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

Summary Studies on the environmental behavior of plutonium in the marine environment require an analytical method with high sensitivity and capability to provide the isotopic composition of 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 atomic ratio of ²⁴⁰Pu/²³⁹Pu in sediment core samples. Using a conical concentric nebulizer and 150-second counting time, we were able to lower the detection limit of Pu down to 0.35 fg. The mass discrimination effect was evaluated using a mixed Pu isotope standard solution with certified a 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 atomic 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 atomic ratio in sediment cores in the Sea of Okhotsk and the NW Pacific. It was found that the Bikini close-in fallout Pu could be transported as far as the Sea of Okhotsk. The results 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.