Anion Effects on Ionization Efficiency Using Gold Nanoparticles as Matrices for LDI-MS

The effects of NaX and NH4X (X = F, Cl, Br, and 1) on 337 nm laser desorption ionization (LDI) ion yields of peptides from size-selected citrate-capped gold nanciparticles (AuNPs) are investigated. The presence of NaF, NaCl, NaBr, and NH4X do not significantly affect analyte ion abundances, whereas addition of NaI strongly suppresses analyte ion yields. The LDI ion yields from AuNPs treated with beta-mercaptoethanol, glutathione, or tiopronin are much less sensitive to the presence of NaI, suggesting that covalently bound ligands protect the NP from halide adsorption. In addition, peptide [M - H](-) ion yields are significantly higher from AuNPs treated with oxyanions (NOx- and SO42-); however, thiolate-functionalized AuNPs do not show increased negative ion yields, suggesting that these ligands also protect the NP surface from oxyanion physisorption. These studies clearly illustrate the sensitivity of AuNPs to the presence of passivating impurities.