STABLE-ISOTOPE DILUTION - MASS-SPECTROMETRY FOR DETERMINING TOTAL SELENIUM LEVELS IN PLANTS, SOILS AND SEWAGE SLUDGES

Quantitation of selenium in plants, soils and sludges was achieved by isotope dilution-mass spectrometry using a benchtop instrument. Sample s for analysis were spiked with Se-76 isotope solution. Plant material was digested on a heating block at 150 degrees C using a mixture of n itric acid and hydrogen peroxide. Selenium in soils and sludges was re leased by treatment with nitric acid followed by digestion with nitric and hydrofluoric acids. Selenium in the digests was reduced to Se(IV) with hydrochloric acid and derivatised with nitro-1, 2-phenylenediami ne to 5'-nitropiazselenol. Analysis by gas chromatography-mass spectro metry using selected ion monitoring was validated using certified refe rence materials (CRMs) and gave results within the certified range wit h a low standard deviation. The CRMs plant (Chinese cabbage leaves) an d soil (Chinese soil) were found to contain (+/-95% confidence limits) 0.091(+/-0.007) mu g g(-1) and 1.67(+/-0.04) mu g g(-1)Se respectivel y. The certified values were 0.083(+/-0.008) mu g g(-1) and 1.56(+/-0. 12) mu g g(-1) respectively. The selenium content of four different fr eely drained acid Scottish soils under grasslands was in the range 0.5 -0.8 mu g g(-1) air-dried soil. Sewage sludges were found to contain m easurably more selenium than the soils, and samples of three sludges t aken from sites in the UK contained between 1.1 and 3.5 mu g g(-1) dry matter.