Measurement of the sulphur isotope ratio (34S/32S) in uranium ore concentrates (yellow cakes) for origin assessment

A novel method has been developed for the measurement of the S-34/S-32 isotope ratio in uranium ore concentrate (yellow cake) samples for origin assessment in nuclear forensics. The leachable sulphate is separated and pre-concentrated by anion exchange separation followed by the S-34/S-32 ratio measurement by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The effect of sample composition on the accuracy was investigated and optimized. Matching of the sample to the bracketing standard was necessary to obtain accurate results. The method was validated by the measurement of standard reference materials (IAEA-S-2, IAEA-S-3 and IAEA-S-4) and the delta S-34 value could be determined with an uncertainty between 0.45 parts per thousand and 1.9 parts per thousand expressed with a coverage factor of 2. The method was then applied for the analysis of uranium ore concentrates of world-wide origin. In the studied materials distinct S-34/S-32 isotope ratios could be observed, which can be a promising signature for the nuclear forensic investigations to identify the source of unknown nuclear materials.