Role of oxygen in the determination of oxide-forming elements by electrothermal atomic absorption spectrometry. Part 1. Effect of oxygen on the reactions of thallium

Abstract

To study the effect of oxygen on the chemical reactions of oxide-forming elements in a graphite furnace a method was developed that allows a separation between the oxygen treatment and the subsequent measurement. This method demonstrated that the effects caused by oxygen are due to a change in the surface properties of the furnace. Thallium forms residues in untreated and oxygen-treated furnaces after the drying step as Tl₂O₃ on the surface. In untreated furnaces the reduction to the atomic state occurs via the volatile suboxide Tl₂O, which is the reason for losses of atomizable material. Furnaces treated with oxygen avoid the formation of the volatile suboxide owing to the chemically modified graphite surface and enhance the absorbance considerably. The modification of the graphite surface is caused by chemisorption of oxygen on active sites of the graphite which are destroyed in a high-temperature step. Hence, the reduction process is shifted to higher temperatures with lower losses of atomizable material. Details of a pre-treatment that avoids the losses of material without the use of oxygen are also given.