Determination of Pu isotopes in urine samples by a pseudo-isotope dilution method using non-isotopic tracer ²³⁷Np

Plutonium (Pu) isotopes are extremely hazardous radioactive materials. Rapid analysis of Pu in urine from contaminated persons is important for radiobiological verification and exposure dose assessment. Since Pu is classified as a nuclear fuel nuclide, the use of ²⁴²Pu tracer for isotope dilution (ID) quantification is commonly controlled and regulated, which can affect timely exposure dose assessment. In this study, a pseudo-isotope dilution (PID) method using a non-isotopic tracer (NIT) ²³⁷Np was developed. The method accuracy was validated using both spiked samples and urine standard reference materials. Uncertainties throughout the experiment, instrument analysis, and activity calculation were characterized by Monte Carlo simulation (MCS). The results showed that the activity concentrations of ²³⁹Pu, ²⁴⁰Pu, and ²³⁷Np quantified by PID were consistent with those measured by the ID method. For spiked samples with theoretical ²³⁹Pu concentrations of 0.028 pg/mL and 0.014 pg/mL, the coefficients of variation (CVs) derived by the PID method were 0.043 and 0.074, respectively, which were comparable to those quantified by the ID method with CVs of 0.042 and 0.045. From the parameter sensitivity analysis, the ICP-MS signal intensities of ²³⁷Np and ²⁴⁰Pu isotopes after standard addition and the ²³⁷Np signal intensity before the standard addition were identified as the key factors affecting the quantification of ²³⁹Pu, ²⁴⁰Pu and ²³⁷Np, respectively. Due to its considerable accuracy, precision and extensive application conditions, the PID method is judged to be a promising tool for radiological assessment of contaminated persons in an internal exposure situation and for providing decision-making information in emergency medical treatments.