Internal energy transfer in soft laser desorption/ionization: MALDI and silicon nanostructures

In this dissertation, the systematic exploration of internal energy (IE) transfer in three soft laser desorption/ionization (SLDI) methods: matrix-assisted laser desorption/ionization (MALDI), desorption/ionization on silicon (DIOS) and desorption/ionization from silicon nanowires is discussed. A series of well-characterized benzyl-substituted benzylpyridinium (BP) salts and a benzyl triphenylphosphonium (BTP) salt were used as thermometer ions (TIs). In MALDI, these TIs were studied in three common matrixes, alpha-cyano-4-hydroxycinnamic acid (CHCA), 3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid, SA) and 2,5-dihydroxybenzoic acid (DHB). Both the survival yield (SY) and the calculated IE values indicated that the order of increasing IE transfer was CHCA, SA and DHB corresponding to 'cold', 'intermediate' and 'hot' matrix environments, respectively. Both the calculated IE values and the corresponding effective IE distributions demonstrated only a mild dependence on the laser pulse length (4 ns vs. 22ps). All TIs indicated lower IE transfer with the ps laser. For the BTP ions that have two competing fragmentation channels to produce tropylium ions (F1) and benzyl triphenylphosphine ions (F2), low fluence was moderately in favor of producing tropylium ions with an ion abundance ratio of F1 to F2 between 1 and 6. As the laser fluence was increased, for CHCA there was a dramatic shift in favor of the tropylium production that resulted in the ion abundance ratio of F1 to F2 of about 10 and 30 for the ns and the ps laser, respectively. In DIOS, the nanoporous silicon surfaces were silylated resulting in trimethylsilyl-, amine-, perfluoroalkyl-, and perfluorophenyl modifications. In remarkable contrast with MALDI, no change in the IE transfer was observed during a substantial increase in the laser fluence. Derivatization of the silicon surface did not affect the survival yields significantly but had a discernable effect on the threshold fluence: The effective IE distributions determined for DIOS and MALDI from CHCA revealed a lower effective IE value for the latter. Using the ps laser, the effective IE distribution was always narrower for DIOS, whereas for ns laser excitation the width depended on the surface modification. A comparison of ion yields indicated that the desorption/ionization from silicon nanowires required 5--8 times lower laser fluence for ion production than either MALDI or DIOS. In contrast however, the SY values showed that the IE transferred to the TIs was more (ps laser) or comparable (ns laser) to MALDI but it was significantly less than in DIOS. The IE transfer was only slightly dependent on laser fluence and on wire density. Unlike in MALDI from CHCA and in perfluorophenyl-derivatized DIOS, for desorption from SiNWs the effect of laser pulse length on IE transfer was found to be negligible.