INTERFERENCE OF HUMIC SUBSTANCES ON THE SPECIATION ANALYSIS OF INORGANIC SELENIUM IN WATERS AND SOILS BY DPCSV

A speciation scheme allowing the study of selenium speciation in envir onmental samples has been developed in order to study the transfer mec hanism in the system water/soil/plant/animal. This scheme is based on a set of sample treatment procedures followed by Se(TV) determination by Differential Pulse Cathodic Stripping Voltammetry (DPCSV). Se(IV) m ay be determined with a detection limit close to 25 ng l(-1) and a lin ear response in the range 25-4000 ng l(-1). However, humic substances, present in some natural waters and soils, which are adsorbed at the m ercury drop electrode (HMDE) surface may alter the signal. This may be caused by a competition between adsorption of organic matter and merc ury(II) selenide formation at the electrode surface. As a consequence the detection limit has been increased to ca. 250 ng l(-1) in the pres ence of 1 mg l(-1) fulvic acids; the linear response range is then shi fted to 250-10000 ng l(-1). After an extensive study of these interfer ences and using standard additions procedures, the Se(TV) content of v arious waters and soil extracts has been determined by DPCSV with a go od reproducibility (RSD about 1%). Accuracy is satisfactory comparing the results obtained by DPCSV to those obtained by Hydride Generation/ Quartz Furnace Absorption Atomic Spectrometry (HG/QFAAS).