Determination of selenium in solid samples by continuous subcritical water extraction, flow injection derivatisation and atomic fluorescence detection

Abstract

A method for the determination of selenium in solid samples is proposed. The method involves two main steps: (a) continuous leaching of the analyte by subcritical water (250 °C, 200 bar, 0.7 ml min^–1), which is complete in 15 min and (b) continuous derivatisation (hydride formation) and detection by atomic fluorescence. The design of the derivatisation manifold, based on flow injection, enables speciation analysis of inorganic selenium (as Se^IV/Se^VI) and organo-selenium compounds by appropriate selection of the reagent streams. Both conversions of organic into inorganic Se by oxidation and reduction of the Se^VI formed as well as that of Se^VI initially present in the samples to Se^IV is accelerated by irradiation with focused microwaves using 1 m reactors and a flow-rate of 4.4 ml min^–1. The linear range of the calibration curves is between 10–50 000 ng ml^–1 with RSD values lower than 4.5%. The method has been applied successfully to sludge samples from an interlaboratory study. The drastic conditions of extraction convert organic Se into inorganic, so the method does not enable inorganic-Se/organic-Se speciation in solid samples.