Experimental evaluation of data integration methods for isotopic analysis of uranium with ultra-trace levels using TIMS

Isotopic analysis of ultra-trace levels of uranium (1 ng, 100 pg, 30 pg, and 5 pg) was performed using thermal ionization mass spectrometry (TIMS). The data integration procedure was optimized through experimental evaluation of three data integration methods. In Method I, the isotope ratios of uranium were calculated using pre-integrated ion signal intensities. Statistical integration of the pre-calculated uranium isotope ratios for each data set was performed with and without applying weighting for Methods III and II, respectively. Methods I and III efficiently minimized the deviation of the uranium isotope ratios caused by mass fractionation. The integration range was required to be reduced to 25–100% of the maximum intensity for data sets containing samples with small weights (i.e., 5 pg) for the best accuracy. Method III, with an integration range over 25%, was recommended for individual microparticle analysis by TIMS due to its accuracy, precision, and possibility of uncertainty estimation. The excellent agreement of the isotope ratios of uranium in microparticles (approximately 2–3 μm in diameter) with the certified values verified the applicability of the optimized procedure to the TIMS isotopic analysis of uranium for nuclear safeguards purposes.