Determination of selenite and selenate in drinking water: a fully automatic on-line separation/pre-concentration system coupled to electrothermal atomic spectrometry with permanent chemical modifiers

A time-based flow injection (FI) separation pre-concentration system coupled to an electrothermal atomic absorption spectrometer (graphite furnace) has been developed for the direct ultra-trace determination of selenite and selenate in drinking water. The pre-concentration of both forms of selenium is carried out onto a micro-column packed with an anionic resin (Dowex 1X8) that is placed in the robotic arm of the autosampling device. Selenite and selenate are sequentially eluted with HCl 0.1 M and HCl 4 M, respectively. The interference of large quantities of chloride during selenium atomisation is prevented by using iridium as a "permanent" chemical modifier. The features of the pre-concentration separation system for both species are: 53% efficiency of retention and an enhancement factor of 82 for a pre-concentration time of 180 s (sample flow rate=3 ml min^-1) with HCl elution volumes of 100 (mu)l. The detection limit (3 s) is 10 ng l^-1 for the two species and the relative standard deviation (n=10) at the 200 ng l^-1 level is 3.5% for selenite and 5.6% for selenate. The addition of selenite and selenate stock standard solutions to tap water samples yields a 97–103% recovery of both species.