Fourier Transform Mass Spectrometry. Evolution, Innovation, and Applications 9780841214415, 9780841212060, 0-8412-1441-7

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Fourier Transform Mass Spectrometry

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

ACS SYMPOSIUM SERIES

359

Fourier Transform Mass Spectrometry Evolution, Innovation, and Applications Michelle V Buchanan EDITOR Oak

Developed from a symposium sponsored by the Division of Analytical Chemistry at the 192nd Meeting of the American Chemical Society, Anaheim, California, September 7-12, 1986

American Chemical Society, Washington, DC 1987

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Library of Congress Cataloging-in-Publication Data Fourier transform mass spectrometry: evolution, innovation, and applications/Michelle V. Buchanan, editor. p. cm.—(ACS symposium series, ISSN 0097-6156; 359) "Developed from a symposium sponsored by the Division of Analytical Chemistry at the 192nd Meeting of the American Chemical Society, Anaheim, California, September 7-12, 1986." Includes bibliographies and indexes ISBN 0-8412-1441-7 1. Ion cyclotron resonance spectroscopy— Congresses. 2. Fourier transform spectroscopy— Congresses. I. Buchanan, Michelle V , 1951. II. American Chemical Society. Division of Analytical Chemistry. III. American Chemical Society. Meeting (192nd: 1986: Anaheim, Calif.) IV. Series. QD96.154F68 1987 543'.0873'015157-dc19

87-26107 CIP

Copyright © 1987 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of the first page of each chapter in this volume indicates the copyright owners consent that reprographic copies of the chapter may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc., 27 Congress Street, Salem, M A 01970, for copying beyond that permitted by Sections 107 or 108 of the US. Copyright Law. This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating a new collective work, for resale, or for information storage and retrieval systems. The copying fee for each chapter is indicated in the code at the bottom of the first page of the chapter. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by ACS of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission, to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. Registered names, trademarks, etc., used in this publication, even without specific indication thereof, are not to be considered unprotected by law. PRINTED IN T H E UNITED STATES OF A M E R I C A

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

ACS Symposium Series M. Joan Comstock, Series Editor 1987 Advisory Board Harvey W. Blanch

Vincent D. McGinniss

University of California—Berkeley

Battelle Columbus Laboratories

Alan Elzerman Clemson University

John W. Finley

James C. Randall

Nabisco Brands, Inc.

Exxon Chemical Company

Marye Anne Fox

E. Reichmanis

The University of Texas—Austin

AT&T Bell Laboratories

Martin L. Gorbaty

C. M. Roland

Exxon Research and Engineering Co.

U.S. Naval Research Laboratory

Roland F. Hirsch

W. D. Shults

US. Department of Energy

Oak Ridge National Laboratory

G. Wayne Ivie

Geoffrey K. Smith

USDA, Agricultural Research Service

Rohm & Haas Co.

Rudolph J. Marcus

Douglas B. Walters

Consultant, Computers & Chemistry Research

National Institute of Environmental Health

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Foreword The

A C S SYMPOSIUM SERIES was founded in 1974 to provide a

medium for publishing symposia quickly in book form. The format of the Series parallels that of the continuing ADVANCES IN CHEMISTRY SERIES except that, in order to save time, the papers are not typeset but are reproduced as they are submitted by the authors in camera-ready form. Papers are reviewed under the supervision of the Editors with the assistance of the Series Advisory Board an symposia; however, verbatim reproductions of previously pub lished papers are not accepted. Both reviews and reports of research are acceptable, because symposia may embrace both types of presentation.

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Preface FOURIER TRANSFORM (FT) TECHNIQUES have been applied to a variety of spectroscopic methods in the past twenty years, as computer technology has rapidly advanced. It was generally believed that mass spectrometry would not benefit from FT techniques because the multiplex or Fellgett's advantage is realized only when the technique is detector-noise limited and not when it is signal-noise limited. In 1974, however, Comisarow and Marshall applied FT technique spectrometry, which did conventional mass spectrometers. FTICR, or FTMS as it is now commonly called, retains the unique features of conventional ICR, including ion trapping and multiple resonance capabilities, while circumventing its restrictions, including slow scan speeds, low resolution, and limited mass range. As a result, FTMS has evolved into a versatile and powerful spectroscopic technique. In addition, it possesses a number of unique capabilities which give it great potential for becoming a major tool for both analytical applications and basic physical and chemical studies. The purpose of this symposium was to bring together researchers who are investigating new applications of FTMS, as well as those who are developing instrumental advances in FTMS, in order to report on the current analytical capabilities of FTMS and those projected for the future. The chapters cover a wide variety of applications, ranging from basic studies of photodissociation of ions to the analysis of high molecular weight biopolymers. Although several review articles have appeared in the literature, this is the first book dedicated exclusively to FTMS. It is meant to serve as a general introduction to the technique as well as a summary of current applications. I hope that this book will spark the interest of chemists, biologists, physicists, and others to learn more about FTMS and help them ascertain whether this technique will solve problems in their own laboratories. I thank the authors who have given their time to participate in the symposium and the publication of this book. Marc Wise, Robert Hettich, and Elizabeth Stemmler of the Organic Spectroscopy Group here at Oak Ridge have also contributed substantially by helping compile the glossary

ix

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

and proof manuscripts; their efforts are greatly appreciated. Finally, I thank Lavonn Golden for secretarial assistance. M I C H E L L E V. B U C H A N A N

Oak Ridge National Laboratory Oak Ridge, TN August 1987

χ In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 1

Principles and Features of Fourier Transform Mass Spectrometry Michelle 1

Analytical

1

V.

Buchanan

Chemistry

Division,

and M e l v i n B.

2

Comisarow

O a k Ridge N a t i o n a l

Laboratory,

Oak

Ridge, TN 37831-6120 2

Department of C h e m i s t r y ,

University

British Columbi

of B r i t i s h C o l u m b i a ,

V 6 T 1Y6

Vancouver,

Canad

Fourier transform mass spectrometry (FTMS) is a rapidly growing technique of increasing analytical importance. Foremost among i t s many attributes are its high mass resolution and wide mass range capabilities, as well as its ability to store ions. This relatively new technique has been employed in a wide variety of applications, ranging from the exact mass measurement of stable nuclides to the determination of peptide sequences. The future holds considerable promise both for the expanded use of FTMS in a diverse range of chemical problems, as well as advances in the capabilities of the technique itself. Fourier transform mass spectrometry (FTMS) i s an e x c i t i n g technique that combines the operating features of several d i f f e r e n t types of conventional mass spectrometers into a single instrument and possesses a number of unique c a p a b i l i t i e s , as w e l l . Originally developed by Comisarow and Marshall i n 1974 (1-3), FTMS i s derived from scanning ion cyclotron resonance mass spectrometry (ICR) (4) by the a p p l i c a t i o n of Fourier transform (FT) techniques. I t should be noted that both i n t h i s book and i n the general l i t e r a t u r e , the terms Fourier transform i o n cyclotron resonance (FT-ICR) mass spectrometry and Fourier transform mass spectrometry are used synonymously. Although ICR has h i s t o r i c a l l y been a valuable t o o l for the study of gas-phase chemical reactions (5), p r i o r to the introduction of FTMS, the a n a l y t i c a l applications of ICR had been r e s t r i c t e d by low mass r e s o l u t i o n , l i m i t e d mass range, and slow scanning speeds (6-8). By employing Fourier transform techniques (9) i n conjunction with a trapped ion c e l l (10, 11), FTMS has circumvented these l i m i t a t i o n s and, i n f a c t , has the p o t e n t i a l of becoming an important a n a l y t i c a l technique. This chapter i s intended to serve as an overview of the general p r i n c i p l e s and features of FTMS. C a p a b i l i t i e s pertinent to a n a l y t i c a l studies w i l l be s p e c i f i c a l l y highlighted. In addition 0097-6156/87/0359-0001 $06.00/0 © 1987 American Chemical Society

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

2

FOURIER T R A N S F O R M MASS S P E C T R O M E T R Y

to the papers c i t e d i n t h i s chapter, the reader i s r e f e r r e d to a number of other reviews (12-16) which provide a d d i t i o n a l references to d e t a i l e d reports of applications and research i n the f i e l d of FTMS. P r i n c i p l e s of Fourier Transform Mass Spectrometry Figure 1 i s a schematic of a trapped ion c e l l with cubic geometry, which i s commonly used i n FTMS (8, 11). The c e l l i s contained w i t h i n a high-vacuum chamber (pressures of 10" t o r r or less) which i s centered i n a homogeneous magnetic f i e l d . Magnetic f i e l d strengths used f o r FTMS are t y p i c a l l y 1-7 Τ, with 2-3 Τ f i e l d s generated by superconducting magnets being the most common. Like conventional mass spectrometry, ions may be formed i n the FTMS c e l l by a number of methods, including electron impact (1), chemical i o n i z a t i o n (17-20), l a s e r i o n i z a t i o n and desorption (21-23), and particle induced desorption spectrometry and plasm are trapped i n the c e l l , held i n the r a d i a l d i r e c t i o n (xy plane) by the magnetic f i e l d and along the axis of the magnetic f i e l d (z-axis) by small voltages (0.5 to 5 V) applied to the trapping plates. Either p o s i t i v e or negative ions may be trapped i n the c e l l simply by changing the p o l a r i t y of the voltage applied to the c e l l plates. The frequency of the c y c l i c motion of ions,ω , w i t h i n the c e l l i s given by the cyclotron equation: ω - KqB/m (1) where Κ i s a p r o p o r t i o n a l i t y constant, q i s the charge of the ion, m i s i t s mass, and Β i s the magnetic f i e l d strength. Because the magnetic f i e l d strength i s constant i n the FTMS experiment, ions of d i f f e r e n t mass w i l l have unique cyclotron frequencies. For example, at a magnetic f i e l d strength of 3 Τ, an ion with a mass to charge r a t i o (m/z) of 18 w i l l have a cyclotron frequency of 2.6 MHz, while an ion at m/z 3,000 w i l l have a frequency of 15.6 KHz. Because of momentum conservation, the i n i t i a l i o n v e l o c i t y upon i o n formation i s the same as the v e l o c i t y of i t s neutral precursor. For a macroscopic ensemble of ions, there i s no net coherent cyclotron motion even though the ion o r b i t s are nonzero. Without coherent motion, a signal cannot be detected. By applying a very short, high i n t e n s i t y , broadband radiofrequency signal (2) ("chirp") to the excite (or transmitter) plates of the c e l l , the ions absorb energy, which accelerates them into l a r g e r o r b i t s and causes them to move together (coherent motion). The o r b i t i n g packet of ions induces a small a l t e r n a t i n g current ("image current") i n the receiver plates (28). This s i g n a l i s converted into a voltage, amplified, d i g i t i z e d and stored i n a computer (28). The frequency components of the image current correspond to the cyclotron frequencies of the ions present i n the c e l l . I f ions of only one mass to charge r a t i o were present i n the c e l l , the detected s i g n a l (time domain signal) would resemble a single frequency sine wave. However, i n the FTMS experiment, a l l ions i n the c e l l are excited v i r t u a l l y simultaneously (3) and detected simultaneously (2, 3, 7). The r e s u l t i n g time domain spectrum i s very complex because the signals of a l l ions are superimposed. In 6

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

1.

B U C H A N A N AND COMISAROW

Principles and Features of FTMS

3

order to recover the frequency information, the complex time-domain spectrum i s subjected to a Fourier transform algorithm. This generates a frequency domain spectrum, which can be r e a d i l y converted to the f a m i l i a r mass spectrum using the cyclotron r e l a t i o n s h i p given above, Equation 1 (3, 29, 30). Another advantage of the Fourier transform technique i s that data a c q u i s i t i o n time i s greatly reduced because ions of a l l masses are detected simultaneously (7). Because frequency can be so p r e c i s e l y measured, the exact mass of an i o n can be determined very accurately i n the FTMS experiment (8). T y p i c a l l y , low p a r t s - p e r - m i l l i o n accuracy can be achieved i n the presence or even i n the absence of an i n t e r n a l mass c a l i b r a n t (13) . In a d d i t i o n , a high degree of mass accuracy can be maintained f o r days without r e c a l i b r a t i o n provided that the magnetic f i e l d remains stable. More d e t a i l e d information on the theory of FTMS (1, 16 28 31-33) and the p r i n c i p l e s of Fourier transforms applied to spectroscopi i n the l i t e r a t u r e . The Basic FTMS Experiment In p r a c t i c e , the basic FTMS experiment i s conducted using a series of computer-controlled pulse sequences, as depicted i n Figure 2. In t h i s experiment, i o n formation and detection are separated i n time. This i s i n contrast to conventional mass spectrometers, which r e l y on s p a t i a l dispersion of ions. In the simplest case, the i n i t i a l pulse involves the formation of ions, whether by electron i o n i z a t i o n or another technique. The second i s the frequency sweep (2) e x c i t a t i o n pulse which brings the ions into coherent motion, as described previously. This i s followed by a detection period during which the s i g n a l (image current) (28) from the ions i s received. A quench pulse i s then applied to the c e l l to remove a l l the ions from the c e l l p r i o r to s t a r t i n g the next pulse sequence. The entire experiment sequence can take less than a second (a minimum of about 10 ms f o r an e l e c t r o n i o n i z a t i o n experiment), and the series of pulses may be repeated as many times as desired f o r signal averaging and enhanced signal-to-noise r a t i o . A d d i t i o n a l pulses or delays between pulses may be inserted i n t h i s basic pulse sequences i n order to perform more complex experiments, as w i l l be discussed l a t e r . In a d d i t i o n to the cubic c e l l geometry (11) depicted i n Figure 1, a v a r i e t y of other c e l l geometries have been devised (4, 35, 36). One that has attacted considerable i n t e r e s t f o r a n a l y t i c a l purposes i s the dual c e l l (37), depicted i n Figure 3. In t h i s design, two d i f f e r e n t i a l l y pumped cubic c e l l s share a common trapping plate which contains a small (2 mm) o r i f i c e . This arrangement allows the c e l l s to be operated at a pressure d i f f e r e n t i a l of 10 . Ions may therefore be formed at higher pressures i n the "source" c e l l and transferred to the "analyzer" c e l l f o r detection under low pressure conditions. In the FTMS experiment, low pressures are required f o r high r e s o l u t i o n mass measurement and other high performance c a p a b i l i t i e s , as o u t l i n e d below. 3

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

4

FOURIER T R A N S F O R M MASS S P E C T R O M E T R Y

Figure 1. Schematic representation of a cubic trapped ion c e l l commonly used i n FTMS. Coherent motion of ions i n the c e l l i n ­ duces an image current i n the receiver p l a t e s . The time domain signal i s subjected t Fourie transfor algorith t y i e l d mass spectrum.

ION EXCITATION

ION FORMATION

ION DETECTION

π

QUENCH

υ ι

VARIABLE DELAY

Figure 2. Simple pulse sequence used i n elementary FTMS experi­ ments, including ion formation, e x c i t a t i o n , and detection. A quench pulse i s used to eject ions from the c e l l p r i o r to repeat­ ing the sequence. The v a r i a b l e delay between ion formation and e x c i t a t i o n can be used f o r ion storage and manipulation, as described i n the text. CONDUCTANCE LIMIT ANALYZER CELL SAMPLE INLET "

-ELECTRON FILAMENT

NSSSSSSSSSSSS1 SUPERCONDUCTING MAGNET PUMP

PUMP

Figure 3. Schematic representation of a dual c e l l used for FTMS. The two d i f f e r e n t i a l l y pumped c e l l s are joined by a trapping plate containing a small o r i f i c e that i s capable of supporting a 10)3 pressure d i f f e r e n t i a l . Ions may be detected i n e i t h e r c e l l . How­ ever, i f desired, ions formed i n the "source" region under higher pressure conditions may be transferred to the "analyzer" c e l l for detection under lower pressure conditions, as described i n the text.

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

1.

B U C H A N A N AND COMISAROW

Principles and Features of FTMS

5

Features of FTMS FTMS i s mechanically simple, yet remarkably v e r s a t i l e . The instrument can be operated i n any number of modes simply by changing the pulse sequences used to control the spectrometer. As a r e s u l t , a wide v a r i e t y of experiments can be performed with a sample i n a very short period of time. For example, by a l t e r i n g j u s t a few parameters using the computer keyboard, one may switch from p o s i t i v e to negative ion detection, from e l e c t r o n i o n i z a t i o n to chemical i o n i z a t i o n , and from a single stage experiment to a multiple stage experiment (e.g., MS/MS). In addition, a number of features makes FTMS p a r t i c u l a r l y w e l l - s u i t e d f o r a n a l y t i c a l studies, including high mass r e s o l u t i o n and wide mass range, a b i l i t y to store, manipulate and s e l e c t i v e l y detect ions, and c o m p a t i b i l i t y with a v a r i e t y of i o n i z a t i o n techniques. Examples of these features are given below and a d d i t i o n a l examples may be found i n previous reviews o

High r e s o l u t i o n and wide mass range. One of the most outstanding features of FTMS i s i t s a b i l i t y to achieve very high mass r e s o l u t i o n , which was predicted early i n the development of the technique (7, 39). Mass r e s o l u t i o n i n the FTMS v a r i e s inversely with mass and increases proportionally to observation time and magnetic fie-ld strength (8, 40). An increase i n operating pressure causes c o l l i s i o n a l damping of the coherent motion of the ions w i t h i n the c e l l and a rapid decay of the image current s i g n a l , r e s u l t i n g i n a decrease i n resolution. Therefore, very low pressure (10" t o r r or lower) i s required i n order to monitor the s i g n a l f o r a s u f f i c i e n t l y long period of time to obtain high mass resolution. Mass r e s o l u t i o n i n excess of 500,000 (FWHM) at mass 100 can be r e a d i l y obtained with most commercial FTMS instruments and r e s o l u t i o n of 200,000,000 at m/z 40 (41) has been reported. This compares with the highest mass r e s o l u t i o n of commercial sector instruments of about 120,000. In addition, high r e s o l u t i o n measurements can be made i n a matter of seconds with an FTMS and the instrument can be switched between medium and high r e s o l u t i o n modes simply by changing a few parameters through the computer. With conventional magnetic sector instruments, which must be mechanically adjusted (14), t h i s change can be very time-consuming. Further, with conventional mass spectrometers, high mass r e s o l u t i o n i s achieved by narrowing the ion transmission s l i t s , which r e s u l t s i n a decrease i n the s i g n a l . In FTMS, increased r e s o l u t i o n i s achieved simply by accumulating the s i g n a l f o r longer periods of time and r e s u l t s i n an increase i n the signal to noise r a t i o (S/N) as w e l l (42). Thus, high r e s o l u t i o n spectra can often be obtained on very small quantities of sample using FTMS. In a d d i t i o n to high mass r e s o l u t i o n , another important feature of FTMS i s i t s wide mass range (7,8). From examination of the cyclotron equation (Eq. 1), the mass range of FTMS appears to have no upper l i m i t . However, an instrumental upper l i m i t i n excess of 100,000 has been suggested (16), based on a d e t a i l e d study of i o n motion (30). From a p r a c t i c a l standpoint, the cyclotron 8

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

β

FOURIER TRANSFORM MASS SPECTROMETRY

frequencies of high mass ions (> 5,000 daltons) are low enough that environmental (1/f) noise can severely i n t e r f e r e with t h e i r detection. Improved e l e c t r o n i c s have been devised to help circumvent t h i s problem, and ions at mass 16,241 from cesium iodide c l u s t e r s have been detected using an FTMS equipped with a 3 Τ magnet (43). A l t e r n a t i v e l y , as higher f i e l d magnets are developed and employed, the upper mass range of FTMS w i l l be extended as well. The low mass l i m i t i s governed by the rate of signal digitization. For example, according to Nyquist theory, i f a 5.2 MHz d i g i t i z e r i s used, the highest frequency s i g n a l which can be processed without d i s t o r t i o n i s 2.6 MHz. As previously mentioned, at a f i e l d strength of 3 T, t h i s frequency corresponds to about 18 daltons. Lower masses can be obtained, however, by use of a faster s i g n a l d i g i t i z e r or with lower magnetic f i e l d s . A l t e r n a t i v e l y , a s p e c i a l narrow-band e x c i t a t i o n (heterodyne) technique which reduces the d i g i t i z a t i o n requirements of the s i g n a l can be used to obtain lower masses (7,8). The combination o exact mass c a p a b i l i t i e s makes FTMS an important t o o l f o r determining the chemical formula of an unknown molecule. This can be e s p e c i a l l y important for higher molecular weight compounds such as polymers and b i o l o g i c a l molecules. Resolution of 150,000 at m/z 1180 has been reported (37). High r e s o l u t i o n i s also required for the separation of isobaric ions and can be of importance i n the analysis of mixtures. For example, Figure 4 i s a spectrum obtained from c i g a r e t t e smoke t a r , which contains many thousands of compounds. The sample was deposited on a d i r e c t i n s e r t i o n probe, and the spectrum was c o l l e c t e d using medium r e s o l u t i o n conditions. Closer inspection of the area around mass 124 (expansion shown i n Figure 5) reveals three well-resolved isobaric ions. Under the moderate r e s o l u t i o n conditions used to acquire the spectrum ( r e s o l u t i o n of 24,000 at m/z 124) molecular formulae f o r the three compounds can r e a d i l y be established. Using the high r e s o l u t i o n c a p a b i l i t i e s of FTMS, the same approach can be used to i d e n t i f y i s o b a r i c species at much higher masses. The requirement of low pressure f o r high r e s o l u t i o n mass measurement can l i m i t the performance of FTMS when used i n conjunction with chromatographic techniques, such as gas chromatography or s u p e r c r i t i c a l f l u i d chromatography, or with i o n i z a t i o n techniques that require higher source pressures, such as f a s t atom bombardment or high pressure chemical i o n i z a t i o n . Using a j e t separator to reduce the pressure w i t h i n the i o n c e l l , mass r e s o l u t i o n f o r benzene (m/z 78) of 24,000 has been obtained, compared to 8000 without the j e t separator (44). Pulsed valves have also been used to l i m i t the gas load i n the i o n c e l l by admitting the GC e f f l u e n t only during the i o n i z a t i o n event and allow the gas to be pumped away p r i o r to the i o n detection step (45). Separation of the i o n i z a t i o n region from the analyzer region i s another means of increasing mass r e s o l u t i o n by reducing the pressure i n the analyzer region and has the advantage of r e t a i n i n g high mass r e s o l u t i o n with l i t t l e or no loss s e n s i t i v i t y (12). This separation can be accomplished using a tandem quadrupole-FTMS arrangement (46, 47), an external i o n i z a t i o n c e l l (48, 49) or the

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

100

\

120

CATECHOL

/

140 160 MASS IN A.M. U.

LIMONENE

GUAIACOL

180

200

ROOM TEMP PROBE

NICOTINE

10 EV

220

Figure 4. Medium r e s o l u t i o n low energy (10 eV) electron i o n i z a t i o n spectrum of tobacco smoke t a r desorbed from a heated s o l i d s probe.

80

PYRIDINE

PHENOL

TOLUENE

CRESOL

N-METHYLPYRROLIDINE

MAINSTREAM TAR 2R1 CIGARETTE

FOURIER T R A N S F O R M MASS SPECTROMETRY

8

dual c e l l design (37) described e a r l i e r . Using a d i f f e r e n t i a l l y pumped dual c e l l interfaced with a GC, mass r e s o l u t i o n of 30,000 at m/z 174 has been reported (37). This i s s u f f i c i e n t to allow chemical formulae of GC peaks to be determined. Recently, s u p e r c r i t i c a l f l u i d chromatography has been s u c c e s s f u l l y interfaced to an FTMS equipped with a dual c e l l (50). Ion storage and manipulation. Once formed, ions may be trapped w i t h i n the i o n c e l l f o r as long as 5 χ 10* seconds (51) p r i o r to detection. During t h i s time and p r i o r to the e x c i t a t i o n pulse and subsequent detection, a number of processes can be used to probe molecular structure. For example, i f a short delay (tens to hundreds of msec) i s introduced a f t e r i o n formation, the low mass fragment ions formed from a molecule can react with neutrals to y i e l d i o n i c products (17 52) These ions i n turn can serve as proton t r a n s f e r agent phenomenon has been c a l l e reagent gas i s required as i n conventional chemical i o n i z a t i o n (CI) techniques. Low pressures (10" torr) of a reagent gas may be introduced during the delay between i o n formation and e x c i t a t i o n to provide more s e l e c t i v e CI reactions, such as hydrogen/deuterium exchange reactions (18) or to provide a source of low energy electrons f o r electron capture negative i o n CI reactions (53, 54). As i n the case of " s e l f - C I " , the extent of these CI reactions may be c o n t r o l l e d by changing the delay time between i o n formation and excitation. CI reactions using reagent gas pressures of greater than 10" t o r r have also been performed using pulsed valve i n t r o d u c t i o n of the reagent (55), i n a manner s i m i l a r to pulsed valve GC/FTMS discussed previously. The CI products can be detected with higher mass r e s o l u t i o n because the reagent gas i s pumped away p r i o r to the detection step. The dual c e l l (37) and external source designs (35, 46-48) are p a r t i c u l a r l y w e l l - s u i t e d f o r high pressure CI reactions. In the case of the dual c e l l , the reagent gas may be introduced into the source c e l l using either pulsed valves or a s t a t i c pressure of reagent gas. A f t e r a s p e c i f i e d delay time to allow the CI reactions to occur, the product ions are transferred to the lower pressure analyzer c e l l , where they are detected. Chemical i o n i z a t i o n reactions using s o - c a l l e d non-conventional reagents have also been performed using FTMS. These would include compounds with l i m i t e d v o l a t i l i t y that are not amenable f o r use i n conventional high pressure CI sources. Chemical i o n i z a t i o n i n the FTMS experiment i s accomplished by multiple c o l l i s i o n s obtained by introducing longer delay times a f t e r i o n formation, e l i m i n a t i n g the need f o r high concentration of CI reagent ions. For example, metal ions can be r e a d i l y formed by laser impact on metal targets (56). These ions have been shown to react s e l e c t i v e l y with organic compounds and have great p o t e n t i a l f o r providing s t r u c t u r a l information (57, 58). Ions can also be manipulated a f t e r formation by s e l e c t i v e e x c i t a t i o n using a radiofrequency pulse. A high-power r f pulse may be used to excite an i o n (or group of ions) to a s u f f i c i e n t l y large 6

6

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Principles and Features of FTMS

9

o r b i t a l ( s ) to cause n e u t r a l i z a t i o n on the c e l l plates (52). This process, c a l l e d double resonance (52, 59), has a number of applications. For example, suspected reactant ions may be s e l e c t i v e l y ejected from the c e l l using double resonance to confirm t h e i r p a r t i c i p a t i o n i n observed ion-molecule reactions (17, 52). Another important a p p l i c a t i o n of double resonance i s to increase the dynamic range of FTMS, which i s generally l i m i t e d to about 10 to 10 . By ejecting a major ion i n a spectrum, the dynamic range of the measurement can be increased, allowing the less abundant components to be observed. Double resonance can also be used to i s o l a t e ions of a single mass i n the c e l l so that a d d i t i o n a l gas phase reactions may be performed. An important example of t h i s type of process i s c o l l i s i o n a l a c t i v a t i o n (60, 61), or MS/MS, a technique of increasing a n a l y t i c a l importance (62, 63). A f t e r the i o n of i n t e r e s t i s i s o l a t e d , a second radiofrequency pulse (lower i n energy than the ion e j e c t i o higher k i n e t i c energy s u i t a b l e target gas, such as argon, which causes the i o n to fragment. The mass spectrum of these fragment ions i s c h a r a c t e r i s t i c of the o r i g i n a l ion. At present, the r e s o l u t i o n with which a parent i o n may be selected i s generally l i m i t e d to u n i t mass. However, as w i l l be discussed l a t e r , new e x c i t a t i o n techniques should overcome t h i s l i m i t a t i o n . With conventional MS/MS instruments, several types of mass analyzers are l i n k e d together to perform the i s o l a t i o n and analysis steps (62). FTMS performs these steps i n one trapped ion c e l l . In addition, the i s o l a t i o n and e x c i t a t i o n steps may be repeated to obtain MS . Recently, four stages of MS/MS or MS has been reported using FTMS (64). The ultimate l i m i t a t i o n of the number of MS/MS stages possible by FTMS i s loss of the i o n s i g n a l (due to collisional inefficiencies, ion-molecule reactions, and i o n e j e c t i o n ) , not hardware or software (15). Therefore, i n extended multiple MS/MS experiments, such as the MS experiment, i t i s advantageous to u t i l i z e conditions that minimize the number of d i f f e r e n t daughter ions produced. Extensive fragmentation reduces the percentage of the ion current c a r r i e d by the daughter ion to be c o l l i s i o n a l l y dissociated i n the next step. FTMS has several advantages f o r selected MS/MS experiments. F i r s t , the energy of the c o l l i s i o n can be v a r i e d with the power of the e x c i t a t i o n and i s proportional to B r , where Β i s the magnetic f i e l d strength and r i s the radius of the cyclotron motion. The a b i l i t y to a l t e r the energy of the c o l l i s i o n can be of p a r t i c u l a r use i n s t r u c t u r a l studies because c o l l i s i o n a l spectra are s e n s i t i v e to energy (60). Typical c o l l i s i o n a l energies used i n FTMS range up to a few hundred electron v o l t s , which i s considerably lower than for conventional tandem sector instruments, which can a t t a i n several thousand electron v o l t s . In addition, the c o l l i s i o n a l energies obtainable with FTMS decrease as mass increases (15) . However, by using larger c e l l s and greater magnetic f i e l d strength, higher c o l l i s i o n a l energies can be acquired. For example, by employing a larger c e l l , high-energy charge s t r i p p i n g of benzene has been performed (65). 3

4

n

5

5

2

2

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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FOURIER T R A N S F O R M MASS S P E C T R O M E T R Y

One d i s t i n c t i o n of FTMS f o r c o l l i s i o n a l d i s s o c i a t i o n i s that these reactions proceed by multiple c o l l i s i o n s , which can enhance the occurrence of rearrangement processes. A f t e r e x c i t a t i o n , the population of ions has a f i n i t e k i n e t i c energy d i s t r i b u t i o n and t h i s spread i n i o n k i n e t i c energy i s greater at lower e x c i t a t i o n energies (66). Because of t h i s , the daughter i o n spectra obtained with FTMS can be less reproducible than with higher energy sector instruments. Another advantage of FTMS f o r MS/MS experiments i s that high r e s o l u t i o n daughter i o n spectra can be obtained. Using a single c e l l and narrow band (heterodyne)(7, 17) detection, daughter i o n resolutions of several thousand have been reported (67, 68). A t e n - f o l d increase i n r e s o l u t i o n f o r daughter i o n spectra was reported when pulsed valves were used to introduce the c o l l i s i o n gas and allow i t to be pumped away p r i o r to mass analysis (12) . Use of a d i f f e r e n t i a l l y pumped dual c e l l has allowed much higher r e s o l u t i o n daughter i o shows a spectrum obtaine d i s s o c i a t i o n of the m/z 120 ions from 1,3,5-trimethylbenzene (mesitylene) and acetophenone. The parent ions were formed by electron impact at 10 eV i n the high pressure source c e l l , and ions of lower mass were ejected by double resonance pulses. The m/z 120 ions were excited to an energy of 125 eV (laboratory frame of reference) and c o l l i d e d with argon, present i n the source c e l l at a pressure of 6 χ 10" t o r r . The daughter ions were then pulsed across the conductance l i m i t into the lower pressure (1 χ 10" t o r r ) analyzer c e l l , where they were detected with a r e s o l u t i o n of 211,000 (FWHM). S i m i l a r experiments with negative ions have y i e l d e d daughter ion spectra with r e s o l u t i o n i n excess of 320,000 (FWHM) at m/z 46 from 2,4-dinitrotoluene. Other means of manipulating ions trapped i n the FTMS c e l l include photodissociation (70-74), surface induced d i s s o c i a t i o n (75) and electron impact e x c i t a t i o n ("ΕΙΕΙ0")(76) reactions. These processes can also be used to obtain s t r u c t u r a l information, such as isomeric d i f f e r e n t i a t i o n . In some cases, the information obtained from these processes gives i n s i g h t into structure beyond that obtained from c o l l i s i o n induced d i s s o c i a t i o n reactions (74) . These and other processes can be used i n conjunction with FTMS to study gas phase properties of ions, such as gas phase a c i d i t i e s and b a s i c i t i e s , electron a f f i n i t i e s , bond energies, r e a c t i v i t i e s , and spectroscopic parameters. Recent reviews (4, 77) have covered many examples of the a p p l i c a t i o n of FTMS and ICR, i n general, to these types of processes. These processes can also be used to obtain s t r u c t u r a l information, such as isomeric d i f f e r e n t i a t i o n . 6

8

I o n i z a t i o n of Non-volatile Samples. In addition to electron impact i o n i z a t i o n and chemical i o n i z a t i o n , which were discussed e a r l i e r , FTMS can be used i n conjunction with a v a r i e t y of other i o n i z a t i o n techniques. The wide mass range and high mass r e s o l u t i o n c a p a b i l i t e s of FTMS make i t p a r t i c u l a r l y w e l l - s u i t e d f o r the analysis of high molecular weight compounds, such as polymers and b i o l o g i c a l samples. These compounds t y p i c a l l y have l i m i t e d v o l a t i l i t y due to high molecular weight and/or p o l a r i t y and are

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Principles and Features of FTMS

B U C H A N A N AND COMISAROW

ISOBARIC IONS AT NOMINAL MASS 124

C7

11

RESOLUTION 24,000 FWHH

H8 02 124.1246

124.0519

C9

H16

C8 H12 O l 124.0883

123.95

124.00

124^5

^

^24.10

124.20

124.15

Figure 5. Expansion of the region around m/z 123 of Figure 4 showing three well-resolved i s o b a r i c ions. CID OF ACETOPHENONE AND 1..3, 5-TRIMETHYLBENZENE

125 eV

RES=211K

89 M/Z

to ζ

105.0699

2 m UJ > M/Z 105,0335

105.02

105.04 105.06 MASS IN A.M. U.

Τ

Γ

105.08

105.10

Figure 6. High-resolution CAD daughter ion spectrum of a mixture of acetophenone and 1,3,5-trimethylbenzene from m/z 120 parent ions.

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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FOURIER T R A N S F O R M MASS SPECTROMETRY

d i f f i c u l t to ionize using conventional techniques. A number of soc a l l e d "soft i o n i z a t i o n " techniques have been developed f o r the analysis of high molecular weight compounds by mass spectrometry, including l a s e r desorption, secondary i o n methods, and plasma desorption. A l l of these techniques are applicable to FTMS. In addition, f i e l d desorption, one of the f i r s t "soft i o n i z a t i o n " techniques, has recently been used with FTMS (78). The combination of FTMS and lasers (79) i s p a r t i c u l a r l y w e l l suited for i o n i z a t i o n of non-volatile samples (80) because the FTMS experiment i s amenable to non-continuous ion production provided by pulsed lasers. Further, l a s e r desorption o f f e r s a means of generating high mass ions with a minimum of fragmentation. After i o n i z a t i o n , the v e r s a t i l e features of FTMS can be used to obtain s t r u c t u r a l information on these high mass ions. Metal ions have been produced from metal targets using a pulsed Nd/YAG l a s e r and reacted with organics f o r subsequent study by FTMS (21, 81, 82). Further desorption and subsequentl study, FTMS spectra of polar and z w i t t e r i o n i c compounds were obtained using a pulsed TEA C0 l a s e r (22). Since that time, pulsed lasers have been employed f o r the i o n i z a t i o n of nonv o l a t i l e s i n a number of studies (19, 37, 84-86). For example, the molecular weight d i s t r i b u t i o n s of polymers with average molecular weights of up to 6000 daltons have been determined using a pulsed C0 l a s e r (87). Solutions of the polymers were doped with e i t h e r KBr or NaCl and the r e s u l t i n g spectra revealed no significant fragmentation, simplifying molecular weight characterization. Laser desorption has been employed to ionize nucleosides, oligosaccharides and glycosides, generating both p o s i t i v e and negative ions i n the r e s u l t i n g FTMS spectra (88). Laser desorption FTMS has also been used to obtain sequence information on mixtures of peptides (89). Selective e x c i t a t i o n of the molecular ions produced by l a s e r desorption was followed by c o l l i s i o n a l a c t i v a t i o n to y i e l d complete sequence information on a c y c l i c decapeptide and for 12 of 15 amino acids of a l i n e a r peptide. Oligonucleotides have been studied using l a s e r desorption FTMS and c o l l i s i o n a l a c t i v a t i o n , as w e l l (90). Figure 7 shows a negative ion daughter spectrum obtained from the adenine base o r i g i n a l l y contained i n the tetranucleotide d(AGCT), or deoxy(adenosine-guanosine-cytidinethymidine). This tetranucleotide, which has a molecular weight of 1173 daltons, was ionized using the fundamental l i n e of a Nd/YAG laser at 1064 nm. The fragment anion at m/z 134 was i s o l a t e d and i d e n t i f i e d as adenine, based on c o l l i s i o n a l d i s s o c i a t i o n , forming a daughter ion at m/z 107, which corresponds to a loss of HCN. S t r u c t u r a l information on ions produced by l a s e r desorption has also been obtained by using i n f r a r e d multiphoton d i s s o c i a t i o n processes (73). A pulsed C0 l a s e r was used to form the ions, which were further fragmented e i t h e r by using sequential pulses from the same l a s e r or by using a gated continuous wave i n f r a r e d laser. Laser desorption also produces neutral species which can be subsequently ionized by electron i o n i z a t i o n or by " s e l f - C I " processes. Laser desorption/electron i o n i z a t i o n has been used to 2

2

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In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Principles and Features of FTMS

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CID OF M/Z 134 FROM d(AGCT)

Figure 7. Daughter ion spectrum from the m/z 134 anion isolated from the tetranucleotide deoxy(adenosine-guanosine-cytidinethymidine). Ions were generated by laser desorption from a d i r e c t i n s e r t i o n probe.

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

FOURIER TRANSFORM MASS SPECTROMETRY

14

ionize erythromycin (91) and produced a very clean spectrum with about two orders of magnitude less sample than f a s t atom bombardment (FAB). In addition, greater molecular weight information was obtained with the LD/EI/FTMS spectrum than with the FAB spectrum. P a r t i c l e induced desorption methods are commonly used to ionize l o w - v o l a t i l i t y compounds. Cesium i o n desorption (or cesium ion secondary i o n mass spectrometry, SIMS) uses a primary beam of cesium ions to desorb and ionize a n o n - v o l a t i l e sample. This technique has been used with FTMS to produce pseudomolecular ions of vitamin B , { ( B ) + Cs - 2CN} , at m/z 2792 (24) and molecular ions of beta-cyclodextrin (m/z 1135) (92, 93). Detection l i m i t s of 1 0 " mol f o r the peptide gramicidin S has been demonstrated using Cs SIMS with FTMS (25), and a d d i t i o n a l s t r u c t u r a l information was obtained using MS/MS processes. Fast atom bombardment has been used with an FTMS instrument equipped with an externa of cytochrome C at m/z plasma desorption (PD) (95) has been used i n conjunction with FTMS (27, 75, 96, 97), and spectra of compounds with molecular weights up to 2000 daltons have been observed (27, 75). This technique has generally been used only with time-of-flight (TOF) mass spectrometers because the ion currents obtainable are t y p i c a l l y too low f o r scanning instruments. PD i o n i z a t i o n with FTMS o f f e r s a number of advantages over PD/TOF, including higher mass r e s o l u t i o n and the c a p a b i l i t y of i o n manipulation to probe molecular structure. +

1 2

1 2

2

13

+

Future Developments and Applications of FTMS Two important features of FTMS that w i l l be widely e x p l o i t e d i n the future are the u l t r a - h i g h mass r e s o l u t i o n and the wide mass range of the technique. I t should be noted that to a considerable extent, the need for greater mass range and higher mass r e s o l u t i o n has been stimulated by the development of s p e c i a l i z e d i o n i z a t i o n techniques such as l a s e r desorption, f a s t atom bombardment, plasma desorption, and secondary i o n (SIMS) methods. As these and other processes f o r the production of gas phase ions from i n v o l a t i l e s o l i d samples are developed and r e f i n e d , FTMS w i l l be exceptionally useful f o r the mass analysis of these materials. For example, the analysis of biopolymers by mass spectrometry i s an area that i s growing r a p i d l y at present (98). For ordinary p r o t e i n and nucleotide sequencing, mass spectrometry can be used as an adjunct to t r a d i t i o n a l biochemical techniques. However, f o r modified or Nterminus blocked proteins, f o r which the conventional Edman degradation method often f a i l s , mass spectrometry w i l l be important i n i t s own r i g h t (98). S i m i l a r l y , FTMS could be used to sequence modified nucleotides. The u l t r a - h i g h mass r e s o l u t i o n c a p a b i l i t i e s of FTMS w i l l also be used i n the future f o r accurate determination of nuclide masses. I t i s possible that a l l the known stable nuclides w i l l have t h e i r masses reexamined by FTMS. A recent example of t h i s type of measurement i s the determination of the difference i n mass between H e and H by FTMS (99), which may be used i n conjunction with 3

+

3

+

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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15

other data to e s t a b l i s h the mass of the antineutrino. Another i n t e r e s t i n g use of u l t r a - h i g h r e s o l u t i o n FTMS i s the d i r e c t determination of the r e l a t i v i s t i c mass difference between the ground state and metastable excited state of ions, such as A r (100). FTMS also has the p o t e n t i a l of becoming an important t o o l for determining molecular structure. T r a d i t i o n a l l y , mass spectrometry has been rather l i m i t e d i n i t s a b i l i t y to determine the structure of an unknown compound unambiguously. Additional structural methods, such as nuclear magnetic resonance or crystallography, are commonly used i n conjunction with mass spectrometry to elucidate the i d e n t i t y of a molecule. However, when the amount of sample i s severely l i m i t e d or when the sample i s a component i n a complex mixture, mass spectrometry i s often one of the few a n a l y t i c a l techniques that can be used. The a b i l i t y to trap and manipulate ions i n the FTMS makes t h i s a p o t e n t i a l l y powerfu has been described as where reactions can be used to "pick apart" a molecule systematically using sequential CAD, photodissociation, chemical reactions, or other techniques. As s e l e c t i v e and s e n s i t i v e processes for these reactions are developed, FTMS has the p o t e n t i a l of y i e l d i n g d e t a i l e d information on the structure of a molecule which i s currently only obtainable using techniques that require considerably larger sample s i z e s . I t should also be noted that reactions of trapped ions with neutrals can be also be devised for the step-wise synthesis of a p a r t i c u l a r species i n the FTMS (102, 103) . As noted e a r l i e r , the development of the dual c e l l (37), tandem quadrupole-FTMS (46, 47) and external i o n i z a t i o n c e l l (48, 49) has f a c i l i t a t e d the coupling of FTMS and chromatographic methods. Advances i n i n t e r f a c i n g separation techniques with FTMS w i l l be important i n the analysis of mixtures, e s p e c i a l l y where high mass r e s o l u t i o n i s required. For example, liquid chromatographic introduction of mixtures i s o l a t e d from b i o l o g i c a l systems d i r e c t l y into an FTMS for analysis would eliminate the need for laborious sample clean up. The future of FTMS from an instrumental standpoint also shows considerable p o t e n t i a l . The performance of any FTMS instrument i s dependent upon the performance of the computer, magnet, and the vacuum system. While no s u b s t a n t i a l improvements are on the horizon for the price/performance r a t i o of vacuum components, the price/performance r a t i o of computers and magnets improve each year. I t i s reasonable to expect that the s t e a d i l y decreasing p r i c e of computers and magnets w i l l lower the cost of FTMS instruments and promote t h e i r more widespread a p p l i c a t i o n . In addition, with the recent surge of research i n higher temperature superconducting magnets, i t i s possible that much smaller, less expensive, and easier to maintain magnets might be a v a i l a b l e i n the future. The commercial development of small, lower performance FTMS instruments based on lower f i e l d magnets or other means of i o n trapping are also a p o s s i b i l i t y i n the future. F i n a l l y , i n addition to computers becoming more powerful, f a s t e r , and less expensive, new mathematical- and computer-based +

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

FOURIER T R A N S F O R M MASS SPECTROMETRY

16

concepts are being developed to increase the c a p a b i l i t i e s of FTMS. One example, which i s outlined i n a subsequent chapter i n t h i s volume (104), i s Stored Waveform Inverse Fourier Transform techniques, or SWIFT. These new techniques have the p o t e n t i a l to reduce s u b s t a n t i a l l y the previous l i m i t a t i o n s of FTMS with respect to areas such as dynamic range, accurate isotopic r a t i o s , and high r e s o l u t i o n parent ion s e l e c t i o n i n MS/MS experiments. The high performance c a p a b i l i t i e s of FTMS, combined with the a b i l i t y to manipulate trapped ions, makes the FTMS an extremely v e r s a t i l e and powerful t o o l . The future promises to bring even greater c a p a b i l i t i e s to t h i s r e l a t i v e l y new technique. More applications of FTMS to diverse f i e l d s w i l l be seen, as w e l l as the maturation of FTMS into a routine a n a l y t i c a l technique. Acknowledgments Research sponsored (MVB under Interagency Agreemen Health and Environmental Research, U.S. Department of Energy under contract DE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc. Support i s also g r a t e f u l l y acknowledged (MBC) from the Natural Sciences and Engineering Research Council of Canada. L i t e r a t u r e Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

M. B. Comisarow and A. G. Marshall, Chem. Phys. Lett. (1974) 4, 282-293. M. B. Comisarow and A. G. Marshall, Chem. Phys. Lett. (1974) 26, 489-490. M. B. Comisarow and A. G. Marshall, Can. J . Chem. (1974) 52, 1997-1999. K. P. Wanczek, I n t . J . Mass Spectrom. Ion Proc. (1984) 60, 11-60. J . E. Bartmess and R. T. McIver, Jr., in "Gas-Phase Ion Chemistry", ed. M. T. Bowers, Academic Press, New York, 1979, V o l . 2, p. 81-121. J . M. H. Henis, Anal. Chem. (1969) 41, 22-32A. M. B. Comisarow, Adv. Mass Spec. (1978) 7, 1042-1046. M. B. Comisarow, Adv. Mass Spectrom. (1980) 9, 1698. A. G. Marshall, i n "Fourier, Hadamard, and H i l b e r t Transforms in Chemistry", ed. A. G. Marshall, Plenum Press, New York, 1982, pp 1-43. R. T. McIver, J r . , Rev. S c i . Instrum. (1970) 41, 555. M. B. Comisarow, I n t . J . Mass Spectrom. Ion Phys. (1982) 37, 251-257. A. G. Marshall, Acc. Chem. Res. (1985) 18, 316. C. L. Johlman, R. L. White, and C. L. Wilkins, Mass Spectrom. Rev. (1983) 2, 389. C. L. Wilkins and M. L. Gross, Anal. Chem. (1981) 53, 1661A. M. L. Gross and D. L. Rempel, Science (1984) 226, 261. M. B. Comisarow, Anal. Chim. Acta (1985) 178, 1. G. Parisod and M. B. Comisarow, Adv. Mass Spectrom. (1980) 8A, 212-223.

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44.

Principles and Features of FTMS

S. Ghaderi, P. S. Kulkarni, Ε. B. Ledford, and M. L. Gross, Anal. Chem. (1981) 53, 428. T. J. C a r l i n and B. S. F r e i s e r , Anal. Chem. (1983) 55, 571. D. A. McCrery, T. M. Sack, and M. L. Gross, Spectrosc. Int. J. (1984) 3, 57. R. B. Cody, R. C. Burnier, and B. S. F r e i s e r , Anal. Chem. (1982) 54, 96. D. A. McCrery, Ε. B. Ledford, J r . , and M. L. Gross, Anal. Chem. (1982) 54, 1435. M. P. I r i o n , W. D. Bowers, R. L. Hunter, R. S. Rowland, and R. T. McIver, Jr., Chem. Phys. Lett. (1982) 93, 375. M. E. Castro and D. H. R u s s e l l , Anal. Chem. (1984) 56, 578. I . J . Amster, J . A. Loo, J . J . P. Furlong, and F. W. McLafferty, Anal. Chem. (1987) 59, 313. D. F. Hunt, J . Shabanowitz, R. T. McIver, R. L. Hunter, and J . E. P. Syka, Anal Chem (1985) 57 765 J . A. Loo, E. R B. H. Wang, F. W Anal Chem. (1985) 57, 1880. M. B. Comisarow, J . Chem. Phys. (1978) 69, 4097. Ε. B. Ledford, Jr., S. Ghaderi, R. L. White, R. B. Spenser, P. S. K u l k a r n i , C. L. Wilkins, and M. L. Gross, Anal. Chem. (1980) 52, 463. Ε. B. Ledford, D. L. Rempel, and M. L. Gross, Anal. Chem. (1984) 56, 2744. A. G. Marshall, M. B. Comisarow, and G. Parisod, J . Chem. Phys. (1971) 71, 4434. A. G. Marshall, Chem. Phys. Lett. (1979) 63, 515. A. G. Marshall and D. C. Roe, J . Chem. Phys. (1980) 73, 1581. P. R. G r i f f i t h s , "Fourier Transform Techniques i n Chemistry", Plenum Press, New York, 1978. R. P. Grese, D. L. Rempel, and M. L. Gross, t h i s volume. F. H. Laukein, M. Allemann, P. Bischofberger, P. Grossmann, Hp. K e l l e r h a l s , and P. K o f e l , t h i s volume. R. B. Cody, J . A. Kinsinger, S. Ghaderi, I . J . Amster, F. W. McLafferty, and C. E. Brown, Anal. Chim. Acta (1985) 178, 43. Ε. B. Ledford, J r . , S. Ghaderi, C. L. Wilkins, and M. L. Gross, Adv. Mass Spectrom. (1981) 8B, 1707. M. B. Comisarow and A. G. Marshall, J . Chem. Phys. (1975) 62, 293. M. B. Comisarow and A. G. Marshall, J . Chem. Phys. (1976) 64, 110. K. -P. Wanczek, 1987 Pittsburgh Conference on A n a l y t i c a l Chemistry and Applied Spectroscopy, A t l a n t i c C i t y , ΝJ, paper No. 526. R. L. White, Ε. B. Ledford, Jr., S. Ghaderi, C. L. Wilkins, and M. L. Gross, Anal. Chem. (1980) 52, 1525. I . J . Amster, F. W. McLafferty, M. E. Castro, D. H. R u s s e l l , R. B. Cody, Jr., and S. Ghaderi, Anal. Chem. (1986) 58, 458. Ε. B. Ledford, Jr., R. L. White, S. Ghaderi, C. L. Wilkins, and M. L. Gross, Anal. Chem. (1980) 52, 2450.

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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FOURIER T R A N S F O R M MASS SPECTROMETRY

45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75.

T. M. Sack and M. L. Gross, Anal. Chem. (1983) 55, 2419. D. F. Hunt, J . Shabanowitz, R. T. McIver, Jr, R. L. Hunter, and J . E. P. Syka, Anal. Chem. (1985) 57, 765. R. T. McIver, J r . , R. L. Hunter, and W. D. Bowers, I n t . J . Mass Spectrom. Ion Proc. (1985) 64, 67. P. K o f e l , M. Allermann, Hp. K e l l e r h a l s , and K. P. Wanczek, Int. J . Mass Spectrom. Ion Proc. (1985) 65, 97. J . M. A l f o r d , P. E. Williams, and R. E. Smalley, I n t . J . Mass Spectrom. Ion Proc. (1986) 72, 33-51. R. B. Cody, Jr. and J . A. Kinsinger, t h i s volume. M. Allemann, Hp. K e l l e r h a l s , and K. P. Wanczek, Chem. Phys. Lett. (1980) 75, 328. M. B. Comisarow, V. Grassi, and G. Parisod, Chem. Phys. Lett. (1978) 57, 413. M. V. Buchanan, I. B. Rubin, and M. B. Wise, Biomed. Environ. Mass Spectrom. (1987) 14 395 M. V. Buchanan an T. M. Sack and M 31st Annual Conference of Mass Spectrometry and A l l i e d Topics, Boston, May 1983, p. 396. R. A. Forbes, E. C. Tews, B. S. F r e i s e r , M. B. Wise, and S. P. Perone, Anal. Chem. (1986) 58, 684. R. C. Burnier, G. D. Byrd, and B. S. F r e i s e r , Anal. Chem. (1980) 52, 1641. D. A. Peake and M. L. Gross, Anal. Chem. (1985) 57, 115. L. R. Anders, J . L. Beauchamp, R. C. Dunbar, and J . D. Baldeschwieler, J . Chem. Phys. (1966) 45, 1062. F. W. McLafferty, Science (1981) 214, 1981. R. T. McIver, J r . and W. D. Bowers, i n "Tandem Mass Spectrometry", ed. F. W. McLafferty, Wiley, New York, 1983, pp. 287-302. F. W. McLafferty,(ed.), "Tandem Mass Spectrometry", Wiley, New York, 1983. R. L. Cooks and G. L. G l i s h , Chem. & Engineering News. Nov. 30, 1980, p. 40. J . C. Kleingeld, Ph.D. thesis, University of Amsterdam (1984). D. L. Bricker, T. A. Adams, Jr., and D. H. R u s s e l l , Anal. Chem. (1983) 55, 2417. W. T. Huntress, M. M. Moseman, and D. D. Ellerman, J . Chem. Phys. (1971) 54, 843. R. B. Cody and B. S. Freiser, Anal. Chem. (1982) 54, 1431. R. L. White and C. L. Wilkins, Anal. Chem. (1982) 54, 2211. M. B. Wise, Anal. Chem.. in press. B. S. F r e i s e r , Anal. Chim. Acta (1985) 178, 137. C. H. Watson, G. Baykut, M. A. B a t t i s t e , and J . R. Eyler, Anal. Chim. Acta (1985) 178, 125. W. D. Bowers, S. S. Delbart, R. L. Hunter, and R. T. McIver, Jr., J . Am. Chem. Soc. (1984) 106, 7288. C. H. Watson, G. Baykut, and J . R. Eyler, t h i s volume. R. L. Hettich and B. S. Freiser, t h i s volume. F. W. McLafferty, I. J . Amster, J . J . P. Furlong, J . A. Loo, Β. H. Wang, and E. R. Williams, t h i s volume.

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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B U C H A N A N AND COMISAROW

76. 77. 78. Allied 79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98. 99. 100. 101.

Principles and Features of FTMS

R. B. Cody, J r . and B. S. F r e i s e r , Anal. Chem. (1987) 59, 1054. M. T. Bowers, (ed.), "Gas Phase Ion Chemistry", Vols. 1 and 2 (1979) and Vol. 3 (1984), Academic Press, New York. H. B. Linden, H. K n o l l , and K. P. Wanczek, paper presented at the 35th Annual Conference on Mass Spectrometry and Topics, Denver, CO, May 24-29, 1987. M. A. Posthumus, P. G. Kistemaker, H. L. C. Meuzelaar, and M C. Ten Noever deBrauw, Anal. Chem. (1978) 50, 985. R. S. Brown and C. L. Wilkins, t h i s volume. D. B. Jacobson, G. C. Byrd, and B. S. F r e i s e r , J . Am. Chem. Soc. (1982) 104, 2321. G. D. Byrd and B. S. F r e i s e r , J . Am. Chem. Soc. (1982) 104, 5944. W. E. Reents, J r . , A. M. Mujsce, W. E. Bondybey, and M. L. Mandrich, J Chem Phys (1987) 86 5568 C. L. Wilkins, D Anal. Chem. (1985 E. C. Brown, P. Kovacic, C. A. W i l k i e , R. B. Cody, J r . , and J. A. Kinsinger, J . Polym. S c i . . Polym. Lett. Ed. (1985) 23, 435. D. A. McCrery and M. L. Gross, Anal. Chim. Acta (1985) 178, 105. R. S. Brown, D. A. Weil, and C. L. Wilkins, Macromolecules (1986) 19, 1255. D. A. McCrery and M. L. Gross, Anal. Chim. Acta (1985) 178, 91. R. B. Cody, J r . , I . J. Amster, and F. W. McLafferty, Proc. N a t l . Acad. S c i . USA. (1985) 82, 6367. R. L. Hettich and M. V. Buchanan, paper presented at the 35th Annual Conference on Mass Spectrometry and Allied Topics, Denver, CO, May 24-28, 1987. R. E. Shomo, II, A. G. Marshall, and C. R. Weisenburger, Anal. Chem. (1985) 57, 2940. M. E. Castro, L. M. M a l l i s , and D. H. R u s s e l l , J . Am. Chem. Soc. (1985) 107, 5652. D. H. R u s s e l l , Mass Spectrom. Rev. (1986), 5, 167. D. F. Hunt, J . Shabanowitz, J . R. Yates, III, N-Z Zhu, D. H. R u s s e l l , and M. E. Castro, Proc. Natl. Acad. S c i . USA (1987) 84, 620. R. D. Macfarlane, Anal. Chem. (1983) 55, 1247A. J . C. Tabet, J . Rapin, M. P o r e t i , and T. Gaumann, Chimia. (1986) 40, 169. S. K. Viswanadham, D. M. Hercules, R. R. Weller, and C. S. Giam, Biomed. Environ. Mass Spectrom. (1987) 14, 43. K. Biemann, Anal. Chem. (1986) 58, 1289A. E. Lippma, R. Pikver, E. Suurmaa, J . Past, J . Puskar, I. Koppel, and A. Tammik, Phys. Rev. Lett. (1985) 54, 285. M. Bamberg and K. P. Wanczek, paper presented at the 35th Annual Conference on Mass Spectrometry and Allied Topics, Denver, CO, May 24-29, 1987. T. C a r l i n , "The Fourier Transform Mass Spectrometer as a Gas Phase Chemical Laboratory", Ph. D. Thesis, Purdue U n i v e r s i t y , Aug. 1983.

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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102. B. S. Freiser, Talanta (1985) 32(8B), 697. 103. T. C. Jackson, D. B. Jacobsen, and B. S. Freiser, J . Am. Chem. Soc., (1984) 1 0 6 , 1252. 104. A. G. Marshall, T.-C. L. Wang, L. Chen, and T. L. Ricca, this volume. R E C E I V E D October 9, 1987

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 2

New Excitation and Detection Techniques in Fourier Transform Ion Cyclotron Resonance Mass Spectrometry 1,2

4

1

Alan G. Marshall , Tao-Chin Lin Wang , Ling Chen , and Tom L. Ricca

3

1

Department of Chemistry, Ohio State University, Columbus, OH 43210 Department of Biochemistry, Ohio State University, Columbus, OH 43210 Chemical Instrument Center Ohio State University Columbus OH 43210

2

3

FT/ICR experiments have conventionally been carried out with pulsed or frequency-sweep excitation. Because the cyclotron experiment connects mass to frequency, one can construct ("tailor") any desired frequency-domain excitation pattern by computing its inverse Fourier transform for use as a time-domain waveform. Even better results are obtained when phase-modulation and time-domain apodization are used. Applications include: dynamic range extension via multiple-ion ejection, high-resolution MS/MS, multiple-ion simultaneous monitoring, and flatter excitation power (for isotope-ratio measurements). The analytically important features of Fourier transform ion cyclotron resonance (FT/ICR) mass spectrometry (1) have recently been reviewed (2-9): ultrahigh mass resolution 01,000,000 at m/z < 200) with accurate mass measurement even in gas chromatography/mass spectrometry experiments; sensitive detection of low-volatility samples due to 1,000-fold lower source pressure than in other mass spectrometers; versatile ion sources (electron impact (EI), self-chemical ionization (self-CI), laser desorption (LD), secondary ionization (e.g., Cs -bombardment), fast atom bombardment (FAB), and plasma desorption (e.g., 252cf fission); trapped-ion capability for study of ion-molecule reaction connectivities, kinetics, equilibria, and energetics; and mass spectrometry/mass spectrometry (MS/MS) with a single mass analyzer and dual collision chamber. +

Ion Motion in Crossed Static Magnetic and Oscillating Electric Fields The basic principle of FT/ICR mass spectrometry is that a moving ion in an applied static magnetic field undergoes circular motion, in a plane perpendicular to that f i e l d , at a "cyclotron" frequency, UJ, 4

Current address: Section on Analytical Biochemistry, Building 10, Room 3D-40, National Institute of Mental Health, 9000 Rockville Pike, Bethesda, M D 20892 0097-6156/87/0359-0021 $06.00/0 © 1987 American Chemical Society

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

22

FOURIER TRANSFORM MASS SPECTROMETRY

3>

c

= qB/m

(1)

in which Β is the magnetic f i e l d strength (Tesla), q is the ionic charge (Coul), and m is the ionic mass (kg). The cyclotron motion is excited and detected as shown schematically in Figure 1 (10). First, i f an electric f i e l d oscillating (or rotating) at an angular frequency, (*>(=2τπί, in a plane perpendicular to the magnetic field axis, then "resonant" ions (i.e., those for which w = -

en

> i< _l

-I Trap switch „ before chirp

' ΟΌΙ

0.03

0.05

„ Trap switch after chirp "^0.07

0.09

^switch

Figure 3· (a). Trap switch time sequence. (b). Relative i n t e n s i t y of benzene molecular ion versus trap switch time for a chirp test e x c i t a t i o n . 100,000 ions were formed i n i t i a l l y . ELI(kHz) EHl(MHz) ATl(dB) VTR(V) DLl(ms) 0 2 2 0.44 47 Δ Δ 0 2 2 1.04 47 Ο Ο 0 2 5 0.44 47 χ...χ 200 0.350 2 0.44 47 •...• 0 2 2 0.44 2000 (Reproduced with permission from Réf. 23. Copyright 1986 Elsevier Science Publishers B.V.)

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

3.

GRESE ET AL.

A Routt to Instrument Improvements

41

rate and 0 to 2 MHz sweep width. Other conditions were a 1.2 Τ magnetic f i e l d , a 0.0254 m cubic trap, and a 1.0 V trap p o t e n t i a l . When the z-mode position and v e l o c i t y at the beginning of the excitation i s compared with the corresponding information at the conclusion of the e x c i t a t i o n , i t i s found that i n a l l the cases t r i e d for benzene (m/q = 78) and nitrogen (m/q = 28), the z-mode energy i s increased by the e x c i t a t i o n . Those benzene ions with i n i t i a l ion z-mode energies 50 percent of the trappable maximum are l o s t i n the z-direction whereas ions with i n i t i a l z-modes of 40 percent or less are not l o s t . Those nitrogen ions with i n i t i a l z-mode energies 17.5$ or more are l o s t whereas an i o n with an i n i t i a l z-mode energy of 15% i s not l o s t . The d i f f e r e n t i n i t i a l z-mode energies required f o r loss i s consistent with the hypothesis that f o r lower mass ions a larger f r a c t i o n of the i o n population i s ejected. The numerical results are consistent with the result of an experiment i n which th (OH,,*) and benzene (C control experiment, the two ions were excited by low amplitude consecutive RF burst pulses of varied time. The s i g n a l r a t i o was e s s e n t i a l l y constant over the range of orbits for which signals were detectable. In contrast, f o r a chirp from 10 kHz to 2 Mhz at 2.094 kHz/ysec of varied amplitude, the abundance r a t i o of d^ ) are u t i l i z e d , we experimentally observe a degradation of both r e s o l u t i o n and line-shape, presumably because the length of the homogeneous magnetic f i e l d region i s approximately 60 mm. The use of large c e l l s i n FTMS i s advantageous, f i r s t l y because the volume of the quadrupolar e l e c t r i c f i e l d region i s maximized and line-broadenings are therefore reduced, and secondly because the maximum trapping time increases with ά^ . This l a s t point i s p a r t i c u l a r l y important i f one i s interested i n large ion clusters which b u i l d up v i a gas-phase ion-neutral reactions. The average number of ion-neutral c o l l i s i o n s C (and hence the maximum c l u s t e r size) which can occur before the ion c l u s t e r s are l o s t at the side walls i s proportional to the square of the product of magnetic f i e l d strength Β and c e l l diameter d^ according to (3) z

z

2

C

α

(Û^B)

2

A

(D-

Here V i s the trapping voltage; note that C i s pressure-independent. An example of large negative ion c l u s t e r s b u i l d i n g up from smaller parent ions i n the ICR c e l l i s given i n Figure 2a, where the Re - and Re -groups are selected from the negative ion-cluster mass spectrum of decacarbonyldirhenium. A f t e r electron capture and loss of one or several carbonyl groups, negative rheniumcarbonyl clusters polymerize up to R e ( C O ) " at mass M -= 4,216 amu; they can s t i l l be observed with u n i t resolution. P o s i t i v e rheniumcarbonyl clusters up to M - 8,828 amu are p l o t t e d i n Figure 2b. The average number of c o l l i s i o n s experienced by the parent ion of mass 652 amu i s approximately C - 15,000. The average c o l l i s i o n number f o r smaller ions, such as water, can be as high as 400,000. 17

18

1

8

31

A l l broadband spectra were amplified by a low-noise preamplifier with a dynamic range > 1,000, and a broadband main amplifier. The amplified time-domain s i g n a l i s d i g i t i z e d by a 20 MHz, 9 b i t Bruker ADC with 128 Κ words of buffer memory, and Fourier transformed by a Bruker array processor (128 Κ word FFT i n 8 sec). In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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L A U K I E N E T AL.

Instrumentation and Applications

83

Figure 1. C y l i n d r i c a l ICR c e l l with - d - 60mm. The voltages indicated are suitable f o r p o s i t i v e ions and can simply be reversed for the study of negative ions. z

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

84

I

1

I

FOURIER T R A N S F O R M MASS SPECTROMETRY

1

3950

I

1

I

1

I

1

I

3980

1

I

1

I

1

I

1

4010

1

1

I I I

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

I I I I I I I I I I I I I I I I I I I

4040

4070

4100

4130

4160

4200

4240

n/2

Figure 2a. Negative rheniumcarbonyl c l u s t e r s with unit resolution above 4,200 amu (2xl0~ mbar, 65 eV EI, CID a c t i v a t i o n , 1,000 scans). 8

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

5.

85

Instrumentation and Applications

L A U K I E N ET AL.

The advantages of d i f f e r e n t i a l ion a c c e l e r a t i o n and detection i n terms of r f power and s i g n a l homogeneity have been elucidated by Dunbar (4) . In our c e l l design, r f voltages of up to 400 V o l t (peak-to-peak) can be applied d i f f e r e n t i a l l y , which allows f o r rapid acceleration and ejection. Pulsed-Valve CI and CID-Experiments. Chemical I o n i z a t i o n (CI), s e l f - C I (SCI), and d i r e c t or desorption CI (DCI) experiments i n FTMS can be done equally w e l l with the differentially-pumped external ion source described below, or with a pulsed-valve single c e l l arrange­ ment (5,6). In our experiments, we admit a pulse of reagent gas v i a a p i e z o e l e c t r i c pulsed valve with a minimum opening time of about 2.5 ms ( 7 ) . Unlike solenoid pulsed valves, the performance of p i e z o e l e c t r i c pulsed valves i s not disturbed by the strong magnetic f i e l d of 4.7 Tesla. Immediately following t h i s pulse of reagent gas, the pressure i n the ICR c e l l temporaril a time delay of 1 - 2 second ICR c e l l i s pumped down to 2x10" mbar by a 330 1/s turbomolecular pump, permitting enhanced resolution, and accurate mass determinations without i n t e r n a l c a l i b r a n t s . Examples of both, pulsed-valve p o s i t i v e ion CI, using methane and ammonia as reagent ions, and pulsed-valve negative ion CI, using a N 0/CH mixture, are shown i n Figure 3 a,b. In both examples the molecular i o n was not stable with respect to e l e c t r o n impact at 70 eV, but the CI spectra c l e a r l y show abundant quasi-molecular-ion peaks. P i e z o e l e c t r i c pulsed-valve i n l e t systems are equally u s e f u l i n c o l l i s i o n - i n d u c e d d i s s o c i a t i o n (CID) experiments (8) where the CID target gas (usually Argon) i s pulsed, and subsequently pumped away to permit high-resolution, high-accuracy a c q u i s i t i o n of FTMS spectra. Recently, low pressure CI and SCI experiments, which take advantage of the long r e a c t i o n times ( t y p i c a l l y 10 to 60 seconds) possible i n an FTMS, have been demonstrated (9). Figure 4 exhibits low-pressure EI and DCI spectra of R i b o f l a v i n (Vitamin B2) , taken without a pulsed valve. Only the DCI spectrum taken a t 2x10' mbar contains the quasi-molecular ion at M - 377. 2

4

8

SIMS. Secondary Ion Mass Spectrometry i s p a r t i c u l a r l y suited for i o n i z a t i o n of n o n v o l a t i l e , polar, and thermally l a b i l e molecules. L i q u i d SIMS, using l i q u i d g l y c e r o l matrices, i s best done i n the differentially-pumped external ion source, because matrix effects and the high vapor pressure of g l y c e r o l make l i q u i d SIMS unsuitable for single c e l l low-pressure FTMS. S t a t i c SIMS, however, which does not require any matrix, and demands only low primary beam i n t e n s i t i e s , does not deteriorate the u l t r a - h i g h vacuum i n the FTMS single c e l l . An example of a s t a t i c SIMS spectrum of Valinomycin using primary 2-4 kV Cs -ions i s shown in Figure 5, including a h i g h - r e s o l u t i o n spectrum of the quasi-molecular (M+Na)-peak at mass 1133.6 amu. In t y p i c a l s t a t i c SIMS experiments 1 μΐ of sample s o l u t i o n (concentration about 10" mol/1) i s deposited on an etched s i l v e r surface and dried. A f t e r i n s e r t i o n o f the s i l v e r target into the combined ΕΙ/SIMS c e l l v i a the s o l i d sample i n l e t , the target i s bombarded by Cs f o r 0.02 - 10 msec with an i n t e n s i t y of 1-10 nanoAmp. Nearly a l l ions created by s t a t i c SIMS are trapped at an ambient pressure of 1 - 5 χ 10" mbar. +

3

+

9

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

86

FOURIER T R A N S F O R M MASS SPECTROMETRY

(CH ) Si-0-Si(CH )

147

3

3

3

3

70 eV EI

1131 CC

J00

130

160

180

220

2S0

280

147 self CI

ec

i

—,

see

J3C

309

l 221 ι eo

^ i90

I

220

2S0

2eo

3)0

CH^- CI pulsed volve

1149

(M+H)

H

147

Λ-*-,—l βΟ

100

no

1B0

190

220

2i>C

280

3)0

Figure 3a. 70 eV EI spectrum, s e l f - C I , and pulsed-valve p o s i t i v e i o n spectrum of hexamethyldisiloxane.

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

CH CI A

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

I il

2

A

385

Figure 3b. 70 eV EI and pulsed-valve negative ion N 0/CH -CI FTMS spectrum of c h o l e s t e r y l acetate.

59

2

neg ion N 0/CH^-CI

H

427 |(M+H)

FOURIER T R A N S F O R M MASS S P E C T R O M E T R Y

88

I'I'MI'I'I'1 I' I' I'I I'I'I I I'I'I I I 'I'I I I'I'I I'I I'I UO 150 190 230 270 310 350 390 RHU 1

b)

CI

1

1

1

1

1

1

1

1

1

1

(NH )-Spectrum 3

377 120

243 257 I ' I

1

I ' I ' I ' I ' I

120

1

I

1

I ' I

160

11 200

240 RHU

' I'I I I I'I I I'I'I'I I I 280 320 360 400 1

1

1

1

1

Figure 4. Low-pressure EI and DCI FTMS spectra (Vitamin B2). Direct probe i n l e t , about 210°C.

1

1

of R i b o f l a v i n

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Magnet Requirements f o r FTMS In t h i s section the importance of both the magnetic f i e l d strength Β and of the magnetic f i e l d homogeneity w i l l be discussed. Even though a n a l y t i c a l FTMS experiments can be done i n r e s i s t i v e , l o w - f i e l d magnets, we s h a l l r e s t r i c t our discussion to superconducting magnets, which permit superior mass r e s o l v i n g powers and absolute mass accuracies. Using a h o r i z o n t a l 4.7 Tesla cryoshimmed supercon magnet with a bore diameter of 15 cm, we have obtained two-parameter mass c a l i b r a ­ tions with absolute mass accuracies of better than 1.5 ppm without, and better than 0.4 ppm with an i n t e r n a l c a l i b r a n t over a mass range from 18 to 502 amu (10). Absolute accuracies with an i n t e r n a l c a l i b r a n t are s l i g h t l y better because the c a l i b r a n t and the unknown compound are measured under i d e n t i c a l p h y s i c a l conditions, i n p a r t i c u l a r with the exact same number of ions i n the c e l l , r e s u l t i n g i n i d e n t i c a l space-charg Due to the extrem associated e l e c t r o n i c s , pp reproduce absence of reference compounds f o r many days. F i e l d Strength. A l l other conditions being equal, the mass resolving power increases l i n e a r l y with f i e l d strength according to (11) m/Am

= qBr/m

(2),

where r i s the e f f e c t i v e signal-decay time. I f the linewidth i s dominated by pressure broadening, as i s t y p i c a l l y the case for pressures above 10" mbar, p a r t i c u l a r l y f o r higher masses over M = 100, then the r e s o l u t i o n indeed improves l i n e a r l y with B. In the collisional-broadening regime i t i s possible e i t h e r to reduce the pressure or to increase the f i e l d strength i n order to improve resolution. The signal-to-noise r a t i o , S/N, i n FTMS i s given by (12) 9

Nq [ pB7R ;

2

S/N

(3), 2md 72kTAf x

i f a m p l i f i e r noise i s neglected. Here Ν i s the t o t a l number of ions, ρ i s the cyclotron radius, R i s the resistance, Τ i s temperature, and Af i s the detection bandwidth. C l e a r l y , higher f i e l d strength increases the FTMS s e n s i t i v i t y l i n e a r l y . Nevertheless, the increase of S/N with Β i s not as dramatic as i n NMR, where s e n s i t i v i t y increases with the seven-fourth power of the f i e l d strength (13). I f one i s interested i n high-mass FTMS i t i s advantageous to work at the highest possible f i e l d strength, because the higher the cyclotron frequency of a given mass, the less noise i s introduced during s i g n a l p r e a m p l i f i c a t i o n . Moreover, the gain of most r f power a m p l i f i e r s used f o r a c c e l e r a t i o n , as w e l l as the gain of the signal a m p l i f i c a t i o n c i r c u i t r y , tend to f a l l o f f f o r lower frequencies, i . e . higher masses. F i n a l l y , Equation 1 e x h i b i t s another important advantage of high f i e l d s i n FTMS. Both the maximum trapping time and the maximum number of c o l l i s i o n s (and gas-phase reactions) increase q u a d r a t i c a l l y with B. Consequently increased magnetic f i e l d strength o f f e r s experimental access to larger ion c l u s t e r s (Figure 2 ) . In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Instrumentation and Applications

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Homogeneity. Residual s p a t i a l magnetic f i e l d inhomogeneities l i m i t the maximum mass resolving power to a pressure-independent upper l i m i t , despite the fact that c e r t a i n inhomogeneities are averaged by the i o n motion i n the c e l l (14). The experimental and t h e o r e t i c a l l i m i t of in/Am i n our 4.7 Tesla cryoshimmed supercon magnet i s approximately 10 , as shown i n Figure 6, where p o s i t i v e methane ions were measured with a mass resolving power of 2 Χ 10 . I n supercon­ ducting magnets without cryoshims t h i s l i m i t i s l i k e l y to be one order of magnitude lower, and i n electromagnets the upper resolution l i m i t i s about 10 . Theoretical c a l c u l a t i o n s (14) show that the maximum mass resolving power can increase by a factor between 20 and 100 to 10 - 1 0 , i f room-temperature shims are used i n addition to the standard cryoshims. 8

8

5

9

10

Software Requirements f o r FTMS S t a r t i n g from the genera i s a research grade a n a l y t i c a a software system which offers v e r s a t i l i t y and d i r e c t control over almost a l l experimental parameters. Phasing. In order to take f u l l advantage of FTICR's high-resolution c a p a b i l i t i e s , automatic and manual phasing have been implemented from the beginning. Consequently, i n the high-resolution mode absorption spectra which require phasing can be obtained instead of magnitude spectra. Marshall (15) has pointed out that absorption spectra are narrower than magnitude spectra by a factor of JT. An example of automatic zeroth-order, and i n t e r a c t i v e f i r s t - and second-order phasing of an absorption mode spectrum i s given i n Figure 7, and a t y p i c a l absorption mode high-resolution spectrum a t mass M — 131 i s shown i n Figure 8. The phasing problem i n broadband FTMS has not yet been solved, and usually only magnitude spectra are displayed. Rapid Scan/Correlation ICR. In order to provide a d d i t i o n a l experimental f l e x i b i l i t y , the CMS 47 can be operated i n the rapid scan/cross correlation mode (16,17) without any hardware modifications. I n t h i s mode the receiver plates of the ICR c e l l are incorporated i n an r f capacitance bridge c i r c u i t which detects energy absorption of the ions during a low-power rapid frequency scan using sweep rates of approximately 1 MHz/s. Rapid scan/correlation ICR can be u t i l i z e d advantageously when very large mass ranges are to be detected, and p a r t i c u l a r l y when large mass ranges extend into the low mass/high frequency region below mass M » 20. An example of a cross c o r r e l a t i o n spectrum i s given i n Figure 9. With standard FT-NMR phasing techniques both absorption and magnitude spectra can be obtained (16). Rapid scan/cross c o r r e l a t i o n ICR has the a d d i t i o n a l advantage that i n t e n s i t y r a t i o s are reproduced more accurately than i n FTICR. V e r s a t i l e Pulse Sequence. One of the great strengths of FTMS i s the f l e x i b i l i t y to s e l e c t i v e l y accelerate, a c t i v a t e , and eject ions i n any combination and any sequence without hardware modifications. This v e r s a t i l i t y makes FTMS the method of choice f o r MS/MS and hence for e s t a b l i s h i n g pathways and rate constants f o r gas-phase ion-molecule reactions, and to correlate t h i s data with s t r u c t u r a l information. Recently up to (MS) has been demonstrated (18). 5

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

92

FOURIER T R A N S F O R M MASS S P E C T R O M E T R Y

+

+

Figure 7. High-resolution absorption mode peaks of N and C0 : a) unphased, b) a f t e r automatic zeroth-order phasing, c) after i n t e r a c t i v e f i r s t - and second-order phasing. 2

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Instrumentation and Applications

93

Ce

m / i m : HpOQOOO

130.9905

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5

50Z

Figure 9. a) Rapid scan spectrum b) cross c o r r e l a t i o n spectrum.

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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To e x p l o i t the advantages of FTMS f u l l y we have implemented several predefined e j e c t i o n , a c t i v a t i o n and a c c e l e r a t i o n options, l i k e single shots, covers, sweeps over some mass window, sweeps around some unaccelerated mass window, t i c k l i n g b e l t s with phase inversion, parent ion s e l e c t i o n chirps, a c t i v a t i o n shots f o r daughter ion production, etc. Using automation software routines f o r the ASPECT 3000 pulse generator, we can u t i l i z e up to nine e j e c t i o n / a c t i v a t i o n pulses, each combining up to four options. Mass (or frequency) information i s taken from several v a r i a b l e user-defined l i s t s , such that each option i n each event may be used on a large number of masses. The variable mass and delay l i s t s can be defined either v i a alphanumeric keyboard input, or they can be defined i n t e r a c t i v e l y from the spectrum v i a cursor. Each a c c e l e r a t i o n / e j e c t i o n event can contain up to 4095 d i f f e r e n t steps. A l l events can be separated by f i x e d or incrementable delays. Computer Interpretation applications and high throughput, g CMS 47 three so-called jobs can run i n timesharing mode. For instance, job 1 may be acquiring data, job 2 may p l o t previously taken data, while the user i s d e f i n i n g the parameters of the next experiment i n job 3 i n a menu-driven fashion. F i n a l l y , f o r routine applications, our software provides a database management system c a l l e d BASIS f o r storage and manipulation of chemical information. BASIS can access generally available s p e c t r a l l i b r a r i e s from three d i f f e r e n t spectroscopic techniques (MS, H-NMR and F 3C-NMR, IR), and permits the c r e a t i o n of new l i b r a r i e s . For structure e l u c i d a t i o n and substructure search of unknown compounds, l i b r a r y search algorithms allow the r e t r i e v a l of i d e n t i c a l and s t r u c t u r a l l y s i m i l a r spectra. 1

External Ionization Typical pressures i n the analyzer region of FTICR mass spectrometers are about three orders of magnitude lower than i n most other types of mass spectrometers. For instance i n our single c e l l CMS 47 we can r o u t i n e l y obtain pressures of 3x10" mbar using a 330 1/sec Balzers turbomolecular pump. I t i s very d i f f i c u l t to couple e x i s t i n g , high gas-load GC-MS, LC-MS, l i q u i d SIMS, or FAB ion sources d i r e c t l y into a s i n g l e - c e l l configuration. Other i o n i z a t i o n techniques l i k e s t a t i c SIMS and Laser Desorption (LD) can be operated with much improved throughput i n a medium-pressure source. I t i s therefore evident that a differentially-pumped ion source, operating at 10" to 10" mbar, i s a desirable feature of an FTICR mass spectrometer, p a r t i c u l a r l y f o r routine applications. Three approaches are presently followed to implement differentially-pumped ion sources : the dual c e l l which i s described elsewhere i n t h i s book, and external i o n i z a t i o n with (19) and without (20) tandem quadrupoles. External i o n i z a t i o n has the advantage of completely removing the i o n i z a t i o n region from the narrow magnet bore while r e t a i n i n g a l l the features which make FTMS such an a t t r a c t i v e technique, l i k e high r e s o l u t i o n , accurate mass determinations, wide mass range, long trapping times, s e l e c t i v e e j e c t i o n / a c t i v a t i o n / a c c e l e r a t i o n etc. Our external ion source, which uses simple a c c e l e r a t i n g and focusing elements, greatly improves access to the ionizer, thereby f a c i l i t a t i n g the coupling of various i n l e t systems, without the need 10

5

6

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Ion Source Probe

95

Instrumentation and Applications

ICR-Cell

Ion Transfer

Faraday Cup

Ion Current Monitor

Ion Beam

Deflection

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Acceleration

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Gas Flow Restriction

ND-.YAG Laser

Figure 10. Schematics of i o n t r a n s f e r d i f f e r e n t i a l l y pumped external ion source with LD.

mechanism

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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for expensive and experimentally more complex r f quadrupoles. I t does not provide f o r mass preseparation outside the magnet, but mass s e l e c t i o n can be obtained with FTMS double resonance techniques. Typical i o n current transmission e f f i c i e n c i e s are of the order of 30%. The i o n transfer e f f i c i e n c y from source to analyzer i s mass-independent (20). For s p e c i a l a n a l y t i c a l a p p l i c a t i o n s , t i m e - o f - f l i g h t e f f e c t s during i o n transmission can be u t i l i z e d to separate c e r t a i n mass windows by s e t t i n g appropriate time i n t e r v a l s between EI or LD i o n i z a t i o n and the c e l l trapping pulse (21). Figure 10 shows a schematic drawing of the i o n transfer mechanism i n the CMS 47 with external i o n i z a t i o n . Figure 11a presents the mass spectrum of Ultramark 1621 using electron impact i n the external source, and Figure l i b depicts a broadband spectrum and the high-resolution window of the negatively charged molecular ion of Euroshift-FOD a f t e r l a s e r desorption i n the external i o n source. Moreover, the external i o n source can be employed f o r medium-pressure CI and SCI as an a l t e r n a t i v techniques described above Anticipated Developments i n FTMS Instrumentation With the successful implementation of differentially-pumped external ion sources, FTMS i s r a p i d l y becoming a routine mass spectrometric technique. Medium-pressure interfaces f o r the coupling of GC, LC, FAB, and l i q u i d SIMS into the external i o n i z e r are c u r r e n t l y under development, and should become a v a i l a b l e i n the near future. Recently, new 2D-methods f o r the analysis of complex mixtures have been developed f o r t i m e - o f - f l i g h t mass spectrometry (22), which could also be u t i l i z e d i n external i o n i z a t i o n FTMS. Specifically, the combination of IR-laser desorption of n o n v o l a t i l e neutrals, followed by adiabatic cooling to 2° Κ i n a supersonic j e t , and subsequent compound-selective Resonance-Enhanced Multiphoton I o n i z a t i o n (REMPI) could increase the role of FTMS i n the analysis of b i o l o g i c a l mixtures. The coupling of supersonic j e t s to the external ion source would also be of i n t e r e s t i n ion- and neutral c l u s t e r experiments. F i n a l l y , i t i s conceivable that u l t r a - h i g h r e s o l u t i o n FTICR w i l l f i n d a d d i t i o n a l applications i n p a r t i c l e , nuclear, and atomic and molecular physics. We believe that some of the instrumental prerequisites f o r these applications w i l l include a) working at lower pressures below 1 0 " mbar, b) better magnetic f i e l d homogeneity using a d d i t i o n a l room-temperature shims and employing improved shimming techniques (14), and c) u l t r a low-noise preamplifiers capable of detecting single or very few ions. 11

Acknowledgments FHL wishes to thank Professor William Klemperer at Harvard U n i v e r s i t y for h i s encouragement and guidance. The authors thank Robert A. Forbes f o r h i s suggestions and f o r reading the manuscript. Finally the authors are g r a t e f u l to Michelle V. Buchanan f o r her tremendous e f f o r t s i n organizing t h i s FTMS book. L i t e r a t u r e Cited 1.

Wanczek, K.P. I n t . J . 11-60.

Mass Spectrom. Ion Processes

1984, 60,

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

5.

LAUKIEN ET AL.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

Instrumentation and Applications

99

J e f f r i e s , J.B.; Barlow, S.E.; Dunn, G.H. I n t . J . Mass Spectrom. Ion Processes 1983, 54, 169. Francl, T.J.; Fukuda, E.K.; McIver, R.T. I n t . J . Mass Spectrom. Ion Physics 1983, 50, 151. Dunbar, R.C. I n t . J . Mass Spectrom. Ion Processes 1984, 56, 1. McIver, R.T. 30th An. Conf. Mass Spectrom. All. Top. A.S.M.S., Honolulu 1982. F r e i s e r , B.S. 30th An. Conf. Mass Spectrom. All. Top. A.S.M.S., Honolulu 1982. Zwinselman, J . J . ; Allemann, M.; K e l l e r h a l s , Hp. Spectrospin ICR A p p l i c a t i o n Note 1984, IV. Cody, R.B.; F r e i s e r , B.S. Anal. Chem. 1982, 54, 1431. Grossmann, P.; Allemann, M.; K e l l e r h a l s , Hp. Spectrospin ICR A p p l i c a t i o n Note 1986, V I . Klass, G.; Allemann, Α.; Bischofberger, P.; K e l l e r h a l s , Hp. Spectrospin ICR A p p l i c a t i o n Note 1983, II. Marshall, A.G.; Comisarow 1979, 71(11),4434 Comisarow, M.B. J. Chem. Phys. 1978, 69 (9), 4097. Hoult,D.I.; Richards,R.E. J. of Magn. Reson. 1976, 24, 71. Laukien,F.H. I n t . J . Mass Spectrom. Ion Processes 1986, 73, 81. Marshall,A.G. In Fourier. Hadamard and H i l b e r t Transforms in Chemistry: Marshall,A.G.,Ed.; Plenum Press: New York, 1982; Chapter 1. Parisod,G.; Gaumann,T. Chimia 1980, 34, 271. McIver,R.T.; Hunter,R.L.; Ledford,Ε.B.; Locke,M.J.; Francl,T.J. Int. J. Mass Spectrom. Ion Phys. 1981, 39, 65. Forbes,R.A.; Laukien,F.H.; Wronka,J. submitted to I n t . J . Mass Spectrom. Ion Processes. Hunt,D.F.; Shabanowitz,J.; Yates,J.R.; McIver,R.T.; Hunter,R.L.; Syka,J.E.P.; Amy,J. Anal. Chem. 1985, 57, 2728. Kofel,P.; Allemann,M.; Kellerhals,Hp. I n t . J . Mass Spectrom. Ion Processes 1985, 65, 97. Kofel,P.; Allemann,M.; Kellerhals,Hp. I n t . J . Mass Spectrom. Ion Processes 1986, 72, 53. Frey,R. 34th An. Conf. Mass Spectrom. All. Top. A.S.M.S., C i n c i n n a t i , 1986.

RECEIVED

June

15,

1987

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 6

Fourier Transform Mass Spectrometry of Large (m/z >5,000) Biomolecules 1

1

1

2

Curtiss D. Hanson , Mauro E. Castro , David H . Russell , Donald F. Hunt , and Jeffrey Shabanowitz 2

1

Department of Chemistry, Texas A&M University, College Station, TX 77843 Department of Chemistry 2

Recent experimental results demonstrate the high mass (>10,000 amu) capabilities of Fourier transform mass spectrometry, however the data reveal non-theoretical limits in the resolution at high mass. These advances can be attributed to the development of methods for coupling high pressure ion sources to the ultra-high vacuum FT-ICR analyser. Specifically, external ion sources permit the utilization of liquid matrix secondary ionization mass spectrometry for the desorption of large involatile, thermally labile biomolecules with ion detection by FT-ICR. Limitations in the mass resolution arise from the inability to effectively trap the ions and produce a coherent packet of ions for detection. The lack of spatial and phase coherence of the injected ions leads to the loss of the frequency domain signal. According to our model, a principle factor contributing to the lack of spatial and phase coherence is field inhomogeneities coupled with the kinetic energies of the ions along the Z-axis.

The development of an ion excitation scheme for ion cyclotron resonance (ICR) compatible with Fourier transform data analysis methods has greatly increased the analytical utility of the method. Although the potential utility of Fourier transform mass spectrometry (FTMS) for the analysis of large biomolecules was recognized early, the development of suitable ionization methods and experimental hardware for biomolecule F T M S proved to be a rather difficult task. One consideration in designing a system for analysis of biomolecules is the high vacuum requirements of ion detection by F T - I C R methods. The requirements for maintaining high vacuum (10~ torr or less) for ion trapping and high resolution mass measurements has led to the adaption of ionization methods for F T M S such as laser desorption and C s ion SIMS. Although these ionization methods are quite useful for the analysis of some biomolecules, the success with molecules larger than 2,500 daltons has been limited. 1,2

3

4,5

1,6

8

4

+

5

0097-6156/87/0359-0100S06.00/0 © 1987 American Chemical Society

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

β. HANSON ET AL.

101

FTMS of Large Biomolecules

The introduction of the tandem quadrupole-Fourier transform mass spectrometer , the dual cell analyzer, and the external ion source opens new possibilities for analyzing biomolecules by F T M S . The first real success with biomolecules was sample ionization by liquid matrix SIMS and introduction of the sample ions to the ion cell by the tandem quadrupole ion injection system. Although the transmission efficiency (fraction of ions trapped in ion cell relative to the total secondary ion yield) of the device has not been fully characterized, especially for large molecules, it is clear that the sample detection levels are comparable to the most sensitive magnetic sector instruments and time-of-flight mass analyzers. An additional advantage of the tandem quadrupole-FTMS system ( Q - F T M S ) has been recently demonstrated by the photodissociation of large peptides. 7,8

9

10

7

8,11

11

The work performed on the Q - F T M S reveals a fundamental problem with the method, viz., the inability to preform high resolution mass measurement at high mass. Although impressive mass resolution data has been reported for ions of 1500-2000 daltons the limited duration of the frequency domain signal fo resolution measurements. Simila experiments performed in a Nicolet FTMS-1000 system equipped with a single section ion c e l l . In an attempt to understand this problem, we will consider the physics of trapping ions in the ion cell and the effect of the initial velocity of the ions and field inhomogeneities on the ion trapping. 11

11

12

Detection of High Mass Ions using F T M S A major thrust of recent work on F T M S of large biomolecules has dealt with questions concerning the lifetime of molecular ions formed by highenergy particle bombardment. Chait and Field have reported that a large fraction of the molecular ions of chlorophyll A formed by C f fission fragment ionization decomposes with lifetimes of less than a few microseconds. A later study on the [M+H] ion of insulin showed that extensive dissociation occurs on both the nanosecond and microsecond time scale. These results raise questions concerning the utility of F T M S for the analysis of large biomolecules. For example, to acquire a low resolution F T mass spectrum from m/z 100 to m/z 10,000 requires data acquisition times of 300-500 ms. On the basis of Chait and Field's study it is reasonable to suspect that molecular ions of large molecules may not survive the long times required for F T detection. The problem is even more severe when considering the requirements for acquiring high mass resolution data at high mass, e.g.. conditions that require data acquisition times of several seconds to even tens of seconds. 2 5 2

13

+

14

+

Earlier studies on the detection of C s ( C s I ) cluster ions were directed at addressing the general problem discussed above. Although there is ample sensitivity to detect high mass (>m/z 10,000) C s ( C s I ) cluster ions in the low and high mass resolution modes, there are questions concerning how well the behavior of such cluster ions models the behavior of large organic molecules. The analysis of biomolecules by FTMS methods at Texas A & M University has been limited to C s desorption ionization from a solid-state matrix in order to maintain the high vacuum requirements. Due to the low ion yields for [M+H] type ions by keV energy particle bombardment from the solid-state, the number of successful analyses with molecules larger n

15

+

n

+

+

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

102

FOURIER TRANSFORM MASS SPECTROMETRY 12

than 2000-2500 amu are limited. Owing to the differential pumping arrangement of the ion source and the analyzer, sample ionization on the tandem quadrupole-FTMS instrument at the University of Virginia can be performed by desorption ionization from a liquid matrix, i.e., keV energy Cs ion particle bombardment of a liquid matrix. Early studies with this instrument demonstrate both the sensitivity and mass resolution of the method. Recent modifications to the Q - F T M S instrument have greatly increased the signal-to-noise ratio and resolution for high mass samples. The modifications involved milling out the ion source block to increase the gas flow in the vicinity of the sample probe. Aberth has discussed the importance of gas pressure on the performance of particle bombardment ion sources. The second modification involved pulsing of the trapping plate voltage of the ion cell from 1 to 10 volts during the ionization step. These two relatively simple modifications have increased the sensitivity of the instrument by roughly two orders of magnitude and extended the working mass range for organic sample +

8,11

11

16

The data for neurotensi the available sensitivity and mass resolution. This spectrum was obtained from a 100 pmol sample dissolved in thioglycerol/glycerol matrix and by using a primary C s ion beam irradiation time of 4 ms. The secondary ions formed on particle bombardment were injected into the ion cell by the rfonly quadrupole rods operated with a low-mass cut-off of ca. m/z 400. As noted above, the trapping voltage was set to zero during ionization and then pulsed positive (to 3 volts for this sample) following the ionization step. The purpose of pulsing the trapping voltage positive following ionization is to restrict the motion of the ions in Z-direction (vide infra). In the broadband (low mass resolution) mode the only ion detected was the [M+H] ion, and in the narrowband mode a mass resolution of ca. 90,000 (resolution specified at F W H M ) was obtained. Note also that the signal-tonoise ratio in the broadband mode is > 100:1 for a sample size of 100 pmol, and in the narrowband (high resolution) mode the signal-to-noise ratio is greater than 500:1. Owing to the pulsed ionization used for F T M S , the total ionization time required to produce this spectrum was approximately 80 ms. That is, the C s ion beam was turned on for 4 ms to produce each mass spectrum and 20 individual ionization events were signal-averaged. +

11

+

+

Tandem quadrupole-FTMS data for two 2000-6000 mass range peptides are shown in Figure 2. Melittin (26 residues) and glucagon (29 residues) were analyzed in earlier work, but even with long sample ionization times (5 s) the signal-to-noise in the region of the [M+H] was low (

/

2

γ1W i » ^

ι'Ι r r

(M+Au)*j

ι

5 pmol

o-

LLILO >

_l UJo_ CCo

100 fmol

Au!

Au* Au+ ιο

o-

iL 200

400

600

MASS

800

f "f'W V 1000

1200

1400

IN A . M . U

F i g u r e 1. SIMS/FTMS spectrum o f g r a m i c i d i n S and g l u t a t h i o n e (1:2) on a g o l d t a r g e t . T o t a l samples d e p o s i t e d were as f o l l o w s : 2 χ 1 0 " mol ( t o p ) , 5 χ 1 0 " mol ( m i d d l e ) , 1 0 " mol (bottom). (Reproduced from r e f . 28. C o p y r i g h t 1987 American Chemical Society.) 1 0

1 2

1 3

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

200

ι ιΊ ιV

— ι —

tift

— Orn

VA

—Vol-

' r ' V f r ι ι "ι ' i"i

Leu-

* ν τ|'—' ι — i f

- Pro —

ι—f—r

Phe

+

+

Τ—1' V r ι — J — ι — ι — ι — r -

Leu •

1200 600 800 1000 MASS I N A.M.U. F i g u r e 2. SIMS/FTMS/MS spectrum o f g r a m i c i d i n S. A f t e r C s i o n i z a t i o n , m/z 100-1100 i o n s a r e e j e c t e d from t h e c e l l and t h e (M + N a ) i o n s , m/z 1164, a r e c o l l i s i o n a l l y d i s s o c i a t e d by admitting a i r . (Reproduced from r e f . 28. C o p y r i g h t 1987 American C h e m i c a l Society.)

400

Phe

7.

McLAFFERTY ET AL.

Tandem FTMS of Large Molecules

121

method to d a t e i s c o l l i s i o n a l l y a c t i v a t e d d i s s o c i a t i o n (CAD) (4043). For energy-dependent mass a n a l y z e r s CAD s p e c t r a have the d i s a d v a n t a g e t h a t t r a n s l a t i o n a l energy from d i s s o c i a t i o n spreads the k i n e t i c energy of the r e s u l t i n g i o n s , l o w e r i n g r e s o l u t i o n (vide supra). FTMS mass a n a l y s i s i s e n e r g y - i n d e p e n d e n t , a v o i d i n g t h i s problem. M u l t i k i l o v o l t g r a z i n g c o l l i s i o n s have the advantage t h a t r e l a t i v e abundances of i o n s from h i g h e r energy p r o c e s s e s are n e a r l y independent of p r e c u r s o r i o n i n t e r n a l energy (41, 43, 4 4 ) , but these e n e r g i e s cannot be a c h i e v e d i n FTMS f o r l a r g e r (a few hundred d a l t o n s ) i o n s . CAD of 10-100 eV k i n e t i c energy i o n s w i t h e f f i c i e n t product c o l l e c t i o n over l a r g e a n g l e s ( p o s s i b l e w i t h t r i p l e q u a d r u p o l e and FTMS i n s t r u m e n t a t i o n ) has the advantage t h a t the s u b s t a n t i a l e f f e c t of c o l l i s i o n energy on product i o n abundances p r o v i d e s an e x t r a parameter f o r c h a r a c t e r i z a t i o n (42, 45). S u r f a c e Induced D i s s o c i a t i o n of r e q u i r i n g a c o l l i s i o pressure. T h i s problem can be reduced by p u l s e d v a l v e i n t r o d u c t i o n of the c o l l i s i o n gas ( 4 6 ) , and e l i m i n a t e d by u s i n g the e x c i t i n g t e c h n i q u e of S u r f a c e Induced D i s s o c i a t i o n ( S I D ) , r e c e n t l y i n t r o d u c e d by Cooks (47, 4 8 ) . SID a l s o has the u n u s u a l advantage of b e i n g a convenient means of a d d i n g a v a r i a b l e , and r e l a t i v e l y narrow, range of i n t e r n a l e n e r g i e s to the i o n . P r e l i m i n a r y work by our l a b o r a t o r y (49) has e f f e c t e d SID by a c c e l e r a t i n g i o n s c o l l e c t e d i n one s i d e of the d u a l c e l l through the conductance l i m i t o r i f i c e to s t r i k e a m e t a l s u r f a c e i n the other c e l l . Results f o r toluene molecular ions using d i f f e r e n t a c c e l e r a t i n g e n e r g i e s are s i m i l a r to those r e p o r t e d by Cooks ( 4 7 ) , and d i s s o c i a t i o n e f f i c i e n c i e s appear to be r e l a t i v e l y h i g h . Other A c t i v a t i o n . E f f i c i e n t (>10%) CAD of p o s i t i v e i o n s appears p o s s i b l e u s i n g c o l l i s i o n s w i t h e l e c t r o n s ( 5 0 ) . Recent r e s u l t s by Cody and F r e i s e r a r e v e r y p r o m i s i n g ( 5 1 ) , c o n f i r m e d by i n i t i a l e x p e r i m e n t s w i t h our FTMS i n s t r u m e n t a t i o n ( 4 9 ) . Laser p h o t o d i s s o c i a t i o n appears to be p a r t i c u l a r l y a p p l i c a b l e f o r FTMS i o n c h a r a c t e r i z a t i o n , w i t h 193 nm p u l s e s from an e x c i m e r l a s e r g i v i n g f r a g m e n t a t i o n ( w i t h c o n v e r s i o n y i e l d s as h i g h as 25%) t h a t appears to be a s s o c i a t e d w i t h s p e c i f i c a b s o r p t i o n and d i s s o c i a t i o n a t groups such as p h e n y l a l a n i n e h a v i n g h i g h a b s o r p t i v i t i e s ( 5 2 ) . Recent r e s u l t s by Hunt and coworkers d e s c r i b e d i n t h i s volume, c o n f i r m e d i n our l a b o r a t o r y , i n d i c a t e t h a t t h i s i s a v e r y p r o m i s i n g method f o r i o n s up to mass 2000 u s i n g 193 nm l a s e r radiation. A c t i v a t i o n of Large Ions. The m u l t i k i l o v o l t CAD s p e c t r a of l a r g e r i o n s (m/z >~1500) can be dominated by the l o s s of s m a l l n e u t r a l s p e c i e s such as Η„0 and NH^ (53, 5 4 ) , c o n s i s t e n t w i t h t h e o r e t i c a l p r e d i c t i o n s t h a t bond c l e a v a g e s at m o l e c u l a r e x t r e m i t i e s w i l l be i n c r e a s i n g l y f a v o r e d over c e n t r a l c l e a v a g e s i n such " f e n d e r bender" c o l l i s i o n s of i n c r e a s i n g l y l a r g e i o n s ( 5 5 ) . Increasing amounts of added i n t e r n a l energy are n e c e s s a r y to e f f e c t e r g o d i c d i s s o c i a t i o n because of the i n c r e a s i n g number of degrees of freedom among which the added energy must be d i s t r i b u t e d . Recent s t u d i e s by Biemann w i t h an e l e g a n t tandem d o u b l e - f o c u s i n g mass

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

FOURIER TRANSFORM MASS SPECTROMETRY

122

s p e c t r o m e t e r (56) c o n f i r m t h a t CAD o f l a r g e p e p t i d e i o n s becomes v e r y i n e f f i c i e n t above about mass 2500. As r e p o r t e d i n t h i s volume, R u s s e l l has shown t h a t h i g h e r mass c o l l i s i o n gases a r e more e f f e c t i v e because they c a r r y away l e s s o f t h e c o l l i s i o n energy. We have proposed (57) an a d d i t i o n a l m e t h o ^ f o r such f a s t d i s s o c i a t i o n u s i n g i o n s such as ( p e p t i d e + H^) (eq 1 ) . Our s t u d i e s have shown t h a t n e u t r a l i z a t i o n o f an i o n i z e d s a t u r a t e d heteroatom s i t e such as a q u a t e r n a r y ammonium i o n g i v e s an unstable hypervalent species. +

+

RNHR NHR NHR 1

2

3

+ 3H + e"

+

-> R - N + H ^ R ^ N ^ - R ^ N ^ ^ +

(1)

+

-> R-N H -R -N*H -R -N H -R 2

1

2

2

+

2

+

(2)

3

+

Thus t h e m u l t i p l y p r o t o n a t e d s p e c i e s , R C0-N H -R' , on n e u t r a l i ­ z a t i o n (eq 2) o f t h e c e n t r a l charge s i t e c o u l d y i e l d (eq 3) t h e s i n g l y charged odd- and e v e n - e l e c t r o n p r o d u c t s R -C0* + H N-R' . The e f f e c t i v e n e s s c o u l d be reduced by competing p r o t o n l o s s o r s t a b i l i z a t i o n by z w i t t e r i o n f o r m a t i o n , -> R -C0~-N H -R' . However, n e u t r a l i z a t i o n w i t h e l e c t r o n s ( e f f e c t e d as proposed above f o r e l e c t r o n CAD) would d e p o s i t v i b r a t i o n a l energy c o r r e s p o n d i n g to t h e i o n i z a t i o n energy (~5 eV) o f t h e new n e u t r a l s i t e , enhancing i t s n o n - e r g o d i c d i s s o c i a t i o n . 2

+

2

+

+

+

2

The s e l e c t e d p r i m a r y i o n s can a l s o be c h a r a c t e r i z e d by i o n m o l e c u l e r e a c t i o n s w i t h the FTMS-2000 d u a l c e l l and/or s u r f a c e induced r e a c t i o n s ( 4 8 ) . R e a c t i o n s s p e c i f i c f o r p a r t i c u l a r f u n c t i o n a l i t i e s can be used t o count t h e number o f these i n a p r i m a r y i o n such as the number o f hydrogen atoms t h a t can be exchanged w i t h a d e u t e r i u m - c o n t a i n i n g species (59). Further, t h e i r p o s i t i o n may w e l l be d i s t i n g u i s h e d by t h e r e a c t i o n ' s e f f e c t on fragment i o n s i n MS-II s p e c t r a . Hadamard T r a n s f o r m Enhancement i n Measurement For tandem mass s p e c t r o m e t r y t h e m u l t i c h a n n e l c a p a b i l i t y o f d i s s o c i a t i n g any c o m b i n a t i o n o f parent i o n s t o form a c o l l e c t i v e MS-II spectrum o f daughter i o n s , p o s s i b l e on F o u r i e r - t r a n s f o r m and i o n t r a p i n s t r u m e n t s , can be used t o i n c r e a s e the s i g n a l / n o i s e and/or speed o f d a ^ a ^ c o l l e c t i o n ( 6 0 ) . A Hadamard t r a n s f o r m advantage o f 0.5 η * i n s i g n a l / n o i s e o r 4/n i n r e q u i r e d time i s p o s s i b l e i n measuring i n d i v i d u a l MS-II s p e c t r a o f η parent i o n s ; η c o l l e c t i v e s p e c t r a u s i n g d i f f e r e n t c o m b i n a t i o n s o f 0.5n o f the p a r e n t i o n s a r e measured, and the i n d i v i d u a l c o n t r i b u t i o n s a t each s p e c i f i c mass i n each MS-spectrum i s c a l c u l a t e d from t h e η s i m u l t a n e o u s e q u a t i o n s r e p r e s e n t i n g t h e summed i n t e n s i t y v a l u e s a t each mass. F o r MS/MS d e t e c t i o n o f many t a r g e t compounds which o c c u r i n f r e q u e n t l y , "no peak" i n f o r m a t i o n can r a p i d l y reduce the number o f p o s s i b l e t a r g e t compounds p r e s e n t ( 4 9 ) . Any peak i n t h e MS-II spectrum o f a t a r g e t parent i o n which i s n e g l i g i b l e i n t h e c o l l e c t i v e MS-II s p e c t r a o f a l l parent i o n s shows t h a t t h e c o r r e s p o n d i n g t a r g e t compound i s n e g l i g i b l e ( 6 0 ) .

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

3

2

8

n

IN

A.M.U.

1500

2

2

2 5 2

4

8

n

20Û0

2

g

2

5

2

2

F i g u r e 3. PD/FTMS n e g a t i v e i o n spectrum o f p o l y ( b u t e n e - l - s u l f o n e ) from bombardment by 100 MeV f i s s i o n fragments from Cf. Major i o n s e r i e s s e p a r a t e d by 120 d a l t o n s ( C , H , 56; S 0 , 64) c o r r e s p o n d to CH CH=CHCH2(S0 C4H ) S02" and H 0 S C H ( C H ) C H ( S 0 C H ) S 0 H (Reproduced f r o n r e f . 61. C o p y r i g h t 1987 American C h e m i c a l Society.)

1000 MASS

1

Ci

1

a. 3

>

W H

H

*i w

>

r

124

FOURIER TRANSFORM MASS SPECTROMETRY

D e g r a d a t i o n Mechanisms o f Ε-Beam R e s i s t s . Research s u p p o r t e d by IBM has the o b j e c t i v e o f e l u c i d a t i n g the r e a c t i o n mechanisms from h i g h energy r a d i a t i o n o f p o l y ( o l e f i n - s u l f o n e s ) . These have by f a r the h i g h e s t s e n s i t i v i t y (G v a l u e ) as Ε-beam r e s i s t polymers. The f o r m a t i o n o f submicron-dimension images i n such r e s i s t s w i t h focused h i g h energy e l e c t r o n s i s used t o produce m i c r o c i r c u i t s . From a n a l y s i s o f the condensed p r o d u c t s o f the r a d i a t i o n d e g r a d a t i o n an " u n z i p p i n g " ( d e p o l y m e r i z a t i o n ) mechanism was proposed. Our p r e l i m i n a r y study o f the p r i m a r y p r o d u c t s f r o n ^ ^ bombardment o f p o l y ( b u t e n e s u l f o n e ) by 10 keV C s i o n s , o r by Cf f i s s i o n p r o d u c t s ( F i g u r e 3 ) , i n the h i g h vacuum FTMS c e l l i n d i c a t e s an e n t i r e l y d i f f e r e n t mechanism. The spectrum p r o d u c t s can be e x p l a i n e d by a r a d i c a l - c h a i n r e a c t i o n propagated a c r o s s the polymer c h a i n s u t i l i z i n g pre-formed hydrogen bonds, c o n s i s t e n t w i t h the h i g h r a d i a t i o n s e n s i t i v i t y ( 6 1 ) . +

Acknowledgments The a u t h o r s a r e i n d e b t e d t o Β. T. C h a i t , R. B. Cody, J r . , R. J . C o t t e r , F. H. F i e l d , T. Gâumann, S. G h a d e r i , D. F. Hunt, D. P. L i t t l e j o h n , R. D. M a c f a r l a n e , C. J . McNeal, P. R o e p s t o r f f , B. U. R. S u n d q v i s t , and J . C. Tabet f o r h e l p f u l a d v i c e , and t o the N a t i o n a l I n s t i t u t e s o f H e a l t h (Grant GM16609) and the Army Research O f f i c e (Grant DAAL03-86-K-0088) f o r generous f i n a n c i a l support. Literature Cited 1.

2. 3. 4.

5. 6. 7. 8. 9. 10. 11. 12.

Jonsson, G. P.; Hedin, A. B.; Håkansson, P. L . ; Sundqvist, B. U. R.; Säve, B. G. S.; Nielsen, P. F . ; Roepstorff, P.; Johansson, K . - E . ; Kamensky, I . ; Lindberg, M. S. L. Anal. Chem. 1986, 58, 1084-1087. McLafferty, F. V. Interpretation of Mass Spectra, Third Edition; University Science Books: Mill Valley, CA, 1980. Facchetti, S., Ed. Mass Spectrometry of Large Molecules; Elsevier: Amsterdam, 1985. Burlingame, A. L . ; Castagnoli, Ν., Jr., Eds., Mass Spectro­ metry in the Health and Life Sciences; Elsevier: Amsterdam, 1985. McLafferty, F. W., Ed., Tandem Mass Spectrometry; John Wiley: New York, 1983. Coutant, J . E . ; McLafferty, F. W. Int. J . Mass Spectrom. Ion Phys. 1972, 8, 323-339. Boerboom, A. J . H. In ref. 5, Chapter 11. Macfarlane, R. D. Anal. Chem. 1983, 55, 1247A-1264A. Chait, B. T . ; Field, F. H. J . Am. Chem. Soc. 1984, 106, 1931-1938. Pannell, L. K.; Sokoloski, Ε. Α.; Fales, Η. M.; Tate, R. L. Anal. Chem. 1985, 57, 1060-1067. Alai, M.; Demirev, P.; Fenselau, C . ; Cotter, R. J . Anal. Chem. 1986, 58. 1303-1307. Benninghoven, Α., Ed. Ion Formation from Organic Solids; Springer-Verlag: Berlin, 1983.

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

7. McLAFFERTY ET AL. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40.

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Benninghoven, Α.; Niehuis, E . ; Friese, T . ; Greifendorf, D.; Steffens, P. Org. Mass Spectrom. 1984, 19, 346. Benninghoven, Α.; Wollnik, H . , Private Communications, 1986. Walter, Κ.; Boesl, U.; Schlag, E. W. Int. J . Mass Spectrom. Ion Processes, submitted. Haddon, W. F . ; McLafferty, F. W. Anal. Chem. 1969, 41, 3136. Comisarow, M. B.; Marshall, A. G. Chem. Phys. Lett. 1974, 25, 282-283. Gross, M. L.; Rempel, D. L. Science 1984, 226, 261-268. Marshall, A. G. Acc. Chem. Res. 1985, 18, 316-322. Cody, R. B., Jr.; Kinsinger, J . Α.; Ghaderi, S.; Amster, I. J.; McLafferty, F. W.; Brown, C. E. Anal. Chim. Acta 1985, 178, 43-66. Laude, D. Α., Jr.; Johlman, C. L.; Brown, R. S.; Weil, D. Α.; Wilkins, C. L. Mass Spectrom Rev 1986 5 107-166 Russell, D. H. Mas Hunt, D. F . ; Shabanowitz Russell, D. H.; Castro, M. E. Proc. Natl. Acad. Sci. U.S.A., 1987, 84, 620-623. Lange, W.; Jirikowsky, M.; Benninghoven, A. Surf. Sci. 1984, 136, 419-436. Castro, M. E.; Russell, D. H. Anal. Chem. 1984, 56, 578-581. Castro, M. E.; Mallis, L. M.; Russell, D. H. J . Am. Chem. Soc. 1985, 107, 5652-5657. Wandass, J . H.; Gardella, J . A. J . Am. Chem. Soc. 1985, 107, 6192-6195. Amster, I. J.; Loo, J. Α.; Furlong, J . J . P.; McLafferty, F. W. Anal. Chem. 1987, 59, 313-317. McCrery, D. Α.; Ledford, E. G.; Gross, M. L. Anal. Chem. 1982, 54, 1435-1437. Cotter, R. J.; Tabet, J . - C . Int. J . Mass Spectrom. Ion Phys. 1983, 53, 151. Cody, R. B.; Amster, I. J.; McLafferty, F. W. Proc. Natl. Acad. Sci. U.S.A. 1985, 82, 6367-6370. Hunt, D. F . ; Shabanowitz, J.; Yates, J . R., III; Mclver, R. T . , J r . ; Hunter, R. L.; Syka, J . E. P.; Amy, J. Anal. Chem. 1985, 57, 2728-2733. Kofel, P.; Allemann, M.; Kellerhals, H. P.; Wanczek, K. P. Adv. Mass Spectrom. 1986, 10, 885-886. Ghaderi, S.; Littlejohn, D. P. Adv. Mass Spectrom. 1986, 10, 875. Amster, I. J.; McLafferty, F. W.; Castro, M. E.; Russell, D. H.; Cody, R. B., Jr.; Ghaderi, S. Anal. Chem. 1986, 58, 483485. Brown, I. G.; Galvin, J . E . ; Gavin, B. F . ; MacGill, R. A. Rev. Sci. Instrum. 1986, 57, 1069-1084. Tabet, J . C . ; Rapin, J.; Poretti, M.; Gaumann, T. Chimia 1986, 40, 169-171. Viswanadham, S. K.; Hercules, D. M.; Weller, R. R.; am, C. S. Biomed. Environ. Mass Spectrom. 1987, 14, 43-45. Loo, J. Α.; Williams, E. R.; Amster, I. J.; Furlong, J . J. P.; Wang, B. H.; McLafferty, F. W.; Chait, B. T . ; Field, F. H. Anal. Chem., accepted. McLafferty, F. W.; Bente, P. F . , III; Kornfeld, R.; Tsai, S.C.; Howe, I. J . Am. Chem. Soc. 1973, 95, 2120. In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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126 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61.

McLafferty, F. V.; Kornfeld, R.; Haddon, W. F . ; Levsen, K.; Sakai, I . ; Bente, P. F . , III; Tsai, S.-C.; Schuddemage, H. D. R. J . Am. Chem. Soc. 1973, 95, 3886-3892. Dawson, P. H.; Douglas, D. J. In ref. 5, Chapter 6. Todd, P. J.; McLafferty, F. W. In ref. 5, Chapter 7. McLafferty, F. W.; Proctor, C. J. Org. Mass Spectrom. 1983, 18, 272. McLuckey, S. Α.; Cooks, R. G. In ref. 5, Chapter 15. Carlin, T. J.; Freiser, B. S. Anal. Chem. 1983, 55, 571-574. Mabud, Μ. Α.; DeKrey, M. J.; Cooks, R. G. Int. J . Mass Spectrom. Ion Proc. 1985, 67, 285-294. DeKrey, M. J.; Kenttamaa, Η. I . ; Wysocki, Β. H.; Cooks, R. G. Org. Mass Spectrom. 1986, 21, 193-195. McLafferty, F. W.; Williams, E. R. Proc. 1986 Conf. Chemical Defense Res., Aberdeen Proving Ground, MD, 1987. Cody, R. B.; Freiser, B. S. Anal. Chem. 1979, 51, 547-551. Cody, R. B.; Freiser Bowers, W. D.; Delbert Chem. 1986, 58, 969-972. Amster, I. J.; Baldwin, Μ. Α.; Chang, M. T . ; Proctor, C. J.; McLafferty, F. W. J . Am. Chem. Soc. 1983, 105, 1654-1655. Neumann, G. M.; Derrick, P. J . Org. Mass Spectrom. 1984, 19, 165-170. Bunker, D. L.; Wang, F.-M. J . Am. Chem. Soc. 1977, 99, 74577459. Biemann, K. Anal. Chem. 1986, 58, 1288A-1300A. McLafferty. F. W. In Mass Spectrometry in the Analysis of Large Molecules; McNeal, C. J., Ed.; John Wiley: New York, 1986, pp 107-120. Wesdemiotis, C . ; Danis, P. O.; Feng, R.; Tso, J.; McLafferty, F. W. J . Am. Chem. Soc. 1985, 107, 8059-8066. Ingemann, S.; Nibbering, Ν. M. M. J . Org. Chem. 1983, 48, 183-191. McLafferty, F. W.; Stauffer, D. B.; Loh, S. Y.; Williams, E. R., Anal. Chem., accepted. Loo, J. Α.; Wang, B. H . ; Wang, F. C.-Y.; McLafferty, F. W.; Klymko, P. Macromolecules 1987, 20, 698-700.

RECEIVED May 12, 1987

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 8

Analytical Applications of Laser Desorption-Fourier Transform Mass Spectrometry for Nonvolatile Molecules R. S. Brown and C. L. Wilkins Department of Chemistry, University of California, Riverside, CA 92521

Laser desorptio (LD-FTMS) result mers are presented. Successful production of molecular ions of peptides with masses up to ~2000 amu is demonstrated. The amount of structurally useful fragmentation diminishes rapidly with increasing mass. Preliminary results of laser photodissociation experiments in an attempt to increase the available structural information are also presented. The synthetic biopolymer poly(phenylalanine) is used as a model for higher molecular weight peptides and produces ions approaching m/z 4000. Current instrument resolution limits are demonstrated utilizing a poly(ethyleneglycol) polymer, with unit mass resolution obtainable to almost 4000 amu.

L a s e r d e s o r p t i o n F o u r i e r t r a n s f o r m mass s p e c t r o m e t r y (LD-FTMS) has emerged an an a l t e r n a t i v e s o f t i o n i z a t i o n method f o r samples w h i c h a r e d i f f i c u l t t o a n a l y z e by more c o n v e n t i o n a l mass s p e c t r o m e t r i c methods. S i n c e i t s i n i t i a l d e m o n s t r a t i o n (J_) f o r o r g a n i c samples, t h e r e has been s t e a d y p r o g r e s s i n a p p l i c a t i o n s o f LD-FTMS t o a wide v a r i e t y o f samples. These have i n c l u d e d such d i v e r s e s u b s t a n c e s as s i m p l e p e p t i d e s ( 2 , 3 ) , sugars ( 4 ) , polymers ( 2 , 5 , 6 ) , p o r p h y r i n s (7,8) and o l i g o s a c c h a r i d e s (9,10) o f v a r i o u s m o l e c u l a r w e i g h t s . A l t h o u g h s t i l l a c o m p a r a t i v e l y new t e c h n i q u e , f o r many samples LD-FTMS compares f a v o r a b l y w i t h more e s t a b l i s h e d i o n i z a t i o n methods such as f a s t atom bombardment (FAB) and f i e l d d e s o r p t i o n ( F D ) , t h e former h a v i n g become the method o f c h o i c e f o r most n o n - v o l a t i l e p o l a r samples. LD-FTMS o f f e r s s e v e r a l advantages over FAB, i n c l u d i n g the l a c k o f need f o r a l i q u i d m a t r i x , w h i c h i s a source of h i g h c h e m i c a l background i n the FAB method. A d d i t i o n a l l y , the absence o f a l i q u i d m a t r i x removes the n e c e s s i t y t h a t the sample be s o l u b l e . Indeed, i n s o l u b l e samples can be a p p l i e d d i r e c t l y i n s o l i d form t o a sample probe and a n a l y z e d by LD-FTMS w i t h good r e s u l t s ( 6 ) . Many o r g a n o m e t a l l i c s , m e t a l l o p o r p h y r i n s and polymers do n o t y i e l d s p e c t r a when examined by 0097-6156/87/0359-0127$06.00/0 © 1987 American Chemical Society

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FAB-MS, due t o t h e i r i n s o l u b i l i t y i n the l i q u i d m a t r i x . Thus, LD-FTMS s h o u l d be c o n s i d e r e d complementary t o FAB-MS. Due t o the r e l a t i v e l y s m a l l number of l a b o r a t o r i e s equipped w i t h LD-FTMS, the method i s not as w e l l known or as h i g h l y d e v e l ­ oped as FAB i o n i z a t i o n which i s now a v a i l a b l e f o r most commercial magnetic s e c t o r and quadrupole i n s t r u m e n t s . The p u l s e d n a t u r e of the l a s e r s t y p i c a l l y employed are i d e a l l y matched w i t h mass s p e c t r o m e t e r s c a p a b l e of s i m u l t a n e o u s d e t e c t i o n or r a p i d a n a l y s i s . Thus FTMS or time of f l i g h t (TOF) i n s t r u m e n t s are best s u i t e d f o r LD a p p l i c a t i o n s . However, the l a t t e r possesses l i m i t e d mass r e s o ­ l u t i o n . T h i s c h a p t e r w i l l attempt t o p r e s e n t the c u r r e n t c a p a b i ­ l i t i e s and l i m i t a t i o n s of LD-FTMS and t o address i t s f u t u r e potential. Experimental The g e n e r a l e x p e r i m e n t a s e n t e d i n d e t a i l elsewher be p r o v i d e d h e r e . For sample d e s o r p t i o n / i o n i z a t i o n , a p u l s e d c a r ­ bon d i o x i d e l a s e r i s f o c u s s e d t o a p p r o x i m a t e l y a 1 mm diameter spot v i a a ZnSe l e n s onto a s t e e l c i r c u l a r probe t i p (10 mm diame­ t e r ) a t t a c h e d t o a d i r e c t i n s e r t i o n probe h o l d i n g the sample. F o l l o w i n g the l a s e r p u l s e , an a p p r o p r i a t e d e l a y ( u s u a l l y 3-5 seconds) i s i n t r o d u c e d t o a l l o w the h i g h l o c a l p r e s s u r e ( ΐ ο - Μ ο t o r r u n c o r r e c t e d i o n i z a t i o n gauge r e a d i n g ) of desorbed n e u t r a l s to be pumped away, l e a v i n g the i o n s w h i c h are t r a p p e d t o be d e t e c t e d w i t h improved r e s o l u t i o n . Such d e l a y s s h o u l d be s u b s t a n t i a l l y reduced i n the newer d i f f e r e n t i a l l y pumped d u a l c e l l d e s i g n . S u b s e q u e n t l y , the i o n s a r e e x c i t e d t o a l l o w t h e i r image c u r r e n t s to be r e c o r d e d u s i n g a h i g h speed a n a l o g t o d i g i t a l (A/D) con­ v e r t e r . The t r a n s i e n t s i g n a l thus o b t a i n e d i s then a p o d i z e d and F o u r i e r t r a n s f o r m e d t o produce the mass spectrum f o r the i o n s r e s u l t i n g from a s i n g l e l a s e r p u l s e . A l t h o u g h s i g n a l a v e r a g i n g may be employed, the c u r r e n t s p e c t r a a l l r e s u l t from s i n g l e l a s e r p u l s e s u s i n g ~ l - 5 μg of sample. No d e f i n i t i v e comparisons of the e f f e c t s of l a s e r s o p e r a t i n g a t d i f f e r e n t wavelengths f o r l a s e r d e s o r p t i o n have been r e p o r t e d . For p h o t o d i s s o c i a t i o n e x p e r i m e n t s , a s p e c i a l probe was c o n s t r u c t e d t o r e p l a c e the 12.7 mm d i a m e t e r d i r e c t i n s e r t i o n probe n o r m a l l y employed. I t c o n s i s t s of a h o l l o w s t a i n l e s s s t e e l tube w h i c h has a 38 mm f o c a l l e n g t h q u a r t z l e n s vacuum s e a l e d a t one end and an extended h o l l o w probe t i p a t the o t h e r . The beam of a Lambda P h y s i k exciraer l a s e r o p e r a t i n g a t 308 nm was passed through the probe and l e n s i n t o the FTMS c e l l t h r o u g h a s m a l l h o l e i n the c e n t e r of the t r a p p l a t e as shown i n F i g u r e 1. - 6

Sample P r e p a r a t i o n 50-100 μg of each p e p t i d e was d e p o s i t e d onto the s t a i n l e s s s t e e l probe t i p from a m e t h a n o l / a c e t i c a c i d s o l u t i o n (1 drop a c e t i c a c i d i n 1 ml of methanol) r e s u l t i n g i n sample consumption of ~1 μg per l a s e r p u l s e based on a 1 mm l a s e r spot and u n i f o r m sample coverage. S i m i l a r amounts of the o t h e r samples were a p p l i e d from a methanol s o l u t i o n . S e n s i t i v i t y i s v e r y dependent upon sample c o m p o s i t i o n and the r e s u l t s p r e s e n t e d a r e t y p i c a l f o r these 2

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c l a s s e s o f compounds. P o l y ( p h e n y l a l a n i n e ) was i n s o l u b l e and was d e p o s i t e d from a methanol s u s p e n s i o n produced by s o n i c a t i o n . A p p r o x i m a t e l y 500 μg o f p o l y ( p h e n y l a l a n i n e ) was r e q u i r e d t o i n s u r e adequate coverage o f t h e probe t i p , due t o t h e i r r e g u l a r s u r f a c e produced as a r e s u l t o f e v a p o r a t i o n o f t h e s o l v e n t . R e s u l t s and D i s c u s s i o n P e p t i d e s . One o f t h e a p p l i c a t i o n s t h a t has b e n e f i t e d most from the development o f s o f t i o n i z a t i o n methods f o r mass s p e c t r o m e t r y has been the a n a l y s i s o f p o l a r b i o l o g i c a l m a t e r i a l s . P e p t i d e s , i n p a r t i c u l a r , have been e x t e n s i v e l y s t u d i e d by a v a r i e t y o f s o f t i o n i z a t i o n methods, p r i m a r i l y FAB, w i t h e x c e l l e n t r e s u l t s b e i n g o b t a i n e d f o r p e p t i d e s i n t h e 1000-10000 d a l t o n range. LD-FTMS a l s o has proved t o be v e r y s u c c e s s f u l f o r a n a l y s i s o f s i m p l e pep­ t i d e s and an e x a m i n a t i o n o f these r e s u l t s s h o u l d h e l p d e l i n e a t e the a n a l y t i c a l p o t e n t i a The LD-FTMS spectru decapeptid Angiotensi i n F i g u r e 2 and i s a good example o f t y p i c a l r e s u l t s f o r p e p t i d e s of t h i s s i z e . An abundant p r o t o n a t e d m o l e c u l a r i o n (M+H) i s o b s e r v e d , as w e l l as t h e N-terrainus sequence i o n s (Zg)" " t o (Zj) w i t h d e c r e a s i n g r e l a t i v e i n t e n s i t y f o r the s m a l l e r fragments; t h e r e a r e no abundant fragment i o n s w i t h m/z lower than 900. A l t h o u g h a low abundance C-terrainus sequence i o n (Ag)" " i s o b s e r v e d , t h e N-terminus i o n s u s u a l l y predominate, i n a d d i t i o n t o fragments r e s u l t i n g from l o s s e s o f n e u t r a l s such as H2O, NH3 and 002· T h i s i s i n c o n t r a s t w i t h FAB r e s u l t s i n w h i c h C-terminus i o n s a r e u s u a l l y observed ( 1 1 ) . As m o l e c u l a r w e i g h t i s i n c r e a s e d , l e s s f r a g m e n t a t i o n i n d i c a ­ t i v e o f t h e p e p t i d e ' s sequence i s o b s e r v e d , as c a n be seen i n t h e spectrum o f t h e u n d e c a p e p t i d e , Substance P, i n F i g u r e 3. Here, the p r o t o n a t e d m o l e c u l a r i o n dominates t h e spectrum w i t h the sodium c a t i o n i z e d s p e c i e s (M+Na) a l s o observed. Fragment i o n s a r e l i m i t e d t o t h e f i r s t sequence i o n from t h e N-terminus as w e l l those a r i s i n g from l o s s e s o f H2O and NH3. ( Z 9 ) and (A9)"*" i o n s a t m/z = 1094 and m/z = 1087 a r e a l s o observed i n low r e l a t i v e abun­ dance, as i s an i o n which may a r i s e from a minor i m p u r i t y a t m/z 1104. T h i s r e d u c t i o n i n u s e f u l f r a g m e n t a t i o n a t h i g h e r masses i s a l s o observed i n FAB-MS. Use o f h i g h e r l a s e r power d e n s i t y i n c r e a s e s s t r u c t u r a l l y r e l e v a n t f r a g m e n t a t i o n a t t h e expense o f the m o l e c u l a r i o n and r e s u l t s i n lower s i g n a l t o n o i s e r a t i o s (i_*e_. decreased s e n s i t i v i t y ) . The e f f e c t s o f l a s e r power on p o r p h y r i n f r a g m e n t a t i o n a l s o have been r e p o r t e d CO. The LD spectrum o f t h e t r i d e c a p e p t i d e n e u r o t e n s i n ( F i g u r e 4 ) , shows s e v e r a l d i f f e r e n c e s . Here f r a g m e n t a t i o n r e s u l t s almost e x c l u s i v e l y from s m a l l n e u t r a l l o s s e s . A p r o t o n a t e d m o l e c u l a r i o n i s s t i l l one o f t h e major i o n s observed (m/z = 1673) and some c a t i o n i z e d ( K ) fragment s p e c i e s a r e apparent as w e l l as a s m a l l amount o f c a t i o n i z e d m o l e c u l a r i o n (1000 amu) i s q u e s t i o n a b l e due t o the preponderance o f m o l e c u l a r c o m p o s i t i o n s a t these masses w h i c h a r e w i t h i n even a 1 ppm e r r o r . +

+

2

2

Conclusions The a n a l y t i c a l a p p l i c a t i o n s o f LD-FTMS have j u s t begun t o be developed. As more work i s done, LD-FTMS w i l l c o n t i n u e t o be a p p l i e d t o a n a l y s i s o f d i f f i c u l t samples. I t i s a p r o m i s i n g t e c h n i q u e f o r s t a b l e samples such as p o l y m e r s , o r g a n o m e t a l l i c s and

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

FOURIER TRANSFORM MASS SPECTROMETRY

138

(A)

443

r

4

N(CH CH ) 2

3

(M-CI)

2

(M-HCI*Naj] 465

481

ο 150

200

250

r

, M i

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ι

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Γ 400

300 350 MASS IN A.M.U.

450

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(Β)

(M-Clf-(HCOOH) ,397

-(C H ) -(C H )N(C H ) 326 ?

2

5

2

443

4

4

465 48I ο . 200

150

250 SS IN A

F i g u r e 9·

m .

4 0 0

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( a ) L a s e r d e s o r p t i o n p o s i t i v e i o n spectrum o f Rhodamine Β u t i l i z i n g o n l y t h e CO2 l a s e r w i t h e j e c t i o n o f a l l i o n s below m/z = 443. (b) P o s i t i v e i o n spectrum as i n ( a ) but w i t h UV i r r a ­ d i a t i o n (308 nm) a f t e r t h e pump down d e l a y .

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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p o r p h y r i n s , as w e l l as many i n s o l u b l e samples. C o n t i n u e d work i n the a r e a i s e x p e c t e d t o improve b o t h t h e mass range and s e n s i t i v i t y w h i l e b e t t e r i n s t r u m e n t a t i o n and u n d e r s t a n d i n g o f t h e i o n dynamics o f FTMS s h o u l d r e s u l t i n t h e r e a l i z a t i o n o f the h i g h r e s o l u t i o n promise o f FTMS a t h i g h mass. G i v e n i t s p r e s e n t s t a t e of development, LD-FTMS c a n p r o v i d e u n i q u e c a p a b i l i t i e s f o r s o l u t i o n o f a v a r i e t y o f a n a l y t i c a l problems. Acknowledgments Support from t h e N a t i o n a l I n s t i t u t e s GM-30604 i s g r a t e f u l l y acknowledged.

o f H e a l t h under g r a n t

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

15.

McCrery, D.A.; Ledford E . G . Jr.; Gross M.L Anal Chem 1982, 54, 1435-1437 Wilkins, C . L . ; Weil Chem. 1985, 57, 520-524. Wilkins, C . L . ; Yang, C.L.C. Int. J. Mass Spectrom. Ion Processes 1986, 72, 195-208. McCrery, D.A.; Gross, M.L. Anal. Chim. Acta, 1985, 178, 103-116. Brown, R.S.; Weil, D.A.; Wilkins, C.L. Macromolecules 1986, 19, 1255-1260. Brown, C.E.; Kovacic, P.; Wilke, C . E . ; Cody, R.B.; Kinsinger, J.A. J . Polymer Sci. Poly. Lett. E d . , 1985, 23, 453-463. Brown, R.S.; Wilkins, C.L. Anal. Chem. 1986, 58, 3196-3199. Brown, R.S.; Wilkins, C.L. J. Am. Chem. Soc. 1986, 108, 2447-2448. Coates, M.L.; Wilkins, C.L. Biomed. Mass Spectrom. 1985, 12, 424-428. McCrery, D.A.; Gross, M.L. Anal. Chim. Acta, 1985, 178, 91-103. König, W.A.; Aydin, M.; Schulze, V.; Rapp, V . ; Hohn, M.; Pesch, R.; Kalikhevitch, V.N. Int. J. Mass Spectrom. Ion Phys. 1983, 46, 403-406. Cody, R.B., J r . ; Amster, I.J.; McLafferty, F.W. Proc. Natl. Acad. Sci. USA 1985, 82, 6367-6370. Alai, M.; Demirev, P.; Fenselau, C . ; Cotter, R. Anal. Chem. 1986, 58, 1303-1307. Jonsson, G.P.; Hedin, A.B.; Hakansson, P . L . ; Sundquist, B.U.; Save, G.B.S.; Nielsen, P . F . ; Roepstorff, P.; Johansson, K . E . ; Kamensky, I . ; Lindberg, M.S.L. Anal. Chem. 1986, 58, 1084-1087. Bowers, W.D.; Delbert, S.S.; McIver, R.T. Anal. Chem. 1986, 58, 969-972.

RECEIVED September 14, 1987

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 9

Infrared Multiphoton Dissociation of Laser-Desorbed Ions 1

Clifford H . Watson, Gökhan Baykut , and John R. Eyler Department of Chemistry, University of Florida, Gainesville, FL 32611 Output from both gated continuous wave and pulsed carbon dioxide laser surfaces and the transform ion cyclotron resonance mass spectrometer. Pulsed CO laser irradiation was most successful in laser desorption experiments, while a gated continuous wave laser was used for a majority of the successful infrared multiphoton dissociation studies. Fragmentation of ions with m/z values in the range of 400-1500 daltons was induced by infrared multiphoton dissociation. Such photodissociation was successfully coupled with laser desorption for several different classes of compounds. Either two sequential pulses from a pulsed carbon dioxide laser (one for desorption and one for dissociation), or one desorption pulse followed by gated continuous wave irradiation to bring about dissociation was used. 2

The t e c h n i q u e of l a s e r d e s o r p t i o n (LD) has been w i d e l y used i n mass s p e c t r o m e t r y (J_,_2) to d e s o r b and to i o n i z e h i g h m o l e c u l a r weight o r o t h e r n o n v o l a t i l e samples, most o f t e n u s i n g t i m e - o f - f l i g h t (3_,_£) or F o u r i e r t r a n s f o r m i o n c y c l o t r o n resonance (FTICR) (5_,_6j mass s p e c t r o m e t e r s f o r mass a n a l y s i s . In t h i s t e c h n i q u e , h i g h l y f o c u s s e d l a s e r i r r a d i a t i o n , most o f t e n w i t h a power d e n s i t y of a t l e a s t 1 0 W / c m , i s used to desorb and i o n i z e a s o l i d sample t h a t has been i n s e r t e d i n t o the h i g h vacuum system of the mass s p e c t r o m e t e r . The p r i m a r y advantage o f l a s e r d e s o r p t i o n i s t h a t abundant m o l e c u l a r or p s e u d o m o l e c u l a r i o n s are produced f o r many d i f f e r e n t c l a s s e s of compounds. P o s i t i v e p s e u d o m o l e c u l a r i o n s are most o f t e n formed by attachment o f a c a t i o n , t y p i c a l l y a p r o t o n , p o t a s s i u m , o r sodium i o n , to the p a r e n t m o l e c u l e . Negative pseudomolecular ions can a l s o be formed by l a s e r d e s o r p t i o n , u s u a l l y by l o s s o f a proton, or by e l e c t r o n attachment t o m o l e c u l e s w i t h a p o s i t i v e electron a f f i n i t y . O f t e n l i t t l e f r a g m e n t a t i o n o c c u r s (making t h i s 8

2

Current address: Ion Spec, One Longstreet, Irvine, C A 92714

0097-6156/87/0359-0140S06.00/0 © 1987 American Chemical Society

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

9. WATSON ET AL.

IR Multiphoton Dissocation of Laser-Desorbed Ions

141

one of the " s o f t " i o n i z a t i o n t e c h n i q u e s ) , and the s t r o n g m o l e c u l a r i o n s i g n a l (M , (M + H) , (M + Na) , (M + K) , M~*, or (M H)~) p r o v i d e s m o l e c u l a r weight i n f o r m a t i o n . Although an abundant molecular i o n peak i s important in determination of the m o l e c u l a r weight of a compound, fragmentation is often desirable to characterize the molecular structure. S e v e r a l t e c h n i q u e s have been a p p l i e d t o a c h i e v e f r a g m e n t a t i o n ; most n o t a b l e are c o l l i s i o n i n d u c e d d i s s o c i a t i o n {7) and photodissocia­ t i o n (8_ι_9_) · Thus, i t would be advantageous to combine one of these t e c h n i q u e s w i t h l a s e r d e s o r p t i o n to o b t a i n both m o l e c u l a r w e i g h t and structural information. One p o s s i b l e drawback of collision i n d u c e d d i s s o c i a t i o n i s d i f f i c u l t y i n d i s s o c i a t i n g l a r g e r i o n s (1012) because of an i n a b i l i t y to impart s u f f i c i e n t i n t e r n a l energy to them i n c o l l i s i o n s w i t h much l i g h t e r t a r g e t g a s e s . F o u r i e r t r a n s f o r m ICR mass s p e c t r o m e t r y i s i d e a l l y s u i t e d f o r LD i o n i z a t i o n because, as w i t h t i m e - o f - f l i g h t (TOF) mass spectrom e t e r s , a complete mas pulse. In a d d i t i o n , FTIC t i o n than can be a t t a i n e d by TOF, even a f t e r m o d i f i c a t i o n s t o the TOF i n s t r u m e n t such as the a d d i t i o n o f r e f l e c t i n g g r i d s . H i g h mass r e s o l u t i o n can be q u i t e i m p o r t a n t i n the d e s o r p t i o n and fragmenta­ t i o n of l a r g e m o l e c u l e s , i n c l u d i n g those o f b i o l o g i c a l r e l e v a n c e . E a r l i e r s t u d i e s from our l a b o r a t o r y (13,14) have shown the f e a s i b i l i t y of c o u p l i n g g a t e d c o n t i n u o u s wave (cw) C 0 l a s e r s with an FTICR mass s p e c t r o m e t e r to p h o t o d i s s o c i a t e i o n s t r a p p e d there. The a b i l i t y t o h o l d i o n s i n a c o n s t r a i n e d i r r a d i a t e d volume p e r m i t s f a c i l e p h o t o d i s s o c i a t i o n of i o n s w i t h low to moderate m o l e c u l a r weights. Combination o f these two l a s e r t e c h n i q u e s (LD t o produce i o n s and p h o t o d i s s o c i a t i o n (PD) t o fragment them) was thus o f g r e a t interest. T h i s c h a p t e r d i s c u s s e s the s u c c e s s of t h i s c o u p l i n g when a p p l i e d t o compounds w i t h m o l e c u l a r w e i g h t s i n the range of 100-600 daltons. +

+

+

+

2

Experimental

Section

E x p e r i m e n t s were c a r r i e d out i n a N i c o l e t FT/MS - 1000 mass spec­ trometer. The s t a n d a r d l a s e r d e s o r p t i o n i n t e r f a c e s u p p l i e d by the m a n u f a c t u r e r was used. As shown by the s o l i d l i n e i n F i g u r e 1 , l i g h t from a Lumonics TE 860 g r a t i n g - t u n e d p u l s e d C 0 (infrared) l a s e r entered the vacuum chamber t h r o u g h a ZnSe window and was f o c u s e d by a ZnSe l e n s o f 1.25 cm d i a m e t e r and 5 cm f o c a l l e n g t h onto a s o l i d s probe c o n t a i n i n g the a n a l y t e . An A p o l l o 570 con­ t i n u o u s wave g r a t i n g - t u n e d C 0 ( i n f r a r e d ) l a s e r , gated by a t r i g g e r p u l s e from the FTICR e l e c t r o n i c s , was a l s o employed f o r l a s e r de­ sorption. However, i t s o u t p u t was of i n s u f f i c i e n t power d e n s i t y t o g e n e r a t e i o n s r e l i a b l y and so i t s usage was limited. 2

2

Both the p u l s e d and cw l a s e r s were used f o r p h o t o d i s s o c i a t i o n o f gaseous i o n s produced by l a s e r d e s o r p t i o n . The cw l a s e r had a maximum o u t p u t power o f 50 w a t t s a t 10.61 m i c r o m e t e r s and a beam d i a m e t e r of 6 mm. The p u l s e d l a s e r p r o d u c e d 2.6 j o u l e s i n a p u l s e o f 1 m i c r o s e c o n d d u r a t i o n a t 10.61 m i c r o m e t e r s and had a 2 χ 3 cm r e c t a n g u l a r beam shape. M o d i f i c a t i o n s of the FTICR vacuum chamber t h a t f a c i l i t a t e i o n i r r a d i a t i o n have been r e p o r t e d p r e v i o u s l y ( 1 3 ) . In the "one l a s e r experiment" (Figure 1 ), the ions were d e s o r b e d by a s i n g l e l a s e r p u l s e u s i n g a f o c u s s i n g l e n s . A mirror

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

142

FOURIER TRANSFORM MASS SPECTROMETRY

outside the vacuum chamber was then r o t a t e d so t h a t a second u n f o c u s s e d l a s e r p u l s e e n t e r e d the a n a l y z e r c e l l and i r r a d i a t e d the s t o r e d i o n s . The e x t e r n a l m i r r o r was then r e t u r n e d t o i t s o r i g i n a l p o s i t i o n f o r the next e x p e r i m e n t . U n f o r t u n a t e l y , a very l a r g e K s i g n a l was o b s e r v e d f o r both the f o c u s s e d and u n f o c u s s e d p u l s e s , and the u n f o c u s s e d l a s e r p u l s e o f t e n desorbed contaminant n e u t r a l s from the c e l l p l a t e o p p o s i t e the beam r e f l e c t o r shown i n F i g u r e 1. Difficulty in eliminating K i o n s and contaminant n e u t r a l s formed d u r i n g the second l a s e r p u l s e , and problems w i t h a l i g n m e n t o f the l a s e r beam when moving the e x t e r n a l m i r r o r s u g g e s t e d the use o f a second l a s e r , the cw C 0 l a s e r , to p h o t o d i s s o c i a t e ions produced by the p u l s e d l a s e r . In the "two l a s e r " experiment, shown i n F i g u r e 2, the c o n v e n t i o n a l N i c o l e t s o l i d s probe was r e p l a c e d by a N a C l window mounted on a hollow s t a i n l e s s s t e e l tube t h a t extended i n t o the vacuum chamber t o w i t h i n a few m i l l i m e t e r s of the FT ICR cell. T y p i c a l l y 1-5 mg of a n a l y t e was d i s s o l v e d i n 3-5 ml o f e i t h e r acetone or e t h a n o deposited using a micropipett from the window and n e a r e s t to the c e l l . T h i s assembly s e r v e d as both sample support and as a second window f o r l a s e r i r r a d i a t i o n o f the i o n s . F o l l o w i n g sample p r e p a r a t i o n , the tube assembly was i n s e r t e d i n t o the vacuum chamber through the s o l i d s probe i n l e t . As shown i n F i g u r e 2, the p u l s e d l a s e r beam was d i r e c t e d i n t o the c e l l u s i n g the same o p t i c s as i n F i g u r e 1 to desorb the i o n s i n the normal manner. The (gated) cw l a s e r beam e n t e r e d the vacuum chamber through the s a l t window mounted on the t u b e . Ions were formed by LD u s i n g f o c u s s e d l i g h t from the p u l s e d l a s e r . After a d e l a y o f 1-3 seconds t o a l l o w the i n i t i a l p r e s s u r e b u r s t due t o d e s o r b e d n e u t r a l s t o d i s s i p a t e , the cw l a s e r i r r a d i a t e d the i o n s f o r 0.5-3 seconds u n t i l s i g n i f i c a n t p h o t o d i s s o c i a t i o n was o b s e r v e d . +

+

2

The hollow tube c o u l d be r o t a t e d , and i t s t i p was d i v i d e d i n t o 8 d i f f e r e n t a r e a s i n which t h e r e was no o v e r l a p o f the p u l s e d l a s e r beam. Samples d e p o s i t e d on each a r e a produced s i g n i f i c a n t i o n s i g n a l f o r a range o f 3-20 l a s e r p u l s e s , depending on the sample and p r e p a r a t i o n c h a r a c t e r i s t i c s . The s t a n d a r d e x p e r i m e n t a l method used i n v o l v e d c o l l e c t i o n of an LD, an LD/PD, and a second LD mass spectrum. T h i s p r o c e d u r e p r o v i d e d a r e f e r e n c e spectrum w i t h no p h o t o d i s s o c i a t i o n both b e f o r e and a f t e r the LD/PD spectrum, and e l i m i n a t e d the p o s s i b i l i t y of o b s e r v e d photofragments o r i g i n a t i n g from some unknown s o u r c e . Photodissociation of l a r g e r ions (>200 d a l t o n s ) formed by e l e c t r o n impact was also studied. These i o n s were produced by electron i o n i z a t i o n of vapor s u b l i m e d from the s t a n d a r d heated solids i n s e r t i o n probe. Probe temperatures o f 150-200°C were employed. T h i s p r o c e s s gave r i s e t o a low abundance of m o l e c u l a r ions. T h e r e f o r e a v a r i a b l e l e n g t h (100-1000 ms) r e a c t i v e delay time was utilized t o produce ions by ion/molecule reactions. Fragment i o n s formed by e l e c t r o n impact r e a c t e d w i t h the n e u t r a l background m o l e c u l e s to produce the v a r i o u s i o n s o f i n t e r e s t . Samples were o b t a i n e d from commercial s o u r c e s , or a c q u i r e d from v a r i o u s r e s e a r c h l a b o r a t o r i e s . Sample p u r i t y was c o n f i r m e d by wide mass range s p e c t r a , and the samples were used w i t h o u t f u r t h e r purification.

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

WATSON ET AL.

IR Multiphoton Dissocation of Laser-Desorbed Ions 143

beam reflector

2 1

/ν

ZnSe-window

probe

ZnSe-lens ICR-cell

laser

F i g u r e 1. C o n f i g u r a t i o n used f o r s i n g l e l a s e r experiments w i t h a p u l s e d CO2 l a s e r . D u r i n g t h e f i r s t l a s e r p u l s e , t h e beam was f o c u s s e d t h r o u g h t h e l e n s and formed i o n s by l a s e r d e s o r p t i o n . A r o t a t a b l e m i r r o r d i r e c t e d t h e second, u n f o c u s s e d l a s e r beam i n t o t h e c e l l t o d i s s o c i a t e t r a p p e d i o n s by m u l t i p h o t o n d i s s o c i ­ ation. (Reproduced from r e f . 18. C o p y r i g h t 1987 American Chemical Society.)

laser 2

D

laser 1 ZnSe-lens ZnSe-window

0

t c

=

=

ICR-cell

probe / NaCI-window

F i g u r e 2. C o n f i g u r a t i o n used f o r double l a s e r e x p e r i m e n t . A p u l s e d CO l a s e r ( l a s e r 1) was used t o desorb and i o n i z e t h e sample ana a cw CO2 l a s e r ( l a s e r 2) was used t o d i s s o c i a t e t h e trapped i o n s . (Reproduced from r e f . 18. C o p y r i g h t 1987 American Chemical Society.)

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

144

FOURIER TRANSFORM MASS SPECTROMETRY

R e s u l t s and

Discussion

Photodissociation of large ions. Before attempting photod i s s o c i a t i o n o f moderate m o l e c u l a r weight i o n s formed by laser desorption, i n i t i a l e x p e r i m e n t s were c a r r i e d out to v e r i f y t h a t i o n s i n the 500-1500 d a l t o n range o f m o l e c u l a r weight c o u l d , i n fact, be made t o d i s s o c i a t e by the absorption of laser irradiation. These l a r g e r i o n s , w i t h t h e i r many degrees of freedom and h i g h e r d e n s i t y of s t a t e s , are e x p e c t e d to appear " b l a c k " i n the i n f r a r e d r e g i o n ; t h a t i s , t h e i r i n f r a r e d a b s o r p t i o n spectrum s h o u l d be n e a r l y c o n t i n u o u s , l e a d i n g to a h i g h p r o b a b i l i t y t h a t they w i l l absorb i n f r a r e d photons. The q u e s t i o n o f whether i n t e r n a l e x c i t a ­ t i o n produced by i n f r a r e d a b s o r p t i o n would be r e l a x e d by r a d i a t i v e or c o l l i s i o n a l p r o c e s s e s f a s t e r than such energy might accumulate i n c e r t a i n modes l e a d i n g t o bond r u p t u r e was of great i n t e r e s t . Three compounds were s t u d i e d u s i n g e l e c t r o n impact i o n i z a t i o n t o see i f e a r l i e r work (13,14 200 d a l t o n range c o u l d i h i g h e r m/z v a l u e s . An i o n o f nominal m/z 442 was produced from c i s - d i c h l o r o trans-dihydroxo-bis-2-propanamine/platinum (IV) (CHIP) by ion/molecule reactions (IMR) o f the major e l e c t r o n impact (EI) fragment i o n w i t h the n e u t r a l m o l e c u l e . As shown i n F i g u r e 3, t h i s i o n d i s s o c i a t e d when i r r a d i a t e d by the cw l a s e r f o r c a . 500 ms t o produce two daughter photofragment i o n s , m/z 366 and m/z 311, by l o s s of v a r i o u s l i g a n d s ( E q u a t i o n 1 ) . m/z

441

+ nhv

+ m/z

366

and

m/z

311

(1)

Shown i n F i g u r e 4 i s the p h o t o d i s s o c i a t i o n o f p r o t o n a t e d Ν,Ν'bis(4,6-dimethoxysalicylidene)-4-trifluoromethyl-o-phenylenediimina t o c o b a l t ( I I ) (CoSALOPH), which i s a l s o produced by ion/molecule r e a c t i o n s o f e l e c t r o n impact fragments w i t h the n e u t r a l m o l e c u l e . T h i s i o n d i s s o c i a t e d by l o s s o f a methyl group ( E q u a t i o n 2 ) . m/z

562

+ nhv

+ m/z

547

(2)

The i o n produced by e l e c t r o n attachment t o t r i s ( p e r f l u o r o - n n o n y l ) t r i a z i n e d i s s o c i a t e d when i r r a d i a t e d by l i g h t from the cw l a s e r as shown i n F i g u r e 5 ( E q u a t i o n 3 ) . m/z

1485

m/z

1066

(3)

T h i s i s the h i g h e s t m o l e c u l a r weight compound f o r which i n f r a ­ r e d m u l t i p h o t o n d i s s o c i a t i o n has been o b s e r v e d i n our l a b o r a t o r i e s to date. A s i m i l a r pathway was i n d i c a t e d by the c o l l i s i o n - i n d u c e d d i s s o c i a t i o n spectrum o f t h i s i o n . L i m i t e d CID e x p e r i m e n t s were p e r f o r m e d on some of the o t h e r l a s e r - d e s o r b e d i o n s d i s c u s s e d here and i n g e n e r a l the CID and PD pathways were the same. Use of a s i n g l e l a s e r f o r d e s o r p t i o n / d i s s o c i a t i o n . Photodissoci­ a t i o n o f s m a l l gaseous i o n s u s i n g a p u l s e d CC>2 l a s e r has been reported (15), so an attempt was made both t o form i o n s and to p h o t o d i s s o c i a t e them w i t h s e q u e n t i a l l a s e r p u l s e s from the same p u l s e d l a s e r u s i n g the i r r a d i a t i o n scheme shown i n F i g u r e 1. As

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

9. WATSON ET AL.

IR Multiphoton Dissocation of Laser-Desorbed Ions 145

cis-dichloro- tronsdihydroxobis-2-propanomineplatirKjm ( IV)

R(0H) (CI)(C H N) * 2

3

9

3

,442 (CH ) CH-NH 3

2

2 n

OH I /Cl

(CH^CH-NH/^CI OH

444

200

250

300

200

250

300

350

400

450

500

350

400

450

500

MASS IN A. M. U

F i g u r e 3. Top. Ion/molecule r e a c t i o n product formed by r e a c t i o n of c i s - d i c h l o r o - t r a n s - d i h y d r o x o - b i s - 2 - p r o p a n a m i n e p l a t i n u m ( I V ) w i t h i t s 50 eV e l e c t r o n impact fragments. Bottom. D i s s o c i a t i o n p a t t e r n produced upon i r r a d i a t i o n w i t h cw CO2 l a s e r , same experi m e n t a l c o n d i t i o n s and d e l a y times as i n t h e t o p spectrum. (Reproduced from r e f . 18. C o p y r i g h t 1987 American Chemical Society.)

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

146

FOURIER TRANSFORM MASS SPECTROMETRY CoSALOPH derivative, electron impact

(M + l )

Ô C = N H.CCI

/

ί >

H CO

Ν

Λ

/

,N=C

\

< )

P C H ,

OCH

3

Ι

3

1 400

450

+

500

1

550

600

CoSALOPH derivative, electron impact/photodissociation

((M + I)-CH ) 3

+

(M + l)

400

450

500 MASS IN A M U

+

550

f

600

F i g u r e 4. Top. P r o t o n a t e d N , N - b i s ( 4 , 6 - d i m e t h y l s a l i c y l i d i n e ) 4 - t r i f l u o r o m e t h y l - o - p h e n y l e n e d i i m i n a t o c o b a l t ( I I ) (CoSALOPH) formed by r e a c t i o n o f t h e n e u t r a l m o l e c u l e w i t h fragment i o n s produced by e l e c t r o n impact a t 50 eV. Bottom. D i s s o c i a t i o n o b t a i n e d upon i r r a d i a t i o n w i t h t h e cw C 0 l a s e r , same e x p e r i m e n t a l c o n d i t i o n s and d e l a y times as i n t h e top spectrum. (Reproduced from r e f . 18. C o p y r i g h t 1987 American Chemical Society.) 2

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

WATSON ET AL.

IR Mxdtiphoton Dissocation of Laser-Desorbed Ions

Tris ( per f luorononyl) -s-trioz ine,

M"

electron impact, negative ions

N ^ N

600

800

1000

1200

Ji

1400

Tris(perf luorononyl)-s-triazine, e.i. /photodissociation

600

800

(M-CeW

1000 MASS IN A . M . U .

1200

1400

F i g u r e 5. Top, N e g a t i v e m o l e c u l a r i o n o f t r i s - n - n o n y l f l u o r o t r i a z i n e formed by c a p t u r e o f t h e r m a l e l e c t r o n s . Bottom. D i s s o c i a t i o n o b t a i n e d upon i r r a d i a t i o n w i t h cw l a s e r , same e x p e r i m e n t a l c o n d i t i o n s and d e l a y t i m e s as i n t h e t o p spectrum. (Reproduced from r e f . 18. C o p y r i g h t 1987 American Chemical Society.)

American Chemical Society Library Î155 1 6th St, N.W . In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: DC, 1987. Washin gton, O.a 20Washington, 036

148

FOURIER TRANSFORM MASS SPECTROMETRY

r e p o r t e d i n the e x p e r i m e n t a l section, a large Κ s i g n a l always appeared d u r i n g the p u l s e d l a s e r d e s o r p t i o n e x p e r i m e n t . The i n t e n ­ s i t y of the K s i g n a l was reduced somewhat by a d j u s t i n g v a r i o u s d e l a y times, and e j e c t i n g t h i s i o n d u r i n g the l a s e r beam d u r a t i o n and f o r 50 ms f o l l o w i n g the end of the l a s e r p u l s e . Time r e s o l v e d LD mass s p e c t r o m e t r y (_4) has shown t h a t s i g n i f i c a n t amounts o f K a r e formed f o r up t o 30 m i l l i s e c o n d s f o l l o w i n g the l a s e r d e s o r p t i o n pulse. Under these e j e c t i o n c o n d i t i o n s , p h o t o d i s s o c i a t i o n of the t - b u t y l p y r i d i n i u m c a t i o n (from the p e r c h l o r a t e s a l t ) produced by l a s e r d e s o r p t i o n was observed. As shown i n F i g u r e 6 the t b u t y l p y r i d i n i u m c a t i o n , m/z 136, d i s s o c i a t e s under p u l s e d laser i r r a d i a t i o n w i t h l o s s o f C^H t o produce an i o n o f m/z 80, which i s most l i k e l y p r o t o n a t e d p y r i d i n e ( E q u a t i o n 4 ) . +

+

Q

m/z

136

+ nhv

m/z

80

(4)

The magnitude of the p h o t o d i s s o c i a t i o t h i s case. A new FT ICR t h a t w i l l p e r m i t passage of the l a s e r beam through the c e l l w i t h o u t s t r i k i n g any of the c e l l p l a t e s . G e n t l e f o c u s s i n g o f the p u l s e d l a s e r beam s h o u l d then i n c r e a s e the l a s e r f l u e n c e i n the c e n t e r of the c e l l , and improve p h o t o d i s s o c i a t i o n e f f i c i e n c y . Use of two lasers for desorption/dissociation. Ν,Ν'-bis(4,6dimethoxysalicylidene)-4-trifluoromethyl-o-phenylenediiminato c o b a l t ( I I ) (CoSALOPH): T h i s compound was s e l e c t e d as a t e s t case s i n c e i t s PD pathways were known (see E q u a t i o n 2 and F i g u r e 4) and s i g n i f i c a n t amounts of (M + H ) were o b s e r v e d f o l l o w i n g l a s e r desorption. P h o t o d i s s o c i a t i o n behavior observed f o l l o w i n g l a s e r d e s o r p t i o n was i d e n t i c a l to t h a t seen when e l e c t r o n impact was used t o produce the i o n s ( E q u a t i o n 2 and F i g u r e 4 ) . +

The fragmentation observed f o r t h i s i o n i s not of great a s s i s t a n c e i n a s s i g n i n g molecular s t r u c t u r e , since only a methyl group i s l o s t from the p e r i p h e r y . T h i s i l l u s t r a t e s one p o t e n t i a l weakness i n the use of i n f r a r e d m u l t i p h o t o n d i s s o c i a t i o n (IRMPD) for structure elucidation: the d i s s o c i a t i o n pathways a c c e s s e d a r e those ( u s u a l l y few) of lowest a c t i v a t i o n energy (9J. I f these pathways c o r r e s p o n d t o the l o s s o f s m a l l pendant groups from a large molecular i o n , no m e a n i n g f u l s t r u c t u r a l i n f o r m a t i o n can be obtained. However, s i m i l a r b e h a v i o r would a l s o be e x p e c t e d f o r these i o n s w i t h c o l l i s i o n - i n d u c e d d i s s o c i a t i o n , s i n c e i n e f f i c i e n t c o l l i s i o n a l t r a n s f e r of energy would a l s o only a c c e s s the lowest a c t i v a t i o n energy f r a g m e n t a t i o n pathways. Sucrose: O b s e r v a t i o n of the m o l e c u l a r i o n produced by l a s e r d e s o r p t i o n ( 16) and LD f o l l o w e d by e l e c t r o n impact (Hein, R. E.; Cody, R. Β., N i c o l e t Instrument Corp., p e r s o n a l communication, 1986.) has been r e p o r t e d f o r sucrose. The negative i o n mass spectrum f o l l o w i n g l a s e r d e s o r p t i o n r e v e a l s an (M - 1 ) " i o n a t m/z 341. When i r r a d i a t e d , t h i s i o n d i s s o c i a t e s w i t h p r o d u c t i o n o f i o n s a t m/z 251, 249, 179 and 161 as shown i n F i g u r e 7. The i o n a t m/z 161 c o r r e s p o n d s t o the l o s s o f a water molecule from one mono­ s a c c h a r i d e u n i t . The i o n a t m/z 179 most l i k e l y r e s u l t s from l o s s o f one monosaccharide u n i t . S i n c e both g l u c o s e and f r u c t o s e have i d e n t i c a l m o l e c u l a r formulae, i t i s not p o s s i b l e a t t h i s time t o assign a d e f i n i t i v e i d e n t i t y to t h i s i o n .

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

9. WATSON ET AL.

IR Multiphoton Dissocation of Laser-Desorbed Ions

Tert-butylpyridinium perchlorote, laser desorption

0

c,o

1 ^-

3

CoOH

+

+

CH N0 2

(5)

i n a s t e p w i s e f a s h i o n from Fe(CO) by f i r s t g e n e r a t i n g MFe(CO) f o l l o w e d by c o l l i s i o n - i n d u c e d d i s s o c i a t i o n t o g e n e r a t e t h e b a r e M F e , r e a c t i o n s 6 and 7 [ 3 1 - 3 3 ] . Next, a s e r i e s o f i o n e j e c t i o n p u l s e s a r e used t o i s o l a t e t h e +

M

+

+

MFe(C0)

Fe(C0) + c

>

5

C

v

I

D

+

MFe(C0) _ +

> MFe

5

+

+

x

XCO

(5 -X)C0

(6) (7)

ion of i n t e r e s t . By t h i s t i m e t h e r e a g e n t gas has been pumped away, p e r m i t t i n g t h e i o n s t o be s t o r e d e f f i c i e n t l y f o r t h e r e l a t i v e l y l o n g t r a p p i n g t i m e s used f o r p h o t o dissociation. F o l l o w i n g an a p p r o p i a t e d e l a y p e r i o d ( w i t h o r w i t h o u t l i g h t ) , d e t e c t i o n y i e l d s t h e f u l l mass s p e c t r u m , and f i n a l l y a quench p u l s e e l i m i n a t e s a l l o f t h e i o n s from t h e c e l l and t h e whole s e q u e n c e i s r e p e a t e d .

Information

from

Photodisgocjatjon

I n o r d e r t o o b s e r v e p h o t o d i s s o c i a t i o n p r o c e s s 1, t h r e e c r i t e r i a must be met: f i r s t , t h e i o n must a b s o r b a p h o t o n ; s e c o n d , t h e p h o t o n must have s u f f i c i e n t e n e r g y t o c a u s e f r a g m e n t a t i o n ; and t h i r d , t h e quantum y i e l d f o r p h o t o d i s s o c i a t i o n must be n o n - z e r o . Figure 2 i s useful in u n d e r s t a n d i n g the i n f o r m a t i o n i n h e r e n t i n a photod i s s o c i a t i o n experiment. I f the f i r s t allowed e l e c t r o n i c s t a t e o f AB l i e s a t an e n e r g y above t h a t r e q u i r e d t o generate the p r o d u c t s P * and P * ( l e f t s i d e of F i g u r e 2 ) , the observed p h o t o d i s s o c i a t i o n onset i s s p e c t r o s c o p i c a l l y d e t e r m i n e d and y i e l d s o n l y an upper e n e r g y l i m i t f o r t h e p r o c e s s e s i n r e a c t i o n 1. In o t h e r words, even i f t h e r e i s s u f f i c i e n t e n e r g y i n t h e p h o t o n to c a u s e i o n f r a g m e n t a t i o n , c l e a r l y t h e p r o c e s s w i l l not o c c u r i f t h e i o n does not a b s o r b t h e p h o t o n . Alter+

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

10.

HETTICH AND FREISER

Photodissociation of Transition Metal Ions

161

\Reagent Gas Pulse /Ionization Synthesis

CID Excitation

Ion Ejection Separations

Detection

Quench

Product jAnalysis Structure J ' Determination,

• Reaction Time

F i g u r e 1.

P u l s e sequenc s p e c i f i c metal-containing

AB

+

•

ions.

— P.\ P

?

+

- AB

F i g u r e 2.

+

Energy l e v e l diagram d e p i c t i n g cases where photod i s s o c i a t i o n t h r e s h o l d s a r e d e t e r m i n e d by s p e c t r o s c o p i c f a c t o r s ( l e f t s i d e ) and thermodynamic f a c t o r s (right side).

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

162

FOURIER TRANSFORM MASS SPECTROMETRY

n a t i v e l y , i f an a l l o w e d e l e c t r o n i c s t a t e f o r t u i t o u s l y l i e s a t t h e same e n e r g y as t h e l o w e s t e n e r g y p r o c e s s o r , as shown i n F i g u r e 2 ( r i g h t s i d e ) , t h e r e i s a h i g h d e n s i t y o f allowed s t a t e s having energies i n the v i c i n i t y of the thermodynamic t h r e s h o l d , t h e o b s e r v e d p h o t o d i s s o c i a t i o n o n s e t s h o u l d a c c u r a t e l y r e f l e c t t h e d i s s o c i a t i o n energy f o r p r o c e s s 1 p r o v i d e d t h e quantum y i e l d i s n o n - z e r o . Ons e t s f o r h i g h e r energy p r o d u c t s w i l l a l s o r e f l e c t t h e thermochemistry i f rapid i n t e r n a l conversion to a v i b r a t i o n a l l y e x c i t e d ground s t a t e o c c u r s r a n d o m i z i n g t h e energy. Thus, i f an i o n a b s o r b s l i g h t o v e r a broad r a n g e , say from t h e i r t o t h e uv, t h e n p h o t o d i s s o c i a t i o n c a n b e g i n t o o c c u r when t h e p h o t o n e n e r g y i s s u f f i c i e n t t o cause f r a g m e n t a t i o n . I n any e v e n t , t h e p h o t o d i s s o c i a t i o n s p e c t r u m i s an i n d i r e c t measure o f t h e t r u e g a s - p h a s e a b s o r p t i o n s p e c t r u m and, as such, p r o v i d e s a " f i n g e r p r i n t " of the ground s t a t e s t r u c t u r In g e n e r a l , t h i s a good d e s c r i p t i o n f o r o r g a n i c i o n s . As an example, t h e p h o t o d i s s o c i a t i o n s p e c t r u m o f b e n z o y l c a t i o n o b t a i n e d by m o n i t o r i n g p r o c e s s 8 has two maxima a t 260 nm and 310 nm C H CO 6

5

+

+ hv

C

H

> 6 5

+

+

0

0

(

8

)

1 1 1 a t t r i b u t e d to the t r a n s i t i o n s A^ 1u 1 2 . r e s p e c t i v e l y , and an o n s e t for p h o t o d i s s o c f a t i o n a t 350 nm o r 3.5 eV [ 1 8 ] . The o n s e t i s c o n s i d e r a b l y h i g h e r t h a n t h e a c t u a l e n t h a l p y o f 2.3 eV r e q u i r e d t o decarbonylate the benzoyl i o n . In c o n t r a s t t o t h e o r g a n i c i o n s , a l l o f o u r work t o d a t e on t h e c h e m i c a l s y s t e m s I, I I , and I I I , s u g g e s t t h a t t h e s e m e t a l c o m p l e x e s have a h i g h d e n s i t y o f low l y i n g e l e c t r o n i c s t a t e s and, t h e r e f o r e , a b s o r b b r o a d l y y i e l d i n g p h o t o d i s s o c i a t i o n o n s e t s w h i c h r e f l e c t t h e thermochemistry. I n p a r t i c u l a r , v a l u e s o b t a i n e d by o b s e r v i n g p h o t o d i s s o c i a t i o n o n s e t s a r e f o u n d i n g e n e r a l t o be i n good agreement w i t h t h o s e o b t a i n e d by o t h e r t e c h n i q u e s [15]. C e r t a i n l y , e x c e p t i o n s t o t h i s w i l l be f o u n d . F i n a l l y , a l t h o u g h the i n f o r m a t i o n c o n t e n t o f these photod i s s o c i a t i o n spectra i s great with regard to i o n s t r u c t u r e and t h e r m o c h e m i s t r y , t h e h i g h d e n s i t y o f low l y i n g e l e c t r o n i c s t a t e s makes band a s s i g n m e n t s v i r t u a l l y i m p o s s i b l e a t t h i s t i m e . I n f a c t such an i n t e r p r e t a t i o n w i l l p r o v i d e a r e a l c h a l l e n g e t o t h e o r i s t s f o r y e a r s t o come. B

a

n

d

A

B

(I)

P h o t o d i s s o c i a t i o n o f ML* One o f t h e f i r s t examples from o u r l a b o r a t o r y [13] which suggested t h a t p h o t o d i s s o c i a t i o n t h r e s h o l d s c o u l d y i e l d q u a n t i t a t i v e m e t a l - l i g a n d bond e n e r g i e s was from a c o m p a r i s o n o f t h e p h o t o d i s s o c i a t i o n s p e c t r a o f FeOH and F e C 0 o b t a i n e d by m o n i t o r i n g r e a c t i o n s 9 and 10, r e s p e c t i v e l y , and shown i n F i g u r e 3. The two s p e c t r a a r e r e m a r k a b l y s i m i l a r w i t h two a b s o r p t i o n maxima o b s e r v e d +

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

10.

HETTICH AND FREISER

Photodissociation of Transition Metal Ions

163

Energy (eV)

(a)

5.0

4.0

3.0

5.0

4.0

3.0

(b)

2Ό

Energy (eV)

20

Wavelength (nm)

Figure

3.

+

( a ) P h o t o d i s s o c i a t i o n spectrum o f FeOH o b t a i n e d by m o n i t o r i n g r e a c t i o n 9 as a f u n c t i o n o f wavelength. No p h o t o d i s s o c i a ­ t i o n i s observed a t wavelengths greater than 390 nm. (b) P h o t o d i s s o c i a t i o n spectrum o f F e C 0 g e n e r a t e d by e l e c t r o n i m p a c t on Fe(CO) . The o b s e r v a t i o n o f p h o t o d i s s s o c i a t i o n a t w a v e l e n g t h s g r e a t e r t h a n 660 nm was c o n f i r m e d by u s i n g a c u t o f f f i l t e r . (Reproduced from r e f . 13. C o p y r i g h t 1984 American Chemical S o c i e t y . ) +

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

164

FOURIER TRANSFORM MASS SPECTROMETRY

FeOH

+

+

hv

->

Fe

+

+

OH

(9)

FeCO

+

+

hv

>

Fe

+

+

CO

(10)

n e a r 290 nm and 335 nm, s u g g e s t i n g t h a t t h e t r a n s i t i o n s are metal l o c a l i z e d . The F e O H s p e c t r u m , however, has a p h o t o d i s s o c i a t i o n t h r e s h o l d o f 390 ± 1 0 nm, w h i l e F e C 0 i s o b s e r v e d t o have a low i n t e n s i t y l o n g w a v e l e n g t h a b s o r p t i o n t a i l from about 400 t o 700 nm. The c u t o f f o b s e r v e d a t 390 nm f o r F e O H y i e l d s an a b s o l u t e bond e n e r g y D ° ( F e - 0 H ) = 73±3 k c a l / m o l , w h i c h i s i n e x c e l l e n t agreement w i t h a v a l u e o f 76±5 k c a l / m o l o b t a i n e d by m e a s u r i n g t h e i o n i z a t i o n p o t e n t i a l o f FeOH [35] and a v a l u e o f 77±8 k c a l / m o l o b t a i n e d by b r a c k e t i n g t h e p r o t o n a f f i n i t y o f FeO [13]· A p p e a r a n c e p o t e n t i a l measurements on FeCO* from F e ( C 0 ) wer D ( F e - C 0 ) s 60 k ? a l / m o however, were l a t e r r e i n t e r p r e t e yiel D°(Fe ) 38 k c a l / m o l [ 3 7 ] . Observation of photodissociation re­ a c t i o n 10 a t "700 nm o r 41 k c a l / m o l p r o v i d e s a d d i t i o n a l support f o r the lower value. As shown i n T a b l e I , t h e e v i d e n c e i s m o u n t i n g t h a t p h o t o d i s s o c i a t i o n t h r e s h o l d s do i n f a c t y i e l d a c c u r a t e m e t a l - 1 i g a n d bond e n e r g i e s . I f there i s a disagreement between t h e p h o t o d i s s o c i a t i o n v a l u e and t h e v a l u e from an a l t e r n a t i v e technique, t h e p h o t o d i s s o c i a t i o n value tends to be l o w e r i n c o n t r a s t , f o r example, t o t h e b e n z o y l c a t i o n case d i s c u s s e d above. A r r i v i n g a t a l o w e r v a l u e by p h o t o d i s s o c i a t i o n suggests the p o s s i b i l i t y that the p r e c u r s o r i o n may be g e n e r a t e d w i t h e x c e s s i n t e r n a l energy [4b,38]. To c i r c u m v e n t t h i s p r o b l e m , once formed t h e p r e c u r s o r i o n s a r e p e r m i t t e d t o undergo t h e r m a l i z i n g c o l l i s i o n s w i t h a r g o n ( s e e e x p e r i m e n t a l s e c t i o n ) and t h e e f f e c t on t h e t h r e s h o l d , i f any, i s n o t e d . In addition, whenever p o s s i b l e , t h e p r e c u r s o r i o n o f i n t e r e s t was g e n e r a t e d from more t h a n one n e u t r a l s o u r c e . For the m a j o r i t y o f t h e s y s t e m s s t u d i e d , however, n e i t h e r t h e c o l l i s i o n a l c o o l i n g s t e p n o r t h e s y n t h e s i s from d i f f e r e n t n e u t r a l p r e c u r s o r s had a m e a s u r e a b l e e f f e c t on t h e threshold. One n o t a b l e a c c e p t i o n i s F e C H as d i s c u s s e d below. T r a n s i t i o n - m e t a l m e t h y l i d e n e s have been i m p l i c a t e d a s intermediates i n a v a r i e t y c f important c a t a l y t i c t r a n s ­ f o r m a t i o n s i n c l u d i n g o l e f i n m e t a t h e s i s , t h e Ζiegler-Natta p o l y m e r i z a t i o n o f o l e f i n s , o l e f i n homologation, and t h e heterogeneous F i s c h e r - T r o p s c h process. Likewise, the i n c r e a s i n g l i t e r a t u r e on b a r e M C H * i n t h e g a s phase h a s shown t h e s e s p e c i e s t o have I n t e r e s t i n g p h y s i c a l and chemical p r o p e r t i e s [39-42]. I t i s perhaps not s u r p r i s ­ ing, t h e r e f o r e , that the photochemistry of these s p e c i e s has p r o v e n t o be among t h e most i n t e r e s t i n g . Our i n i t i a l work on M C H * was f o r M = Fe and Co [14], b u t t h e s e s t u d i e s have r e c e n t l y been expanded t o i n c l u d e M = Rh, Nb, and L a [ 1 6 ] . The m e t h y l i d e n e s c a n be +

+

+

+

+

+

2

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

10.

HETTICH AND FREISER

Table

I.

Bond Energy

Photodissociation of Transition Metal Ions

165

Determinations +

D°(A -B)(kcal/mol) photodissociation

A+-B +

Fe -CH Fe -CH Fe -C Co -CH Co -CIT Co -C Nb -CH Nb -CH Nb -C Rh -CH Rh -CIT Rh -C La -CH La -CH La -C Fe -CH Co -CH^ Fe -0 Fe -S Co -S Ni -S V -C H +

other 96±5 (b) 115±20 ( c ) 89 (c) 85±7 (d)

82±5( ' a) 101±559(i) 74>x>59(i)

59±5 ( j ) 71>x>61(k) 49±6 (m) >49 (1) >62 (f) 58±7 62±6 52 >60 >60

(o) (o) (p) (1) (q) (q)

( a ) R e f e r e n c e 1 4 . ( b ) R e f e r e n c e 4 0 . ( c ) Beauchamp, J . L . , p r i v a t e communication, ( d ) A r m e n t r o u t , P.B.; Beauchamp, J . L . J . Chem. Phvs. 1 9 8 1 , H , 2 8 1 9 · ( e ) J a c o b s o n , D.B. F r e i s e r , B.S. J . Am. Chem. S o c . 1 9 8 5 , 107 5 8 7 0 . ( f ) R e f e r e n c e 15. (g) Reference 3 7 . (h) R e f e r e n c e 8. ( i ) C a r l i n , T . J . Ph.D. T h e s i s , Purdue u n i v e r s i t y , 1984. ( j ) J a c o b s o n , D.B.; F r e i s e r , B.S. J . Am. Chem. S o c . 1984 , 106 3900. (k) Reference 3 1 . (1) Reference 16. (m) L e c h , L.M.; F r e i s e r , B.S. u n p u b l i s h e d r e s u l t s , (n) R e f e r e n c e 3 3 . ( o ) R e f e r e n c e 3 1 . ( p ) J a c o b s o n , D.B.; F r e i s e r , B.S. u n p u b l i s h e d r e s u l t s , ( q ) B u c k n e r , S.; MacMahon, T.; F r e i s e r , B.S. u n p u b l i s h e d r e s u l t s . f

f

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

166

FOURIER TRANSFORM MASS SPECTROMETRY

formed from a v a r i e t y o f d i f f e r e n t n e u t r a l p r e c u r s o r s d e p e n d i n g on t h e o v e r a l l t h e r m o c h e m i s t r y . In fact the r e a c t i o n o r l a c k o f r e a c t i o n o f the metal ions w i t h the compounds used, w h i c h i n c l u d e d c y c l o h e p t a t r i e n e (70 k c a l / m o l ) , e t h y l e n e o x i d e (78 k c a l / m o l ) , c y c l o p r o p a n e (93 k c a l / m o l ) , propene (101 k c a l / m o l ) , and methane (112 k c a l / mol) where t h e e n e r g y shown i n p a r e n t h e s i s i s t h a t r e ­ q u i r e d t o remove a CH u n i t [ 4 3 ] , p r o v i d e d t h e f i r s t i n d i c a t i o n o f t h e i r m e t a l i o n - m e t h y l i d e n e bond e n e r g i e s . P h o t o d i s s o c i a t i o n of these MCH s p e c i e s p r o c e e d s by t h r e e pathways, r e a c t i o n s 11—13· These r e s u l t s were un­ +

p

-> M MCH

+ 2

+

hv

+

MC

+ +

CH

+

(11)

2

H

(12)

+

e x p e c t e d i n t h e f i r s t s t u d i e s on FeCH and C o C H * , b e c a u s e they were i n d i r e c t c o n t r a s t το t h e l o w - e n e r g y ( 0 100 ev) c o l l i s i o n - i n d u c e d d i s s o c i a t i o n r e s u l t s y i e l d i n g e x c l u s i v e c l e a v a g e o f CH- t o f o r m M , and b e c a u s e b o t h F e C H * and CoCH p h o t o d l s s o c i a t e by CH- l o s s , exclusively. A ^ s i m i l a r d i f f e r e n c e between t h e p h o t o d i s s o c i a t i o n and c o l l i s i o n - i n d u c e d d i s s o c i a t i o n pathways was o b s e r v e d f o r LaCH , whereas o b s e r v a t i o n o f b o t h t h e c a r b i d e as w e l l as t h e b a r e m e t a l i o n f o l l o w i n g c o l l i s i o n i n d u c e d d i s s o c i a t i o n o f R h C H * and NbCH more c l o s e l y p a r a l l e l s t h e p h o t o d i s s o c i a t i o n pathways o b s e r v e d for these ions. By m o n i t o r i n g t h e p h o t o d i s s o c i a t i o n t h r e s h o l d s f o r each o f t h e t h r e e p h o t o p r o d u c t s i n r e a c t i o n s 11-13, bond e n e r g i e s f o r M - C, M -CH, and M -CH^ were o b t a i n e d ( s e e T a b l e I ) . One o f t h e s t r i k i n g d i s a g r e e m e n t s between a bond e n e r g y d e t e r m i n e d by p h o t o d i s s o c i a t i o n and one from t h e ion-beam e x p e r i m e n t i s D ° ( F e - C H ) where p h o t o ­ d i s s o c i a t i o n y i e l d s 82±5 k c a l / m o l [ 1 4 ] compared t o t h e 96±5 k c a l / m o l r e p o r t e d e a r l i e r [ 4 0 ] . As m e n t i o n e d i n one of t h e above s e c t i o n s , a p o s s i b l e n e g a t i v e d e t e r m i n a t e e r r o r i n the p h o t o d i s s o c i a t i o n experiment can a r i s e i f the p r e c u r s o r i o n i s i n t e r n a l l y e x c i t e d p r i o r t o photon absorption. I f so, c o l l i s i o n a l r e l a x a t i o n o f t h e i o n s s h o u l d r e s u l t i n a h i g h e r energy o n s e t . F i g u r e 4 shows the l o n g w a v e l e n g t h t h r e s h o l d r e g i o n f o r F e C H photod i s s o c i a t i n g t o F e , taken at d i f f e r e n t pressures, w h i c h i s t h e " w o r s t c a s e " we have e n c o u n t e r e d t o d a t e . O b s e r v a t i o n o f p h o t o d i s s o c i a t i o n beyond 400 nm i n F i g u r e 4 would i m p l y D ° ( F e - C H ) < 71 k c a l / m o l w h i c h i s u n r e a s o n a b l y low, s i n c e t h e r e a c t i o n o f F e w i t h e t h y l e n e o x i d e t o f o r m F e C H * r e q u i r e s a bond e n e r g y o f 78 k c a l / mol o r g r e a t e r [ 4 3 J . A d d i n g a r g o n and r a i s i n g t h e pressure c l e a r l y r e s u l t s i n a decrease of d i s s o c i a t i o n i n the t a i l r e g i o n (> 350 nm) and a s l i g h t e n h a n c i n g o f t h e peak maximum n e a r 330 nm. W h i l e t h e s i g n i f i c a n t p r e s s u r e +

+

+

+

+

+

+

+

2

+

+

+

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

10.

HETTICH AND FREISER

Photodissociation of Transition Metal Ions

167

q u e n c h i n g may i n d i c a t e t h e p r e s e n c e o f i n t e r n a l l y o r k i n e t i c a l l y e x c i t e d F e C H * i o n s , an a l t e r n a t i v e e x p l a n a t i o n i s t h a t t h e I o n i s a b s o r b i n g two p h o t o n s (multiphoton process) [29] although t h i s i s b e l i e v e d t o be l e s s l i k e l y . Presumably, a s u f f i c i e n t l y high p r e s s u r e of Ar would, i n any e v e n t , c o m p l e t e l y quench t h e t a i l , but r e d u c e d t r a p p i n g e f f i c i e n c y p r e c l u d e d d o i n g t h i s experiment. The w a v e l e n g t h a t w h i c h Ar b e g i n s t o r e d u c e t h e p h o t o a p p e a r a n c e o f F e and w h i c h c o r r e s p o n d s t o t h e e x t r a p o l a t e d o n s e t o f t h e a b s o r p t i o n band i s about 350 nm or 82 k c a l / m o l . The e a r l i e r r e p o r t e d v a l u e o f D ° ( F e -CH ) = 96±5 k c a l / m o l seems h i g h on t h e b a s i s o f t h e i n t e n s e p h o t o a p p e a r a n c e o f F e a t 340 nm even i n t h e p r e s e n c e o f A r , w h i c h r e q u i r e s D ° ( F e - C H ) < 84 kcal/mol. The p h o t o d i s s o c i a t i o n r e s u l t , however, i s cons i s t e n t w i t h t h e 77-87 k c a l / m o l range s u g g e s t e d from ion-molecule r e a c t i o s t r a t e s the value o d e t e r m i n e t h e s e i m p o r t a n t thermodynamic p a r a m e t e r s . F i n a l l y , we a r e c o n t i n u i n g t o s t u d y t h e u n d e r l y i n g r e a s o n s why m e t h y l i d e n e s a r e p a r t i c u l a r l y s u s c e p t i b l e t o these long wavelength effects. Two i s o m e r s o f CoC H w h i c h c a n be p o s t u l a t e d as t h e p r o d u c t s o f v a r i o u s i o n - m o l e c u l e r e a c t i o n s [ 3 ] a r e shown by s t r u c t u r e s I and I I . S t r u c t u r e I can e a s i l y r e a r r a n g e t o s t r u c t u r e I I upon a c t i v a t i o n , as i s e v i d e n t +

+

+

+

2

+

1 0

II

I +

from t h e p r i m a r y r e a c t i o n s o f C o w i t h 1-pentene, s p e c i f i c a l l y r e a c t i o n s 16 and 17. T h e s e same p r o d u c t s a r e o b s e r v e d i n t h e CID o f C o - p e n t e n e , w i t h t h e e x c e p t i o n +

that

+

o n l y a s m a l l amount o f CoC H « was o b s e r v e d [3]· The i o n o f s t r u c t u r e I can p r e s u m a b l y be made by r e a c t i o n 18 [ 3 ] . D i s p l a c e m e n t o f p r o p e n e s h o u l d a l l o w t h e i o n i c p r o d u c t o f r e a c t i o n 18 t o be g e n e r a t e d w i t h

l i t t l e excess i n t e r n a l energy, p r e f e r e n t i a l l y s t r u c t u r e I over s t r u c t u r e I I .

favoring

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

168

FOURIER TRANSFORM MASS SPECTROMETRY

P h o t o d i s s o c i a t i o n o f CoC H . * from r e a c t i o n 18, s t r u c t u r e IV, g e n e r a t e s f i v e p h o t o p r o d u c t s , r e a c t i o n s 19-23. The p e r c e n t a g e s o f p r o d u c t s were t a k e n a t x m

2 ,

+

->C0C H 5

- ϋ > Ο ο Ο Co

1

62*

hv

ή

+

Η

11* > C o C H

4

22*

CH

(20)

4

H

(21)

H

(22)

+

C

2 4

+

C

3 6

+

Co'

x

(19)

2

+ 6

3

H

•

6

>CoC H 2

+

8

a

(23)

H

5 10

+

The p h o t o d i s s o c i a t i o n s p e c t r u m o f C o - p e n t e n e . 320 nm. F i g u r e 5, i n d i c a t e s peak maxima a t 320 nm (σ = 0.02 A ) and 370 nm. A l l fiv w a v e l e n g t h s out t o a appearance of Co i s due s o l e l y t o r e a c t i o n 23, t h e n o b s e r v a t i o n of Co a t 430 nm w o u l d i m p l y D°(Co -C H J < 66 k c a l / m o l . T h i s v a l u e must be e x p r e s s e d w i t h c a u t i o n , however, s i n c e t h e p r i m a r y p h o t o p r o d u c t s can a l s o f u r t h e r d i s s o c i a t e to give Co . +

+

1

+

II.

Photodissociation

of

ML^

The i o n o f s t r u c t u r e I I can be made p r e s u m a b l y by r e a c t i o n 24 [ 3 ] . The h i g h s t a t i c p r e s s u r e o f a r g o n

(24)

s t a b i l i z e s the c o n d e n s a t i o n of C o n t o CoC-Hg . P h o t o d i s s o c i a t i o n o f CoC H * frOm r e a c t i o n 24 ( F i g u r e 6) y i e l d s o n l y tnrèe p h o t o p r o d u c t s , r e a c t i o n s 25-27, w h i c h a r e a l l o b s e r v e d a t l e a s t out t o 430 nm. The r e l a t i v e a b u n d a n c e s o f t h e s e p h o t o p r o d u c t s were t a k e n a t

61* C H 2

4

(25)

->C0C H < 3

hv

6

15*

C

H

3 6

(26)

—> COC H/ -> Co (27) 5 10> 24* .2 L . = 320 nm(a= 0.05 k ) · No CoC.Hg o r CoC H« are o U s e r v e d , s u p p o r t i n g t h e f a c t t h a t t h i s i o n e x i s t s as s t r u c t u r e I I . The s p e c t r a o f t h e s e two i s o m e r s I and I I a r e a r e s i m i l a r , but t h e 370 nm peak o b s e r v e d i n t h e s p e c t r u m o f C o - p e n t e n e i s a b s e n t i n t h e s p e c t r u m o f Co (propene) ( e t h y l e n e ) . The enhanced p h o t o d i s s o c i a t i o n 2

( C

H

Y

+

+

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

•

HETTICH AND FREISER

Photodissociation of Transition Metal Ions 169

Energy (Kcal/mol) 90

80

70

Effect of Pressure on FeCH£

Wavelength (nm) F i g u r e 4.

Long wavelength t h r e s h o l d r e g i o n f o r Fed^" d i s s o c i a t i n g to F e at various pressures.

1-

photo-

+

ENERGY (Kcol/mol) 110 ~i—ι

250

90 1

300

1

350

70 1

400

Γ

450

WAVELENGTH (nm) F i g u r e 5. P h o t o d i s s o c i a t i o n spectrum o f C0C5H-LQ r e a c t i o n 18.

g e n e r a t e d from

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

170

FOURIER TRANSFORM MASS SPECTROMETRY

c r o s s s e c t i o n o b s e r v e d f o r s t r u c t u r e I I ( i e . 0.05 A*) i s t y p i c a l o f a t r e n d we have o b s e r v e d t h a t p h o t o d i s s o c i a t i o n c r o s s s e c t i o n tends to i n c r e a s e w i t h the a d d i t i o n of a second l i g a n d [15]. The d i f f e r e n c e i n c r o s s s e c t i o n s a l o n e , however, would not be s u f f i c i e n t t o d i s t i n g u i s h t h e two i s o m e r s , A photoappearance t h r e s h o l d f o r Co , r e a c t i o n 27, c o u l d not be o b t a i n e d due t o t h e s e c o n d a r y d i s s o c i a t i o n o f CoC H / and CoC^H.."" t o y i e l d Co . R e c e n t l y we r e p o r t e d t h a t N i ( C H . ) c o u l d a l s o be d i s t i n g u i s h e d from t h r e e o t h e r i s o m e r s f N i - b u t e n e , Ni i s o b u t e n e , n i c k e l a c y c l o p e n t a n e c a t i o n ) on t h e b a s i s o f i t s u n i q u e p h o t o d i s s o c i a t i o n s p e c t r u m and p h o t o p r o d u c t s [15]· In p a r t i c u l a r N i i C - H j . ) * p h o t o d i s s o c i a t e s t o g i v e two p r o d u c t s o f about e q u a l abundance (by l o s s o f C H^ and C.Hg). Continuous e j e c t i o n of Ni -C H . allows r e a c t i o n 28 t o be m o n i t o r e d As shown i n F i g u r e 7 +

1

+

+

2

J

2

+

+

2

+

2

||-

N i * -||

+

hv

i

> ,

r e a c t i o n 28 i s o b s e r v e d f o r < 370 nm i m p l y i n g D°(Ni* "-2 p i i ) = 77±5 k c a l / m o l . The f a c t t h a t t h i s v a l u e i s net s i g n i f i c a n t l y d i f f e r e n t t h a n t w i c e D ° ( N i - C H ) = 37±2 k c a l / m o l s u g g e s t s t h a t s y n e r g i s t i c e f f e c t s a r e not very pronounced i n t h i s i o n . W h i l e t h i s may p r o v e t o be t y p i c a l , i t i s not e x p e c t e d t o be t h e r u l e . F o r example, p r e l i m i n a r y s t u d i e s on t h e b i s - b e n z e n e i o n s , ( 5 6)o ' suggest a n e g l i g i b l e s y n e r g i s t i c e f f e c t f o r M = V [l5J and Sc [ 4 4 1 , a n e g a t i v e e f f e c t f o r L a ( i . e . , D°(La* -C.H ) > D ( L a C H . - C.H.), and a p o s i t i v e e f f e c t f o r Cu ? i . e. , B°t C u - C . H, ) < D°( CuC. H."* - C H ) [44]. c

H

+

2

M

4

C

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+

+

+

fi

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+

+

6

6

III.

6

6

6

6

P h o t o d i s s o c i a t i o n o f MFe+

E x a m i n a t i o n o f t h e b o n d i n g and e n e r g e t i c s o f s m a l l b a r e c l u s t e r s has become a t o p i c o f c o n s i d e r a b l e i n t e r e s t in recent years. An e x c i t i n g a r r a y o f methods has been d e v e l o p e d t o g e n e r a t e c l u s t e r s o f v a r y i n g s i z e s and c o m p o s i t i o n f o r s t u d y i n t h e gas phase. Some of t h e s e new methods i n c l u d e s p u t t e r i n g t e c h n i q u e s [ 4 5 ] , gas e v a p o r a t i o n techniques [46], supersonic expansion techniques w i t h oven [47] and p u l s e d l a s e r s o u r c e s [ 4 8 ] , and m u l t i p h o t o n d i s s o c i a t i o n o f m u l t l n u c l e a r o r g a n o m e t a l l i c compounds [ 4 9 ] . As d e s c r i b e d above, our l a b o r a t o r y dem o n s t r a t e d t h e s y n t h e s i s o f s m a l l h o m o n u c l e a r and h e t e r o n u c l e a r t r a n s i t i o n m e t a l c l u s t e r i o n s ±n _s_i£_u. by u s i n g FTMS [ 3 1 - 3 3 ] . F o r example, r e a c t i o n s 6 and 7 have been used t o s y n t h e s i z e a wide v a r i e t y o f M F e s p e c i e s . The c l e a r a d v a n t a g e s o f t h i s method a r e t h a t not o n l y i s there a great deal of s e l e c t i v i t y i n generating s p e c i f i c c l u s t e r s , but once formed t h e f u l l power o f FTMS can be a p p l i e d t o s t u d y the c h e m i s t r y and p h o t o c h e m i s t r y o f t h e s e species in d e t a i l . T h i s a p p r o a c h i s t y p i f i e d by ext e n s i v e s t u d i e s on t h e r e a c t i v i t i e s o f C o F e [ 3 2 ] and VFe [33] with alkenes. I n a soon t o be p u b l i s h e d +

+

+

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

10. HETTICH AND FREISER

Photodissociation of Transition Metal Ions

ENERGY 110

100

171

( KCAL/MOL )

90

80

70

IK-II

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WAVELENGTH ( NM ) Figure

7.

T h r e s h o l d r e g i o n f o r N i ( C H J photod i s s o c i a t i n g t o N i . I n t h sie x p e r i m e n t , was c o n t i n u o u s l y e j e c t e d . Ν10 Η 2

+

-

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2

4

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

L

172

FOURIER TRANSFORM MASS SPECTROMETRY

paper [ 1 6 ] , we expand on t h i s work by r e p o r t i n g on t h e p h o t o d i s s o c i a t i o n o f MFe (M = Se, T i , V, C r , Fe, Co, N i , Cu^ Nb, T a ) . To summarize t h o s e r e s u l t s , b o t h M and Fe a r e o b s e r v e d as p h o t o p r o d u c t s , w i t h t h e m e t a l h a v i n g t h e l o w e s t i o n i z a t i o n p o t e n t i a l p r e d o m i n a t i n g . The p h o t o d i s s o c i a t i o n s p e c t r a r e v e a l broad a b s o r p t i o n i n t h e u l t r a v i o l e t and v i s i b l e r e g i o n s w i t h a range o f c r o s s s e c t i o n s from 0.06 A f o r VFe t o 0.62 A f o r f o r CrFe . Bond e n e r g i e s o b t a i n e d by o b s e r v i n g p h o t o a p p e a r a n c e ons e t s a r e i n t h e range o f 48 k c a l / m o l f o r S c F e t o 75 kcal/mol f o r VFe ( s e e T a b l e I ) . T h e s e s t u d i e s c a n be r e a d i l y e x t e n d e d t o o t h e r d i m e r s e r i e s , s u c h as MV , M C r , and MCo g e n e r a t e d i n a n a l o g y t o r e a c t i o n s 6 and 7 from V ( C 0 ) , C r ( C 0 ) , and C o ( C 0 ) N 0 , r e s p e c t i v e l y , as w e l l as t o t r i m e r s [ 3 4 ] and n i g h e r o r d e r clusters. C l e a r l y , t h e s u r f a c e has j u s t been s c r a t c h e d ! +

+

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+

+

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Acknowledgment i s mad S c i e n c e s i n t h e O f f i c e o f B a s i c Energy S c i e n c e s i n t h e u n i t e d S t a t e s Department o f Energy (DE-AC02-80ER10689) f o r s u p p o r t i n g t h i s r e s e a r c h and t o t h e N a t i o n a l S c i e n c e F o u n d a t i o n ( C H E - 8 3 1 0 0 3 9 ) f o r c o n t i n u e d s u p p o r t o f FTMS methodology. The a u t h o r s a l s o w i s h t o thank M i c h e l l e Buchanan f o r h e r i n v i t a t i o n t o c o n t r i b u t e t o t h i s symposium.

References 1.

2. 3. 4.

5. 6. 7. 8. 9. 10. 11.

For a comprehensive review on gas-phase metal ion chemistry see: Allison, J . in Progress in Inorganic Chemistry, Ed. Lippard, S. J., Wiley - Interscience, New York, Vol. 34, 628, 1986. Halle, L.F.; Houriet, R.; Kappes, M.; Staley, R.H.; Beauchamp, J . L . J . Am. Chem. Soc. 1982, 104, 6293. Jacobson, D.B.; Freiser, B.S.; J. Am. Chem. Soc. 1983, 105, 5197. (a) Larsen, B.S.; Ridge, D.P. J . Am. Chem. Soc. 1984, 106, 1912. (b) Freas, R.B.; Ridge, D.P. J . Am. Chem, Soc., 1980, 102, 7129. (c) Armentrout, P.B.; Beauchamp, J.L.; J. Am. Chem. Soc., 1980, 102, 1736. Jacobson, D.B.; Freiser, B.S. J . Am. Chem. Soc. 1985, 107, 72. Houriet, R.; Halle, L.F.; Beauchamp, J . L . Organonetallics 1983, 2, 1818. Allison, J.; Freas, R.B.; Ridge, D.P., J. Am. Chem. Soc., 1979, 101, 1332. Armentrout, P.B.; Beauchamp, J . L . J . Am. Chem. Soc. 1981, 103, 784. Aristov, N.; Armentrout, P.B. J . Am. Chem. Soc 1984, 106, 4065. Jones, R.W.; Staley, R.H. J. Am. Chem. Soc. 1982, 104, 2296. Uppal, J . S . ; Staley, R.H. J . Am. Chem. Soc. 1982, 104, 1235.

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

10. HETTICH AND FREISER

12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

26. 27. 28.

29. 30. 31. 32. 33. 34. 35. 36. 37.

Photodissociation of Transition Metal Ions 173

McLuckey, S.A., Schoen, A.E.; Cooks, R.G. J. Am. Chem. Soc. 1982, 104, 848. Cassady, C . J . ; Freiser, B.S. J . Am. Chem. Soc. 1984, 106, 6176. Hettich, R . L . ; Freiser, B.S. J . Am. Chem. Soc. 1986, 108, 2537. Hettich, R . L . ; Jackson, T . C . ; Stanko, E . M . ; Freiser, B.S. J . Am. Chem. Soc. 1986, 108, 5086. Hettich, R . L . ; Freiser, B.S. J. Am. Chem. Soc., in press. Dunbar, R.C. "Gas Phase Ion Chemistry;" Bowers, M.T., E d . ; Academic Press, Inc.; New York, 1984; Vol. 3, Chap. 20. Freiser, B.S.; Beauchamp, J . L. J . Am. Chem. Soc. 1976, 98, 3136. Dunbar, R.C.; Hutchinson B.B J Am Chem Soc 1974, 96, 3816 Burnier, R.C.; Freiser 1979, 18, 906. Comisarow, M.B. Adv. Mass Spec. 1980, 8, 1698. Gross, M . L . ; Rempel, D.L. Science 1984, 226, 261. Lande, Jr.,D.A.; Johlman, C . L . ; Brown, R.S.; Weil, D.A.; Wilkins, C.L. Mass Spec. Rev. 1986, 5, 107. Cody, R.B.; Burnier, R.C.; Freiser, B.S. Anal. Chem. 1982, 54, 96. Cody, R.B.; Burnier, R.C.; Reents, Jr., W.D.; Carlin, T.J.; McCrery, D.A.; Lengal, R.K.; Freiser, B.S. Int. J . Mass Spec. Ion Phys. 1980, 33, 37. Cody, R.B.; Burnier, R.C.; Freiser, B.S. Anal. Chem. 1982, 54, 96. Carlin, T.J.; Freiser, B.S. Anal. Chem. 1983, 55, 571. The photodissociation spectra of a l l of the ions showed no pressure dependence, except for slight quenching of the low energy t a i l region, indicating that photodissociation in these cases is probably due to one-photon excitation and that the precursor ions do not have substantial internal energy. Freiser, B . S . ; Beauchamp, J . L . Chem. Phys. Lett. 1975, 35, 35. Dunbar, R.C. Chem. Phys. Lett. 1975, 32, 508. Jacobson, D.B.; Freiser, B.S. J . Am. Chem. Soc. 1984, 106, 4623. Jacobson, D.B.; Freiser, B.S. J . Am. Chem. Soc. 1985, 107, 1581. Hettich, R . L . ; Freiser, B.S. J . Am. Chem. Soc. 1985, 107, 6222. Jacobson, D.B.; Freiser, B.S. J . Am. Chem. Soc. 1984, 106, 5351. Murad, E. J . Chem. Phys. 1980, 73, 1381. Distefano, G.J. Res. Natl. Bur. Stand, Sect. A. 1970, 74A, 233. Halle, L.F.; Armentrout, P.B.; Beauchamp, J . L . Organometallics 1982, 1, 963. In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

174

38.

39. 40. 41. 42. 43.

44. 45.

46.

47.

48.

49.

FOURIER TRANSFORM MASS SPECTROMETRY

Metal ions can be made with excess internal and/or kinetic energy. See, for example, (a) Kang, H . ; Beauchamp, J . L . J . Phys. Chem. 1985, 89, 3364. (b) Halle, L.F.; Armentrout, P.B.; Beauchamp, J . L . J . Am Chem. Soc., 103, 962 (1981). (a) Stevens, A . E . ; Beauchamp, J . L . J . Am. Chem. Soc. 1980, 100, 2584. (b) Stevens, A . E . ; Beauchamp, J . L . J . Am. Chem. Soc. 1979, 101, 6449. Armentrout, P.B.; Halle, L.F.; Beauchamp. J . L . J . Am. Chem. Soc. 1981, 103, 6501. (a) Jacobson, D.B.; Freiser, B.S. J . Am. Chem. Soc. 1985, 107, 2605. (b) Jacobson, D.B.; Freiser, B.S. J . Am. Chem. Soc. 1985, 107, 4375. (a) Carter, A . E . ; Goddard III, W.A. J . Am. Chem. Soc. 1986, 108, 2180. (b) Shim, I . ; Gingerich, K.A. J . Chem. Phys. 1982, 76, 3833. A l l heats of formatio values) are taake K.; Steiner, B.W.; Herron, J . T . J . Phys. Chem. Ref. Data. Suppl. 1 1977, 6. Lech, L . M . ; Tews, E . C . ; Huang, Y . ; Freiser, B.S. unpublished results. (a) Katakuse, I . ; Ichihara, T . ; F u j j i t a , Y . ; Matsuo, T . ; Sakurai, T . ; Matsuda, H. Int. J . Mas Spec. Ion Proc. 1985, 67, 229. (b) Freas, R.B.; Campana, J . E . J . Am. Chem. Soc. 1985, 107, 6202. (a) Sattler, K . ; Muhlbach, J.; Recknagel, E. Phys. Rev. Lett. 1980, 45, 821. (b) Abe, H.; Schulze, W.; Tesche, B. Chem. Phys. 1980, 47, 95. (c) Godenfeld, I . ; Frank, F . ; Schulze, W.; Winter, B. Int. J . Mass Spec. Ion Proc. 1986, 71, 103. (a) Riley, S . J . ; Parks, E . K . ; Mao, C.R.; Poppo, L . G . ; Wexler, S. J . Phys. Chem. 1982, 86, 3911. (b) Bowles, R.S.; Park, S.B.; Otsuka, N.; Andres, R.P. J. Mol. Catal. 1983, 20, 279. (a) Bondybey, V . E . ; English, J . H . J . Chem. Phys. 1982, 74, 6978. (b) Morse, M.D.; Hansen, G.P.; Langridge-Smith, P.R.R.; Zheng, L . S . ; Geusic, M . E . ; Michalopoulos, D . L . ; Smalley, R.E. J . Chem. Phys. 1984, 80, 5400. (c) Smalley, R.E. Laser Chem. 1983, 2, 167. Leopold, D.G.; Vaida, V. J . Am. Chem. Soc. 1983, 105, 6809.

RECEIVED May 12,

1987

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 11

Fourier Transform Mass Spectrometry Studies of Negative Ion Processes Michelle V . Buchanan and Marcus B. Wise Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, T N 37831-6120 FTMS has been used to examine negative ion chemical ionization (CI) reactions which can be used to differentiate isomeric compounds. A number of unusual anions formed in a conventional high pressure CI source were successfully produced in the FTMS with much lower reagent gas pressures. Exact mass measurements were used to establish the elemental compositions of these anions. Swept double resonance experiments were performed to elucidate ionic precursors in gas phase reactions. In some cases, the observed anions were found to arise from reactions with trace impurities that are present in the vacuum system or the CI reagent gas, such as water and oxygen. N e g a t i v e i o n c h e m i c a l i o n i z a t i o n (NICI) p r o c e s s e s c a n be v e r y u s e f u l f o r the d i f f e r e n t i a t i o n o f isomeric p o l y c y c l i c aromatic hydrocarbons (PAH) ( 1 ) . These compounds a r i s e from b o t h n a t u r a l and a n t h r o p o g e n i c s o u r c e s , and a r e commonly found i n complex m i x t u r e s i n t h e environment ( 2 ) . The p h y s i c a l and b i o l o g i c a l p r o p e r t i e s o f PAH a r e known t o v a r y w i t h m o l e c u l a r s t r u c t u r e (3) . Thus, i t i s o f t e n i m p o r t a n t t o be a b l e t o d i s t i n g u i s h i s o m e r i c PAH i n complex m i x t u r e s . C o n v e n t i o n a l e l e c t r o n impact i o n i z a t i o n mass s p e c t r o m e t r y i s o f t e n l i m i t e d i n i t s a b i l i t y t o i d e n t i f y PAH unambiguously because t h e s e compounds t y p i c a l l y y i e l d i n t e n s e m o l e c u l a r i o n s w i t h l i t t l e o r no a d d i t i o n a l f r a g m e n t a t i o n which would g i v e i n s i g h t i n t o t h e i d e n t i t y o f a p a r t i c u l a r isomer. I n t h e NICI t e c h n i q u e ( 1 ) , methane was used as a b u f f e r gas t o produce t h e r m a l e l e c t r o n s , which were p r e f e r e n t i a l l y c a p t u r e d by an a n a l y t e s p e c i e s t o form a n i o n s . I n t h e case o f PAH, i t was o b s e r v e d t h a t t h e i o n i z a t i o n b e h a v i o r o f these compounds under NICI c o n d i t i o n s was r e l a t e d t o t h e e l e c t r o n a f f i n i t y (EA) o f t h e compound. Compounds w i t h EA v a l u e s g r e a t e r than about 0.5 eV c a p t u r e d e l e c t r o n s t o form m o l e c u l a r a n i o n s , whereas PAH w i t h lower EA v a l u e s d i d n o t i o n i z e . This difference i n i o n i z a t i o n behavior

0097-6156/87/0359-0175$06.00/0 © 1987 American Chemical Society

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under NICI c o n d i t i o n s a l l o w e d a number o f i s o m e r i c PAH t o be distinguished readily. For example, the p o t e n t c a r c i n o g e n b e n z o ( a ) p y r e n e (EA « 0.64 eV ( 4 ) ) c o u l d be s e l e c t i v e l y d e t e c t e d and q u a n t i t a t i v e l y determined i n the presence o f i t s r e l a t i v e l y b e n i g n isomer, b e n z o ( e ) p y r e n e (EA = 0.32 eV ( 4 ) ) , w i t h o u t i n t e r f e r e n c e . A number o f o t h e r c l a s s e s o f a r o m a t i c compounds have a l s o been s t u d i e d t o determine the g e n e r a l i t y o f the o b s e r v e d 0.5 eV i o n i z a t i o n threshold. For example, azaarenes (which have a n i t r o g e n i n the a r o m a t i c r i n g ) were found t o f o l l o w the same t r e n d as the PAH, a l l o w i n g many isomers o f t h i s c l a s s o f compounds t o be d i s t i n g u i s h e d on the b a s i s o f r e l a t i v e e l e c t r o n a f f i n i t y v a l u e s . Thus f a r , two c l a s s e s o f compounds have been found t o be e x c e p t i o n s t o the o b s e r v e d t r e n d i n i o n i z a t i o n , i n c l u d i n g condensed a r o m a t i c systems which c o n t a i n a s a t u r a t e d five-membered r i n g and aminos u b s t i t u t e d PAH ( a r o m a t i c amines). I n the f i r s t c a s e , compounds such as f l u o r e n e were n o t p r e d i c t e d t o i o n i z e under NICI c o n d i t i o n s because they are t y p i c a l l reduce (5) and would b a f f i n i t y v a l u e s ( 6 ) . A l t h o u g h m o l e c u l a r a n i o n s were n o t o b s e r v e d f o r t h e s e types o f compounds, a n i o n s c o r r e s p o n d i n g t o (M + 14)" were r e a d i l y formed. A s i m i l a r type o f b e h a v i o r was o b s e r v e d i n the case o f aromatic amines. Amino-substituted PAH, such as aminof l u o r a n t h e n e s , which would be p r e d i c t e d t o have EA v a l u e s g r e a t e r t h a n 0.5 eV, d i d y i e l d m o l e c u l a r a n i o n s i n a manner s i m i l a r t o nons u b s t i t u t e d PAH. However, a r o m a t i c amines w h i c h have lower EA v a l u e s were a l s o o b s e r v e d t o produce a n i o n s under NICI c o n d i t i o n s , although not molecular anions. For example, i n the NICI spectrum o f 1 - aminoanthracene, the base peak c o r r e s p o n d s t o (M + 14)" , while f o r 2 - aminoanthracene, the base peak c o r r e s p o n d s to (M + 12)" . The f o r m a t i o n o f two d i f f e r e n t a n i o n s from these i s o m e r i c compounds suggested t h a t d i f f e r e n t i a t i o n o f p o s i t i o n a l isomers o f a r o m a t i c amines might be p o s s i b l e u s i n g the NICI technique. The quadrupole mass s p e c t r o m e t e r which was used f o r these initial studies could provide only l i m i t e d insight into the f o r m a t i o n o r s t r u c t u r e s o f these u n u s u a l i o n s from f l u o r e n e and amino PAH. Experiments u s i n g s u b s t i t u t e d f l u o r e n e s , such as 1-, 2-, and 9 - m e t h y l f l u o r e n e , 9 - p h e n y l f l u o r e n e , and c a r b a z o l e , r e v e a l e d t h a t the (M + 14)" i o n d i d not form i f the C-9 p o s i t i o n was blocked. Knowing t h a t the (M + 14)" i o n was formed by a r e a c t i o n a t the C-9 c a r b o n , two p o s s i b l e s t r u c t u r e s c o u l d be drawn f o r t h i s anion. One p o s s i b i l i t y would be 9 - m e t h y l f l u o r e n e ( s t r u c t u r e I ) , w h i c h c o u l d a r i s e from the a d d i t i o n o f CH to fluorene. Similar f o r m a t i o n o f adducts from methane b u f f e r gas under NICI c o n d i t i o n s has been r e p o r t e d (7, 8 ) . A l t e r n a t i v e l y , f l u o r e n e c o u l d l o s e the two hydrogens a t the 9 - p o s i t i o n and add oxygen t o form 9 - f l u o r e n o n e (structure II) 2

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F o u r i e r t r a n s f o r m mass s p e c t r o m e t r y (FTMS) i s a p o w e r f u l and v e r s a t i l e t e c h n i q u e f o r i n v e s t i g a t i n g gas phase p r o c e s s e s ( 9 - 1 2 ) . Combined w i t h i o n t r a p p i n g , the a b i l i t y t o p e r f o r m double resonance and i o n e j e c t i o n experiments i s h e l p f u l i n p r o b i n g gas phase r e a c t i o n pathways. Furthermore, the h i g h r e s o l u t i o n and e x a c t mass measurement capabilities o f FTMS p e r m i t empirical molecular formulae o f p r o d u c t i o n s t o be e a s i l y e s t a b l i s h e d . Collisionally a c t i v a t e d d i s s o c i a t i o n (MS/MS) r e a c t i o n s a r e o f t e n h e l p f u l i n p r o b i n g i o n s t r u c t u r e s , as w e l l . I n t h i s paper, FTMS has been used to examine t h e r e a c t i o n pathways i n v o l v e d i n a number o f NICI r e a c t i o n s t h a t can be used t o d i f f e r e n t i a t e i s o m e r i c compounds. E l u c i d a t i o n o f these pathways might make i t p o s s i b l e t o d e v i s e a d d i t i o n a l r e a c t i o n s which would d i s t i n g u i s h a w i d e r v a r i e t y o f i s o m e r i c compounds u s i n g NICI p r o c e s s e s . The c o n d i t i o n s r e q u i r e d to o b t a i n NICI s p e c t r a on the N i c o l e t FTMS 2000 w i l l be compared t o those o f more c o n v e n t i o n a l h i g h CI F i n a l l y th experiment sequences, whic phase r e a c t i o n pathway described. Experimental C o n v e n t i o n a l h i g h p r e s s u r e NICI s p e c t r a were o b t a i n e d u s i n g a H e w l e t t - P a c k a r d 5985B quadrupole GC/MS, as d e s c r i b e d p r e v i o u s l y (1) . Methane was used as the CI r e a g e n t gas and was m a i n t a i n e d i n the s o u r c e a t 0.2-0.4 t o r r as measured through the d i r e c t i n l e t w i t h a thermocouple gauge. A 200 eV e l e c t r o n beam was used t o i o n i z e t h e CI gas, and t h e e n t i r e s o u r c e was m a i n t a i n e d a t a temperature o f 200° C. Samples were i n t r o d u c e d i n t o t h e s p e c t r o m e t e r v i a the gas chromatograph w h i c h was equipped w i t h a 25 meter f u s e d s i l i c a c a p i l l a r y column d i r e c t l y i n t e r f a c e d w i t h the ion source. For a l l e x p e r i m e n t s , a column c o a t e d w i t h bonded 5% m e t h y l p h e n y l s i l i c o n s t a t i o n a r y phase, (Quadrex, I n c . ) was used and h e l i u m was employed as the c a r r i e r gas a t a head p r e s s u r e o f 20 lbs. M o l e c u l a r s i e v e / s i l i c a g e l t r a p s were used t o remove water and i m p u r i t i e s from the c a r r i e r gas. The FTMS experiments were performed with a Nicolet I n s t r u m e n t s , I n c . FTMS-2000 equipped w i t h two d i f f e r e n t i a l l y pumped 2 i n c h c u b i c c e l l s and o p e r a t e d a t a nominal magnetic field strength of 3 tesla. The d i f f e r e n t i a l pumping system i s equipped w i t h two 1,000 L/s o i l d i f f u s i o n pumps. A m e t a l p l a t e w i t h a 2 mm d i a m e t e r h o l e s e r v e s as a vacuum conductance l i m i t between t h e c e l l s and s u p p o r t s a p r e s s u r e d i f f e r e n c e o f a p p r o x i m a t e l y 1000:1. For low p r e s s u r e CI experiments ( 1 0 " t o 10" t o r r ) , the CI reagent gases were i n t r o d u c e d i n t o t h e s o u r c e c e l l o f the s p e c t r o m e t e r t h r o u g h a b a t c h i n l e t system equipped w i t h 1 l i t e r e x p a n s i o n volumes and m o l e c u l a r leaks. F o r h i g h e r p r e s s u r e experiments (> 10" t o r r ) , the CI gas was i n t r o d u c e d i n t o the system b y means of a p u l s e d v a l v e arrangement. The r e a g e n t gases were i o n i z e d w i t h e l e c t r o n e n e r g i e s t y p i c a l l y a t 70 eV, a l t h o u g h e n e r g i e s as low as 10 eV were used. A l l samples were i n t r o d u c e d i n t o the s p e c t r o m e t e r w i t h a h e a t e d d i r e c t i n s e r t i o n probe, which was o p e r a t e d a t temperatures between ambient and 250° C. P r e s s u r e s i n the FTMS c e l l s were measured w i t h i o n i z a t i o n gauges. U n l e s s mentioned 6

4

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o t h e r w i s e , t h e FTMS c e l l s were m a i n t a i n e d a t ambient temperature (about 30°C). A l l s t a n d a r d compounds were o b t a i n e d c o m m e r c i a l l y and used as received. L a b e l l e d w a t e r , D 0 (99.78 atom % D) and H 0 (97.3 atom % 0 ) , were o b t a i n e d from KOR I s o t o p e s (Cambridge, MA) and MSD I s o t o p e s ( M o n t r e a l , Canada), r e s p e c t i v e l y . L a b e l l e d oxygen, 0 (52.6 atom % 0 ) , was a l s o o b t a i n e d from MSD I s o t o p e s . Water samples were s u b j e c t e d t o s e v e r a l freeze-pump-thaw c y c l e s t o remove v o l a t i l e gases p r i o r t o i n t r o d u c t i o n i n t o t h e FTMS. S e v e r a l experiment sequences were w r i t t e n f o r t h e FTMS-2000 s p e c i f i c a l l y f o r t h e NICI s t u d i e s . One sequence a l l o w s i o n s t o be formed i n t h e source c e l l under r e l a t i v e l y h i g h s t a t i c p r e s s u r e s (up t o 1 0 " t o r r ) f o l l o w e d by t r a n s f e r o f t h e p r o d u c t i o n s i n t o t h e analyzer c e l l f o r detection. An i m p o r t a n t feature of this e x p e r i m e n t sequence i s t h e use o f a r e l a t i v e l y h i g h t r a p p i n g p o t e n t i a l (> 2.5 v o l t s ) i n t h e source c e l l and a much lower t r a p p i n g p o t e n t i a l (< e f f e c t o f g r e a t l y improvin under t h e h i g h p r e s s u r e CI c o n d i t i o n s , w h i l e a l l o w i n g them t o be d e t e c t e d i n t h e a n a l y z e r under t h e i n f l u e n c e o f weaker e l e c t r i c f i e l d s f o r improved e x a c t mass measurements. For e x p e r i m e n t s r e q u i r i n g h i g h e r p r e s s u r e s o f CI r e a g e n t gas, a second experiment sequence was w r i t t e n w h i c h i n c o r p o r a t e s t h e use o f t h e p u l s e d v a l v e s s u p p l i e d w i t h t h e FTMS. I n t h i s sequence, t h e p u l s e d v a l v e s a r e opened f o r 6 msec, a l l o w i n g a h i g h p r e s s u r e p u l s e o f r e a g e n t gas t o e n t e r t h e source c e l l . A v a r i a b l e delay i s i n t r o d u c e d between t h e time t h e v a l v e s a r e opened and t h e time t h e e l e c t r o n beam i s t u r n e d on. T h i s e n a b l e s t h e p r e s s u r e a t w h i c h t h e i n i t i a l i o n i z a t i o n o c c u r s t o be v a r i e d . A g a i n , a f t e r i o n f o r m a t i o n i n t h e s o u r c e , t h e p r o d u c t s may be t r a n s f e r r e d i n t o t h e a n a l y z e r c e l l f o rdetection i f desired. I n o r d e r t o h e l p e l u c i d a t e r e a c t i o n pathways by w h i c h t h e p r o d u c t anions a r e formed i n t h e NICI r e a c t i o n s , a F o r t r a n program was w r i t t e n t o p e r f o r m a swept double resonance e x p e r i m e n t . With t h i s program, t h e i n t e n s i t y o f a p r o d u c t i o n i s m o n i t o r e d w h i l e a s u s p e c t e d r e a c t a n t i o n i s c o n t i n u o u s l y e j e c t e d from t h e FTMS c e l l , p r o h i b i t i n g i t from p a r t i c i p a t i n g i n t h e f o r m a t i o n r e a c t i o n . E j e c t i o n o f a p r e c u r s o r i o n r e s u l t s i n a d e c r e a s e i n t h e measured i n t e n s i t y o f the product i o n . Experimentally t h i s information i s o b t a i n e d by c o l l e c t i n g a s e r i e s o f s p e c t r a i n w h i c h t h e mass o f t h e e j e c t e d i o n i s i n c r e m e n t e d by a s p e c i f i e d amount. T h i s program a l l o w s t h e u s e r t o s p e c i f y b o t h t h e mass l i m i t s and mass increment o f t h e i o n e j e c t i o n event. The number o f s p e c t r a c o l l e c t e d a t each i o n e j e c t i o n mass may a l s o be d e t e r m i n e d by t h e u s e r . Results are t a b u l a t e d on a p r i n t e r i n u n i t s o f a b s o l u t e i o n i n t e n s i t y f o r t h e p r o d u c t i o n o f i n t e r e s t v e r s u s t h e mass o f t h e e j e c t e d i o n . A g r a p h i c r e p r e s e n t a t i o n o f t h e data i s a l s o g e n e r a t e d on a p l o t t e r . Two v e r s i o n s o f t h i s program a l l o w one t o e i t h e r e j e c t i o n s formed d u r i n g t h e e l e c t r o n beam o r d u r i n g t h e i o n t r a p p i n g time a f t e r t h e beam. A l l experiment sequences and F o r t r a n programs d e s c r i b e d above a r e a v a i l a b l e from t h e a u t h o r s on 8 i n c h f l o p p y d i s k s . In a d d i t i o n , source codes and documentation f o r each program i s available. 1 8

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2

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1 8

1 8

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R e s u l t s and D i s c u s s i o n Comparison o f Chemical I o n i z a t i o n C o n d i t i o n s . A comparison o f t h e c o n d i t i o n s used i n the c o n v e n t i o n a l h i g h p r e s s u r e NICI experiment and t h e FTMS experiment a r e o u t l i n e d i n Table I . One d i f f e r e n c e between t h e two i n s t r u m e n t s i s t h e reagent gas p r e s s u r e used f o r chemical i o n i z a t i o n . I n t h e case o f c o n v e n t i o n a l C I e x p e r i m e n t s , the CI reagent i s i n t r o d u c e d a t r e l a t i v e l y h i g h p r e s s u r e s ( a few t e n t h s o f a t o r r ) t o generate a plasma o f i o n s , e l e c t r o n s , n e u t r a l s and r a d i c a l s i n t h e i o n i z a t i o n source ( 1 3 ) . I n t h e case o f FTMS, however, CI r e a c t i o n s c a n be performed i n s e v e r a l d i f f e r e n t manners. First, reagent gases may be i n t r o d u c e d i n t o t h e s p e c t r o m e t e r a t r e l a t i v e l y low p r e s s u r e s ( 1 0 " t o 1 0 " t o r r ) , and a d e l a y time (up t o s e v e r a l seconds) i n s e r t e d between reagent i o n f o r m a t i o n and d e t e c t i o n o f p r o d u c t i o n s t o a l l o w gas phase r e a c t i o n s to occur (14) A l t e r n a t i v e l y p u l s e d v a l v e s (15 16) may be u s e d t o i n t r o d u c e b r i e FTMS c e l l t o approximat and s t i l l m a i n t a i n most o f t h e h i g h performance c a p a b i l i t i e s o f t h e FTMS. F i n a l l y , w i t h t h e d i f f e r e n t i a l l y pumped d u a l c e l l d e s i g n o f the FTMS 2000 ( 1 7 ) , a somewhat h i g h e r s t a t i c p r e s s u r e ( 1 0 " t o 10" t o r r ) o f t h e C I reagent gas may be m a i n t a i n e d i n t h e source c e l l , w h i l e pressure i n the analyzer c e l l i s t y p i c a l l y three orders o f magnitude lower. The i o n s formed i n t h e source c e l l under C I c o n d i t i o n s c a n be t r a n s f e r r e d i n t o t h e a n a l y z e r c e l l , a l l o w i n g them to be d e t e c t e d a t p r e s s u r e s w h i c h p e r m i t much h i g h e r performance o f the FTMS, including high mass r e s o l u t i o n and e x a c t mass measurement. I n t h i s s t u d y , most s p e c t r a were o b t a i n e d w i t h a s t a t i c p r e s s u r e o f 1 0 " t o 1 0 " t o r r o f reagent gas i n t h e source c e l l , a l t h o u g h p u l s e d v a l v e s were used t o generate h i g h e r p r e s s u r e s i n a few experiments. R e a c t i o n times o f up t o 30 seconds were used b e f o r e i o n d e t e c t i o n i n o r d e r t o observe a n i o n f o r m a t i o n . 6

8

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T a b l e I . Comparison o f NICI C o n d i t i o n s Parameter Pressure E l e c t r o n Beam Detection Temperature

quadrupole 0.2 -0.4 t o r r continuous < msec ambient - 300°C

FTMS 10" - 10" t o r r < .001 - > 1 s e c . > msec ambient - 250°C 6

4

A n o t h e r d i f f e r e n c e between c o n v e n t i o n a l h i g h p r e s s u r e C I and the FTMS CI experiment i s t h e d u r a t i o n o f t h e e l e c t r o n beam event used t o form p r i m a r y i o n s a n d secondary e l e c t r o n s . I n the c o n v e n t i o n a l CI s o u r c e , t h e e l e c t r o n beam i s on c o n t i n u o u s l y d u r i n g the experiment. The FTMS, i n c o n t r a s t , uses a p u l s e d e l e c t r o n beam, and t h e d u r a t i o n o f t h e e l e c t r o n beam event may be v a r i e d from l e s s than a m i l l i s e c o n d t o over a second. I n t h e NICI s t u d i e s u s i n g t h e FTMS, t h e e l e c t r o n beam was t y p i c a l l y l e f t on f o r t e n m i l l i s e c o n d s . However, i t was found t h a t i n some c a s e s , w h i c h w i l l be d i s c u s s e d l a t e r , i t was n e c e s s a r y t o use a l o n g e r beam t i m e , up to 1 s e c , i n o r d e r t o observe t h e p r o d u c t i o n s n o r m a l l y produced

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FOURIER TRANSFORM MASS SPECTROMETRY under the h i g h p r e s s u r e NICI c o n d i t i o n s used on the quadrupole instrument. F i n a l l y , the time r e q u i r e d f o r the d e t e c t i o n o f i o n s a l s o d i f f e r s between the quadrupole i n s t r u m e n t and the FTMS. I n the quadrupole i n s t r u m e n t , i o n s are t y p i c a l l y d e t e c t e d i n l e s s than a millisecond after l e a v i n g the i o n s o u r c e , whereas the FTMS experiment n o r m a l l y r e q u i r e s tens o f m i l l i s e c o n d s t o a c q u i r e a medium r e s o l u t i o n spectrum. Therefore, i t i s conceivable that s h o r t - l i v e d a n i o n s , w h i c h may be observed w i t h the quadrupole i n s t r u m e n t , may n o t be r e a d i l y d e t e c t e d w i t h the FTMS. I n o r d e r t o a s s e s s whether the FTMS c o u l d be used t o s i m u l a t e c o n v e n t i o n a l h i g h p r e s s u r e NICI r e a c t i o n s , a m i x t u r e o f f l u o r e n e , f l u o r a n t h e n e , and benzo(a)pyrene was i n v e s t i g a t e d . T h i s m i x t u r e was i n t r o d u c e d i n t o the i n s t r u m e n t u s i n g the h e a t e d s o l i d s probe and the r e s u l t i n g spectrum i s shown i n F i g u r e 1. T h i s spectrum was o b t a i n e d w i t h what might be c a l l e d " t y p i c a l " CI c o n d i t i o n s f o r the FTMS 2000, w i t h the sourc p r e s s u r e o f 7 χ 10" t o r a d e l a y time o f 30 s a f t e r i o n f o r m a t i o n t o a l l o w p o s t - i o n i z a t i o n r e a c t i o n s to occur. F o r a l l t h r e e compounds, the a n i o n s formed were i d e n t i c a l t o those observed u s i n g the quadrupole i n s t r u m e n t , w i t h m o l e c u l a r a n i o n s a t m/z 202 and 252 produced from f l u o r a n t h e n e and b e n z o ( a ) p y r e n e , r e s p e c t i v e l y , w h i l e an a n i o n a t m/z 180, c o r r e s p o n d i n g t o (M + 1 4 ) ' , was produced from f l u o r e n e . In the studies outlined below, collisionally activated d i s s o c i a t i o n (MS/MS) experiments were attempted as a means o f h e l p i n g e s t a b l i s h the i d e n t i t i e s o f a number o f a n i o n s . I n every case t r i e d , however, the p a r e n t a n i o n s appeared t o undergo e l e c t r o n detachment w i t h no daughter i o n p r o d u c t i o n observed. 6

F o r m a t i o n o f M + 14 Anions From F l u o r e n e . One o f the f i r s t NICI experiments performed on the FTMS was the i n v e s t i g a t i o n o f the peak a t m/z 180 observed from f l u o r e n e , t o determine whether the s p e c i e s formed was s i m i l a r t o 9 - m e t h y l f l u o r e n e ( I ) o r 9-fluorenone ( I I ) . E x a c t mass measurement o f t h i s a n i o n y i e l d e d a v a l u e o f 180.059, w h i c h compared f a v o r a b l y t o t h a t e x p e c t e d f o r the fluorenone s t r u c t u r e o f C H 0 (mass 180.058) r a t h e r than the 9 - m e t h y l f l u o r e n e structure of C H (mass 180.094). To v e r i f y t h i s c o n c l u s i o n , a m i x t u r e o f f l u o r e n e and 9-methyl f l u o r e n e was i n t r o d u c e d v i a the s o l i d s probe. The r e s u l t i n g NICI spectrum (18) i s shown i n F i g u r e 2, i n w h i c h two d i s t i n c t i o n s are observed. One c o r r e s p o n d s t o the peak e x p e c t e d f o r 9-methyl f l u o r e n e and the o t h e r t o 9 - f l u o r e n o n e , i n d i c a t i n g t h a t under NICI c o n d i t i o n s , f l u o r e n e l o s e s two hydrogens a t the C-9 p o s i t i o n and adds an oxygen atom t o form a s p e c i e s s i m i l a r to 9 -fluorenone. I t s h o u l d be n o t e d that i n this p a r t i c u l a r spectrum, argon was used as the b u f f e r gas, i n s t e a d o f methane, d e m o n s t r a t i n g t h a t methane does n o t p a r t i c i p a t e i n the gas phase r e a c t i o n . In addition, reactions using C H and CD as r e a g e n t gases a l s o f a i l e d t o i n d i c a t e the p a r t i c i p a t i o n o f methane i n the f o r m a t i o n o f the m/z 180 a n i o n . Experiments u s i n g 9,9-d f l u o r e n e ( m o l e c u l a r weight 168) a l s o y i e l d e d a n i o n s a t m/z 180, which corresponds t o (Μ + 0 - 2D) , f u r t h e r s u p p o r t i n g the c o n c l u s i o n t h a t the r e a c t i o n o c c u r s a t the C-9 p o s i t i o n . 1 3

1 4

8

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1 3

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200 220 MASS IN A.M.U.

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Figure 1. FTMS NICI spectrum of a mixture of fluorene [(M+14) at m/z 180], fluoranthene [M~ at m/z 202)], and benzo(a)pyrene [(M~ at m/z 252)]. Methane was used as the reagent gas at a static pressure of 7 χ 10" torr. β

179.b

179.7

179.8

179.9

180.0 180.1 180.2 MASS IN A.M.U.

180.3

180.4

Figure 2. FTMS NICI spectrum of a mixture of fluorene and 9-methyl fluorene, using argon as the reagent gas. Observed anions at 180.058 and 180.094 correspond to C Η O" and C Η Ο", respectively. (Reproduced with permission from ref. 18. Copyright 198^ vfôley.)

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182

Knowing t h a t the a n i o n a t (M + 14) mass u n i t s e x h i b i t e d oxygen i n c o r p o r a t i o n , experiments were conducted w i t h l a b e l l e d r e a g e n t gases, i n c l u d i n g H 0 and 0 , t o determine t h e s o u r c e o f t h e oxygen. I n i t i a l experiments u s i n g t h e s e l a b e l l e d r e a g e n t s and t h e c o n d i t i o n s t y p i c a l l y used f o r the NICI r e a c t i o n s on the FTMS gave no i n d i c a t i o n t h a t e i t h e r water o r m o l e c u l a r oxygen were r e a c t i n g w i t h f l u o r e n e i n t h e gas phase. I n c r e a s i n g t h e i o n t r a p p i n g time p r i o r t o d e t e c t i o n a l s o d i d n o t y i e l d any s p e c i e s i n w h i c h l a b e l l e d oxygen was i n c o r p o r a t e d . The s p e c i e s a t (M + 1 4 ) ' was s t i l l o b s e r v e d i n b o t h o f t h e s e c a s e s , b u t no i o n a t (M + 1 6 ) ' , c o r r e s p o n d i n g t o (M - 2H + 0 ) ' , was observed. When the e l e c t r o n beam time was i n c r e a s e d t o l o n g e r t i m e s (up t o 500 msec), a d d i t i o n a l a n i o n s were observed. T h i s i s i l l u s t r a t e d i n F i g u r e 3, w h i c h i s the NICI spectrum o b t a i n e d from a r e a c t i o n o f 9 , 9 - d - f l u o r e n e ( m o l e c u l a r w e i g h t 168) w i t h oxygen-18 u s i n g an e l e c t r o n beam time o f 200 msec A s i g n i f i c a n t anion i s observed a t m/z 166, due t o l o s s o 9 , 9 - d - f l u o r e n e . Anothe l o s s o f a hydrogen from t h e f u l l y p r o t o n a t e d f l u o r e n e w h i c h was p r e s e n t as an i m p u r i t y i n t h e sample. Spectra obtained using a much s h o r t e r e l e c t r o n beam time d i d e x h i b i t t h e c h a r a c t e r i s t i c a n i o n a t m/z 180 c o r r e s p o n d i n g t o (Μ + 0 - 2D) from the d e u t e r a t e d fluorene was o b s e r v e d , b u t t h e analogous anion with 0 i n c o r p o r a t e d was n o t o b s e r v e d . A d d i t i o n a l a n i o n s were p r e s e n t i n the NICI spectrum o f 9,9-d fluorene obtained with 0 as the r e a g e n t gas and u s i n g a 200 msec beam time w h i c h d i d i n d i c a t e the i n c o r p o r a t i o n o f 0 , as shown i n F i g u r e 3. A n i o n s a t m/z 181, 183, and 185 were found t o c o r r e s p o n d t o t h e g e n e r a l f o r m u l a o f (Μ + 0 - Η ) " , where m/z 181 a r i s e s from f l u o r e n e w i t h 0 i n c o r p o r a t i o n , m/z 183 a r i s e s from d - f l u o r e n e w i t h 0 , as w e l l as f l u o r e n e and 0 , and m/z 185 i s from d - f l u o r e n e and 0 . To h e l p i d e n t i f y the p r e c u r s o r i o n s from which t h e s e p r o d u c t s were formed, a swept double resonance e x p e r i m e n t was performed. The r e s u l t s o f t h i s e x p e r i m e n t i n d i c a t e d that 0 " was t h e i o n w h i c h r e a c t s w i t h t h e 9,9 - d - f l u o r e n e t o produce t h e a n i o n c o r r e s p o n d i n g t o (M + 0 - H) a t m/z 185, and that 0 " produces t h e a n i o n a t m/z 183. An i n t e r e s t i n g o b s e r v a t i o n was the f a c t t h a t t h i s r e a c t i o n i n v o l v e d the l o s s o f a hydrogen from t h e r i n g system i n s t e a d o f t h e l o s s o f a d e u t e r i u m from the l a b e l l e d C-9 c a r b o n . T h i s i s i n c o n t r a s t t o the r e a c t i o n d i s c u s s e d e a r l i e r where t h e f o r m a t i o n o f t h e m/z 180 a n i o n from f l u o r e n e was d e t e r m i n e d t o be due t o the a d d i t i o n o f oxygen t o t h e C-9 c a r b o n w i t h the l o s s o f the two hydrogens a t t h a t p o s i t i o n . The r e a c t i o n o f f l u o r e n e w i t h w a t e r was a l s o s t u d i e d u s i n g l o n g e r e l e c t r o n beam t i m e s , w i t h the major a n i o n produced b e i n g (M - H)" . Swept double resonance was a g a i n u s e d t o s t u d y t h i s r e a c t i o n and t h e r e s u l t s a r e shown i n F i g u r e 4. In this experiment, perdeuterofluorene (molecular weight 176) was i n t r o d u c e d v i a t h e s o l i d s probe and H 0 as used as t h e NICI b u f f e r gas. The a n i o n a t m/z 174, c o r r e s p o n d i n g t o (M - D)" , was m o n i t o r e d as masses from 15 t o 22 amu were e j e c t e d a t 0.2 amu intervals. Four "peaks", c o r r e s p o n d i n g t o d e c r e a s e s i n t h e i n t e n s i t y o f t h e p r o d u c t i o n a t m/z 174 were o b s e r v e d i n t h e r e s u l t i n g swept double resonance p l o t , a t 17, 18, 19, and 20 amu, 1 8

1 8

2

2

1 8

2

2

1 8

2

1

8

2

1 8

1 6

1 6

1 8

2

1 8

2

1 8

2

1 8

1 6

1 8

2

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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183

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200

180 190 MASS IN A. M. U.

1 220

1 8

F i g u r e 3. FTMS NICI spectrum o f 9,9-d f l u o r e n e u s i n g 0 a r e a g e n t gas (1 χ 1 0 " t o r r ) and a 200 msec e l e c t r o n beam. 2

5

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

2

FOURIER TRANSFORM MASS SPECTROMETRY

184

" ι — ι — ι — ι — ι — 1 16

17 MASS

18 OF

19 I ON

20

r 21

22

EJECTED

F i g u r e 4. Swept double resonance experiment on p e r d e u t e r o fluorene using K 0 as a r e a g e n t gas. Mass 174 was m o n i t o r e d as masses from 15 t o 21 amu were s e q u e n t i a l l y e j e c t e d a t 0.2 amu i n t e r v a l s . 1 8

2

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Negative Ion Processes 1 6

16

1 8

18

corresponding t o masses OH", 0D", OH", and 0 D " , r e s p e c t i v e l y . T h i s experiment c o n f i r m s t h a t t h e s e f o u r i o n s (which o r i g i n a t e from the b u f f e r gas, H 0 , and t r a c e s o f H 0, as w e l l as from t h e H/D exchange r e a c t i o n s o f t h e s e two s p e c i e s w i t h t h e d e u t e r a t e d f l u o r e n e ) r e a c t w i t h n e u t r a l f l u o r e n e i n the gas phase to produce the a n i o n a t m/z 174. A summary o f the observed r e a c t i o n s w i t h f l u o r e n e i s g i v e n i n T a b l e I I . I n none o f the NICI experiments conducted w i t h the FTMS to date h a s 0 from e i t h e r l a b e l l e d water o r oxygen e v e r been o b s e r v e d t o be i n c o r p o r a t e d i n t o f l u o r e n e t o form the analogous a n i o n , (M + 0 - 2H) a t m/z 182. Thus, the p r e c u r s o r f o r t h i s r e a c t i o n has n o t been i d e n t i f i e d . These experiments have i n v o l v e d changing a v a r i e t y o f experimental c o n d i t i o n s , i n c l u d i n g cell temperature, p r e s s u r e , t r a p p i n g p o t e n t i a l , r e a c t i o n delay time, e l e c t r o n beam t i m e , and e l e c t r o n energy. Recently, small q u a n t i t i t e s of 0 were doped i n t o the methane r e a g e n t gas used i n the h i g h p r e s s u r e sourc at m/z 182 was observed a l t h o u g h the e x t e n t o f 0 i n c o r p o r a t i o n was r e l a t i v e l y low. I t i s p o s s i b l e t h a t the f o r m a t i o n o f the M + 14 a n i o n from f l u o r e n e may p r o c e e d b y d i f f e r e n t pathways i n the two i n s t r u m e n t s . 1 8

2

2

1 8

1 8

1 8

2

1 8

R e a c t i o n s o f 1- and 2-aminoanthracene. As p o i n t e d out e a r l i e r , h i g h p r e s s u r e NICI s p e c t r a o b t a i n e d w i t h t h e quadrupole mass s p e c t r o m e t e r y i e l d e d a p o s s i b l e means o f d i f f e r e n t i a t i n g p o s i t i o n a l isomers o f aminoanthracene, w i t h 1-aminoanthracene p r o d u c i n g a base peak a t (M + 14) mass u n i t s and 2-aminoanthracene, a t (M + 12) mass units. I n i t i a l FTMS s t u d i e s o f NICI s p e c t r a o f 2-aminoanthracene o b t a i n e d w i t h the c e l l a t ambient temperature yielded a base peak a t m/z 192, c o r r e s p o n d i n g t o (Μ - H)~, as w e l l as a n i o n s a t m/z 193 (M" ) and m/z 204. U s i n g the c o n v e n t i o n a l h i g h p r e s s u r e CI s o u r c e on the quadrupole mass s p e c t r o m e t e r , which was o p e r a t e d a t 200° C, the base peak was m/z 205, w i t h a n i o n s a l s o o b s e r v e d a t m/z 204 (about 60% r e l a t i v e abundance) and m/z 192 (about 50% r e l a t i v e abundance). A s t u d y was conducted on t h e quadrupole mass s p e c t r o m e t e r t o i n v e s t i g a t e the e f f e c t s o f i o n s o u r c e temperature on the p r o d u c t i o n s observed under n e g a t i v e i o n CI c o n d i t i o n s f o r 2-aminoanthracene and the r e s u l t s are shown i n F i g u r e 5. A marked i n c r e a s e i n the f o r m a t i o n o f m o l e c u l a r a n i o n s was o b s e r v e d a t lower temperatures. This o b s e r v a t i o n i s c o n s i s t e n t w i t h other r e p o r t s o f enhanced m o l e c u l a r a n i o n l i f e t i m e s a t low source temperatures f o r o t h e r t y p e s o f compounds under n e g a t i v e i o n CI c o n d i t i o n s , such as a z u l e n e ( 1 9 ) . A t h i g h e r source t e m p e r a t u r e s , the abundance o f the m o l e c u l a r a n i o n d e c r e a s e d as the i n t e n s i t y o f (M + 12)" i n c r e a s e d . T h i s same temperature e f f e c t was observed i n the case o f (M + 14)" at m/z 207 f o r 1-aminoanthracene as w e l l . Based on t h i s information, the temperature o f the FTMS c e l l was i n c r e a s e d t o 200° C t o more e f f e c t i v e l y reproduce the conditions o f i o n f o r m a t i o n i n the c o n v e n t i o n a l h i g h p r e s s u r e CI s o u r c e . The FTMS n e g a t i v e i o n CI s p e c t r a o f 1-aminoanthracene e x h i b i t e d a n i o n s a t m/z 192 (base p e a k ) , 207, and 208 when the c e l l was m a i n t a i n e d a t 200° C and methane was used as the r e a g e n t gas a t p r e s s u r e s o f about 10" t o r r o r h i g h e r . As p o i n t e d out above, as the c e l l temperature was d e c r e a s e d , the i n t e n s i t y o f the a n i o n a t 6

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Table I I SUMMARY OF NICI REACTIONS OF FLUORENE CHARACTERIZED BY FTMS

H 0 (OH") 2

(M+0-2H) M/Z 207

OR

2 H 0 (OH") 2

^

(M-H) 192

M/z

0 (0") 2

MW

> (M+O-H) M/Z 208

193 18

°2 -

1R lb

> (N+ 0-H) M/Z 210

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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F i g u r e 5. E f f e c t o f source temperature on a n i o n f o r m a t i o n from 2-aminoanthracene u s i n g c o n v e n t i o n a l h i g h p r e s s u r e C I .

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187

188

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m/z 192 i n c r e a s e d r e l a t i v e t o t h a t o f the m/z 207 and 208 a n i o n s . Swept double resonance experiments i n d i c a t e d t h a t b o t h the m/z 192 and m/z 207 anions were formed by r e a c t i o n o f OH" with 1-aminoanthracene. Exact mass measurements s u g g e s t e d t h a t these i o n s c o r r e s p o n d e d t o C H N (M - H) and C H N 0 (Μ + 0 - 2H), respectively. U s i n g swept double resonance, the a n i o n a t m/z 208 was found t o be l i n k e d t o the r e a c t i o n o f 0" w i t h n e u t r a l 1-aminoanthracene. The measured mass o f the m/z 208 c o r r e s p o n d e d to C H NO (Μ + 0 - H). Additionally, the analogous anion (M + 0 - H) a t m/z 210 was observed when 0 was i n t r o d u c e d as the r e a g e n t gas. The r e a c t i o n s observed f o r 1-aminoanthracene are sumarized i n Table I I I . For 2-aminoanthracene, m/z 192 was a l s o o b s e r v e d t o be the base peak a t a c e l l temperature o f 200° C, u s i n g methane as the r e a g e n t gas. I n a d d i t i o n , a n i o n s a t m/z 204 and 205 were observed ( a p p r o x i m a t e l y 50% and 11% r e l a t i v e abundance r e s p e c t i v e l y ) When H 0 was used as the CI r e a g e n t 208 were observed. As resonance a g a i n showed t h a t the (M - H) a n i o n was produced from the r e a c t i o n o f OH" w i t h the p a r e n t m o l e c u l e . The a n i o n a t m/z 207 was found t o be l i n k e d t o OH' as w e l l , and e x a c t mass measurement r e v e a l e d t h a t t h i s i o n c o r r e s p o n d e d t o an e m p i r i c a l f o r m u l a o f C H 0 N , o r (Μ + 0 - 2H) . The f o r m a t i o n o f the m/z 208 a n i o n was found t o a r i s e from the r e a c t i o n o f 0" w i t h 2aminoanthracene. A c c o r d i n g t o e x a c t mass measurements, t h i s a n i o n corresponded to C H N O o r (Μ + 0 - H) . The p a r t i c i p a t i o n o f oxygen i n t h i s r e a c t i o n was v e r i f i e d u s i n g 0 as the CI reagent gas, which produced an a n i o n a t m/z 210, corresponding to (M + 0 - H). E x a c t mass measurement o f the m/z 205 a n i o n i n d i c a t e d t h a t t h i s s p e c i e s c o r r e s p o n d e d t o C H N 0 , o r (Μ + 0 - 4H). Double resonance experiments d i d not i d e n t i f y a l i n k t o an oxygenc o n t a i n i n g p r e c u r s o r , i n c l u d i n g OH", 0", o r 0 ". The measured mass o f the m/z 204 a n i o n i n d i c a t e d an e m p i r i c a l f o r m u l a o f C H N , o r (Μ + Ν - 3H). The m/z 205 and 204 a n i o n s were o n l y o b s e r v e d when methane was used as the CI r e a g e n t , and n o t when e i t h e r oxygen o r water were used. Reactions with C H as the r e a g e n t gas f a i l e d t o show any p a r t i c i p a t i o n o f methane i n the f o r m a t i o n o f e i t h e r o f these anions. A d d i t i o n a l experiments were conducted w i t h the h i g h p r e s s u r e CI s o u r c e on the quadrupole mass s p e c t r o m e t e r u s i n g s m a l l amounts of 0 , 0 , and H 0 doped i n t o the methane r e a g e n t gas t o a s c e r t a i n whether oxygen was i n c o r p o r a t e d i n t o the a n i o n s observed from 2-aminoanthracene. When a m i x t u r e o f 0 and CH was used as the NICI r e a g e n t gas ( p r o d u c i n g OH" and 0 " ) , the abundances o f m/z 192, 205 and 207 were observed t o i n c r e a s e , s u g g e s t i n g t h a t oxygen was r e s p o n s i b l e f o r the f o r m a t i o n o f these i o n s i n the h i g h p r e s s u r e CI s o u r c e . When t h i s experiment was r e p e a t e d by a d d i n g 0 t o the methane reagent gas, an a n i o n a t m/z 209 was observed, f u r t h e r i n d i c a t i n g t h a t oxygen was p a r t i c i p a t i n g i n the f o r m a t i o n o f the m/z 207 a n i o n . An i n c r e a s e i n the 207 a n i o n was a l s o o b s e r v e d i n t h i s spectrum. However, the low mass r e s o l u t i o n o f the quadrupole mass s p e c t r o m e t e r d i d not p e r m i t the d e t e r m i n a t i o n o f whether t h i s i n c r e a s e was due t o the i n c o r p o r a t i o n o f 0 i n t o the 1 4

1 4

1 0

1A

9

1 0

1 8

1 8

o

2

1A

9

1 A

1 0

1 8

2

1 8

1A

7

2

1 4

1 3

A

1 8

2

2

2

2

A

2

1 8

2

1 8

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

8

2

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189

Table I I I SUMMARY OF NICI REACTIONS 1- AND 2- AMINOANTHRACENE CHARACTERIZED BY FTMS

ο

ο

Η 0 (OH") 2

->

(Μ-ΗΓ

ΗΗ (Μ+Ο-ΗΓ MW 166

Μ/ζ 183

Η 0 (0Η~) 2

-» 18

(M-D)" Μ/Ζ 166

18

0ο ( 0") 4

^

1δ

(Μ+ 0-Η) Μ/Ζ 185

(M-D)~ Μ/ζ 17α

MW 176

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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190

m/z 205 a n i o n o r i f i t was from the o r i g i n a l m/z 207 a n i o n . When a m i x t u r e o f water and methane was used as t h e NICI r e a g e n t g a s , p r o d u c i n g OH", o n l y (M-H)~ was enhanced. The FTMS s t u d i e s i n d i c a t e d t h a t t h e 207 a n i o n was l i n k e d t o OH" . I n these h i g h pressure experiments, i t i s p o s s i b l e t h a t the formation o f t h i s a n i o n may p r o c e e d i n a s t e p w i s e f a s h i o n and t h e presence o f an a d d i t i o n a l r e a c t a n t , such as 0 , may be r e q u i r e d . 2

Conclusions U s i n g t h e d u a l c e l l FTMS, i t h a s been shown t h a t a number o f u n u s u a l i o n s formed i n t h e h i g h p r e s s u r e C I source r e s u l t from r e a c t i o n w i t h t r a c e i m p u r i t i e s p r e s e n t i n the vacuum system o r from the CI r e a g e n t gas, i n c l u d i n g water and oxygen. I n some c a s e s , t h e s e i m p u r i t i e s are r e s p o n s i b l e f o r the p r o d u c t i o n o f a n i o n s which can be used t o d i f f e r e n t i a t e i s o m e r i c s t r u c t u r e s o f m o l e c u l e s . This work suggests t h a t doping C I r e a g e n t gases w i t h c o n t r o l l e d amounts o f these i m p u r i t i e t h e s e r e a c t i o n s so the isomeric d i f f e r e n t i a t i o n . FTMS h a s been shown t o be a p o w e r f u l t o o l f o r s t u d y i n g NICI reactions. I n many c a s e s , t h e o b s e r v e d NICI s p e c t r a were v e r y complex, i n v o l v i n g a number o f r e a c t i o n s . The a b i l i t y t o form the a n i o n s under h i g h p r e s s u r e c o n d i t i o n s and d e t e c t them a t l o w p r e s s u r e s u s i n g t h e d u a l c e l l c o n f i g u r a t i o n , combined w i t h s e v e r a l specialized experimental sequences, p l a y e d a major role i n u n r a v e l l i n g these r e a c t i o n s . The r e s u l t s o f these s t u d i e s have shown t h a t f o r s i m p l e e l e c t r o n c a p t u r e r e a c t i o n s i n which m o l e c u l a r a n i o n s a r e formed, t h e i o n i z a t i o n i n the lower p r e s s u r e FTMS source i s s i m i l a r t o t h a t i n the h i g h p r e s s u r e CI source o n the quadrupole mass s p e c t r o m e t e r . I n cases where r e a c t i o n s o t h e r t h a n s i m p l e e l e c t r o n c a p t u r e o c c u r , t h e observed a n i o n s , o r p o s s i b l y even t h e pathways b y which they are formed, sometimes d i f f e r between the two types o f sources. I n o r d e r t o m i n i m i z e these d i f f e r e n c e s , a h i g h p r e s s u r e C I source f o r t h e FTMS 2000 i s b e i n g d e s i g n e d and constructed i n our laboratory. T h i s source s h o u l d a l l o w a much b e t t e r means o f s t u d y i n g c h e m i c a l i o n i z a t i o n r e a c t i o n s observed i n c o n v e n t i o n a l h i g h p r e s s u r e CI sources u s i n g FTMS. Acknowledgments The authors thank Dr. E l i z a b e t h Stemmler f o r o b t a i n i n g t h e quadrupole d a t a . Research sponsored b y t h e O f f i c e o f H e a l t h and E n v i r o n m e n t a l Research, U.S. Department o f Energy under c o n t r a c t DE-AC05-840R21400 w i t h M a r t i n M a r i e t t a Energy Systems, I n c .

Literature Cited 1. 2. 3.

M. V. Buchanan and G. Olerich, Org. Mass Spectrom. 19, 486 (1984). R. E. Laflamme and R. A. Hites, Geochim. Cosmochim. Acta (1978), 42, 289. M. L. Lee, M. V. Novotny and K. D. Bartle, "Analytical Chemistry of Polycyclic Aromatic Compounds", Academic Press, New York, 1981.

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

11. BUCHANAN AND WISE 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

Negative Ion Processes

191

J . M. Younkin, L. J. Smith, and R. N. Compton, Theor. Chim. Acta (Berl.) 41, 157 (1976). I. Bergman, Trans. Faraday Soc. 50, 829 (1954). A. Streitwieser, J r . , "Molecular Orbital Theory for Organic Chemists," pp.175-185, John Wiley and Sons, New York, (1961). D. Stockl and H. Budzikiewicz, Org. Mass Spectrom. 17, 376 (1982). G. W. Dillow and I. K. Gregor, Org. Mass Spectrom. 21, 386 (1986). A. G. Marshall, M. B. Comisarow, and G. J. Parisod, J . Chem. Phys. 71, 4434 (1979). A. G. Marshall, Acc. Chem. Res. (1985) 18, 316. M. L. Gross and D. L. Rempel, Science, (1984), 226, 261. C. L. Wilkins and M. L. Gross, Anal. Chem., (1981), 53, 1661A. A. G. Harrison, "Chemical Ionization Mass Spectrometry", pp.75-80, CRC Press, Inc., Boca Raton, FL, (1983). S. Ghaderi, P. S and M. L. Gross, Anal T. J. Carlin and B. S. Freiser, Anal. Chem. 55, 571 (1983). D. A. McCrery, T. M. Sack and M. L. Gross, Spectrosc. Int. J. 3, 57 (1984). R. B. Cody, J . A. Kinsinger, Sahba Ghaderi, I. J . Amster, F. W. McLafferty, and C. E. Brown, Anal. Chim. Acta. 178, 43 (1985). M. V. Buchanan, I. B. Rubin, M. B. Wise, and G. L. Glish, Biomed. and Environ. Mass Spectrom., 14, 395 (1987). E. P. Grimsrud, S. Chowdhury, and P. Kebarle, J . Chem. Phys. (1985) 83, 3983.

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Glossary A/D C o n v e r t e r : A n a l o g t o d i g i t a l c o n v e r t e r ; changes a v a r i a b l e electrical s i g n a l i n t o a d i g i t a l r e p r e s e n t a t i o n t h a t c a n be p r o c e s s e d b y a computer. Aliasing: A r t i f i c i a l s h i f t o f a h i g h f r e q u e n c y s i g n a l t o a lower f r e q u e n c y ( f o l d - b a c k ) due t o s a m p l i n g t h e s i g n a l a t a r a t e o f l e s s t h a n two p o i n t s p e r c y c l e ; see N y q u i s t f r e q u e n c y . Analog S i g n a l :

A continuously variable

signal.

Analyzer C e l l : G e n e r a l term used f o r t h e lower p r e s s u r e t r a p p e d ion c e l l i n dual c e l instruments Apodization: Weightin domain s i g n a l i n o r d e r t o improve t h e peak shape o f the f r e q u e n c y domain spectrum by r e d u c i n g the s i d e - l o b e s . Array Processor: Computer c i r c u i t d e s i g n e d f o r p e r f o r m i n g numeric computations on l a r g e a r r a y s o f d a t a . Bandwidth: Frequency l e s s t h a n 3 dB. Base Peak:

range

over which a s i g n a l

fast

i s a t t e n u a t e d by

Most i n t e n s e peak i n a mass spectrum.

Baseline Correction: W e i g h t i n g scheme used b a s e l i n e i n F o u r i e r transformed s i g n a l s . Beam Time: The l e n g t h o f time d u r i n g an experiment c y c l e .

that

t o achieve

a

flat

t h e e l e c t r o n beam i s on

Broadband E x c i t a t i o n : I r r a d i a t i o n o f i o n s w i t h a wide range o f r a d i o f r e q u e n c i e s ( t y p i c a l l y i n t h e range o f kHz t o MHz) t o b r i n g them i n t o c o h e r e n t motion i n o r d e r t o d e t e c t a wide mass range. CAD: Collisionally Activated Dissociation; dissociation of a ion caused by s e l e c t e d e x c i t a t i o n o f that i o n and subsequent c o l l i s i o n with a target neutral species. CI:

Chemical

Ionization.

CID: Collision Induced i n t e r c h a n g e a b l y w i t h CAD). Coherent cell.

Motion:

In-phase

Dissociation

(sometimes

used

movement o f i o n s w i t h i n a t r a p p e d i o n

0097-6156/87/0359-0192$06.00/0 © 1987 American Chemical Society

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Glossary

193

C o l l i s i o n Energy: Energy t r a n s f e r r e d t o an i o n upon w i t h a gas or surface. C o l l i s i o n Gas: A target gas ( t y p i c a l l y c o l l i s i o n a l l y activated d i s s o c i a t i o n of ions.

argon)

collision

used f o r

Conductance L i m i t : O r i f i c e i n a m e t a l p l a t e s e p a r a t i n g t h e two c e l l s i n a d u a l c e l l FTMS i n s t r u m e n t ; a l l o w s d i f f e r e n t i a l pumping o f t h e two c e l l s . Co-processor: A d d i t i o n a l c i r c u i t added t o a computer t o i n c r e a s e the speed o f c a l c u l a t i o n s . Cross S e c t i o n : place.

Measure o f t h e p r o b a b i l i t y o f an event

Cyclotron Equation: Invers and c y c l o t r o n f r e q u e n c c o n s t a n t , q i s i o n i c charge, Β i s magnetic f i e l d

taking

strength.

C y c l o t r o n Frequency: The a n g u l a r f r e q u e n c y o f t h e o r b i t a l m o t i o n o f an i o n i n a c o n s t a n t magnetic f i e l d ; i o n s o f d i f f e r e n t masses have unique c y c l o t r o n f r e q u e n c i e s . Cyclotron Motion: O r b i t a l m o t i o n o f an i o n o r c h a r g e d p a r t i c l e i n a magnetic f i e l d . Dalton:

1 2

An atomic mass u n i t based on C - 12 d a l t o n s .

Damping: Loss o f c o h e r e n t i o n c y c l o t r o n m o t i o n p r i m a r i l y due t o c o l l i s i o n s with n e u t r a l molecules; r e s u l t s i n l o s s o f s i g n a l . Daughter Ion: parent ion.

P r o d u c t i o n from t h e r e a c t i o n o r d i s s o c i a t i o n o f a

Delay: L e n g t h o f time between two s e q u e n t i a l e v e n t s i n an FTMS experiment sequence. D i f f e r e n t i a l Pumping: The use o f two o r more independent pumping systems t o c r e a t e d i f f e r e n t p r e s s u r e r e g i o n s w i t h i n a vacuum system. D i r e c t Mode: transient.

Direct

digitization

of a

broadband

Double Resonance: Use o f a p p r o p r i a t e r a d i o f r e q u e n c y e j e c t s e l e c t e d i o n ( s ) from t h e t r a p p e d i o n c e l l . D u a l C e l l FTMS : FTMS i n s t r u m e n t c o n f i g u r a t i o n d i f f e r e n t i a l l y pumped t r a p p e d i o n c e l l s .

that

signal

pulses to

uses two

Dynamic Range: The d i f f e r e n c e between t h e maximum and minimum number o f i o n s t h a t c a n be d e t e c t e d w i t h o u t s i g n a l d i s t o r t i o n .

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

FOURIER TRANSFORM MASS SPECTROMETRY

194 EI:

Electron ionization.

EIEIO: E l e c t r o n Impact E x c i t a t i o n o f Ions from O r g a n i c s ; uses low energy e l e c t r o n s t o c o l l i s i o n a l l y a c t i v a t e and d i s s o c i a t e ions. Electron A f f i n i t y : Energy r e q u i r e d t o remove an e l e c t r o n from an a n i o n i n i t s ground s t a t e . eV:

Electron volt.

Excitation Amplifier: the a m p l i t u d e o f the synthesizer.

Electronic r f signal

Excitation Level: Magnitude FTMS c e l l t o i n d u c e c o h e r e n

c i r c u i t designed to increase g e n e r a t e d by the f r e q u e n c y

o f the r f v o l t a g e

a p p l i e d t o the

Excite Signal: Radio f r e q u e n c y p u l s e a p p l i e d t o the t r a n s m i t p l a t e s o f the FTMS c e l l t o induce c o h e r e n t m o t i o n o f the i o n s . Experiment Sequence: S e r i e s o f l i n k e d e v e n t s c o n t r o l l e d by the computer which includes i o n f o r m a t i o n , m a n i p u l a t i o n , and d e t e c t i o n , among o t h e r s . E x t e r n a l Source: I o n i z a t i o n r e g i o n which i s p h y s i c a l l y s e p a r a t e d from the FTMS c e l l and u s u a l l y i n d e p e n d e n t l y pumped. FAB:

F a s t Atom Bombardment.

F a s t F o u r i e r Transform: An a l g o r i t h m w h i c h c a l c u l a t e s the Fourier transform of a d i g i t i z e d s i g n a l e f f i c i e n t l y with respect to time and computer memory space. F i x e d Frequency E x c i t a t i o n : Selective e x c i t a t i o n of a single mass by a p p l y i n g a f i x e d f r e q u e n c y t o the t r a n s m i t p l a t e s o f the FTMS c e l l . Forward F o u r i e r Transform: Mathematical transformation s i g n a l from the time domain t o the f r e q u e n c y domain.

of

a

F o u r i e r Transform: M a t h e m a t i c a l method o f d e t e r m i n i n g the f r e q u e n c y components ( a m p l i t u d e and phase o f r e a l and i m a g i n a r y components) o f a complex time domain s i g n a l . Frequency Domain Spectrum: Graphic representation of i n f o r m a t i o n i n the form o f a m p l i t u d e v e r s u s f r e q u e n c y . Frequency S y n t h e s i z e r : Specialized controlled) which generates f i x e d signals.

spectral

c i r c u i t ( u s u a l l y computer o r swept r a d i o f r e q u e n c y

Frequency C h i r p : F a s t broadband r a d i o f r e q u e n c y sweep used i n d u c e c o h e r e n t i o n m o t i o n i n the FTMS c e l l .

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

to

195

Glossary FTICR: F o u r i e r Transform i n t e r c h a n g e a b l y w i t h FTMS). FTMS : with

Ion Cyclotron

F o u r i e r Transform Mass S p e c t r o m e t r y FTICR).

Resonance

(used

(used i n t e r c h a n g e a b l y

GC/MS: Gas Chromatography/Mass S p e c t r o m e t r y ; t e c h n i q u e which u t i l i z e s a gas chromatograph i n t e r f a c e d t o mass s p e c t r o m e t e r , e s p e c i a l l y f o r the on-line a n a l y s i s o f mixtures. Heterodyne: P r o c e s s o f m i x i n g two f r e q u e n c i e s ( s i g n a l and r e f e r e n c e ) t o g e t h e r t o produce a lower f r e q u e n c y s i g n a l w h i c h c a n be d i g i t i z e d a t a s l o w e r r a t e than t h e o r i g i n a l s i g n a l ; commonly used i n FTMS t o a t t a i n u l t r a h i g h mass r e s o l u t i o n s p e c t r a . ICR:

I o n C y c l o t r o n Resonance Spectrometry

Image C u r r e n t : Time-varyin r e c e i v e r p l a t e s o f t h e FTMS c e l l by c o h e r e n t i o n motion. I n v e r s e F o u r i e r Transform: M a t h e m a t i c a l t r a n s f o r m a t i o n from t h e f r e q u e n c y domain t o t h e time domain. Ion E j e c t i o n : E x c i t a t i o n o f an i o n t o a s u f f i c i e n t l y l a r g e o r b i t a l t o cause n e u t r a l i z a t i o n on t h e c e l l p l a t e s ; used t o remove i o n s s e l e c t i v e l y from t h e FTMS c e l l . I o n C y c l o t r o n Resonance: Type o f mass s p e c t r o m e t r y i n w h i c h i o n masses a r e determined on t h e b a s i s o f t h e i r unique orbital f r e q u e n c i e s i n a magnetic f i e l d . Ion P a r t i t i o n i n g : D i s t r i b u t i o n o f t h e i o n p o p u l a t i o n between the s o u r c e and a n a l y z e r c e l l s i n a d u a l c e l l FTMS i n s t r u m e n t . I s o b a r i c Ions: Ions o f t h e same n o m i n a l d i f f e r e n t elemental composition.

mass b u t h a v i n g

LC/MS: Combined l i q u i d chromatography/mass spectrometry; t e c h n i q u e w h i c h u t i l i z e s a l i q u i d chromatograph i n t e r f a c e d t o a mass s p e c t r o m e t e r f o r t h e o n - l i n e a n a l y s i s o f m i x t u r e s . LDI:

Laser Desorption I o n i z a t i o n .

m/z:

Mass t o charge

ratio.

Magnitude Transform: Absolute value representation of points i n a f r e q u e n c y domain spectrum. Mixed Mode D e t e c t i o n : E x c i t a t i o n o f i o n s w i t h a broadband radiofrequency sweep and heterodyne d e t e c t i o n o f a narrow f r e q u e n c y (mass) range. Molecular Ion: electron.

I o n produced

when a m o l e c u l e

g a i n s o r l o s e s an

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

FOURIER TRANSFORM MASS SPECTROMETRY

196

MPI: M u l t i p h o t o n I o n i z a t i o n ; i o n i z a t i o n o f a m o l e c u l e by two o r more s e q u e n t i a l photon a b s o r p t i o n p r o c e s s e s . MS I :

F i r s t s t a g e o f mass s e l e c t i o n i n an MS/MS experiment.

MS I I :

Second s t a g e o f mass s e l e c t i o n i n an MS/MS experiment.

MS/MS: Mass spectrometry/mass two s t a g e s o f mass s e l e c t i o n .

spectrometry;

technique

utilizing

n

MS : A b b r e v i a t i o n f o r m u l t i p l e s t a g e mass spectrometry/mass s p e c t r o m e t r y ; a t e c h n i q u e u t i l i z i n g (n) s t a g e s o f mass s e l e c t i o n . M u l t i p l e x Advantage: Time advantage spectral information simultaneously.

gained

by o b t a i n i n g a l l

Narrowband E x c i t a t i o n : frequencies to the purpose o f e x c i t i n g a s m a l l window o f i o n masses. NICI: N e g a t i v e I o n Chemical interchangeably.

Ionization;

NCI i s sometimes used

Non-resonant I o n : An i o n whose c y c l o t r o n f r e q u e n c y i s d i f f e r e n t t h a n t h e e x c i t a t i o n f r e q u e n c y a p p l i e d t o t h e FTMS c e l l . N y q u i s t Frequency: H i g h e s t f r e q u e n c y which c a n be a c c u r a t e l y represented a t a s p e c i f i e d sampling r a t e ; according t o sampling theory, the Nyquist frequency corresponds t o one h a l f t h e sampling frequency. Parent Ion: I o n which d i s s o c i a t e s o r r e a c t s t o form i o n s and n e u t r a l s .

daughter

P a r t i c l e Desorption: Use o f a h i g h energy p a r t i c l e t o desorb and i o n i z e m o l e c u l e s from a s u r f a c e . Photodissociation:

F r a g m e n t a t i o n o f an i o n u s i n g photons.

P r e s s u r e Broadening: Increase i n s p e c t r a l l i n e w i d t h s c o l l i s i o n a l damping o f t h e time domain s i g n a l . Proton A f f i n i t y : Negative o f the enthalpy addition o f a proton to a neutral species.

change

due t o

for

the

Pulsed Valves: S o l e n o i d d r i v e n v a l v e s commonly used w i t h FTMS t o admit a b r i e f b u r s t o f gas i n t o t h e vacuum chamber; t y p i c a l l y used t o a v o i d o p e r a t i o n o f t h e i n s t r u m e n t under h i g h s t a t i c pressure c o n d i t i o n s i n order t o maintain high performance capabilities. Q-FTMS:

See Tandem quadrupole/FTMS.

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

197

Glossary

Quench P u l s e : A d i r e c t current p o t e n t i a l applied t o the trapping p l a t e s o f t h e FTMS c e l l t o remove a l l i o n s p r i o r t o t h e s t a r t o f an experiment sequence. R e a g e n t l e s s CI: Ion-molecule r e a c t i o n s between t r a p p e d fragment i o n s and n e u t r a l m o l e c u l e s (M) which r e s u l t i n t h e f o r m a t i o n o f i o n s from t h e l a t t e r . Resolution: The degree o f s e p a r a t i o n between two a d j a c e n t peaks i n a spectrum; t y p i c a l l y d e f i n e d as t h e mass o f t h e peak d i v i d e d by t h e w i d t h o f t h e peak a t h a l f i t s maximum h e i g h t . Resonant Ion: An i o n i n t h e FTMS c e l l whose c y c l o t r o n f r e q u e n c y i s equal t o the frequency o f the r f e x c i t a t i o n s i g n a l a p p l i e d t o the c e l l . r f Burst: Applicatio c e l l ; may be used f o rf: S/N:

ejection

Radio frequency. Signal-to-noise

ratio.

S e c t o r MS: A mass s p e c t r o m e t e r t h a t uses one o r more e l e c t r i c and/or magnetic dispersion units to e f f e c t spatial mass separation. Self-CI: Ion-molecule r e a c t i o n s between t r a p p e d daughter i o n s and n e u t r a l p a r e n t m o l e c u l e s (M) w h i c h r e s u l t i n t h e f o r m a t i o n o f i o n s from t h e l a t t e r , g e n e r a t i n g a prominent (M + H ) peak. +

SID:

S u r f a c e Induced D i s s o c i a t i o n .

Signal Amplifier: An e l e c t r o n i c c i r c u i t w h i c h i n c r e a s e s t h e a m p l i t u d e o f t h e image c u r r e n t s i g n a l i n d u c e d i n t h e r e c e i v e r p l a t e s o f t h e FTMS c e l l . S i g n a l Averaging: A d d i t i o n o f s u c c e s s i v e s p e c t r a l scans f o r improved s i g n a l - t o - n o i s e by r e d u c i n g t h e l e v e l o f random n o i s e ; applied to transients p r i o r to Fourier transformation. SIMS:

Secondary I o n Mass S p e c t r o m e t r y .

S i n g l e Resonance: E x p e r i m e n t a l sequence u s i n g d e t e c t i o n events w i t h o u t i o n e j e c t i o n s t e p s . Single C e l l : cell.

FTMS i n s t r u m e n t d e s i g n

i o n f o r m a t i o n and

w h i c h uses one t r a p p e d i o n

Source C e l l : Designation o f the trapped i o n c e l l l o c a t e d i n the h i g h e r p r e s s u r e r e g i o n o f a d u a l c e l l FTMS i n s t r u m e n t . Space Charge: Coulombic i n t e r a c t i o n o f i o n s and/or e l e c t r o n s i n an FTMS c e l l , w h i c h c a n l e a d t o s i g n a l d i s t o r t i o n .

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

FOURIER TRANSFORM MASS SPECTROMETRY

198

S u r f a c e Induced D i s s o c i a t i o n : c o l l i s i o n with a surface. SWIFT: T:

F r a g m e n t a t i o n o f an i o n i n d u c e d by

S t o r e d Waveform I n v e r s e F o u r i e r Transform. T e s l a ; u n i t o f magnetic f i e l d

Tandem MS:

strength.

Two o r more s u c c e s s i v e s t a g e s o f mass

spectrometry.

Tandem Quadrupole FTMS: Use o f a quadrupole mass f i l t e r f o r i n t r o d u c i n g i o n s i n t o t h e FTMS f o r subsequent a n a l y s i s ; u s e d t o circumvent pressure l i m i t a t i o n s . Thermal Energy: Energy ambient t e m p e r a t u r e .

o f an e l e c t r o n ,

Time o f F l i g h t : TOF s e p a r a t i o n b a s e d on t h known k i n e t i c energy t o t r a v e r s e a s p e c i f i c

i o n , or molecule a t

distance.

Time Domain S i g n a l : Complex s i n u s o i d a l i o n image c u r r e n t w h i c h i s d i g i t i z e d and t r a n s f o r m e d t o y i e l d t h e f r e q u e n c y domain mass spectrum. Transient: current.

Time domain

signal equivalent

t o t h e i o n image

Transmitter Plates: P l a t e s i n t h e FTMS c e l l w h i c h a r e used t o couple r a d i o frequency e x c i t a t i o n to ions. Trapped I o n C e l l :

An e l e c t r o s t a t i c - m a g n e t i c i o n s t o r a g e

Trapping E f f i c i e n c y : c e l l to store ions.

device.

A measure o f t h e a b i l i t y o f a t r a p p e d i o n

Trapping P o t e n t i a l : The magnitude o f t h e v o l t a g e a p p l i e d t o t h e t r a p p i n g p l a t e s o f t h e FTMS c e l l t o reduce i o n m o t i o n p a r a l l e l t o the magnetic f i e l d . T r a p p i n g Frequency: The f r e q u e n c y o f i o n m o t i o n i n t h e z - a x i s o f the FTMS c e l l ( p a r a l l e l t o t h e magnetic f i e l d ) ; t h i s f r e q u e n c y i s much lower t h a n t h e c y c l o t r o n f r e q u e n c y . Z-Mode E x c i t a t i o n : E x c i t a t i o n o f ions a t t h e i r trapping f r e q u e n c i e s i n a d i r e c t i o n p a r a l l e l t o t h e magnetic f i e l d . Zero F i l l : A d d i t i o n o f a b l o c k o f z e r o p o i n t s t o a time domain spectrum p r i o r t o t r a n s f o r m a t i o n o f t h e d a t a ; o f t e n improves r e s o l u t i o n when t h e number o f d a t a p o i n t s i s l i m i t e d .

RECEIVED August

26,

1987

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Author Index Allemann, M , 81 Amster, I. Jonathan, 116 Baykut, Gôkhan, 140 Bischofberger, P., 81 Brown, R. S., 127 Buchanan, Michelle, V., 1,175 Castro, Mauro E., 100 Chen, Ling, 21 Cody, Robert 9 Comisarow, Melvin Eyler, John R. Freiser, Ben S., 155 Furlong, Jorge, J. P., 116 Grese, Richard P., 34 Gross, Michael, L., 34 Grossmann, P., 81 1 Hanson Crutiss D., 100

Hettich, Robert L., 155 Kellerhals, Hp., 8 Kinsinger, James Α., 59 Kofel, P., 81 Laukien, Frank H., 81 Loo, Joseph Α., 116 Marshall Alan, G., 21 McLafTerty, Fred W., 116 Rempel Do L. 34

Wang, Bing R , 116 Wang, Tao-Chin Lin, 21 Watson, Clifford H., 140 Wilkins, C.L., 127 Williams, Evan R., 116 Wise, Marcus Β., 175

Affiliation Index Texas A & M University, 100 University of British Columbia, 1 University of California—Riverside, 127 University of Florida, 140 University of Nebraska, 34 University of Virginia, 100

Cornell University, 116 Harvard University, 81 Nicolet Analytical Instruments, 59 Oak Ridge National Laboratory, 1,175 Ohio State University, 21 Purdue University, 155 Spectrospin A G , 81

Subject Index

Aminoanthracene, 1- and 2-, reactions, effects Biomolecules, 100-114 of ion source temperature, 185,187/ Analytical Fourier transform mass spectrometry, instrumentation and application examples, 81 -98 Calibration Apodization, 27,28 accuracy improvement, 62 Applications, future, 14,15 equations, 62 Aromatic amines, negative ion chemical internal calibrant, 62 ionization, 176

200

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Author Index Allemann, M , 81 Amster, I. Jonathan, 116 Baykut, Gôkhan, 140 Bischofberger, P., 81 Brown, R. S., 127 Buchanan, Michelle, V., 1,175 Castro, Mauro E., 100 Chen, Ling, 21 Cody, Robert 9 Comisarow, Melvin Eyler, John R. Freiser, Ben S., 155 Furlong, Jorge, J. P., 116 Grese, Richard P., 34 Gross, Michael, L., 34 Grossmann, P., 81 1 Hanson Crutiss D., 100

Hettich, Robert L., 155 Kellerhals, Hp., 8 Kinsinger, James Α., 59 Kofel, P., 81 Laukien, Frank H., 81 Loo, Joseph Α., 116 Marshall Alan, G., 21 McLafTerty, Fred W., 116 Rempel Do L. 34

Wang, Bing R , 116 Wang, Tao-Chin Lin, 21 Watson, Clifford H., 140 Wilkins, C.L., 127 Williams, Evan R., 116 Wise, Marcus Β., 175

Affiliation Index Texas A & M University, 100 University of British Columbia, 1 University of California—Riverside, 127 University of Florida, 140 University of Nebraska, 34 University of Virginia, 100

Cornell University, 116 Harvard University, 81 Nicolet Analytical Instruments, 59 Oak Ridge National Laboratory, 1,175 Ohio State University, 21 Purdue University, 155 Spectrospin A G , 81

Subject Index

Aminoanthracene, 1- and 2-, reactions, effects Biomolecules, 100-114 of ion source temperature, 185,187/ Analytical Fourier transform mass spectrometry, instrumentation and application examples, 81 -98 Calibration Apodization, 27,28 accuracy improvement, 62 Applications, future, 14,15 equations, 62 Aromatic amines, negative ion chemical internal calibrant, 62 ionization, 176

200

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

201

INDEX Cell design cylindrical cell, 83/ hyperbolic, 48 hyperbolic Penning trap, 48,49/ large cells, 82 single-cell Fourier transform mass spectrometry, 82 Cesium ion desorption, 14 Chemical ionization (CI) high-pressure, 8 non-conventional reagents, 8 selective, 8 Chromatography, combination with FTMS, 6 Cigarette smoke tar, 7/,l 1/ Coherent motion, description, 2,107,109/ Coherent phase relationship, 107,109/ Collision-induced dissociation (CID) EI and DCI spectra of riboflavin static SIMS, 85 Collisional activation (CA) daughter ion resolution, 73,75/ ion selection for MS-I in MS-MS, 51 resolution, MS-I, 53 See also Mass spectrometry-mass spectrometry Collisionally activated dissociation (CAD), disadvantages, 121 Cubic cell, frequency variations, 45 Cubic trap, modified, 43/ Cyclotron equation, 2 Cyclotron frequency, explanation, 21,22 Cyclotron motion, schematic, 22

D Daughter ion spectra, resolution 10 Development cycle, 36,38/ Double laser desorption-dissociation MW'-bis(4,6-dimethoxysalicylidene)-4-trifluoromethyl-o-phenylencdiiminato cobalt(II) (CoSALOPH), 148 sucrose, 148,150/151 Double resonance, application, 9 Dual cell, schematic, 3,4/ Dual-cell Fourier transform mass spectrometer, 59-77 Dual-cell Fourier transform mass spectrometry, identification of cocaine in human urine, 64,66/67/ Dynamic range enhancement, 29/ Dynamic range extension, 28 Dynamic range limit, 29/

Ε

Ejection resolution derivation of expression for, 52 limits, 52,53 MS-I resolution, 53 Electron impact, description, 105 Electron impact excitation of ions from organics (EIEIO), spectrum of W-methylaniline, 73,75/ Electron impact ionization c/s-dichloro-/rflrts-dihydroxo-bis-2-propanamine-platinum (IV) (CHIP), 144,145/ tris(perfluoro-w-nonyl)triazine, 144,147/ Electron impact spectrum of epoxy resin extract, 73,74,76/ Electrostatic focusing lenses ion trajectory, 113/114 possible use, 114

space charge-related, 45 systematic, 45 Euroshift-FOD, negative ions, 97/ Excitation frequency-sweep, problems, 25,26/ fundamentals, 22,23/ pseudorandom noise, 25 random noise, 25 stored waveform inverse Fourier transform (SWIFT), 25,26/27 Excitation waveforms, frequency-domain spectra of time-domain excitation waveforms, 22,24/ External ionization advantages, 94 ion-transfer mechanism, 98

F Fast atom bombardment-mass spectrometry (FAB-MS), comparison with laser desorption FTMS, 127,128 Fourier transform-ion cyclotron resonance (FT-ICR), See Fourier transform mass spectrometry Fourier transform mass spectrometry analytically important features, 21 basic experiment, 3 development, 1 precision, 3 principles and features, 1-20 problems, 105 versatility, 5 Fourier transform mass spectrometry-negative ion chemical ionization (FTMS-NICI) description, 180-190 typical CI conditions, 180,181/ spectrum of 9,9-d fluorene, 182,183/ spectrum of a mixture of fluorene and 9-methyl fluorene, 180,181/ 2

Ε-beam resist polymers, products, 123,124/

2 5 2

C f fission

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

202

FOURIER TRANSFORM MASS SPECTROMETRY

Fourier transform mass spectrometry-negative Ion motion distribution in cell, 108 ion chemical ionization—Continued helix model, 108 spectrum of a mixture of fluorene, path in a uniform magnetic field, 108,109/ fluoranthene, and physics, 105-108 benzo(a)pyrene, 180,181/ pitch of helical motion, 108,110/ Frequency domain signal, origin, 107 Ion population Frequency domain spectrum, generation, 3 distribution, 105 initial dimensions, 105,106/ Ion signal G dependence on ion z-oscillation amplitudes, 46/ Gas chromatography-Fourier transform mass inert ion, benzene molecular ion spectrometry (GC-FTMS) dual-cell instrumentation, 60 signal, 42,43/ molecular ion of naphthalene, 60-62,61/ Ion storage, 8 resolution, 60 Ion synthesis, specific Gas chromatography-mass spectrometry example, 160 (GC-MS), chromatograms fo peppermint flavor extract, 64,65/,66 Gas-phase photodissociation of Ionic cross section determination, 156,157 metal ion complexes and clusters Ionization methods, SIMS, 117 instrumentation, 157-158 ionic cross section determination, 159 photoappearance curves, 158-159 photodissociation spectra, 158 Gas-phase transition metal ion chemistry Large ions analogies to solution organometallic formed by electron impact, photodissociation, 142,144 chemistry, 156 activation, 121-122 characteristics, 155-156 Large molecules analysis, problems, 116,117 tandem Fourier transform mass H spectrometry, 116-124 Laser Hadamard transform advantage carbon dioxide description, 122 laser desorption spectrum of measuring individual MS-II spectra, 122 Rhodamine B, 137,138/ High molecular weight biomolecules, 100-114 use, 137 High-mass ions, resolution, 50 Laser-desorbed ions, infrared multiphoton dissociation, 140-154 Laser desorption (LD) advantages, 140 description, 140 ionization of non volatiles, 12 spectrum generated by, 12 Image current, description, 2 uses, 12 Inhomogeneities Laser desorption-Fourier transform mass electrostatic and magnetic, 107 spectrometry (LD-FTMS) magnetic, 107,111 analytical applications, 129-133 stray magnetic fields, 111 angiotensin I, 129,130/ Instrumentation, developments, applications, 70,127 anticipated, 98 caffeine, accurate mass measurement, 71/ Intermediate cell comparison with fast atom bombardmentdescription, 53 mass spectrometry (FAB-MS), 127 schematic, 54/ segmented tandem cell, 55 description of experimental technique, 128 Inverse Fourier transformation, 25,26/ gramicidin A , 133 Ion beam trajectories, effect of a 500 gauss negative ion spectra of stained and cross magnetic field, 111,112/ unstained copper, 70,71 / Ion manipulation, 8 neurotensin, 129,131/

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

INDEX Laser desorption-Fourier transform mass spectrometry—Continued nonvolatile molecules, 127-139 peptides, 129-133 renin substrate, 129,132/ sample preparation, 128 table, 133 undecapeptide, 129,131/ zircon, 70-72,74/ Laser photodissociation, 121 Laser-desorption mass spectrometer, description, 141 Laser-desorption mass spectrometry one laser experiment, 141-142,143/ potassium ion contamination, 142.148 two laser experiment, 142,143/

203 Metal ion-ligand species photodissociation information from, 160,161/, 162 requirements, 160 uses, 159 Metal-ligand bond energies determination, 156,157,165/ M C H , 164-167 problems, 166-167,169/ using photodissociation thresholds, 164 Modified cells, 41,42 Molecular ions, lifetime, 101 2 +

Ν Negative ion chemical ionization (NICI)

M Magnet requirements field strength, 90 homogeneity, 91 Magnetic fields bottlenecks, 111-114,113/ inhomogeneities, 111 -114,113/ machined 304 stainless steel, 111 stray, 111-114,113/ unworked 304 stainless steel, 111 Magnetic inhomogeneities effects, 107,111 reduction in oxygen-free copper, 114 Mass calibration law accuracy, 45 assumptions, 47 equation, 45 model, 45 Mass range, 5,6 Mass spectrometers comparison of sector and Fourier transform instruments, 35,36 conventional, 3 double-focusing, history, 34,35 Fourier transform, advantages, 36 resolution, 35 Mass spectrometry-mass spectrometry advantages of FTMS, 9-10 approaches to ion activation, 72 description, 9 electron impact excitation of ions from organics (EIEIO), 72 high-resolution, 30 ion selection for MS-I in MS-MS, 51 multiple, 30 parent ion selection, 72,73,74/ surface-induced dissociation, 121 using C A D , 118,121 See also Collisional activation (CA)

compare , 179,179 technique, 175 use in differentiation of polycyclic aromatic hydrocarbons, 175,176 Negative ion chemical ionization (NICI) Fourier transform mass spectrometry, 1- and 2-aminoanthracene, reactions, 185,188-189,189/ reactions of fluorene, 185 Negative ion chemical ionization (NICI) reactions, Fortran program to perform swept double resonance experiment, 178 Negative ion chemical ionization (NICI) spectra experimental sequences, 178 experimental conditions, 177-179 Negative ion laser desorption mass spectrum hesperidin, 151,152/ yV-[2-(5,5-diphenyl-2,4-imidazolidinedion3-yl)ethylJ-7-acetoxy-1 -naphthalene sulfonamide, 151,153/ Nyquist criterion, 27

Ο Orbit size, optimum, 47 Organometallic chemistry, 156

Peptides higher molecular weight, reproducibility problems, 134 polyethylene glycol), 134,135/-136/ spectrum of poly(phenylalanine), 134 L D - F T M S spectra, 129-133 cytochrome-C (equine), 102,104/

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

204

FOURIER TRANSFORM MASS SPECTROMETRY

Peptides L D - F T M S spectra—Continued Scaling, 50 glucagon, 102,103/ Secondary ion mass spectrometry-Fourier insulin (bovine), 102,104/ transform mass spectrometry insulin (bovine and porcine), 102,104/ (SIMS-FTMS), 117,118/, 119/ melittin, 102,103/ Secondary ion mass spectrometry-Fourier neurotensin, 102,103/ transform mass spectrometry-mass Phase modulation, 27 spectrometry (SIMS-FTMS-MS), 120/ Photoappearance curves, 158-159 Self-CI, description, 73,74/ Photodissociation Signal-to-noise ratio, 90 description, 134,137 Single laser desorption-dissociation, large ions /-butylpyridinium cation, 148,149/ c/s-dichloro-/rarts-dihydroxo-bis-2Software requirements propanamine-platinum (IV) database management, 94 (CHIP), 144,145/ phasing, 91,92/,93/ procedure, 142 rapid scan-correlation ICR, 91,93/ protonated N,N'-bis(4,6-dimethoxysalicylidene)-4-trifluoromethyl 0-phenylenediiminato cobal (CoSALOPH), 144,146/ Static secondary ion mass spectrometry, metal ion-ligand species Fourier transform mass spectrometry information from, 160,162,161/ spectrum of valinomycin, 89/ requirements, 160 Steel, stainless uses, 159 magnetic fields in machined cell, 111 M F e , 170-172 magnetic fields in unworked cell, 111 ML+ Stored waveform inverse Fourier transform bond energies, 164,165/ (SWIFT) Co -pentene, 167,168,169/, 171/ spectra of F e O H a n d F e C O , 162-16- [,163/ advantages, 30 excitation, 25,26/,27 M L , 168-170 problems, 27 Ni(C H ), 171/ selective ejection of most abundant Photodissociation spectra, 158 ions, 29/ Plasma desorption (PD) simultaneous ejection and (252 Cf-PD) excitation, 30,31/ advantages, 118 use of two successive SWIFT waveforms, 30 experimental techniques, 118 Supercritical fluid chromatography-Fourier Polycyclic aromatic hydrocarbons, transform mass spectrometry (SFC-FTMS) behavior related to electron advantages, 68 affinity, 175 instrumentation, 68 Probe, special for photodissociation separation of caffeine and methyl experiments, 128,130/ stéarate, 68,69/ Problems of FTMS \2 Surface-induced dissociation (SID), 121 nonuniform tuning of mass spectra, 37— Swept double resonance, reaction of fluorene possible solutions, 41 with water, 182,184/ Pseudomolecular ions, formation, 140 Swept double resonance experiment, Fortran Pulse sequence, 3,4/ program, 178 Pulsed-valve CI Synchronization, excitation waveform and ion description, 85 cyclotron motion, 37,38/ examples, 86/,87/ +

+

+

+

2

Rapid scan-correlation ICR, software requirements, 91,93/ Resolution, 65 daughter ion spectra, 10 high-mass ions, 50 Resonance-enhanced multiphoton ionization (REMPI), 98

Τ Tandem Fourier transform mass spectrometry, large molecules, 116-124

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

INDEX Tandem quadrupole-Fourier transform mass spectrometry (Q-FTMS) problems, 101 modifications to spectrometer, 102 peptides, 102-105 signal-to-noise ratio, 102 Time domain spectrum, formation, 2 Time-of-flight mass spectrometry, 98 Transition metal ion chemistry, gas phase characteristics, 155-156 metal-ligand bond energies, determination, 164,165t Transmission efficiency, definition, 101 Trapped ion cell, schematic, 2,4/ 1,3,5-Trimethylbenzene, 10

U Ultramark 1621, positive ions, 96/

Ζ Z-mode excitation mass discriminating z-losses, 39 mechanism, 37 model, 37,39 reduction of 39 "trap-switch" experiment, 39,40/ Z-motion, 39 Zero-filling, 27

Production and indexing by Linda R. Ross Jacket design by Caria L. Clemens Elements typeset by Hot Type Ltd., Washington, DC Printed and bound by Maple Press, York, PA

In Fourier Transform Mass Spectrometry; Buchanan, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.