Evaluation of iodine speciation and ¹²⁹I/¹²⁷I ratios at low concentrations in environmental samples using IC-ICP-MS

Subterranean radioiodine contamination at the Hanford Site in Washington State is believed to be present as iodide, iodate, and organo-I species, with iodate being the predominant form. Because these species have different sediment-sorption characteristics, understanding their distribution is important for developing an accurate understanding of iodine migration in the subsurface. Herein, we report a novel, rapid technique for simultaneous iodine speciation (iodide/iodate) and isotopic ratio (¹²⁹I/¹²⁷I) measurements using ion chromatography (IC) joined with collision/reaction cell inductively coupled plasma mass spectrometry (ICP-MS), collectively referred to as IC-ICP-MS. This approach employs online dynamically regenerated eluent suppression post chromatographic separation of the samples and collision cell technology, with pure oxygen as a collision gas for the active suppression of ¹²⁹Xe (which naturally exists in the argon supplied to the ICP source) to rapidly (< 15 min) achieve precise and reproducible results. Speciated standard reference materials yielded detection limits for ¹²⁷I of approximately 23.8 ng/L for iodate and 24.3 ng/L for iodide, and for ¹²⁹I of approximately 1.81 ng/L for iodate and 2.62 ng/L for iodide. The method was demonstrated by analyzing groundwater samples from six wells from ¹²⁹I-contaminated regions of the Hanford Site; iodate was the primary species for both ¹²⁷I and ¹²⁹I. Small quantities of ¹²⁷I-iodide were also detected in most of the samples, but all ¹²⁹I-iodide results were below the detection limit. An interference from molybdenum prevented the estimation of organo-iodine concentrations but did not affect the iodate and iodide results. This new analytical capability will enable rapid, simultaneous characterization of speciated inorganic iodine in vadose zone sediments and groundwater samples at levels below the US federal drinking water standard for ¹²⁹I of 1 pCi/L (~ 5.6 ng/L).