Determination of radioactive cesium isotope ratios by triple quadrupole ICP-MS and its application to rainwater following the Fukushima Daiichi Nuclear Power Plant accident

Abstract

A method was developed for the determination of ¹³⁴Cs/¹³⁷Cs and ¹³⁵Cs/¹³⁷Cs in rainwater samples using a triple quadrupole ICP-MS, with the objective of investigating radioactive cesium isotope ratios released by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. The high background caused by Ba ions and Xe impurities in argon plasma gas has made it difficult to carry out determination of the ¹³⁴Cs/¹³⁷Cs and ¹³⁵Cs/¹³⁷Cs ratios on conventional ICP-MS instruments without prior chemical separation. In this study, nitrous oxide was used as the reaction gas for reducing the isobaric interference by ¹³⁴Ba, ¹³⁵Ba, ¹³⁷Ba and ¹³⁴Xe, and an additional quadrupole in front of the reaction cell was shown to effectively remove Sn and Sb that could have produced possible interference such as SbO and SnO by the N₂O reaction. The detection limit for ¹³⁴Cs, ¹³⁵Cs and ¹³⁷Cs in solution containing Ba was successfully improved. In order to confirm the applicability of this method to the measurement of ¹³⁴Cs/¹³⁷Cs and ¹³⁵Cs/¹³⁷Cs ratios of rainwater, four samples were collected for analysis from 40–200 km away from the FDNPP. The measured ¹³⁴Cs/¹³⁷Cs ratios in the samples are consistent with the values determined by Ge semiconductor analysis to within the analytical error, suggesting that the developed method can provide reliable isotopic data without any correction of the mass-discrimination effect. No variation was found in the ¹³⁵Cs/¹³⁷Cs ratios of the four samples, suggesting either the same contamination source or a uniformly consistent mixing ratio between contamination sources. The measured ¹³⁵Cs/¹³⁷Cs ratios in the samples are different from global fall-out values and from that of Chernobyl. This result indicates that the value of the rainwater samples can be used as a radiocesium tracer in the environment.