A combined TIMS and ICP-MS study for Th0.5Np0.5O2 thorium neptunium mixed oxide analysis

Abstract

This paper presented thorium and neptunium quantification of a new synthesized powder, a (Th,Np)O2 mixed oxide. Thorium was quantified by Thermal Ionization Mass Spectrometry using Isotope Dilution (ID-TIMS) which is one of the most accurate methodology to determine actinide mass fraction. The (sample-spike) mixture isotope ratio was measured using the TIMS reference method for major isotope ratio determination, i.e. the total evaporation method. An expanded uncertainty below 1% was estimated for the Th mass fraction. The study of the uncertainty sources showed that the main one comes from the spike mass fraction uncertainty. As Np quantification with ID-TIMS is difficult due to the lack of commercially-available spike solution, the Np mass fraction was obtained by ICP-MS measurements using a gravimetric standard addition method combined with an analytical internal standard. The developed method used an element initially present in the sample, here Th, as an internal standard for ICP-MS measurements. This helps reducing the number of manipulation as well as cross contamination, which is an important risk when experiments are performed in glove boxes. Also, equations are simplified. An expected uncertainty about 1.8% was estimated for Np. Finally, the Th–Np mixed oxide was found to have a Th0.4836(50)Np0.5164(50)O2 stoichiometry. This study can be extended to other complicated samples or to the characterization of in-house certified reference material.