poten¬

Sample gases

tiometer type, 220.

Q-values, 31, 32.

Reflection of ions from solids, isotopes,

identifi¬

19. Radiolysis, study of products by mass spectrometer, 238, 437. re-arrangements,

263.

abundance,

tions in, 279. 95—

98. -, limits to abundance of, Rates of reaction, isotope ef¬ fects, 457. mechanisms,

deter¬

— paths, study of alternative, 433-435.

186. Scanning, in trochoidal instru¬ ment, 17.

87. -, detection of small changes

—, rapid to obtain approximate spectral pattern, 232.

in, 83, 84-

— speed, increase of between peaks, 229.

on, 64.

— time, as a function of resolv¬

tion, 61—63.

ing power, 217.

—- —, — due to fragmentation

Scintillation detection, 195, 213.

of polyatomic molecules, 75,

— detector, use of coincidence techniques to reduce “noise”,

spectrometry, 432—440. carried

out

within

the sample system, 436. Re-arrangement ions 262—275. — reactions, study of by isotop¬ ic labelling, 455.

213.

-, — in measurement of,

Sealed sample tubes, opening in

vacuo, 153, 155—157.

—- —, — in spectrographs, 60. -, scanning errors, 65.

Seals, dissimilar materials, 137.

Repeller voltage, effect of varia¬

—, glass to metal, 135—137, 144.

tions on peak height, 43 1.

— products, study of by mass

for

liquids,

-accuracy in measurement,

59-71.

mination of, 453—457.

systems

volatile

— temperature, control of, 475.

76.

97.

and

determination of, 65.

-, errors due to fractiona¬

— isotopes, detection of,

handling

— introduction, gases, 156.

absolute

-, effect of source magnet

Rare gases, intermolecular reac¬

for

— system, for general use, 184—

442. Relative

Radio-frequency spectrometers,

fragmentation

147-161.

—, sensitivity selection, 223,224.

cation of, 447.

of

paraffins, 329, 330.

—, multi-channel, 220, 226 •—,

Quinones, spectra of, 360.

“Random”

age determination, 452, 453. Rules

—, —, uses of, 234.

-of mass spectra, tests of,

Radioactive

138. Rubidium-strontium method of

--of hydrocarbons, 291. mass spectra, 246—250.

138—140.

—, treatment for vacuum work,

for transient phenomena,

use of C.R.O., 222.

spectrometry, 424.

Reactions

of in mass analysis, 27.

Recorders, cathode-ray oscillo¬

—, spectrum of, 403.

Reaction

anthracenes, 345.

— in thio-ethers, 413.

—, graded glass, 136.

-, effect on ion current, 105.

— using metal gaskets, 141—142.

“Replacement” re-arrangement,

— using rubber O-rings, 138MO.

268. Resolution, as function of slit widths, 53.

Secondary electrons, 207. —

—, definitions of, 51.

emission,

suppression

of,

197-198.

-—, geometrical limit of, 8, 51.

— mass standards, 32.

—, “replacement”-, 268.

-—, “inherent”, 8.

Second derivative of ionization

—, “specific”, 269.

Resonance capture of electrons,

Re-arrangements,

in

alcohols,

347, 351, 352. — in amides, 396. — in amines, 389, 390.

Respiration studies, 485. Retarding

potential

difference

method, 287.

— in carboxylic acids, 372.

-for ionization po¬

-— in esters, 377, 381, 385.

tential determination, 465.

-— in esters of dibasic acids, 378.

probability curves, 467-471. Sector magnetic field, 8.

287.

Reutersward’s spectrograph, 13.

-, comparison with 180° instrument, 10. -, non-uniform, 16. Sensitivity of A.C.

and D.C.

amplifiers as detectors, 204206.

639

SUBJECT INDEX Separation of isotopes, see “Iso¬

Subtraction of spectra, mixture

tope separation.”

analysis, 320.

Sex attractants, identification of,

gen palladium, 484.

Succinate, diethyl-, spectrum of,

183.

379.

Shunt-selection, automatic, 224. 225.

lar weight, 317. dance and age, 94.

Silicon compounds, re-arrange-

—

ments in, 421.

compounds,

mass spectra

amines, trimethyl silyl ethers

287. —,

and amines, 353.

Thiophenes,

mass

spectra

of,

—, rearrangement in, 411. Threshold law for ionization, 462, 467.

isotopic

molecules

Single-focussing, 10.

of,

Thomson’s spectrograph, 2

— hexafluoride, negative ions,

— derivative^ of alcohols and

spectra

410, 411.

of, 410-413.

-, spectra of, 421—422.

mass

412, 413. —, rearrangements in, 413.

— isotopes, correlation of abun¬

Signal-to-noise ratio, 203.

Thio-esters, spectra of, 387. Thio-ethers,

Sulphur atom, effect on molecu¬

-, —, sensitivity indicator,

Thermionic ions from hydro¬

abundances containing,

of

300—

301.

Time constant of a viscous-flow leak, 131. Time-of-flight

spectrometers

Sintered discs, introduction of -, natural abundance varia¬ liquid through, 157, 162, 181. tions, 94.

-, with linear ion path, 23—26.

Skeletal re-arrangement, 266.

Suppressor, ion-, 200.

Toluenes,

Small currents, measurement by

—, secondary electron, 198—199.

“interval integration”, 214-

Surface ionization source, 111—

-, measurement by rate-ofcharge method, 214, 215.

115. -,

-, statistical fluctuations in, 214.

of,

114,

115.

Small samples, analysis of, 482.

-, filaments for, 112. -,

Solid

samples,

-,

inorganic,

analysis of,

secondary

ions

in,

112-113. Synchrometer, The Mass-, 21,

tions, 282. base,

“Teflon-bucket”

Trioxane, meta-stable transition,

sample introduction, 163.

Terephthalate, dimethyl-, spec¬

Tropylium ion, 265, 266.

of products of, 436. of

study of, 167, 170. -, study of, 479.

polymers,

diffusion, 444. mass

Trochoidal mass spectrometer,

Thermal decomposition, study degradation

ions,

16, 21.

Steroids, introduction of, 167. deuteriated,

organic

— -, focussing in, 17.

Stearic acid, spectrum of, 373. Stevenson’s rule, 284-

261, 312. Triply-charged

Tesla coil as leak detector, 479.

—

spectra, 267.

re-arrange¬

ment in, 266.

Stearate, methyl-, spectrum of, 373.

of, 353,354.

283.

Tert-butylbenzene,

duction of entire sample, 182. tra, 246—250.

Transition probability, methods

Temperature, variation of spec¬

241.

Statistical theory of mass spec¬

216,

Trimethyl silyl ethers, spectra

trum of, 376.

“Static’ ’ Vacuum system, intro¬

spectra,

of obtaining, 467.

method

trum with source-, 429, 430. definition,

mass

of

constant, 251.

Styrenes,

mass

spectrum of, 404-

Sputtering of positive ions, 442.

ion,

ling”. Transient

222.

Tchitchibabine’s

21, 22, 41, 42.

of parent

—, isotopes as, 70. Tracer work, see “Isotopic label¬

Spectrometers, 180°, 9, 10.

—

dilution factor toler¬

—, perfect, 99.

study of ion-molecule reac¬

Stability of bonds, Hammett’s

air

able, 87, 88.

“Specific” re-arrangement, 269.

—, sector, 9, 10.

in

samples, 178. Tracers,

Tandem magnetic sectors, use in

—, high resolution, 15, 18, 20,

if volatile, 175. -, — identification

limited,

104.

principle of additivity,

239, 240. Trace impurities, identification,

36. space-charge

-intensity, measuring of,

-, fila¬

-, theory of, 111.

114Sources,

of

ment temperature in, 114.

introduction,

164-165. Solids,

monitoring

113.

—, identification of, 183—184.

Total ion current, sampling of,

— ionization, 239, 240.

-, multifilament design,

Smells, detection of, 173.

in,

203.

-, examination of, 178—184. ization chamber, 182—183.

re-arrangement

264-265. 240.

advantages

-, efficiency of, 181.

-, recirculation through ion¬

using a magnetic field, 19—23.

Trouton’s Rule, 145. Tuned amplifier for recording derivative spectra, 236. Tungsten

filaments,

composi¬

tion of, 108.

— emission source, 111—115.

-, conditioning of, 108, 109.

— transpiration, 126.

-, study of gases in, 483.

640 Uni-molecular

SUBJECT INDEX

decomposition,

247-249.

— diaphragm, 86.

Upper atmosphere studies, 483. Vacuum locks, 166. — research, use of mass spec¬ trometer, 482. — septum sample tubes,

—, double

inlet

151,

alternate

Vacuum-spark source, 115—117. 116,

117. — system, use of glass in, 133— 137.

Virtual slits, 53.

introduction of each of two

Viscous flow, 124.

samples, 86.

-leaks, 130.

— using molten metal, 143—144. of temperature, measurement of, 474.

-, advantages of, 116, 117.

electrometer,

204. for

Vapour pressure as a function

154.

-for solids analysis,

Vibrating-reed

Valves, Alpert, 140.

Volatile liquids, examination at low temperature, 151. Volatility

of

organic

com¬

pounds, 145-147.

-of inorganic substances, 476-478.

Wilson seals, 139.

Velocitron, The 23. Velocity filter, 11.

Xylenes, re-arrangement in, 264.

— selection, 11. — spectrum, 5.

-, use of metal in, 133—137.

“Vertical” ionization, 242.

— valves, 140.

-potential, 459.

Zero-drift

in

D.C.

amplifiers,

204. Zone refining, 184, 191—194.

DATE DUE

DEC ? ! 2006

QD 95 .B4 Beynon, J. H. (John Herbe M?^?„?R?.