TIME-OF-FLIGHT MASS-SPECTROMETRY OF BIOORGANIC MOLECULES BY LASER-ABLATION OF SILVER THIN-FILM SUBSTRATES AND PARTICLES

A laser desorption/ionization (LDI) technique, which uses laser ablati on of a thin silver film substrate under vacuum conditions to desorb a nd ionize bioorganic molecules, was developed for molecular mass and s tructural reactivity analysis in time-of-flight mass spectrometry (TOF -MS). After a sample overlayer is deposited by solvent evaporation on a thin silver film substrate, it is subjected to 355 or 532 nm Nd:YAG laser light by back-irradiation, Photoablation of the silver film subs trate occurs with sufficient laser fluence, producing Ag-n(+) (n = 1-9 ) cluster cations which can react with the desorbed bioorganic molecul es in the gas phase to form M+ or [M + H](+) and [M + Ag](+) ions for TOF-MS analysis. This LDI technique has been successfully applied to d ithizone, benzo[e]pyrene, 1,4,8,11-tetraazocyclotetradecane, dicyclohe xyl-18-crown-6, [5]-helicene dendrimer, gramicidin S, substance P and melittin, One advantage of this method over conventional LDI technique s is that the sample does not need to have appreciable spectral absorp tion at the laser wavelength, The use of silver in thin-film substrate s affords analyte-dependent efficiencies that may serve for the direct and accurate mass analysis of specific groups of bioorganic molecules in sample mixtures. In a new sample preparation method, gramicidin S is added to a Tollen's reagent mixture for direct impregnation on to s ilver particles during their formation and growth in the colloidal sol ution. These silver particles provide a silver matrix for the analyte molecules, which can enhance the LDI efficiency to produce greater [M + H](+) and [M + Ag](+) signals. (C) 1998 John Wiley & Sons, Ltd.