Precise Determinations of 234U/238U Isotope Ratios from Small Sample Sizes by MC-ICPMS for Groundwater Hydrological Study

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.