Roles of Water, Acidity, and Surface Morphology in Surface-Assisted Laser Desorption/Ionization of Amino Acids

The roles of adsorbed water, acidity, terminal OH groups, and surface morphology in surface-assisted laser desorption/ionization (SALDI) of amino acids from porous graphite and silicon substrates are examined. The SALDI yields and relative intensity ratios of protonated arginine, tryptophan, histidine, methionine, glutamine, and glycine are found to be very similar using porous graphite and porous silicon, despite the large differences in substrate electronic structure and surface chemistry. SALDI does not occur using initially pristine boron-doped Si(100) substrates. However, adsorption of water at 130 K to Si(100) containing adsorbed amino acids produces a SALDI signal similar to that observed from porous graphite and porous silicon surfaces containing aqueous amino acid solutions adsorbed at 300 K. The SALDI yields from all substrates are greatly reduced after removal of physisorbed and chemisorbed water and are completely quenched from Si(100) and porous Si once the surface terminal hydroxyls are removed via recombinative desorption. The ion yields of all amino acids increased greatly with reduction of the solution pH, indicating important roles of surface/interface layer acidity and proton affinity of the desorbing amino acid. Multiphoton-induced ionization of interfacial water and terminal-OH-derived surface states may be important in SALDI. Surface morphologies that lead to the adsorption of water matrices with dispersed analytes are most effective because they enhance and maximize protonated complex formation and escape.