Rapid determination of Pu isotopes and atom ratios in small amounts of environmental samples by an on-line sample pre-treatment system and isotope dilution high resolution inductively coupled plasma mass spectrometry

Abstract

Isotope dilution (ID) high resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) combined with an on-line sample pre-treatment method was applied to the trace determination of Pu-239, Pu-240 and Pu-240/Pu-239 ratios in small amounts of environmental samples. To reduce the interference effect of U-238, a microconcentric nebulizer, MCN-6000(R), was used as a sample introduction method, which showed a 5-fold lower UH+/U+ ratio than the conventional pneumatic nebulizer. U-238 separation and preconcentration of Pu were performed sequentially on two extraction resins, Sr-Spec(R) and TEVA-Spec(R), which were sequentially combined in PrepLab(TM), an integrated liquid handing system. By reducing the final sample volume to 2.4 ml, injecting it into the HR-ICP-MS system and employing the MCN-6000, the analysis of Pu isotopes is feasible in 1 g of soil and sediment having the environmental level of Pu. The detection limits for Pu-239, Pu-240 and Pu-242 by this method were about 4 fg ml(-1) (9.2 mu Bq ml(-1)), 3 fg ml(-1) (25 mu Bq ml(-1)) and 6 fg ml(-1) (0.87 mu Bq ml(-1)), respectively. These represent total amounts of 9.6, 7.2 and 14 fg for Pu-239, Pu-240 and Pu-242 in a final sample volume. After digestion of the sample, the U-238 elimination and Pu preconcentration procedures were fully automated. Sample preparation is therefore far simpler and the full sample analysis process can be achieved within about 5 h. The analytical reliability was confirmed using several certified or standard reference materials (NIST SRM-4350b, IAEA-6, IAEA-300, IAEA-367, IAEA-368, IAEA-375).