Precise isotope ratio measurements for uranium, thorium and plutonium by quadrupole-based inductively coupled plasma mass spectrometry

Precise long-term measurements of uranium and thorium isotope ratios was carried out in 1 mu g/L solutions using a quadrupole inductively coupled plasma mass spectrometer (ICP-QMS). The isotopic ratios of uranium (U-235/Pu-238 = 1, 0.02 and 0.00725) were determined using a crossflow nebulizer (CFN, at solution uptake rate of 1 mL/min) and a low-flow microconcentric nebulizer (MCN, at solution uptake rate of 0.2 mL/min) over 30 h. For 1 mu g/L uranium solution (U-235/U-238 = 1) relative external standard deviations (RESDs) of 0.05% and 0.044% using CFN and MCN, respectively, can be achieved. Additional short term isotope ratio measurements using a direct injection high-efficiency nebulizer (DIHEN) of 1 mu g/L uranium solution (U-235/U-238 = 1) at a solution uptake rate of 0.1 mL/min yielded an RSD of 0.06-0.08%. The sensitivity of solution introduction by DIHEN for uranium, thorium and plutonium (145 MHz/ppm, 150 MHz/ppm and 177 MHz/ppm, respectively) increased significantly compared to CFN and MCN and the solution uptake rate can be reduced to 1 mu L/min in DIHEN-ICP-MS. Isotope ratio measurements at an ultralow concentration level (e.g. determination of Pu-240/Pu-239 isotope ratio in a 10 ng/L Pu waste solution) were carried out for the characterization of radioactive waste and environmental samples.