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.

Unusual redox stability of pentavalent uranium with hetero-bifunctional phosphonocarboxylate: insight into aqueous speciation.

The +5 state is an unusual oxidation state of uranium due to its instability in the aqueous phase. As a result, gaining information about its aqueous speciation is extremely difficult. The present work is an attempt in that direction and it provides insight into the existence of a new pentavalent species in the presence of hetero-bifunctional phosphonocarboxylate (PC) chelators, other than the carbonate ion, in the aqueous medium. The aqueous chemistry of pentavalent uranium species with three environmentally relevant PCs was probed using electrochemical and DFT methods to understand the redox energy and kinetics of conversion of the U(VI)/U(V) couple, stability, structure, stoichiometry, binding modes, etc. Interestingly, pentavalent uranium complexes with PCs are quite persistent over a wide range of pH starting from acidic to alkaline conditions. The PC chelators block the cation-cation interaction (CCI) of U(V) through strong hetero-bidentate chelation and intermolecular hydrogen bonding (IMHB) interactions which stabilize the pentavalent metal ion against disproportionation. For uranyl species in the presence of PCs, acting as chelators, CV plots were obtained at varying pH values from 2 to 8. The obtained results indicate an irreversible single redox peak involving U(VI) to U(V) conversion and association of a coupled chemical reaction with the electron transfer step. ESI-MS studies were performed to understand the speciation effect on the U(VI)/U(V) redox couple with varying pH. Speciation modelling of U(V) with the PC ligands was carried out, which indicated that the U(V) is redox stable in nearly 47% of the pH region in the presence of the PCs as compared to the carboxylate-based chelators. The free energy and reduction potential of the U(V) complexes and the reduction free energy and disproportionation free energy for the U(VI)/U(V) couple were determined by DFT computations in the presence of the PCs. In situ spectroelectrochemical spectra were recorded to provide evidence for the existence of U(V) species with PCs in the aqueous medium and to acquire its absorption spectra. The present study is highly significant for understanding the coordination chemistry of pentavalent uranium species, accurate modelling of uranium, and isolation of U(V).

Outcome of Surgical Treatment for Metastatic Spinal Cord Compression: A Single-Center Retrospective Study.

Objective The spinal column is one of the most prevalent regions for metastasis, with an increasing frequency of spinal metastases. Spinal cord metastatic tumor damages the vertebral body, weakens the spinal support, and exerts mass effect on the spinal cord. Overzealous surgical intervention does not provide any additional benefit in most of the spinal metastasis due to shorter life expectancy. The principal goal of this study is to analyze the outcome of various surgical treatments offered to patients with metastatic spinal cord compression (MSCC). Methods Retrospective cohort study including all patients that underwent surgical intervention for MSCC from March 2013 to March 2020. Results A total of 198 patients were included, 113 males and 85 females; the mean age was 62 years. The most common primary cancer was prostate (21.71%) followed by hematological (20.07%) and lung (16.66%). At 6-month postsurgery, 68.68% of patients were Frankel grade D or E (vs. 23.23% preoperatively), 16.6% were grade C (vs 57% preoperatively), and 14.64% were grade A or B (vs. 19.69% preoperatively). Pain on numeric rating scale was decreased from 6.38 ± 3.08 to 3.39 ± 0.73 at 24 hours postsurgery and 1.94 ± 0.67 at 6 months. Conclusion This study found that the majority of patients, undergoing minimally invasive spinal stabilization and decompression for metastatic spinal tumors, have better quality of life, analgesia, and mobility. In conclusion, treatment for spinal metastases should be individualized and a multidisciplinary approach is needed.

Outcome of percutaneous pedicle screw fixation for traumatic thoracic and lumbar fractures -six years experience.

OBJECTIVES: This study discusses the experience at our centre with treating traumatic thoracolumbar fractures using percutaneous pedicle screw fixation. METHODS: We reviewed the case notes and imaging retrospectively between January 2013 and June 2019. RESULTS: A total of 257 patients were included, 123 males and 134 females aged between 17 and 70; the mean age was 47.6 years. The majority of injuries were from fall from significant height. The fractures involved the thoracic and lumbar vertebra in 98 and 151 of the cases respectively. Percutaneous pedicle screw fixation was performed either one or two levels above and below the fracture depending on the level of injury. Forty two cases were treated with additional short pedicle screws at the level of fracture. More than 15% (39) of the patients presented with a neurological deficit; more than 80% (32) of those showed post-operative improvement in their neurology as per Frankel Grading system. The mean operative time was 117 min ± 45 min; mean length of hospital stay was 7.2 days ± 3.8 days, with significant improvement in pain. CONCLUSIONS: Percutaneous pedicle screw fixation is a safe surgical option with comparable outcomes to open surgery and a potential reduction in perioperative morbidity.

Chemical and Redox Speciation of Uranyl with Three Environmentally Relevant Bifunctional Chelates: Multi-Technique Approach Combined with Theoretical Estimations.

Carbon and phosphorous are two primary elements common to the bio-geosphere and are omnipresent in both biotic and abiotic arenas. Phosphonate and carboxylate are considered as building blocks of glyphosate and humic substances and constituents of the cellular wall of bacteria and are the driving functionalities for most of the chemical interactions involving these two elements. Phosphonocarboxylates, a combination of both the functionalities in one moiety, are ideal models to dig deep into for understanding the chemical interactions of the two functional groups with metal ions. Phosphorous and carbon majorly exist as inorganic/organic phosphate and carboxylate, respectively, in the bio-geosphere. Aquatic contamination is a major concern for uranium, and the presence of complexing agents would alter the uranium concentrations in aquifers. Determination of solution thermodynamic parameters, speciation plots, redox patterns, Eh-pH diagrams, coordination structures, and molecular-level understanding by density functional theory calculations was carried out to interpret the uranyl (UO22+) interaction with three environmentally relevant phosphonocarboxylates, namely, phosphono-formic acid (PFA), phosphono-acetic acid (PAA), and phosphono-propanoic acid (PPA). UO22+ forms 1:1 complexes with the three phosphonocarboxylates in the monoprotonated form, having nearly the same stability, and the complexes [UO2(PFAH)], [UO2(PAAH)], and [UO2(PPAH)] involve chelate formation of five, six, and seven membered rings, respectively, through the participation of an oxygen each from the carboxylate and phosphonate, strengthened by an intra-molecular hydrogen bonding through the proton of the phosphonate moiety with uranyl oxygen. The complex formations are favored both enthalpically and entropically, with the latter being more contributive to the overall free energy of formation. The redox speciation showed an aqueous soluble complex formation over a wide pH range of 1-8. Electrospray ionization mass spectrometry and extended X-ray absorption fine structure established the coordination modes, which are further corroborated by density functional calculations. The knowledge gained from the present studies provide potential inputs in framing the cleanup, sequestering, microbial, and bio-remediation strategies for uranyl from aquatic environments.

Characterization of Thorium-Pyrazinoic acid complexation and its decorporation efficacy in human cells and blood.

Thorium (Th) exposure to the human beings is a radiochemical hazard and the chelation therapy by suitable drugs is the major prevention approach to deal with. The present studies aimed at usage of pyrazinoic acid (PCA), which is a prodrug to treat tuberculosis, for its usage as decorporating agent for thorium from human body. The present studies provide a comprehensive knowledge on the chemical interaction and biological efficacy of pyrazinoic acid (PCA) for decorporation of Thorium from the human body. The thermodynamic parameters for Th-PCA speciation are determined by both experiment and theory. The potentiometric data analysis and Electro-Spray Ionization Mass Spectrometry (ESI-MS) studies revealed the formation of MLi (i = 1-4) species with the decrease in stepwise stability constants. All the species formations are endothermic reactions and are predominantly entropy-driven. Biological experiments using human erythrocytes, whole blood and normal human lung cells showed cytocompatibility and decorporation ability of PCA for Thorium. Density functional calculations have been carried out to get insights on interaction process at molecular level. The experimental results and theoretical predictions found to be in line with each other. Present findings on complexation of Th by PCA and its evaluation in human cells and blood would further motivate determination of its safety levels and decorporation efficacy in animal models.

Aquatic interaction of uranium with two naturally ubiquitous pyrazine compounds: Speciation studies by experiment and theory.

The present studies interpret the speciation of uranyl (UO22+) with the most ubiquitous class of natural species named pyrazines in terms of stability, speciation and its identification, thermodynamics, spectral properties determined by a range of experimental techniques and further evidenced by theoretical insights. UO22+ forms ML and ML2 kind of species with a qualitative detection of ML3 species, while the ESI-MS identified the formation of all the complexes including ML3. Both the ligands act as bidentate chelators with a difference in ring size and coordinating atoms in the complex formed. The ML3 complexes involve the third ligand participation as monodentate via carboxylate only due to the restricted coordination number and space around the UO22+ ion to accommodate three ligand molecules in its primary coordination sphere. All the complexes are found to be endothermic and purely entropy driven formations. The complex formations showed redshift in the absorption spectra and the shift was further enhanced from ML to ML2 formation. The UO22+ ion redox properties are used to explore the redox potential and heterogeneous electron-transfer kinetic parameters as a function of pH and concentration of UO22+ in presence of pyrazine carboxylates. Interestingly, the cyclic voltammograms identified the ligands also as redox sensitive. The theoretical calculation gave inputs to understand the complex formation at the molecular level with major emphasis on geometry optimization, energetics, bonding parameters, molecular orbital diagrams and bond critical point analyses. The experimental observations in combination with theoretical addendum provided detailed knowledge on the interaction of UO22+ with pyrazine-2-carboxylate and pyrazine-2,3-dicarboxylates.

Reduction in Coordination Number of Eu(III) on Complexation with Pyrazine Mono- and Di-Carboxylates in Aqueous Medium.

The denticity, flexibility, and steric hindrance of the ligand are key factors in deciding the mode and number of coordination around a metal ion on complex formation. The thermodynamic aspects of lanthanide complexation with various multidentate ligands provides a significant insight into understand the coordination chemistry of lanthanides in framing the relevant metal organic networks for the applications in biological, biochemical and medical aspects. The pyrazine carboxylic acids are known to form many structurally important complexes and further can form chelates with coordination number of eight for europium in which more water molecules can be knocked out from the primary coordination sphere than demanded by denticity of the ligand. The present studies aimed at ESI-MS characterization and determination of the thermodynamic parameters (log ß, ΔG, ΔH, and ΔS), luminescence properties of europium complexes with pyrazine-2-carboxylate and pyrazine-2,3-dicarboxylate in aqueous solutions by experiment as well as theory. Time resolved luminescence spectroscopy supported by DFT calculations are carried out to optimize the stable geometries of the complexes with various modes of binding and coordination. Furthermore, the thermodynamic parameters estimated theoretically have been used to trace the path of complex formation.

Abdominal Vascular Injury During Posterior Lumbar Discectomy, Experience from Three Cases and Review of Literature.

STUDY DESIGN: Case series and review of literature. OBJECTIVE: To report three cases of vascular injury during posterior lumbar disc surgery, two of these occurred during open discectomy, and one during an endoscopic surgery. Aim is to highlight importance of early diagnosis and prompt steps taken to prevent morbidity and mortality. SUMMARY OF BACKGROUND DATA: Vascular injury during lumbar discectomy is rare injury. Also no case has been reported so far occurring during an endoscopic discectomy. METHODS: Three patients were treated for lumbar intervertebral disc prolapse, two had undergone open posterior discectomy while the third patient underwent endoscopic discectomy and all of them were diagnosed to have suffered an abdominal vascular injury. RESULTS: In two cases pseudo-aneurysm was found while third case was diagnosed as an arterio-venous fistula. All the three cases were managed with endovascular stenting and followed for a minimum period of 2 years. No further complications were detected. CONCLUSION: Vascular injury during a disc surgery is difficult to diagnose due to the rarity of occurrence and subtle signs which need to be recognized to start early management. LEVEL OF EVIDENCE: 5.

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.

Tailored bifunctional polymer for plutonium monitoring.

Monitoring of actinides with sophisticated conventional methods is affected by matrix interferences, spectral interferences, isobaric interferences, polyatomic interferences, and abundance sensitivity problems. To circumvent these limitations, a self-supported disk and membrane-supported bifunctional polymer were tailored in the present work for acidity-dependent selectivity toward Pu(IV). The bifunctional polymer was found to be better than the polymer containing either a phosphate group or a sulfonic acid group in terms of (i) higher Pu(IV) sorption efficiency at 3-4 mol L(-1) HNO3, (ii) selective preconcentration of Pu(IV) in the presence of a trivalent actinide such as Am(III), and (iii) preferential sorption of Pu(IV) in the presence of a large excess of U(VI). The bifunctional polymer was formed as a self-supported matrix by bulk polymerization and also as a 1-2 µm thin layer anchored on a microporous poly(ether sulfone) by surface grafting. The proportions of sulfonic acid and phosphate groups in both the self-supported disk and membrane-supported bifunctional polymer were found to be the same as expected from the mole proportions of monomers in polymerizing solutions used for syntheses. α radiography by a solid-state nuclear track detector indicated fairly homogeneous anchoring of the bifunctional polymer on the surface of the membrane. Pu(IV) preconcentrated on a single bifunctional bead was used for determination of the Pu isotopic composition by thermal ionization mass spectrometry. The membrane-supported bifunctional polymer was used for preconcentration and subsequent quantification of Pu(IV) by α spectrometry using the absolute efficiency at a fixed counting geometry. The analytical performance of the membrane-supported-bifunctional-polymer-based α spectrometry method was found to be highly reproducible for assay of Pu(IV) in a variety of complex samples.

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.

An insight into the influence of low dose irradiation pretreatment on the microbial decolouration and degradation of Reactive Red-120 dye.

The influence of low dose irradiation pretreatment on the microbial decolouration and degradation of Reactive Red-120 (RR-120) dye was investigated in detail by using Pseudomonas sp. SUK1. About 27%, 56% and 66% decolouration of 150 ppm RR-120 dye solution was observed by applying 0, 0.5 and 1 kGy doses, respectively, in the first step followed by microbial treatment for 24 h under static condition. Similarly, about 70%, 88% and 90% TOC removal was observed by applying 0, 0.5 and 1 kGy doses, respectively, in the first step followed by the microbial treatment for 96 h under static condition. The radiation induced fragmented products of RR-120 at doses of 0.5 and 1 kGy were investigated by FTIR and electrospray ionization-MS analysis. The induction of the enzymes viz. laccase, tyrosinase, azoreductase and NADH-2,6-dichlorophenol indophenol reductase was studied in the decolourised solution obtained after irradiating 150 ppm RR-120 dye solution with 0 and 1 kGy doses followed by the microbial treatment for 96 h under static condition. The enzymatic degradation products were studied by FTIR, HPLC and GC-MS. The toxicity study of the treated dye solution on plants revealed the degradation of RR-120 into non-toxic products by combined radiation-microbial treatment. This study explores a reliable and promising way to use industrially viable dose (≤1 kGy) and microbial strain viz. Pseudomonas sp. SUK1 for permissible safe disposal of dye solutions from textile industries.

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.