Mass Spectrometry Volume 2

A Review of the Literature Published between July 1970 and June 1972

By D. H. Williams

The Royal Society of Chemistry

Copyright © 1973 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-268-2

Contents

Chapter 1 Alternative Methods of Ionization and Analysis By J. M. Wilson,
Chapter 2 Kinetic and Energetic Studies of Organic Ions By I. Howe,
Chapter 3 Reactions of Specific Functional Groups By J. H. Bowie,
Chapter 4 Natural Products By T. J. Mead, H. R. Morris, J. H. Bowie, and I. Howe,
Chapter 5 Organometallic and Co-ordination Compounds By M. I. Bruce,
Chapter 6 Computerized Data Acquisition and Handling By S. D. Ward,
Chapter 7 Gas Chromatography–Mass Spectrometry By C. J. W. Brooks and B. S. Middleditch,
Author Index, 337,

CHAPTER 1

Alternative Methods of Ionization and Analysis

BY J. M. WILSON

1 General Introduction

In Volume 1 of this Report, the material covered fell fairly neatly into four categories, Chemical Ionization (CI), Field Ionization (FI), Negative Ion Studies (NI) and Ion Cyclotron Resonance Spectroscopy (ICR). For this article the scope has been broadened to cover Photoionization (PI). In the previous article the emphasis was on the impact of these methods on structural studies, i.e. on the structure of the molecules ionized or of the ions produced. These methods are now being extended, and some discussion of energetic and kinetic studies on ions is included here.

There is much more overlap than previously between the sections. The details of the ion–molecule reactions of importance in Cl work can be understood better if the primary ions can be produced in well-defined electronic states and within a narrow energy range; this can be achieved by using a vacuum mono-chromator and photoionizing the molecules. There is also a certain amount of overlap between ICR and CI, and the ease with which the reactions of negative ions can be studied by ICR leads to a degree of overlap between these two sections.

The first section, on electron and photon impact, covers methods of determining ionization and appearance potentials and other applications of photoionization sources. The other sections cover the techniques of FI, CI, NI, and ICR. The advantages of FI and CI in molecular weight determination are becoming more and more obvious. The use of the fragmentation patterns in these modes is still at the development stage. For the sequencing of peptides, CI, FI, and PI have all been claimed to give better results than EI. Of these, FI has the advantage that it is possible to obtain spectra by field desorption of free peptides although, for clarity of spectra and certainty in sequencing, CI appears to be better if suitable derivatives are prepared.

Further developments in ICR spectroscopy have led to more accurate measurements of photodetachment energies of negative ions, and there is now a considerable number of measurements of acidities and basicities in the gas phase. We are now reaching the stage where our knowledge of gas-phase ions can help in our understanding of the behaviour of ions in the condensed phase.

2 Ionization Potentials and Other Applications of Photoionization

The four principal methods of determination of ionization potentials are by observation of absorption spectra, photoionization yields, photoelectron spectra, and electron impact ionization yields. Although the methods differ vastly in the accuracy of results they can produce, even the least accurate, electron impact, is still widely used. The various methods which can be used for accurate measurements of small ions in the gas phase have been described by Herzberg.

The availability of efficient sources of monochromatic light and the ease and accuracy with which ionization potentials and energies of both electronically and vibrationally excited states can be measured, has led to an explosion of publications of results of photoelectron spectra during the past two years. Much of this work is covered in other reviews and will not be considered here. The one advantage of electron impact work was the versatility of the instrument, but even that advantage is now doubtful, since a photoelectron spectrometer can be used to measure ionization potentials of radicals and dissociation energies of ions. Tetrafluorohydrazine decomposed at 225 °C in a silica tube adjacent to the photon source and a photoelectron spectrum of NF2 was obtained. In a photoelectron spectrometry study of HF and DF, the dissociation limit of the H — F bond could be observed at the end of the vibrational series of the second IP, and was measurable with greater accuracy than is usually possible by electron impact AP measurements. It is only in small molecules, however, that the vibrational structure can be sufficiently sharp to show up dissociation limits. It is of interest that the first IP as measured in this study was 0.07 eV lower than a previous photoionization measurement.

Lloyd has pointed out that some molecules may have a, much lower cross-section for the first IP than for the second. This is particularly true of the Group V trihalides. In AsCl3 there is only a 0.6 eV difference between the two levels as measured by photoelectron spectroscopy, but the electron impact method gives a result between these two. The same reason is suggested for the 0.7 eV difference between El and PI measurements of the first IP of 1-methylcyclopentene and methylenecyclopentane; the low cross-section state is lost in the thermal tail of the El ionization efficiency curve.

This ‘thermal tail’ is a result of the spread of thermal energy in the electron beam. Tails have been observed in photoionization efficiency curves, but these are ascribed to the presence of thermally excited molecules. Because of this effect the values of I(Me4Si) and D(Si — C) are only upper limits, but they are 0.3 eV less than the values obtained by electron impact. The thermal effect on ionization efficiency curves has been calculated to be as much as 0.1 — 0.2 eV.

There have been a number of recent attempts to improve the accuracy of ionization and appearance potential measurements by electron impact methods. One group prefers to use an ICR spectrometer for this work, for two reasons: (a) the electron kinetic energy will not be distorted by large electric fields and (b) because of the long residence time of the ions the appearance potential measurements should not be affected by a ‘kinetic shift’. The results show a small but reproducible difference in IP between cis– and trans-pent-2-ene. An on-line acquisition method has been described which involves averaging the ionization efficiency curve over ten scans, followed by mathematical smoothing, followed by correction for the electron energy spread by the EED method. This is a purely mathematical treatment which can give ionization efficiency curves with breaks corresponding to vibrational states. Alternative mathematical methods have been produced involving the second differential of the ionization efficiency curve or a Fourier transform of the first differential. The authors claim that the latter method gives a superior result in terms of clarity of fine structure than does the EED method.

There is a report of success in obtaining good ionization curves using the RPD method which should produce effectively monoenergetic electrons. Breaks are found in the curves which correlate with excited states of neutral products.

An indirect method of determining ionization potentials of radicals has been suggested which relies on the relative abundances of fragment ions in the mass spectra of a series of compounds. In any process such as the abundance of R1+ will be greater than that of R2+ if I(R1) <I(R2). The authors have produced the results shown in Table 1 for cyclohexane derivatives. The ionization potential of the cyclohexyl radical must therefore lie between those of s-butyl and t-butyl, i.e. at about 7.2 eV. This is 0.4 eV less than the only measured value.

The importance of photoionization and photoelectron spectroscopy measurements is not only for the measurement of ionization and appearance potentials. There are applications which are more general to mass spectra. It has been suggested that photoelectron spectra can be used as a rough guide to the internal energy distribution of ions produced by electron impact at higher electron energies. This suggestion has been discussed and treated in general with great reservations. The principal reason given is that there are a number of compounds which have fragment ions with appearance potentials which coincide with regions in the photoelectron spectrum where there is zero photoelectron yield. It has also been shown that photoionization and Penning ionization give rise to quite different relative population of the various accessible electronic states of the ion.

The relationship between the photoelectron spectrum and the mass spectrum produced by the same photon beam is one which will obviously require investigation. In the coincidence technique, single-event counting methods are used and only photoelectrons and ions which coincide in time are registered. There are two types of coincidence experiment; it is possible for each mass to measure a kinetic energy spectrum of the electrons produced during its formation; alternatively, for each electron energy level, a mass spectrum can be determined. Experiments of the first type have been described. Eland has described in detail the theoretical limitations of the method and has produced preliminary results from a system designed for experiments of the second type.

In a review of photoionization, Reid has discussed instrumentation, analytical applications, ion efficiency curves, and studies of unimolecular ion decomposition and of ion–molecule reactions. The mass spectra tend to be similar to electron impact spectra in that they are formed by unimolecular decomposition of a series of molecular ions of differing energy content, but a photon source does have the advantages of a well-defined narrow energy range and of low source temperatures (there is no hot filament in the region of the ionization chamber).

PI spectra can be used to detect small differences between stereoisomers which would give identical spectra under electron impact. The endo– (1) and exo– (2) isomers of tricyclo[3,2,1,0]octane can be differentiated by small intensity differences in spectra obtained using the helium 584 Å line. If the hydrogen Lyman α line is used (10.19 eV) the differences become more marked. The difference between the isomeric ketones (3) and (4) is much more striking. The PI spectra of peptide derivatives are interesting in that, compared with El spectra, the total number of peaks is reduced, especially in the low-mass region, and the molecular ion and higher-mass ions are more abundant. The sequence ions are more often formed by fission of C — C bonds as in (5), but one disadvantage of the method would appear to be that some of the important lower-mass sequence ions may be missing.

[FORMULA NOT REPRODUCIBLE IN ASCII]

Photoionization has certain advantages for the study of ion–molecule reactions, and they are the usual ones of energy resolution and low temperature. Lyman α radiation has been used to study the C3H6–C4D10 system, because it can ionize C3H6 and not C4D10. The principle reactions are

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

The second process is much more probable with propylene than with cyclopropane. In spectra of ethanol run using 10.68 eV radiation the only primary ion is EtOH[??], and in the absence of other ions it is easier to measure the pressure dependence of the solvated protons produced.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

Potapov and Sorokin have used a vacuum monochromator to examine the effect of photon energy on ion-molecule reactions. The rate of the reaction

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

is invariant with photon energy between the first and second ionization potentials of methanol. This is not true of the rate of formation of Me[??]D2 because there are two reactions involved, and the fragment-ion current will be affected by photon energy.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

The same authors have shown that the yield of MeCH[??]H from MeCHO does not vary from 10.2 — 12.5 eV, the reason suggested being that in this region there is no excitation of higher vibrational states of the ground-state ion. The yield of the reaction

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

decreases with increasing photon energy because the excited vibrational states of NH3[??] are accessible and are less reactive.

Some of the most spectacular results of photoion–molecule reactions have come out of the studies on ethane and propane. In the medium-pressure mass spectrum of ethane at 298 K using argon resonance radiation, (C2H6)+2 accounts for 25% of total ionization. At higher temperatures the abundance of this ion decreases, as does that of C4H+11). In a conventional electron impact source at 498 K, C4H+9 is the only abundant C4 ion observed. In a similar experiment (C3H8)+2 can be obtained from propane.

3 Chemical Ionization

A number of workers in this field have now published experimental details of ion sources used for chemical ionization. A quadrupole mass spectrometer has some advantages for this type of application, particularly when used as a g.c.–m.s. combination. The advantage of chemical ionization for gas chromatography work is that the ion source can take all or most of the column effluent, since the source works at higher pressures and faster pumping speeds than usual. A molecular separator should therefore be unnecessary, if the carrier gas is also used as reactant gas. The problem which one faces is to be able to register a complete chromatogram, since so much carrier gas is being ionized. This problem can be solved by using the quadrupole mass spectrometer as a crude mass filter, rejecting all ions below m/e 60. The output will then be the total ionization produced by the sample, with no contribution from the carrier (reactant) gas. Using such a system, Biemann and co-workers could get spectra from 1 µg of fatty acid ester.

Schoengold and Munson have obtained good Ci spectra using about 0.1 µl of sample. They have used both methane and helium as carrier gases and find that only the former gives true Cl spectra. The mass spectra obtained in the presence of 1 Torr of helium were very similar to electron impact spectra. Charge exchange to helium produces ions of very high internal energy which undergo drastic decomposition, but the cross-section for charge exchange is an order of magnitude less than for reactions of organic ions, so this is one of the few cases where electron impact ionization of the sample can compete with ion–molecule reactions under these conditions. A more sophisticated combined EI–CI system has been designed which incorporates two sources in a quadrupole mass spectrometer. The electron impact source monitors the gas in the source housing region outside the ionization chamber of the Cl source. The system can be programmed to run alternate El and Cl spectra but can also produce simultaneous EI–CI spectra which are claimed to have the advantages of both methods.

Many of the applications of chemical ionization are to compounds which have molecular ions of very low abundance in their electron impact spectra. The electron impact mass spectra of the barbiturates are often unhelpful; the molecular ions are undetectable and the alkyl groups are often eliminated to give identical spectra from different molecules, e.g. (6) and (7). In the chemical ionization spectra of the barbiturates the (M+ 1)+ ions have a relative abundance of 46 — 87% of total ionization and can be used for quantitative analysis of mixtures. Another group of biologically important compounds, the prostaglandins, give characteristic CI spectra. The base peak of PGA1(8) is (M – 17)+, which is probably formed by dehydration of the protonated molecular ion. More heavily hydroxylated prostaglandins do not form a stable (M + 1)+ ion and are more easily characterized as O-methylated derivatives.

The most successful applications of CI have been to nitrogen compounds, usually because reaction 1 produces a stable cation which does not decompose thermally, whereas in oxygen compounds reaction 3 is sometimes quantitative even at low temperatures. The molecular weight of the steroid aminoglycoside holacurtine (9) was very easily determined by isobutane CI. The more abundant ions in the spectrum can be easily explained: protonation on nitrogen gives the stable (M + 1)+ ion; protonation at oxygen joining the aglycone to the

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

[FORMULA NOT REPRODUCIBLE IN ASCII] (2)

[FORMULA NOT REPRODUCIBLE IN ASCII] (3)

amino-sugar leads to fission of either C — O bond and production of the ions at m/e 158 and 317. An interesting suggestion is made as to the origin of the ion of m/e 176, i.e. that it is formed by protonation of the neutral fragment formed by fission, as in Scheme 1. For this to be true (since the intensity of the peak at m/e 176 is of the same order of magnitude as other ions in the spectrum), it would require that the concentration of the neutral species of mass 176 was similar to that of the parent molecule in the ionization chamber, i.e. it requires an extremely high ionization efficiency. Since the sensitivity of this method is not much greater than that for electron impact, it is much more probable that the ion of m/e 176 is formed by a hydrogen rearrangement process from (M + H)+.

(Continues…)Excerpted from Mass Spectrometry Volume 2 by D. H. Williams. Copyright © 1973 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.