Anal. Chem. 1994,66, 725-729
Resonance Ejection Ion Trap Mass Spectrometry and Nonlinear Field Contributions: The Effect of Scan Direction on Mass Resolution Jon D. Williams, Kathleen A. Cox, and R. Graham Cooks’ Chemistry Department, Purdue University, West Lafayette, Indiana 4 7007 Scott A. McLuckey,’ Kevin J. Hart, and Douglas E. Goeringer Analytical Chemistty Division, Oak Ridge National Laboratory, Oak RMge, Tennessee 3783 1-6365
Mass resolution obtained with the commonly employed stretched quadrupole ion trap geometry is shown to bedependent upon scan direction when resonance ejection is used as the mass analysis method. Data are also presented showing that the energy absorption profde for ions subjected to resonance excitation broadens with the amplitude of ion oscillation and shifts to higher frequencies, an observation consistent with behavior predicted for ion motion in a quadrupole electric field with relatively small higher-order field components. The observed effect of scan direction on mass resolution is therefore reasoned to arise from an effect analogous to Doppler focusing or defocusing, depending on scan direction relative to the direction of the ion frequency shift with oscillatory amplitude. Application of a dipolar supplementary ac voltage of appropriate frequency to the end-cap electrodes of a quadrupole ion trap allows ions to be resonantly ejected at a selected value of the Mathieu parameter qz. Resonance ion ejection used in conjunction with a ramp of the amplitude of the radio frequency (rf) voltage has been used to extend the mass-tocharge range of the ion trap.’ A reverse-then-forward rf scan (i.e., decreasing-then-increasing rf amplitude), performed while a fixed-frequency ac signal is applied, allows unwanted ions to be removed from the trap in preparation for MS/MS experiments.2 If the electron multiplier is turned on during the reverse rf ramp, signal is observed at intervals which correspond to ejection of unwanted ions. This observation, along with the demonstration that parent ion scans can be performed using the trap,3provides evidencethat mass analysis is possible by resonance ejection using a reverse rf scan. This type of scan inverts the order in which ions are ejected from the trap (e.g., m/z 1000 exits before m/z 999 and so on). In this paper, comparisons of spectra obtained with forward and reverse rf scans are made using a commercially available quadrupole ion trap in which the end caps are spaced symmetrically such that the inter-end-cap distance (22,) is 1.48 mm greater than the 14.14-mm spacing required for a pure quadrupole field.4 In the case of most commercial ion traps, ro = 10.0 mm and 22, = 15.62 mm, which deviates from (1)
the “theoretical” relationship of 22, = 2’/*r0. Deviations of the spacings and shapes of the ion trap electrodes from the ideal geometry used to produce an exact quadrupole field are known to introduce higher-order fields such as hexapole and octopole fieldsS5 It is noteworthy that much of the recent development of the quadrupole ion trap as an analytical mass spectrometer has been made with the ion trap geometry (herein termed the “stretched” geometry) used in this study. It is therefore important to determine the effect of the presence of higher-order fields on the performance characteristics of this commonly employed ion trap. Improved understanding of the influence that such fields have on the behavior of ions-from ion accumulation to ion detection-may lead to improvements in quadrupole ion trap mass spectrometry. This paper emphasizes the effect of nonlinear fields on the both the resonance ejection and resonance absorption peak shapes.
EXPERIMENTAL SECTION Evaluation of resonance ejection peak shapes using reverse rf scans was done by employing cluster ions generated from aqueous solutions of CsBr by Cs+bombardment. Saturated solutions of 1 pL were applied to the sample probe and dried in vacuo. The potentials of the injection lenses were 9 , 8 , and -25 V applied to the ion volume, extractor, and central element of the einzel lens.6 The helium bath gas pressure was 1.0 X 10-4 Torr (uncorrected) unless otherwise stated. A single scan sequence (Figure 1) was used for comparing reverse and forward scan experiments. It consisted of four periods: (A) ionization, (B) isolation of the ions of interest by a reverse-then-forward rf isolation scan, (C) a reverse rf scan, and (D) a forward rf scan. Ion ejection and hence mass analysis was selected to occur in either the forward or the reverse scan segment by application of a 184.6-kHz supplementary ac signal. When applied to the end caps during (C), a reverse mass analysis scan resulted, otherwise a forward mass analysis scan occurred sincethis signal was always applied during (D). The ionization period was varied from 10 to 600 ms. The mass-to-charge range of the trap was extended to 1300 Da/charge by the choice of supplementary ac frequency.
Kaiser,R.E.,Jr.;Cwks,R.G.;Stafford,G.C.,Jr.;Syka,J.E.P.;Hemberger,(4) Louris, J.; Schwartz, J.; Stafford, G.; Syka, J.; Taylor, D. Proceedings of the
P. H. Int. J . Mass Spectrom. Ion Processes 1991, 106, 79. (2) Cox,K.A.;Williams, J. D.;Cooks,R.G.;Kaiser,R.E., Jr. Biol. MassSpectrom. 1992, 21, 226. (3) Johnson, J. V.; Pedder, R. E.; Yost, R. A. Int. J. MassSpectrom. Ion Processes 1991, 106, 197.
0003-2700/94/0366-0725$04.50/0 0 1994 Amerlcan Chemical Society
40th ASMS Conferenceon Mass Spectrometryand Allied Topics, Washington, DC. May 31-June 5, 1992; p 1003. ( 5 ) Franzen, J. Int. J. Mass Spectrom. Ion Processes 1991, 106, 63. (6) Kaiser, R. E., Jr.; Louris, 3. N.; Amy, J. W.; Cooks, R. G. Rapid Commun. Mass Spectrom. 1989, 3, 225.
Analytical Chemlsw, Vol. 86, No. 5, Mer&
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Figure 1. Timing sequence used to obtain resonance ejection data: (A) ionization; (B) isolation of the ions of interest by a reverse-thenforward rf isolation scan; (C) a reverse rf scan: (D) a forward rf scan.
Diminution of the scan rate for both the forward and reverse rf scans was used to improve resolution and was accomplished by using the offset-DAC method.7 Forward scan rates ranged from 11 110 to 370 Da/s and reverse scan rates ranged from -1 1 110 to -370 Da/s. The sign associated with the scan rate is used to indicate whether the rf amplitude applied to the ring electrode was increased or decreased with time. Data acquisition was performed using a digital storage oscilloscope (Tektronix Model 5 10) to acquire the amplified electron multiplier output signal since the ITMS data system allows only forward rf scan spectra to be acquired. A digital pulse was sent from the ITMS electronics to trigger the scope, which was set to acquire 5000 data points in 100 ms. The digital pulse (and the ac signal) was activated at the beginning of either stage 3 or stage 4 of the scan function (Figure l), depending on whether reverse or forward scans were of interest. The acquired data were downloaded to a PC computer for further processing. The spectra were plotted as voltage produced by the preamplifier versus ion ejection time. The time axis can be converted to a relative mass-to-charge value by the following conversion: 2Imeasured ejection time (s)] [scan rate magnitude (Da/s)]. The factor of 2 corresponds to the mass-to-charge range extension factor. These massto-charge values are not precise since no attempt was made to calibrate the mass-to-charge scale; however, the differences between two mass-to-charge values allow accurate measurement of the spacing between adjacent peaks and accurate peak width measurements. Since the ion traps used in this study were not equipped for direct power absorption measurements, an indirect means using ion dissociation via resonance excitation was used to obtain resonance absorption spectra. The molecular ions of tetraethylsilane (Aldrich Chemical, Co., Milwaukee, WI) and n-butylbenzene (Eastman Kodak Co., Rochester, NY) both formed by electron ionization within the ion trap were used as substrates. In both cases, samples were admitted to a pressure of approximately 5 X lO-' Torr (uncorrected) via a variable leakvalve followingseveral freeze-pumpthaw cycles. Helium buffer gas was held throughout the measurement at a pressure of 1.5 X lo4 Torr (uncorrected), and the temperature of the system was maintained at 100 OC. To acquire the resonance absorption peak shape, a scan function (7) Schwartz, J . C.;Syka, J. E.P.;Jardine, I. J . Am. SOC.MussSpectrom. 1991, 2. 198.
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typical for MS/MS was used which included an ion formation period, ion isolation by combined rf and dc fields, a cooling period of 10 ms to minimize the effect of holding ions near a boundary during ion isolation, a collisionalactivation period, and a mass analysis period. A resonance absorption profile was acquired by measuring the parent ions that remained after an arbitrary period of collisional activation as a function of the frequency of the supplementary ac signal applied in dipolar mode to the end-cap electrodes. A family of profiles was acquired by systematically varying the amplitude of the supplementary ac signal. In the case of the molecular ion of tetraethylsilane, each profile was acquired using a duration of collisionalactivation of 6 ms. In the case of n-butylbenzene, where a wider range of amplitudes could be studied, the collisional activation period for each amplitude was set at the timenecessary todissociate 55-65%of theions at theoptimum frequency. Both types of experiments used Finnigan (San Jose, CA) ITMS ion trap mass spectrometers.
RESULTS AND DISCUSSION Figure 2 compares portions of forward and reverse scan mass spectra which include the cluster ions Cs3BrZ+ ( m l z 556.5-%OS), Cs4Br3+ ( m / z768.4-774.4) and CssBrd+( m / z 980.2-988.2). The absolute value of the scan rate was 11 100 Da/s in each experiment to allow direct comparison of the resolution. The supplementary ac voltage was also the same, 8 V&p in each experiment, to facilitate comparison of ion abundances. As expected, higher mass ions are ejected before lighter ions when a reverse scan ramp is performed; however, there is a dramatic decrease in mass resolution in the reverse scan compared to that in the forward scan spectrum. At this high scan rate, the mass resolution achieved by the reverse scan was far too low to resolve isotopic species that differ by 2 Dalcharge. Changing the scan rate to 2380 Da/s for a forward rf scan and -2380 Da/s for a reverse rf scan,
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produced the signals shown in Figure 3. The acvoltage applied to the end-cap electrodes in both cases was reduced to 4.75 VO-~.The resolution for the forward scan spectrum improved to 3500 (m/Am, fwhm definition) while the resolution in the reverse scan improved but remained insufficient to resolve the isotopomers. However, when the supplementary ac voltage was decreased further to 1.5 VO-~, the isotopomers were resolved in the reverse resonance ejection scan (Figure 4). Similar effects in resolution enhancement were observed by slowing the scan speed to 370 and -370 Da/s. This difference in resolution for forward and reverse resonance ejection is not expected for a "perfect" quadrupole ion trap. Solutions for the forced-damped Mathieu equation, which describes resonantly excited ion motion in a quadrupolar field when small amounts of dipolar field components are superimposed, predict that the frequency line shape, in terms of the difference between the frequency of the applied ac and the frequency of the ion, is symmetric.* Furthermore, to first order, the resonance absorption peak shape is expected to be
Lorentzian. Thus, changing the direction of the rf scan from a forward to a reverse scan (i.e., from increasing to decreasing the ion frequency to match the frequency of the supplementary ac voltage applied to the end-cap electrodes) should not produce the observed disparity in mass resolution. Likewise, the direction of a sweep of the supplementary ac frequency under conditions of fixed ring-electrode rf amplitude is not expected to affect peak shape for a pure quadrupole field. However, it has been observed experimentally in the stretched ion trap geometry that the resonance absorption line shape both for atomic and polyatomic ions is asymmetri~.~-*Although such curves, herein referred to as "resonance absorption" plots, were acquired over a series of discrete excitation frequencies and thus do not indicate the effects of scan rate or direction, they can nevertheless be highly revealing of the effects of nonlinear fields on resonance excitation processes. That is, the magnitude of the ion secular frequency shift due to the presence of nonlinear fields increases with the distance ions travel from the center of the ion trap, as pointed out by F r a n ~ e n .Thus, ~ because the ion oscillatory amplitude in the z-direction is a function of the supplementary ac voltage, resonance absorption plots acquired as a function of resonance excitation amplitude can reveal the shift in ion secular frequency with magnitude of ion oscillation. The indirect means described in the experimental section using ion dissociation via resonance excitation was used to obtain resonance absorption spectra. Two sets of resonance absorption plots are shown here to illustrate the effect of resonance excitation amplitude on ion secular frequency. The first set of plots, shown in Figure 5 , was obtained using the molecular ion of tetraethylsilane, an ion with a relatively low critical energy for decomposition.12 In this case, collision-induceddissociation can be effected with a relatively low parent ion oscillatory amplitude in the presence of helium. These plots, therefore, represent the resonance absorption profiles near the center of the ion trap. Note that (8) Gocringer, D. E.; Whittcn, W. B.;Ramsey, J. M.; McLuckey, S. A.; Glish, G. L. Anal. Chem. 1992.64. 1434. (9) Charles, M. J.; Glish, G. L.; McLuckey, S. A,, unpublished results,Oak Ridge National Laboratory, 1992. (10) March, R. E. Org. Muss Spedrom. 1993, 1151. (1 1) Todd, J. F. J.; Penman, A. D.; Thorner, D. A.; Smith, R. D. Rupid Commun. Mass Spectrom. 1990, 4, 108. (12) Nourse, B. D.; Kenttamaa, H. I. J . Phys. Chem. 1990, 94, 5809.
Analytical Chemistry, Vol. 68, No. 5, March 1, 1994
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the resonance absorption peak shape is asymmetrical with a somewhat shallower slope on the low-frequency side. This peak shape is qualitatively similar to resonance absorption plotsg-l employing resonance ejection but the asymmetry is much less pronounced. Presumably this is because the ions are closer to the center of the ion trap where the contributions from nonlinear fields are smallest. Note also that as the amplitude of the resonance excitation signal is increased, the peak tends to become broader and the maximum shifts to higher frequency. Resonance absorption profiles at higher amplitudes than shown here become increasingly difficult to measure for this particular parent ion because 100% of the parent ions are dissociated, making the optimum frequency difficult to measure. For this reason, a second set of plots was acquired for a parent ion, ionized n-butylbenzene,with a higher critical energy for decomposition. This set of curves, shown in Figure 6, spans a larger range of resonance excitation amplitudes and shows dramatically the shift in both peak shape and position with increasing excursions from the center of the ion trap. A shift in the optimum frequency of several hundred hertz is noted over the range of 140-320 mV along with a significant increase in the width of the absorption profile. Note that the peak shift in both plots is toward higher frequencies as predicted by ~imu1ation.l~ The resonance absorption profiles of Figures 5 and 6, though acquired using static excitation frequencies, can nevertheless provide a basis for interpreting the dynamic situation encountered in a resonance ejection scan that makes scan direction an important consideration. In a reverse scan, the decreasing rf amplitude causes the secular frequencies of the ions to decrease and thus approach the ac frequency applied to the end-cap electrodes from the high-frequency side. As the ions come into resonance with the applied signal their ion oscillatory motion increases in amplitude and their secular frequencies shift to higher values due to the octopole field, which grows stronger with distance from the trapcenter. These shifts move the secular frequencies of these ions away from the resonant excitation frequency and hence delay the growth of their trajectories. The larger these shifts the more dispersed (13) Franzen, J., private communication 1993.
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the ion cloud (for a single m / z value) becomes, and this translates into degraded mass resolution. Reduction of the supplementary ac voltage for reverse scan experiments affords some improvement in resolution (cf. Figures 3 and 4) because the optimum absorption frequency for ejection is contained over a much narrower region of frequencies. When a forward resonance ejection scan is performed, the change in rf amplitude increases the secular frequencies of the ions as they approach the ac signal from the low-frequency side. As the ions begin to come into resonance and are displaced from the center of the trap, their frequencies are further increased by interaction with the octopole component. These frequency shifts assist in driving the ions toward the frequency of the supplementary ac signal, which causes their trajectories to continue to grow rapidly until they are ejected from the trap. In this case, the ions "run into" the ac signal and their ejection is compacted in time, resulting in a sharper peak. Mass resolution is therefore improved by the presence of an octopole field in the forward scan. Note that resonance with the octopole field is not the basis of the effects discussed here. Alternatives to the explanations of the scan anisotropy given here, including the differences in the initial conditions of the ions prior to ejection, were considered. Consider the results from recent ion tomography experiments:14at higher qzvalues, ions of a single m / z value are held tightly in the center of the trap, but as the potential on the ring electrode is decreased (i.e,, qz is decreased in a reverse scan), the cloud begins to spread in both the axial and radial directions. This spreading of the trajectories will tend to increase the width of theejection profile for these ions and so might decrease the resolution. Hence, the initial spatial distributions for the forward and reverse scans might differ. An experiment using CsBr cluster ions was performed to determine whether changes in initial conditions, altered by quickly ramping the rf from high amplitude to low amplitude (in
