The selenium species selenite, selenate, selenomethionine (SeMet), and trimethylselenonium iodide (TMSe+) were separated in aqueous solution by ion chromatography. The separation was performed on an Ionpac CS5 cation exchange column by elution with 10 mM oxalic acid and 20 mM potassium sulphate, pH 3. The Se-78 and Se-82 isotopes were used for the inductively coupled plasma mass spectrometry (ICP-MS) detection. Using the chromatographic system on urine diluted 1 + 1, a large shift in retention times was observed. TMSe+ could be separated from the other species, but the signal from SeMet co-eluted with the selenate signal. The calibration curve was linear in the range 5-50 mu g l(-1) TMSe+ determined by spiking a urine pool. The precision at the 30 mu g l(-1) level was 1.9%. The limit of detection and determination were 0.8 and 2.6 mu g l(-1), respectively. All calculation are based on the Se-82 isotope. In urine, a large interference was observed close to the retention time of TMSe+ when monitoring the Se-78 isotope. The interference was ascribed to the sodium content of the urine. Thus, the Se-82 isotope should be used for selenium speciation in urine on this chromatographic system. Urine samples from nine volunteers were analyzed. TMSe+ concentrations above the limit of detection was only found in two samples and constituted less than 10% of the total selenium content of the samples. (C)2000 Elsevier Science B.V. All rights reserved