Simultaneous Spectrofluorimetric Determination of Selenium(IV) and (VI) by Flow Injection Analysis

Abstract

A simple, sensitive, highly selective, automatic spectrofluorimetric method for the simultaneous determination of selenium(IV) and (VI) as selenite–selenate by flow injection analysis (FIA) has been developed. The method is based on the selective oxidation of the non-fluorescent reagent 2-(α-pyridyl)thioquinaldinamide (PTQA) in acidic solution (1.5–3.0 M H ₂ SO ₄ ) by Se ^IV to give an intensely fluorescent oxidation product (λ _ex = 350 nm; λ _em = 500 nm). Selenium(VI) is reduced on-line to Se ^IV , in a reduction coil installed in a photo-reactor, which is then treated with PTQA and the fluorescence due to the sum of Se ^IV and Se ^VI is measured; Se ^VI is determined from the difference in fluorescence values. Various analytical parameters, such as effect of acidity, flow rate, sample size, dispersion coefficient, temperature, reagent concentration and interfering species were studied. The photo-reduction conditions were optimized, with an FIA procedure, for Se ^VI on the basis of its reduction efficiency. The calibration graphs were rectilinear for 0.1–2.4 µg ml ^{- 1} of Se ^VI and 10 ng ml ^{- 1} –2.2 µg ml ^{- 1} of Se ^IV , respectively. The method was applied to the determination of Se in several Standard Reference Materials (alloy, sediments and tea), as well as in some environmental waters (tap and surface water), food samples (flour and egg), a biological sample (human hair), soil sample and in synthetic mixtures. Up to 25 samples per hour can be analysed with an RSD ≈ 0.1–2%.