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 ²³⁹Pu, ²⁴⁰Pu and ²⁴⁰Pu/²³⁹Pu ratios in small amounts of environmental samples. To reduce the interference effect of ²³⁸U, a microconcentric nebulizer, MCN-6000^®, was used as a sample introduction method, which showed a 5-fold lower UH⁺/U⁺ ratio than the conventional pneumatic nebulizer. ²³⁸U separation and preconcentration of Pu were performed sequentially on two extraction resins, Sr-Spec^® and TEVA-Spec^®, 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 ²³⁹Pu, ²⁴⁰Pu and ²⁴²Pu by this method were about 4 fg ml⁻¹ (9.2 µBq ml⁻¹), 3 fg ml⁻¹ (25 µBq ml⁻¹) and 6 fg ml⁻¹ (0.87 µBq ml⁻¹), respectively. These represent total amounts of 9.6, 7.2 and 14 fg for ²³⁹Pu, ²⁴⁰Pu and ²⁴²Pu in a final sample volume. After digestion of the sample, the ²³⁸U 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).