地下水分析のための微少量ウランによる高精度²³⁴U/²³⁸U同位体比分析法の確立

抄録

A new analytical technique for the precise determination of ²³⁴U/²³⁸U isotope ratios was developed for low uranium concentration natural samples by MC-ICPMS to detect their natural isotopic variation. A reproducibility in ²³⁴U/²³⁸U ratio measurements of about 0.18% was obtained from approximately 1.6 ng uranium through the correction of the mass discrimination effect with a reference ²³⁵U/²³⁸U ratio without ²³³U–²³⁶U double spike method. The precision and sample amounts required were improved by more than one order of magnitude compared to those achieved by the conventional alpha spectrometry, and the analysis time of less than 10 minutes per sample was shorter than that of TIMS. The validity of our method was confirmed from a coral reference material JCp-1, with 1.1414±0.0025 and 1.1426±0.0025 as the ²³⁴U/²³⁸U activity ratio, which was consistent with previously reported values. This technique was applied to water samples, and a natural ²³⁴U/²³⁸U isotopic variation was observed in two spring water samples with different redox conditions and groundwater ages. The data obtained in this study demonstrated that this technique will be useful for the hydrological study using ²³⁴U/²³⁸U ratios for samples with small uranium contents including groundwater research.