High-performance liquid chromatography–atomic absorption spectrometry interface for the determination of selenoniocholine and trimethylselenonium cations: application to human urine

Abstract

The operation of a prototype high-performance liquid chromatography–atomic absorption spectrometry (HPLC–AAS) interface based on thermochemical hydride generation (THG) was characterized for the determination of selenonium compounds. Methanolic solutions of analytes containing selenium were nebulized by a thermospray effect, pyrolysed in a methanol–oxygen kinetic flame in the presence of excess of hydrogen and atomized in a micro-diffusion flame maintained at the entrance to an unheated quartz T-tube. Factorial models for predicting the performance of the interface–detector combination, at different levels of five interface operating variables, indicated that the interface is compatible with both reversed- and normal-phase HPLC eluents, and that variations in the five operating parameters within relatively wide ranges does not affect the analyte response appreciably (less than 50% variation in response). Co-injection of trimethylselenonium iodide [(CH₃)₃ Sel] with a 10-fold excess of other potential interferent onium ions did not affect the THG process significantly. A modified apparatus was used to study the composition of the gases produced from the pyrolysis of (CH₃)₃ Sel or SeO₂ in the presence of either H₂ or He. The product gases were condensed, acidified and channelled through two consecutive trapping solutions to recover Se^IV and hydrogen selenide (H₂Se), separately, from the product mixture. The analysis of these trapping solutions by HPLC–AAS demonstrated that, in the presence of H₂, both (CH₃)₃ Sel and SeO₂ were thermochemically reduced to H₂Se whereas Se^IV was the major product in post-pyrolysis atmospheres of He. A rapid isocratic HPLC separation was developed for the determination of selenoniocholine and trimethylselenonium cations and applied to human urine. Recoveries of both analytes, when present at levels of between five- and ten-times the normal background level of total Se, were 77% or better. The low cost, high reproducibility and robust nature of this system make it a good candidate for the routine determination of several selenium compounds including other selenonium cations, selenoamino acids and Se^IV organometalloid species.