Anal. Chem. 1998, 70, 3545-3547
Fourier Transform Detection in a Cylindrical Quadrupole Ion Trap Ethan R. Badman, J. Mitchell Wells, Huy A. Bui, and R. Graham Cooks*
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1393
Broad-band nondestructive ion detection based on induced image current measurement is performed in a quadrupole ion trap having cylindrical geometry. Spectra of krypton and acetophenone are shown to demonstrate the first use of nondestructive detection with a cylindrical ion trap. Recently, we reported1,2 the first use of a quadrupole ion trap with cylindrical geometry3 operated in the mass-selective instability mode.4 This simplified geometry is of interest due to its ease of miniaturization and, hence, its potential applicability to fieldportable instrumentation.5-7 After optimization of the geometry to minimize higher-order field contributions,8 the cylindrical ion trap (CIT) was found to provide performance comparable to that of a standard hyperbolic trap, viz., unit mass resolution over a mass/charge range of ∼600 Th (1 Th ) 1 Da/unit charge).9 Here we report the first use of a CIT in a nondestructive mode of detection, a method that may yield improvements in sensitivity, resolution, and flexibility of tandem mass spectrometry through the ability10 to remeasure ion populations. The method reported here for broad-band nondestructive ion detection is related to the Fourier transform (FT) technique for ion cyclotron resonance developed by Comisarow and Marshall.11 Its application to a hyperbolic trap has been described previously by our group12,13 and earlier, in a narrow-band mode, by Syka and Fies.14 Narrow-band nondestructive detection using marginal (1) Gill, L. A.; Wells, J. M.; Badman, E.; Cooks, R. G. Proceedings of the 45th ASMS Conference on Mass Spectrometry and Allied Topics, Palm Springs, CA, June 1-5, 1997; p 127. (2) Wells, J. M.; Badman, E. R.; Cooks, R. G. Anal. Chem. 1998, 70, 438-444. (3) Langmuir, D. B.; Langmuir, R. V.; Shelton, H.; Wuerker, R. F.; U.S. Patent 3,065,640, 1962. (4) Stafford, G. C.; Kelley, P. E.; Syka, J. E. P.; Reynolds, W. E.; Todd, J. F. J. Int. J. Mass Spectrom. Ion Processes 1984, 60, 85-98. (5) Hemberger, P. H.; Alarid, J. E.; Cameron, D.; Leibman, C. P.; Cannon, T. M.; Wolf, M. A.; Kaiser, R. E. Int. J. Mass Spectrom. Ion Processes 1991, 106, 299-313. (6) Hemond, H. F. Rev. Sci. Instrum. 1991, 62, 1420-1425. (7) Wise, M. B.; Thompson, C. V.; Buchanan, M. V.; Merriweather, R.; Guerin, M. R. Spectroscopy 1993, 8, 14-22. (8) Wang, Y.; Franzen, J. Int. J. Mass Spectrom. Ion Processes 1992, 112, 167178. (9) Cooks, R. G.; Rockwood, A. L. Rapid Commun. Mass Spectrom. 1991, 5, 93. (10) Amster, I. J. J. Mass Spectrom. 1996, 31, 1325-1337. (11) Comisarow, M. B.; Marshall, A. G. Chem. Phys. Lett. 1974, 25, 282-283. (12) Cooks, R. G.; Cleven, C. D.; Horn, L. A.; Nappi, M.; Weil, C.; Soni, M. H.; Julian, R. K. Int. J. Mass Spectrom. Ion Processes 1995, 146, 147-163. (13) Soni, M.; Frankevich, V.; Nappi, M.; Santini, R. E.; Amy, J. W.; Cooks, R. G. Anal. Chem. 1996, 68, 3314-3320. (14) Syka, J. E. P.; Fies, W. J. U.S. Patent 4,755,670, 1988. S0003-2700(98)00270-4 CCC: $15.00 Published on Web 07/09/1998
© 1998 American Chemical Society
oscillators and tuned transformer circuits has also been implemented by other groups.15-18 At least one other group is currently exploring the use of a CIT with FT detection.19 Our image current method uses a small pin electrode, embedded in and electrically isolated from the exit end-cap electrode of the ion trap, to detect the image current induced by an oscillating ion cloud.20 The pin arrangement reduces the capacitance between the detector and the ring electrode and, hence, minimizes pickup of the rf trapping voltage which would otherwise saturate the preamplifier.13 A short dc pulse is applied to the entrance end-cap electrode in order to cause trapped ions to oscillate coherently in the z-direction at their characteristic secular frequencies.12 The secular frequency in the z-direction is dependent upon the m/z ratio of the ions, at a fixed rf trapping voltage and frequency.21 After amplification, the image current signal is recorded using a fast digital oscilloscope and Fourier transformed to obtain an intensity vs frequency spectrum. Simple calibration procedures are used to convert this spectrum to a mass spectrum. EXPERIMENTAL SECTION The experimental setup used for FT nondestructive detection in a cylindrical ion trap (FT-CIT) is shown in Figure 1. The basic geometry (Figure 1a) is as described previously:2 the ring electrode has radius (r0) of 1.0 cm, and the center-to-end-cap distance (z0) is 0.909 cm. The entrance end-cap electrode has a center hole (2 mm in diameter) and is machined so as to allow the standard Finnigan electron gate to be mounted and used to control the ionizing electron beam. The filament block is held at -12.5 V relative to ground, which causes electrons emitted from the filament to travel toward the electron gate. Electrons are then admitted to or repelled from the ion trap by switching the electron gate between +180 and -180 V, respectively. The modified exit end-cap electrode has a stainless steel pin electrode 3 mm in diameter which is centered in a 5-mm-diameter cylinder of Teflon to provide electrical insulation. The pin electrode is mounted on a threaded drive mechanism to allow variation of the distance in (15) Fischer, E. Z. Phys. 1959, 156, 1-26. (16) Rettinghaus, G. Z. Angew. Phys. 1967, 22, 321-326. (17) Parks, J. H.; Pollack, S.; Hill, W. J. Chem. Phys. 1994, 101, 6666-6685. (18) Goeringer, D. E.; Crutcher, R. I.; McLuckey, S. A. Anal. Chem. 1995, 67, 4164-4169. (19) Arkin, R. C.; Laude, D. A. Proceedings of the 45th ASMS Conference on Mass Spectrometry and Allied Topics, Palm Springs, CA, June 1-5, 1997; p 123. (20) Frankevich, V. E.; Soni, M. H.; Nappi, M.; Santini, R. E.; Amy, J. W.; Cooks, R. G. U.S. Patent 5,625,186, 1997. (21) March, R. E.; Hughes, R. J. Quadrupole Storage Mass Spectrometry; John Wiley and Sons: New York, 1989.
Analytical Chemistry, Vol. 70, No. 17, September 1, 1998 3545
Figure 1. (a) Geometry of cylindrical ion trap showing embedded pin electrode detector, the position of which can be varied in the axial (z)-direction. b) Timing sequence for the FT-CIT experiment.
the z-direction from the electrode surface to the tip of the pin electrode. The control electronics used for trap operation and signal detection are as described previously.13 The system is based on the Finnigan ITMS (Finnigan MAT, San Jose, CA), and the scan function used is created with the Finnigan data system, which provides control of ionization and cooling times, in addition to triggers for the dc pulser and the digital oscilloscope. A typical scan function is shown in Figure 1b; it employs an ionization period (tion), followed by a period for ion cooling (tcool), a trigger for the dc excitation pulse, and a trigger to begin data collection with the oscilloscope. Axial modulation was not used in any of the experiments. The pulser used to apply the short dc pulse is custom-built22 and based on a DEI Inc. (Ft. Collins, CO) HV1000 high-speed pulse switch. This switch provides pulses of up to 950 V, with pulse widths as long as 10 µs and rise times on the order of 1020 ns. The heights and widths of the pulses used in these experiments ranged from 8 to 60 V and from 0.5 to 5 µs, respectively. Image currents induced on the pin electrode were converted to voltages across a 10-MΩ resistor and amplified with a custombuilt preamplifier based on two Burr-Brown INA 111 instrumentation op-amps as described previously.13 For this work, the preamp was operated with a gain of 500 and a bandwidth of 130 kHz. The output voltage was filtered and amplified further with a low-pass filter/amplifier (model 4302, Ithaco, Ithaca, NY), set at a gain of 10 and a high-frequency break point of 400 kHz, and finally (22) Lammert, S. A.; Cooks, R. G. J. Am. Soc. Mass Spectrom. 1991, 2, 487491.
3546 Analytical Chemistry, Vol. 70, No. 17, September 1, 1998
Figure 2. Background-subtracted frequency spectrum of (a) krypton and (b) acetophenone. For (a), the distance from the end-cap to pin tip was 2.03 ( 0.05 mm; tion ) 20 ms; dc pulse ) 29 V, 0.8 µs; tcool ) 60 ms; PKr ) 3 × 10-6 Torr; low-mass cutoff, 23 Th. For (b), the distance from the end cap to pin tip was 2.03 ( 0.05 mm; tion ) 20 ms; dc pulse ) 27 V, 1.3 µs; tcool ) 60 ms; PKr ) 2 × 10-6 Torr; low-mass cutoff, 23 Th.
recorded using a Tektronix TDS 540 digitizing oscilloscope (Tektronix, Beaverton, OR) at a sampling rate of 1 MHz. Each time domain spectrum consisted of 15 000 data points and was averaged in the oscilloscope for 100 scans before being downloaded via a GPIB interface to a PC. An in-house program was used to manage the scope-to-PC interface; it employed a fast Fourier transform algorithm to calculate the frequency spectrum from the averaged time domain data. Background spectra were obtained by disabling the dc pulser and recording the frequency spectrum as described above. Phase matching of the oscillating ion cloud between repetitive spectral measurements is required to ensure that averaging will be constructive. Phase matching results from the fixed times used for ionization, cooling, and the dc pulse in each mass scan, causing the ions in each scan to have the same phase during detection. The long cooling time after ionization allows ions to be cooled to the center of the trap and reproduces the initial conditions in each scan. Spectra were obtained by leaking headspace vapors of acetophenone (Fischer Scientific, Fair Lawn, NJ) and krypton (Airco,
Murray Hill, NJ) into the manifold through a variable leak valve (Granville-Phillips, Boulder, CO). No helium buffer gas was used for the data reported here. All pressure measurements are uncorrected. RESULTS AND DISCUSSION Figure 2 shows the background-subtracted frequency spectrum of the two samples studied. The two most abundant isotopes of krypton are evident in its spectrum (Figure 2a) recorded using a distance from the end-cap to the pin electrode tip of 2.03 ( 0.05 mm, an ionization time of 20 ms, a dc pulse amplitude of 29 V with a duration of 0.8 µs, and an ion cooling time of 60 ms. Krypton pressure was 3 × 10-6 Torr, and the low-mass cutoff was nominally 23 Th, as read from the data system display. The two measured peaks correspond to 86 and 84 Th and occur at 88.30 and 91.79 kHz, respectively. The full width at half-maximum values are 0.3 and 0.2 kHz. The merging of the 82 and 83 Th peaks with that at 84 Th is the result of the higher order fields present in the device. Krypton spectra recorded using the CIT in the mass-selective instability mode also without helium buffer gas or axial modulation show only the two resolved isotopic peaks at m/z 84 and 86 as well (data not shown). The background-subtracted frequency spectrum of acetophenone (Figure 2b) was recorded under similar conditions. The distance from the end-cap electrode to the pin tip was 2.03 ( 0.05 mm, the ionization time was 20 ms, the dc pulse amplitude was 27 V with a duration of 1.3 µs, and the ion cooling time was 60 ms. Acetophenone pressure was 2 × 10-6 Torr (uncorrected), and the nominal low-mass cutoff was 23 Th. The two most abundant peaks appear at 60.70 and 71.37 kHz, respectively, which correspond to 120 and 105 Th. The full width at half-maximum values are 0.2 kHz for both peaks, which represents a resolution of approximately 300. The peak at 100 kHz is the aliased frequency from the 1.1MHz rf drive frequency applied to the ring electrode. The aliased rf peak varies in intensity over the averaging time; therefore, as seen when comparing Figure 2, background subtraction may not always completely remove this peak from the spectrum. Other
low-frequency noise peaks occur and are of unknown origin. Ion signals were distinguished from noise by comparison with a background spectrum recorded with the dc pulser turned off. As expected, the ion frequencies in Figure 2 differ from the theoretical frequencies calculated for the hyperbolic trap. However, experimental calibration of frequency vs qz was conducted using krypton and argon and showed an approximately constant offset from the corresponding theoretical curve for a hyperbolic trap. Values of qz for the experimental conditions used were calculated by first determining the nominal rf voltage applied to the ring electrode according to the low-mass cutoff chosen with the data system and the mass of the ion of interest. The rf voltage then could be used in the Matthieu equation to calculate the nominal qz value for each ion of interest. A plot of experimental frequency vs nominal qz value allowed prediction of ion frequencies in the CIT. Further attempts are being made to optimize the sensitivity and resolution of the FT-CIT. This will include optimization of the pin position and of the ionization and cooling times, as well as the dc pulse shape and manifold pressure. Preliminary investigation showed that the dc pulse shape has the greatest effect on peak amplitude and shape and that the pin position has a significant effect on the measured frequency of the trapped ions. The effect of pin position on the measured frequencies of the ions is certainly a result of the contributions of higher order fields in the device and will be explored in future experiments. The data reported here show that nondestructive detection is applicable to cylindrical ion traps and that further development is warranted. ACKNOWLEDGMENT This work was supported by the Office of Naval Research Grant N00014-94-K-2002, the U.S. Department of Energy Grant DE-FG0294ER14470, and Finnigan Corp.
Received for review March 10, 1998. Accepted June 1, 1998. AC980270O
Analytical Chemistry, Vol. 70, No. 17, September 1, 1998
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