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Separation of different fractions of humic substances (HS) by their molecular weight was carried out by size exclusion chromatography (SEC). The fixation of inorganic iodide by HS of these fractions was determined by online coupling of SEC with inductively coupled plasma mass spectrometry (ICP-MS). Possible changes of HS during this transformation process of HS/iodine species could be followed by UV absorption. A 129 I-labeled sodium iodide solution was used to determine the transformation of iodide into iodine fixed on humic substances. In the case of natural samples, iodide was exclusively fixed by HS fractions where natural HS/iodine species have also been observed in the original sample. In wastewater samples from sewage disposal plants organoiodine compounds were also identified which were not affected by the transfer of iodide. A strong influence of the transformation process was seen on the microbiological activity of the sample and a complicated transfer mechanism by the formation of different intermediate HS/iodine species was observed. In general, the results contribute to a better understanding of the biogeochemical cycle of the important trace element iodine, but they are also important for the possible fixation of radioactive iodine, e.g. emitted from nuclear plants, in natural aquatic systems. On the other hand, it must be assumed that substantial amounts of iodine are removed as HS/iodine species during water processing, where humic substances are usually separated. This reduces the iodine content in drinking and mineral waters with respect to the natural level of this essential trace element.

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Two different sample treatment methods are used in connection with inductively coupled plasma isotope dilution mass spectrometry for accurate and precise determinations of iodine traces in food samples. 129I-enriched iodate is applied as a spike compound for the isotope dilution step. Extraction of iodine by tetramethylammonium hydroxide (TMAH) solution at high temperatures in a closed vessel is one of the sample treatment methods. The other one is a complete decomposition of the sample with a mixture of perchloric acid and nitric acid using microwave assistance. By analyzing different certified reference materials (three milk powders with different iodine levels, BCR CRM 63, 150, and 151; bovine liver BCR CRM 185), the accuracy of ICP-IDMS with both sample treatment methods could be demonstrated. The relative standard deviation was typically in the range of 0.6-2.8% for iodine concentrations between 0.1 microgram g-1 and 5 micrograms g-1. The detection limit was 8 ng g-1 using sample weights of 0.8 g. In a round robin test, using two different types of infant food samples, the results of the two ICP-IDMS methods and of an ICP-MS method without the isotope dilution technique, but applying the TMAH extraction procedure, agree very well with the mean of results of all participating laboratories also using ICP-MS/TMAH. However, the ICP-IDMS method is faster, more precise, widely independent of matrix effects, and, therefore, relatively accurate, which makes this method especially attractive for use as a routine method.

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