Uranium isotope analysis by MC-ICP-MS in sub-ng sized samples

Abstract

This study describes the multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) method for the determination of n(²³³U)/n(²³⁸U), n(²³⁴U)/n(²³⁸U), n(²³⁵U)/n(²³⁸U), and n(²³⁶U)/n(²³⁸U) isotope ratios in purified uranium solutions. The used Neptune Plus™ MC-ICP-MS is equipped with the so-called “Jet Interface”, which comprises a high capacity interface pump and specially designed sample and skimmer cones. Together with a high-efficiency sample introduction system (Aridus II™, Cetac Technologies, NE, USA), absolute U registration efficiencies of typically 1.5% to 2.5% can be achieved under routine operation. A mass bias correction, background/memory effect correction, hydride formation correction, yield and dead time correction of the multi-collector ion counters, and peak-tailing correction are performed for isotope ratio measurements. The effect of potential molecular interferences is assessed by the screening of concentrations of interfering elements in uranium fractions. The new MC-ICP-MS makes possible the determination of n(²³⁵U)/n(²³⁸U) with relative expanded uncertainties (k = 2) lower than 0.2% and allows achieving detection limits of 1 × 10⁻⁷ for n(²³³U)/n(²³⁸U) and 6 × 10⁻⁸ for n(²³⁶U)/n(²³⁸U) in solution aliquots containing 0.4 ng to 0.6 ng uranium. The relative expanded uncertainties (k = 2) of n(²³⁴U)/n(²³⁸U) and n(²³⁶U)/n(²³⁸U) ratios (>10⁻⁵) are below 1% and the absolute accuracy for minor isotope ratios n(²³⁴U)/n(²³⁸U) and n(²³⁶U)/n(²³⁸U) measured in CRMs is below the 1 ppm level. The developed method has been validated by comparative measurements with the previously used thermal ionisation mass spectrometry (TIMS) method by analysing test samples from an inter-laboratory comparison exercise. The MC-ICP-MS method has been applied for analysis of U isotope ratios in U fractions obtained from cotton swipe samples.