Direct determination of urinary iodine by inductively coupled plasma mass spectrometry using isotope dilution with iodine-129.

An inductively coupled mass spectrometric method was developed for the direct determination of iodine in urine. The application of isotope dilution analysis with added 129I offers new possibilities for automatic and accurate determinations. The sample preparation consists of dilution with an ammonia solution containing 129I. The validation was made by comparison with the results obtained in another laboratory by a spectrophotometric method based on the Sandell-Kolthoff reaction. Different regression models, including maximum likelihood estimation, were used to compare the methods. None of the models revealed analytical bias between the two methods. The urine samples analyzed for validation were from three persons previously exposed to an iodine bath and covered a concentration range of 0.2 to 2.8 micromol/L. A detection limit of 0.02 micromol/L, a within-run CV of 2.5%, and a between-run CV of 11.9% were estimated for the proposed method.