Trace iodine quantitation in biological samples by mass spectrometric methods: The optimum internal standard

Accurate quantitation of iodine in biological samples is essential for studies of nutrition and medicine, as well as for epidemiological studies for monitoring intake of this essential nutrient. Despite the importance of accurate measurement, a standardized method for iodine analysis of biological samples is yet to be established. We have evaluated the effectiveness of 72Ge, 115In, and 129I as internal standards for measurement of iodine in milk and urine samples by induction coupled plasma mass spectrometry (ICP-MS) and of 35Cl18O4−, 129I−, and 2-chlorobenzenesulfonate (2-CBS) as internal standards for ion chromatography-tandem mass spectrometry (IC-MS/MS). We found recovery of iodine to be markedly low when IC-MS/MS was used without an internal standard. Percent recovery was similarly low using 35Cl18O4 as an internal standard for milk and unpredictable when used for urine. 2-Chlorobenzebenzenesulfonate provided accurate recovery of iodine from milk, but overestimated iodine in urine samples by as much as a factor of 2. Percent recovery of iodine from milk and urine using ICP-MS without an internal standard was ∼120%. Use of 115In predicted approximately 60% of known values for both milk and urine samples. 72Ge provided reasonable and consistent percent recovery for iodine in milk samples (∼108%) but resulted in ∼80% recovery of iodine from urine. Use of 129I as an internal standard resulted in excellent recovery of iodine from both milk and urine samples using either IC-MS/MS and ICP-MS.