Abstract.A method for the determination of free iodide in human serum was developed. For this purpose iodide from pooled serum samples was separated from the organic manner by SEC. The iodide fraction subsequently was freezedried and analyzed by ion chromatography for quantification. Investigations for recovery and precision were carried out and were found to show sufficient results. For quality assurance ICP-MS was taken additionally as an total I-detector [1], using native and iodide-spiked serum samples. The iodide results of ICP-MS as well as those of IC were well corresponding. Iodine containing SEC-fractions from iodide-spiked samples showed no increased I-values except that in the iodide fractions, proving that there was no iodide conversion into other I-species (and vice versa) during the whole procedure.Free iodide from two serum pools of different healthy persons was determined as 2.25 and 2.43 gg I-/L, respectively. The values are related to total iodine levels determined by ICP-MS. For comparative reasons a table of individual iodine and iodide values is presented.Key words: iodide, iodine, ion chromatography, quality control, human serum.Iodine is an essential trace element [2, 31. I-containing hormones (thyroxine, tri-iodothyronine [4,5]) are strongly influencing an extended range of biochemical reactions. Usually different iodine species (e.g. IOn-, I-) are absorbed from the food and reduced to iodide (or remains iodide) in the gastro-intestinal tract [5]. The iodide pool in serum is considered as I-source for the complete synthesis of thyroxine and tri-iodothyronine. The sufficiency of I-supplementation can be seen directly by monitoring iodide in serum or secondarily by monitoring the urinary I--excretion [6]. The latter is state of the art because the direct iodide determination in serum was
