Selenium is an environmentally important element that is difficult to determine in natural waters owing to its low concentrations. Research has shown that selenium levels are typically <0.5 µg l–1 in most waters sampled. The increased environmental interest in selenium has promoted the modification of analytical methods with the aim to improve analysis at the sub-µg l–1 level. The determination of selenium by hydride generation has been widely documented by a number of authors. Selenium may be reduced to a gaseous hydride by reaction with sodium tetrahydroborate and introduced directly into the base of the ICP torch. This allows almost 100% efficiency of sample injection compared with only 2–5% by standard nebulization, hence improving sensitivity. Hydride generation represents a greatly improved method of selenium analysis and detection limits of 1 µg l–1 or lower may be obtained. However, when measuring selenium in most natural waters, samples must be preconcentrated to further lower detection limits into the usual range of occurrence. Coprecipitation with lanthanum hydroxide is a rapid and efficient method of concentrating selenium. In natural groundwaters and similar environments, selenium occurs mostly as SeVI(selenate) although small amounts of SeIV(selenite) may also be present. This research has shown that the dominant form of selenium (selenate) is not effectively precipitated with lanthanum hydroxide. Previously documented methods have therefore been modified here by reducing all Se to selenite prior to preconcentration and analysis. A method is described that allows selenium levels as low as 0.06 µg l–1 to be analysed rapidly and accurately. The selenium in each sample is reduced to selenite by heating with HCl and KBr. Selenite is then coprecipitated with lanthanum hydroxide and separated by centrifugation. The resulting precipitate is dissolved in an acid solution and introduced into the ICP in the form of gaseous hydride. Standard solutions show very good linear calibration across the range 0–10 µg l–1.\u
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