Measurement of the Isotopic Composition of Uranium in Geological Samples by MC-ICP-MS Using a TOPO-Based Extraction Chromatographic Protocol

The precise determination of uranium (U) isotope ratios in geological samples is crucial for tracing redox processes and reconstructing paleoenvironments. However, the widespread application of U isotopes is often limited by the difficulty of obtaining a 233 U– 236 U double spike for correction of instrumental isotopic fractionation. In this study, a single-column purification procedure was used to separate U from geological samples using a self-made tri- n -octylphosphine oxide (TOPO) extraction resin. This procedure takes advantage of the high adsorption selectivity of U on the TOPO resin, achieving low procedural blanks, quantitative U recovery (100.1 ± 1.6%), and effective separation from major and matrix elements (e.g., Ti, Fe, Zn, Pb, Zr, and Th). A combined standard-sample bracketing and internal normalization (C-SSBIN) method, using NIST SRM 981 Pb as the normalization element, was applied to correct instrumental isotopic fractionation during U isotope measurements by MC-ICP-MS. Repeated measurements of the standard solutions GBW04428 and HPS U yielded a long-term external reproducibility (2SD) for δ 238 U of ±0.064‰, comparable to that achieved by double spike methods in previous studies. Repeated analyses of a suite of geological and environmental reference materials (BCR-2, GSR-1, GSR-3, SGR-1b, GSR-12, NOD-P-1, JCP-1, and seawater) yielded δ 238 U values in excellent agreement with published data, demonstrating the accuracy and robustness of the protocol. Overall, this protocol enables the quantitative recovery of high-purity U and high-precision U isotope determination, providing an alternative to the double spike technique and promoting its wider application in geochemistry.