Thorium Complexation with Aliphatic and Aromatic Hydroxycarboxylates: A Combined Experimental and Theoretical Study.

Hydroxycarboxylic acids, viz., α-hydroxyisobutyric acid (HIBA) and mandelic acid (MA), have been widely employed as eluents for inner transition metal separation studies. Both extractants have identical functional groups (OH and COOH) with different side-chains. Despite their similarities in binding motifs, they show different retention behaviors for thorium and uranium in liquid chromatography. To understand the mechanism behind the trend, a detailed study on the aqueous phase interaction of thorium with both extractants is carried out by speciation, spectroscopy, and density functional theory-based calculations. Potentiometric titration experiments are carried out to reveal the stability and species formed. Electrospray ionization mass spectrometry is performed to identify the formation of different species by Th with both HIBA and MA. It is seen that for Th-HIBA and Th-MA, the dominating species are ML3 and ML4, respectively. A similar pattern observed in potentiometric speciation analysis supports the tendency of Th to form higher stoichiometric species with MA than with HIBA. The difference in the dominating species thus helps in explaining the reversal in the retention behavior of uranium and thorium in the reverse-phase liquid chromatographic separation. The results obtained are corroborated with extended X-ray absorption fine structure spectroscopic measurements and density functional theory (DFT) calculations.

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.

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

RATIONALE: Determining the speciation of platinum-benzoylthiourea (Pt-BTU) in the gas phase is a challenging task due to various reaction pathways and the conformational flexibility of the BTU ligand. METHODS: Electrospray ionization mass spectrometry (ESI-MS) experiments and density functional theory (DFT) based calculations were carried out to shed light on this complex reaction in the gas phase using K2 PtCl4 salt and BTU. Various Pt complexes were studied in both positive and negative ion modes of ESI-MS using a quadrupole-time-of-flight mass spectrometer. The effects of the ESI-MS experimental parameters such as capillary voltage, pH and electrolyte on the peak intensity of the Pt-BTU complex were investigated. DFT calculations employing B3LYP functional with the 6-311++G** basis set were used to characterize the geometric parameters and fragmentation patterns of various Pt complexes in the gas phase. RESULTS: In the positive ion mode, complexes with differing numbers of BTU ligands coordinated to the metal ion were observed, whereas, in the negative ion mode, no species associated with BTU or with the solvent (acetonitrile) molecules were found. It was also found that Pt forms complexes with the BTU ligand in different stoichiometric ratios. For both Pt(BTU)2 and Pt(BTU)3 complexes, the BTU ligand undergoes deprotonation followed by bi-dentate coordination. DFT calculations suggest that BTU can coordinate to Pt in both cis and trans isomeric forms, which are nearly iso-energetic with a slight preference towards the trans-isomer. The preference of trans-BTU binding is attributed to the exclusive retention of intra-molecular hydrogen bonding which is absent in the cis-form. CONCLUSIONS: Experimental and theoretical calculations have shown that the gas-phase interaction of BTU to Pt is very complex. The BTU ligand can coordinate to Pt in both mono-dentate and bi-dentate modes, the latter mode being favorable upon deprotonation of the BTU ligand. Furthermore, many close lying species with different geometric isomeric forms are found to be possible due to the presence of intra- and inter-molecular hydrogen bonding.

Electrospray ionisation mass spectrometric studies for the determination of palladium after pre-concentration by disposable pipette extraction.

RATIONALE: Electrospray ionisation mass spectrometric (ESI-MS) analysis of Pd in complex matrices is difficult due to the multiplicity of matrix effects. Two different approaches, internal standard and matrix separation, were investigated for developing a reliable analytical procedure for the trace level determination of Pd in simulated high-level liquid waste (SHLLW) solutions. METHODS: An ESI mass spectrometer with a quadrupole-time-of-flight analyser was used to study the speciation of the palladium-benzoylthiourea (Pd-BTU) complex and to determine the Pd content. The Pd-BTU complex was selectively pre-concentrated using disposable pipette extraction (DPX). Extraction parameters as well as ESI-MS parameters such as concentration of BTU, acidity, composition of medium and capillary voltage, etc., were optimized based on the major species [Pd(BTU)(2)S](+). RESULTS: The method gave quantitative and selective pre-concentration of the Pd-BTU complex from SHLLW. Linearity from 5 ppb to 200 ppb and a limit of detection of 0.012 ppb were obtained for Pd. No interference from the neighboring elements, viz. ruthenium, rhodium, silver and cadmium, was observed during the determination of Pd based on the [Pd(BTU)(2)S](+) peak. The ESI signal intensity was not influenced by the presence of the many other elements in the SHLLW solution. CONCLUSIONS: Good sensitivity, tolerance to matrix concentration and the absence of interference from neighboring elements make the method very promising for the determination of Pd at low levels in complex samples. We have demonstrated the capability of ESI-MS for the quantification of Pd in complex matrices and its potential for providing data on speciation, using the Pd-BTU complex.

Determination of uranium in seawater samples by liquid chromatography using mandelic acid as a complexing agent.

The determination of uranium at different stages of the recovery process as well as in seawater is important in its recovery study. A previous study developed a high-performance liquid chromatography (HPLC) method for uranium determination in seawater using α-hydroxy isobutyric acid as a chelating agent. However, this method causes turbidity in process samples containing high amounts of iron, resulting in the clogging of the HPLC column. In the present work, use of mandelic acid as a chelating agent for uranium has been explored. Elution conditions were optimized for the separation of iron [Fe(III)] and uranium [U(VI)] by studying the effect of an ion interaction reagent, the concentration of mandelic acid, and methanol content in the mobile phase. Different parameters were optimized to develop off- line pre-concentration of uranyl-mandelate on the reversed stationary phase. The method offers quantitative recovery of uranium and linearity in the U(VI) concentration range of 0.5 ppb to 500 ppb and can be used for the determination of U(VI) in process samples with Fe/U amount ratios up to 3,000. The method has been successfully used for the determination of U(VI) in seawater samples and process samples. The developed methodology was validated by comparing the results with those of isotope dilution-thermal ionization mass spectrometry.

Reversed-phase liquid chromatography using mandelic acid as an eluent for the determination of uranium in presence of large amounts of thorium.

Studies were carried out for the separation of uranium (U) and thorium (Th) on reversed-phase (RP) C18 columns using mandelic acid as an eluent. Retention of thorium-mandelate on the unmodified stationary phase was found to be greater than that of uranyl-mandelate under the pH conditions employed. Th retention capacity of the stationary phase was determined as a function of pH and MeOH content of the mobile phase. The optimised parameters allowing U elution prior to Th were utilized for the determination of small amounts of U in the presence of large amounts of Th. The method has been used for the determination of U in synthetic samples with Th/U amount ratios up to 100,000 (10 microg/g of U) without any pre-separation, employing a particulate C18 column. Effect of concentration of ion interaction reagents (IIRs) on the retention was studied to understand the mechanism of adsorption of their mandelate complexes onto the stationary phase. The experiments conducted unequivocally prove that thorium-mandelate complex is neutral whereas uranyl-mandelate complex is anionic in nature.