Isotopic determination of selenium in biological materials with inductively coupled plasma mass spectrometry

Abstract

A method for the isotopic determination of selenium in biological matrices is described. The method is based on hydride generation inductively coupled plasma mass spectrometry (ICP-MS). The development is specifically related to the requirements of stable isotope tracer studies in human subjects. The method is based on isotope dilution using ⁸²Se as the in vitro spike and can quantify the ⁷⁴Se and ⁷⁷Se contents of samples. It involves wet oxidation (HNO₃-H₂O₂ or HNO₃-HClO₄) of the ⁸²Se-spiked matrix, reduction to selenite by boiling with HCl followed by measurement of the isotope ratios (⁸²Se/⁷⁷Se and ⁷⁴Se/⁷⁷Se) in the gas stream (H₂Se) generated from on-line reduction of the sample selenite with NaBH₄. Compared with the isotopic signal resulting from a selenite solution containing 5 ng ml^–1 of Se, the total sample blank contributions at m/z= 74, 77 and 82 were less than 5% of the respective isotope signal. Worst-case absolute detection limits were 0.2–0.9 ng of Se, depending on the isotope used. Ion beam intensity ratios were measured with an over-all precision [relative standard deviation (RSD)] of 1% for both isotope pairs. Measured ratios (MR_a/b) were stable during a given day's operation within the expected precision of the measurements but varied for different days. The magnitude of MR_a/b was generally independent of the nature of the matrix. Highly linear relationships were found between ion beam intensity ratios (MR_a/b) and the corresponding true isotope ratios for calibration solutions whose isotope ratios had been altered by as much as one order of magnitude. The precision/accuracy of the isotopic analysis was established by replicate measurements of the Se content of several biological matrices [National Bureau of Standards Standard Reference Material (NBS SRM) 1577a Bovine Liver, human plasma, red blood cells and human urine], and comparison of the results with independent measurements obtained using hydride generation atomic absorption spectrometry (AAS). The following data were obtained (mean ± SD, n= 3–5; first result, hydride generation ICP-MS; second result, hydride generation AAS): NBS SRM 1577a Bovine Liver, 0.697 ± 0.002, 0.69 ± 0.01 µg g^–1; plasma, 0.098 ± 0.001, 0.135 ± 0.008 µg g^–1; red blood cells, 0.211 ± 0.002, 0.216 ± 0.012 µg g^–1; and urine, 0.0473 ± 0.0003, 0.0489 ± 0.0003 µg ml^–1. It was concluded that the proposed method could be used as the measurement method for studies of Se metabolism in human subjects using the concept of stable isotope tracers. Compared with other available methods of isotopic analysis, this method possesses the added advantage of requiring no chemical separation steps as the hydride generation is sufficient for removal of any potential matrix-related interferences.