Mechanism of cobalt atomization from different atomizer surfaces in graphite-furnace atomic-absorption spectrometry

Abstract

The mechanism of cobalt atomization from different atomizer surfaces in graphite-furnace atomic-absorption spectrometry has been investigated. The atomizer surfaces were pyrolytically coated graphite, uncoated electrographite, and glassy carbon. The activation energy of the rate-determining step in the atomization of cobalt (taken as the nitrate in aqueous solution) in a commercial graphite furnace has been determined from a plot of log k s vs. 1/ T (for T values greater than the appearance temperature), where k s is a first-order rate constant for atom release, and T is the absolute temperature. The activation energy E a, can be correlated either with the dissociation energy of CoO (g) or with the heat of sublimation of Co (s), formed by carbon reduction of CoO (s), the latter being the product of the thermal decomposition of Co(NO 3) 2. The mechanism for Co atomization seems to be the same for the pyrolytically coated graphite and the uncoated electrographite surfaces, but different for the glassy carbon surface. The suggested mechanisms are consistent with the chemical reactivity of the three atomizer surfaces, and the physical and thermodynamic properties of cobalt and its chemical compounds in the temperature range involved in the charring and atomization cycle of the graphite furnace.