8. Jelus, B. Munson, and C . Fenselau, Anal. Chem., 46, 729 (1974). B. Jelus, B. Munson, and C. Fenselau, Biomed. Mass Spectrom., 1, 96 (1974). D. F. Hunt and J. F. Ryan, Chem. Commun.,620 (1972). D. F. Hunt, T. M. Harvey, and J. W. Russell, J. Chem. SOC., Chem. Commun., 151 (1975). S.K. Searles and P. Kebarle, J. fhys. Chem.. 72, 742 (1968). M. A. French, L. P. Hills, and P. Kebarle, Can. J. Chem., 51, 456 (1973). W. W. Harrison and C. W. Magee, Anal. Chem., 46,461 (1974). F. H. Field, M. S. B. Munson, and D. A. Becker, Adv. Chem. Ser., 56, 167 (1966). D. F. Hunt and C. N. McEwen. Org. Mass Spectrom., 7, 441 (1973). S.K. Stuart and L. W. Sieck, J. Chem. Phys., 53, 794 (1970). N. Einolf and B. Munson, ht. J. Mass Spectrom. /on Phys., 9, 141 (1972). J. L. Moruzzi and A. V. Phelps, Chem. fhys., 45,4617 (1966). J. H. Futrell and T. 0. Tiernan, ton-Molecule Reactions", J. L. Franklin, Ed., Plenum Press, New York 1972, Vol. 2, pp 485-551. I. Dzidic, D. I, Carroll, R. N. Stillwell, and E. C. Horning, J. Am. Chem. Soc., 96, 5258 (1974). F. H. Field, J. Am. Chem. Soc., 92, 2672 (1970). i. Dzidic, D. I. Carroll, R. N. Stillwell, and E. C. Horning, Abstracts 22nd Annual Conference on Mass Spectrometry and Allied Topics, Phiiadelphia, Pa., May 1974, No. V-1. R. C. Dougherty, J. Dalton, and F. J. Biros, Org. Mass Spectrom., 6, 1171 (1972).
LITERATURE CITED M. S.8. Munson and F. H. Field, J. Am. Chem. Soc., 68, 2621 (1966). A. L. Burlingame. R. E. Cox, and P. J. Derrick, Anal. Chem., 46, 248R (1974). F. H. Field, "Ion Molecule Reactions", J. L. Franklin, Ed., Plenum Press, New York. 1972, pp 261-312. F. H. Field, "MIP International Review of Science", Physical Chemistry, Voi. 5 , A. Maccoll, Ed., Butterworth, London, 1972, p 133. M. S.B. Munson, Anal. Chem., 43(13), 28A(1971). D. F. Hunt, Prog. Anal. Chem., 6, 359 (1973). G. W. A. Milne and M. J. Lacey, "Modern lonization,Techniques in Mass Spectrometry," Crit. Rev. Anal. Chem., 45 (1974). D. Beggs, M. L. Vestal, H. M. Fales, and G. W. A. Milne, Rev. Sci. lnstrum., 42, 1578 (1971). H. M. Fales, Y . Nagai, G. W. A. Milne, H. B. Brewer, Jr., and J. J. Pisand, Anal. Biochem., 43, 288 (1971). D. F. Hunt, Adv. Mass Spectrom.: 6, 517 (1974). C. N. McEwen, T. M. Harvey, and D. F. Hunt, Abstracts 22nd Annual Conference on Mass Spectrometry and Allied Topics, Philadelphia, Pa., May 1974, No. X-5. E. C. Horning, M. G. Horning, D. i. Carroll, i. Dzidic, and R. N. Stillwell, Anal. Chem., 45, 936 (1973). D. I. Carroll, I. Dzidic. R . N. Stiilwell, M. G. Horning, and E. C. Horning, Anal. Chem., 46, 706 (1974). I. Dzidic, D. I. Carroll, R . N. Stillwell. and E. C. Horning, Abstracts 22nd Annual Conference on Mass Spectrometry and Allied Topics, Philadeiphia, Pa.. May 1974, No. L-4. E. C. Horning, D. I. Carroll, I. Dzidic, K. D. Haegele, M. G. Horning, and R . N. Stillwell, J. Chromatogr.,99, 13 (1974). E. C. Horning, D. I. Carroll, i. Dzidic, K. D. Haegeie, M. G. Horning. and R. N. Stillwell, J. Chromatogr. Sci., 12, 725 (1974).
;
RECEIVEDfor review March 19, 1975. Accepted May 23, 1975. Acknowledgment is made to the National Science Foundation in the US.Army Research Office for financial support of this research.
Imaging Detector for Mass Spectrometry J. H. Beynon,' D. 0. Jones,2 and R. G. Cooks Depatfment of Chemistry, Purdue University, West La fayette, Ind. 47907
An imaging ion detector, consisting of an array of channel electron multipliers, a phosphor, an optical system, a vidicon camera, and a data acquisition unit, has been fitted to a Mattauch-Herzog mass spectrometer. The detector repiaces the conventional photographic plate in such instruments and has far greater sensitivity, although, in its present form, it gives lower resolution. its use in simultaneous multiple ion monitoring Is demonstrated and its application in ion kinetic energy spectrometry is foreseen. Deiiterious effects of fringing magnetic fields on the resolution of the device and saturation effects in the channeitron are discussed. The feasibility of achieving the enhanced sensitivity inherent in imaging detection in mass spectrometry is established.
The photographic plate has played an important part in the practice of mass spectrometry since the earliest days. Specially prepared plates were used by Thomson (I, 2 ) in the detection system of the parabola spectrograph and similar plates are still used today on high resolution instruments (3-5) in which the arrangement of electric and magnetic fields used produces a plane of focus, as in the Mattauch-Herzog geometry (6). On the other hand, the electrical detection system used on other high and low resolution mass spectrometers has been continuously developed with respect to both its sensitivity and accuracy since the time when currents were detected using gold leaf electroscopes.
Permanent address, Chemistry Department, University College of Swansea, Singleton Park, Swansea SA2 8PP, United Kingdom. Permanent address, David 0. Jones Company, Chelan, Wash. 98816. 1734
*
The advent of the thermionic vacuum tube meant that dc amplifiers were designed, capable of detecting and recording currents down to A when used with input resistors of 1011R and simple Faraday cup detectors. The use of specially designed vibrating reed units in these electrometer amplifiers led to a reduction in drift that moved the limit of detection to approximately A. Further improvement in sensitivity resulted with the introduction of electron multiplier detectors of various types (7, 8 ) , of similar detectors in which the primary signal is converted into a scintillation before measurement (9) and of ion counting techniques (IO). These developments further increased the sensitivity to the point that the arrival of individual ions a t the detector could be monitored. A rate of arrival of 1 ion per second can be measured and this rate represents a curA. These electrical detectors give the rent of 6 X added advantages of wide dynamic range and of good linearity and in both these respects are more convenient and accurate than the photographic plate. Although the inherent sensitivity of the photographic plate is poor by modern standards (it requires the arrival of about 500 ions of 10 keV energy to produce perceptible blackening of the plate), its comparative cheapness and the fact that it can record an entire mass spectrum continuously without the need for scanning the mass peaks successively across a slit offsets this limitation and accounts for its continued use. The effective increase in sensitivity that is achieved is equivalent to the Felgett or multiplex advantage (11) that arises when the slit can be removed in optical spectrometry and an interferometric method used to record spectra. The extent of the advantage depends upon the ratio of the dispersion of the spectral range to be scanned to the slit width; there is more advantage in using a photographic plate in mass spectrometry for plotting en-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
tire high resolution mass spectra than for investigating the characteristics of a single mass peak. There are many examples in work of the type done in this laboratory where several mass peaks need to be compared under different conditions or where wide metastable peaks need to be investigated at the best possible sensitivity. The recently developed imaging detectors employing arrays of channel electron multipliers (12, 13) offer the opportunity of combining the sensitivity of the multiplier detector with the multiplexing advantage of a focal plane detector. These detectors consist of an array of miniature electron multipliers in a regular pattern as illustrated in Figure 1. They may consist of an array of single multipliers giving a gain of the order of lo7 when the appropriate voltage is applied across them or of two stage multipliers arranged in a chevron pattern ( 1 4 , 1 5 ) ,each arm of the chevron giving a gain of 103-104. With this latter arrangement, the possibility of feedback of cascading electrons is reduced and the signal-to-noise ratio improved. Commercial units are now available in which individual multipliers are spaced 40 ~ror less apart. The experiments described in this paper were carried out using a single stage array having a spacing of 290 p (dimension d, Figure 1). The mode of operation of an imaging detector is as follows: An ion impinging on the sensitive surface of one of the multipliers in the array produces a number of electrons which are accelerated for a short distance through the device before striking the active surface again. Multiplication of the number of electrons occurs each time there is a collision with a wall surface. A shower of electrons emerges from the rear of this channel in the multiplier array where it comes under the action of an electric field between the multiplier array and a screen of phosphorescent material similar to that used in conventional oscilloscopes. The electron shower produces a scintillation when it strikes the phosphor. A vidicon storage device is used to collect the signal from this scintillation and to add to it at the same position on the vidicon surface any further signals due to the arrival of further showers of electrons from the same multiplier. Adjacent storage positions on the vidicon are used to integrate the signals received from adjacent channels in the multiplier array. The signal intensity received a t the vidicon depends upon the number of electrons arriving at an individual position on the phosphor surface. This is proportional to the gain of the multiplier channel and to the number of ions entering the device at this individual channeltron position as well as the voltage between the rear of the electron multiplier and the phosphor surface. The gains of the individual multipliers are not all the same and some method must be found that averages out the gain differences. For a spectrometer system in which point images are produced, this can be achieved by scanning of the signal across the multiplier array so as to produce an averaged signal but, for the line images in a mass spectrum, this averaging is automatically achieved because the signal from any line is averaged over the number of individual multipliers in any column (resolution element) on which the ions making up the line signal fall. Experience in using imaging detectors has already been obtained in other fields. They have been used in conjunction with optical spectrographs in which only very low light levels can be achieved (16). Applications are to be found, for example, in flame spectrometry (17, 18). The only difference between these and the present application of the device is that photons fall upon the multiplier input rather than ions. Imaging detectors are also used in a commercial ESCA spectrometer (Hewlett-Packard Model 5900) where they give an effective increase in sensitivity of approxi-
Figure 1. The upper part of the diagram illustrates the geometrical arrangement of the individual electron multipliers making up the channeltron array. The distance d was 290 p for our experiments. The central part of the diagram shows the appearance of the image on the phosphor that is placed behind the channeltron strip when four ion species fall simultaneously on the front face of the channeltron array. The lower part of the figure illustrates the peak shapes obtained when the images from the phosphor are viewed with the vidicon camera
mately an order of magnitude. In this application, low energy electrons fall on the multipliers, but otherwise the mode of action is the same. Preliminary experiments aimed a t investigating the use of an imaging detector in mass spectrometry have also been described (19) and this paper extends the information from these preliminary studies. Several groups have used channeltron detectors in the nonimaging mode on mass spectrometers (20, 21). The experiments discussed below were designed to'test the applicability of the device as an energy analyzer in ion kinetic energy (IKE) spectrometers (22). In any such application, of course, the energy resolution of the device must be sufficient to match that of the electric or magnetic sector being used to disperse the beam, otherwise the imaging detector will itself limit the attainable resolution. For the RMH-2 spectrometer, modified for the IKE work (23), for example, the ultimate energy resolution of the electric sector in combination with th& accelerating voltage system and lens system used to produce the ion beam is about 1 part in IO4.An energy change of this amount produces a displacement of the beam in the plane of energy focus of about 50 microns. As seen from the figures quoted above, an imaging detector that would not limit the resolution when used with such a sector is now commercially available although the channel plate available to us had somewhat poorer specifications. It has been pointed out that the presence of magnetic fields can seriously affect the performance of an imaging detector (19). I t is, thus, obviously preferable to use the device either at the energy resolving slit of a double-focusing instrument or a t the final collector in an instrument in which this is located a considerable distance from the magnetic field as in the case of Nier-Johnson geometry (24). It was, nevertheless, most convenient for us to test the device on a Mattauch-Herzog instrument, despite the fact that the multiplier had to be placed in the fringe field of the magnet where the focal plane is located, because of the work load on our Nier-Johnson instrument. We also had an interest in using the detector for the simultaneous study of ions of several different, mass to charge ratios, an application in which the Mattauch-Herzog geometry is superior.
EXPERIMENTAL ARRANGEMENT Figure 2 is an overall view of the detector and mass spectrometer and Figure 3 is a schematic diagram showing the way in which the detector is fitted to the mass spectrometer. The mass spectrometer is a model CEC-21-11OB manufactured by Consolidated Electrodynamics Corporation
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
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ion on multipliei
\
vidicon
Schematic representation showing the relative positions of the channeltron array, phosphor, lens, and vidicon camera Flgure 2.
The front face of the channeltron Is located In the focal plane of the mass spectrometer (a CEC 21-llOB) and does not affect the use of the normal electron multiplier detector which is also shown, in schematic form, on the diagram. A viewing window Isolates the lens and vidicon camera from the vacuum. VI and Vz represent the high voltages across the channeltron multiplier array and between the rear surface of the channeltron and the phosphor, respectively
Flgure 4.
Peaks at m/e
14.5 and 15 in the
mass spectrum of ethane
The peak at m/e 14.5 makes up 0.2% of the total ion current under the conditions used. The spectrum was taken with an ion current which was only lo-' of that normally used
adjustment of the position so as to achieve optimum focus and also the retraction of the device out of the focal plane when it is desired to use the photoplate detector system. The fixed part of the mounting carries all the electrical feed-throughs and terminates in a flange that holds the viewing window as shown in Figure 4. The channeltron and phosphor are carried on the four vacuum-sealed sliding rods shown in the Figure. Outside the vacuum system, the rods are connected to a metal ring. The heavy window flange is also used to support the optical system and the lens.
THE DETECTOR
fQQd-throughinaulotor
8
mount ( l n r ~ r t e d )
L i e n s mount (withdrawn)
Flgure 3.
Illustration of the mechanical arrangement used
The tube holding the flange for the viewing window is welded to a plate that forms part of the magnet housing. This plate carries the high voltage feedthrough insulators for supplying the voltages to the channeltron and phosphor. The lens and vidicon arrangement are attached to the window flange. A mount held on four vacuum-sealed, sliding rods and holding the phosphor and channeltron array enables the lens to be moved relative to the phosphor
(25). This instrument employs a modification of the Mattauch-Herzog double-focusing geometry such that the focal plane is located approximately one half of a magnet gap width beyond the edge of the magnet pole face. Mass spectra can be recorded either on photoplates, the emulsion of the photoplate being located in the focal plane, or using an electron multiplier. When the multiplier is being used, the photoplate is replaced by an aluminum mask that prevents secondary electrons from reaching the multiplier and distorting the peak shapes. For the present series of experiments, a hole was cut in this mask so that the front surface of the imaging detector could be located in the focal plane while the mask was in place. This gives the further advantage that the imaging detector and the electron multiplier could be used simultaneously, if desired. The channeltron multiplier and phosphor are capable of being moved, as a unit, relative to the focal plane. This movement is achieved by means of a sliding vacuum seal and allows both for fine 1736
The channeltron strip used in these experiments is an early version developed by Bendix Corporation. I t has an active area 25 mm long and 6 mm wide and the individual channels are arranged in a honeycomb pattern, the distance from center to center of individual channels in the same row being 0.29 mm. The effective resolution of the arrangement is, however, 0.15 mm because of the closer spacing of adjacent columns caused by the honeycomb arrangement. The phosphor was supplied by Galileo Electro Optics Company. It has an active area measuring 35 mm by 16 mm and was made of type P-20 phosphor with a tin oxide conductive coating. The phosphor plate had a narrow chromium border by which electrical contact could be made to the phosphor. I t was rigidly mounted 1 mm away from the rear face of the channeltron.
OPTICAL SYSTEM Light from the scintillations produced a t the phosphor is transmitted through the window in the vacuum system shown in Figure 3 and is optically coupled through a Nikon camera lens of focal length 105 mm, using black PVC sewer pipe, to the active face of the vidicon tube. The focal length of the lens was chosen to give a 2:l image reduction on the vidicon because the active area on the vidicon face was about 12.7 mm in width while the channeltron was about 25 mm wide and a t the same time to enable the lens to be positioned far enough away from the phosphor that it did not interfere with the mechanism used to move the channeltron assembly into and away from the focal plane of the mass spectrometer. Each detector element on the silicon target vidicon can be represented as a series combination of a capacitor and a photosensitive resistor. The vidicon uses an electron beam to sweep all these elements and to charge the capacitor to a
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
predetermined voltage level. When the beam has passed, the capacitors discharge a t a rate determined by the value of the series resistance, which decreases with increasing light intensity. The next sweep of the electron beam restores the charge lost from the capacitor and an amplifier responds to the charging current which is proportional to the integrated light intensity between sweeps (26, 27). The total active area of the vidicon used was 12.7 by 10.2 mm. An area 12.7 by 5.1 mm was used to record the signal from the phosphor; an equal area was used to monitor the background illumination. Signal was prevented from falling on the area reserved for background measurements by offsetting the vidicon from the optic axis in a vertical direction.
ffl
DATA PROCESSING An optical multichannel analyzer (OMA) model 1205A manufactured by Princeton Applied Research Corporation was used to digitize and store data obtained from the same manufacturer's vidicon camera unit model 1205B. The 12.7-mm length of the active area of the vidicon tube is scanned every 32.8 msec. The signals obtained are digitized and stored in a 500-channel memory. A variable delay time can be chosen between successive scans and the data from all scans can be accumulated in the appropriate channels until the memory is filled. (The total capacity is lo5 counts per channel.) A second memory unit can be used to collect and store background signals over the same number of scans. The differences between the signals contained in corresponding memory channels can then be displayed. This background subtracted averaged signal gives the best ,available signal-to-noise ratio for the observation of small peaks using this system. A variety of data accumulation and display modes is available. Signals from either a single scan or from multiple scans (up to a possible total of lo4) can be viewed in real time on an oscilloscope (we used a Hewlett-Packard Model 130C). Averaged spectra were plotted on a Hewlett-Packard Model 7044A X-Y pen recorder. The method of subtraction of signal and background used in the OMA limited the smallest current that could be detected. Subtraction of the signals contained in two memories was carried out in the arithmetic unit which subtracts the stored sample and background signals prior to display. The averaged signal was, in the limiting case, very small and would itself have filled only a small part of a memory. However, there was no means using the PAR device alone by which signal and noise could be subtracted prior to storage and no means of returning the background subtracted averaged signal to the memory for further successive accumulation and subtraction procedures. Such a successive approximation approach, because it would allow integration of signal over a much greater number of scans, could be expected to produce substantially better signal-to-noise ratio than we were able to achieve. At the level of background noise in most of our experiments, the overall time for which accumulation was possible was limited to less than three minutes. Changing the accumulation procedures as discussed above to allow collection of data over a period of several hours is an obvious way of achieving a further improvement in signal-to-noise ratio of an order of magnitude. The problem is less acute for larger signals when a great improvement in signal-tonoise ratio by accumulation procedures is not required.
OPERATING CONDITIONS Except as otherwise indicated, experiments were performed under the following conditions. (1) Muss spectrometer: source pressure, 1 X 10-5 Torr; analyzer pressure, -1 X Torr; electron emission current, 150 wA; ion acceler-
Figure 5. Effect of phosphor voltage upon signal strength As the voitage increases, the signal increases steadily with no deterioration in the observed noise level
ating voltage, 7.5 kV; electron multiplier voltage, 2000 V. (2) Detector: fully inserted with front of channeltron grounded; VI (see Figure 2), 1.7 kV; V2, 1.7 kV. (3) Optics: lens aperture, F 2.5.
PERFORMANCE CHARACTERISTICS The sensitivity of the detector was measured in terms of the absolute ion current detectable a t a given signal-tonoise ratio in a given time. Figure 4 shows a plot obtained using the optical multichannel analyzer in the region of mass-to-charge ratio 14.5 in the mass spectrum of ethane. ~ + up 0.2% of the total ion current from Ions of C Z H ~make ethane under the conditions used in our experiments. The total ion current, measured on the beam monitor of the A, so that the CEC 21-llOB spectrometer was 4 X current corresponding to the peak a t mle 14.5 corresponds A. The spectrum was obtained in a to a current of 8 X time of 52 sec corresponding to 1580 scans each of 32.8 msec duration. The signal-to-noise ratio of the peak is estimated to be 1O:l. Figure 5 shows the effect upon the observed signal strength of increasing the phosphor voltage. It can be seen that there is a steady increase in signal which continued further as the phosphor voltage was raised to 2.6 kV, the maximum voltage used. At this voltage, a signalto-noise improvement of six times was observed over the value obtained at 1.7 kV. The effect of varying the voltage across the channeltron was much less marked and optimum signal-to-noise ratio was obtained a t about 1.7 to 1.8 kV. Thus, it is calculated that a signal of approximately 2.6 X A could be recorded at a signal-to-noise value of two in a period of less than a minute. This corresponds to an arrival rate of 40 ions sec-l. A further improvement by a factor of 10 would be obtained if integration of signal could be extended to several hours, as discussed above. A well-known problem with channeltron multipliers is their lack of linearity over a wide range of ion currents. Recent developments have alleviated this problem but the channel plate used in these experiments is of early vintage. Saturation effects were observed for ion currents above A/mass unit. Due to the fact that the ion beams can be deflected by the fringing fields because of the voltage applied across the channeltron array, there is also a slight displacement of the mass scale as this voltage is changed. This can be seen on the various scans in Figure 5 . In several experiments, the sensitivity of the detector was shown to vary with the position of the channeltron. The effect was most marked for higher masses and seems to be a consequence of the variation in the magnet field
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
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in these experiments by their most modern counterparts can further improve the attainable performance both in terms of resolution and of detection sensitivity. Cooling of the vidicon tube offers another possible means of increasing signal-to-noise ratio and is used in astronomy for just this purpose. The use of chevron-type channeltrons should improve the linearity and dynamic range. Despite the fact that the sensitivity data were obtained with less than optimum experimental conditions, an ion current of 40 ions/ second could be measured with a signal-to-noise ratio of 2 over a measurement time of less than a minute. Flgure 6. Spectrum obtained from a sample of a partially fluorinated hydrocarbon in the region of m/e 5 0 A range of seven mass numbers is accommodated simultaneously on the channeltron array
strength experienced by the detector. A 35% increase in the signal due to m/e 15 in ethane, run under standard conditions could be obtained by withdrawing the detector 5 mm. Much larger increases were observed at higher masses when the detector was removed from the focal plane into a region of lower magnetic field strength. At present, the magnetic field effect limits measurements made a t the full accelerating voltage (8 kV) to masses below 100 even when signal strength is optimized by varying the detector position. Of course, this problem does not exist in mass spectrometers using different geometries. Figure 6 shows a series of ions in a fluorocarbon in the range m/e 49-55 taken with the detector position optimized. In this mass-to-charge region, seven peaks could be focused on the channeltron simultaneously at full accelerating voltage.
CONCLUSIONS These experiments confirm that the imaging detector can be used on a mass spectrometer for applications involving detection of ion beams at very high sensitivity, for determining shapes of metastable peaks and for comparing directly the behavior of different peaks as experimental conditions are varied, for example, by introducing a collision gas. The channeltron with its fixed number of detection elements of a given size per unit length may be the resolution limiting component of such a detection system. The resolution of the detector used in our experiments, while too low for applications involving very high mass resolution, is adequate for most IKE work in which it is required to separate beams of different energy-to-charge ratio or to investigate the detailed energy profile of a single peak. Replacement of the individual components of the system used
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ACKNOWLEDGMENT We thank J. W. Amy, W. E. Baitinger, W. 0. Perry, and T. Y. Ridley for helpful advice and discussions. LITERATURE CITED (1) J. J. Thomson, Phi/. Mag., 21, 225 (1911). (2) J. Koenigsberger and J. Kutchewski, Phys. Zeit., 11, 666 (1910). (3) R. E. Honig in "Trace Analysis by Mass Spectrometry", A. J. Ahearn, Ed., Academic Press, New York, 1972, Chapter 4. (4) J. M. Hayes, Anal. Chem., 41, 1966 (1969). (5) D. M. Desiderio in "Mass Spectrometry: Techniques and Applications", J. W. A. Milne, Ed., Wiiey-lntersciencs, New York, 1971, Chapter 2. (6) J. Mattauch and R. F. K. Herzog, Z.Phys., 89, 789 (1934). (7) J. S.Allen, Phys. Rev., 55, 966 (1939). (8) J. S.Allen, Rev. Sci. Instrum., 18, 739 (1947). (9) N. R. Daly. A. McCormick, and R. E. Powell, Rev. Sci. Instrum., 39, 1163 (1968). (IO) F. A. White, "Mass Spectrometry in Science and Technology", John Wlley, New York, 1968. pp 95-107. (1 1) R. Braceweii, "The Fourier Transform and its Applications", McGraw Hill, New York (1965). (12) G. W. Goodrich and W. C. Wiiey, Rev. Sci. Instrum., 33, 761 (1962). (13) D. S.Evans, Rev. Sci. hstrum., 36,375 (1965). (14) G. W. Goodrich, U.S. Patent 3,374,380, (15) W. B. Colson, J. McPherson, and F. T. King, Rev. Sci. Instrum., 44, 1694 (1973). (16) W. G. Hyzer, Res./Dev., 25 (12), 34 (1974). (17) K. W. Busch, N. G. Howeii, and G. H. Morrison, Anal. Chem., 46, 575 (1974). (18) D. G. Mitchell, K. W. Jackson, and K . M. Aidous, Anal. Chem.. 45, 1215A (1973). (19) C. E. Giffln, H. G. Boettger, and D. D. Norris, paper presented at the 22nd Annual Conference on Mass Spectrometry and Allied Topics, American Society for Mass Spectrometry, Philadelphia, Pa., 1974. (20) H. H. Tuithof and A. J. H. Boerboom, Int. J. Mass Specfrom. Ion Phys., 15, 105 (1974). (21) W. H. Aberth and R. R. Sperry, Rev. Sci. Instrum., 45, 128 (1974). (22) J. H. Beynon and R. G. Cooks, J. Phys. E, 7, 10 (1974). (23) J. H. Beynon, W. E. Baltinger, J. W. Amy, and T. Komatsu, Int. J. Mass Specborn. Ion Phys., 3, 147 (1969). (24) E. G. Johnson and A. 0. Nier, Phys. Rev., 91, 10 (1953). (25) C. F. Robinson, Rev. Sci. Instrum., 28, 777 (1957). (26) R . E. Santlni, M. J. Milano, and H. L. Pardue, Anal. Chem., 45, 915A (1973). (27) M. J. Miiano and H. L. Pardue, Anal. Chem., 47, 25 (1975).
RECEIVEDfor review February 7, 1975. Accepted May 21, 1975. This work was supported by the Research Corporation and the National Science Foundation.
1975
