Simultaneous spectrofluorimetric determination of selenium (IV) and (VI) by flow injection analysis

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-(alpha-pyridyl)thioquinaldinamide (PTQA) in acidic solution (1.5-3.0 M H2SO4) by SeIV to give an intensely fluorescent oxidation product (lambda ex =350 nm; lambda em = 500nm). Selenium (VI) is reduced on-line to SeIV, in a reduction coil installed in a photo-reactor, which is then treated with PTQA and the fluorescene due to the sum of SeIV and SeVI is measured; SeVi 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 SeVI on the basis of its reduction efficiency. The calibration graphs were rectilinear for 0.1-2.4 micrograms ml-1 of SeVI and 10 ng ml-1-2.2 micrograms ml-1 of SeIV, 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 approximately 0.1-2%.