Use of enriched 74Se and 77Se in combination with isotope pattern deconvolution to differentiate and determine endogenous and supplemented selenium in lactating rats.

A quantitative methodology has been developed to differentiate between endogenous and supplemented selenium in lactating rats using two enriched selenium isotopes. Lactating rats were fed for 2 weeks with formula milk containing one enriched Se isotope, (77)Se, as the metabolic tracer. The isotopic composition of selenium in serum and urine samples was then measured by collision cell ICP-MS after the addition of a solution containing another enriched isotope, (74)Se, as quantitation tracer, before analysis. Isotope pattern deconvolution allowed the transformation of measured Se isotopic abundances into concentrations of natural abundance (endogenous) selenium and enriched (77)Se (supplemented) present in the samples. The proposed methodology was validated using serum and urine reference materials spiked with both (77)Se and (74)Se. The obtained results are discussed in terms of selenium exchange and half-life in lactating rats (11-12 days) and selenium levels in serum in comparison with non-supplemented rats and control rats after maternal feeding.