Anal. Chem. 1993, 65, 1609-1614
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Laser Desorption in a Quadrupole Ion Trap: Mixture Analysis Using Positive and Negative Ions M. L. Alexander, P. H. Hemberger,’ M. E. Cisper, and N. S. Nogar Chemical and Laser Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
We demonstrate the generation of both negative and positive ions by laser desorption directly within a quadrupole ion trap and the application of this method to analyze complex samples containing compounds such as explosive residues, metal complexes, and polynuclear aromatic hydrocarbons. In some cases the ability to rapidly switch between positive and negative ion modes provides sufficient specificity to distinguish different compounds of a mixture with a single stage of mass spectrometry. In other experiments, we combine intensity variation studies\with tandem mass spectrometry experiments and positive and negative ion detection to further enhance specificity. INTRODUCTION The coupling of laser desorption with mass spectral detection has produced a powerful method for the analysis of complex liquid or solid samples14 and has found diverse areas of application from analysis of superconductors5 to complex biomo1ecules.u This process typically assumes one of two forms: (1)the production of neutral atoms, molecules, and fragments by thermal desorption, followed by postionization methods or (2) direct production of ionized species as a result of the interaction of a high power, 2106 W/cm2,laser pulse with a solid surface, matrix, or sample adsorbed on the surface.9 This interaction may initiate desorption, multiphoton ionization, dissociation, and many other processes. The result is the formation of a neutral plasma containing positive ions, neutral species,electrons, and negative ions.lOJ1 The high heating rates (21010K/s)and short irradiation times (nanoseconds) permit the desorption of thermally labile or high molecular weight molecules with little or no thermal degradation. Molecules with high molecular weights are most commonly desorbed from a matrix of molecules having a large (1) Land, D. P.; Pettiette, Hall, C. L.; Hemminger, J. C.; McIver, R. T., Jr. Acc. Chem. Res. 1991,24,42. (2) Biarnason. A.: DesEnfants, R. E. I.; Barr, M. E.; Dahl, L. F.
Organometallics 1990, 9, 657. (3)Young, C. E.; Pellin, M. J.; Calaway, W. F.; Joergensen, B.; Schweitzer, E. L.; Gruen, D. M. Nucl. Instrum. Methods Phys. Res. 1987, B27, 119. (4) Muller, J. F.; Berthe, C.; Magar, J. M. Fresenius’ 2.Anal. Chem. 1981,308,312. (5) Estler, R. C.; Nogar, N. S. J. Appl. Phys. 1991, 69, 1654. (6) HillenkamD. F.: Karas. M.: Beavis, R C.: Chait, B. T. Anal. Chem. 1991; 63, 1193A. ’ ’ (7) Stahl.. B.:. Steuo. .. M.:. Karas,. M.:. HiUenkamD, F. Anal. Chem. 1991, 63,‘1463. (8) Karas, M.; Bachmann, D.; Bahr, U.; Hillenkamp, F. Int. J. Mass Spectrom. Ion Processes 1987, 78, 53. (9) Sneddon, J.;Mitchell, P. G.; Nogar, N. S. In LaserInducedPlasmas and Applications; Radziemski, L. J., Cremers, D. A., Ed.; Dekker: New York, 1989; Vol. 21, pp 347-76. (10) Ready, J. F. Effects of High-Power Laser Radiation; Academic Press: New York, 1971; 433 pp. (11) Moenke-Blandenburg, L. Laser Microanalysis; John Wiley & Sons: New York, 1989; Vol. 105, 288 pp. 0003-2700/93/0365-1609$04.00/0
absorption at the laser wavelength and the ability of accepting or donating protons or metal ions. This technique, matrixassisted laser desorption ionization: enhances the desorption process and reduces fragmentation of the analyte. Laser desorption techniques have been combined with virtually all types of mass analyzers, including sector,12timeof-flight,13 single and tandem quadrupole,14 and ion storage mass spectrometer^.'^ Most laser desorption work involves the mass spectrometric detection of positive ions, although a few have looked a t negative ions.16J7 The combination of laser desorption/ionization techniques with trapped ion methodologies-Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR)18J9 and quadrupole ion trap mass spectrometry20.21-provides an easy route to combine several spectroscopicand mass spectrometric techniques into a single analytical sequence. Lasers have long been used with ion cyclotron resonance (ICR) mass spectrometers for desorption22923 and for photodissociation.24 The ability to use one laser (XeC1excimer, 308nm) for consecutive or concurrent laser desorption and photodissociation in a single mass analysis scan has also been demonstrated with an FT-ICR.25 The combination of lasers with ion trap mass spectrometers is a more recent development. Early experiments used a quadrupole ion store (QUISTOR) with a quadrupole mass filter for fundamental studies of ion spectroscopy.2628 More recently, a number of laser desorption techniques have been examined with ion trap mass spectrometers.29JO These experiments demonstrated the ability to desorb biological molecules directly within the ion trap with a COZlase90 and the desorption of trimethylphenyl ammonium iodide within the ion trap using a Nd:YAG laser (1064 nm) followed by (12) Strobel, F. H.; Solouki, T.; White, M. A.; Russell, D. H. J. Am. SOC. Mass Spectrom. 1991,2, 91. (13) Cornish, T.; Cotter, R. J. Rapid Commun. Mass Spectrorn. 1992, 6, 242. (14) Perchalski, R. J.; Yost, R. A.; Wilder, B. J. Anal. Chem. 1983,55, 2002. (15)Allison, J.; Stepnowski, R. M. Anal. Chem. 1987,59, 1072A. (16) Buchanan, M. V.; Wise, M. B. Fuel 1987, 66, 954. (17) Oehme, M. Anal. Chem. 1983,552290. Mass Spectrom. 1991,2, (18) Hettich, R.; Buchanan, M. J. Am. SOC. 402. (19) Yang, L. C.; Wilkins, C. L. Org.Mass Spectrom. 1989, 24, 409. (20) Heller, D. N.; Lys, I.; Cotter, R. J.; Uy, 0. M. Anal. Chem. 1989, 61, 1083. (21) Goeringer, D. E.; Whitten, W. B.; Ramsey, J. M. Int. J. Mass Spectrom. Ion Processes 1991, 106, 175. (22) Gord, J. R.; Bemish, R. J.; Freiser, B. S. Int. J. Mass Spectrom. Ion Processes 1990, 102, 115. (23) Weller, R. R.; MacMahon, T. J.; Freiser, B. S. Oxford Ser. Opt. Sci. 1990, I , 249. (24) Dunbar, R. C. Chem. Phys. Lett. 1986, 125, 543. (25) Speir, J. P.; Gorman, G. S.; Amster, I. J. In Laser Ablation; Springer-Verlag: Oak Ridge, TN, 1991; pp 174-9. (26) March, R. E.; Hughes, R. J. Quadrupole Storage MQSSSpectrometry; Wiley Interscience: New York, 1989. (27) Hughes, R. J.; March, R. E.; Young, A. B. Int. J.Mass Spectrom. Ion Processes 1982,42, 255. (28) Hughes, R. J.; March, R. E.; Young, A. B. Can. J.Chem. 1983,61, 834. (29) McIntosh, A.; Donovan, T.; Brodbelt, J. Anal. Chem. 1992, 64, 2079. (30) Louris, J. N.; Amy, J. W.; Ridley, T. Y.; Cooks, R. G. Int. J.Mass Spectrom. Ion Processes 1989,88, 97. 0 1993 American Chemical Soclety
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mass spectrometry/mass spectrometry (MS/MS)on the laserdesorbed i0ns.3~ Laser ablation ion trap mass spectrometry has also been demonstrated for elemental analysis of metal samples.32 Resonance-enhanced multiphoton ionization experiments with an ion trap can reveal various isotopic combinations of NO20 and, in combination with MSIMS experiments, have been shown to enhance specificity in the analysis of mixtures of substituted benzenes.33 Laser photodissociation of organic ions has also been used to track ion trajectories within the ITMS.34 All of the experiments combining laser desorption and ion trap mass spectrometers have been conducted with positive ions. The ability to store, collisionally activate, and mass analyze negative ions created by laser desorption in an ion trap has only recently been reported.35 In this work, we describe how positive and negative ions created by laser desorption in a quadrupole ion trap can be used to rapidly detect compounds of environmental significance. Compounds such as polynuclear aromatic hydrocarbons (PAHs), explosive residues, and transition metal complexes can often be detected directly from complex matrices. In some cases the ability to rapidly switch between positive and negative ion modes provides adequate specificity to distinguish different compounds within a mixture with a single stage of mass spectrometry. In other experiments, we combined tandem mass spectrometry experiments with positive and negative ions to further enhance specificity. We demonstrate a route to determine oxidation-state information for metal complexes. This technique could be very valuable in screening environmental samples for EPA-listed toxic metals where the maximum allowable limits are often based on oxidation state. For example, the allowable limit of Cr(VI) in soil is 1000-fold lower than that for Cr(IV).36837 In addition to the selection of laser-desorbedpositive or negative ions and MS/MS experiments, control of laser power provides a third variable to control selectivity. The versatility provided by the combination of laser sampling with several mass spectroscopic tools using the ion trap mass spectrometer shows promise for the direct analysis of complex mixtures.
EXPERIMENTAL SECTION The combined laser/ion trap apparatus has been previously described34and is only briefly outlined here. A Finnigan ion trap mass spectrometer is mounted within a custom vacuum housing. The ion trap control and detection electronics are standard Finnigan components. A Phrasor Model G61 channel electron multiplier with conversiondynode was used for positive/ negative ion detection. Biasing the conversion dynode to approximately +4 kV allows for the detection of negative ions in a fashion similar to that previously d e ~ c r i b e d .The ~ ~ ion trap was modified for desorption experiments by drilling opposed 2.5-mm holes in the ring electrode as described by Cotter.20 Samples were introduced to the ion trap with a direct insertion probe guided through one of the holes. The surface of the probe tip (2.2-mm diameter) was flush with the inner surface of the ring electrode, and the material on the probe tip was desorbed by the laser beam which passed through the opposite hole. The (31) Glish, G. L.; Goeringer, D.E.; Asano, K. G.; McLuckey, S. A. Int. J. Mass Spectrom. Ion Processes 1989, 94, 15. (32) Gill, C. G.; Daigle, B.; Blades, M. W. Spectrochim. Acta 1991, 46B, 1227. (33) Goeringer, D. E.; Glish, G. L.; McLuckey,S. A. Anal. Chem. 1991, 63,1186. (34) Hemberger, P. H.; Nogar, N. S.; Williams, J. D.; Cooks, R. G.; Syka, J. E. P. Chem. Phys. Lett. 1992,191,405. (35) Hemberger, P. H.; Alexander, M. L.; Cisper, M. E.; Nogar, N. S. Proceedings of the4OthASMSAnnual ConferenceonMssSpectrometry and Allied Topics;ASMS Washington, DC, 1992; p 1001. (36) Bartlett, R. J.; Kimble, J. M. J.Enuiron. Qual. 1976,5, 383. (37) Bartlett, R. J.; Kimble, J. M. J. Enuiron. Qual. 1976, 5, 379. (38) McLuckey, S. A.; Glish, G. L.; Kelley, P. E. Anal. Chem. 1987,59, 1670.
'4 120
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Flguro 1. (a) Spatlal profile of the focused laser beam, as monitored wlth a Retlcon photodiode array on an equhralent beam line. (b) Schematlc of the ion trap reglon showing laser lrradlatlon of the probe tip.
probe tip material most often used was 304L stainless steel, although a Macor tip was used for experiments with the metal complexes. The end of the probe tip was machined having a concave contour with a 5-mm radius of curvature so that a finely divided sample could be tamped into it. Helium, at an uncorrected gauge pressure of 1-2 X Torr, was used as a buffer gas in all experiments. In general, this was found to be the optimum pressure for best intensity and resolution of the fragment ions at m/z 219, 502, and 614 of perfluorotributylamine. Ion gauge pressures were later calibrated against an absolute pressure gauge (MKS Model 390HA-00001 Baratron capacitance manometer) using Nz and He as calibration gases.' Using this calibration, an ion gauge pressure of 1 X le5Torr of He corresponds to an absolute pressure of about 2 X lo-' Torr. The fundamental RF level during desorption was set to a low mass cutoff of 20 Da, corresponding to a calculated RF voltage of about 250 V,,+ Energy for laser desorption was supplied by a XeCl excimer laser (Questek 2200). This laser produces output pulses with a 15-ns duration at 308 nm. The beam was focused by a 60-cm lens, and the size of the laser beam at the probe tip was found to be 1X 2 mm and Gaussian in the z-direction as measured with a Reticon photodiode array on an equivalent beam line (Figure 1). Laser energy, at the probe tip was typically 1 mJ/pulse, corresponding to a power density of 3 x 106W/cm2. Laser timing was controlled with a Stanford Research Model 535digital delay/ pulse generator, which was triggered by a pulse generated at the start of the data acquisition sequence by the ITMS electronics. The period between desorption and acquisition of mass spectra was varied from 0 to 100 ms with the delay generator. The data shown here were taken at delay times ranging between 20 and 40 ms and at random phase with respect to the fundamental RF. We have recently found that both sensitivity and reproducibility are enhanced when the laser is fired at very short delay times with respect to turning on the fundamental RF voltage.39 Other recent work has demonstrated the feasibility of phase-locking a pulsed laser with the fundamental RF40 and with auxiliary AC
Table I. PAHs in SuDelco Standard 4-8905Ma 9 benzo[alanthracene(228) 1naphthalene (128) 10 chrysene (228) 2 acenaphthylene(152) 11benzo[b] fluoranthrene(252) 3 acenaphthene (154) 4 fluorene (166) 12 benzo[k]fluoranthrene (252) 5 phenanthrene (178) 13 benzo[alpyrene (252) 14 dibenzo[a,h]anthracene (278) 6 anthracene (178) 7 fluoranthrene (202) 15 benzo[ghi]perylene(276) 16 indeno[l,2,3-cdlpyrene(276) 8 pyrene (202) Nominal molecular weights follow in parentheses.
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voltage.41 Such phase-locked techniques will improve the reproducibility and ion collection efficiencies of laser desorption and photodissociation experiments in the ion trap mass spectrometer. One laser shot per mass scan was used, and in general, the results here are the average of 10 spectra. The trimethylphenylammonium iodide was obtained from ChemService and used without further purification. The sample was dissolved in methanol to make a 1pg/pL solution. Approximately 1pL of this solution was loaded and dried on the probe tip. The polynuclear aromatic hydrocarbon mixture (Table I) was obtained from Supelco (4-8905M). This mixture, which contains 16 polynuclear aromatic hydrocarbons (PAH), at 2000 pg/mL in a methylene ch1oride:benzene solution (5050), was diluted 10-fold with methanol, and 2 p L of that solution was loaded onto the probe tip and dried. The PAH in soil standard was prepared by adding 10 pL of the stock Supelco standard to 50 mg of the soil. This resulted in a soil mixture that was about 400 ppm by weight for each of the 16 PAHs. A small amount of methanol was added to this mixture to ensure that the PAH standard was homogeneously distributed. The methanol was allowed to evaporate, and a portion of the soil was tamped into the end of the probe tip. The dimethyl methylphosphonate was obtained from Aldrich and used without purification. The soot sample was obtained by detonating 300 g of a mixture of trinitroto1uene:nitroguanidine(5050) in a 1.5 m3tank in uucuo and collectingthe residue from the walls of the tank.42The nickel and chromium acetylacetonates were obtained from Aldrich. These compounds were dissolved in methanol to a final concentration of 1 pg/mL.
RESULTS Trimethylphenylammonium iodide was studied first as a benchmark compound. Laser desorption ion trap mass spectra of this compound obtained on our system were in qualitative agreement with the work of Glish e t al.3l and so are not reproduced here; peaks were observed at m/z 120, 121, and 136 (the intact cation). The agreement between the previous work, which used the fundamental wavelength from a Nd:YAG laser (1064 nm), and the results reported here strongly suggest that this particular desorption process is power, not wavelength, dependent. The dependence of total ion current on buffer gas pressure in our system was also substantially the same: a logarithmic plot of ion current vs He pressure was approximately linear over a range from lo4 to Torr. These data were acquired with a delay of 20 ms between laser shot and the beginning of the mass scan. No significant changes in the mass spectra were observed with this, or longer, delays. At delay times less than 20 ms, some degradation of ion signal and mass resolution were observed, (39) Eiden, G. C.; Cisper, M. E.; Alexander, M. L.; Hemberger, P. H.; Nogar, N. S. J.Am. SOC.Mass Spectrom., in press. (40) Doroshenko, V.; Cornish, T.; Cotter,R. J. Proceedings of the 40th Annual Conference on Mass Spectrometry and Allied Topics; ASMS: Washington, DC, 1992; p 1753. (41) Alexander, M. L.; Hemberger, P. H.; Cisper, M. E.; Nogar, N. S.; Williams, J. D.; Syka, J. E. P.; Cooks, R. G. Presented at the Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, FACSS XIX, Philadelphia, P A (1992). (42) Greiner, N. R.; Hermes, R. Proceedings of the Ninth Symposium (International)onDetonation; Portland, OR, Aug. 28-Sep 1,1992; Office of Naval Research Arlington, VA, 1992; pp 1107-84.
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ANALYTICAL CHEMISTRY, VOL. 05, NO. 11, JUNE 1, 1993
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standard as described above was deposited on the probe tip. The negative ion signal was larger than the positive ion signal for these compounds. The dependence of ion intensity on laser pulse energy was also measured for this mixture of compounds. The results are shown in Figure 3. The plotted ion signal is the sum of all negative ion peaks. The threshold laser energy dependence for the mixture of PAHs in Figure 3 shows marked differences from that displayed by the trimethylphenylammonium iodide. Instead of a single, smooth increase over this power range, there is an apparent series of desorption thresholds (plateaus followed by sharp rises). These discontinuities may result from different cross sections for different components of the mixture or the onset of different desorption processesat higher laser energies. This is supported by the mass spectra observed at various incident laser pulse energies. At low fluences, spectra of molecular, pseudomolecular, and other ions are observed; above 1.5 mJ/ pulse, the mass spectra show a shift in ion distribution to low mass carbon clusters (C246). Although these mass spectra are not typical of PAH mass spectra taken by other meth0ds,16J7they are reproducible under constant conditions of laser energy, sample loading, and buffer gas pressure. The formation of C,- and C,H- has also been observed from the laser desorption of PAHs in a laser microprobe mass analyzer at much higher power densities (108 W/cm2) at 265 nm.44 Typical negative ion mass spectra of PAHs are shown by the data in Figure 4,which shows the laser desorption negative ion mass spectra obtained from the standard mixture loaded onto the probe tip at a level of 400 p g of each PAH. Although the correspondence between the spectra obtained from the neat mixture and the spectrum of a soil sample spiked with the PAH mixture is quite good, the individual peaks within the mass spectra are not the simple molecular or pseudomolecular anions ([M - HI-, M-, [M + HI-)observed in negativeion chemicalionization mass spectra.45 Severalpeaks in these spectra appear to correspond to anions of the type [Mz-1, as may be the case for the peak at m/z 256 for M = naphthalene or to [Mz + H-I for the peaks at m/z 281, 295, and 309. These peaks are labeled with numbers corresponding to the chemicals identified in Table I. Accuratemeasurements of the mass of the peaks in this figure are not reliable because (44)Balasanmugam, K.;Viswanadham, S.;Hercules,D. M. Anal. Chem. 1983,55, 2426. (45) Daishima, S.; Iida, Y.; Shibata, A.; Kanda, F. Org. Mass Spectrom. 1992, 27, 571.
mlz Figure 4. Negative ion spectra for desorptionlionbtion of a mixture of PAHs applied directly to the sample probe tip (upper spectra) and for a sample containing 400 ppm of mixed PAHs in dirt. The numbers assigned to the peaks correspond to the PAHs listed In Table I.
of the method used to collect these spectra, which is essentially a screen capture of the mass spectrum as displayed in real time, and at this time it is uncertain whether these peaks correspond to the odd-electron anion PAH dimers or the hydride-bound dimers. There have been few reports of mononegative radical dimers of organic molecules in the gas phase. Negative ion dimers of substituted nitrobenzenes have been created in a high-pressure chemical ionization source,46 and the negative ion dimer of toluquinone has been generated in a drift-mode ion cyclotron resonance spe~trometer.~7 Oddelectron anion dimers of PAHs are predicted to be stable with a symmetrical sandwich structure48and although the negative ion dimer of anthracene has been studied in a there have been no reports of anionic dimers of PAHs in the gas phase. Regardless of the structure of the ions in Figure 4, PAH dimer ions of either structure are of interest. Work is presently underway to modify the data acquisition routine so that mass measurement can be made with greater precision and to confirm the structure of these novel ions. Figure 5 shows the laser desorption positive ion mass spectra of the neat mixture of PAHs compared to a soil sample spiked a t a level of 400 pg of each PAH per gram of soil. These spectra demonstrate excellent correspondence with good signal to noise. The dominant positive ion in the laser desorption spectra of PAHs is often M+.50951 Both the radical cation and the protonated molecule have been observed in fast atom bombardment mass spectrometry of PAHs from a liquid matrix.52 The results shown in Figure 5 indicate the formation of M H+, which is not unreasonable in light of the moderately high proton affinities of the PAHs (ranging from 193.2 kcal/mol for naphthalene to 213.0 kcal/mol for perylene53) and the relatively high amounts of these PAHs on the probe tip. Although there are some uncertainties in the mass assignments in both the positive and negative ion
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(46) Burinsky, D. J.;Fukuda, E. K.; Campana, J. E.J. Am. Chem.Soc. 1984, 106, 2770. (47) Comita, P. B.; Brauman, J. I. J. Am. Chem. SOC. 1987,109,7591. (48) Howarth, 0.W.; Fraenkel, G. K. J.Am. Chem. SOC.1966,88,4514. (49) Shida, T.; Iwata, S. J . Chem. Phys. 1977, 67, 1779. (50) Hercules, D. M.; Day, R. J.; Balasanmugan, K.; Dang, T. A.; Li, C. P. Anal. Chem. 1982,54, 280A. (51) Novak, F. P.; Balasanmugan, K.; Viswanadham, K.; Parker, C. D.; Wilk, Z. A.; Mattern, D.; Hercules, D. M. Int. J. Mass Spectrom. Ion Phys. 1983,53, 135. (52) Dass, C. J. Am. SOC.Mass Spectrom. 1990, 1, 405. (53) Meot-ner, N. M. J. Phys. Chem. 1980, 84, 2716.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 11, JUNE 1, 1993 I
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mass spectra of the PAHs at this time, the point is that, in either mode, the spectra of the pure compounds agree quite well with the PAH spectra in soils,demonstrating the potential for this methodology in the direct analysis of trace contaminants from complex matrices. The laser desorption spectra can also be successfullyapplied to the detection of more volatile compounds; Figure 6 shows the laser desorption ion trap mass spectrum of a soil sample that has been spiked with dimethyl methylphosphonate (DMMP). This spectrum shares several important ions with the electron impact (EI) spectrum of DMMP64 with the notable exception of the molecular ion. The molecular ion is 34% of the base peak in the E1 spectrum; in the laser desorption experiments, protonated DMMP is the base peak, and the molecular ion is not observed. It is interesting to note that we were not able to observe any ions when the DMMP was loaded directly onto the probe tip. It is likely that the DMMP (bp 181 "C) evaporates from the probe tip before interrogation with the laser. The soil apparently acts as a sorbent medium to retain the DMMP. This phenomenon may point to a more general technique in which volatile compounds, which may otherwise be lost through volatilization, may be trapped on an inert matrix for analysis by laser desorption mass spectrometry. The technique of alternate pulsed positive ionlpulsed negative ion mass spectrometry is well-established in GC/ MS techniques, especially for the analysis of halogenated compoundsa55A similar methodology can be easily implemented with laserlion trap instrumentation. Negative and (54) Wils, E. R. J. Fresenius' J. Anal. Chern. 1990, 338, 22.
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mlz Figure 8. (a) Positive ion spectra for desorptionllonlzatlonof chromium acetylacetonate, Cr(acac)3, at 1.5 mJ/pulse. (b) Negathre ion MS/MS spectrum of acetyl acetonate obtained by desorptlonllonization of chromium acetyl acetonate, Cr(acack, at 1.5 mJlpuise.
positive ion signals can provide complementary data to determine the constituents of a complex mixture. Soot from a detonated mixture of trinitrotoluene and nitroguanidine (NIGU) was examined using both negative and positive ion mass spectra, as shown in Figure 7. The molecular ion of NIGU is observed almost exclusivelyin the negative ion mode whereas urea (a decomposition product from the explosion) is observed in the positive ion mass spectrum at mlz 60 along with several nondiagnosticpeaks at lower mass. These results agree very well with those obtained by thermal desorption direct-insertion probe mass spectrometry of this sample.42 The complementary information provided by positive/ negative ion laser desorption mass spectrometry can also be used to characterize transition metal complexes. The laser desorption positive ion mass spectrum of chromium acetyl acetonate a t 0.5 mJ/pulse shows primarily the molecular ion. Using higher laser energies (1.5 mJ/pulse), the complex is forced to decompose, and elemental information can be obtained in the positive ion mode (Figure 8a). By switching (55) Hunt,D. F.; Stafford, G. C., Jr.; Crow, F. W.; Russell, J. W. Anal. Chern. 1976,48, 2098.
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to negative ion mode a t this laser energy, the intact acetyl acetonate negative ion is observed and can be interrogated with MS/MS techniques to yield structural information, as shown in Figure 8b. Parallel results are obtained with nickel acetyl acetonate that thermally decomposes at 182 "C, which provides a further demonstration of the gentle desorption that can be obtained via laser irradiation.
CONCLUSIONS The combination of laser sampling and ion trap mass spectrometry promises to be a powerful analytical technique for the direct analysis of complex samples. This work demonstratesthat the mass analysis of laser-desorbedpositive and negative ions provides complementary mass spectral information for mixture analysis: negative ion spectra emphasize organic content and molecular information, while
the positive ion spectra yield useful structural and elemental information. Studies of ion signal vs laser pulse energies near the threshold for ion production demonstrate different behaviors for the systems investigated here. Desorption of TMA yields a single,smooth threshold signal,while the same measurement for a mixture of PAHs gives a dramatically different result: a series of thresholds marked by sharp rises and plateaus in the signal vs pulse energy curve. These stops were seen to correspond to qualitative changes in the mass spectrum. This suggeststhe use of laser pulse energy near threshold as another dimension to help unravel the mass spectra of complex mixtures. Detailed studies of threshold behavior and the corresponding mass spectral changes are underway.
RECEIVED for review November 9, 1993.
5, 1992. Accepted March
