(MIKE) Spectrometer

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Instrumentation

Design and Performance of a Mass-analyzed Ion Kinetic Energy (MIKE) Spectrometer J . H. Beynon, R. G. Cooks, J . W. A m y , W. E. Baitinger, and T. Y. Ridley Department of Chemistry, Purdue University, West Lafayette, Ind. 47907

Ion kinetic energy spectrometry allows the study of both unimolecular and bimolecular reactions and is a unique source of information on thermochemistry, molecular structure, isotopic quantitation, and reaction dynamics

Analytical determinations of a qualitative and quantitative nature, as well as more fundamental studies on gaseous ions, form the province of mass spectrometry. Traditionally, mass spectrometers have been used to measure ion abundance as a function of ion mass-to-charge ratio. Variations on this theme include measurements of the ratio of abundances of ions of particular mass-to-charge ratios, as in isotope ratio and isotope incorporation studies, or the abundance of particular ions as a function of time as in mass chromatography, or the abundance of particular ions as a function of ionizing voltage, as in measurements of appearance potentials. These measurements are all incomplete in the sense that only the product ion is defined; nothing is learned of its origin. This ambiguity becomes most serious when attention is directed to the chemistry of gaseous ions, a subject of growing importance. The ambiguity can be removed in several ways. For example, double resonance techniques provide such information in the special case of ion cyclotron resonance instruments. Probably the most direct and useful

Figure 1. Ion optical arrangement used in MIKE spectrometer showing location of pumping and sample introduction ports 1, Sample introduction; 2, ion source; 3, source diffusion pump line; 4, source slit; 5, source isolation valve; 6, y, z deflectors; 7,13, collision gas inlet lines; 8, magnetic sector; 9, 12, 15, ion pump lines; 10, intermediate slit; 11, intermediate electron multiplier; 14, electrostatic sector; 16, final collector slit; 17, electron multiplier

method.is,to accelerate the reactant ions to a known kinetic energy (done routinely in many types of mass spectrometers as a prelude to mass analysis) and then to measure the energies of the resulting product ions that are formed in a field-free region of the spectrometer. Partitioning of energy in proportion to the masses of the ionic and neutral fragments occurs and serves to define the ratio of reactant to product ion masses. Thus, by a combination of mass and ion kinetic energy measurements, the mass of each product and reactant ion can be determined so that the ionic reaction can be defined. Metastable ions are ions which fragment after acceleration, that is, they have life times of the order of 10 _ 5 sec. The detection of their products, as the well-known diffuse peaks which occur in low abundance at .nonintegral mass positions in magnetically scanned mass spectra, in-

volves kinetic energy analysis. This energy determination, however, is indirect and does not provide unambiguous identification of the reaction. Nevertheless, it has sufficient value to account for the importance of the so-called metastable peaks in analytical determinations, especially structural determinations on organic compounds, as well as in fundamental investigations by mass spectrometry. The development of techniques for observing metastable peaks with high sensitivity by variation of the accelerating voltage at constant electric sector voltage (1-3) really represents a method for performing both ion kinetic energy and mass analysis. These techniques employ a double-focusing mass spectrometer of conventional geometry (ion source/electrostatic sector/magnetic sector/detector). Fragmentations of metastable ions occurring in the field-free region prior to the electrostatic sector are detect-

A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973 · 1023 A

ed by varying the ion accelerating voltage if the electrostatic and mag­ netic field strengths are fixed at ap­ propriate values. It is also possible to perform ion ki­ netic energy analysis without mass analysis if a detector is placed behind the electrostatic sector in such an in­ strument. The energy spectra so ob­ tained are termed ion kinetic energy (IKE) spectra (4-6), although the term has a wider meaning too. Provided only that sufficient sensi­ tivity is available, and it often is, it is our contention that because of the ex­ plicit information available, all appli­ cations of mass spectrometry, includ­ ing analytical applications, stand to benefit if kinetic energy analysis is combined with mass analysis. Also, any measurement done by using nor­ mal mass spectra can be better done by studying metastable ions whose kinetic energies and masses can read­ ily be measured. We have used a modified doublefocusing sector mass spectrometer of conventional geometry in metastable ion studies, including the investigaion of IKE spectra. These methods, ap­ plied in many laboratories, have greatly advanced the subject of ion chemistry (7). In this paper a new in­ strument for the combined study of ionic mass and ion kinetic energy is described. The instrument has the geometry: ion source/magnetic sec­ tor/electrostatic sector/detector. This is not a new mass spectrometer geom­ etry (8). The Purdue group was the first to suggest its use in metastable ion studies (9). Since then, this ap­ plication has been made with com­ mercially available reversed sector mass spectrometers (10, 11), and two other reversed sector instruments have also been described (12, 13). A feature which distinguishes the present instrument is its capability for high energy resolution. The above-mentioned instruments per­ form some of the functions to which the new method is particularly suit­ ed, especially with regard to defining ionic reactions and facilitating rela­ tive abundance measurements. How­ ever, the wealth of information avail­ able in kinetic energy distributions and in the details of ion kinetic ener­ gy peak shapes is only accessible by use of high energy resolution mea­ surements. These provide two further types of information: the amount of kinetic energy (T) released during any fragmentation can be accurately measured from the peak width, and the kinetic energy (Qr) converted into internal energy in an ion-molecule reaction can be determined. The "reversed" geometry allows the selection of a given ion and the subse­ quent study of all reactions leading from this ion. The name mass-ana­

lyzed ion kinetic energy spectrometer (MIKES) is applied to the instru­ ment (14). This geometry is comple­ mentary to the conventional geometry in that the latter is suited to the study of all reactions leading to a par­ ticular product ion. The present instrument was de­ signed for moderate mass resolution and high energy resolution. It is im­ portant to understand that the in­ strumentation needed for ion kinetic energy spectrometry is equally appli­ cable to the study of both unimolecular and bimolecular reactions, and provision has therefore been made for the introduction of collision gas into selected regions of the instrument. To obtain the maximum amount of in­ formation from ion kinetic energy dis­ tributions, slit heights, as well as the width of the energy resolving slit, are variable. The instrument has been designed for computer control. It is expected that the MIKE spec­ trometer will find an even broader range of applications to chemical analysis, ion structure determination, reaction mechanisms, kinetic studies, and energy partitioning investigations than has the IKE spectrometer (7). Specific advantages of this technique are as follows: there is no limit to the ratio of parent-to-daughter ion mass which can be studied since the accel­ erating voltage is not scanned; all reactions are studied at constant sensitivities; the characterization of a single ion can be done by utiliz­ ing its unimolecular fragmentations, collision-induced fragmentations, and the various high-energy charge ex­ change reactions which it can be made to undergo. The relative abun­ dances and the details of the kinetic energy peak shapes for each of these processes can be utilized in studying the ion in question. The advantages of the above for chemical analysis and ion structure determination are significant. Description of instrument A sketch of the ion optical arrange­ ment used is shown in Figure 1. The ion source, magnet, and electrostatic sector are standard components used in the commercial MS902 mass spec­ trometer (15) manufactured by ΑΕΙ (Scientific Instruments) Ltd. and were donated to us. Mass analysis is accomplished with a 12-in. radius 90° magnetic sector and energy analysis with a 15-in. radius 90° elec­ trostatic sector. In the arrangement used in this instrument, the magnetic sector gives an image magnification of unity, and the electrostatic sector a magnification of 0.793. In the ar­ rangement of magnetic and electric fields used, the beam is deflected in opposite directions in the two sectors. This is different from the Nier-John-

1024 A · ANALYTICAL CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973

son double-focusing geometry and was adopted merely to give the most convenient mechanical arrangement. Provision has been made for fitting an inverting lens of unit magnifica­ tion between the magnetic and elec­ trostatic sectors if it should ever prove necessary to convert the instru­ ment into a double-focusing mass spectrometer of high mass resolution. Fitting this lens would be exactly equivalent to reversing the direction of deflection of the ion beam through the magnetic field. However, as will be emphasized later, high mass reso­ lution is not important in a MIKE spectrometer. What is required is ad­ equate energy resolution of the massanalyzed ion beams, and this can be achieved without recourse to an ar­ rangement of magnetic and electric fields giving simultaneous direction and velocity focus. Th curved wave-guide tubing of rectangular cross section necessary to transmit the ion beam through the magnet was supplied by ΑΕΙ Ltd. All other parts of the vacuum system were fabricated from blocks of 304 stainless steel. The time scale of the instrument follows directly from the geometry and the ion source residence time. Ig­ noring the latter term, the time scale can be summarized conveniently as shown below. Relative Region time Ion chamber Acceleration 1.0 Pre-magnet 22.0 Magnetic sector 34.4 Post-magnet 22.0 Collision region 15.9 ESA 42.9 Post-ESA 4.5 Collector To obtain time in μββο, multiply above numbers by (m/Vx)1·'2 where m is the ion mass (amu), V is the accelerating voltage in volts, and χ is the charge on ion. The source slit, intermediate slit, and the final collector slit (Figure 1) are located at the focal points of the two sectors. A variable slit was fitted as part of the ion source supplied. This was removed and used instead as the final collector slit. The slit mechanism is of the type described by Jones (16, 17) and can be adjusted in width from a maximum of 0.030 in. to a fraction of 0.001 in. As described later, it is usually operated at a set­ ting of 0.005 in. The fixed source slit that is used as a replacement for the variable slit consists of a rectangular aperture of 0.22 in. by 0.005 in. in width. This slit housing was carefully

fitted to fill the aperture connecting the source to the remainder of the ion path. This ensures good differential pumping between the source region and the field-free region in front of the magnet, the only connection being via the slit aperture itself. The intermediate slit housing was similar­ ly made to fit the flight tube to give good differential pumping between the regions on either side of it. The aperture in this slit housing has the same dimensions as those of the source slit. By ignoring aberrations and energy spread in the ion beam, the source and intermediate slits should give a mass resolution of 1200. The resolution achieved in practice is estimated on a 10% valley definition to correspond to m / i m = 1150. The electric sector is equipped with an entrance slit located in the flange which is used to attach it to the colli­ sion region. This slit is 0.050 in. wide and 0.500 in. high and serves as a means of decreasing the acceptance angle of the sector as well as providing differential pumping. The width of the final collector can be adjusted to accommodate the entire width of the beam. The effect of scanning a single mass peak across the final collector by varying the elec­ trostatic sector field is shown in Fig­ ure 2. Intensity begins to be lost when the slit width is reduced to 0.004 in. Curves such as those shown in Figure 2 may be used to determine the dis­ persion of the beam at the final col­ lector. The variable collector slit opens symmetrically about its center; a plot of the electrostatic sector volt­ age at which the edge of the beam is just detected plotted against slit width thus enables the dispersion to be estimated. It was found that a beam deflection of 0.17 in. results from a 1% change in electrostatic sector volt­ age, corresponding to a 1% change in ion energy. If a metastable ion decomposes in the field-free region in front of the electrostatic sector, with conversion of some internal energy into translational energy, this will result in daughter ions having a range of kinet­ ic energies and an increased spread in direction (18, 19). The daughter ion peak is broadened owing to the range of energies, and if the energy converted is sufficiently large, ions deflected in the 2-direction along the length of the slits (in which direction there is no focusing action) may miss the collec­ tor slit. This discrimination leads to a dishing at the center of the peak. The edges of the peak, corresponding to ions with velocities only along the xdirection or which have been deflect­ ed only in the x-y plane, will be unaf­ fected. It is sometimes desirable to in­

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Figure 2. Effect of collector slit width upon width and intensity of kinetic energy peak. Scale is given in terms of electrostatic sector voltage (one plate to ground)

crease this discrimination as much as possible to improve the chances of detecting overlapping peaks or of seeing fine structure caused by formation of products in various vibrationally excited states (20, 21). For this reason, slit heights throughout the instrument have been kept to a maxi­ mum value of 0.22 in., and in addi­ tion, provision has been made for shortening the intermediate and final collector slits independently, either to 0.11 or 0.055 in., whenever desired. This is accomplished by moving a stepped aperture across either slit to provide the required height. Move­ ment of the aperture is controlled by means of a micrometer screw operating through a vacuum bellows assembly. Figure 3 shows the peak owing to daughter ions from the reaction C e H 6 0 + ' — * C 5 H 6 0 + ' + CO

(l)

in phenol, the fragmentation occur­ ring in the field-free region in front of the electrostatic sector. In this reac­ tion, 0.50 eV of internal energy is converted into translational energy, and the peak is dish-topped. Ions formed in this process are not discrim­ inated against in instruments fitted with longer slits than are used in this work. For example, in the RMH-2 mass spectrometer (made by Hitachi/ Perkin-Elmer), which has been used in many studies involving metastable ions, the peak has a rounded shape. The electronic circuits used are mostly based upon commercially available units. The accelerating volt­ age supply is a continuously variable 0-10 kV supply, Model 410B manufac­ tured by John Fluke Manufacturing Co., Inc., the output of which is guar­ anteed stable to 0.005% per hour; it is even better than this after an hour's warm-up period. In a double-focusing mass spectrometer, the accelerating voltage does not need to be this sta­ ble because small variations in ion

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Figure 3. Metastable peak owing to loss of CO from phenol molecular ion. Main beam of stable ions is shown on same energy scale

energy are compensated by the dou­ ble-focusing properties. In the present case, however, setting the instrument to record the current corresponding to a point half way up the side of a mass peak and obtaining a current steady to within ± 1 % of the peak height re­ quires, at a mass resolution of 1000, a stability of accelerating voltage of 5 ppm. To ensure that the various volt­ ages tapped off this supply to provide the necessary voltages for focusing and deflecting electrodes are also maintained steady, the chain of high stability resistors used for this purpose is immersed in an oil bath thermostatedto ±0.1°C. Two alternative methods of driving the electrostatic sector are available. The first of these makes use of two 0-250 V supplies, Model LSDM6 manufactured by Lambda Electronics Corp., which are stable to within 0.001% + 100 μ ν over an 8-hr period. They are arranged to provide equal positive and negative voltages to the sector plates by a voltage divider cir­ cuit. A two-pole, five-way switch al­ lows 0.3, 1.0, 3.0, 10.0, or 30.0% of the supply voltage to be applied across two-ganged, ten-turn helical potenti­ ometers. These helipots can be

ANALYTICAL CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973 · 1025 A

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scanned mechanically with a slow speed motor and gearbox. The electrostatic sector voltage, set at say, 500 V, can be scanned by using this arrangement at any prechosen rate from 0.003 to 10 V/min. This mechanical system of scanning has the advantage that the starting position and range of any scan are extremely reproducible, and it is therefore suitable for use with a computer-controlled system. The alternate system uses a highvoltage operational amplifier approach. The positive sector supply is generated by an Analog-Devices, Inc., Model 180 amplifier, zener coupled to a high-voltage pass transistor. A wirewound, low-temperature coefficient, feedback resistor is used to connect the output to the amplifier summing junction. A reference voltage is fed to the summing junction to set the initial sector voltage, and a scan ramp is also applied to this junction during electronic scanning. The resulting amplifier has a gain of 32; 10-V input produces 320-V output. The negative sector supply is based on a similar system with a gain of — 1.0000. The input to this inverter is the output of the positive supply. Thus, tracking is assured and both supplies vary equal amounts from signal common, which is spectrometer ground. Scanning may be accomplished with several types of ramp generators. These include the following: an operational amplifier integrator circuit, a Solion (Self-Organizing Systems, Inc., Dallas, Tex.) electrochemical cell ramp generator, and a computer via a D to A convertor. The magnet coils are driven by a modified supply originally manufactured by Hitachi Ltd. for use on their RMH-2 mass spectrometer. It can deliver up to 4 amps of current at 80 V which, with the magnet coils connected in parallel, enables a mass of 450 to be transmitted at an ion-accelerating voltage of 8 kV. Higher masses can be studied at the same current if the coils are wired in series. Stability of the magnet and source supplies is sufficient after an hour's warm-up to set the mass analyzer on to any mass peak and have it maintain this setting for a period of days. The ion beam is measured with 16 stage electron multipliers Model R474 manufactured by Hamamatsu Co., Ltd. One of these is fixed in position behind the final collector slit; the other is mounted on a vacuum bellows assembly and can be lowered into position behind the intermediate slit when it is required to plot a mass spectrum or to select a mass peak for subsequent energy analysis of its fragmentation products. Having selected a mass peak for study, the multiplier is

raised out of the ion beam which can then pass into the electrostatic sector. The electron multipliers are pow­ ered by a 0-3 kV continuously vari­ able supply, Model 415B manufac­ tured by John Fluke Manufacturing Co., the output of which is stable to ±0.002% per hour. A high-voltage switch is used to connect this supply to one of the electron multipliers. Two electrometer detectors are used to record the outputs of the electron multipliers. For the multiplier locat­ ed behind the intermediate slit, a Model 417 picoammeter manufac­ tured by Keithley Intruments and ca­ pable of reading currents down to 10 13 amp for full-scale deflection is used. For the final collector, a Model 640 variable capacitor electrometer manufactured by the same company is used. This is the electrometer that is required to read the smallest cur­ rent owing to daughter ions of frag­ mentation reactions. It has a maxi­ mum sensitivity of 1 0 - 1 5 amp for full-scale deflection, a stability of better than 5 x 10 17 amp per day, and can record the arrival of individ­ ual ions. The output of either electrometer can be fed to a pen recorder (Model 165 manufactured by Perkin-Elmer). An R-C filter network of time con­ stants variable from 0.1 to 100 sec can be interposed between either electrometer and the pen recorder for use when the slowest scan speeds are being used to average out statistical noise fluctuations. The accelerating voltage, the volt-

age on either the positive or negative electrostatic sector plate, or the mag­ net voltage can be monitored or mea­ sured by using a switching arrange­ ment which compares the appropriate voltage with a voltage supplied from a 6-dial guarded DC differential volt­ meter (Model 662 manufactured by Keithley Instruments). This has a limit of error of 0.01% of reading and a repeatability of 0.0025%. By using this as a null indicator, meter errors are removed. The ion source region is evacuated by an oil diffusion pump, and the pressure in this region is monitored by a Bayard-Alpert ionization gauge which is also the basis for the vacuum protection system which switches off the ion source supplies, the ion accel­ erating voltage, and the diffusion pump in the event of a failure in the system. Three magnetic ion pumps, manufactured by the Ultek Division of Perkin-Elmer, evacuate the re­ mainder of the system, as shown schematically in Figure 1. Ion pump current serves as an indication of sys­ tem pressure. However, two BayardAlpert ionization gauges are also fit­ ted to the two regions into which colli­ sion gas can be introduced (the fieldfree regions in front of the magnetic sector and electrostatic sector). The pressure can thus be monitored in these regions even when the ion pumps have been isolated from them. Pirani gauge vacuum monitors are fitted to each of the differentially pumped regions and coupled to pro­ tection circuits so that the pumps are

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CALIFORNIA EASTERN LABORATORIES INC. Figure 4. Kinetic energy peak owing to daughter ions formed by unimolecular loss of HCN from molecular ion of sy/nm-triazine. Kinetic energy release cal­ culated from half width of this peak is 0.26 meV (6 c a l / m o l ) . Ion accelerating voltage was 7400 V, and final collector slit width 0.006 in. ANALYTICAL

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automatically switched off in the event of a vacuum failure. Circuits also switch off the electron multiplier power supplies if the vacuum fails. A vacuum valve is provided so that the ionization chamber region may be completely isolated in the event of a filament failure to avoid the necessity of letting the whole system up to at­ mospheric pressure when the source region is vented. Whenever any part of the instrument has to be vented to atmospheric pressure, this is done by introducing dry nitrogen gas through venting valves specially fitted for this purpose. Sample introduction is by a conventional heated reservoir all-glass system, and in addition, systems are provided for introducing collision gases by using materials having sufficient vapor pressures at room temperature.

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Figure 10. Partial MIKE spectrum of m/e 45 ( C 2 H 5 0 + ) from (a) isopropanol and (b) chloromethyl methyl ether showing absence of reaction 45+ -->• 1 9 + + 2 6 in (b)

b r o k e n . T h e r e l e v a n t p e a k s for la­ beled a n d u n l a b e l e d m a t e r i a l are s h o w n in Figure 9. T h e M I K E S technique has particu­ lar utility in s t u d y i n g t h e s t r u c t u r e of a given ion, either as an e n d in itself or as a r o u t e to m o l e c u l a r s t r u c t u r e s . B o t h a b u n d a n c e s a n d k i n e t i c energy releases can be utilized in s u c h a s t u d y as well as t h e kinetic energy d i s t r i b u ­ t i o n s . Figure 10 c o m p a r e s p a r t i a l M I K E s p e c t r a of t h e isomeric C 2 H 5 0 + ions g e n e r a t e d from (a) iso­ p r o p a n o l a n d (b) c h l o r o m e t h y l m e t h ­ yl e t h e r . Only one of t h e two reac­ tions is c o m m o n to b o t h ions, a n d t h e s t r u c t u r e s are r e a d i l y d i s t i n g u i s h e d since t h e y give c o m p l e t e l y different k i n e t i c energy d i s t r i b u t i o n s . A p p l i c a t i o n s of t h e M I K E S t e c h ­ n i q u e to t h e s t u d y of ion-molecule r e a c t i o n s a t high energy m a y be illus­ t r a t e d by t h e b e h a v i o r of t h e p y r i d i n e m o l e c u l a r ion shown in Figure 11. T h e p e a k owing t o loss of H C N is t h e only t r a n s i t i o n seen u n d e r c o n d i t i o n s which allow only u n i m o l e c u l a r r e a c ­ t i o n s . O n a d d i t i o n of air as collision gas, collision-induced e x c i t a t i o n oc­ curs, a n d t h e r e s u l t i n g ions f r a g m e n t by a n u m b e r of different r e a c t i o n channels. Conclusions T h e subject of ion kinetic energy s p e c t r o m e t r y c o n s t i t u t e s a n e w form of s p e c t r o m e t r y w h i c h is e m e r g i n g from a n d c o m p l e m e n t a r y to m a s s

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Figure 1 1 . Reactions undergone by pyridine molecular ion with collision gas pressures of (a) 1 X 1 0 ~ 7 torr and, (b) 1.5 X 1 0 ~ 4 torr. Main peak under both sets of con­ ditions is due to loss of HCN

s p e c t r o m e t r y . It allows t h e s t u d y of both unimolecular and bimolecular r e a c t i o n s a n d is a u n i q u e source of in­ f o r m a t i o n on t h e r m o c h e m i s t r y , m o ­ lecular s t r u c t u r e , isotopic q u a n t i t a ­ tion, a n d reaction d y n a m i c s . T h e use of t h e M I K E S g e o m e t r y possesses u n i q u e a d v a n t a g e s in ion a n d molec­ ular structure determination. Acknowledgment We are grateful to Associated Elec­ t r i c a l I n d u s t r i e s (Scientific I n s t r u ­ m e n t s ) L t d . for c o m p o n e n t s u s e d in t h e i n s t r u m e n t a n d to S. E v a n s for t h e ion optical c a l c u l a t i o n s . T h e assis­ t a n c e of Dick Feeney in t h e m a c h i n i n g a n d D . T . Terwilliger a n d R. M . Elliott w i t h t h e electronics is grate­ fully acknowleged. References (1) M. Barber and R. M. Elliott, presented at the Twelfth Annual Conference on Mass Spectrometry and Allied Topics, ASTM Committee E-14, Montreal, Can­ ada, 1964. (2) J. H. Futrell, K. R. Ryan, and L. W. Sieck, J. Chem. Phys., 43, 1832 (1965). (3) K. R. Jennings, ibid., 4176 (1965). (4) J. H. Beynon, J. W. Amy, and W. E. Baitinger, Chem. Commun., 723(1969). (5) J. H. Beynon, R. M. Caprioli, W. E. Baitinger, and J. W. Amy, Int. J. Mass Spectrom. Ion Phys., 3, 313 (1969). (6) R. W. Kiser, R. E. Sullivan, and M. S. Lupin, Anal. Chem., 41, 1958 (1969). (7) R. G. Cooks, J. H. Beynon, R. M. Caprioli, and G. R. Lester, "Metastable Ions," Elsevier, Amsterdam, the Neth­ erlands, 1973.

(8) F. A. White, F. M. Rourke, and J. C. Sheffield, Appl. Spectrosc, 12,46 (1958). (9) J. H. Beynon, R. M. Caprioli, and T. Ast, Org. Mass Spectrom., 5, 229 (1971). (10) K.ffMaurer, C.Brunnee, G.Kappus, K. Habfast, U. Schroder, and P. Schulze, presented at the Nineteenth Annual Conference on Mass Spectrome­ try and Allied Topics, ASMS, Atlanta, Ga., 1971. (11) Nuclide Corp., technical literature, 1971. (12) T. Wachs, P. F. Bente III, and F. W. McLafferty, Int. J. Mass Spectrom. Ion Ph\s., 9, 333(1972). (13) M. E. S. F. Silva and R. I. Reed, pre­ sented at the Twenty First Conference on Mass Spectrometry and Allied Top­ ics, ASMS, San Francisco, Calif., 1973. (14) J. H. Beynon and R. G. Cooks, Res./ Develop., 22(11), 26(1971). (15) R. D. Craig, B. N. Green, and J. D. Waldron, Chemia, 17, 33 (1963). (16) R. V. Jones, J. Sci. Instrum., 29, 345 (1952). (17) R. V. Jones, ibid., 33, 169 (1956). (18) J. H. Bevnon, R. A. Saunders, and A. E. Williams. Z. Naturforsch, 20a, 180 (1965). (19) J. H. Beynon and A. E. Fontaine. ibid., 22a, 334(1967). (20) T . R . GoversandJ.Schopman, Chem. Phys. Lett., 12, 414 (1971). (21) R. G. Cooks and J. H. Beynon, Chem. Commun., 1282(1971). (22) R. G. Cooks, M. Bertrand, J. H. Beynon, M. E. Rennekamp, and D. W. Setser, J. Amer. Chem. Soc, 95, 1732 (1973). (23) J. H. Beynon, M. Bertrand, and R. G. Cooks, ibid., 1739 (1973). (24) T. W. Shannon and F. W. McLaff­ erty, ibid., 88, 5021 (1966). Research supported by the National Science Foundation.

ANALYTICAL CHEMISTRY, VOL. 45, NO. 12, OCTOBER 1973 · 1031 A