Effect of alkali metals on the accuracy of isotope ratio measurement of uranium by ICP-MS

Abstract

The effect of alkali metals on the accuracy of uranium isotope ratio measurements by inductively-coupled plasma sector field mass spectrometry (ICP-SFMS) was investigated. The isotope ratio measurements were performed on the isotopic reference materials, NBS SRM U-015 and U-350, in solutions containing various amounts of any one of Na, K and Rb. The mass bias of the instrument was corrected using an isotopic standard solution without alkali metals. The observed ²³⁵U/²³⁸U isotope ratios for SRM U-015 began to increase at around 1 μg g⁻¹ for K and Rb, and at around 10 μg g⁻¹ for Na. The deviations from the certified value at the concentration of 300 μg g⁻¹ were evaluated to be 0.8%, 1.9% and 2.5% for Na, K and Rb, respectively. For SRM U-350, the ²³⁴U/²³⁸U, ²³⁵U/²³⁸U and ²³⁶U/²³⁸U isotope ratios were measured in various K concentrations. The deviation per unit mass at 300 μg g⁻¹ K was evaluated as 0.56%.