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

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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 SeIV 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 SeIV 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 SeIV organometalloid species.