Accelerated Articles Anal. Chem. 1995, 67,2739-2742
Parallel Monitoring for Multiple Targeted Compounds by Ion Trap Mass Spectrometry Keiji 0. Asano, Douglas E. Goednger, and Scott A. McLuckey* Chemical and Analytical Sciences Division, Oak Ridge National Laboratoty, Oak Ridge, Tennessee 37831-6365
An approach to monitoring for the presence of several
targeted compounds simultaneouslyusing ion trap mass spectrometry is described. The use of tailored meforms sequentially applied to the ion trap end-cap electrodes allows for mass-selective accumulationof multiple parent ions and simultaneous collisional activation of the ions. Further mass selection and ion activation steps can be performed to enhance specscity. The detection of trace explosives in air is used as an illuslrative case in which 2,4-dinitrotoluene,2,4,6-trinitrotoluene,and (2,4,6-trinitrophenyl)methylnitramineare targeted for detection. The parallel monitoring scheme trades specificity for advantages in speed and duty cycle relative to tandem mass spectrometry experiments performed in series. Targeted compound detection is the analytical chemist’s analogue to finding a needle in a haystack. It is frequently the case, however, that more than one compound is of interest such that the analogy must be extended to include finding several different objects in the hay matrix. Obviously, the ability to search for multiple targeted species in parallel, rather than searching for one targeted component at a time, is desirable in this regard. However, many targeted compound detection schemes do not allow for parallel searches. For example, tandem mass spectrom etry, a technique widely recognized for its merits as a tool for targeted compound generally entails the selection of one parent ion at a time for interrogation. Monitoring for the parent ions from a variety of targeted compounds can be done in parallel (or nearly so in scanning-type instruments), but only one parent ion is generally subjected to tandem mass spectrometry * Phone: 615.5742848. Fax: 6155768559. E-mail: mcluckeysaCornl.gov. (1) McLafferty, F. W., Ed. Tandem Mass Spectrometry; John Wiley and Sons: New York, 1983. (2) Busch, K L.; Glish, G. L.;M c h c k e y , S. A. Mass Spectrometry/Mass Spectrometry: Techniques and Applications of Tandem Mass Spectrometry; VCH Publishers: New York, 1988. (3) Kondrat, R W.: Cooks, R. G. Anal. Chem. 1978,50, 81A. This article not subject to US. Copyright. Published 1995 Am. Chem. Soc.
at a time. The time spent examining each parent ion is not available for interrogating other parent ions should they be present. Furthermore, in trace detection scenarios in which signzcant chemical noise is present in regions of the mass spectrum where parent ions are likely to fall, it may be necessary to perform tandem mass spectrometry directly. That is, parent ion signals may not be sufficiently intense, relative to the chemical noise, to indicate clearly the possible presence of the targeted compound. A well-known strength of tandem mass spectrometry in direct mixture analysis is that it can provide a dramatic reduction in chemical However, most forms of tandem mass spectrometry require that a search for multiple targeted compounds be performed serially. Ion-trapping instruments, most notably the quadrupole ion trap and the ion cyclotron resonance spectrometer, enjoy distinct advantages over most beam-type mass spectrometers in experimental flexibility. For example, the fact that ions comprising a wide range of mass-tocharge values can be stored simultaneously in the ion traps mentioned above and the fact that sophisticated waveforms can be applied to the trapping cells for ion man i p ~ l a t i o n ~allow - ~ for the straightforward implementation of procedures that permit monitoring for multiple targeted compounds in parallel. Scanning beam-type tandem mass spectrometers do not readily lend themselves to such a technique. A parallel monitoring procedure is illustrated here in which various filtered-noise fields (FNFs)~are applied sequentially to the endcap electrodes of a quadrupole ion trap to allow for the parallel monitoring of three parent ions. The trace detection application is that of explosives vapor detection in air in which anions formed (4) Marshall, A. G.; Wang, T.-C. L.; Ricca. T. L./. Am. Chem. SOC.1985,107, 7893-7897. 2935(5) Wang. T.-C. L.; Ricca, T. L.; Marshall, A G. Anal. Chem. 1986,58, 2938. (6) Chen L.; Marshall, A. G. fnt. /. Mass Spectrom. fon Processes 1987,79, 115- 125. (7) Kelley, P. E. U S . Patent 5,134,286, 1992.
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by atmospheric sampling glow discharge ionization* (ASGDI) are injected into and accumulated within a quadrupole ion trap.9 EXPERIMENTAL SECTION Experiments were performed using a Teledyne Electronic Technologies (Mountain View, CA) 3DQ quadrupole ion trap mass spectrometer system which was modified in-house for ion injection from an ASGDI source. This modification entailed removal of the standard filament assembly and replacing it with a three-element ion injection lens system. The ASGDI source was designed to replace the filament support flange which is ordinarily mounted on the front flange of the 3DQ vacuum housing. The central element of the injection lens is comprised of two half-plates, one of which is connected to the electron gate of the 3DQ system and the other to a dc power supply. This lens is used to “gate” ions into the ion trap during the ion accumulation period by holding both half-plates at $200 V. At all other times the halfplates are held at 1.200 and -200 V, respectively, which prevents ions from entering the ion trap entrance end cap. Anion detection was achieved by removing the extraction electrode of the 3DQ and replacing the standard electron multiplier detector with a Galileo Model 4873 conversion dynode electron multiplier detector. The dynode of this detector was moved off-axis to avoid photon background arising from the glow discharge source. The air background pressure within the analyzer is 8 x Torr. In all cases, helium was added to the vacuum system to bring the total background pressure up to roughly 1 mTorr. The voltages applied to the ion trap electrodes were controlled by the 3DQ software version 0.99. The ability to apply sequentially various FNFs is integral to this software. The system allows for frequencies of 10-455 kHz in 1kHz increments. The amplitude of each frequency component is independently variable from 0 to 10 V (p-p). An experiment having a single collisional activation period involved the application of an FNF during ion injection to allow for the accumulation of the parent ions of interest; a second FNF comprised of the fundamental secular frequencies of the parent ions of interest with amplitudes roughly optimized for the conversion of parent ions to product ions and a scan of the ion trap to yield the final product ion spectrum. An experiment involving two stages of collisional activation adds an FNF to select the first-generation product ions of interest after the first collisional activation FNF and adds a second collisional activation FNF comprised of the frequencies of the first-generation products ions selected for further interrogation. For the purpose of illustration, three nitroaromatic compounds, 2,4dinitrotoluene (DNT), 2,4,&trinitrotoluene (TNT), and (2,4,& trinitropheny1)methylnitramine (tetryl), were chosen as targeted compounds in air. To illustrate the effectiveness of the series of FNFs for each targeted analyte, roughly similar numbers of parent ions from each targeted compound were accumulated. In a realworld scenario, of course, it is unlikely that all three compounds would be present at similar levels. Tetryl and TNT vapors were sampled by elevating the temperature of a in. diameter glass tube leading up to the inlet aperture of the ASGDI source. Sufficient quantities of these explosives for analysis were present on the inner surface of this tube from sampling the headspace (8) McLuckey. S. A,; Glish. G. L.: Asano, K. G.; Grant, B. C.Anal. Chem. 1988,
60, 2220-2227. (9) McLuckey. S.A.: Glish. G. L.: Asano, K. G. Anal. Chim. Acta 1989.225. 25-35.
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vapors of these explosives when the tube was held at room temperature. Based on the signal levels observed for TNT compared with those obtained by directly sampling the headspace vapors of TNT (the vapor pressure of TNT is a few ppb by volume at room temperaturelo 1, the concentration of TNT and tetryl vapors giving rise to the signals observed in the data presented here is estimated to be roughly 100 ppt by volume. The vapor pressure of DNT is several orders of magnitude greater than those of the trinitro compounds and therefore generally gives far greater signals. The FNF used for ion accumulation was therefore constructed such that the amplitude of the frequency corresponding to m/z 182 (the mass-to-charge ratio of the molecular anion of DNT) was chosen to eject enough of the DNT anions to yield a number of accumulated parent ions comparable to those of TNT and tetryl. In all cases, an ion accumulation period of 400 ms was used. All other FNF periods were 20 ms in duration and the analytical scan was 15 ms. RESULTS AND DISCUSSION The desirability for discriminating against the accumulation of unwanted ions in ion-trapping instruments has been shown for both ion cyclotron resonance mass spectrometry5 and quadrupole ion trap mass spectrometry.“ Tailored waveforms provide the maximum degree of flexibility in this regard. Marshall et al. pioneered the use of stored waveform inverse Fourier transforms (SWIFT) for this purpose with particular emphasis on ion cyclotron r e s o n a n ~ e . ~Guan - ~ and Marshall have discussed its implementation on a quadrupole ion trapI2 and Julian et al. demonstrated the use of SWIFT with the ion trap.13 Another approach to construction of a tailored waveform yields the socalled filtered-noise field.’ FNFs have been used for multiple ion isolation in the quadrupole ion trap14 using in situ ionization. We recently demonstrated the use of FNFs for the mass-selective accumulation of ions of nonadjacent mass injected into an ion trap from an ASGDI source,15 also using the explosives detection scenario as an illustration. Tailored waveforms can be highly effective in minimizing the deleterious effects of space charge on dynamic range and resolution arising from the accumulation of uninteresting ions. Concatenated tailored waveforms can be used to devise a scheme that allows for the monitoring of several different targeted compounds in parallel, as illustrated in Figure 1. The ion accumulation waveform serves as a mass-selection step whereby the ions of interest are allowed to accumulate with greatly reduced influence from the other ions issuing from the ion source. That is, parent ion selection occurs during ion accumulation as opposed to afterwards. Figure l a shows the mass spectrum resulting from the use of an FNF to selectively accumulate ions of mlz 182 (the molecular anion of DNT), 227 (the molecular anion of TNT), and 241 (the (M - NO$ anion from tetryl, by far the most intense anion formed by ASGDI of this compound). As indicated above, measures were taken to yield comparable signals for these ions. Dionne, B. C.; Roundbehler, D. P.; Achter, E. IL;Hobbs, J. R; Fine. D. H. J. E n e q . Mater. 1986,4, 447. McLuckey. S. A; Goeringer, D. E.; Glish, G. L. J. Am. SOC.Mass Spectrom. 1991,2, 11. Guan, S.; Marshall, A G. Anal. Chem. 1993,65, 1288-1294. 1827-1833. Julian, R. K, Jr.; Cox, IL: Cooks, R G. Anal. Chem. 1993,65, Kenny, D.V.: Callahan. P. J.; Gordon, S. M.; Stiller, S. W. Rapid Commun. Mass Spectrom. 1993,7, 1086-1089. Goeringer. D. E.: Asano, K. G.: McLuckey. S. A,: Hoekman, D.: Stiller, S. R. Anal. Chem. 1994,66, 313.
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Figure 1. (a) Mass spectrum resulting from the use of an FNF to selectively accumulate the molecular anion of DNT (mlz 182), the molecular anion of TNT (mlz 227), and the (M - NOn) anion from tetryl (mlz 241). (b) Spectrum that results when a second FNF is applied between the ion accumulation period and the analytical scan consisting of three frequencies correspondingto the fundamental z-dimension secular frequencies of ions at m/z 182, 227, and 241. (c) Spectrum that results after an FNF is applied to isolate ions at mlzvalues of 213, 210, and 165 subsequent to the collisional activation FNF that lead to (b). (d) Spectrum resulting from the application of a collisional activation FNF comprised of the z-dimension secular frequencies of the ions at mlz values of 213, 210, and 165.
Figure l b shows the spectrum that results when a second FNF was applied between the ion accumulation period and the analytical scan. This FNF consisted of three frequencies corresponding to the fundamental z-dimension secular frequencies of ions at m/z 182 (q2 = 0.38), 227 (q2 = 0.30), and 241 (qz = 0.28). The amplitudes of these frequencies were chosen to yield conversion efficiencies in the tens of percent. Unfragmented parent ions are indicated in the figure and product ions from each parent are indicated by showing the parent identity in parentheses. For example, product ions from the molecular anion of TNT are indicated by VNl7 adjacent to the appropriate peak. These assignments were made based on MS/MS experiments, the results of which have not been reproduced here, in which a single parent ion was accumulated and interrogated under otherwise identical conditions. It is important to note that the spectrum of Figure l b does not constitute the result of three MS/MS experiments run in parallel. Stringent definition of product ion genealogy ordinarily obtained in MS/MS is relaxed in the parallel monitoring experiment leading to Figure lb. For example, the product ion at m/z 210 known to arise from the molecular anion of TNT16 could conceivably arise from the m / z 241 parent ion. duty afforded by the monitoring experiment On the one hand and specificity on the other. Both the gains in speed (16) McLuckey, S. A,; Glish, G. L. Org. Mass Spectrom. 1987,22, 224.
and duty cycle over sequential tandem mass spectrometry experiments and the loss in specificity increase with the number of parent ions being interrogated simultaneously. For example, in the case illustrated here, a factor of 3 gain in speed and duty cycle is realized by the parallel monitoring experiment over monitoring for each individual compound sequentially. There may be scenarios involving a large number of targeted compounds, however, in which the best compromise is to run several parallel monitoring experiments in series. The loss in specificity associated with the parallel monitoring experiment involving a single stage of collisional activation can be regained, in part, by use of one or more additional stages of ion isolation and activation. In the single targeted compound scenario, this simply reduces to an MSn (n > 2) experiment, for which ion-trapping instruments have been shown to be well ~ u i t e d . ~The ~ - ~ability ~ to concatenate FNFs makes parallel monitoring using several ion isolation and collisional activation steps straightforward to implement. Figure ICshows the spectrum that results after an FNF is applied to isolate ions at m/z values of 213,210, and 165 subsequent to the collisional activation G. J.; McLuckey. S. A Int. J. Mass Spectrom. Ion Processes 1990,96,117. (18) McLuckey. S. A; Glish, G. L.; Van Berkel, G. J. Int. J. Mass Spectrom. Ion Processes 1991.106. 213. (19) Johnson, J. V.; Yost, R. A.; Kelley, P. E.: Bradford, D.C. Anal. Chem. 1990, 62, 2162-2172.
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FNF that lead to Figure lb. The ion at mlz 213 is the major product ion formed from collisional activation of the (M- NOe)anion of tetryl and likely corresponds to loss of CHzN. The ions at mlz 210 and m/z 165 correspond to loss of O H from the molecular anions of TNT and DNT,16 respectively. Figure Id shows the spectrum resulting from the application of a collisional activation FNF comprised of the z-dimension secular frequencies of the ions at m/z values of 213, 210, and 165. The secondgeneration product ions are identified parenthetically according to the relevant parent molecule. These assignments are based upon the MS3 spectra acquired with each individual parent ion (data not shown). Specificity is enhanced in the latter experiment relative to the experiment leading to Figure l b without much loss in signal. Specificity is enhanced because the first-generation product ions are required to pass an additional test. For example, a parent ion at mlz 241 that happens to give a product ion at mlz 210 would lead to the conclusion that TNT might be present in the experiment leading to Figure lb. However, the product ion at mlz 210 must then also fragment to give the second-generation product ions expected from TNT, such as the ion at mlz 152, in the experiment leading to Figure Id. Ultimately, the degree to which multiple stages of ion isolation and collisional activation in a parallel monitoring scheme provide enhancements in specificity without unacceptable loss in sensitivity is dependent upon the facility with which the ions fragment and the specificity of the product ions. CONCLUSIONS The capability of ion-trapping instruments to accumulate and store a relatively wide range of ion masses coupled with sequences
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of tailored waveforms for ion isolation and collisional activation allows for direct parallel screening of complex mixtures for multiple targeted compounds. Advantages in speed and duty cycle over sequential tandem mass spectrometry experiments result from the parallel nature of the approach. These advantages come at a cost in specificity,however, since parallel monitoring degrades product ion genealogical information. Both the benefits of parallel monitoring and the potential for false positives increase with the number of targeted compounds. The often high efficiency of collision-induced dissociation in the quadrupole ion trap and the capacity for MSn, however, allow for the possibility of parallel monitoring involving multiple collisional activation stages, thereby enhancing specificity over a scheme involving a single collisional activation step. The optimal conditions of sensitivity, specificity, and speed for a given multiple targeted compound detection scenario may be obtained with a series of parallel monitoring experiments. Ion-trapping mass spectrometers allow for straightforward implementation of such experiments. ACKNOWLEDGMENT This research was sponsored by the US. Department of Energy Office of Safeguards and Security under Contract DEAC05840R21400 with Lockheed Martin Energy Systems, Inc. Received for review May 30, 1995. Accepted July 10, 1995.8
AC950519R Abstract published in Advance ACS Abstracts, August 1, 1995.
