Analysis of the free-jet specrra 01 hydroquinone (para). catecho (onho) and resorcinol (mela) shows the presence or muhiple ongms in each molecular spalnim which are interpreted as belonging IO separate snuctural isomers.
Pulsed laser desorption Is used to volatllke small Momdecules for supersonlc Jet Injection Into a mass spectrometer. Following desorption of the molecular plume Into the Jet, the molecules are Ionized by resonant two-photon lonlzatlon (RPPI) In a tbne-of-flight mass spectrometer. Several ultraviolet wavelengths have been used to study the lonlzatlon properties of dlfferent groups of blomolecules lncludlng the aMllty to obtaln Son lonlzation and laser wavelength selective lonlzatlon. I n addition, fragmentation has been studied as a functlon of laser wavelength and power denstty. Laser RPPI Is shown to be a versatlle lonlzatlon source In whlch fragmentation can be convenlently controlled by adjustlng these two parameters. Those specles lnvestlgated Include several classes of clinkally Important molecules, In particular, catecholamines and thelr derlvatlves, Indoleamlnes, neuroleptic drugs, water-soluble vitamlns, and purlne bases.Resonant two-photon ionization (RPPI) has been shown to be an ionization method with unique properties for mass spectrometry (1-10). The RPPI process involves the absorption of two photons by a molecule where the first photon excites the species to an intermediate electronic state and absorption of a second photon results in ionization. The sum of the two photons must therefore exceed the ionization potential of the molecule. However, for most aromatics studied to date the excess energy deposited in the molecule in the ionization process remains low when near-ultraviolet photons are used. The result is that R2PI can produce relatively soft ionization where either only the molecular ion or minimal fragmentation is observed for identification. Although electron beam ionization can also produce soft ionization, it can do so only with a decrease in ionization efficiency of several orders of magnitude. The efficiency of R2PI is typically on the order of 0.1-1% with resulting production of molecular ions, although in some cases it is estimated to be as high as 25-100%(2, 4). In addition, with increase of the laser beam power density, extensive fragmentation may be induced for structural analysis. The fragmentation can be varied simply by changing the laser intensity, and a t high input energy, fragments as small as C+ have been observed (2, 7). Control over the fragmentation is therefore more versatile than that provided by other ionization methods.The feature that makes R2PI a unique ionization source for mass spectrometry is the potential for wavelength selectivity. Although ions are detected as the final product, the ionization signal reflects the absorption cross section of the resonant intermediate state. Thus, selectivity can be based on either the relative absorptions or ionization potentials of molecules using a single wavelength. In previous experiments increased selectivity has been achieved by tuning the wavelength to produce an optical-ionization spectrum characteristic of a species (49). The key point here is that RPPI can provide preselection of ions prior to mass analysis. At pre...
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