Completely integrated on-line determination of dissolved selenium(IV) and total inorganic selenium in sea-water by flow injection hydride generation atomic fluorescence spectrometry

Abstract

Highly sensitive on-line pre-treatment methods were developed for the direct determination of trace amounts of selenite and total inorganic Se in sea-water. Selenite determination was carried out on samples acidified with HCl and merged on-line with 5% m/v KH2PO4, which was employed as an interference eliminating reagent. For total inorganic Se determination, acidified samples were merged with 30% m/v NaBr, used for reduction of Se(vi), and pre-reduced on-line by microwave irradiation at 210 W for 60 s. After pre-treatment, the sample zone was stored in a coil and then transported at a flow rate of 22 ml min ^–1 for reaction with 1.5% m/v NaBH ₄ . The hydride produced was separated and dried and Se was determined in a mini-hydrogen diffusion flame by non-dispersive atomic fluorescence spectrometry. The interface developed for this purpose makes possible the integration, within the same manifold, of two steps involving very different reaction times. The detection limits were 5 ng l ^–1 for Se(iv) and 4 ng l ^–1 for total inorganic Se. The recoveries of selenite at the 30 ng l ^–1 level were 94-104% and those of total inorganic Se, at the same concentration level, were 92-98%. The methods developed are suitable for the accurate and direct analysis of natural waters, and the results obtained for total inorganic Se determination in a freshwater reference material were in good agreement with the certified value.