Loss of selenium in water samples at natural levels during storage

Selenium losses in river, ground, snow-melt and tap water samples, and the recovery of selenite, selenate and selenomethionine added to purified water have been studied. In 1-litre high-density polyethylene bottles, tap, river and snow-melt water samples (at Se concentrations of 44.5–138 ng/l) could be stored at 4 °C for up to 15 days without Se losses. In similar samples stored at room temperature Se losses of 13–25% after 15 days were found, except for groundwater, which showed no Se losses during storage for 13 months at room temperature or at 4 °C. Selenite and selenate added to purified water were recovered without losses after 15 days at 4 °C, while 7.5% of selenomethionine was lost. The stability of different chemical forms of Se during storage followed the order: selenate > selenomethionine > selenite. It is recommended that unacidified water samples should not be kept in polyethylene bottles at room temperature for more than 1 week, nor stored at 4 °C for more than 2 weeks, before analysis for Se.