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 environmental samples has been developed in order to study the transfer mechanism 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) may be determined with a detection limit close to 25 ng l(-1) and a linear response in the range 25-4000 ng l(-1). However, humic substances, present in some natural waters and soils, which are adsorbed at the mercury drop electrode (HMDE) surface may alter the signal. This may be caused by a competition between adsorption of organic matter and mercury(II) selenide formation at the electrode surface. As a consequence the detection limit has been increased to ca. 250 ng l(-1) in the presence of 1 mg l(-1) fulvic acids; the linear response range is then shifted to 250-10000 ng l(-1). After an extensive study of these interferences and using standard additions procedures, the Se(TV) content of various waters and soil extracts has been determined by DPCSV with a good 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).