P. G. Jaison

Comparative evaluation of three α-hydroxycarboxylic acids for the separation of lanthanides by dynamically modified reversed-phase high-performance liquid chromatography

The resolution for the three homologues of α-hydroxycarboxylic acids viz. lactic acid, α-hydroxyisobutyric acid (α-HIBA) and α-hydroxy-α-methylbutyric acid (α-H-α-MBA), was compared for the individual separation of 14 lanthanide elements under identical experimental conditions. α-HIBA was found to be the best for separation of heavier lanthanides (Tb to Lu) while α-H-α-MBA led to a better separation for lighter lanthanides (La to Eu). All the 14 lanthanides were separated by gradient HPLC employing both α-HIBA and α-H-α-MBA so that there was reasonable resolution among all the peaks and the separation was completed in a short time.

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.

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.

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.

Gas phase reactions of uranyl with α-hydroxyisobutyric acid using electrospray ionization mass spectrometry and density functional theory

Electrospray ionization mass spectrometry (ESI-MS) technique was used to find out the type of species of uranyl with α-hydroxyisobutyric acid in positive and negative ion modes. It was found that the singly charged monomeric and doubly charged dimeric species were present in soft ionization conditions. The molecular level understanding on the structures and energetics were investigated using density functional theory based calculations. It was found that ML3, the most intense species observed in ESI-MS were energetically more favorable as compared to ML1 and ML2.

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.

Comparative Study of Ion Interaction Reagents for the Separation of Lanthanides by Reversed-Phase High Performance Liquid Chromatography (RP-HPLC)

Abstract A study of two ion interaction reagents (IIRs) viz. n-octadecane sulphonate (C18-sulphonate) and eicosyl sulphate (C20-sulphate) was carried out for the separation of lanthanides by reversed-phase high performance liquid chromatography (RP–HPLC). The objective of the study was to identify a suitable IIR offering long term adsorption onto the RP column, thereby obviating the need to introduce the IIR in the mobile phase during the separation of lanthanides. This avoids the rigorous treatment of purified fractions before their mass spectrometric analysis. Resolution was used for comparing different IIRs for separation of lanthanides under identical chromatographic conditions, employing α-hydroxy isobutyric acid (α-HIBA) as an eluent. The volume and composition of IIR solution required for the modification of the column, as well as their long term adsorption, were studied. Columns coated with C18-sulphonate and C20-sulphate allowed the separation of lanthanides without introducing the IIR in the mobile phase. Between these two IIRs, C18-sulphonate offered higher resolution and provided good long term adsorption stability. A RP column modified with C18-sulphonate, as per the optimised procedure, was used for the separation of lanthanides from a geological reference sample without the need to include IIR into the mobile phase.

Speciation of uranium-mandelic acid complexes using electrospray ionization mass spectrometry and density functional theory: Study of uranium-mandelic acid complexes using ESI-MS and DFT

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

DETERMINATION OF LANTHANIDES AND YTTRIUM IN HIGH PURITY DYSPROSIUM BY RP-HPLC USING α-HYDROXYISOBUTYRIC ACID AS AN ELUENT

An HPLC method is presented for the determination of lanthanides viz. lutetium (Lu), ytterbium (Yb), thulium (Tm), erbium (Er), holmium (Ho), terbium (Tb), and yttrium (Y) in high-purity dysprosium oxide (Dy2O3) matrix. In view of their similar chemical and physical properties, determination of rare-earth elements in bulk of a rare-earth element is a difficult task. Studies were carried out on C18 and C8 reversed phase columns employing α-hydroxyisobutyric acid (α-HIBA) as an eluent. n-Octane was used as an ion interaction reagent (IIR). Different chromatographic parameters e.g., ion interaction reagent, pH and concentration of α-HIBA were studied simultaneously to arrive at the optimum condition for the individual separation of lanthanides. When n-Octane sulphonate was used as the IIR, C8-based stationary phase was found to give significantly better resolution as compared to C18-based stationary phase. Under the optimized conditions, it was possible to resolve Y from Dy using α-HIBA as the single eluent. Separation and quantification of rare-earth elements were possible in samples with Dy/Tb and Dy/Y amount ratios up to 200. The method was validated using the simulated sample containing above lanthanides, Y and Dy matrix. Quantification was done using standard addition method. The method was successfully applied for the quantification of lanthanides and Y in high purity Dy2O3 sample. Concentrations of Lu, Yb, Tm, Er, Ho, Tb, and Y were found to be 74 ppmw, 432 ppmw, 22 ppmw, 57 ppmw, 3325 ppmw, 1207 ppmw, and 4463 ppmw, respectively.