Microwave digestion of environmental and natural waters for selenium speciation

A microwave preparation procedure is proposed for selenium speciation in natural and drinking waters. Different chemical reagents were tested, and the conditions for Se speciation were optimized. The effect of the different reagents on various oxidation states of selenium under microwave digestion conditions was investigated. Most of the Se(-II) was converted to selenite when digested with HNO3 and <20% to selenate. The digestion with H2O2/H2SO4 can change most Se species into Se(IV). The concentration of Se(IV) in the samples was then determined by HPLC with a fluorescence detector after derivatization with 2,3-diamino-naphthalene (DAN). The microwave preparation procedure allows Se speciation in water samples. Se(IV) was determined after concentrating the sample under nitrogen protection. The amount of Se(IV) and Se(VI) was measured by adding an equal volume of concentrated hydrochloric acid to water sample to reduce Se(VI) to Se(IV). Then the amount of Se(VI) can be calculated by subtraction. The total selenium can be determined after digestion with H2O2/H2SO4, or after digestion with HNO3 followed by reduction with concentrated hydrochloric acid. Selenium (-II, 0) was calculated by subtracting inorganic Se(IV+VI) from the total. Detection limits of 0.0066 ng and 0.0096 ng Se were obtained for HNO3 and H202/H2SO4 as digestion reagents, respectively. The total Se in the four water samples tested range from 0.20 to 0.90 microg L(-1). Among them the dominant form was Se(VI) with the exception of pond waters where Se(-II) predominated.