Determination of Pu isotopes in sediment cores in the Sea of Okhotsk and the NW Pacific by sector field ICP-MS

Studies on the environmental behavior of plutonium in the marine environment require an analytical method with high sensitivity and capability to provide isotopic compositions for Pu in marine samples. In this work, as part of our on-going project on Pu environmental behavior in the Pacific Ocean, a sector field ICP-MS method combined with an off-line anion-exchange chromatography system was optimized for the determination of Pu and its atom ratio of 240Pu/239Pu in sediment core samples. Using a conikal concentric nebulizer and 150 s counting time, we were able to lower the detection limit for Pu down to 0.35 fg. The mass discrimination effect was evaluated using a mixed Pu isotope standard solution with certified 240Pu/239Pu isotope ratio (NBS-947). The overall performance of the analytical method was validated by the determination of Pu and its isotope composition in an ocean sediment reference material (IAEA-368). Both the 239+240Pu activity and 240Pu/239Pu atom ratio were found to be in good agreement with the certified and/or literature values. As an important application, we employed the analytical method to investigate the vertical profiles of 239+240Pu activity and 240Pu/239Pu atom ratio in sediment cores in the Sea of Okhotsk and the NW Pacific. Significantly, it was found that the Bikini close-in fallout Pu could be transported as far as the Sea of Okhotsk. The results obtained provided evidence to support our hypothesis on the oceanic current transportation of Bikini close-in fallout Pu in the NW Pacific and its marginal seas