Study of Uranyl Tartrate Speciation in Aqueous Solution by Electrospray Ionization Tandem Mass Spectrometry

As a nuclear fuel, uranium plays an important role in national defense and energy, however, the release of uranium into the natural environment from mining to radioactive waste disposal poses a severe threat to public health and environmental protection. Therefore, investigation of uranium in aqueous solution is one of the important research on geological disposal of high level waste. Electrospray ionization tandem mass spectrometry (ESI-MS/MS) has been applied for trace detection of uranyl complex in the last decade because of its characteristics of accurate qualitative and quantitative capability, strong anti-interference capability and high determination efficiency. In this work, uranyl complex were identified by ESI-MS/MS, where the cations were ionized by ESI, positive charged uranyl tartrate undergoes characteristic fragmentation in the gas phase. Target ions were bombarded to generate product ions at high collision energy of 200 V in collision-induced dissociation (CID) mode, energy of which is far beyond normal range. As UO₂⁺ has special mass-to-charge ratio (m/z) compared with other substances and it was split apart hardly, product ion of UO₂⁺(m/z 269.9) was the key basis to identify uranium-containing species in ESI-MS/MS. Pretreatment was unessential for the qualitative method which would not change speciation of original solution. It was found 14 kinds of uranyl tartrate and 5 kinds of inorganic uranyl speciation quickly in uranyl nitrate-tartaric acid solution, all of which could generate UO₂⁺ signal significantly or even uniquely. Each uranyl group could bind a tartrate anion tightly and was able to complex two tartaric acid molecular maximum. Experiment results showed that clusters of uranyl tartrate such as (UO₂HTarH₂Tar)₂²⁺ and (UO₂)₃Tar(HTar)₂(H₂Tar)₂²⁺ were identified by ESI-MS/MS for the first time. Despite of losing the acid molecules, there was other mass fragmentation path of uranyl tartrate cluster ions that stable double bonds were reconstructed in the form of tartaric acid groups of the ions dehydrated. Moreover, the cleavage energy of uranyl tartrate complex bond was sorted in the following order: tartrate uranyl clusters bond energy>tartrate anion-uranyl bond energy>tartrate acid molecule-uranyl bond energy. The study indicates that ESI-MS/MS method has promising perspectives for the exploration of aqueous speciation of uranium with organic acids for environmental sample, and has guiding significance for speciation analysis of other lanthanide elements.