Determination of selenite and selenate in human urine by ion chromatography and inductively coupled plasma mass spectrometry

The selenium species selenite, selenate and selenomethionine were separated in aqueous solution by ion chromatography. The separation was performed on an IonPac AG11 in series with an AS11 anion exchange column by elution with 25 mM sodium hydroxide in 2% methanol. The Se-78 and Se-82 isotopes were monitored in the inductively coupled plasma mass spectrometry (ICP-MS) detector. When the chromatographic system was applied to analysis of urine samples diluted 1 + 1, the selenomethionine signal appeared in the front together with other unresolved selenium species, while the selenite and selenate signals were well separated. Calibration curves obtained after spiking a urine pool with standards in the range 2-50 mu g Se l(-1) were linear for selenite as well as selenate. The precision at the 10 mu g l(-1) level was better than 1%. The limits of detection were 0.4 and 0.8 mu g l(-1) for selenite and selenate, respectively, corresponding to absolute amounts of 8 and 16 pg, respectively. Calculations were based on peak height measurements of the Se-82 isotope. In 23 analysed urine samples, the concentration of selenite was in the range < 0.4-7.1 mu g l(-1), while the range of the total selenium concentration was 12.4-97.6 mu g l(-1). No detectable amounts of selenate were found in any of the samples. There was no correlation between selenite and total selenium concentration. No loss of selenite in the samples was observed within 7 h.