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Anal. Chem. 1986, 58,690-694
Fourier Transform Time-of-Flight Mass Spectrometry Fritz J. Knorr* Surface Science Laboratories, Znc., 465 National Avenue, Mountain View, California 94043
Massoud Ajami and Dale A. Chatfield Department of Chemistry, Sun Diego State University, Sun Diego, California 92182
A new mode of operation of tlme-of-flight mass spectrometry (TOFMS) Is demonstrated. The Ions formed In the source reglon are accelerated with a sinusoidal modulation slgnal, and the electron current from the detector Is modulated in phase with the ion source. The data are gathered as a function of the applied modulation frequency. The normal time domain spectrum Is recovered through Fourier transformation (FT) of the measured slgnal. I n theory the FT mode of operation Is capable of Increasing the average ion current at the detector by a factor of 25 wlth no apparent loss in resolutlon. This increase in signal power should ultimately lead to an increase in sensltlvlty and decreased scan time. The operation of the technique was demonstrated by measurlng the transformed E1 mass spectrum of toluene and compared to the normal scanning mode spectrum. The FT mode dld not demonstrate the anticipated increase In sensitlvity, but this was due to the limltations In the equlpment used In the experiments.
In the late 1960s, time-of-flight mass spectrometers (TOFMS) were competitive with magnetic sector and quadrupole mass filter instruments for general low-resolution mass spectrometric measurements. TOFMS fell from popular use with the demand of gas chromatographic/mass spectrometric (GC/MS) analysis, a market that small and sensitive quadrupole mass analyzers quickly filled. Still, there are some real advantages to TOFMS over other mass spectrometric techniques, including an extremely low-cost dispersive element (an empty tube with a high transmission efficiency),unlimited mms range, and potentially, an extremely rapid scan rate. The availability of high-speed digitizers and expanded memory capabilities has reintroduced TOFMS into laboratories for specialized applications. For pulsed-event applications such as laser desorption and multiphoton ionization processes, high-speed acquisition of data from a TOFMS has produced data unobtainable by other mass spectrometric methods. Despite these advances, there is not a commercially available TOFMS that lends itself to GC/MS applications at a modest cost; thus its use is not widespread. Presently, there are three methods of recording TOF spectra-single scan, signal averaging, and time bin (boxcar) averaging. In the single scan method, the TOF waveform of a single source pulsing event is amplified and displayed on an oscilloscope and recorded photographically. Noise levels are severe, but the scan time is extremely rapid; the entire spectrum arrives in + 2 ~ ( ~ o+t fRttf - (1/2)Rtf2)] d t (8)
equation 8 represents the sum and difference frequencies of the product in eq 7 . We will utilize the integral over the time constant of the electronics to eliminate the high-frequency sum term and assume that the low-frequency term is passed unchanged. The difference term, the measured signal, is then For an ion of drift time tf and for a scan rate of R, the excursions of the interferogram appear a t a frequency Rtf. Recognizing that the sampling frequency v = vo + Rt, we can express c in terms of the applied frequency as in eq 5 €(tf, v) = cos 27rb - (1/2)Rtf)t, (10) The recorded interferogram as a function of frequency for the rapid scan case has a flight time dependent phase shift of 0.5Rtf, but the period of the interferogram is still l / t f . Thus, for rapid scan conditions, the magnitude spectra are unaffected by the scan rate. For this reason we calculate the magnitude spectra of the interferograms.
U C
Figure 3. Electronics consisting of a frequency synthesizer (FS), a general purpose interface bus (GPIB), an Apple IIe microprocessor (pP) and keyboard (KB), two dc power supplies (PS1 and PS2), a radio frequency amplifier (RF), a Nicolet 1080 computer (COMP), an analog stripchart recorder (REC), and two 0.33-pfd coupling capacitors (C). Listed instrument components are the fllament (F), ion source backplate (B), ion accelerating grids (AG), scope anode (SA),detector gate 1 (Gl), and gate anode 1 (Al).
Operation of the TOFMS in the Fourier mode cannot improve on the fundamental resolution of the instrument. Ultimate resolution is dictated by the physics governing the TOF separations process and by the configuration of the instrument. Peak broadening due to instrumental limitations in the time domain can be thought of as a pure TOF spectrum-a series of 6 functions representing a spectrum of infinite resolution-convoluted with an instrumental response function. In the Fourier domain, this relationship is expressed as a multiplication of the interferogram by a frequency transfer function. The frequency transfer function and the instrumental response function are a Fourier transform pair. This frequency transfer function manifests itself in the measured interferogram as an envelope that attenuates the intensity of the interferogram a t high frequencies. The time resolution of the transformed spectrum is roughly proportional of the frequency bandwidth of the interferogram envelope function (10, 11). There are many analogies between FT TOFMS and other Fourier transform techniques (12). The normal mode gate 1 and gate 2 pulsing sequences are analogous to the entrance and exit slits of a dispersive spectrometer. Within that analogy we see the trade-off between resolution and sensitivity intrinsic to all dispersive techniques. FT TOFMS achieves the transform multiplex advantage through simultaneous detection of all ion flight times, and it achieves the transform throughput advantage through essentially continuous source broadcasting. In many respects, FT TOFMS can be thought of as analogous to pulse compression radar techniques (13) in that signal power is increased with no loss in signal bandwidth through an optimum filter scheme.
EXPERIMENTAL SECTION Data, ion interferograms, and time domain spectra were measured with a CVC Model 2001 time-of-flightmass spectrometer having a 1-m-length flight tube (CVC Products, Rochester, NY). The ion source has been redesigned to incorporate both electron ionization (EI) and surface ionization (SI) modes of operation (5). The standard filament E1 source has been modified by enlarging the electron entrance and collector slits to 2 X 8 mm t o increase electron current into the source region. The collimating magnets for the focusing of the electron beam had to be removed due to space limitations. The resolution has been reduced to about 60 by these modifications, but it has been adequate for low-mass E1 and SI experiments. The electronic configuration of the FT TOFMS is schematically illustrated in Figure 3. The frequency source was a Model 200 1-200 MHz frequency synthesizer (Programmed Test Source, Inc., Littletown, MA) interfaced by means of a general purpose in-
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
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Figure 5. Ion interferogram for the E1 spectrum of toluene.
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Figure 4. Detail of interferogram at selected frequencies: (A) 1-2 MHz, (B) 5-6 MHz, (C) 10-11 MHz, and (D) 15-16 MHz. terface bus (GPIB) card (Model 7490A, California Computer Systems, Sunnyvale, CA) to an Apple IIe computer. The output from the frequency synthesizer was amplified with an EN1 320L broadband power amplifier (Electronic Navigation Industries,Inc., Rochester, NY) that provided up to 80 V peak to peak into 50 Q at 10 MHz. The rf output was connected by means of coupling capacitors to grid 1 of the source and to gate 1of the continuous dynode electron multiplier. Offset voltages were provided for grid 1 and gate 1 as shown in Figure 3 to increase ion and electron transmission, respectively. Ion packets are gated by means of grid 1, whereas grids 2, 3, and 4 created an ion accelerating potential of -2.7 kV relative to source potential. Instead of gating the ion beam at the electron multiplier, electrons were diverted from the end of the electron multiplier rail toward anode 1 by application of the rf signal with a dc offset potential of -35 V. Anode 1 and gate 1 are also utilized by the analogue scanner (boxcar averager) circuitry in normal scanning mode operation of the TOFMS as supplied by the manufacturer. The ion current at anode 1 was amplified by the electrometer of the analogue scanner, then digitized and stored as either 1024 or 2048 point interferograms by a Nicolet 1080 Instrument Computer (Nicolet Instrument Corp., Madison, WI). Fourier transforms and magnitude spectra were calculated by using a standard Nicolet software. Data acquisition with the Nicolet 1080 and control of the frequency synthesizer were synchronized by means of the control computer. It was programed in BASIC for two modes of operation: tuning and frequency sweeping. In the tuning mode, the frequency synthesizer was stepped from a frequency that produced a maximum in the interferogram to one that produced a minumum, and back repetitively every 0.5 s. That way, the effects of instrument tuning parameters could be observed in real time. This tuning mode is effective if the mass spectrum being measured contains one intense mlz ion, since that produces a simple and easily predicted interferogram.
RESULTS AND DISCUSSION Figure 4, parts A-D, illustrates details of the shape of the interferograms that were produced at 1-2, 5-6, 10-11, and 15-16 MHz intervals from the E1 mass spectrum of toluene, respectively. The lowest frequency and the highest frequency scans are observed to be sinusoidal in shape rather than the triangular shape that would be anticipated from a purely square-wave-shaped ion packet. The bottoms of these waves are slightly flattened due to the bias voltage that was applied to grid 1 to improve the ion transmission efficiency of this
Figure 6. Transformed TOF spectrum from the interferogram shown in Figure 5. source configuration. This modulation method attenuated the total ion beam intensity by approximately 30% at 1 MHz and by lesser values a t higher frequencies. The degree of attenuation was very dependent upon the amplitude of the rf and bias voltages applied to grid 1and to gate 1. The shapes of the interferograms at 5-6 and 10-11 MHz are a much more complex wave form as would be expected from the constructive and destructive interference pattern composed of ions with similar, but slightly different, flight times. Figure 5 illustrates a typical ion interferogram for the E1 mass spectrum of toluene, representing 2048 data points. This interferogram was recorded between 1 and 13 MHz. For this data set, the oscillator was stepped a t 8 kHz/data point and was held at each frequency for approximately 3.4 s, for a total scan time of 10 min. Note that the magnitude of the interferogram is attenuated at high frequencies, limiting the bandwidth of the signal and the resolution of the transformed spectrum. There can be many sources of this bandwidth limit, including accelerating grid irregularities, modulation signal rolloff, ion packet diffusion, and others. Some of these problems are artifacts of the present preliminary configuration and will be addressed in our future FT TOFMS modifications. The ion interferogram of toluene has an interesting “bow tie” envelope shape resulting from the beat frequency of m / z 91 and 92, which confirms that these two ions are partially resolved. Figure 6 illustrates the magnitude spectrum of the Fourier transform of the interferogram illustrated in Figure 5; only the first 250 points of the transform are shown. This should represent the time domain TOF spectrum. The data were transformed directly without any weighting or apodization, giving rise to a large low-frequency peak, corresponding to base-line drift in the interferogram. Note that, although the peak shape of m / e 91 and 92 are irregular, there is indication that these adjacent masses are partially resolved. The other ions that are present at lower masses in this spectrum can be identified by comparison with those of a time domain TOF spectrum of toluene that was acquired with the analogue scanner (boxcar averager) on the conventional TOFMS, shown in Figure 7. Ions a t mle 17,18,27,28,39,50-51,63, and 65 can be distinguished from background. Also note from Figure 7 that the resolution of the TOFMS is not adequate to fully resolve mle 91 and 92. No explanation can be offered at this
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Figure 8. FT spectrum of background noise at 1.00 MHz modulation frequency.
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time to account for the discrepancies in the relative intensities of corresponding mlz values in these two spectra. There are several important distinctions to be made between the FT mode and the normal scanning mode of operation. First there is a large increase in the average signal power for the FT mode of operation. The interferogram in Figure 5 has an average current of 5 X lo4 A above the dc (background) current of 1.5 X A. The time domain spectrum in Figure 7 has a peak current of 3 X lo4 A, but the average current (eq 2 ) was approximately 2 orders of magnitude lower. It is clear that these results fall far below the predicted values, which leads us to believe that the ion source design is far from optimal for the FT mode of operation. Unfortunately, in spite of this much higher average signal current in the FT mode of operation, a comparison of Figure 6 and Figure 7 indicates that for FT mode operation the resolution is slightly less and the S/N is much worse than for normal scanning mode operation. The slight loss in resolution may well be due to lack of experience in FT mode operation, and thus the use of nonideal source tuning parameters. Source tuning philosophy was to use conditions that were also similar to those used in time domain operation. The decrease in the SIN in the FT mode spectra is a much more serious problem and is thought to be due to fluctuations in the electron emission current from an unregulated power supply. Fluctuations in the electron current would act to multiply the interferogram by the electron emission current function, which is some random noise function. The emission current fluctuations would appear in the transformed spectrum as the Fourier transform of the emission current function convoluted with the TOF spectrum. To test this hypothesis, the output signal was acquired while the modulation frequency was held constant at 1.0 MHz. Figure 8 is the Fourier transform of these data. As was observed in the spectrum of toluene, the maximum intensity occurred at zero flight time and rapidly decayed thereafter, finally reaching a steady-state value of about m l e 50. This would suggest that there is one source of noise that has a 1f f dependence (flicker noise) and a second that is relatively independent of frequency. Emission current fluctuations are not nearly as serious a problem in the time domain spectrum; tbere, the time domain signal, S, in eq 2 is multiplied by the emission current function. Thus,emission current fluctuations only within each time interval contribute to the observed signal. In contrast, the emission current fluctuations in the FT mode are convoluted with the signal and the peaks are broadened as well, resulting in the very noisy
background spectrum in Figure 8. This situation can be vastly improved for FT operation by careful emission current regulation over short scan times and by multiple scan averaging. In Figure 6 there is a peak at high "mass" corresponding to the second harmonic of the m / z (91, 92) doublet. There were no higher harmonics observed in the transforms. The sinusoidal modulation shown in Figure 4 produced near-sinusoidal ion density packets, and not well-defined "on-off' ion gating as was described in the theory section. The second harmonic probably arises due to ion packet shape, nonlinearity in the electron multipler, and the amplification electronics. It should be emphasized that this work is only a preliminary investigation in this new field. We have, however, demonstrated, using a prototype instrument, that the basic approach of FT TOFMS is feasible and that, in principle, it results in higher signal strength compared to normal mode TOF operation. Due to needed refinements in the apparatus, the theoretical increase in average signal power was not achieved, but the sources of the problem have been identified. FT TOFMS is thus shown to be closely related to FT IMS (7) as a velocity dispersive technique. Other velocity dispersive techniques may prove amenable to this Fourier transform approach as well.
ACKNOWLEDGMENT We thank S. B. W. Roeder for the loan of equipment, his encouragement, and many helpful comments throughout this project. LITERATURE CITED (1) Price, D.; Williams, J. E. "Time of Flight Mass Spectrometry"; Pergamon: Oxford, 1969. (2) Holland, J. F. et al. Anal. Chem. 1983, 55, 997A-1012A. (3) Wiley. W. C.; McLaren. I. H. Rev. Scl. Instrum. 1955, 26, 1150-1 157. (4) Studier. M. H. Rev. S O / . InStrUm. 1983, 34, 1367-1370. (5) Chatfield, D. A. 30th Annual Conference on Mass Spectrometry and Allied Topics, Paper MOO-8, Honolulu, HI, 1962. (6) Chatfield, D. A.; Ajami, M.; Marsi, K. S., In preparation. (7) Tom Connor, CVC Corporation, personal communication, 1982. (8) Knorr, F. J.; Eatherton, R. L.; Siems, W. F.; Hili, H. H., Jr. Anal. Chem. 1985. 57, 402-406. (9) Siems, W. F.; Knorr, F. J.; Eatherton, R. L.; Hill H. H., Jr., submitted for publication in Anal. Chem. (IO) Braceweli. R. N. "The Fourier Transform and Its Applications", 2nd ed.; McGraw HIII: New York, 1978. (11) Brigham, E. 0. "The Fast Fourier Transform"; Prentlce Hall: Englewood Cliffs, NJ, 1974. (12) Griffiths, P. R. "Transform Techniques in Chemistry"; Plenum: New York, 1978. (13) Barton, D. K.. Ed. "Radars: Volume 3 Pulse Compression"; Raytheon Co.: Bedford, MA, 1975.
RECEIVED for review June 24, 1985. Resubmitted November 15, 1985. Accepted November 15,1985.
