Validation of isotopic analysis of depleted, natural and enriched uranium using high resolution ICP-OES

Abstract

Analytical procedures for the reliable determination of uranium isotopes employing a commercially available high resolution (HR)-ICP-OES installed in a glove box were developed and validated. To this end, nine commonly used U emission lines were tested for their potential to provide isotopic splitting of the U signal, thus providing isotopic information. Among all emission lines tested, the wavelength region around 424.4 nm offered the largest spread between individual U signals, i.e.²³³U, ²³⁵U, ²³⁶U, and ²³⁸U. This study focused specifically on the reliable determination of the abundances of ²³⁵U at 424.412 nm and ²³⁸U at 424.437 nm in samples containing depleted, natural and slightly enriched U, but also included specimens with rather high (up to ∼90%) ²³⁵U enrichments. Appropriate selection of the spectral background and accurate positioning of the peaks proved essential for obtaining reproducible U isotope ratios. The accuracy of the developed methodology was confirmed by the analysis of eight certified isotopic reference materials as well as eight reference samples that have been characterised in an accredited laboratory in-house using TIMS. In addition, this study highlights the potential of the developed HR-ICP-OES procedure for identifying nuclear sources of various U specimens via the analysis of ²³³U (424.398 nm) and ²³⁶U (424.423 nm). In contrast to the commonly used techniques, the methodology applied in this study does neither require the separation of U from the matrix prior to analysis nor the regular analysis of a reference sample with a known isotopic composition to correct for bias effects as it is the case for more sophisticated and precise mass spectrometric measurements. Nevertheless, HR-ICP-OES provides fit for purpose U isotopic information with reasonable accuracy (typically <1.5%) and precision (∼1%) within a few minutes. As such, HR-ICP-OES can be employed as a reliable, fast screening tool to identify the U isotopic composition and as such, complements the more laborious TIMS analysis routinely employed for this purpose.