Determination of ultratrace 129I in soil samples by Triple Quadrupole ICP-MS and its application to Fukushima soil samples

Abstract

A method was developed for the determination of ¹²⁹I in soil samples using a Triple Quadrupole ICP-MS, with the objective of investigating radioiodine released by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. The determination of ¹²⁹I by ICP-MS is capable of providing a high sample throughput compared to other methods. Nonetheless, the high background caused by ¹²⁹Xe impurities in argon plasma gas and polyatomic ions such as ¹²⁷IH₂ ⁺ and ¹²⁷ID⁺ has made it difficult to carry out determinations of this isotope by conventional ICP-MS instruments. In this study, oxygen was used as a reaction gas for reducing the background intensity of m/z 129, principally by ¹²⁹Xe. The contribution of polyatomic ions such as ¹²⁷IH₂ ⁺ and ¹²⁷ID⁺ could be effectively corrected for by assuming a production ratio of 5 × 10⁻⁹. The detection limit for ¹²⁹I in solution was successfully improved. The measured ¹²⁹I/¹²⁷I ratios in NIST SRM 3231 Level II standard solution are consistent with the certified value of 0.981 × 10⁻⁸ within the analytical error, suggesting the reliability of the method to the measurement of ¹²⁹I/¹²⁷I. In order to confirm the applicability of this method to the measurement of ¹²⁹I/¹²⁷I ratios of Fukushima soils, samples were collected for analysis from 5–60 km away from the FDNPP. Prior to the ICP-MS measurement iodine fractions were separated from soil by pyrohydrolysis and were purified by solvent extraction. The ¹²⁹I/¹²⁷I ratios in the samples are consistent with values determined by accelerator mass spectrometry (AMS) within the analytical error. This method provides a powerful tool for the investigation of radioiodine contamination in Fukushima and elsewhere.