Determination of ²³⁹Pu and ²⁴⁰Pu for Radioactive Liquid Waste by High Resolution Inductively Coupled Plasma Mass Spectrometry

The accurate determination of concentration of ²³⁹Pu and ²⁴⁰Pu is very important not only for estimating the source of radioactive liquid waste, but also for selecting the subsequent disposal strategy. In this paper, a direct method using high resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) to measure the concentration of ²³⁹Pu and ²⁴⁰Pu in radioactive liquid waste was proposed. The main problem in the determination of ²³⁹Pu and ²⁴⁰Pu by HR-ICP-MS is the isobaric interferences caused by uranium (²³⁸U¹H⁺ and ²³⁸U¹H₂⁺) which always exists in the radioactive waste liquid with a higher concentration. This work provided a strategy to eliminate these isobaric interferences without separating the uranium from the radioactive liquid waste. Through the measurement of a series of uranium standard solutions with different concentrations by HR-ICP-MS, it was found that the yields of ²³⁸U¹H⁺ and ²³⁸U¹H₂⁺ had positive correlation with the concentration of uranium. Besides, the ratios were constant: r(²³⁸U¹H⁺/²³⁸U)=4.85×10^-5, r(²³⁸U¹H₂⁺/²³⁸U)=2.99×10^-6. Moreover, after using the yields of ²³⁸U¹H⁺ and ²³⁸U¹H₂⁺ to deduct the isobaric interferences caused by uranium, the measurement results of the 0.1 μg/L plutonium standard solution under different uranium matrix interference were roughly equal, indicating that the method to eliminate the isobaric interferences caused by uranium was reasonable and feasible. Furthermore, in order to get the mass concentration-counting rate standard curves for ²³⁹Pu and ²⁴⁰Pu, a series of the plutonium standard solution at different concentration was also measured. After deducting the isobaric interferences, the mass concentration of ²³⁹Pu and ²⁴⁰Pu in the samples of radioactive liquid waste could be calculated by the standard curve equation, and then the activity concentration of ²³⁹Pu and ²⁴⁰Pu in different samples were also calculated. In addition, the results were also compared with those obtained from the liquid scintillation counting method. The deviations between the results from HR-ICP-MS and the liquid scintillation method are less than 4%, and the relative expansion uncertainty is 4.6% (k=2). The proposed method was proved as very effective to overcome the isobaric interferences caused by ²³⁸U¹H⁺ and ²³⁸U¹H₂⁺ with minimal sample preparation (e.g., no need to separate the uranium), which make it a fast, sensitive and accurate method to analyze ²³⁹Pu and ²⁴⁰Pu in radioactive waste liquid with abundant uranium existing.