Microwave digestion of environmental and natural waters for selenium speciation

A microwave preparation procedure is proposed for selenium speciation in na tural and drinking waters. Different chemical reagents were tested, and the conditions for Se speciation were optimized. The effect of the different r eagents on various oxidation states of selenium under microwave digestion c onditions was investigated. Most of the Se(-II) was converted to selenite w hen digested with HNO3 and < 20% to selenate. The digestion with H2O2/H2SO4 can change most Se species into Se(M. The concentration of Se(IV) in the s amples was then determined by HPLC with a fluorescence detector after deriv atization,with 2,3-diamino-naphthalene (DAN). The microwave preparation pro cedure allows Se speciation in water samples. Se(IV)was determined after co ncentrating 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 afte r digestion with H2O2/H2SO4, or after digestion with HNO3 followed by reduc tion with concentrated hydrochloric acid. Selenium (-II, 0) was calculated by subtracting inorganic Se(IV+VI) from the total. Detection limits of 0.00 66 ng and 0.0096 ng Se were obtained for HNO3 and H2O2/H2SO4 as digestion r eagents, respectively. The total Se in the four water samples tested range from 0.20 to 0.90 mug L-1. Among them the dominant form was Se(VI) with the exception of pond waters where Se(-II) predominated.