Study on plutonium isotopic analysis in environmental samples

A method for determination of plutonium isotope ratio in environmental samples was investigated in our laboratory. Plutonium was separated with complex matrix and purified by using TEVA extraction chromatography resin, and determined by MC-ICP-MS. The elution of Pu from TEVA column was tested. Plutonium could be eluted from TEVA column effectively by 0.025mol·l-1 H2C2O4-0.15mol·l-1HNO3 or 10-2 mol·l-1 ascorbic acid-0.2mol·l-1HNO3. For one TEVA column, the decontamination factors of main environmental matrix , such as Na, K, Ca, Al, Fe, Mg, were between 104-105, and the concentration of total cation ions in the final elution was less than 10 μg·g-1, which could be determined by MC-ICP-MS directly. The decontamination of U, Th and Am was also examined, DFU/Pu=1.97x103,DFTh/Pu=7.19x104,DFAm/Pu=1.52x104 were obtained. The chemical recovery of plutonium during the whole process was ∼30%. Because of the tailing of 238U and the formation of 238UH+ while the determination of plutonium isotope by MS, it was not enough for the decontamination of uranium by using one TEVA column. A UTEVA column was needed before the solution loading in the TEVA resin. The decontamination factor of uranium could reach 1.33x106. The detection limit of Pu by MC-ICP-MS was about 2.5 fg·ml-1. The influence of U on the determination of plutonium isotope ratio was also examined. For the solution containing less than 1 ng·g-1 uranium, Aridus sample introduction system was adopted to improve the sensitivity. Meinhard PFA nebulizer was used for solutions of high uranium concentration(1-500 ng·g-1). The results show that, the 238UH+/U+ is about 8.9x10-5 for the high content of uranium, and about 3.6x10-4 for the low content. According to the method mentioned above, two IAEA reference materials, IAEA-368 and SOIL-6 were analyzed. Purified Pu fractions were obtained by using UTEVA+TEVA extraction chromatography; 240Pu/239Pu were determined by MC-ICP-MS. The results were showed in table 1. Ocean sediment IAEA-368 from Mururoa atoll, which was the former nuclear weapon test site of France, had a very small value of 240Pu/239Pu, that is, the plutonium was weapon grade. SOIL-6 was sampled from Ebensee in Upper Austria, the 240Pu/239Pu was consistent with the global fallout value of 0.176±0.014. (author)