EFFECT OF SOLVENT TYPE ON THE DETERMINATION OF TOTAL IODINE IN MILK POWDER AND HUMAN SERUM BY INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY

A method for the determination of the total iodine content in milk pow der and human serum by inductively coupled plasma mass spectrometry is described. It is shown that the oxidation state of the analyte has a strong impact on the analytical performance. If iodine is present as i odide and nitric acid is used for acidification, the observed ion sign al is not stable and samples spiked with iodide yield erroneous recove ries up to 750%. Hence, as the iodine in milk powder is present almost exclusively as iodide, destruction of the sample with nitric acid is not suitable. It was found that such an excessive spike recovery does not occur if ammonia solution instead of nitric acid is used as the di luent. The proposed sample preparation was tested by analysing three r eference milk powders, namely Certified Reference Materials 150 and 15 1 Spiked Skim Milk Powder (from the Community Bureau of Reference) and Standard Reference Material 1549 Non-Fat Milk Powder (from the Nation al Institute of Standards and Technology). Good agreement with the cer tified values was obtained, indicating that no significant errors are introduced. For human serum, samples treated with nitric acid or with ammonia solution gave results that were not significantly different fr om each other. Results are given for both a 'second-generation' biolog ical reference material, Freeze-Dried Human Serum (University of Ghent ), and for sera from healthy individuals.