Simultaneous determination of ultra-trace level 237Np and Pu isotopes in soil/sediment samples by sector-field ICP-MS with a single-column chromatographic separation

Introduction237Np and Pu isotopes (239Pu and 240Pu) are regarded as highly radiological and biological toxic pollutants in the environment because of their alpha emission, long half-lives and readily enrichment in human bones and livers. These radionuclides were released into the environment by the nuclear weapon tests during the 1950s to 1980s. To assess the distribution and migration of 237Np and Pu isotopes, thus to evaluate their radiation impact in the environment, after nuclear accident, it is important to determine low level 237Np and Pu isotopes in environmental samples. This work presented a determination method of trace level 237Np and 239,240Pu in soil/sediment samples based on anion-exchange resin and SF-ICP-MS.\nMethodIn this method, 0.57 pg of 242Pu was used as recovery tracer for both 237Np and Pu isotopes. A commercial anion-exchange resin AG MP-1M was used for the separation of Np and Pu from U, Th and other interferences. A SF-ICP-MS (Element XR, Thermo Scientific, Geermany) combined with an APEX-Q high efficiency sample introduction system (Elemental Scientific Inc, Omaha, NE, USA) was used for the analysis of 237Np and Pu isotopes. Prior to the analysis by SF-ICP-MS, three procedures were included for the sample preparation, a) HNO3-HF digestion; b) LaF3 and CaF2 co-precipitation and c) chromatographic separation. As the chemical behavior of Np and Pu were affected by their oxidation states, several often used reducing reagents (TiCl3, ascorbic acid+Fe2+, NH4OH•HCl and K2S2O5) were tested and their effects were compared.\nResults and discussionThe chemical fractionation occurring between Np and Pu were evaluated by comparing their recovery. For 20 times of determination of 237Np and 242Pu added in deep soil, the ratio of their recovery, Recovery(237Np)/Recovery(242Pu) was 1.02±0.02, which proved that 242Pu can be used as a tracer for monitoring the recovery of 237Np in this method. For 0.2-4g sediment samples the obtained decontamination factor of 238U was 2.5×105～8.5×106. The 238U concentrations in the final sample solutions were below 5.1 pg/mL and no interference correction was needed for the analysis of 237Np and Pu isotopes. The chemical recovery for 242Pu was 79.6%. The instrument detection limits for 237Np, 239Pu and 240Pu were 0.11 fg/mL, 0.092 fg/mL and 0.14 fg/mL, respectively. The method detection limits for 237Np, 239Pu and 240Pu were 0.26 fg/g, 0.71 fg/g and 0.27 fg/g when 1g of soil/sediment was measured, respectively. To demonstrate the accuracy of our method, 237Np and 239,240Pu concentrations in several standard reference materials were determined. The results showed good consistency with the certificated values and the reported values. The high decontamination factor of 238U, together with the low LODs and the chemical recovery of 79.6% demonstrated the ability of this method for the determination of utral-trace level of Np and Pu isotopes in soil/sediment samples.日本分析化学会第６７年