Speciation of platinum-benzoylthiourea in the gas phase using electrospray ionization mass spectrometry and density functional theory

Speciation of uranium-mandelic acid complexes using electrospray ionization mass spectrometry and density functional theory

RATIONALE

Mandelic acid is a complexing agent employed for the liquid chromatographic separation of actinides. However, the types of species and the structural details of the uranyl-mandelate complexes are still unknown. Understanding the nature of these complex species would provide better insight into the mechanism of their separation in liquid chromatography.

METHODS

Formation of different species of the uranyl ion (UO₂ ) with mandelic acid was studied using electrospray ionization mass spectrometry (ESI-MS) with a quadrupole time-of-flight analyzer. The different species of uranyl nitrate with mandelic acid (MA) at ligand (L) to metal ratios in the range 1-10 were examined in both positive and negative ion modes. The stability of different species with the possible pathways of formation was scrutinized using density functional theory (DFT) calculations.

RESULTS

In negative ion mode, nitrate-containing UO₂ (MA)₁ , UO₂ (MA)₂ and UO₂ (MA)₃ species were found in good abundance. In positive ion mode, under-coordinated uranyl-mandelate species, and solvated (S) species of types UO₂ (MA)₁ (S), UO₂ (MA)₁ (S)₂ and UO₂ (MA)₂ (S), were observed whereas nitrate-containing species were absent. Interestingly, doubly and singly charged dimeric species were also identified in positive ion mode. The theoretically computed energetics of the various species are in close agreement with their experimentally observed intensities in ESI-MS.

CONCLUSIONS

The most intense peak observed in ESI-MS, UO₂ (MA)₃ , was found to be the energetically most favorable amongst different UO₂ (MA)_n type species. Metal-ligand equilibria studied in the two modes yielded similar results. The combined experimental and quantum chemical investigations predict that T-shape complexes may be formed even in the gas phase. Copyright © 2016 John Wiley & Sons, Ltd.

Electrospray ionization mass spectrometric studies on uranyl complex with α-hydroxyisobutyric acid in water-methanol medium

RATIONALE

Hydroxycarboxylic acids are extensively used as chelating agents in the liquid chromatographic separation of actinides and lanthanides. They are also used as model compounds to understand the binding characteristics of humic substances. A systematic study of the speciation of uranyl-α-hyydroxyisobutyric acid (HIBA) in water-methanol is essential, as it is important to understand the various mechanisms responsible for the separation of these species in liquid chromatography.

METHODS

ESI-MS studies were carried out using a tandem quadrupole-time-of-flight mass spectrometer in positive and negative ion mode. The effects of solution composition, solute concentration and supporting electrolyte concentration on the ESI-MS behavior of the uranyl species were studied. Transmission parameters such as the quadrupole ion energy and collision cell energy were optimized for acquiring the spectra of uranyl-HIBA species, ensuring that the spectra reflect the solution equilibrium conditions.

RESULTS

The solution composition and concentration of the uranyl salt were found to influence the major uncomplexed uranyl species. Although the ESI parameters did not influence the species distribution of uranyl-HIBA, the transmission parameters did have a significant effect. The overall trend in the complexation reaction between uranyl and HIBA was studied as a function of ligand-to-metal ratio. The species distribution obtained in positive ion mode was similar to that obtained in negative ion mode.

CONCLUSIONS

The study presents the optimization of the mobile phase conditions and the ESI-MS parameters for the speciation of the uranyl-HIBA system. The methodology was applied to obtaining the distribution of complexed and uncomplexed uranyl species for monitoring the trend in the complexation reaction.