Determination of trace selenium in rocks by hydride generation AAS

Considerably negative interference due to large amounts of iron and titanium in rocks was found when trying to determine trace selenium in sample solutions. However, the addition of EDTA and sodium fluoride mixture to the sample solution was significantly effective to mask the interfering ions derived from rocks. The present method is rapid and convenient: In a Teflon beaker, 0.1 similar to 0.5 g of rock with a mixture of 2 ml of hydrofluoric acid, 2 ml of nitric acid and 1.5 ml of perchloric acid was decomposed by heating on a hot-plate. The mixture transferred into a glass beaker with nitric and perchloric acid, where it was also decomposed until white fumes of perchloric acid appeared. After the reduction of selenate to selenite with 6 ml of 6 mol/l hydrochloric acid by boiling, 2 ml of a 0.1 mol/l EDTA-NaF mixture solution was added; this solution was diluted to 10 ml with water. The sample solution was measured by an atomic-absorption spectrophotometer equipped with a hydride generator. This method was applied to the determination of five Japanese rock reference samples of GSJ (Geological Survey of Japan). The analytical values of selenium by this method almost agree with those by fluorometry and with reference vales.