la4
Anal. Chem. 1989, 6 1 , 184-186
(18) Schneider, H. I n Solute-Solvent Interactlons; Marcel Dekker: New York, 1976. (19) Glover, D. J. J . Am. Chem. SOC.1985, 8 7 , 5275-5279. (20) Glover, D. J. J . Am. Chern. SOC.lSS5,87, 5279-5283. (21) Charlot, G.; Tremlllon, B. I n Les feactbns chimbues dans les solvants et /essets fondus; Gautiers-Villars: Paris, 1963. (22) Kreshkov, A. P.; Smoiova, N. T.; Veveris, A,; Splnce, B. Zh. ,qz. Khim. 1977, 51, 1827.
(23) Bose, K.: Kundu, K. K. Can. J . Chern. 1979, 57, 2470-2475.
RECfor review June 13, 1988. Accepted October 6, 1988. The “Minister0 della Pubblica Istruzione” (M.P.I.) of Italy is acknowledged for the financial support.
CORRESPONDENCE Chemical Ionization of Laser-Desorbed Neutrals in a Fourier Transform Mass Spectrometer Sir: Laser desorption mass spectrometry has demonstrated considerable utility for the analysis of nonvolatile, high molecular weight compounds (1-4),for characterizing molecular adsorbates (5-8), and as a microprobe for molecular and elemental analysis (9-11). Most of the work reported to date uses a pulsed laser to desorb and ionize the sample in a single step. The advantage of this method is that ions formed by the laser pulse can be detected directly by a mass spectrometer. However, since the yield of desorbed neutrals is typically much larger than the yield of desorbed ionic products (12), it seems likely that the sensitivity and selectivity of laser desorption can be improved by ionizing the desorbed neutrals as a separate event. This is referred to as “postionization” to emphasize that ionization occurs as a subsequent step to desorption. Several groups have used resonance enhanced multiphoton ionization (REMPI) to postionize laser-desorbed neutrals (13-15) in the ion source of a time-of-flight mass spectrometer. With this type of instrumentation, Grotemeyer et al. (16) have obtained a mass spectrum of insulin that shows a prominent molecular ion peak and only a few fragment ions. Previous work from this laboratory has demonstrated the utility of electron postionization of laser-desorbed neutrals (LD/EI) in a Fourier transform mass spectrometer for the analysis of molecular species adsorbed onto surfaces (5, 6). Using this method, we have identified a poly(dimethylsiloxane) oil present in the 30 nm thick carbon overlayer of a computer hard disk (17). In this case,electron ionization produced many fragment ions characteristic of the structure of the oil but provided no obviously discernible molecular ion peaks. These results suggested the need for a softer ionization method to identify the laser-desorbed neutrals. The REMPI method can produce soft ionization, but it is highly selective because ionization occurs only when the laser wavelength is tuned to match a strong absorption band of the desorbed species. This makes it difficult to use REMPI for the analysis of unknowns. Thus, analytical applications of laser desorption mass spectrometry would benefit greatly from a more general soft postionization method. Chemical ionization is a well-developed soft ionization method (18), and in 1980 it was used by Cotter to ionize neutrals produced by laser desorption (19). One of the difficulties encountered with these early experiments was coupling a pulsed laser to a magnetic sector mass spectrometer. Laser desorption produces a burst of ions that lasts for less than a millisecond, but a sector mass spectrometer cannot be scanned nearly this fast. Thus, many laser pulses were needed to acquire a complete mass spectrum. These early studies encouraged others to use time-of-flight and Fourier transform
mass spectrometers for pulsed laser experiments. However, there was a report in 1985 that laser desorption of thymidine in a Fourier transform mass spectrometer was not enhanced by the addition of chemical ionization reagent ions to the analyzer cell (20). In this paper we report the f i t successful Fourier transform mass spectrometry (FT-MS) experiments involving chemical ionization of laser-desorbed neutrals. We call this method laser desorption-chemical ionization (LD/CI). A high density of reagent ions is produced and stored in an FT-MS analyzer cell, and chemical ionization reactions occur as a plume of laser-desorbed neutrals passes through the cloud of reagent ions. This method provides an extra measure of control over ionization of the laser-desorbed neutrals because the degree of fragmentation and the relative abundance of the molecular ion can be altered by using different types of reagent ions.
EXPERIMENTAL SECTION The Fourier transform mass spectrometer used in these experiments consists of a vacuum chamber (shown in Figure l), an electromagnet operated at 0.9 T, and a FT-MS data system (IonSpec Model 2000). The vacuum system is pumped by a 150 L/s ion pump and a 330 L/s turbomolecular pump. A cubic trapped ion analyzer cell measuring 5.7 X 5.7 X 5.7 cm is mounted inside the vacuum chamber and centered between the pole caps of the magnet. For these experiments a pulsed valve is used to admit reagent gas to the cell. The valve is an automobile fuel injector (Nissan Model 16603-P8200)which is secured with Torr Seal epoxy (Varian Associates) into a vacuum flange and mounted on the vacuum chamber away from the main field of the magnet. Samples are placed on the end of a direct insertion probe which is inserted into the main chamber through a vacuum lock and brought within 2 cm of the analyzer cell. Laser-desorbed neutrals are ionized either by electrons emitted from a heated rhenium filament or by reagent ions that are stored at the center of the analyzer cell. For both methods, the electron acceleration energy is 70 eV, and the electron current is 10 p A . The voltage on the trapping plates is +3 V and all the other cell plates are biased at 0 V. The control pulses that trigger the laser and the pulsed valve are generated by the data system. The sample of 9-heptadecylacridine is prepared as a thin film by vacuum sublimation (170 “C, loa Torr) onto a highly polished platinum foil. Thymidine is prepared by evaporation of a dilute m) thick gold film which methanol solution onto a 100 nm was vacuum evaporated onto a Macor substrate. Samples are desorbed with a Lambda Physik EMG 103 MSC excimer laser. The 248-nm (KrF*) and 193-nm (ArF*) lines are used for the experiments reported here. The laser beam is focused to a spot having a diameter of about 500 pm. Relatively low laser power densities are used (ca. lo6 W/cm2) in order to reduce the formation of ions directly by the laser through multiphoton ionization or cation attachment processes. Mass spectra are
0003-2700/89/0361-0184$01.50/00 1989 American Chemical Society
330 1 / S PULSED VALVE @ , % t-i ?
FOR
PREAMPLIFIER
GATE V y E
TURBOHOLECULPR PUMP
TO LNZ BAFFLE0 DIFFUSION PUMP
ANALYZER CELL
Flgure 1. Diagram of the FT-MS instrument for studying chemical ionization of laserdesorbed neutrals.
+V
*V
+V
+V Iyz
Figure 3. Comparison of LD/EI (top)and LDlCI (bottom) mass spectra of 9-pentadecylacridine,molecular weight 389 amu.
through the cloud of reagent ions at the center of the analyzer cell. For a general bimolecular proton transfer reaction
AH+ +V
+V
tV
iV
+
B Flgm 2. Sequence of events for chemical ionization of laserdesorbed
neutrals: (a) pulsed valve admhs the methane reagent gas Into the cell while the electron beam is on; (b) after the pulsed valve is closed, the excess reagent gas pumps away and reagent ions are stored in the cell; (c)the laser is fired and desorbed neutrals are ionized by the reagent Ions; (d) protonated sample molecules are trapped in the cell and mass analyzed by FT-MS. obtained with a single laser shot. No signal averaging is necessary. The sequence of events for a LD/CI experiment is illustrated pictorially in Figure 2. Figure 2a shows an FT-MS analyzer cell with methane admitted by a pulsed valve in a 30-ms burst, raising Torr. the pressure in the vacuum chamber from lo4 to Electron ionization of methane under these conditions rapidly leads to the formation of the reagent ions CH6+and C2H6+.One second after the pulsed valve is closed, the pressure in the chamber drops to below 10-8 TOR,and a cloud of reagent ions is left trapped in the analyzer cell, Figure 2b. The laser is then fired at the sample. Figure 2c shows the pressure burst of neutral sample molecules that is caused by the 20-11s laser pulse. As the neutrals pass through the cloud of reagent ions, chemical ionization reactions result in proton transfer to the sample moleculea. In Figure 2d, the sample ions are shown trapped in the analyzer cell by the electric and magnetic fields, while excess reagent ions are ejected from the cell by using high-power radio frequency pulses.
RESULTS AND DISCUSSION Estimated Efficiency. Successful LD/CI experiments require that the reagent ions react with the desorbed neutrals during the short period of time that the desorbed neutrals pass
+ B -,BH+ + A
(1)
the yield of sample ions BH+ is determined by the cross section for the reaction and the number density of the reagent ions. The efficiency of this process can be estimated as follows. If Nois the number of desorbed neutral molecules (B) entering the reagent ion cloud, and N is the number of them that passes through the cloud unprotonated, the number of protonated sample molecules (BH+) is equal to No - N. Assuming that the number of reagent ions is not substantially depleted (i.e., pseudo-first-order conditions), the number of protonated sample molecules produced is
No - N = N o [ l - exp(-acl)]
(2)
where a is the cross section for proton transfer, c is the concentration of reagent ions in the analyzer cell, and I is the distance the sample neutrals travel through the reagent ion cloud. Under our experimental conditions, reasonable values for these parameters are a = 100 A2 (lo-'* m2), c = lo7 ions/cm3 (1013ions/m3), and I = 5 X m. The value for No can be estimated as 10l2,which is the number of sample molecules contained in a monolayer within the area irradiated by the laser (2.5 X lo-' m2). Substituting these values into eq 2 gives (No- N) = 50000 for the number of sample ions produced, which is easily detectable by FT-MS. Thus, the LD/CI method is expected to have a sensitivity in the low picomole range. These calculations suggest that the simultaneous mass detection and ion storage capabilities of a Fourier transform mass spectrometer would be well suited to a LD/CI experiment. Apparently, the previous LD/CI experiments of McCrery and Gross (20) did not work because the density of reagent ions assumed above is roughly 2 orders of magnitude greater than is typically used in an FT-MS instrument (21). LD/EI vs LD/CI. Vacuum sublimation of 9-pentadecylacridine onto the surface of a polished platinum foil produces a thin film that is visible only as a slight cloudiness, with an estimated thickness of less than 1 pm. Figure 3a is a LD/EI mass spectrum (wavelength 248 nm, single laser shot) of this sample that shows numerous fragment ions but no discernible molecular ion peak. With the electron beam turned
186 '0°1
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 2, JANUARY 15, 1989
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50
Figure 4. LD/CI mass spectrum of thymidine, molecular weight 242 amu. The base peak is the protonated thymine fragment.
off, the laser alone was found to contribute approximately 25% to the abundance of the group of ions centered at mlz 180, 192, and 206. For comparison, Figure 3b, a LD/CI mass spectrum taken under the same conditions with methane reagent gas, shows an abundant protonated molecular ion at mlz 390 and a very low abundance of fragment ions. The absolute intensities of the FT-MS signals are almost identical for the LD/EI and LDICI experiments, demonstrating that the ionization efficiencies of the two methods are comparable under these experimental conditions. The LD/CI (wavelength 193 nm, single laser shot) spectrum of thymidine obtained by using methane as the reagent gas is shown in Figure 4. The protonated molecular ion has a relative abundance of ca. 10% of the base peak. Since the methane CI mass spectrum of thymidine (20)gives the protonated molecular ion as the base peak, it is possible that either laser pyrolysis or photodissociation has caused fragmentation of a portion of the thymidine sample in our LD/CI experiment. Most of the reported REMPI postionization studies (8, 11-14) use infrared lasers to desorb the sample, and these have been successfully used to volatilize large, thermally labile peptides (14). At this time it is not clear which type laser is best for laser desorption mass spectrometry. Recently, we have successfully used an excimer laser operated at 193 nm to desorb and ionize peptides (22). A pulsed valve was used in these studies to add reagent gas to the vacuum chamber, but we have also done successful LD/CI experiments with a low, static reagent gas pressure of about 1 x lo-' Torr. It seems that the key to LD/CI is to have a very high density of reagent ions in the analyzer cell to react with the laser desorbed neutrals. After the reaction period, however, the reagent ions must be ejected from the analyzer cell to prevent space charge broadening of the FTMS signals. CONCLUSIONS The Fourier transform mass spectrometer and its precursor, the ion cyclotron resonance spectrometer, have often been used to study reactions between gas-phase ions and the neutrals of volatile compounds (23-26). The experiments reported in
this paper suggest the utility of FT-MS for observing the gas-phase reactions of ions with the laser-desorbed neutrals of low volatility compounds. Protonation is only one class of reactions that can be studied. Future work will explore ionization by charge exchange and reactions of metal ions with laser-desorbed neutrals. LITERATURE CITED Tabet, J.-C.; Cotter, R. J. Anal. Chem. 1984, 56, 1662-1667. Wilkins, C. L.; Weil, D. A,; Yang, C. L. C.; Ijames, C. F. Anal. Chem. 1985, 5 7 , 520-524. McCrery, D. A.; Ledford, E. B., Jr.; Gross, M. L. Anal. Chem. 1982, 5 4 , 1435-1437. Cody, R. B.; Amster, I . J.; McLafferly, F. W. R o c . Mtl. Aced. Sci. U.S.A. 1985, 8 2 , 6367-6370. Sherman, M. G.;Klngsley, J. R.; Dahlgren, D. A,; Hemminger, J. C.; McIver, R. T., Jr. Surf. Scl. 1985, 148, L25-L32. Sherman, M. G.; Kingsley, J. R.; Hemmlnger, J. C.; McIver, R. T., Jr. Anal. Chim. Acta 1985. 178, 78-69. Sherman, M. G.; Land, D. P.; Hemminger, J. C.; McIver, R. T., Jr. Chem. fhys. Lett. 1987, 137, 296-300. Hahn, J. H.; Zenobi, R.; Zare, R. N. J. Am. Chem. SOC. 1987, 109, 2842-2843. -.- -. . . Gardella, J. A.; Hercules, D. M. Fresenius' 2. Anal. Chem. 1981, 303. 297-303. Hercules, D. M.; Novak, F. P.; Viswanadham, S.K.; Wilk, Z. A. Anal. Chim. Acta 1987, 195, 61-71. Arrowsmkh, P. Anal. Chem. 1987, 59, 1437-1444. van Breemen, R. B.; Snow, M.; Cotter, R. Int. J. Mass Spectrom. I o n Processes 1983, 49, 35. Tembreull, R.; Lubman, D. M. Anal. Chem. 1987, 59, 1082-1086. Boesl, U.; Grotemeyer. J.: Walter, K.; Schlag, E. W. Anal. Instrum. 1987. ..., 76. _ ,151-171. Engelke, F.; Hahn. J. H.; Henke, W.; Zare, R. N. Anal. Chem. 1987, 5 9 , 909-912. Grotemeyer, J.; Schlag, E. W. Org. Mass Spectrom. 1987, 2 2 , 758-760. Land, D. P.; Tal, T.-L.; Lindquist, J. M.; Hemminger, J. C.; McIver, R. T., Jr. Anal. Chem. 1987, 59, 2924-2927. Harrison, A. G. Chemical Ionlzatlon Mass Spectrometry; CRC Press: Boca Raton, FL, 1983. Cotter, R. J. Anal. Chem. 1980, 52, 1767-1770. McCrery, D.A.; Gross, M. L. Anal. Chim. Acta 1985, 178, 105-116. Gross, M. L., private communication. Amster, I.J.; Land, D. L.; Hemrninger, J. C.: McIver, R. T., Jr., manuscript in preparation. Baldeschwieler, J. D.: Benz, H.: Llewellyn, P. M. Adv. Mass. Spectrom. 1968, 4, 113-120. Bartmess, J. E.; Scott, J. A., McIver, R. T., Jr. J. Am. Chem. SOC. 1979, 101, 6046-6056. Wight, C. A.; Beauchamp. J. L. J. fhys. Chem. 1980, 84, 503-2506. Freiser, B. S. Talanta 1985, 32, 697-708.
I. J. Amster Donald P. Land J o h n C. Hemminger Robert T. McIver, Jr.* Department of Chemistry University of California Irvine, California 92717 RECEIVED for review July 18,1988. Accepted October 18,1988. We are grateful for grant support from the National Science Foundation (CHE8511999),the National Institutes of Health (GM 34327), and the University of California. D.P.L. acknowledges support in the form of an IBM Research Fellowship.
