Total evaporation method for uranium isotope-amount ratio measurements

Total evaporation (TE) is an analysis technique for the measurement of uranium isotopic abundance ratios using thermal ionization mass spectrometry (TIMS). A small mass dependent bias observed in this analytical technique is determined by an external correction factor using well characterized standards (most often certified reference materials, CRMs). The technique had been demonstrated to be highly precise and accurate for major isotope-amount ratio measurements of uranium and plutonium. We compare the performance of the TE analytical technique for uranium isotope ratio measurements on two TIMS instruments (TRITON and MAT261) using well characterized CRMs from NBL and investigate the dependence of the instrumental mass bias on the amount of sample analyzed. It is concluded that the mass bias during a TIMS uranium isotopic analysis by TE is independent of the amount of material analyzed. Unlike the major ratio, minor isotope ratio measurements by TE are biased high due to peak-tailing from the major isotopes. The biases in the minor isotope ratio data using TE are evaluated using well characterized NBL CRMs. Uranium isotopic measurements on two TIMS instruments using total evaporation technique are compared using well characterized CRMs. The dependence of the mass fractionation on the amount of sample is investigated.