Low Fragmentation Laser Desorption of Thermally Labile Molecules
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Low Fragmentation Laser Desorption of Thermally Labile Molecules Ian M. Atkinson*, Jeffrey A. Shorter"*, Timo Karaiste*, F. Richard Keene*, Jeffrey I. Steinfeld** and Alan E.W. Knight*** *James Cook University of North Queensland, Townsville, Queensland 4811, Australia **Massachusetts Institute of Technology, Cambridge, MA 02139
"***Griffith University, Brisbane, Queensland
4111, Australia
ABSTRACT Thermally labile involatile inorganic complexes and biomolecules have been laser desorbed from silver doped emulsions without significant fragmentation. The silver doped emulsions are saturated with the molecule of interest and then mounted in the source chamber of a skimmed molecular beam apparatus. A pulsed frequency doubled Nd:YAG laser is used to desorb the molecules. The desorbed molecules are entrained in a pulsed supersonic expansion of argon, and are thereby cooled rotationally and vibrationally. A pulsed tunable dye laser is used to ionize the desorbed molecules (either resonantly or non-resonantly) between the acceleration grids of a Wiley-McLaren configuration time-of-flight mass spectrometer. Mass-resolved detection of the ions created from the desorbed species reveals that the desorption process causes almost no fragmentation for all the species studied (various aromatic amino acids and inorganic coordination compounds such as the IRu(2,2'bipyridyl) 31 moiety). While other techniques are successful for introducing relatively involatile materials into mass spectrometers, they often lead to substantial fragmentation of the molecular species of interest. Laser desorption combined with laser ionization can minimize fragmentation during both the vaporization process and the ionization step. Furthermore, a specific merit of our sample handling technique involving silver doped emulsions is that the number density of desorbed species is found to remain sufficiently stable on a shot-to-shot basis so as to permit the recording of wavelength scanned resonance enhanced multiphoton ionization spectra. The combination of laser desorption with resonance enhanced multiphoton ionization spectroscopy has thus provided for the first time a means of recording electronic spectra for a range of vibrationally and rotationally cooled involatile materials. INTRODUCTION A simple technique to volatilize large thermally labile, non-volatile organic and inorganic molecules has been developed. The technique uses a film to deliver a fresh sample to the desorption region and relies on silver particles, acting as chromophores, to convert 532 nm laser pulses into fast temperature pulses. EXPERIMENTAL The apparatus used to perform these experiment was a pulsed supersonic molecular beam instrument equipped with a Wiley-Mclaren configuration time-of-flight mass spectrometer (TOFMS) with some modifications (fig. 1) [1]. These modifications were mainly the addition of the laser desorption equipment; a film based molecule source, the "tape player" delivery system and a desorption laser. The apparatus consists of three differentially pumped cham
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