Investigation of the interaction of molybdenum with a graphite surface by electrothermal atomization atomic-absorption spectroscopy and scanning tunnelling microscopy

Abstract

The Mo memory effect usually observed during electrothermal atomization is explained in terms of a complex mechanism which involves both interaction with strong adsorption sites on the graphite substrate and a diffusion process. There is no correlation between the number of adsorption sites and the number of retained Mo atoms. The effect of HCl concentration on the Mo absorbance signal shows that Mo solutions containing 4–10% v v HCl give signals 1.4 times the average signals given by 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 observed memory effect. However, results indicate that there is a limiting acid concentration at which most Mo species are removed from the graphite. Results on the role of H + on the molybdenum electrothermal atomization lead to the conclusions: 1. 1. The presence of the acid does not permanently modify the graphite sites on which Mo will be adsorbed. 2. 2. There is no evidence that H + can compete with Mo for adsorption sites. 3. 3. The analyte and the acid (H +) must be combined in solution or on the surface in order to cause signal enhancement. 4. 4. The effect of the acid may favour the formation of aggregates by vacancy creation by protons in the liquid phase as well as at the liquid-solid interface. STM studies reveal that most adsorption in non-acidic solutions is on dislocations or rough areas, in which the analyte-surface interaction is stronger (a large retention effect). In acidic solutions the cluster distribution occurs on flat and rough areas of the graphite surface, in good agreement with the assumption that aggregates are formed in the liquid phase or at the liquid-surface interface.