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 ablation of a thin silver film substrate under vacuum conditions to desorb and ionize bioorganic molecules, was developed for molecular mass and structural 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 substrate occurs with sufficient laser fluence, producing Agn+ (n = 1-9) cluster cations which can react with the desorbed bioorganic molecules 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 dithizone, benzo[e]pyrene, 1,4,8,11-tetraazocyclotetradecane, dicyclohexyl-18-crown-6, [5]-helicene dendrimer, gramicidin S, substance P and melittin. One advantage of this method over conventional LDI techniques is that the sample does not need to have appreciable spectral absorption at the laser wavelength. The use of silver in thin-film substrates 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 silver particles during their formation and growth in the colloidal solution. 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.