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

Neptunium-237, owing to its long half-life (t(1/2) = 2.14 x 10(6) year) and similar conservatism to Cs-137, has the potential to replace Cs-137 for water mass circulation studies on decades and even longer time scales. A new method for the determination of Cs-137, Np-237, and Pu isotopes in seawater samples was proposed to solve the difficulty of Np-237 analysis in seawater. The developed method includes the separation technique of ammonium phosphomolybdate (AMP) adsorption for Cs-137 and anion exchange chromatography for Np-237 and Pu, a measurement technique of gamma spectrometry for Cs-137 and SF-ICP-MS for Np-237 and Pu isotopes. Pu-242 as a pseudo isotope dilution tracer for Np, the negligible chemical fractionation between Np-237 and Pu-242 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 Np-237, Pu-239, and Pu-240 were 1.3 x 10(-3), 0.065, and 0.15 mu Bq 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 Np-237 activity concentration in Pacific Ocean seawater sampled near the station site, and we obtained the value of 0.122-0.154 mu Bq L-1.