A new method for isotope ratio measurement of uranium in trace amount by thermal ionization mass spectrometry: The continuous heating method

Abstract

A new method for isotope ratio measurement with thermal ionization mass spectrometry (TIMS), “continuous heating method”, was developed in order to determine an accurate isotope ratio of uranium ranging from sub-picograms to several dozen picograms. In this method, signals were measured during evaporation of an entire sample in the same way as in the “total evaporation method”; however, part of the higher detected signals was used for calculation of the isotope ratio. The continuous heating method can measure the ratios with high reproducibility regardless of the sample amount and the skill of operators because procedures of measurement and data calculation were standardized, enabling accurate correction of the mass discrimination effect. By using the optimized procedures, the relative standard deviation (2 σ) of 235U/ 238U ratio in CRM U350 particle (35% 235U enrichment) with a diameter of about 1 μm was less than 2%. When 235U/ 238U ratios in IRMM 184 (natural uranium) solution samples of uranium of 0.4, 4 and 44 pg measured by the continuous heating method were compared with those measured by the conventional method and the total evaporation method, which were well-known methods of TIMS, there was no difference in the case of measurement of a larger amount of samples. However, the continuous heating method showed the most accurate result for the smallest amount of samples.