IntroductionMeasurement of urinary iodine is still an efficient way to reveal deficiencies in the nutritional supply of the essential trace element iodine. For many years, one of the most frequently used analytical techniques has been the catalytic spectrophotometric method based on the Sandell–Kolthoff reaction. In combination with chloric acid digestion it proved to be a reliable routine method (see, for example, ref.1). The hazard of chloric acid could be avoided by using ammonium persulfate as digestion reagent, according to a recent report.2Owing to this improvement, the Sandell–Kolthoff reaction will keep its importance, especially when carried out in a microtiter plate format.3Despite this fact, the availability of additional methods may be beneficial in certain cases, not least for preparation of certified reference materials. High-performance liquid chromatographic (HPLC) and ion chromatographic (IC) techniques in the form of ion-pair or ion-exchange chromatography combined with electrochemical or post-column reaction detection have been reported for trace analysis of iodide in biological samples like urine or serum.4–8The most promising approach would be the hyphenation of chromatography with mass spectrometric detection either in the form of inductively coupled plasma mass spectrometry (ICP-MS) or of electrospray mass spectrometry (ESI-MS).In several studies IC has been used in combination with ICP-MS for speciation of iodine in aqueous samples.9–11However, only a few studies have addressed the selective and sensitive quantification iodine species in difficult biological matrices as serum or urine. Michalkeet al.12have reported the use of reversed-phase HPLC combined with ICP-MS for iodine speciation in urine. Under the conditions applied in their work, iodide is eluted in the void volume so that interference from other polar iodine species cannot be fully excluded.In the present paper, the advantages and drawbacks of ICP-MS and ESI-MS are compared when hyphenated with ion-exchange chromatography for trace analysis of iodide in biological fluids.