Rapid Method To Determine 137 Cs, 237 Np, and Pu Isotopes in Seawater by SF-ICP-MS.

Neptunium-237, owing to its long half-life ( t 1/2 = 2.14 × 10 6 year) and similar conservatism to 137 Cs, has the potential to replace 137 Cs for water mass circulation studies on decades and even longer time scales. A new method for the determination of 137 Cs, 237 Np, and Pu isotopes in seawater samples was proposed to solve the difficulty of 237 Np analysis in seawater. The developed method includes the separation technique of ammonium phosphomolybdate (AMP) adsorption for 137 Cs and anion exchange chromatography for 237 Np and Pu, a measurement technique of gamma spectrometry for 137 Cs and SF-ICP-MS for 237 Np and Pu isotopes. 242 Pu as a pseudo isotope dilution tracer for Np, the negligible chemical fractionation between 237 Np and 242 Pu of 1.02 ± 0.06 ( k = 2) was obtained by implementing sophisticated control of the redox system and chromatographic elution optimization. The analytical results for the International Atomic Energy Agency Certified Reference Materials (IAEA-443) agreed with the reference values, showing chemical yields of 65-88%, U decontamination factor above 10 6 level, and improved sample throughput (5 days for 12 samples). Meanwhile, the lower method detection limits (MDLs) of 237 Np, 239 Pu, and 240 Pu were 1.3 × 10 -3 , 0.065, and 0.15 μBq L -1 for 15 L seawater, respectively. Results obtained by the developed method can be used to evaluate the impact on the marine ecological system of the planned marine discharge of Fukushima decontaminated wastewater. Working toward that purpose, we are the first to report the 237 Np activity concentration in Pacific Ocean seawater sampled near the station site, and we obtained the value of 0.122-0.154 μBq L -1 .