Mechanisms of lead vaporization from an oxygenated graphite surface using mass spectrometry and atomic absorption

The shift in the appearance of gaseous Pb to higher temperatures with oxygen in the gas phase or on an oxygenated graphite surface in graphite furnace atomic absorption spectroscopy (GFAAS) was investigated by use of mass spectrometry and atomic absorption. The observed pulse shift of Pb using an oxygenated graphite surface in 1 atm of inert gas indicates that the shift observed in an oxygen-containing sheath gas in not entirely due to the oxygen in the gas phase. The absence of a shift with vaporization from an oxygenated surface under vacuum indicates it is not strictly a surface process. Mass spectra data show more CO and CO/sub 2/ are evolved from an oxygenated surface than from an unoxygenated surface. Separate GFAAS studies show that >30% CO/sub 2/ introduced into the Ar sheath gas produces a shift equivalent to that seen off of an oxygenated surface. However, the mass spectra data coupled with diffusion calculations suggest that a maximum of approximately 0.84% CO/sub 2/ exists within the furnace from the CO/sub 2/ desorbed from the graphite surface. Unless other sources of CO/sub 2/ can be found, these data indicate that this desorbed CO/sub 2/ may not be responsible for the shift. Pure CO used as a sheath gas in GFAAS produces an early shift in the PB absorbance pulse and therefore is not responsible for the late shifts that have been observed. The shift may depend on the resulting quantities of CO and CO/sub 2/ in the furnace and the relative oxidative character of the furnace environment.