Modified Total Evaporation Method for Highly Precise Measurement of Uranium Isotopes Ratio

Thermal ionization mass spectrometry (TIMS) is a well-established instrumental technique for accurately analyzing isotopes of elements with low first ionization potential. However, the measurement accuracy is affected due to the existence of the mass fractionation effect. Generally, applying the external standard method or total evaporation method is the most common way to correct the mass fractionation effect. Total evaporation (TE） technology is the most conventional method for uranium major isotope ratio measurement (²³⁵U/²³⁸U) using TIMS. However, due to the strong peak tailing of the major isotopes and other factors, the measurement precision and accuracy of minor isotope ratios (²³⁴U/²³⁸U and ²³⁶U/²³⁸U) are relatively low. In this study, the modified total evaporation (MTE) method for highly precise measurement of uranium isotopes ratios by TIMS was developed based on the research of dynamic heating procedures, dynamic and static receiving procedures, peak tailing correction, efficiency calibration of collectors and mass discrimination calibration. The accuracy and reliability of the MTE method were verified with different reference materials. The experiment results showed that the yield of uranium oxide should be monitored and the related parameters should be precisely controlled to reduce the yield of uranium oxidation during measurement. This action could guarantee the effectiveness of the MTE measurement and improve the uranium samples utilization. The initial strength of ¹⁸⁷Re⁺ played an important role in the ionization efficiency of uranium and the yield of uranium oxide. The optimal signal of ¹⁸⁷Re⁺ should be set in the range of 0.17-0.22 V. When uranium samples with 2% ²³⁵U isotopic abundance were measured by the MTE method using TIMS, the measurement precisions of ²³⁵U/²³⁸U, ²³⁴U/²³⁸U and ²³⁶U/²³⁸U were 0.024%, 0.06%, and 0.19%, and the bias between the measured value and reference value were 0.025%, 0.19%, and 0.38%, respectively. Compared to the TE method, the MTE method showed the same precision for the ²³⁵U/²³⁸U ratio measurement. As for the MTE measurement of ²³⁴U/²³⁸U and ²³⁶U/²³⁸U ratios, the major uranium isotopes (²³⁵U and ²³⁸U) were used as an internal standard to overcome the mass fractionation effect. When the tailing effects were eliminated, the measurement precisions of ²³⁴U/²³⁸U and ²³⁶U/²³⁸U ratios were improved by 5 to 10 times. As for sample amount, the TE method generally requires the uranium amount of less than 1 μg, while 5 μg is required for the MTE method.