Mild sample pretreatment procedures based on photolysis and sonolysis-promoted redox reactions as a new approach for determination of Se(iv), Se(vi) and Se(−II) in model solutions by the hydride generation technique with atomic absorption and fluorescence detection

Abstract

Ultraviolet and ultrasound irradiation-induced redox reactions have been assessed for the determination of Se(IV), Se(VI), selenomethionine and selenocystine in model waters, enriched natural waters and soil/fly ash extracts by the hydride-generation technique with atomic absorption and fluorescence detection. Photooxidation of Se-methionine and Se-cystine into Se(IV) by UV irradiation in HCl medium (pH 0.4) can be achieved within 10 min, whilst Se(IV), the only Se species reactive towards NaBH₄, leading to SeH₂ formation, remains unaffected. Photoreduction of Se(VI) to Se(IV) required at least 45 min under the same conditions. Higher pH values yielded incomplete conversions of the Se species into Se(IV). Se(VI) photoreduction was completely inhibited by the presence of 10000 mg L⁻¹ nitrate as KNO₃. Ultrasonic oxidation of Se-cystine to Se(IV) is seen to occur, the remaining Se species remaining unaffected. According to the above redox reactions, a four-stage model for speciation of Se is proposed. The combination of these reactions could be the basis for a new speciation approach that is advantageous in terms of sensitivity and simplicity over methods involving chromatographic separations. The LOD of each Se species was 40 pg mL⁻¹ and 2 pg mL⁻¹ with detection by atomic absorption and fluorescence, respectively. The repeatability expressed as relative standard deviation was in the range 2–5% for natural waters and 4–10% for phosphate extracts.