Speciation analysis for iodine in milk by size-exclusion chromatography with inductively coupled plasma mass spectrometric detection (SEC-ICP MS)

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Abstract
A method allowing the determination of iodine species in milk and infant formulas was developed. It was based on the coupling of size-exclusion chromatography (SEC) with on-line selective detection of iodine by ICP MS. Iodine species were quantitatively eluted with 30 mM Tris buffer within 40 min and detected by ICP MS with a detection limit of 1 µg l–1 (as I). A systematic study of iodine speciation in milk samples of different animals (cow, goat) and humans, of different geographic origin (several European countries) and in infant formulas from different manufacturers was carried out. Whey obtained after centrifugation of fresh milk or reconstituted milk powders contained more than 95% of the iodine initially present in milk in all the samples investigated with the exception of the infant formulas in which only 15–50% of the total iodine was found in the milk whey. An addition of sodium dodecyl sulfonate (SDS) improved considerably the recovery of iodine from these samples into the milk whey. Iodine was found to be principally present as iodide in all the samples except infant formulas. In the latter, more than half of the iodine was bound to a high molecular (>1000 kDa) species. The sum of all the species recovered from a size-exclusion column accounted for more than 95% of the iodine present in a milk sample. For the determination of total iodine in milk a rapid method based on microwave-assisted digestion of milk with ammonia followed by ICP MS was optimized and validated using CRM 151 Skim Milk Powder.