Direct speciation of selenite and selenate with thermospray sample introduction methods

Thermospray is a high-temperature and high-pressure process compared with pneumatic nebulization. In this study these characteristics of thermospray were exploited for selenium speciation with inductively coupled plasma atomic emission spectrometric (TCP-AES) detection, In aqueous solution, the sensitivity for Se(IV) was found to be lower than that for Se(VI), Addition of methanol into the solution further depressed the sensitivity for Se(IV) to a negligible level compared to that for Se(VI), The low sensitivity for Se(IV) arises from its reduction to Se(0) within the thermospray process, after which selenium metal was trapped on the vaporizer, This loss of SE(IV) can be counteracted by addition of an oxidant (K2S2O8 or K2Cr2O7) into the selenium solution, which prevents Se(IV) from being reduced within the vaporizer. Based on these results, a method for speciation of Se(IV) and Se(VI) was developed. The concentration of Se(VI) is selectively determined in a sample aliquot with 20% (v/ v) methanol added, In another sample aliquot with an addition of 5 mmol/L K2S2O8 or 2 mmol/L K2Cr2O7, the total selenium concentration is defined. The selenite content is calculated by difference. The detection limits for Se(VI) at 195.0 nm were measured to be 2 ng/mL for aqueous solution and 9 ng/mL for 20% (v/v) methanol solution, A concomitant with oxidation or reduction ability, which can obstruct: the selenite reduction within the thermospray vaporizer or counteract the function of an oxidant added, will be a potential interferent in the determination of Se(VI) or total selenium, although the tolerance limits of such interferents as Cr(VI) and Sn(II) were determined to be well above their concentrations in many environmental samples such as natural water, The effects of methanol concentration, oxidant concentration, and nitric acid concentration in solution were studied in detail, Good accuracy and precision of the method were demonstrated for the analysis of spiked tap water samples.