PHOTOCHEMICAL VERSUS THERMAL MECHANISMS IN MATRIX-ASSISTED LASER DESORPTION IONIZATION PROBED BY BACK SIDE DESORPTION/

The contribution of photochemical and thermal effects in the formation and ejection of biomolecular ions in matrix-assisted laser desorption /ionization; (MALDI) is studied by comparing desorption from one-and-t he-same sample in two incident laser geometries-irradiating the sample front, or irradiating the back side through a 2000 Angstrom gold laye r. Such a set-up results in the efficient decoupling of photoexcitatio n from thermal processes. Our results indicate that laser-induced phot ochemistry, e.g. charge transfer reactions between electronically-exci ted matrix molecules and the analyte, plays a major role in the MALDI process.