EFFECT OF PHYSICOCHEMICAL PARAMETERS ON TRACE INORGANIC SELENIUM STABILITY

The stability of Se(IV) and Se(VI) species at 10 and 50 ng ml(-1) concentrations, at different pH values, in the presence or absence of chloride and stored at different temperatures and in different containers was reported. Se(IV) was determined by hydride generation atomic absorption spectrometry (HG-AAS) and Se(VI) by reducing Se(VI) to Se(IV) and calculating the difference between total inorganic Se and Se(IV). Neutron activation analysis (NAA) was used for quality control. Complete loss of Se(IV) occurred after twelve months when samples were stored in polyethylene containers at pH 2 in the absence and in the presence of chloride. Se(VI) was stable under those conditions fos the twelve months tested. The stability of Se(IV) increased at pH 6, with two months being the maximum storage time without risk of Se(IV) loss. The presence of Cl- decreased the risk of losses in some cases. PTFE containers increased Se(IV) losses at pH 6, especially at 10 ng ml(-1). Both species were stable at -20 degrees C for the twelve months tested and losses of Se(IV) and Se(VI) were lower at 40 degrees C than at room temperature. A discussion of these results and data published in the literature are included in this paper.