Analytical considerations in the determination of uranium isotope ratios in solid uranium materials using laser ablation multi-collector ICP-MS

Abstract

Validated analytical measurement protocols for the fast and accurate determination of the uranium (U) isotopic composition (²³⁴U, ²³⁵U, ²³⁶U, ²³⁸U) of solid nuclear materials were developed employing ns-laser ablation (LA) coupled to multi-collector ICP-MS. The accuracy of the analytical procedure was assured by frequent (n = 65) analysis of a pressed pellet of certified isotopic reference material CRM U-030 (∼3 wt% ²³⁵U). The expanded uncertainty (k = 2) for the n(²³⁵U)/n(²³⁸U) ratio was as low as 0.05%, rising to 0.62% and 1.09% for n(²³⁴U)/n(²³⁸U) and n(²³⁶U)/n(²³⁸U) ratios, respectively. LA-MC-ICP-MS measurements of a pressed pellet of certified isotopic reference material CRM U-020 (∼2 wt% ²³⁵U) before analysis of each sample allowed calculation of the ion counter gains and mass bias correction. Both individual spot analysis and line scan analysis were used to measure n(²³⁴U)/n(²³⁸U), n(²³⁵U)/n(²³⁸U), and n(²³⁶U)/n(²³⁸U) ratios in two low-enriched UO₂ pellets from the fourth Collaborative Materials Exercise (CMX-4), four seized low-enriched UO₂ pellets intercepted from illicit trafficking and one metal sample consisting of depleted U. LA-MC-ICP-MS results of all investigated samples matched well with U isotope ratios obtained by thermal ionisation mass spectrometry (TIMS). This independent confirmation of the LA-MC-ICP-MS results by TIMS underpinned the high quality of generated analytical data. Acquisition of several thousand data points within a couple of minutes during line scan analysis yielded detailed information on the spatial distribution of the U isotopic composition of selected UO₂ pellets, revealing straightforwardly their (in˗)homogeneity on the μm-scale. Calculating skewness and half width of the frequency distributions of the n(²³⁵U)/n(²³⁸U) amount ratio allowed the quantitative assessment of the (in-)homogeneity of the investigated samples. This information allows drawing conclusions on the starting materials used for the production of the pellets. From a nuclear forensics perspective, LA-MC-ICP-MS provides quick, accurate results on the spatial distribution of major and minor U isotopes while preserving the sample i.e. piece of evidence, essentially intact.