Uranium isotopes measurement is of increasing interest for environmental sciences, nuclear safeguards, and nuclear forensics. Recently, several studies focused on the minor isotopes of uranium in the environment. In fact, minor isotope as ²³⁶U is of prime interest for the determination of uranium sources investigation due to the important variations of the ²³⁶U/²³⁸U ratio regarding uranium origin. The isotopic variations of the ²³⁶U/²³⁸U isotope ratio between the geochemical background (10^-14 – 10^-12), the uranium ore (10^-12 – 10^-10) and the global fallout (10^-9 – 10^-6), make this ratio a relevant fingerprint of uranium sources in the environment.
However, detection of ²³⁶U in environmental samples is quite challenging due to its extremely low abundance and for this reason specific instrumentation disposing high sensitivity and high-resolution capabilities is required. The latest generation of inductively coupled plasma tandem mass spectrometry (ICP-MS/MS), recently commercialized, has improved performance, including better ion transmissions and an extended dynamic range of mass detection. This allows the measurement of isotopes up to mass to charge ratio of 280. This is a great improvement as uranium ions could now be measured as UO₂⁺ rather than UO⁺ which shows important advantages in terms of sensitivity and hydride formation. In this work we explored the capabilities of the new model of ICP-MS/MS, Agilent 8900, in terms of hydride formation rate and the lowest achievable ²³⁶U/²³⁸U isotope ratio in UO₂⁺ mode. To this end, measurements of certified reference materials and environmental samples were performed.
These improvements as well as the coupling with an efficient desolvation module (APEX Ω) and the addition of oxygen in the collision/reaction cell allowed an important sensitivity increase and an effective decrease of the interferences on ²³⁶U⁺ ion measurements: the sensitivity increased to 4.3×10⁶ cps/ppb while reducing the main interference with an hydride formation rate of 7.2×10^-10.
For the first time a precise determination of the ²³⁶U/²³⁸U isotopic ratio up to 10^-11 was demonstrated by ICP-MS/MS, validated by the analysis of certified reference materials and environmental samples previously measured by AMS. This new limit is a new milestone in the determination of ultra-low ²³⁶U/²³⁸U isotope ratios and open the way to various geochemical applications.