Quantitative determination of selenium metabolites in human urine by LC-DRC-ICP-MS

Abstract

Two selenium metabolites, Se-methylseleno-N-acetylgalactosamine (SeGal-N-Ac) and Se-methylselenogalactosamine (SeGal-NH₂), were quantified in human urine by LC-DRC-ICP-MS. Urine samples were analysed after 1 + 1 dilution in a reversed phase chromatographic system using an eluent consisting of 200 mM ammonium acetate and 5% methanol with a pH of 9.25 and quantified by standard addition. Samples were collected from 8 volunteers before and after 5 days ingestion of 100 μg Se day⁻¹ in form of selenized yeast. The average concentration of (SeGal-NH₂) before and after selenium intake was 1.4 and 1.9 μg Se L⁻¹, respectively, while the average concentration of Se-Gal-N-Ac increased from 2.6 to 11.6 μg Se L⁻¹ before and after selenium consumption. Detection limits calculated on basis of three times the standard deviation on peak areas of 2 μg Se L⁻¹ solutions were 0.1 μg Se L⁻¹ for SeGal-NH₂ and 0.2 μg Se L⁻¹ for SeGal-N-Ac based on peak areas and monitoring ⁸⁰Se. The precision expressed as the relative standard deviation (n = 6) at the 2 μg Se L⁻¹ level was 3.1 and 1.7% for SeGal-NH₂ and SeGal-N-Ac, respectively, while the corresponding values were 1.0 and 0.7% at the 10 μg Se L⁻¹ level. Linearity in urine matrix was examined in the range 0.5–100 μg Se L⁻¹ and correlation coefficients better than 0.999 were obtained. As the cationic compound SeGal-NH₃⁺ may be confounded with the trimethylselenonium ion (TMSe⁺), urine samples were also analysed in a cation exchange chromatographic system in which SeGal-NH₃⁺ was separated from the trimethylselenonium ion. None of the samples contained TMSe⁺ in detectable amounts. Three sample introduction systems were compared—a microconcentric nebuliser in combination with a cyclonic spray chamber (MCN), a direct injection nebuliser (DIN) and an ultrasonic nebuliser (USN). The MCN was most suitable for this purpose.