Nanoparticle-Assisted Uranium Extraction and Precise Isotopic Fingerprinting in Groundwater by TIMS

Groundwater isotopic screening has gained increased attention for determining the α activity ratio of ²³⁴U/²³⁸U. This requires efficient uranium preconcentration from aquifer samples and precise measurement of the minor abundant isotope ratio of ²³⁴U/²³⁸U (approximately 55 ppm). Using polydopamine-coated Fe₃O₄ magnetic nanoparticles (PDA@Fe₃O₄-NPs) to extract uranium from aquifer samples reduces analysis time by over 80% compared to conventional preconcentration methods. Thermal Ionization mass spectrometry (TIMS) with carefully selected detector combinations and a novel methodology was employed to measure the ²³⁴U/²³⁸U isotope ratio. The methodology involves sequentially collecting the ion current of ²³⁵U on both a Secondary Electron Multiplier and a Faraday cup. Internal normalization using the invariant ²³⁸U/²³⁵U ratio (∼137.88) is then applied to address isotopic fractionation in the ²³⁴U/²³⁸U ratio measurements, significantly improving the precision and accuracy to better than 0.1%. Efforts were made to assess all possible sources of uncertainty, identifying the primary contributors so as to strengthen the confidence in the proposed correction methodology and to establish traceability to the SI unit system.