SURFACE STUDIES BY STATIC SECONDARY-ION MASS-SPECTROMETRY - ADSORPTION OF 3-MERCAPTOPROPIONIC ACID AND CYSTEINE ONTO GOLD SURFACES

Static secondary ion mass spectra have been compared for 3-mercaptopropionic acid and cysteine in the crystalline state and adsorbed onto gold surfaces. The fragmentation behavior observed for the solid-state material is found to be different from that observed for the molecular adsorbate systems. In the crystalline state, the fragmentation of 3-mercaptopropionic acid is dominated by scission alpha and beta to the sulfur atom and by OH loss. In the adsorbate system, the sulfur-gold bond is sufficiently strong for the dominant fragmentation steps to involve scission of the S-C bond in the adsorbate and ejection of gold atoms bound to the adsorbate molecules. Similar changes are observed for cysteine when adsorbed onto gold surfaces. The spectra provide clear evidence for a chemically determinate relationship between surface structure and secondary ion structure. Electronic deexcitation of sputtered species after ejection from the surface is thought to be an important mechanism of ionization, with the effect that radical cations are observed in the SIMS s