MECHANISMS OF VAPORIZATION FOR CR USING ELECTROTHERMAL ATOMIZATION

An electrothermal atomic absorption mass spectroscopy (ETAA-MS) system was used to monitor the evolution of species from a graphite platform with respect to temperature. Atomic absorption under vacuum conditions was used to monitor the free metal atoms and allow comparisons to be made with the mass spectroscopy (MS) data. Regular ETAAS was used to obtain absorption profiles under atmospheric conditions. These studies provide evidence for two dominant mechanisms contributing to the analytical signal: (I) the decomposition of chromium carbide produced by carbon reduction of the chromium oxide; and (II) the thermal desorption of adsorbed chromium. Mechanism (I) occurs primarily at or near the site of sample deposition while (II) occurs at other locations in the graphite furnace as well. Gaseous chromium oxides were identified at fairly low temperatures using ETAA-MS. Under 1 atm, these early released species probably redeposit at active sites at several locations in the tube and contribute to the formation of adsorbed Cr on the graphite surface. Gaseous chromium carbides were monitored but not detected by MS in the present study. Thermal pretreatments at temperatures above 1600 K resulted in a shift of the absorption maxima to later times explained as a gradual reduction in the contribution of mechanism (I).