INVESTIGATION OF THE INTERACTION OF MOLYBDENUM WITH A GRAPHITE SURFACE BY ELECTROTHERMAL ATOMIZATION ATOMIC-ABSORPTION SPECTROSCOPY AND SCANNING-TUNNELING-MICROSCOPY

The Mo memory effect usually observed during electrothermal atomizatio n is explained in terms of a complex mechanism which involves both int eraction with strong adsorption sires on the graphite substrate and a diffusion process. There is no correlation between the number of adsor ption sites and the number of retained Mo atoms. The effect of HCl con centration on the Mo absorbance signal shows that Mo solutions contain ing 4-10% nu/nu HCl give signals 1.4 times the average signals given b y aqueous (non-acidic) solutions. Removal of Mo species occurs due to the presence of acid, and this in turn leads to a decrease in the obse rved memory effect. However, results indicate that there is a limiting acid concentration at which most Mo species are removed from the grap hite. Results on the role of H+ on the molybdenum electrothermal atomi zation lead to the conclusions: 1. The presence of the acid does not p ermanently modify the graphite sites on which Mo will be adsorbed. 2. There is no evidence that H+ can compete with Mo for adsorption sites. 3. The analyte and the acid (H+) must be combined in solution or on t he surface in order to cause signal enhancement. 4. The effect of the acid may favour the formation of aggregates by vacancy creation by pro tons in the liquid phase as well as at the liquid-solid interface. STM studies reveal that most adsorption in non-acidic solutions is on dis locations or rough areas, in which the analyte-surface interaction is stronger (a large retention effect). In acidic solutions the cluster d istribution occurs on fat and rough areas of the graphite surface, in good agreement with the assumption that aggregates are formed in the l iquid phase or at the liquid-surface interface. (C) 1997 Elsevier Scie nce B.V.