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(6) Seltz, W. R. Anal. Chem. 1984, 5 6 , 16A-34A. (7) Tiefenthaler, K.; Lukosz, W. Proc. SPIE-lnt. Soc. Opt. Eng. 1084, 574, 215-218. (8) Enokihara, A.; Izutsu, M.; Sueta, T. Appl. Opt. 1088, 2 7 , 109-113. (9) Mltschke, F. Opt. Lett. 1080, 74 (17), 967-969. (10) Russell, A. P.; Fletcher, K. S. And. Chlm. Act8 1085. 770, 209-216. (11) Bailantine, D. S.; Wohitjen. H. Anal. Chem. 1088, 58, 2883-2885. (12) Boltinghouse, F.; Abei, K. An8l. Chem. 1080, 6 7 , 1863-1866. (13) Zhou, Q.; Shahriari, M. R.; Kritz. D.; Sigei, G. H., Jr. Anal. Chem. 1088, 60,2317-2320. (14) Muto, S.; Fukasawa, A.; Kamimura. M.; Shinmura, F.; Ito, H. Jpn. J . Appl. phvs. 1080, 28 (6), L1065-L1066. (15) Zhu, C.; Bright, F. V.; Wyatt, W. A.; Hieftje, G. M. J. Nectrochem. SOC. 1080, 736 (2), 587-570. (16) Posch. H. E.; Wolfbeis, 0. S. Sens. Actuators 1088, 75, 77-83. (17) Behrlnger, Ch.; Lehmann, B.; Haug, J. P.; Seller, K.; Mort, W. E.; Simon, W. Anal. Chlm. Acts 1000, 233, 41-47. (18) Sykes. P. A OuMebOdc to Mechanism ln Organic Chetnlstry. 6th ed.; Longman Group Limited: London, New York, 1986; pp 207-209.
(19) Meyehoff. M. E.; Pretsch, E.; Weki, D. H.; Simon, W. Anal. Chem. 1087, 59, 144-150. (20) Simon, W.; Mcff,W. E.; Seller, K.; Spichiger, U. E. Fresenlus' J . Anal. Chem. 1000, 337, 28-27. (21) Ozawa. S.;Hauser. P. C.; Seller. K.; Tan, S. S. S.; Morf, W. E.; Simon, W. Anal. Chem., in press. (22) Seller, 557-561 K.; Morf, W. E.; Rusterholz, B.; Simon, W. And. Scl. 1080, 5 , - - . - - .. (23) Hyland, R. W.; Hurley, C. W. Natl. Bur. Stand. ( U . S . )1083, Bldg. Sci. Ser. 157. (24) Weast, R. C . , Ed. Handbook of Chemistry and phvslcs, 64th ed.; CRC Press, Inc.: Boca Raton, FL, 1979; pp D128-DI33.
RECEIVED for review October 26,1990. Accepted January 23, 1991. This work was partly supported by the Swiss National Science Foundation, by Ciba-Corning Diagnostics Corp., and by Eppendorf Geratebau, Hamburg.
Pulsed Fast Atom Bombardment Sample Desorption with Multiphoton Ionization in a Supersonic Jet/Reflectron Time-of-Flight Mass Spectrometer Liang Li,* Alan M.Hogg, Alan P. L. Wang, Jian-Yun Zhang, a n d Davinder S . Nagra
Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
Pulsed fast atom bombardment (FAB) has been developed as a means of vaporlzlng thermally labile Mobgkal molecules and polycycllc aromatlc hydrocarbons Into the gas phase. The resuttlng neutral molecules are entrained Into a CO, supersonlc jet that prevents thermal dewqmttlon by colllsknal cooling and carries the molecules Into a reflectron tlmsoffllght mass spectrometer where resonant two-photon lonlzatlon (RPPI) Is performed by a UV laser beam. It Is shown that a varlety of bkkglcal molecdes can be studled, wtth both soft and hard lonlzatlon mass spectra obtainable. For dlpeptides, lt Is found that although the molecular Ion peak can be observed, the Ion peak at 18 mass units lower sometimes Is the domlnant peak In the mass spectrum. Thls Interesting result Is also obtained with laser desorptlon/supersonlc Jet multiphoton lonlzation. However, one advantage of thls FAB method over laser desorptlon Is that the FAB gun can be easily constructed and malntalned at a relatively low cost. I n addltlon, It Is shown that polycyclic aromatic hydrocarbons can be readlly desorbed by FAB and studied by RPPI. The detectlon limit In our present system Is In the low-nanogram reglme. The selectlvtty of thls technlque Is also studied. It Is demonstrated that thls technique can be used for selectlve detectlon of the active substance In a drug tablet. Flnally, lt Is shown that, for organlc compounds, chemical and physical propertles of the sample may play a very Important role In the desorptlon and lonlzatlon processes. I t Is Illustrated that tryptamlne can be selectively desorbed from a mixture of Indole-3-acetlc acid and tryptamine by adding NaOH to the mlxture.
INTRODUCTION Laser-induced multiphoton ionization (MPI) has proven to be a very powerful technique for mass spectrometry ( I ) . With MPI, molecules can be ionized with high efficiency and mass fragmentation patterns can be readily controlled by adjusting laser power density and wavelength. Both soft 0003-2700/91/0363-0974$02.50/0
ionization, where only the molecular ion or the molecular ion with very few fragments is generated, and hard ionization, where extensive fragments along with the molecular ion are produced, can be obtained by MPI. In addition, high selectivity can be achieved by MPI on the basis of the electronic absorption and ionization potential of a molecule. Recently, MPI has been employed as a postdesorption ionization technique for the detection of neutrals sputtered from a surface by a primary beam such as an ion or laser beam (2-6). With a time-of-flight mass spectrometer (TOFMS) for the detection of ions, MPI has demonstrated to be a versatile technique for surface and biological chemical analysis. Among many MPI schemes, resonance-enhanced multiphoton ionization (REMPI) has been most widely used ( I ) . In REMPI, the energy of the ionization laser is tuned to a real intermediate electronic state. Thus, the ionization cross section for a molecule is greatly enhanced, resulting in high detection sensitivity. Moreover, REMPI is a truly optical ionization technique because the molecule can now be selectively ionized by tuning the laser wavelength. REMPI, in particular, resonant two-photon ionization (ROPI), has been uniquely combined with supersonic jet spectroscopy (SJS) (4, 6-9). It has been shown that a two-dimensional detection scheme based on a jet-cooled R2PI wavelength spectrum and MPI mass spectrum can provide a powerful means of molecular identification and structural analysis with high sensitivity and selectivity. To extend this SJS/MPI mass spectrometry technique for the study of thermally labile biological molecules, several methods including supercritical fluid injection ( I O ) and thermospray/heating ( 1 1 ) have been attempted to introduce these molecules into the supersonic jet. More recently, laser desorption (LD) has been successfully used to entrain biological molecules into jet expansions (12, 13). In LD, a pulsed laser (e.g. COz laser) is used to desorb molecules from a substrate such as a ceramic rod placed close to the nozzle orifice. The molecules are then entrained into a pulsed supersonic jet and carried into a time-of-flight mass spectrometer where the ionization takes place. Both the resonant two-photon ionization jet-cooled spectrum and the 0 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63, NO. 10, MAY 15, 199
multiphoton ionization fragmentation mass spectrum can be obtained. The LD/supersonic jet MPI technique has been used for the study of the biological molecules (1, 14) such as catecholamines, indoleamines and their metabolites, drugs, amino acids, small peptides, and nucleosides. In this paper, we wish to report an alternative method for introducing thermally labile and nonvolatile molecules into supersonic jet expansions. The technique used herein is fast atom bombardment (FAB) (15). Instead of using photons from a high-power laser, we have used a FAB gun to generate fast atoms to desorb the sample molecules. We demonstrate here that, like in LD, small biological molecules can be readily desorbed and entrained into a supersonic jet without significant thermal decomposition by using FAB. However, one of the advantages of this FAB method is that the FAB gun can be easily constructed and maintained at a small fraction of the cost of the desorption laser used in the LD method. Although one may directly combine FAB with MPI postionization without the use of a supersonic jet, there are several advantages for the use of a supersonic jet for sample delivery. One of the important advantages is that it provides a means to cool the gas-phase molecules so that a high-resolution jet-cooled wavelength spectrum can be obtained. The rapid cooling of molecules also prevents gas-phase biological molecules from thermal decomposition before they are ionized. Another advantage is that the desorption region in which gas-phase molecules are generated can be spatially separated from the ionization region. This is particularly important for the FAB experiment described here. In the FAB method, glycerol or other liquid matrices are often used for the sample deposition on a substrate for the desorption (15). If the sample along with the matrix is directly placed between the highvoltage repeller plate and extracting grid in a TOFMS, the matrix may cause some problems such as arcing (16,17). A design by Cotter, using a grounded source, pulsed extraction TOFMS, mobviate these problems and has been successfully used for biological molecule detection and for the study of the desorption mechanism (16). However, if the desorption and postionization are performed in the same extraction region, then the ions directly formed from the desorption process may interfere with the ions generated from the neutrals. This is especially true if a high-flux ion or FAB gun is used in order to generate enough sample vapor for obtaining a whole mass spectrum from one pulse rather than using a low-flux gun with a single-ion-count technique. In this work, the experimental setup for the FAB/supersonic jet MPI technique is described and the possible improvement on the performance of the instrument for future experiments is discussed. The capabilities of this method for the detection of thermally labile and nonvolatile molecules are examined. In addition, the analytical power of this technique is demonstrated by studying its sensitivity and selectivity. The ability of this technique for the selective detection of the active substance in a drug tablet is also demonstrated. Finally, the effect of sample chemical properities on the desorption and ionization processes is discussed.
EXPERIMENTAL SECTION The experimental setup for fast atom bombardment/supersonic jet multiphoton ionization (FAB/SJMPI) is shown in Figure 1. The system consists of an angular reflectron time-of-flight mass spectrometer (R. M. Jordan Co., Grass Valley, CA) mounted vertically in a six-port cross pumped by a 6-in. diffusion pump (Varian Associates, Inc., Lexington, MA). A pulsed nozzle (R. M. Jordan Co.) with a 50-ps pulse width is used to form a supersonic jet. C02 is used as the expansion gas throughout this work. The jet expands into the acceleration region of the TOF, and a laser beam perpendicular to both the jet and flight tube ionizes the sample. The 1 m long flight tube is differentially pumped by a 4-in diffusion pump (Varian Associates, Inc.). The
975
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pressure in the flight tube is usually below 2 X lo-' Torr, and the pressure in the ionization region is
