Desorption/Ionization Efficiency of Common Amino Acids in Surface-Assisted Laser Desorption/Ionization Mass Spectrometry (SALDI-MS) with Nanostructured Platinum

Surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) using inorganic nanoparticles has been reported as an organic matrix-free approach. However, the correlation of desorption/ionization (DI) efficiency with analyte chemical structures in SALDI-MS is not clear. In this study, we investigated the DI efficiency of 20 common amino acids and several peptides in SALDI-MS with Pt nanoparticles with thin projections on the surface (termed with Pt nanoflowers, Pt Nfs) on silicon substrates. The fluorocarbon-based hydrophobic perfluorodecyltrichlorosilane (FDTS)-Pt Nf substrates enabled the simultaneous analysis of all 20 common amino acids in negative-ion mode, whereas MALDI-MS was able to detect only two amino acids, proline and glutamic acid, from the same mixture in negative-ion mode. The SALDI-MS produced high ion yields for arginine and proline in positive-ion mode as well as for glutamic acid and aspartic acid in negative-ion mode. A linear correlation was found between the ion yield and the gas-phase proton affinity or acidity of amino acids in SALDI-MS, consistent with the MALDI-MS analysis of amino acids, although the linear correlation in the SALDI-MS was poor in comparison with that of MALDI-MS. It was suggested that the ion yields of amino acids (i.e., the DI process) are mainly determined by the same factors regardless of the ionization method employed in both MALDI performed using organic matrix and organic matrix-free SALDI.