Accelerated Articles Anal. Chem. 1994,66, 313-318
Filtered Noise Field Signals for Mass-Selective Accumulation of Externally Formed Ions in a Quadrupole Ion Trap Douglas E. Goeringer,' Keiji G. Asano, and Scott A. McLuckey Analytical Chemistv Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 3783 1-6365 Don Hoekman Teledyne Electronic Technologies, Mountain View, California 94039-7244 Steven W. Stiller Hitachi Instruments, Inc., San Jose, California 95 134 A new wideband resonance excitation technique, termed a filtered noise field (FNF), is demonstrated for selective accumulation of externally formed ions in the rf quadrupole ion trap. Data obtained for detection of trinitrotoluene (TNT) and black powder vapors in ambient air, via atmospheric sarrplrliRg glow discharge ionization (ASGDI),indicate that a notch-filteredFNF signal applied to the endcap electrodes can prevent accumulation of matrix ions without adversely affecting the ion injection effidency of analyte spedes. ConWptntIy, analyte si@ euWMMmnt can be realized via extended ion accumulation tiiWwven though unwanted ions are pesent in overwhelming alM&Mce. The TNT molecular anion signal obtained follawiy2 a 1-s injection period is a factor of -8 greater than that seen without the FNF; Sz-and SJ- cluster addh sfgrllals, obsmffed without the FNF due to matrix-ioninduced space charge effects, also are isolated completely via the FNF. The subsequent use of concatenated FNF waveforms to enhance detection specificityvia MS/MS is also illustrated. Th&FNF-ion trap techniques described here should be generally applicable to many other ionization sources, whether of internal or external configuration.
Once seen primarily as a tool for applications in physics, such as high-resolution microwave and optical spectroscopy, atomic collision studies, and frequency standards, the radio frequency (rf) quadrupole ion trap has increasingly become viewed as an extremely versatile device for analytical mass spectrometry. Interest in the ion trap mass spectrometer for 0003-2700/94/036&0313$04.50/0 0 1994 American Chemical Society
chemical analysis stems in large part from its sensitivity,'-3 capability for multiple stages of tandem-in-time mass spectrometry (MS/MS and MSn),1,475and adaptability for direct sampling The number of sample types which are compatible with the ion trap has also been increasing steadily due to the growing array of ionization techniques coupled with the device. Beginning with electron ionization (EI) and followed by positiveg and negative5 chemical ionization (CI), the list of methods for ion formation demonstrated with the ion trap now includes photoionization,2 desorption ionization (secondary-ion,lo laser,11-13 and matrix-assisted laser1617 ), (1) Johnson, J. V.;Yast, R. A.; Kelley, P. E.;Bradford, D. C. Anal. Chem. 1990, 62, 2162-2172. (2) Goeringer, D. E.; Whitten, W. B.; Ramsey, J. M. Int. J. Mass Spectrom. Ion Processes 1991, 106, 175-189. (3) Kaiser, R. E.; Cooks, R. G.; Syka, J. E. P.; Stafford, G. C. Rapid Commun. Mass Spectrom. 1990, 4, 30-33.
(4) 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. (5) McLuckey, S. A.; Glish, G. L.; Kelley, P. E.Anal. Chem. 1987,59, 1670-
1674.
(6) Hembergcr, P. H.; Spall, W. D.; Leibman, C. P.; Cannon, T. M. Proceedings of the 36th ASMS Conference on Mass Spectrometry and Allied Topics, San Francisco, CA, 1988; p 638. (7) Lister, A. K.; Cooks, R. G. Proceedings of the 36th Conference on Mass Spectrometry and Allied Topics, San Francisco, CA, 1988; pp 645646. ( 8 ) Wise, M. B.; Thompson, C. V.;Buchanan, M. V.;Merriweather, R.;Guerin, M. R. Spectroscopy 1993, 8, 14-22. (9) Brodbelt, J. S.; Louris, J. N.; Cooks, R. G. Anal. Chem. 1987,59,1278-1285. (10) Kaiser, R. E., Jr.; Louris, J. N.; Amy, J. W.; Cooks, R. 0.Rapfd Commun. Mass Spectrom. 1989, 3, 225-229. (1 1) Heller, D. N.; Lys, I.; Cotter, R. J.; Uy, 0. M. Anal. Chem. 1989,61, 1083-
1086.
(12) Glish, G. L.; Goeringcr, D. E.; Asano, K. G.; McLuckey, S.A. Int. J. Mass Spectrom. Ion Processes 1989, 94, 15-24.
Analyticai Chemistry, Vol. 86, No. 3,Febnrety 1, 1994 515
glow discharge ionization (dd8 and rf19 ), and electrospray20 ionization. A characteristic of the ion trap that can be exploited for chemical analysis when combined with any of the above ionization methods is its capability for ion accumulation. In particular, the sample dynamic range can be extended beyond that established by the range in the number of ions that can be stored by controlling the duration of ion formation or introduction. Such an approach enables both the number of ions accumulated at low sample concentrations to be increased and the number trapped at high sample concentrations to be limited. This approach, known as automatic gain contro121,22 (AGC), has in fact been incorporated into commercial ion trap mass spectrometers to dynamically compensate for changes in sample concentration by varying the electron ionization time. AGC is most effective in situations where analyte ions are the most abundant ions in the mass spectrum-a typical example is GC/MS. AGC is much less effective, however, when analyte ions comprise only a small fraction of the total ion signal-the sample dynamic range then is limited by the matrix ion abundance. Yet, even in such cases, further improvements in detection limits can be realized if ion accumulation can be performed in a selective manner. Such a procedure would serve to enrich the ion population in analyte species by discriminating against matrix ions, much the same as separation methods function to isolate neutral sample components prior to ionization. Consequently, maximum ion accumulation times would be limited by the number of analyte ions rather than by the number of matrix ions. Selectiveionization techniques, such as resonance-enhanced multiphoton ionization, obviously can be effective in reducing the relative abundance of matrix-related ions. However, for situations in which a nonselective ionization method is employed, matrix ion discrimination must necessarily be achieved via other processes. Although a number of methods have been developed for mass-selectiveion isolation subsequent to ion storage, such techniques may be less effective when implemented during ion collection. A more effective tactic is to use mass-selective ejection techniques to accomplish massselective a c c ~ m u l a t i o n . Such ~ ~ ejection methods use the characteristic that the frequency of oscillatory macromotion for trapped ions is mass/charge dependent. Ion ejection at a specific mass/charge value can be accomplished via (13) Alexander, M. L.; Hcmbcrger, P. H.; Cisper, M. E.; Nogar, N . S. Anal. Chem. 1993, 65, 1609-1614. (14) Doroshenko, V. M.; Cornish, T. J.; Cotter, R. J. Rapid Commun. Mass Spectrom. 1992, 12, 753-157. (15) Chambers, D. M.; Gocringer, D. E.; McLuckey, S. A.; Glish, G.L. Anal. Chem. 1993, 65, 14-20. (16) Jonschcr, K.;Currie,G.;McCormack,A. L.; Yatcs, J. R., IIIRapidCommun. Mass Spectrom. 1993, 7, 20-26. (11) Schwartz, J. C.; Bier, M. E. Rapid Commun. MassSpectrom. 1993,7,21-32. (18) McLuckey, S. A.; Glish, G.L.; Asano, K. G.AMI. Chim. Acta 1989, 225,
25-35.
(19) McLuckey,S.A.;Glish,G. L.;Duckworth, D. C.;Marcus,R. K. Anal. Chem. 1992,64, 1606-1609. (20) Van Berkel, G.J.; Glish, G. L.; McLuckey, S. A. Anal. Chem. 1990, 62, 12861295. (21) Stafford, G.C.; Taylor, D. M.; Bradshaw, S.C.; Syka, J. E. P. Proceedings of the 35th Conference on Mass Spectrometry and Ion Processes, Denver, CO, 1987; pp 775-776. (22) Yost, R. A.; McClennen, W.; Snyder, A. P. Proceedings of the 35th ASMS Conference on Mass Spectrometry and Ion Processes, Denver, CO, 1981; pp
189-790.
(23) McLuckcy, S. A.; Gocringer, D. E.;Glish, G.L. J . Am. Soc. MassSpectrom. 1991, 2. 11-21.
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absorption of energy from an auxiliary ac signal in resonance with the secular frequency of the targeted ions, applied to the ion trap end cap^.^^ The mass/charge range for ion ejection can be expanded by sweeping the resonance excitation frequency, albeit with a concomitant reduction in duty cycle at each frequency. Other methods for wide-rangeion ejection, which have no such duty cycle problems, involve the use of widebandwidth excitation signals encompassing an extended mass/charge range. Probably the simplest such technique employs random noise as a signal source.2s However, the method is limited in its degree of selectivity, which is determined by the sharpness of band-reject filters used to %otchn the regions of the spectrum corresponding to the mass/ charge values of interest. At the other extreme of complexity (and selectivity) is the stored waveform inverse Fourier transform (SWIFT)26excitation signal. SWIFT, as applied in ion-trapping mass spectrometers, entails initial conversion of the mass/charge-domain excitation profile to the corresponding frequency spectrum. The frequency spectrum is then transformed to its time-domain representation, which serves as a template for the broadband excitation signal. SWIFT-based techniques have been shown to be extremely versatile in ion cyclotron resonance (ICR) mass spectrometry,27*28 and the theoretical and experimental aspects of SWIFT in quadrupole ion trap mass spectrometry have also been examined.29-31 However, SWIFT requires regeneration of a new waveform each time the excitation profile is modified. A more recently developed wideband resonance excitation technique involves filtered noise field (FNF)32technology. In FNF, a basic waveform having evenly spaced (typically 250, 500, or 1000Hz) frequency components spanning the secular frequency spectrum of the entire ion trap mass/charge range is synthesized digitally. Subsequent digital processing permits the waveform to be shaped so as to effect mass-selective ion accumulation (via mass-selective ejection). Moreover, the user can optimize the FNF signal characteristics interactively because the basic waveform has to be constructed only once and may then be edited in real time. FNF waveforms can also be concatenated so that several functions may be performed sequentially. Many other FNF applications besides mass-selective ion accumulation are possible including mass-selective ion isolation (single m / z , multiple discrete m / z values, continuous m/z range, discontinuous m / z ranges, or any combination), selective reagent ion CI, and selected ion monitoring. However, the results presented in this paper were specificallychosen to illustrate the principles and advantages of using FNF for signal enhancement in the quadrupole ion trap for cases in which ions are formed externally. More specifically, the FNFion trap data demonstrate improved detection of trace organic (24) Fulford, J. E.; Hoa, D.-N.; Hugh-, R. J.; March,R. E.; Bonncr, R. F.; Wong, G. J. J . Vac. Sci. Technol. 1980, 17, 829-835. (25) McLuckey, S. A.;Goeringer,D. E.; Glish, G.L. Anal. Chem. 1992,64,14551460. (26) Marshall, A. G.; Wang, T.-C. L.; Ricca, T. L. J . Am. Chem. Soc. 1985,107, 1893-1891. (27) Wang, T.-C. L.; Ricca, T. L.; Marshall, A. G.Anal. Chem. 1986.58.29352938. (28) Chen, L.; Marshall, A. G.Int. J . Mass Spectrom. Ion Processes 1987, 79,
115-125.
(29) Guan, S;Marshall, A. G.Anal. Chem. 1993,65, 1288-1294. (30) Julian, R. K., Jr.; Cox, K.; Cooks, R. G.A M / . Chem. 1993,65, 1827-1833. (31) Yates, N . A.; Johnson, J. V.; Yost, R. A., personal communication. (32) Kelley, P. E. US.Patent 5,134,286, 1992.
molecules in ambient air via mass-selective ion accumulation from an external, atmospheric sampling glow discharge ionization (ASGDI)33 source. The subsequent use of sequential FNF waveforms to enhance detection specificity via MS/MS is also illustrated. The FNF-ion trap techniques and results described herein should be generally applicable to many other ionization sources, whether of internal or external configuration.
EXPERIMENTAL SECTION Experiments were performed on a Finnigan-MAT (San Jose, CA) ion trap mass spectrometer (ITMS) modified for use with an external source. The previously described instrument incorporates an ASGDI source interfaced to an rf quadrupole ion trap via a three-element lens, the central element electronically pulsed to gate ions into the ion trap through an endcap electrode. The ion trap structure was configured with the standard ITMS electrodes and spacers, so that each endcap electrode did not share a common asymptote with the ring electrode. As a result, small, even multipole (octapole, dodecapole,etc.) fields were superimposed on the pure quadrupole field in the Finnigan-MAT (version 4.1) ITMS software running on an IBM-PC/AT was used to develop ion trap scan functions, and basic timing and control signals were produced by the standard ITMS electronics unit. FNF signals were generated by a Teledyne-Hitachi (Mountain View, CA) HST-1000 module consisting of a custom-designed arbitrary waveform generator board installed in a separate microcomputer. Synchronization of the HST- 1000 with ITMS scan functions was accomplished via the ITMS Trigger signal (TP-8 on the scan acquisition processor (SAP) board): the first FNF waveform was initiated by two adjacent trigger pulses, successive waveforms in a sequence were applied after each additional trigger pulse, and the last waveform was terminated by a final trigger pulse. The two HST-1000 outputs, which are configured for dipolar excitation, were connected directly to the endcapelectrodes. Any signal connected to the auxiliary input of the HST-1000 is applied to the endcap electrodes when an FNF signal is not being generated. A 530-kHz signal from the ITMS frequency synthesizer was connected to the auxiliary input and applied during the data acquisition rf ramp to effect axial modulation. FNF data files supplied with the HST-1000 software (version 1.60) included basic waveforms covering the frequency range from 10 to 500 kHz and having frequency components at 250-, 500-, or 1000-Hz intervals. Application waveforms were synthesized from the basic waveforms using the Waveform Editor, which allows the user to selectively insert or remove individual frequency components. In addition, the relative intensity of individual components could be varied over a dynamic range of 500, and the overall signal gain could be adjusted from 0 to 4095. Because the signal output was ~
~~
~~
(33) McLuckey, S. A.;Glish, G. L.; Asano, K. G.;Grant, B.C . AMI. Chem. 1988,
60,2220-2227. (34) Dawson, P. H.; Whetten, N. R.Int. J. Mass Spectrom. Ion Phys. 1969, 2, 45-59. (35) Franzcn, J. Int. J. Mass Spectrom. Ion Processes 1991, 106, 63-78. (36) Wang, Y.; Franzen, J. Int. J. Mass Spectrom. Ion Processes 1992,112,167-
178.
interrupted for only a very short time while a waveform was being edited, updated, and reloaded into the waveform generator, the effects of modifications were effectively observable in real time. Thus, the approach used to produce FNF signals involved the initial calculation of approximate notch-filter frequencies, corresponding to targeted mass/ charge values, via the ITMS software. Using the Waveform Editor, experimental FNF waveforms having the specified frequency components were then created by modifying the basic FNF waveform. Finally, the resultant FWF signal was applied to the ion trap and empiricallyoptimized by observation of ion signals. A very simple apparatus for sampling the headspace of compounds was used in these experiments. A length (-4 in.) of /d-in.-diameter Teflon tubing served as a transfer line between a small sample vial and the inlet aperture (200-pm diameter) of the ASGDI source. After a few milligrams of trinitrotoluene (TNT) and black powder (which served as a source of sulfur vapor) were placed in the container, the transfer line was inserted above the sample. Because normal ASGDI source pumping produced a nominal inlet flow for sample vapors and ambient air of -5 mL s-l, it is likely that the TNT and sulfur concentrations in this case were actually somewhat lower than that determined by their saturated headspace vapors.
RESULTS AND DISCUSSION Although a number of characteristics make the ASGDIion trap combination particularly useful for analysis of organic compounds in ambient air, under 'typical ASGDI conditions ion signals from trace level analytes are significantly lower than thoseassociated with the samplematrix. The high relative abundance of matrix ions can diminish the signal enhancement obtainable for analyte ions from extended ion injection times. In a previous study,23 the detection of TNT vapors in ambient air was presented as an example of such a situation: TNTrelatedanions,observedat m/z227 ([MI), 210 ([M-OH]-), and 197 ([M -NO]-), made up less than 1% of the ASGDIion trap mass spectrum, background ions between m/z 85 and 135 constituted 10% of the total ion signal, and NO2- (m/z
-
46),0~-(m/z48),CO~-(m/z60),HCO~-(m/z61),andNO~(m/z 62) comprised the balance. Although the TNT-related ion signal was observed to increase with injection time, deviation from linearity occurred at -50 ms with a plateau being reached at 200 ms. Large-amplitude, single-frequency resonance ejection of matrix ions near m/z 60 during injection increased the maximum signal level by 1 order of magnitude and extended the linear dynamic range for injection to - 2 s. However, a significant drawbackof high-amplitude, singlefrequency resonance ejection is the single, narrow (tens of m/z units) range of targeted mass/charge values, which is rather poorly defined. Several other techniques for ion manipulation are feasible for use in ion injection application^.^^ Ions below a specified m/z value can be excluded from the trap during injection by adjusting its low m/z cutoff value. However, the optimum injection efficiency for ions greater than m/z 100 typically
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(37) Duckworth,D.C.;BarshicL,C.M.;Smith,D.H.;McLuckey,S.A.,submitted for publication in Anal. Chem.
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corresponds to cutoff values from m/z 20 to 45. Consequently, under such conditions the ASGDI matrix ions noted above would still be admitted to the ion trap. It is also possible to use the combination rf/dc method, also known as apex isolation, for mass-selective ion accumulation. Despite the effectiveness of the rf/dc technique for ion isolation after storage, its practicality during accumulation is limited because the optimum injection efficiency for ions of a particular m/z typically occurs at fundamental rf voltages much lower than those used for apex isolation of ions having the same m/z. The problems noted above, in addition to those associated with large-amplitude, single-frequency resonance ejection, can be circumvented by sweeping the resonance ejection frequency while maintaining a low m/z cutoff optimized for ion injection. However, the low-duty cycle at each mass/charge value may still allow a significant number of unwanted ions to collect. Furthermore, as with combination rf/dc, this method is incapable of isolating ions at several widely spaced m/z values during ion accumulation while simultaneously excluding ions at intermediate m/z values. Use of random noise as the resonance excitation signal source enables all the difficulties described herein to be overcome, but mass/charge selectivity for ion accumulation is limited by the sharpness of filters used to shape the applied signal. While a number of tactics can be used to effect massselective ion accumulation without recourse to shaped resonance ejection waveforms, FNF- or SWIFT-based methods for mass-selective ion accumulation provide enhanced flexibility. The efficiency of FNF for resonance ejection, either during or after ion colleqion, is largely independent of the fundamental rf amplitude. Thus, the low m/z cutoff parameter can be freely adjusted to optimize the ion injection efficiency. In addition, the phase relationship of all frequencies in the FNF waveform has been mathematically optimized to prevent amplifier dynamic range problems associated with wideband signal generation. Consequently, the duty cycle at each frequency included in any tailored FNF waveform is 100%. Moreover, the design flexibility for shaped FNF waveforms permits extreme selectivity for ion accumulation; multiple, widely separated m/z values, a wide m/z range, multiple, discontinuous m/z ranges, or any combination thereof can be targeted. The series of ASGDI-ion trap experiments described here illustrates the utility of FNF for mass-selective ion accumulation of externally generated ions. Figure 1 shows an ion trap mass spectrum generated via negative ion ASGDI of headspace-sampled TNT and black powder vapors in air; the fundamental rf voltage applied during ion injection corresponded to a low m/z cutoff of 23. The spectrum is the average of five scans, the ion injection period for each scan being 20 ms. In addition to matrix and TNT signals, the spectrum also contains low-levelsignals originating from sulfur cluster anions at m/z values 64, 96, and 128 (Le., S2-,S3-,and Sd-). The spectrum in Figure 2 was obtained under conditions identical to those in Figure 1 except the injection time was 1 s. The matrix-related peaks shown in the latter figure exhibit spacecharge-induced signal degradation very similar to that observed in the previously described ASGDI-ion trap experiments. The TNT molecular anion signal is also only -2 times larger than in Figure 1 despite the factor of 50 longer ion injection time, 516
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mlz Flgure 1. Negative ion mass spectrum of TNT and black powder headspace vapors sampled directly from ak into a glow discharge supported by atmospheric gases at reduced pregsure. Ions formed in the source were pulsed into a quadrupok ion trap: the signal (arbitrary units) corresponds to an ion pulse (injection) period of 20 ms. 2500
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mlz Flgure 2. Negative ion mass spectrum of TNT and black powder acquired under the same conditions as Figure 1 except with an ion injection period of 1 s. The intensity is plotted in the same units as Figure 1.
presumably another effect of ionlion interactions. Comparison of the figures also reveals that the sulfur cluster anion signals at m/zvalues 64 and 96 are completely obscured by the matrix ion signals. Although the TNT signal can be enhanced by excluding the most abundant matrix ions (using nearly any one of the several techniques described above), signal recovery for the sulfur peaks is more problematicdue to their proximity with matrix ion peaks. The above case is an example of a situation in which selective ion accumulation can be advantageous. Modifying a basic FNF waveform for selective accumulation involves reducing the relative amplitude of frequency components corresponding to the masslcharge value(s) targeted for storage. A tailored FNF waveform, notch-filtered for mass-selective accumulation of TNT molecular anions at m/z 227 and sulfur cluster anions at m/z 64,96, and 128,was created from a basic FNF waveform having frequency components of equal amplitude spaced at 1-kHz intervals. Approximate notch-filter frequencies were initially determined with the ITMS software and then experimental FNF waveforms having the specified frequency spectra were created from the basic FNF waveform. The resultant FNF signal was subsequently applied to the ion trap, and the waveform was empirically optimized by observation
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Table 1. Flltered N o h Fleld (FNF) Waveform for Selective Accumulation d Externally Formed Ion, In a Quadrupole Ion Trap
fre uencp (%~z)
133 134-137 138 139 87
88 89-90 91 92
relamplb
m/z
100 0 80 100 100 50 0 50 100
64
fre uency (%~z)
65 66 67-68 69 36 37 38-40 41
96
relamplb
m/z
100 50 0 100 100 150 0 100
128
227
0 25
a Region of the FNF 8 corresponding to m/z values (third column) targeted for ae!!accumulation. The values in this column represent ion mcular frequencies resulting from a fundamental rf voltage which gives a low-massstorage cutoff of m/z 23. *All fr uencies from 10 to 600 kHz not listed have a relative amplituege of 100.
m
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mlz Figure 4. Negative ion mass spectrum of TNT and black powder acquired under the same conditions as Figure 2 except using an FNF signal (see Table 1 and Figure 3) for selective ion accumulation.The intenstty is plotted in the same units as Figures 1 and 2.
520
FREQUENCY (kHz)
Flgure 9. Frequency spectrum of the filtered noise field (FNF) signal used for selective ion accumulation of mlz values 64, 96, 128, and 227. The spectrum corresponds to the FNF waveform described in Table 1.
of targeted ion signals. Table 1 lists the relative amplitudes specified for frequencies in the notch-filtered regions of the optimized FNF; the indicated relationship between mass/ charge and frequency is based on a low m/z cutoff of 23. A relative amplitude of 100 was designated for all frequencies not recorded in the table. Figure 3 shows a frequency spectrum of the resultant FNF signal obtained with a spectrum analyzer at a resolution of 300 Hz. Note that the notch-filtered regions of the spectrum are well-defined and attenuated at least 30 dB compared with the unfiltered spectral regions. Figure 4 shows a mass spectrum obtained under conditions identical to those used to acquire Figure 2, except the tailored FNF signal detailed above was continuously applied to the endcaps during and 5 ms after the ion injection period. The effectiveness of the FNF for selective ion accumulation is illustrated by the appearance of distinct sulfur cluster anion signals (MS/MS of the ions at m/z values 96 and 128 confirmed their identities). Furthermore, the TNT molecular anion signal is -8 times more intense than that in Figure 2. These data indicate that the FNF enables selective ion accumulation from an external source to be performed without adversely affecting ion injection efficiency. Thus, signal enhancement can be realized in the ion trap even when matrix ions are present in overwhelming abundance. A characteristic of the ion trap having potentially important ramifications for selective ion accumulation via the FNF technique is the dispersion in ion secular frequency, duo/
-4ooo 50 "
' " " ' ~ " "100 ' " " ' " " ~ ' 1% ' " ' " ~ ~ ~
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d Z Flgure 5. Dispersion in ion secular frequency, dwold(m/z), vs m/zat an iontrap fundamental rf voltage correspondingto a low-messstorage cutoff of m/z 23.
d(m/z). Figure 5 shows the calculated dispersion over the mass/charge range 50-250 for a fundamental rf voltage corresponding to a low m/z cutoff value of 23 (the negative values result from the decrease in secular frequency with increasing m/z). Because the (absolute value of) dispersion becomes lower with increasing masslcharge, this suggests that ion accumulation selectivity should be inversely related to mass/charge for a fixed FNF notch width. Furthermore, because signal intensity normally decreases with increasing resolution (Le., selectivity), this also suggests ion accumulation efficiency under the same conditions will decline with decreasing mass/charge. The FNF flexibility enables such problems to be circumvented since the waveform can easily be designed to maintain a constant notch width/dispersion ratio. It is widely accepted that MS/MS can often enhance the specificity of analysis beyond that provided by a single stage of mass spectrometry. The conventional method for implementing MS/MS in theion trap involves resonance excitation, by application of a single-frequency signal to the end-cap electrodes, of the fundamental secular motion of parent ions. The kinetically excited ions become collisionally activated via multiple, energetic collisions with the helium bath gas and subsequently dissociate. Use of single-frequency resonance excitation for MS/MS can be problematic under conditions of nonconstant trapped ion number density because ion secular AnaMical Chemistry, Vol. 66, No. 3, February 1, 1994
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mlz Figurr 8.. Comparisonof iontrap MS/MSspectra forthe TNT molecular anion (m/z 227) acquired via resonance excitation using either (a) a single-frequency (136 kHz) signal or (b) a multiple-frequency (134, 135, 136, 137, 138 kHz) FNF signal.
frequency is dependent on trapped ion space charge. Several approaches varying in complexity have been used or suggested
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for overcoming this difficulty, the basis for each being use of an excitation signal that encompasses a continuous range of frequencies. FNF waveforms as used here are a type of broadband signal, albeit not continuous in nature. Nevertheless, multifrequency FNF signals can be used for generating MS/MS spectra and can also be effective in eliminating problems associated with space-charge-induced frequency shifts. For each of the MS/MS spectra shown in Figure 6, TNT molecular anions formed in the ASGDI source were first accumulated using identical, notched FNF signals. Then, the molecular anions were caused to fragment via CID by applying another signal for resonance excitation. In the case of Figure 6a, the ITMS frequency synthesizer was the source of the single-frequency,resonance excitation signal (136 kHz). The FNF excitation signal used to generated the MS/MS spectrum in Figure 6b had equal-amplitude frequency components at 134,135,136,137, and 138 kHz. Comparison of panels a and b reveals essentially equivalent structural information. Furthermore, no changes in the relative ion abundances with ion injection time were apparent in additional experiments, indicating that an FNF resonance excitation signal comprised of multiple-frequency components can produce MS/MS spectra that are essentially independent of the number of ions in the trap.
ACKNOWLEDGMENT This research was sponsored by the U S . Department of Energy Office of Safeguards and Security under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. Received for review September 24, 1993. Accepted November 15, 1993." ~
~
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Abstract published in Adaunce ACS Absrrucrs, December 15, 1993.
