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. Samples for analysis were spiked with (76)Se isotope solution. Plant material was digested on a heating block at 150 degrees C using a mixture of nitric acid and hydrogen peroxide. Selenium in soils and sludges was released 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-phenylenediamine to 5'-nitropiazselenol. Analysis by gas chromatography-mass spectrometry using selected ion monitoring was validated using certified reference materials (CRMs) and gave results within the certified range with a low standard deviation. The CRMs plant (Chinese cabbage leaves) and soil (Chinese soil) were found to contain (+/-95% confidence limits) 0.091(+/-0.007) mug g(-1) and 1.67(+/- 0.04) mug g(-1)Se respectively. The certified values were 0.083(+/-0.008) mug g(-1) and 1.56(+/-0.12) mug g(-1) respectively. The selenium content of four different freely drained acid Scottish soils under grasslands was in the range 0.5-0.8 mug g(-1) air-dried soil. Sewage sludges were found to contain measurably more selenium than the soils, and samples of three sludges taken from sites in the UK contained between 1.1 and 3.5 mug g(-1) dry matter.