Comparison of methods for iodine analysis in foods

Spectrophotometric and ICP-MS methodology for iodine determination was compared. Samples were alkali dry-ashed, dissolved in water, and iodine assayed by spectrophotometry and by ICP-MS. Iodine content in the studied foods ranged from 3 to 1304 μg/100 g. There was no significant difference (p > 0.05) between iodine values determined spectrophotometrically using an external calibration curve and values determined using a standard addition. Foods containing low iodine concentrations (4-25 μg/100 g) also showed no significant difference (p > 0.05) between iodine concentrations determined spectrophotometrically and concentrations determined by ICP-MS. For food items with more than 25 μg/100 g, the spectrophotometric methods yielded markedly higher (p < 0.05) concentrations than the standard ICP-MS method (relative positive bias 25-122%), especially in foods with high sodium and/or iron contents. A catalytic effect of sodium and iron on the Sandell and Kolthoff reaction, leading to false high values in the spectrophotometric determination of iodine was demonstrated. ICP-MS is recommended for iodine determination in foods.