Selenite and Selenate Quantification by Hydride Generation-Atomic Absorption Spectrometry, Ion Chromatography, and Colorimetry

The need for a routine, inexpensive method with nanomolar detection limits for soluble SeO²⁻₃ and SeO²⁻₄ is increasing as Se research on soil-plant-water systems intensifies and speciation of Se valence states is required. This study was undertaken to compare hydride generation-atomic absorption spectrometry (HGAAS), colorimetry using 2,3-diaminonaphthalene (DAN), and ion chromatography (IC) as suitable methods for analysis of SeO²⁻₃ and SeO²⁻₄ in soil extracts and water samples. Selenium standards in solutions of either 1.0 mM KH₂PO₄, CaSO₄, and KNO₃, or in distilled water were analyzed to determine detection limits (DL), limits of quantitation (LOQ), and practicality for routine use of the above methods. A 0.1 mM K₂HPO₄/KH₂PO₄ soil extract spiked with 2.0 or 20 µM SeO²⁻₃ and SeO²⁻₄ was also used to compare the precision of the three methods. The HGAAS method was the most sensitive (0.212 absorbance units/µM) and had the lowest DL for SeO²₃ (0.03 µmol Se/L) of the three methods employed. Detection limits for the DAN and IC methods were significantly higher (18 and 0.24 µmol Se/L, respectively). Poor reproducibility between methods was obtained for SeO²⁻₄ content of the soil extract containing 20 µM SeO²⁻₄, with concentrations ranging from 9.1 ± 1.2 µmol/L determined by HGAAS to 24.6 ± 0.05 µmol/L by IC. The variability and poor recovery of SeO²⁻₄ was due to variabilty in efficiency of the HCl reduction required to reduce SeO²⁻₄ to SeO²⁻₃ before analysis in the HGAAS and DAN procedures. Selenium determinations in samples where low detection limits (<0.1 µM) are required are most effectively accomplished with HGAAS. Colorimetry requires greater effort and time from the analyst to obtain the low detection limits of HGAAS, but is useful in that only a visible light spectrometer is required. Ion chromatography is capable of automated determinations of Se²⁻₃ and SeO²⁻₄ directly in combination with other ions of interest and may be appropriate for soil solution Se analysis where Se speciation is required. Contribution of the Maryland Agric. Exp. Stn. Sci. Art. no. A6380.