Anal. Chem. 1995, 67, 3650-3655
High-Resolution Selected Ion Monitoring in a Quadrupole Ion Trap Mass Spectrometer Greg Wells* and Chuck Huston
Varian Chromatography Systems, Walnut Creek, Califomia 94598
A new method of selected ion monitoring was described that was capable of unit mass isolation throughout the operational mass range of an ion trap mass spectrometer. The trap was first selectively Hied with ions via the application of a broad-band waveform to the end caps during ionization. A second isolation step completed the unit mass isolation, leaving only the desired ions in the trap. Following the accumulation and isolation steps, the intensity of the isolated ions was determined by resonantly scanning them from the trap for detection. As many as 10 different masses could be monitored by sequentially applying these steps. The ionization time for each mass was individually adjusted to optimize the dlling of the trap with the selected ions; this increased the dynamic range for the set of ions. The individual spectra for each mass could then be added together to form a mass spectrum containing all specified ions. Selected ion monitoring is a common mass spectrometry technique that is used to increase the signal-to-noise ratio (S/N) of ions (relative to their full scan values). The S/N improvement results from increased transmission efficiency and a reduction in noise due to the increased dwell time associated with the measurement of the selected ion. Ion trap mass spectrometers generally work on a different principle than transmission instruments.' Ions are formed in the electron ionization mode by pulsing the ionizing electron beam into the trap for a short period of time to ionize the sample. All of the ions that will contribute to the resulting mass spectrum are formed during this time interval. The trapped ions are then sequentially scanned from the trap for detection in increasing mass order. Since the ions must be scanned from the trap for detection, the signal-to-noise ratio for a particular ion can only be improved by increasing the number of ions of a particular mass-to-charge ratio that are stored. The storage of large numbers of ions in the trap will result in Coulombic interactions that degrade the sensitivity and resolution of the device. The need to utilize the limited ion storage capacity of the trap has resulted in the development of several techniques to eliminate unwanted ions from the trap and selectively accumulate only specified target ions. Radio frequency (4isolation techniques have been used to selectively trap and accumulate These techniques establish trap conditions that destabilize * Address reprint requests to Varian Chromatography Systems, 2700 Mitchell Dr., Walnut Creek, CA 94598. (1) March, R. E.; Hughes, R. J. Quadrupole Storage Mass Spectrometry;Wiley: New York, 1991. ( 2 ) Fulford, J. E.: March, R E. Znt. J. Mass Spectrom. Ion Phys 1978,26, 155. (3) Eades, D. E.; Yates, N. A. Yost, R A. Proceedings ofthe 39th ASMS Conference on Mass Spectrometry and Allied Topics, May 19-24, 1991; p 1491.
3650 Analyfical Chemistry, Vol. 67, No. 20, October 15, 1995
unwanted ions. A significant limitation to this approach is the requirement of moving the operating point of the selected ion close enough to at least one instability boundary so that ions adjacent to the selected ion are removed from the trap by instability ejection. Ions trapped in a quadrupolar trapping field have amplitude oscillations whose characteristic (or secular) frequencies are functions of the mass-to-charge ratio of the ion and the frequency and voltage of the rf trapping field.' The radial or axial motion of the ions can be increased, via resonance excitation, by subjecting the ions to a field whose frequency matches the characteristic frequency of the ion motion. The selectivity of resonance ejection was utilized in early studies of ion trapping processes to eliminate interfering ions.5 In these studies, a supplemental voltage was added to the end-cap electrodes of the trap to create a dipole field whose frequency matched the secular frequency of the ion to be ejected. The ions in resonance with this field increased their amplitudes in the axial direction until they were ejected. Swept frequency technique^"^ for ion ejection have met with limited success due to the low duty cycle for ejection. More recently, broad-band resonance excitation techniques have been described that use inverse Fourier transform (SWIFT) waveforms,"12 constructed waveform^,'^-^^ or filtered n o i ~ e . ' ~ - ' ~ The techniques utilizing broad-band resonance excitation are all similar in that the waveforms have numerous frequency components to eject unwanted ions and notches in the frequency spectrum corresponding to the secular frequencies of the target ions. The waveforms are applied during the ionization period to continuously eject the unwanted ions. Efficient isolation of the target ions requires very narrow notches with frequency compo(4) Weber-Grabau, M. US. Patent 4,818,869, 1989. (5) Fulford, J. E.; Hoa, D. N.; Hughes, R J.; March, R. E.; Bonner, R. F.; Wong, G. J. J. Vac. Sci. Technol. 1980 17, 829-835. (6) McLuckey, S. A; Goeringer, D. E.; Glish, G. L. J. Am. SOC.Mass Spectrom. 1991,2, 11-21. (7) Schwartz, J. C.; Jardine, I. Rapid Commun. Mass Spectrom. 1992.6, 313. (8) Marshall, A. G . ; Ricca, T. L.; Wang, T. L. U S . Patent 4,761,545, 1988. (9) Goodman, S.; Hanna, A U S . Patent 4,945,234, 1990. (10) Guan, S.; Marshall. A. G. Anal. Chem. 1993,65,1288-1294. (11) Jullian, R IC; Cooks, R. IC Anal. Chem. 1993,65,1827-1833. (12) Soni, M. H.; Cooks, R G. Anal. Chem. 1994,66, 2488-2496. (13) Franzen, J.; Gabling, R. H.; European Patent Application EP 0 362 432 Al, 1988. (14) Shaffer, B. A; Kamicky, J. ; Buttril, S. E., Jr. Proceedings of the 41st ASMS Conferenceon Mass Spectrometry and Allied Topics, May 31-June 4, 1993; pp 802a-802b. (15) Wells, G.; Huston, C.. submitted for publication in J. Am. SOC.Mass Spectrom. (16) Garrett, A W.; Cisper, M. E.; Nogar, P. H.; Hemberger, P. H. Rapid Commun. Mass Spectrom. 1994,8, 174-178. (17) Kelley, P. E.; Hoekman, D.; Bradshaw, S. Proceedings of the 4Ist ASMS Conference on Mass Spectrometry and Allied Topics, May 31-June 4, 1993, 453a-453b. 0 1995 American Chemical Society 0003-2700/95/0367-3650$9.00/0
nents on each side of the target ion secular frequency to eject the corresponding ions with masses above and below it. The Coulombic innuence of space charge on the ion secular frequency, however, can shift the frequency as though a dc field were present.2 Since there is a finite time required to eject unwanted ions from the trap by resonance excitation, there is some average space charge, throughout the ionization period, in excess of that due only to the target ions. The space charge can shift the secular frequency of the target ions from the theoretical value used to determine the center frequency of the notch. Thus, when narrow notches are used for isolation and a large amount of space charge is present, the secular frequencies of many of the ions may be shifted outside of the notch, causing these ions to be ejected. More recently it has been recognized that, in nonideal ion traps possessing an octopole distortion, the secular frequency of the ion depends on the amplitude.'s Thus, ions formed far from the center of the trap (e.g., during electron ionization or ion injection processes) will have secular frequencies that are different from the values that they would have at the center of the trap. The use of a buffer gas in ion traps for collisional dampening of the ion oscillations is advantageous for both electron ionization5q6J9 and ion injection pr0cesses.~~-~3 It would be expected that ions formed or injected far off center followed by collisionaldampening to the center would experience a time-dependent shift in their secular frequencies. Again, the initial frequencies would be different from those located at the center of the notch, causing ejection of some of the target ions. The most severe limitation to using broad-band waveforms to accomplish both selective ion accumulation and ion isolation is the inability to achieve unit mass isolation at higher masses. In these methods, the rf storage voltage is held constant during the application of the waveforms. Since the electron energy distribution depends on the amplitude of the rf storage voltage, a relatively low value is required to obtain E1 spectra that are similar to those obtained from other types of mass spectrometers. The operating point of an ion (the "q," value) decreases as the mass increases. This results in a decrease in mass dispersion with increasing mass. Therefore, a waveform with a fixed notch width will trap an increasing mass range as the ion mass increases. For example, ions with m/z = 100 and 101 have secular frequencies separated by 2000 Hz at a rf storage voltage of 450 Vpp;ions with m/z = 500 and 501 have frequencies separated by only 100 Hz. A broadband waveform that has a 2 kHz wide notch centered at frequencies corresponding to m/z = 100 and 500 will isolate m/z = 100 from adjacent integer masses, but a range of ions (approximately 20 Da) will be trapped at m/z = 500. This work overcame the limitations that occurred when simultaneous isolation of several ions of greatly differing masstocharge ratios was attempted. The method employed in this work utilized a three-step process in which the trap was first selectively filled with the target ion by the application of a broadband waveform to the end caps of the trap during ionization. The
waveform served to eliminate the unwanted ions from the trap and accumulated ions in a small mass range about the target ion. A second isolation step ejected all remaining unwanted ions and left only the desired ions in the trap. Following the storage and isolation steps, the intensity of the isolated ions was determined by resonantly scanning the ions from the trap for detection. As many as 10 different masses could be monitored by sequentially applying these steps. The ionization time for each mass was individually adjusted to optimize the filling of the trap with the target ions, thus increasing the dynamic range for the set of ions. The individual scans for each mass could then be added together to form a mass spectrum containing all specified ions.
(18) Franzen. J. Int. J. Mass Spectrom. Ion Processes 1991,106, 63-78. (19) Stafford, G. C.;Kelley, P. E.; Stephens, D. R US. Patent 4,540,884, 1985. (20) Van Berkel, G. J.; Glish, G. L.; McLuckey, S. A Anal. Chem. 1990,62, 1284- 1295. (21) Louris, J. N.; Amy, J. W.; Ridley, T.Y.; Cooks, R G.Int. J. Mass Spectrom. Ion Processes 1989,88, 97. (22) McLuckey, S. A; Glish, G. A; Asano, K G . Anal. Chim. Acta 1989,225, 25. (23) Pedder, R E.; Yost, R A; Weber-Grabau, M. Proceedings of the 37th ASMS Conferenceon M a s Spectromety and Allied Topin, May 21-26,1989; p 468.
The value of BZis a function of the operating point in (az,qJ space and can be computed from a well-known continuing fraction.' In this work no dc field was applied; thus az = 0. Figure 1 shows the general scan function used for this w 0 r k . 2 ~ - ~A~ broad-band multifrequency waveform WF1 was
EXPERIMENTAL SECTION All experiments were performed using a Varian Saturn 3 ion trap (Varian Chromatography Systems, Walnut Creek, CA) and prototype Selected Ion Storage (SIS) software. The Saturn system contained a built-in arbitrary waveform generator (Wave-Board) that applied a waveform to the endcap electrodes to create a dipolar field. Memory on the Wave-Board allowed for the storage and application of 10 different scan groups and their associated waveforms. Each scan group contained the instrument scan parameters that were required to optimally fill the trap and isolate the selected ions. Additional scan groups and waveforms could be downloaded to the instrument at specified times during a chromatographic run (up to a maximum of 200 acquisition segments) or in real time instrument control. For data taken at slower or faster scan rates than are found in the standard ion trap, data were acquired using a Tektronix digitizing signal analyzer, DSA 601 with a 7A22 differential ampKer (Tektronix Inc., Beaverton, OR). RESULTS AND DISCUSSION The motion of an ion in a quadrupole field can be determined from the solution to the Mathieu equation.' The stable solutions to the equation are characterizedby the parameters q, and a, that define the operating point of the ion within the stability region. These parameters are defined as1
+ 2z012)/Q2 a, = -16eU/m(rt + 2zt)/Q2 q, = 8eV/m(r:
where Vis the amplitude (0 to peak) of the rf potential applied to the ring electrode, U is the dc potential, m is the mass-tocharge ratio (m/z),ro is the radius of the ring electrode, zo is the inscribed radius of the endcap electrodes, and Q is the rf drive frequency. The secular frequency of an ion, w,, can be determined from the value of Bz: w, = cB,/2)Q
(3)
(24) Bolton,B.; Wells, G.; Wang, M. Proceedings of the 4lstASMS Conference on Mass Spectrometry and Allied Topics, May 30-June 1, 1993; p 133.
Analytical Chemistry, Vol. 67, No. 20, October 15, 1995
3651
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applied during the ionization period and for a short "cool time" after the end of ionization. This waveform was constructed of discrete frequency components to resonantly eject ions of masstocharge ratio above and below the target ion. This step filled the trap with predominantly the target ions. The remaining ions with masses below the target ion were next removed by resonantly scanning them out of the trap by applying a singlefrequency dipole field (485 kHz) to the end caps of the trap and moving the operating point of the ion to qz = 0.901 by increasing the rf trapping voltage T/: The remaining ions with masses above the target ion were removed by lowering the operating point of the ion to qz = 0.84 coincident with the application of a broad-band waveform containing frequencies between 20 and 400 kHz with a 500 Hz spacing. Upon reaching q2 = 0.84 the rf field was held constant for 2 ml while the waveform remains on. The isolated target ions were then scanned from the trap for detection. This process could be repeated for a maximum of 10 different ions to produce a contiguous series of "group spectra". Each analytical scan was preceded by a prescan which also selectively filled the trap with the target ion and isolated the target ion as in the analytical scan. Thus, the ionization time in the analytical scan was based on the intensity of the mass isolated ion obtained in the prescan. After the individual group spectra were stored to the MS workstation disk, each unique set of group spectra could be added to form a single merged spectrum that contained up to 10 ion mass-intensity pairs. Figure 2A shows a background spectrum of polysiloxane column bleed from a DB5 column U&W Scientific, Folsom CAI, showing target ions m/z = 406, 415, and 431 taken in the ion profile mode. Figure 2B shows the selected ion storage of m/z = 415 using a broad-band notched waveform applied to the trap during the ionization period.15 A notch width of 3.0 kHz and a cutoff mass of 48 Da was used. The observed mass range stored was approximately 10 Da. Narrower notches would improve the isolation slightly but at the expense of the loss of a significant number of the target ions. No measurable ions were stored if a notch width of less than 1 kHz was used. Figure 2C shows the sequential isolation of m/z = 406, 415, and 431 (insets). The increased ionization time and the attendant increase in signal-to(25) Bolton. B.; Wells, G.; Wang, M. Proceedings ofthe 42stASMS Conference on Mass Spectrometry and Allied Topics, May 29-June 3, 1994; p 710. (26) Wells, G. Quadrupole Ion Trap Improved Technique For Ion Isolation. US. Patent No. 5,198,665, 1993.
3652
Analytical Chemisty, Vol. 67,No. 20, October 75, 7995
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noise ratio was approximately the same for the latter two methods. However, the two-step method used in Figure 2C has the additional advantage of unit mass isolation. This method of selective ion accumulation and isolation has previously been studied as a means of parent ion isolation for GC/MS/MS and was found to have excellent sensitivity and linearity in a variety of complex mat rice^.^^-^^ Individual spectra could not be merged in the profile mode since these data were not stored to the hard disk. Figure 3A shows the sequential acquisition of the three target ions prior to merging the spectra. Figure 3B shows the results of merging the individual spectra to form merged spectra files containing m/z = 406, 415, and 431. An important application of selected ion monitoring is the detection of co-eluting compounds; particularly isotopically labeled compounds. Figure 4A shows the unmerged chromatogram and spectra of 500 pg of phencyclidine (PCP) and 50 pg of deuterated phencyclidine @-5 PCP). The ion monitored for the unlabeled PCP was m/z = 200 (loss of C3H7); the D-5 PCP was monitored using m/z = 205. In this example the peak width at half-maximum was approximately 2 s. Eight data points were obtained during (27) Schachterle, S.; Brittain, R D. Proceedings ofthe 41st ASMS Conference on Mass Spectrometry and Allied Topics, May 30-June 1, 1993, p 178. (28) Schachterle, S.; Brittain. R. D., Mills, J.J Chromatogr. A 1994,683, 185193. (29) Feigel, C.; Brittain, R D. Proceedings ofthe 42stASMS Conference on Mass Spectrometry and Allied Topics, May 29-June 3, 1994; p 253. (30) Bolton. B.; Brittain, R D. M. Proceedings of the 42st ASMS Conference on Mass Spectrometry and Allied Topics, May 29-June 3, 1994; p 720.
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the half-width (16 across the entire peak), which is adequate for quantitation. Each data point is the result of averaging five microscans of 50 ms duration. Averaging several microscans increases the signal-to-noise ratio and increases the stability of the mass intensity. Reducing the number of microscans would allow more data points to be obtained across a chromatographic peak. The maximum ionization time and peak centroidmg represents the majority of the scan cycle time. Experiments that are currently in progress have decreased the cycle time to 16 ms while maintaining the same signal-to-noiseratio. This would allow an acquisition rate greater than 60 Hz for a single mass or over 10 Hz for six different masses. This data frequency is adequate for most applications of capillary chromatography. Figure 4B shows the chromatogram and spectra from the merged group spectra. The small difference in retention times between PCP and D-5 PCP could easily be seen as a change in relative ion ratios. The difference in retention times was even more apparent in Figure 5, which shows the reconstructed singleion mass chromatograms for m/z = 200 and 205. The minimum detectable amount of PCP was 5 pg (101 S/N) injected oncolumn, and the linear range extended to 1.0 ng (no data greater than 1.0 ng were obtained). Sequential isolation and detection of each ion had the inherent advantage of allowing the analytical ionization time to be adjusted independently for each ion, thus increasing the useful dynamic range for the set of selected sample ions. The independent adjustment of the ionization time resulted in the complete filling of the trap with ions of a single mass-tocharge ratio. The signal increased linearly with ionization time, and the noise decreased (at most) by the square root of the number of scans that are co-
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Figure 5. Reconstructed single-ion mass chromatogram for m/z = 200 and 205.
added. Therefore, the signal-to-noiseratio will improve faster by maximizing the number of ions of a given mass-tocharge ratio than by averaging several mass scans. The utility of this method of selected ion monitoring can be better appreciated when the affects of space charge due to background ions from the sample matrix and column are considered. Excess space charge, particularly from masses greater than the target mass, creates a dc field that results in a h i t e value of a, (eq 2), even if there is no applied dc voltage to the trap electrodes. The secular frequency (0, in eq 3) depends on the operating point (az,43. The effect of the space charge is to shift the secular frequency to smaller values (toward higher mass), If the space charge distortion is severe, mass peak shapes Analytical Chemistty, Vol. 67, No. 20, October 15, 1995
3653
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! , , , . , 2ie 215 m/z Figure 6. Spectra of D-5 PCP with polysiloxane column bleed background: (A) ionization time 1000 ps.; (B) ionization time 5000 ps; (C) ionization time 5000 ps with m/z = 205 isolated. I
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will be distorted. Figure 6A shows the spectrum of D-5 PCP (m/z = 205) in the presence of polysiloxane column bleed (m/z = 207). Increasing the ionization time (Figure 6B) to increase the sensitivity results in a loss of mass resolution due to space charge effects. When the ion was mass isolated to remove the excess space charge from the m/z = 207 ion, the resolution and sensitivity was restored (Figure 6C). Ions with masses below the target ion do not affect the target ion since they are first removed by resonant scanning. The broad-band waveform (wF2 in Figure 1) was applied at an operating point (qz= 0.901) greater than than the final value (qr = 0.84). At this point, the highest frequency component of the waveform was in resonance with ions several daltons above the target ion mass. The rf field was lowered to slowly reduce the operating point to the final value, resulting in the ejection of high-mass ions in decending mass order. As the unwanted space charge was ejected from the trap, the secular frequency of the target ion shifted back to higher frequencies (toward lower mass) and the resolution and mass position was restored. A notched broad-band waveform utilizing a 2 kHz notch width would still not separate m/z = 205 from m/z = 207 sufficiently to prevent distortion of the PCP ion, particularly when the mass from the polysiloxane is in great excess. An extension of this technique can be used to obtain highresolution selected ion monitoring. It is known that increased mass resolution is possible by reducing the rf scan rate during mass a n a l y ~ i s . ~It~is- ~also ~ possible to increase the resolution during the mass isolation step34by reducing the rates at which the rf field is changed. Polysiloxane column bleed interferes with 3654 Analytical Chemistry, Vol. 67, No. 20, October 15, 7995
the measurement of ions of hexachlorobenzene (HCB) at the nominal integer mass of m/z = 282. HCB has a mass of 281.8134 Da and the polysiloxane ion has a mass of m/z = 282.0510 Da. Figure 7A shows the high mass resolution scan of these ions at a scan rate of 550 Da/s (10 times slower than the normal scan rate) following isolation of a mass range of 1 Da. Isolation of the HCB ion and high mass resolution scanning (Figure 7B) would reduce space charge effects as in the previous example.34 Although these ions are resolved, the slow scan rate results in the peak width (at the base) being increased from the normal 180 to 750 ps/Da. If the same number of ions are isolated and scanned out for detection, the peak sensitivity would be reduced by over a factor of 4 due to the slower scan rate. Since it is possible to achieve high mass resolution during the isolation step, it should not be necessary to also scan the ions out of the trap under high-resolution conditions. In Figure 7C the HCB ion is first isolated under high-resolution conditions as in Figure 7B, followed by mass scanning at a rate of 11000 Da/s (90 ps/Da). A factor of 8 increase in sensitivity was expected to be obtained in this manner. The observed increase was only a factor of 7. It is believed that the peak response at these scan rates was limited by the bandwidth of the electrometer, since the predicted factor of 4 increase was observed when the scan rate was reduced to 180 ps/Da. (31) Schwartz, J. C.; Syka, J. E. P.; Jardine, I.]. Am. SOC.Mass Spectrom. 1991, 2, 198-204. (32) Williams, J. D.; Cox, IC A; Cooks, G. R.; Kaiser, R E., Jr.; Schwartz, J. C. Rapid Commun. Mass Spectrom. 1991,5,327-329. (33) Londry. F. A; Wells, G. J.; March, R. E. Rapid Commun. Muss Spectrom. 1993,7,43-45. (34) Schwartz, J. A; Jardine, I. Rapid Commun. Mass Spectrom. 1992,6,313317.
A significant unknown in this method is the ultimate accuracy of the mass isolation step at very high resolution. The effects of space charge on resolution and mass accuracy become more pronounced as the number of ions in the trap is increased. TraldP reported mass shifts that are related to ion structure. These effects are not completely understood and are the subject of current research. CONCLUSIONS A new method of selected ion monitoring was demonstrated that combined the sensitivity enhancement of selective ion accumulation and the specificity of unit mass isolation. Future improvements to this technique would involve the use of higher resolution in the isolation step and faster scanning during the (35) Traldi, P.; Cummto , 0.;Bortolini.0. Rapid Commun. Mass Spectrom. 1992, 6, 410.
detection step to reduce the cycle time. Increasing the ionization efficiency would reduce the ionization time required and further reduce the total cycle time for a given mass. The current maximum ionization time that is used is 25 ms. An increase in ionization efficiency by a factor of 5 would allow the maximum ionization time to be reduced to 5 ms while maintaining the same number of ions produced, and thus maintaining a constant signalto-noise ratio.
Received for review April 20, 1995. Accepted August 2, 1995.@ AC950390L Abstract published in Advance ACS Abstracts, September 1, 1995
Analytical Chemistty, Vol. 67, No. 20, October 15, 1995
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