Determination of total selenium and Se-77 in isotopically enriched human samples by ICP-dynamic reaction cell-MS

This paper describes an analytical method for the simultaneous quantitative determination of total selenium (Se) and Se-77 in isotopically enriched human plasma, urine and faeces by inductively coupled plasma- dynamic reaction cell- mass spectrometry ( ICP- DRC- MS). The samples originated from a human study in which a single dose of 327 mug Se-77 ( 99.3% pure) had been given as intrinsically Se-77-labelled yeast, following administration for six weeks of 300 mug d(-1) of selenium also as selenised yeast with natural isotope abundance.

Prior to analysis, the plasma and urine samples and the digested faecal samples were diluted using an aqueous diluent containing 0.5% Triton X-100, 2% nitric acid and 3% methanol. Selenium was detected as Se-76, Se-77 and Se-80 by ICP- DRC- MS. Selenium originating from the natural isotope abundance yeast and other selenium sources from the diet was determined as Se-80, which was unaffected by the isotope enrichment.

The degree of enrichment of Se-77 was estimated from the measured Se-77 signal intensity ( natural abundance plus enrichment) minus the natural abundance of this isotope, which was calculated from measurement of Se-76. Quantification of the enriched amount of selenium Se-77 was carried out against standard additions calibration curves ( natural isotope abundance) by correcting the slope of the Se-77 calibration curve according to the 99.3% abundance of this isotope in the enriched fraction.

The limits of detection for selenium with natural abundance were 0.1 mug l(-1), 0.2 mug l(-1) and 6 mug kg(-1) and the minimum detectable increase in Se-77 was 0.38 mug l(-1), 0.58 mug l(-1) and 15 mug kg(-1) ( corresponding to 0.21%, 0.63% and 0.61% of the mean total selenium concentrations in this study) in plasma, urine and faeces, respectively.

The accuracy was controlled by analysis of the reference materials Seronorm Serum and BCR 185 Bovine Liver.