Selenium speciation in urine by ion-pairing chromatography with perfluorinated carboxylic acids and ICP-MS detection

Abstract

Five aqueous standards, selenomethionine (SeMet), methylselenomethionine (MeSeMet), methylselenocysteine (MeSeCys), selenogammaaminobutyric acid (SeGaba) and the trimethylselenonium ion (TMSe), were separated in ion-pairing chromatographic systems based on perfluorinated carboxylic acids in methanol. Two different perfluorinated carboxylic acids, heptafluorobutanoic acid (HFBA) and nonafluoropentanoic acid (NFPA), were used as ion-pairing agents in the separation. The selectivities of the ion-pairing agents were different. The separation was performed on a microbore column, which was connected to the ICP-MS via a laboratory-made direct injection nebuliser. This nebulisation system allowed methanol concentrations of 50% in the eluent when a flow rate of 50 µl min⁻¹ was used. The detection limits in urine were between 0.8 and 1.7 µg l⁻¹ corresponding to absolute detection limits of between 2.3 and 5.1 pg. Urine samples from different individuals before and during supplementation with selenomethionine were analysed. Several species were separated in the different urine samples. A major component eluting at the beginning of the chromatogram was predominant in many samples, especially after selenium consumption. This species was not identified and solid phase extraction experiments suggested that it was neutral. When different urine samples were spiked with the available standards, co-elution of species with TMSe, MeSeMet or SeMet was observed in some samples. None of these species were major compounds in urine samples—even after massive consumption of selenium-containing supplements. The selenium species in the urine samples showed a limited stability, as they changed during storage at +4 °C as well as −18 °C.