Anal. Chem. 1986, 58, 1299-1303
1299
Pulsed Laser Desorption with Resonant Two-Photon Ionization Detection in Supersonic Beam Mass Spectrometry Roger Tembreull a n d David M. Lubman*
Department of Chemistry, The University of Michigan, A n n Arbor, Michigan 48109
Laser desorptlon Is used as a means of volatlllzlng hlghmeltingpoint or thermally labile compounds Into a supersonlc Jet expanslon. The neutral desorbed species are carrled Into the acceleratlon reglon of a tlme-of-flight mass spectrometer (TOFMS) where they are lonlzed by resonant two-photon Ionization wlth an ultravlolet laser. I n thls soft lonlratlon process only the molecular Ion Is detected In the TOFMS for several large polynuclear aromatlc hydrocarbons and nonvolatlles such as adenine and cytoslne. The desorptlon process In thls work uses a shock mechanism In thick samples In whlch approximately a monolayer Is desorbed per laser pulse so that extended repetltlve desorptlon can be obtalned for each sample. The effect of dlfferent wavelengths for produclng neutral molecules wlthout extensive Ionization-fragmentatlon processes occurrlng Is also Investigated. I n addltlon, the translational cooling of desorbed neutrals In the supersonlc expanslon Is lnvestlgated based upon the Ion packet width detected In the TOFMS.
Laser multiphoton ionization has been shown to be an ionization tool with unique properties for mass spectrometry (1-12). In particular, the process used in our experiments is resonant two-photon ionization (R2PI) where one photon excites a molecule to a resonant intermediate electronic state and a second photon ionizes the molecule. This method is often capable of producing efficient soft ionization where only the molecular ion is produced with efficiencies that may range from several percent or higher within the laser beam during the pulse time (1-5). In addition, wavelength selectivity may be obtained based upon the electronic absorption spectra of the intermediate state of molecules in this two-photon ionization process or based upon the fact that the sum of the two photons must be greater than the ionization potential in order to efficiently produce ions. These advantages have been demonstrated in conjunction with supersonic beam injection, which provides internally ultracold molecules with sharp spectral features for identification ( I , 3, 4 ) . However, a t present these methods have been confined mainly to analysis of volatile species or molecules with reasonable vapor pressures a t less than 250 "C. Thus, a means of volatilizing molecules that either need strong heating (>300"C) and may be susceptible to pyrolysis or are thermally labile need be developed for injection into supersonic beams for mass spectrometry. The first group of molecules includes large polynuclear aromatic hydrocarbons (PNAH's), which are environmentally important molecules to detect since they are potential carcinogens, and which are also important for analysis since they compose a large fraction of complex coal and oil samples. The second group of molecules includes thermally labile biomedical molecules, which are important in clinical and pharmaceutical analysis. A means of volatilizing these species into a mass spectrometer followed by efficient and selective ionization could revolutionize analysis in these fields. There have been several recent attempts to dissolve nonvolatile and biological molecules into supersonic expansions.
Levy and co-workers have used thermospray injection to volatilize tryptophan and then entrain it in a jet of He followed by laser multiphoton ionization (13). The parent ion of tryptophan could be detected; however, significant numbers of tryptophan-solvent complexes were also formed and detected in the mass spectrometer. More recently supercritical fluid injection of C 0 2 into supersonic beams was used to volatilize several PNAH's and tryptophan followed by laser ionization a t 266 nm (14). In all cases the parent ion was detected with no fragmentation, and only minor clustering was observed. However, supercritical fluids are effective for dissolving PNAH's with melting points < 300 "C, although for larger compounds such as coronene the solubility in supercritical COBis not great (15). Thermospray and supercritical fluid injection, though, have the distinct disadvantage in that both inject large amounts of solvent into the mass spectrometer system, which requires fairly substantial pumping capability. An alternative method is to use laser desorption to volatilize molecules into the gas phase followed by jet expansion into a mass spectrometer for analysis. Laser desorption using infrared C02lasers has been used to volatilize and simultaneously ionize molecules on surfaces based upon a rapid heating effect in which the molecules desorb before they have time to kinetically decompose (11-25). Both neutrals and ions desorb in a ratio that depends on the surface temperature; however, at power densities < lo6 W/cm2 the neutrals form the dominant species while the ions may form
