High-precision determination of ²³⁴U/²³⁸U activity ratios in natural waters and carbonates by ICPMS

A method has been developed for precise measurement of ²³⁴U/²³⁸U activity ratios in natural waters and carbonates using quadrupole inductively coupled plasma mass spectrometry. A recovery of 80-85% of seawater U is achieved by Fe(III) coprecipitation followed by extraction chromatography with a supported dipentyl pentane phosphonate material; 90-95% of U is recovered from carbonates, which are dissolved in HNO₃ and subjected to the same extraction chromatographic preparation. Isotopic measurements are made via recirculating pneumatic nebulization of small volumes of solutions containing 0.5-5 mg/L U. ²³⁴U/²³⁵U is measured as a proxy for determination of ²³⁴U/²³⁸U; iridium is added to sample solutions and the ion ratio ¹⁹¹Ir⁴⁰Ar⁺/¹⁹³Ir⁴⁰Ar⁺ is measured for internal mass discrimination correction. ²³⁴U/²³⁸U activity ratios in the range 1.143-1.154 are observed for 13 seawater and contemporary corals, in agreement with the established marine ²³⁴U/²³⁸U activity ratio. For sample sizes of 5-25 μg U, ICPMS uncertainties of +/- 0.2-0.5% relative, 2σ standard error, approach those obtained for < 0.1 μg U by thermal ionization mass spectrometry. Measurements of ²³⁴U/²³⁸U activity ratios in bottled waters, Lake Erie surface waters, mollusk fossils, and fertilizers are also demonstrated.