Species determination of selenium in natural waters

A method is described for the determination in natural waters of selenite, selenate, dimethyl selenide, and dimethyl diselenide. The detection limits are in the parts per trillion range. The volatile methyl species are removed from the sample with a stripping gas. The inorganic forms are selectively reduced to the hydride, and also stripped from the sample. A liquid nitrogen trap is used to collect both the selenides and the generated hydrides. Separation of the methyl species is accomplished by gas chromatography. All the species are detected by an atomic absorption spectrometer equipped with a quartz tube furnace.