Anal. Chem. 1998, 70, 4896-4901
A Miniature Cylindrical Quadrupole Ion Trap: Simulation and Experiment Ethan R. Badman, Rudolph C. Johnson, Wolfgang R. Plass,† and R. Graham Cooks*
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1393
A cylindrical quadrupole ion trap (r0 ) 2.5 mm, z0 ) 2.88 mm, ∼1/64 of the volume of commercial hyperbolic ion traps) has been constructed, its geometry optimized, and its performance examined in the mass-selective instability scan mode. Spectra of ionized perfluorotributylamine and o-dichlorobenzene show a resolution (m/∆m, 50% valley definition) of ∼100. The instrument has been coupled to a membrane introduction system to test its applicability for on-line reaction monitoring and to determine detection limits. Simulations using the ion trap simulation program are used to explore the effects of geometry on performance and to validate the experimental results.
The imperative for minimization of analytical instrumentation has led to the development of small mass spectrometers that employ various types of mass analyzers. Magnetic,1-3 time-offlight,4,5 quadrupole,6,7 and even FT-ICR8 mass spectrometers have been the subject of miniaturization efforts. Two groups have built extremely small linear quadrupoles (with inner radii on the order of 200 µm) using silicon micromachining methods.9-11 Silicon micromachining methods have also been used to develop an array of electrostatic energy analyzers for charged particle analysis.12 † On leave from the II. Physikalisches Institut, Justus-Liebig Universita¨t Giessen, 35392 Giessen, Germany. (1) Sinha, M. P.; Tomassian, A. D. Rev. Sci. Instrum. 1991, 62, 2618-2620. (2) Kogan, V. T.; Kazanskii, A. D.; Pavlov, A. K.; Tubol’tsev, Y. V.; Chichagov, Y. V.; Gladkov, G. Y.; Il’yasov, E. I. Instrum. Exp. Technol.-Engl. Tr. 1995, 38, 106-110. (3) Sinha, M. Presented at the 46th ASMS Conference on Mass Spectrometry and Allied Topics, Orlando, FL, May 31-June 4, 1998. (4) Bryden, W. A.; Benson, R. C.; Ecelberger, S. A.; Phillips, T. E.; Cotter, R. J.; Fenselau, C. Johns Hopkins APL Technol. Dig. 1995, 16, 296-310. (5) Ecelberger, S. A.; Cotter, R. J.; Cornish, T. J.; Fenselau, C.; Bryden, W. A. Proceedings of the 45th ASMS Conference on Mass Spectrometry and Allied Topics, Palm Springs, CA, 1997; p 469. (6) Ferran, R. J.; Boumsellek, S. J. Vac. Sci. Technol. A 1996, 14, 1258-1265. (7) Orient, O. J.; Chutjian, A.; Garkanian, V. Rev. Sci. Instrum. 1997, 68, 13931397. (8) Miller, G.; Koch, M.; Hsu, J. P.; Ozuna, F. Proceedings of the 45th ASMS Conference on Mass Spectrometry and Allied Topics, Palm Springs, CA, 1997; p 1163. (9) Syms, R. R. A.; Tate, T. J.; Ahmad, M. M.; Taylor, S. Electron. Lett. 1996, 32, 2094-2095. (10) Brennan, R.; Chutjian, A.; Fuerstenau, S.; Hecht, M.; Orient, O.; Wiberg, D.; Yee, K. Proceedings of the 45th ASMS Conference on Mass Spectrometry and Allied Topics, Palm Springs, CA, 1997; p 470. (11) Taylor, S.; Tunstall, J. J.; Syms, R. R. A.; Tate, T.; Ahmad, M. M. Electron. Lett. 1998, 34, 546-547. (12) Stalder, R. E.; Boumsellek, S.; Van Zandt, T. R.; Kenny, T. W.; Hecht, M. H.; Grunthaner, F. E. J. Vac. Sci. Technol. A 1994, 12, 2554-2558.
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The relative simplicity and small size of the quadrupole ion trap make it an ideal candidate for miniaturization. Previous work in this laboratory in collaboration with scientists at Los Alamos National Laboratory13,14 has included development of both halfand quarter-sized (internal radius 5.0 mm and 2.5 mm) hyperbolic ion traps. These instruments demonstrated the extended mass/ charge range consequent upon reduction in the trap dimensions. This is expressed in the mass analysis equation
m/e )
8V qejectΩ (r02 + 2z02) 2
(1)
where m/e is the mass-to-charge ratio of the ion, V is the voltage of the applied rf, qeject is the value of the Mathieu parameter qz when the ion is ejected from trap, Ω is the angular frequency of the applied rf, and r0 and z0 are the inscribed radius and centerto-end-cap distance, respectively, of the ion trap electrode structure. Other performance characteristics were rather poor due to the large relative size of the entrance and end-cap electrode apertures. Because of the relative difficulty in accurately machining hyperboloid shapes on a small scale, a quadrupole ion trap with a cylindrical ring electrode and flat end-cap electrodes was selected for the present work. Note that the cylindrical ion trap (CIT) itself is not new.15 It has been used as a mass analyzer in the mass-selective stability mode16-18 and more recently for ion storage.19-24 (13) Kaiser, R. E.; Cooks, R. G.; Moss, J.; Hemberger, P. H. Rapid Commun. Mass Spectrom. 1989, 3, 50-53. (14) Kaiser, R. E.; Cooks, R. G.; Stafford, G. C.; Syka, J. E. P.; Hemberger, P. H. Int. J. Mass Spectrom. Ion Processes 1991, 106, 79-115. (15) Langmuir, D. B.; Langmuir, R. V.; Shelton, H.; Wuerker, R. F.; United States Patent No. 3,065,640, 1962. (16) Benilan, M. N.; Audoin, C. Int. J. Mass Spectrom. Ion Phys. 1973, 11, 421432. (17) Bonner, R. F.; Fulford, J. E.; March, R. E.; Hamilton, G. F. Int. J. Mass Spectrom. Ion Phys. 1977, 24, 255-269. (18) Mather, R. E.; Waldren, R. M.; Todd, J. F. J.; March, R. E. Int. J. Mass Spectrom. Ion Phys. 1980, 33, 201-230. (19) Wood, T. D.; Ross, C. W.; Marshall, A. G. J. Am. Soc. Mass Spectrom. 1994, 5, 900. (20) Mikami, N.; Miyata, Y.; Sata, S.; Toshiki, S. Chem. Phys. Lett. 1990, 166, 470-474. (21) Mikami, N.; Sato, S.; Ishigaki, M. Chem. Phys. Lett. 1991, 180, 431-435. (22) Mikami, N.; Sato, S.; Ishigaki, M. Chem. Phys. Lett. 1993, 202, 431-436. (23) Grebner, T. L.; Neusser, H. J. Int. J. Mass Spectrom. Ion Processes 1994, 137, L1-L6. (24) Ji, Q.; Davenport, M. R.; Enke, C. G.; Holland, J. F. J. Am. Soc. Mass Spectrom. 1996, 7, 1009-1017. 10.1021/ac980908w CCC: $15.00
© 1998 American Chemical Society Published on Web 11/04/1998
We recently reported the first use of a cylindrical ion trap (r0 ) 1.0 cm) in the mass-selective instability mode.25,26 Parallel work is ongoing elsewhere.27 Our results suggest that the CIT performance matches that routinely sought28,29 from hyperbolic ion traps: a mass range of ∼600 Th (1 Th ) 1 Da/unit charge30), high sensitivity, better than unit mass resolution, and the ability to perform multiple stages of mass isolation and analysis (MSn). Even more recently, we reported the first use of the CIT in a nondestructive mode of detection.31 Here we report the development of a miniature CIT (r0 ) 2.5 mm) operated in the massselective instability mode with a trapping volume 1/64th that of the full-scale trap. EXPERIMENTAL SECTION Experiments were performed using an extensively modified Finnigan ITD 700 (Finnigan Corp., San Jose, CA) ion trap mass spectrometer system. The instrument was adapted for tandem mass spectrometry using procedures described previously.32 The CIT was mounted on ion optical rails inside a large vacuum manifold. The system was designed so that the standard Finnigan ITD electron ionization source, gate and lens system, and electron multiplier detector assembly could all be used. In addition, the trap was driven and data were recorded using custom-modified Finnigan electronics. The basic design of the CIT is as described previously.26 The cylindrical electrode was machined into a 23/4-in. inch conflat flange, chosen to match the size and mounting arrangement of the standard electrodes. The trap electrodes and mounting supports were machined from 304 stainless steel while machinable glass ceramic (Macor, Corning Inc., Corning, NY) was used for electrode spacing and electrical isolation. The inner radius (r0) of the cylindrical ring electrode was chosen to be 2.5 mm, or onefourth the dimension of the commercial ring electrode. As with the full-sized CIT, the ring/end-cap spacing has a large effect on the higher-order components of the electric field in the trap.26 Best performance is achieved with the smallest distance between the two, consistent however with the requirement that electrical isolation be maintained. The ring/end-cap spacing was chosen as 1.25 mm, the smallest size that could be conveniently and accurately machined. The entrance and exit end-cap electrodes each have a 1.0-mm-diameter hole in the center to allow entrance of the ionizing electron beam and egress of ions to the detector, respectively. This size is half that used in the full-sized device and therefore, proportionately, the apertures are twice as large. Smaller holes were tested and found to reduce significantly the (25) 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. (26) Wells, J. M.; Badman, E. R.; Cooks, R. G. Anal. Chem. 1998, 70, 438-444. (27) 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. (28) Johnson, J. V.; Yost, R. A.; Kelley, P. E.; Bradford, D. C. Anal. Chem. 1990, 62, 2162. (29) Charles, M. J.; Glish, G. L. In Practical Aspects of Ion Trap Mass Spectrometry; March, R. E., Todd, J. F. J., Eds.; CRC Press: Boca Raton, FL, 1995; Vol. III, Chapter 3. (30) Cooks, R. G.; Rockwood, A. L. Rapid Commun. Mass Spectrom. 1991, 5, 93. (31) Badman, E. R.; Wells, J. M.; Bui, H. A.; Cooks, R. G. Anal. Chem. 1998, 70, 3545-3547. (32) Louris, J. N.; Cooks, R. G.; Syka, J. E. P.; Kelley, P. E.; Stafford, G. C.; Todd, J. F. J. Anal. Chem. 1987, 59, 1677-1685.
number of electrons that can be gated into the trap volume for ionization, therefore, greatly reducing the ion signal detected. The z0 dimension, the distance from the center of the trap to the end-cap electrode, was determined by calculation of the higherorder field components using the Poisson/Superfish33 code developed at Los Alamos National Laboratory and an in-house multipole expansion fitting program.34 The Poisson program was used to generate the potential array inside the volume defined by the trap electrodes, after which the multipole expansion fitting program was used to fit the potential array to a multipole expansion and to calculate the higher-order field coefficients. The potentials were fit on a series of 20 ellipses from 0.75 mm from the trap center to within 0.125 mm of the electrodes. Using this method, it was determined that a z0 value of 2.88 mm best approximated the fields present in the CIT previously characterized.26 The higher-order field content of the miniature trap (reported here as percent An/A2, where A2 corresponds to the quadrupole and n ) 4 or 6 corresponds to the octapole and dodecapole field coefficients in the expansion) was +8.65% for A4/ A2 and -16.12% for A6/A2. The high negative percentage for the dodecapole term was seen also in the full-sized CIT26 and cannot be reduced to zero without introducing a large negative octapole component. The relatively high percent of positive octapolar field is used to help compensate for this dodecapole contribution; all other higher-order terms are considered to be of negligible importance. Simulations of the effects of the higher-order fields consequent upon manipulation of the geometry, specifically the z0 dimension, were performed as discussed below. Two external arbitrary wave form generators (model 33120A, Hewlett-Packard, Palo Alto, CA) were used to generate the 1.60 MHz sine wave used as the rf drive frequency applied to the ring electrode and as the dipolar ac frequency used for the resonance ejection experiments. A balun amplifier was used to generate the (90° phase difference needed to apply the dipolar ac signal to the end-cap electrodes. Slow-scanning capabilities were achieved through use of an in-house attenuation board, described previously14 and now integrated into the ITD electronics. Briefly, the board attenuates the rf-DAC signal generated by the mass control circuit which defines the applied rf. The rf is ramped to the attenuated value in the same time as required for the normal value to be reached, allowing the ramp to be slowed at the expense of the available mass range. The home-built membrane introduction probe35,36 used for the detection limit study employed a 0.64-mm-i.d./1.19-mm-o.d. zeolitefilled silicone capillary membrane (Silastic Laboratory Tubing, Dow Corning Corp., Midland, MI). A flow injection module consisting of a peristaltic pump (model M312, Gilson, Middleton, WI) was used to control delivery of the sample solution. The sample temperature was controlled separately (model FIA-TC1, MIMS Technology Development Inc., Palm Bay, FL) and was typically 91 °C in these experiments. The probe was inserted directly into the vacuum manifold to allow the sample to pervapo(33) Billen, J. H.; Young, L. M. Proceedings of the 1993 Particle Accelerator Conference, 1993; pp 790-792. (34) Plass, W. R., unpublished results, 1998. (35) Bier, M. E.; Kotiaho, T.; Cooks, R. G. Anal. Chim. Acta 1990, 231, 175190. (36) Bauer, S.; Solyom, D. Anal. Chem. 1994, 66, 4422-4431.
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rate through the membrane into the manifold so that it could be ionized via internal EI in the ion trap volume. Simulations of ion motion and mass spectra were performed with the ion trap simulation program (ITSIM) for Windows 95/ NT, developed in this laboratory.37 The program allows the motion of multiple ions in an ion trap of user-selected geometry to be followed under various operating conditions. With ITSIM up to 600 000 ions can be generated in the ion trap with different original spatial distributions. Their trajectories are calculated using a first-order Euler method or, in a new approach, using a standard fourth-order Runge-Kutta method. The simulations take into account the rf trapping field and supplementary dc and ac fields used for ion isolation and resonant ejection. Statistical corrections for collisions with the helium buffer gas and Coulombic interactions between the ions can be included in the simulation but neither was used in this study. A complete mass spectrum is generated by simulating the ejection process for an appropriate collection of ions. The inclusion of nonlinear field effects, resulting from the cylindrical geometry and the end-cap holes, could previously only be calculated using a multipole expansion. The newest update to ITSIM allows the electric fields to be calculated from numerical potential values computed with the Poisson program. This method was used for all simulations. Separate Granville-Phillips (Boulder, CO) leak valves were used to introduce headspace vapors of organic compounds and helium buffer gas into the vacuum manifold. Organic compounds were obtained from Aldrich (Aldrich, Milwaukee, WI). All pressure measurements are uncorrected. RESULTS AND DISCUSSION Operating Frequency. Initially, the 1/64th size CIT was tested using an rf drive frequency of 1.1 MHz, the standard value used in the Finnigan ITD system. This proved to be unacceptable because the minimum drive voltage allowed by the rf electronics was too high to trap ions below ∼80 Th. This is a result of the extremely small size of the device. The mass analysis equation (eq 1) shows that, for a fixed operating point, qeject, and fixed rf frequency and voltage, a decrease in the dimensions of the ion trap (i.e., r0 and z0) causes ions of higher mass/charge to be ejected. This also has the effect of increasing the low-mass cutoff that is available relative to that available with a standard hyperbolic ion trap operated at the same rf frequency. To overcome this problem, the rf drive frequency was increased to 1.60 MHz by applying a signal from the external wave form generator to the ITD rf amplifier to replace the internally generated 1.10-MHz signal. This change required replacing the ITD inductor coil by a smaller coil and changing the capacitance so that the resonance circuit could be properly tuned.14 All data and simulations shown use the 1.60-MHz rf drive frequency. The rf frequency was not optimized further; the value of 1.60 MHz allowed the low-mass fragment ions of the compounds of interest to be trapped and did not require extensive modification to the rf generation and control electronics. Typical Spectra. Figure 1 shows mass spectra of a calibration compound, perfluorotributylamine (PFTBA), and of o-dichlorobenzene. The abscissa for these and all subsequent spectra have been calibrated using ions of known mass/charge ratio (the change in (37) Bui, H. A.; Cooks, R. G. J. Mass Spectrom. 1998, 33, 297-304.
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Figure 1. (a) Mass spectrum of PFTBA recorded with the miniature cylindrical ion trap: PPFTBA 1 × 10-6 Torr, PHe 2 × 10-5 Torr; ionization time 25 ms; low-mass cutoff 41 Th; resonance ejection, 775 kHz and variable amplitude 1.3 (50-100 Th), 1.5 (100-140 Th), and 3.0 V (140 Th and up). (b) Mass spectrum of o-dichlorobenzene: Pdichlorobenzene 1 × 10-6 Torr, PHe 2 × 10-5 Torr; ionization time 25 ms; low-mass cutoff 49 Th; resonance ejection, 775 kHz, fixed resonant ac amplitude, 4.5 V.
size, geometry, and frequency obviously does not allow for automatic calibration through the instrument control system). Each spectrum is an average of 30 scans of ∼50 ms each. Resonance ejection at 775 kHz was used, and the amplitude of the resonant ac signal was adjusted to achieve the best resolution. The selected ac frequency corresponds to a value qz of 0.907 (i.e., to an axial modulation experiment since this qz value corresponds approximately to the stability boundary in the axial direction). Values of this magnitude also give good performance in hyperbolic ion traps. Because an ac ramp was not available on the instrument, portions of the spectrum were recorded separately and reconstructed to give the full spectrum shown. The fragment ions up to 90 Th were ejected using a 1.3 V sine wave, the fragments from 90 to 131 Th using 1.5 V, and the higher mass ions using 3.0 V. With this technique, a modest improvement in both intensity and resolution was achieved, especially for the higher mass ions. Calibration of the mass/charge scale using the PFTBA spectrum is straightforward. The mass spectrum of o-dichlorobenzene (Figure 1b) was recorded using resonance ejection at 775 kHz and a constant voltage of 4.5 Vp-p over the entire mass range. This spectrum compares well with that recorded using a 1.0-cm hyperbolic ion trap (unpublished data) except that there is some loss in resolution in the small trap. Two significant results can be seen in the spectra shown in Figure 1: the large signal intensities obtained with the device and the relatively long ionization timessin the tens of milliseconds rangesrequired for optimum performance. Even with the 64-fold decrease in volume available for trapping ions, the miniature CIT provides high signal intensities and quite good signal-to-noise (S/ N) ratios. The long ionization times needed for optimal performance may be a result of low ionization efficiency in the device, but this does not pose significant problems in performance or
Figure 2. Mass spectrum of o-dichlorobenzene taken at reduced (half) scan speed: Pdichlorobenzene 1 × 10-6 Torr, PHe 2 × 10-5 Torr; ionization time 25 ms; low-mass cutoff 53 Th; resonance ejection, 775 kHz, 3.0 V. Figure 4. Selected ion monitoring of m/z 91 and 92 from toluene solutions recorded using capillary membrane introduction: (a) 1 ppm, ionization time 60 ms; (b) 500 ppb, ionization time 100 ms. PHe 2 × 10-5 Torr; resonance ejection, 775 kHz, 1.3 V.
Figure 3. Experimental stability diagram for the miniature CIT, mapped using the 131 Th fragment ion of PFTBA: PPFTBA 9 × 10-7 Torr; no resonance ejection or helium buffer gas used.
operation of the device. The smaller size of the end-cap holes compared to the full-sized CIT may also reduce the ionization efficiency, but the holes were kept small in order to minimize local higher-order field effects in the region of the end-cap electrodes. As shown in Figure 1b, operation under normal resonance ejection conditions at the normal scan rate, determined by the electronics, the rf frequency, and the reduced trap size, gave a resolution that allowed separation of ions spaced by 2 Th. The lower mass resolution is the main disadvantage of the miniature trap in comparison with full-scale devices. Slow Scans. To gain back some of the resolution lost in the course of reduction in the size of the trap, a slower rf ramp was used in the mass-selective instability scan. This is an established method for increasing resolution.14,38 The measured ramp rate of ∼14 Th/ms (which includes the effect of the dimensions of the miniature CIT and the frequency used) was slowed by a factor of 2-7 Th/ms, a rate that is still slightly faster than the rate of 5.55 Th/ms used with the commercial hyperbolic ion trap in the ITMS and ITD systems. The spectrum of o-dichlorobenzene (Figure 2), recorded using the slower scan, indeed shows (38) Schwartz, J. C.; Syka, J. E. P.; Jardine, I. J. Am. Soc. Mass Spectrom. 1991, 2, 198-204.
Figure 5. Simulated mass spectrum of o-dichlorobenzene. The simulation was performed with 5000 ions using ITSIM with the electric field calculated from a numerically determined potential distribution appropriate to the CIT geometry.
the improvement expected. The full width half-maximum for the 146-Th ion is now 1.4 Th, corresponding to a resolution (m/∆m, 50% valley definition) of ∼100. One feature that is apparent when Figure 1b is compared with Figure 2 (o-dichlorobenzene at the standard scan rate vs the reduced scan rate) is the decrease in S/N ratio as the scan rate is slowed. This effect has been noted previously.38 Further improvement in resolution was achieved at even slower scan rates, along with a further decrease in S/N ratio. Experimental Map of the Stability Diagram. To better characterize the miniature CIT and to compare it with the standard hyperbolic ion trap, the upper half of the stability diagram was mapped (the lower half could not be mapped due to limitations in the control software and the dc supply available). The experimental stability diagram was obtained using the 131-Th fragment ion of PFTBA, created using an ionization time of 50 ms and isolated using rf/dc isolation. Then, after a 5-ms delay time, the rf was ramped at a rate of 14 Th/ms to record the mass spectrum. By choosing an rf voltage and varying the dc voltage, Analytical Chemistry, Vol. 70, No. 23, December 1, 1998
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Figure 6. Simulated partial mass spectrum of o-dichlorobenzene for ion traps of r0 2.5 mm and (a) z0 ) 2.68 mm (b) z0 ) 2.88 mm, (c) z0 ) 3.13 mm.
or vice versa, the stability boundary was determined for each rf voltage as that voltage at which the ion signal became indistinguishable from the noise. The rf and dc voltages applied to the CIT were systematically varied, and the results are shown in Figure 3. Both the rf and dc power supplies were calibrated to provide an accurate determination of the stability diagram. Typically in a hyperbolic ion trap, the stability of an ion in expressed by its az and qz values.39 Approximate values for az and qz can be calculated for the miniature CIT using eqs 2 and 3,
az ) -8eA2u/mΩ2rn2
(2)
qz ) +4eA2V/mΩ2rn2
(3)
where A2 is the quadrupole expansion coefficient and rn is the corresponding normalization radius used in the expansion calculation. For the experimentally determined stability diagram, eqs 2 and 3 give qz ) 0.731 and az ) 0.146 for the apex and qz ) 0.874 for the upper boundary at az ) 0. For comparison, the theoretically determined values of az and qz at the apex are 0.782 and 0.150, respectively, and qz is 0.908 for the boundary in the axial direction. The shape of the stability diagram compares very well with that determined experimentally for a hyperbolic ion trap40 which contains similar errors in qz and az values compared to the theoretical values. This suggests that the field in the CIT is very similar to that in the hyperbolic trap and is encouraging in respect to the performance that can be expected from the CIT. Membrane Introduction. The CIT mass analyzer was used in conjunction with a membrane introduction mass spectrometry (MIMS) system to make a preliminary determination of the lower limit of detection of the device. The CIT was operated with a low-mass cutoff of 36 Th and scanned from 78 to 104 Th. The helium pressure was 2 × 10-5 Torr, and resonance ejection was performed at 775 kHz and 2.5 V. Toluene solutions in water were injected multiple times for 3.5 min at a flow rate of 2.5 mL/min and with a membrane temperature of 91 °C. Figure 4 shows a plot of ion current vs time for the 91- and 92-Th ions at (a) 1 ppm, 60-ms ionization time and (b) 500 ppb, 100-ms ionization time. The data demonstrate the reproducibility of the signals and the (39) March, R. E.; Londry, F. A. In Practical Aspects of Ion Trap Mass Spectrometry; March, R. E., Todd, J. F. J., Eds.; CRC Press: Boca Raton, FL, 1995; Vol. I, Chapter 2. (40) Johnson, J. V.; Pedder, R. E.; Yost, R. A. Rapid Commun. Mass Spectrom. 1992, 6, 760-764.
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signal-to-noise levels achieved. Further development in the ionization and ion injection processes of the CIT should allow decreased limits of detection. Simulations. A simulation of the o-dichlorobenzene mass spectra calculated using ITSIM is shown in Figure 5 for comparison with the experimental data shown in Figure 2. The simulated mass spectrum used 5000 ions with masses and relative abundances chosen to match those that occur in the standard mass spectrum. The ions were generated with an initial Gaussian spatial distribution with fwhmx,y ) 0.4 mm, fwhmz ) 0.24 mm, and zero initial velocity. No collisions were allowed, and a mass-selective instability scan without resonance ejection was simulated. The rf ramp rate was set to match the slow-scan rate used in Figure 2. Higher-order field effects were modeled using a numerical potential distribution and not the multipole expansion method. The data allow comparison of the resolution and peak shapes with experiment (Figure 2). The two spectra agree well in peak shapes and S/N ratios, even though resonance ejection was not simulated. Using ITSIM, simulations were also performed to compare the effect of the CIT geometry on resolution and signal intensity. Figure 6 shows simulated results which display the effect of different trap geometries, and hence higher-order fields, on the performance of the miniature CIT. Three different cylindrical geometries were simulated with three different z0 dimensions but a fixed r0 dimension (2.5 mm) and a fixed ring-end-cap gap (1.25 mm). An rf ramp rate of 14 Th/ms was applied, and 1000 ions of m/z 75 were used in these simulations. These simulation data validate the fact that the experimental geometry was in fact optimized in terms of resolution and sensitivity. Future. The resolution of the miniature CIT is poorer than the full-sized trap but adequate for many purposes. Other performance criteria are surprisingly good for so small a mass analyzer. Detection limits using MIMS for sample introduction are below 1 ppm and the dynamic range, as judged from the data in Figure 3, is on the order of 103. Dynamic range and detection limits can be traded against resolution by adjusting the scan speed. Future studies will aim at further improvement in performance as well as miniaturization of the entire control and data acquisition system, as well as the vacuum manifold, for field portability. Use of a set of miniature cylindrical ion traps may allow greater flexibility in MSn experiments41 and merit acquisition of other data, such as the parent ion scan. Further refinement of the simulation (41) McLuckey, S. A.; Glish, G. L.; VanBerkel, G. J. Int. J. Mass Spectrom. Ion Processes 1991, 106, 213-235.
program, ITSIM, will also allow more precise study of the effect of the local higher-order fields (especially those due to the endcap holes) on ion motion, trapping efficiency, and resolution. Development of the miniature CIT for use with nondestructive Fourier transform detection is also underway. ACKNOWLEDGMENT This work was supported by the Office of Naval Research and by Finnigan Corp. R.C.J. acknowledges a Phillips Petroleum Co.
Fellowship. We thank Alexander Kalimov and Matthias Wollnik for the original version of the field interpolator used with ITSIM and Jon Amy for advice on the electronics.
Received for review August 13, 1998. Accepted October 13, 1998. AC980908W
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