Evaluating SF-ICP-MS and reaction cell ICP=MS for the determination of radioactive cesium isotopes

After the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in 2011, intensive studies on the distribution of released fission products, in particular 134Cs, 136Cs and 137Cs, in the environment have been conducted. Radioactive Cs isotope ratio, especially, 134Cs/137Cs has been found wide application in the source identification of released radioactive Cs in the environment. However, due to the short half-life of 134Cs (t1/2 = 2.06 y), the determination of 134Cs will be practically difficult in the near future. Due to the long half-life of 2x106 y and low energy beta-decay, the analysis of 135Cs, a high yield fission product up to 6.535% from thermal neutron fission of 235U, has been a great challenge in the field of environmental radioactivity study as suitable radiometric analytical method is not available. If 135Cs could be determined, the use of 135Cs/137Cs as a powerful geochemical tracer to study the environmental behavior and fate of the FDNPP-sourced radioactive Cs can be expected. In this work, we report our recent study on the evaluation of analytical potential of ICP-MS for the determination of radioactive Cs isotopes. The most troublesome problem for ICP-MS analysis of 135Cs and 137Cs is the isobaric interferences from Ba isotopes. Furthermore, the Mo in the environmental samples could result in the polyatomic interferences, 95Mo40Ar+ and 97Mo40Ar+ for 135Cs and 137Cs analysis. In addition, the high concentration of stable Cs (133Cs) in environmental samples will also interfere with 135Cs analysis due to the peak tailing effect. To overcome these difficulties, we made investigations using SF-ICP-MS and reaction cell quadrupole ICP-MS for 135Cs and 137Cs analysis. Our study focuses on: (1) estimation of instrumental detection limit and abundance sensitivity for Cs analysis; (2) possibility to eliminate MoAr+ polyatomic interferences; and (3) suppression of Ba isobaric interferences using cool plasma technique and Cs-Ba separation using collision-reaction cell technique and ion-exchange chromatography. Detailed results will be presented. This work was supported by the Sumitomo Environmental Foundation and partly supported by the Agency for Natural Resources and Energy, the Ministry of Economy, Trade and Industry, Japan.7th International Symposium on Radiation Safety and Detection Technolog