Palladium nitrate–magnesium nitrate modifier for electrothermal atomic absorption spectrometry. Part 4. Interference of sulfate in the determination of selenium

Abstract

The possible sources of sulfate interferences in the determination of selenium, even when stabilized temperature platform furnace conditions were used, were investigated using sulfuric acid, sodium sulfate and magnesium sulfate as model compounds. The molecular spectrum of CS was observed when sodium sulfate was volatilized. The most likely mechanism of interference in this instance is an expulsion of the analyte element with the violently volatilized matrix early in the atomization stage. Sulfuric acid and magnesium sulfate decompose at elevated temperatures with the formation of SO₃, which was reduced at least in part to SO₂ in the graphite tube. The most likely interference mechanism in these instances is the formation of SeO₂ in the presence of excess of SO₃ and its volatilization during the pyrolysis stage. Addition of barium nitrate to the palladium nitrate–magnesium nitrate modifier reduced the background absorption and also the sulfate interference significantly. In the presence of barium, sulfate is bound at least in part as barium sulfate, which decomposes in the atomization stage. This means that SeO₂, if it is formed, is volatilized at temperatures which are high enough for its atomization. The proposed mixed modifier allows an interference-free determination of selenium in mineral waters with high sulfate content.