Determination of the Elemental Iodine in Human Breast Milk by Inductively Coupled Plasma mass Spectrometry

Adequate iodine status in lactating women is defined by a maternal median urinary iodine concentration (UIC) ≧ 100 µg/L. However, the above-mentioned criterion does not account for the secretion of iodine into breast milk and could not truly reflect the amount of iodine delivered to the infants. Measuring breast milk median iodine concentration (BMIC) is crucial, but the method to measure BMIC has not been developed and validated in Taiwan. We adopted the ammonia dilution method without prior sample digestion to measure BMIC by inductively coupled plasma mass spectrometry (ICP-MS). Samples and iodate calibrators were prepared into an aqueous solution containing Triton X-100, 0.5% ammonia solution, and tellurium (¹²⁸Te) as the internal standard. Precision, accuracy, serial dilution, and recovery tests were performed for method validation. The range of intra-assay and inter-assay coefficient of variation for the four human breast milk samples with different iodine concentrations were 3.2–4.7% and 2.3–5.5%, respectively. The standard NIST 1549 milk powder was prepared into three different concentrations of 50 µg/L, 100 µg/L, and 200 µg/L to assess the accuracy; the bias was < 5%. A recovery of 95–105% was achieved for four human breast milk samples spiked with sodium iodide solution. The serial dilution test confirmed linearity up to 0.998. The limit for detection and quantification was 0.78 µg/L and 2.34 µg/L, respectively. The results of the current study confirmed that this ICP-MS method is accurate and reliable in measuring BMIC.