COMBINED XPS AND SIMS STUDY OF AMINO-ACID OVERLAYERS

Factors influencing the SIMS fragmentation patterns are studied for three simple amino acids-glycine, α-alanine, and serine-deposited onto Ag substrates from aqueous solution. Secondary ion emissions are measured for 1 keV Ar+ ions incident at 70° from sample normal as a function of substrate preparation and solution concentration. Studies by XPS and X-ray induced AES prior to SIMS analysis show that the amino acids adsorb in a film on the Ag surface and that the film thickness increases with solution concentration. In addition, considerable amounts of amino acid can be deposited on the surface from a water film retained during extraction from a concentrated solution. On acid etched samples, positive ion fragments of mass AgM, Ag(M - 45), Ag, M + 1, and M - 45 are observed, where M is the molecular, weight of the parent amino acid. With the exception of the (M + 1)+ fragment, these peak intensities behaved similarly for the different surface concentrations. When the adsorbed film grows too thick, the positive molecular ion emissions drop considerably; this substantiates the need for proximity between the Ag substrate and the amino acid molecule. © 1979.