Ultra-trace plutonium determination in small volume seawater by sector field inductively coupled plasma mass spectrometry with application to Fukushima seawater samples

Long-term monitoring of Pu isotopes in seawater is required for assessing Pu contamination in the marine environment from the Fukushima Dai-ichi Nuclear Power Plant accident. In this study, we established an accurate and precise analytical method based on anion-exchange chromatography and SF-ICP-MS. This method was able to determine Pu isotopes in seawater samples with small volumes (20-60 L). The U decontamination factor was 3 × 107-1 × 108, which provided sufficient removal of interfering U from the seawater samples. The estimated limits of detection for 239Pu and 240Pu were 0.11 fg mL−1 and 0.08 fg mL−1, respectively, which corresponded to 0.01 mBq m−3 for 239Pu and 0.03 mBq m−3 for 240Pu when a 20 L volume of seawater was measured. We achieved good precision (2.9%) and accuracy (0.8%) for measurement of the 240Pu/239Pu atom ratio in the standard Pu solution with a 239Pu concentration of 11 fg mL−1 and 240Pu concentration of 2.7 fg mL−1. Seawater reference materials were used for the method validation and both the 239+240Pu activities and 240Pu/239Pu atom ratios agreed well with the expected values. Surface and bottom seawater samples collected off Fukushima in the western North Pacific since March 2011 were analyzed. Our results suggested that there was no significant variation of the Pu distribution in seawater in the investigated areas compared to the distribution before the accident.