Sample preparation method for low-level total 129I measurements by ICP-MS

Trace-level (ng/L) measurements of iodine’s isotopic (129I and 127I) and chemical species distributions are needed for an accurate understanding of subsurface radioiodine migration at former plutonium production complexes such as the Hanford and Savannah River sites. A previously developed method that uses ion chromatography coupled to inductively coupled plasma mass spectrometry (ICP-MS) can measure speciated forms of 129I at levels below the drinking water standard (~ 5.6 ng/L), but an interference from molybdenum (Mo) prevents the assay from quantifying the total 129I concentration in many relevant sample matrices. In this work, a preparative cation exchange method was developed to eliminate the Mo interference. The method requires little sample volume, is simple and fast to perform, and was shown to recover 96–99% of 129I in test solutions containing approximately 20 µg/L Mo and 2–4 µg/L of either 129I− or 129IO3−. Method efficacy was further demonstrated by processing 11 groundwater and surface water samples with interfering levels of Mo from the Hanford and Savannah River sites. Total 129I concentrations in the processed samples ranged from approximately 20–1175 ng/L and generally agreed well with values independently obtained by radiochemistry methods. The sample preparation method enables quantification of total 129I by ICP-MS in complex samples and provides a convenient approach for characterizing radioiodine distributions at contaminated nuclear waste sites.