Analytical Possibilities of Total Reflection X-ray Spectrometry (TXRF) for Trace Selenium Determination in Soils

Selenium content of soils is an important issue due to the narrow range between the nutritious requirement and toxic effects upon Se exposure. However, its determination is challenging due to low concentrations within complex matrices that hamper the analysis in most spectroscopic techniques. In this study, we explored the possibilities of several analytical approaches combined with total reflection X-ray (TXRF) spectrometry for soil Se determinations. The direct analysis of a solid suspension using 20 mg of fine ground material (<50 mu m) has a relatively high Se limit of detection (LOD) of 1 mg/kg (worldwide Sc average in soils = 0.4 mg/kg) and is therefore only suitable for seleniferous soils. Several fast and simple analytical strategies were developed to decrease matrix effects and improve the LOD for Sc determination in soil digests. On one hand, the application of a liquid liquid extraction procedure using ethyl ether and the introduction of a Cr absorbent in the instrument configuration were carried out to avoid the associated problems on TXRF analysis of soil extracts due to the high Fe concentrations (similar to 700 mg/L). On the other hand, a dispersive liquid liquid microextraction procedure (DLL-ME) before the TXRF analysis of the soil digest was also developed. The effects of various experimental parameters such as sample volume, effect of major elements present in the soil matrix (Fe), and Se concentration in the sample were investigated. The LOD using this analytical methodology (0.05 mg/kg of Se) was comparable to or lower than those obtained in previous works using other popular spectrometric techniques such as GFAAS, ICPMS, and AFS. The calculated Se concentration for JSAC-0411 ([Se] = 1.32 +/- 0.27 mg/kg) using the combination of DLLME and TXRF ([Se] = 1.40 +/- 0.23 mg/kg) was in agreement with the certified value.