Analytical strategies for inorganic and organic selenium speciation in natural waters [Stratégies analytiques pour la spéciation du sélénium inorganique et organique dans les eaux naturelles]
Résumé
Selenium is today recognized as a trace element, it is thus essential to life, but in higher doses it becomes toxic for Human and animals. The concentration range between its indispensable role and its toxicity is very narrow. Selenium appears in its natural cycle in several oxidation states and as a variety of inorganic and organic compounds. In waters, selenite and selenate are the species most often searched and detected. It has been shown that inorganic selenium doesn't always account for total concentration in water. Therefore identification and quantification of dissolved organic species, i.e selenomethionine, selenocystine, trimethylselenonium..., is essential to complete selenium speciation. Our lab is interested since 1991 in selenium environmental chemistry in aquatic systems. Attention is paid here to our recent analytical developments based on high performance liquid chromatography - inductively coupled plasma mass spectrometry (HPLC-ICPMS), allowing environmental speciation at the ten ng (Se). L-1 level. We focus on two ways to lower Se detection limits. On the one hand, detector performances are improved with the help of recent ICPMS generation equipped with collision/reaction cell, operating conditions being optimized by experimental design methodology. Sample introduction by hydride generation step instead of pneumatic nebulisation, also results in improved detection power. On the other hand, we are developping preconcentration procedures to artificially lower detection limits. We select working stationary phases that allow the simultaneous preconcentration of inorganic and organic selenium species to keep the advantage of HPLC-ICPMS direct speciation.