Atomization of aluminium oxide in electrothermal atomic absorption analysis

Abstract

To verify the validity of assumptions made and conclusions reached for the proposed theory of carbothermal reduction of metal oxides by gaseous carbides, aluminium oxide (Al 2O 3) vaporization and related gaseous reaction occurring in a tube furnace used for atomic absorption analysis has been studied theoretically and experimentally. The Langmuir model of vaporization from the open surface at normal pressure and calculations of the equilibrium composition of vapours over (Al 2O 3) with the concentration of the oxygen impurity in the sheath gas taken into account form the basis of the estimations. Experimental studies have been performed using a laboratory-made set-up composed of a dual channel, scanning spectrometer, an atomizer with independent temperature control of the furnace and vaporizer, and a vaporizer whose temperature can be controlled by feedback of the analytical signal. It has been shown that the basic vapour components at Al 2O 3 atomization are Al, Al 2O and AlO. The relation between these components is determined by the content of oxygen in the sheath gas. A characteristic for metallic or metal-lined graphite furnaces at an oxygen concentration in the analytical zone above 10 −3% is the formation of AlO compounds. A reduction in the partial pressure of oxygen through reactions in the graphite furnace is accompanied by an increase in the fraction of Al 2O molecules. The results obtained enable one to reject the assumption of the existence of a “super-equilibrium” concentration of gaseous carbon and significant quantities of volatile carbides in the atmosphere of the graphite furnace, two assumptions which formed the basis of the theory of reduction of oxides by carbides (“ROC” model) under discussion. The results of the theoretical estimations and the detection of a molecular absorption band with a maximum at 255 nm in the carbon-free furnace clearly show that earlier assignment of the band to a carbon-containing compound was incorrect and that the absorption band must be attributed to Al 2O.