Iodine-129 Analysis of NTS Near-Field Groundwater Samples on the Multi-Collector ICP-MS

Abstract

Iodine was chemically separated from NTS near-field groundwater samples and analyzed for its {sup 129}I/{sup 129}I ratio on a Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICPMS). The measured ratios were then compared to {sup 129}I/{sup 129}I ratios for identical samples run on the Accelerator Mass Spectrometer (AMS). The MC-ICPMS results in some cases differed from the AMS values by more than an order of magnitude. The disparity in the results is due to isobaric interferences at mass 129 from polyatomic species and {sup 129}Xe in the MC-ICPMS plasma gas. It is anticipated that the interferences can be largely eliminated by (1) improving the molybdenum separation chemistry using a {sup 92}Mo-spike precipitation method, and (2) introducing O{sub 2} to the plasma gas to reduce the {sup 129}Xe interference. The MC-ICPMS detection limit for {sup 129}I/{sup 127}I (measured on reference standards) is on the order of 1E-6 or slightly lower. Most near-field groundwater samples have {sup 129}I/{sup 127}I ratios >1E-6, and should be measurable on the MC-ICPMS once the isobaric interference issues are resolved.