MECHANISMS OF VAPORIZATION FOR SILVER AND GOLD USING ELECTROTHERMAL ATOMIZATION

Electrothermal atomization (ETA) of Ag and Au within graphite tubes was studied at atmospheric pressure. Vaporization for Ag and Au appears to occur from varying sizes of microdroplets or from adsorbed atoms, depending on the conditions of the analysis. Using as criteria the shifting of the peaks to later times with increasing concentrations, a fractional order of release for both metals was suggested under normal ETA analytical conditions. However, a first order process was obtained when formation of adsorbed atoms was expected. The activation energies for desorption (E(a)) of Ag and Au are mass dependent. The decreasing influence of the graphite as the droplet size becomes larger might explain the increase in E(a) observed at increasing concentrations. The highest E(a) values obtained for both metals approach the respective DELTAH(v) and indicate desorption from the surface of large microdroplets. The lowest E(a) might represent the interaction between individual adsorbed atoms and the graphite surface. The dry and thermal pretreatment stages affect E(a) and the shape of the absorption profiles because they probably produce changes in the surface topology of these metals before atomization takes place. Chemisorbed O2 seems to reduce the size of the microdroplets in the case of Ag, but the effect on Au has not been completely rationalized. The tendency of the Ag and Au microdroplets to disperse on graphite seems to be reduced by mechanical roughening of the furnace surface, which probably increases the number of active sites on graphite.