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

A method was developed for the determination of Cs-134/Cs-137 and Cs-135/Cs-137 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 Cs-134/Cs-137 and Cs-135/Cs-137 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 Ba-134, Ba-135, Ba-137 and Xe-134, 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 Cs-134, Cs-135 and Cs-137 in solution containing Ba was successfully improved. In order to confirm the applicability of this method to the measurement of Cs-134/Cs-137 and Cs-135/Cs-137 ratios of rainwater, four samples were collected for analysis from 40-200 km away from the FDNPP. The measured Cs-134/Cs-137 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 Cs-135/Cs-137 ratios of the four samples, suggesting either the same contamination source or a uniformly consistent mixing ratio between contamination sources. The measured Cs-135/Cs-137 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.