Evaluation of a new generation quadrupole-based ICP-MS for uranium isotopic measurements in environmental samples

Pushing Limits of ICP-MS/MS for the Determination of Ultralow 236U/238U Isotope Ratios

Determination of uranium isotope ratios is of great expedience for assessing its origin in environmental samples. In particular, the ²³⁶U/²³⁸U isotope ratio provides a powerful tool to discriminate between the different sources of uranium (uranium ore, geochemical background, and uranium from anthropogenic activities). However, in the environment, this ratio is typically below 10^-8. This low abundance of ²³⁶U and the presence in large excess of major isotopes (mainly ²³⁸U and ²³⁵U) complicates the accurate detection of ²³⁶U signal by mass spectrometry and thus highly sensitive analytical instruments providing high abundance sensitivity are required. This work pushes the limits of triple quadrupole-based ICP-MS technology for accurate detection of ²³⁶U/²³⁸U isotope ratios down to 10^-10, which is so far mainly achievable by AMS. Coupled with an efficient desolvating module, N₂O was used as the reaction gas in the collision reaction cell of the ICP-MS/MS. This configuration allows a significant decrease of the uranium polyatomic interferences (²³⁵UH⁺ ions) and an accurate determination of low ²³⁶U/²³⁸U isotope ratios. This new methodology was successfully validated through measurements of certified reference material from 10^-7 to 10^-9 and then through comparisons with AMS measurement results for ratios down to 10^-10. This is the first time that ²³⁶U/²³⁸U isotope ratios as low as 10^-10 were determined by ICP-MS/MS. The possibility of measuring low ²³⁶U/²³⁸U isotope ratios can offer a large variety of geochemical applications in particular for the determination of uranium sources in the environment.

A study on the preparation and application of a core-shell surface imprinted uranyl magnetic chelating adsorbent

A core-shell surface imprinted uranyl magnetic chelating adsorbent (UMCA) was synthesized by combining the sol-gel process with the surface molecular imprinting technique (SMIT). A specific salophen and uranyl-salophen were designed and synthesized. Then, the synthesized uranyl-salophen complex was used as a template (in which uranyl is the target analyte), 3-aminopropyltriethoxysilane as a functional monomer and tetraethylorthosilicate as a cross-linker. The obtained UMCA was characterized by a variety of modern analytical and detection techniques. The adsorbent can be used for the solid-phase extraction of uranyl with good selectivity, high adsorption capacity, magnetic separation characteristics and good reusability. The chelating sorbent was successfully applied for the separation of uranyl, followed by multiphase photocatalytic resonance fluorescence method determination in several environmental water samples with a relative standard deviation of <5.48% and spiked recoveries of 92.5% to 103.0%. The adsorption mechanism was preliminarily discussed.

Isotopic investigation of the colloidal mobility of depleted uranium in a podzolic soil

The mobility and colloidal migration of uranium were investigated in a soil where limited amounts of anthropogenic uranium (depleted in the 235U isotope) were deposited, adding to the naturally occurring uranium. The colloidal fraction was assumed to correspond to the operational fraction between 10 kDa and 1.2 μm after (ultra)filtration. Experimental leaching tests indicate that approximately 8-15% of uranium is desorbed from the soil. Significant enrichment of the leachate in the depleted uranium (DU) content indicates that uranium from recent anthropogenic DU deposit is weakly bound to soil aggregates and more mobile than geologically occurring natural uranium (NU). Moreover, 80% of uranium in leachates was located in the colloidal fractions. Nevertheless, the percentage of DU in the colloidal and dissolved fractions suggests that NU is mainly associated with the non-mobile coarser fractions of the soil. A field investigation revealed that the calculated percentages of DU in soil and groundwater samples result in the enhanced mobility of uranium downstream from the deposit area. Colloidal uranium represents between 10% and 32% of uranium in surface water and between 68% and 90% of uranium in groundwater where physicochemical parameters are similar to those of the leachates. Finally, as observed in batch leaching tests, the colloidal fractions of groundwater contain slightly less DU than the dissolved fraction, indicating that DU is primarily associated with macromolecules in dissolved fraction.