Effect of phase ratio on van't Hoff analysis in reversed-phase liquid chromatography, and phase-ratio-independent estimation of transfer enthalpy

On physical meaning of van't Hoff equation and its applicability in chromatography

The van't Hoff equation is a widely used tool to study adsorption thermodynamics in chromatography. Recent experimental data and some theoretical arguments questioned the accuracy of thermodynamic characteristics determined by using this equation. The present report addresses these concerns and shows that if certain requirements are satisfied, the van't Hoff analysis provide reliable results. These requirements include isothermality of the column not perturbed by the percolation of the mobile phase, mass transfer kinetics allowing sufficient proximity to equilibrium, pressure below 200 bar for low molecular weight compounds and below 50 bar for macromolecules, and knowledge of the phase ratio at all experimental temperatures to allow accurate conversion of the retention factor to the equilibrium constant. Physical meaning of adsorption enthalpy and entropy obtained by means of the van't Hoff analysis particularly in the case of heterogeneous adsorbents is discussed and recommendations on how to perform experiments to obtain reliable results are given.

Binding characteristics and conformational changes in alpha-2-macroglobulin by the dietary flavanone naringenin: biophysical and computational approach

In the present study, we investigated the interaction of alpha-2-macroglobulin (α2M) with naringenin using multi-spectroscopic, molecular docking, and molecular simulation approaches to identify the functional changes and structural variations in the α2M structure. Our study suggests that naringenin compromised α2M anti-proteinase activity. The results of absorption spectroscopy and fluorescence measurement showed that naringenin-α2M formed a complex with a binding constant of (kb)∼104, indicative of moderate binding. The value of ΔG° in the binding indicates the process to be spontaneous and the major force responsible to be hydrophobic interaction. The findings of FRET reveal the binding distance between naringenin and the amino acids of α2M was 2.82 nm. The secondary structural analysis of α2M with naringenin using multi-spectroscopic methods like synchronous fluorescence, red-edge excitation shift (REES), FTIR, and CD spectra further confirmed the significant conformational alterations in the protein. Molecular docking approach reveals the interactions between naringenin and α2M to be hydrogen bonds, van der Waals forces, and pi interactions, which considerably favour and stabilise the binding. Molecular dynamics modelling simulations also supported the steady binding with the least RMSD deviations. Our study suggests that naringenin interacts with α2M to alter its confirmation and compromise its activity.Communicated by Ramaswamy H. Sarma.

Correctness of the van 't Hoff analysis for homogeneous or heterogeneous retention

It is already known that the physical meaning of the numerical values of the calculated enthalpy (ΔH) and entropy (ΔS) change in chromatography via the van 't Hoff plot analysis is rather questionable. In the former work of these authors, it has been demonstrated that by serially coupling two reversed-phase columns of the same inner diameter but of different retention mechanism, the obtained thermodynamic values are not related to the numerical results obtained on the individual columns respectively. Since surface heterogeneity of the stationary phase is intrinsically present in chiral chromatography (enantioselective and non-selective sites), the calculation of ΔH and ΔS should be revisited in that field. In this study, more details of the pressure dependence were investigated. Using special POPLC columns, the effect of the column length on the calculated thermodynamic values was investigated. The calculated values differ by 30-400% when the column length is increased from 4 cm to 12 cm. The dependence of the calculated results on the applied flow rate was already highlighted earlier, and here the emphasis was put on the instrument on which the separation was performed. It turns out that there is a difference when using a Shimadzu HPLC system compared to using a Waters Acquity UPLC system. Because the Eyring method serves as an alternative route to calculate the thermodynamic values from chromatographic data, the differences obtained by the two methods were also investigated.

Separation of Molar Weight-Distributed Polyethylene Glycols by Reversed-Phase Chromatography—Analysis and Modeling Based on Isocratic Analytical-Scale Investigations

The separation of polyethylene glycols (PEGs) into single homologs by reversed-phase chromatography is investigated experimentally and theoretically. The used core–shell column is shown to achieve the baseline separation of PEG homologs up to molar weights of at least 5000 g/mol. A detailed study is performed elucidating the role of the operating conditions, including the temperature, eluent composition, and degree of polymerization of the polymer. Applying Martin’s rule yields a simple model for retention times that holds for a wide range of conditions. In combination with relations for column efficiency, the role of the operating conditions is discussed, and separations are predicted for analytical-scale chromatography. Finally, the approach is included in an efficient process model based on discrete convolution, which is demonstrated to predict with high accuracy also advanced operating modes with arbitrary injection profiles.

Covalent immobilization of beta2 adrenergic receptor through trans-methylation reaction by SNAP-tag and its application in anti-asthmatic compound screening from Raphani Semen

Beta2-adrenergic receptor (β₂-AR) is believed as an attractive target for anti-asthmatic drugs. Its crystal structure and pharmacological activity have been clearly investigated. Yet the number of the approved anti-asthmatic drugs has declined in recent years. This work reports on the preparation of an immobilized β₂-AR column through the specific trans-methylation reaction between SNAP tag and the benzyl-guanine derivative and application in anti-asthmatic compound screening from Raphani Semen. The characterization of the immobilized β₂-AR was performed by scanning electron microscopy (SEM) and receptor-ligand interaction analysis by chromatographic methods. SEM analysis showed that the receptor has been successfully coated on the surface of PEGA amino microspheres. Binding constants of salbutamol and terbutaline calculated from frontal analysis within the temperature range of 10-30 ℃ confirmed the feasibility of the method in a thermodynamic viewpoint. Hydrogen bond was verified as the main driving force for drug-receptor interaction analysis. Sinapine was identified as the potential bioactive compound in Raphani Semen that specifically bind with β₂-AR with a specific binding site of Ser 207. Taking together, the immobilized β₂-AR column is promising in exploring drug-protein interaction analysis and anti-asthmatic drug screening.

Hydrogel Coating with Temperature Response Retention Behavior and Its Application in Selective Separation of Liquid Chromatography

We reporte the double-layer hydrogel-coated mesoporous silica material as a new stationary phase for liquid chromatography. The method of combining physical coating and chemical coating was to apply hydrogel coating on the surface of silica, and finally, a new type of liquid chromatography stationary phase with in situ coating of the functional hydrogel on silica was obtained. This hydrogel-functionalized liquid chromatography stationary phase also exhibits a certain temperature responsiveness. Experimental results show that this temperature response is mainly due to changes in the hydrogen bonding between the stationary phase and the analyte at different temperatures in the column oven, which leads to changes in retention behavior. The hydrogel-coated mesoporous silica microspheres showed excellent selectivity for many polar analytes. An excellent column efficiency was obtained (139 000 plates/m for terephthalic acid) after optimization of chromatographic conditions. In addition to rapid separation of some analytes, this new hydrogel stationary phase also has certain superiority in chromatographic performance compared with other new excellent liquid chromatography stationary phases functioned by three-dimensional cross-linking systems. The important thing is that this strategy is relatively easy to prepare a new stationary phase with different properties.

Enantiomeric Separation and Thermodynamic Investigation of (R)-N-Tert-Butoxy Carbonyl-Piperidine-3-Carboxylic Acid Hydrazide, a Key Starting Material of Zidebactam

A new selective, accurate and precise chiral high-performance liquid chromatography method for the separation of (R)-N-tert-butoxy carbonyl-piperidine-3-carboxylic acid hydrazide (RE) and its enantiomer was developed. RE is a key starting material of novel β-lactam enhancer drug Zidebactam. Chiral resolution of more than 10 was achieved on Chiralpak IA column using mobile phase consisting of n-hexane, ethanol in the ratio of 70:30, v/v. The flow rate of the mobile phase was 1.0 mL min-1 and the column oven temperature was 30°C. Detection was carried out at 225 nm. The developed method was validated as per the International Conference on Harmonization guideline. Limit of detection and limit of quantification of the enantiomeric impurity (S)-N-tert-butoxy carbonyl-piperidine-3-carboxylic acid hydrazide (SE) was 2.5 and 7.5 μg mL-1, respectively. Mean recovery of the SE was 96.83 ± 1.4%. The effect of thermodynamic parameters on the chiral separation was evaluated.

A thermodynamic study of capillary electrochromatographic retention of aromatic hydrocarbons on a lauryl acrylate porous polymer monolithic column with measured phase ratio

The phase ratio of a chromatographic system is an important measurement that has long been estimated or calculated, but rarely directly measured. This study utilized a nanoflow liquid chromatography instrument to more accurately measure the phase ratio for a lauryl acrylate porous polymer monolith. Direct measurement of the phase ratio, and its dependence on temperature, allows for a better understanding of the thermodynamics of retention of small analytes. This study investigates the retention of an alkyl benzene series, toluene to octylbenzene, via capillary electrochromatography. The phase ratio was determined to be 0.202 at 303 K and 0.213 at 333 K. Using the directly measured phase ratio, entropic contributions to retention can also be obtained. Therefore, the Gibbs free energy calculations from these measurements and methods can give insight to modes of retention. The free energy of retention for toluene is -3.97 kJ/mol at 303 K and -3.78 kJ/mol at 333 K. The trends for enthalpy, entropy, and Gibbs free energy of transfer show that retention is enthalpically driven in this capillary electrochromatography (CEC) porous polymer monolith system.

The study of the chromatographic behavior and a mass spectrometric approach to differentiating the diastereomer pair of the β-O-4 lignin dimer series

Lignin and lignans are natural products found in plant cell walls. Lignin research has historically focused on lignin degradation techniques in the hope of converting lignin into useful aromatic carbon feedstocks. In contrast, investigations of lignans existing as natural product dimers, have been focused on thier interesting biological activities. Many lignan compounds are chemically identical to dimers derived from lignin, and both lignin and lignan dimers can possess multiple chiral centers leading to observations of diastereomer pairs where one diastereomer exhibits the bulk of the activity. For example, the G-(β-O-4')-G dimer was reported to have a pro-angiogenic activity with one diastereomer of the pair showing enhanced pro-angiogenic activity. Traditional analytical techniques such as nuclear magnetic resonance (NMR) can differentiate the diastereomer pairs of β-O-4 compounds; however, isolation of a pure sample is often required for analysis. This work was aimed at exploring the potential use of tandem mass spectrometry to differentiate diastereomer pairs in the β-O-4 dimer series. Each diastereomer pair in the nine-dimer series was separated by HPLC and interrogated by tandem mass spectrometry. To understand the chromatographic behavior of the diastereomer pair in the β-O-4 dimer series, three commercially available reverse phase HPLC columns were evaluated. A temperature programming experiment using water/acetonitrile isocratic elution showed that the chromatographic retention mechanism of these diastereomers was hydrophobically driven with analytes having more methoxy groups exhibiting larger ΔH⁰ and higher octanol-water partition coefficient values. Tandem mass spectrometry performed on each of the diastereomers produced fragment ions having different ion abundances. A mechanistic study based on the ion abundance of "sequence-specific ions" and "-48 ions" was used to assign a configuration to each of the pairs of diastereomers in the nine-dimer series.

High-performance liquid chromatographic evaluation of strong cation exchanger-based chiral stationary phases focusing on stationary phase characteristics and mobile phase effects employing enantiomers of tetrahydro-ß-carboline and 1,2,3,4-tetrahydroisoquinoline analogs

In this study, we present results obtained on the enantioseparation of some cationic compounds of pharmaceutical relevance, namely tetrahydro-ß-carboline and 1,2,3,4-tetrahydroisoquinoline analogs. In high-performance liquid chromatography, chiral stationary phases (CSPs) based on strong cation exchanger were employed using mixtures of methanol and acetonitrile or tetrahydrofuran as mobile phase systems with organic salt additives. Through the variation of the applied chromatographic conditions, the focus has been placed on the study of retention and enantioselectivity characteristics as well as elution order. Retention behavior of the studied analytes could be described by the stoichiometric displacement model related to the counter-ion effect of ammonium salts as mobile phase additives. For the thermodynamic characterization parameters, such as changes in standard enthalpy Δ(ΔH°), entropy Δ(ΔS°), and free energy Δ(ΔG°), were calculated on the basis of van't Hoff plots derived from the ln α vs. 1/T curves. In all cases, enthalpy-driven enantioseparations were observed with a slight, but consistent dependence of the calculated thermodynamic parameters on the eluent composition. Elution sequences of the studied compounds were determined in all cases. They were found to be opposite on the enantiomeric stationary phases and they were not affected by either the temperature or the eluent composition.

Silver (I)-dimercaptotriazine functionalized silica: A highly selective liquid chromatography stationary phase targeting unsaturated molecules

Silver(I)-mercaptopropyl (Ag-MP) functionalized silica gel has demonstrated its effectiveness in separating various unsaturated organic compounds including unsaturated fatty acid ethyl esters (FAEEs), triglycerols (TAGs) and long-chain alkyl ketones (alkenones). While Ag-MP stationary phase displays many advantages over the conventional silver ion-impregnated silica gel (e.g., stability, high recovery, etc.), potential drawbacks of Ag-MP include relatively low retentions for unsaturated molecules, which could limit chromatographic resolutions under certain circumstances. In this study, we evaluate a new silver-thiolate stationary phase: silver(I)-dimercaptotriazine (Ag-DMT) functionalized silica gel targeting the separation of unsaturated compounds. We show Ag-DMT affords substantially higher retention factors, peak resolutions and capacities for TAGs and FAEEs than Ag-MP does. Ag-DMT also yields higher purity eicosapentaenoic acid (EPA) from fish oil FAEE mixtures than Ag-MP. In addition, Ag-DMT resolves double bond positional and cis/trans-isomers of C_18:1 fatty acid methyl esters (FAMEs) as well as unsaturated methyl/ethyl alkenones with different number of double bonds. Based on van't Hoff plots, enthalpy changes during the adsorption of unsaturated FAEEs onto Ag-DMT are ~2 times higher than those on Ag-MP. Such difference may be attributed to the stronger electron-withdrawing effect of the thiol group on DMT, which results in more positively charged silver ions hence greater interactions with unsaturated molecules. The stronger interaction between double bonds and Ag-DMT is further corroborated by density-functional theory (DFT) calculations. Ag-DMT shows its high stability for repeated uses in the separation of TAGs over 319 runs, with peak resolutions decreasing by < 3%. Collectively, our data demonstrate the exceptionally high efficiency of Ag-DMT column for separating unsaturated molecules.

Effective removal of tetracycline from water by batch method using activated carbon, magnetic carbon nanocomposite, and membrane hybrid technology

In this study two adsorbents (activated carbon; AC and magnetic activated carbon; MCN) were prepared from Dalbergia sissoo sawdust and used as potential adsorbents for the removal of tetracycline (TC) from water. Both the adsorbents were characterized by instrumental techniques like energy dispersive x-ray (EDX), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR), x-ray diffraction (XRD), surface area analyzer and thermal gravimetric/Differential thermal analysis (TG/DTA). The effect of antibiotic initial concentration, contact-time, pH, adsorbent-dose, and temperature were evaluated to determine optimum adsorption conditions. The optimum TC concentration for both AC and MCN was 120 mg/L while optimum time of saturation for both adsorbents was 120 min. The optimum pH determined was five while optimum adsorbent dose was 0.1 g. The adsorption isothermal data of both sets of experiments was best explained by Langmuir model. The kinetic data was well explained by pseudo-second order kinetics model. The ΔH° (enthalpy change) and ΔSo (entropy change) were; −14.989 and 25.174 kJ/mol for AC and −11.628 and 51.302 kJ/mol for MCN respectively. The values of Gibbs free energy (ΔG°) calculated for AC were 7.36, −7.99, −7.36, −7.61, and −8.12 kJ/mol while for MCN these were −15.02, −15.53, −16.05, −16.56, and −17.07 kJ/mol corresponding to temperatures; 298, 303, 313, 323, and 333 K. To control fouling in ultra-filtration, nano-filtration, and reverse osmosis membranes caused by TC, and both adsorbents, a continuous stirred reactor was connected in series with membrane pilot plant. The improvement brought about by both adsorbent in parameters like % retention and permeate flux was also evaluated. Comparatively, better improvement was brought about in % retention and permeates flux by MCN.

High-temperature liquid chromatography for evaluation of the efficiency of multiwalled carbon nanotubes as nano extraction beds for removal of acidic drugs from wastewater. Greenness profiling and comprehensive kinetics and thermodynamics studies

The retention behavior of a series of acidic drugs, namely ketoprofen (KET), naproxen (NAP), diclofenac (DIC), and ibuprofen (IBU), on the heat-resisting ZORBAX 300SB-C₁₈ column, was studied thermodynamically using high-temperature liquid chromatography (HTLC). A perfect correlation of the compounds' lipophilicity and the calculated thermodynamic indicators evidenced its contribution to the retention behavior. Besides, the steric fitting has a subsidiary effect on IBU retention. Isocratic HTLC separation of the four compounds was achieved using an aqueous mobile phase containing 30% acetonitrile-0.2% acetic acid-0.2% triethylamine at 60 °C. This method has been utilized to monitor the adsorption efficiency of multiwalled carbon nanotubes (MWCNTs) for the removal of the four NSAIDs from water. Different variables affecting the remediation process have been optimized such as the time of contact, pH, ionic strength, temperature, and the mass of MWCNTs. The kinetics and thermodynamics of the adsorption were investigated. The adsorption was evidenced to take place via pseudo-second-order kinetics and the intraparticle diffusion is the rate-controlling step. The thermodynamic investigation showed that the adsorption process is exothermic and enthalpy-driven, and the adsorption is more extensive at a lower temperature. The MWCNTs showed excellent adsorption efficiency of about 76.4 to 97.6% at the optimum conditions. The obtained results are promising and encouraging for the full-scale application of MWCNTs for remediation of NSAIDs-related water pollution. The green analytical chemistry metric "AGREE" and the analytical eco-scale score tool confirmed that the developed protocol is greener and more favorable to the environment and user than most of the reported literature.

Enantioseparation of Eight Pairs of Tetralone Derivative Enantiomers on Cellulose Based Chiral Stationary Phase by HPLC

Background:

Tetralone derivatives, important resources for the development of new drugs which can act in the treatment of central nervous system disorders or participate in synthesis reaction for the synthesis of various pharmaceuticals, have great research value and a bright prospect in exploitation.

Methods:

A novel chiral HPLC method for efficient enantioseparation of eight tetralone derivative enantiomers was developed on cellulose based CHIRALPAK IC chiral stationary phase under normal mode by investigating the effects of type and content of organic modifier, column temperature and flow rate on retention and enantioselectivity. Besides, the specificity, linearity, stability, precision, accuracy and robustness of this method were also validated.

Results:

Satisfactory enantioseparation was obtained for all enantiomers in n-hexane/2-propanol mobile phase system at ambient temperature. The thermodynamic study indicated that the solute transfer from the mobile to stationary phase was enthalpically favorable, and the process of enantioseparation was mainly enthalpy controlled. This method met the requirements for quantitative determination of tetralone derivative enantiomers.

Conclusion:

This study can provide great and important application value for enantioseparation of eight pairs of newly synthesized tetralone derivative enantiomers under normal mode using CHIRALPAK IC chiral column.

Teicoplanin-Modified HPLC Column as a Source of Experimental Parameters for Prediction of the Anticonvulsant Activity of 1,2,4-Triazole-3-Thiones by the Regression Models

The cell membrane is a complex system that consists of lipids, proteins, polysaccharides, and amphiphilic phospholipids. It plays an important role in ADME processes that are responsible for the final pharmaceutical effects of xenobiotics (bioavailability, activity). To study drug-membrane interaction at the molecular level, several high-performance liquid chromatography (HPLC) membrane model systems have been proposed which are mimicking mainly its lipid character. The aim of this work was to study interactions of new synthesized antiepileptic compounds of 4-alkyl-5-(3-chlorophenyl)-2,4-dihydro-3H-1,2,4-triazole-3-thione derivatives with Chirobiotic column containing glycoprotein ligand attached to the silica matrix. The affinity of the analytes to immobilized glycoprotein ligand was examined chromatographically in reversed-phase mode. The thermodynamics of interactions between bioactive compounds and teicoplanin was studied in terms of the van't Hoff linear relationship ln k vs. 1/T in the range of 5-45 °C. Change in enthalpy (ΔH°), change in entropy (ΔS°) and change in Gibbs free energy (ΔG°) were estimated utilizing graphical extrapolation and interpolation methods. The density functional theory (DFT) approach and docking simulations were used to get the molecular interpretation and prove the obtained experimental results. Cross-correlations of chromatographic and thermodynamic parameters with non-empirical topological and quantum chemical indices suggest that the polarizability of analytes appears to be responsible for the interactions of the tested molecules with teicoplanin and, ultimately, their retention on the column. Experimental and theoretical parameters were subjected to statistical analysis using regression models. Partial least squares (PLS) regression model showed the usefulness of the experimentally measured parameter φ0 (MeOH) to discriminate between anticonvulsant active and inactive 1,2,4-triazole-3-thione derivatives. Obtained results point out the usefulness of interaction of potential anticonvulsants with glycoprotein class of compounds to anticipate their activity.

Does phase ratio in reversed phase high performance liquid chromatography vary with temperature?

Phase ratio Φ for an high performance liquid chromatography (HPLC) column is a parameter defined as the ratio between the volume of the stationary phase V_s and the void volume of the column V_m. Together with the equilibrium constant K of the separation process, phase ratio is part of the retention factor k (k = KΦ). Although a considerable number of studies have been dedicated to the evaluation of V_s and V_m with the goal of obtaining the value for Φ, there are still debatable results regarding the true value of Φ, which for a column with a specific stationary phase may vary with the composition of the mobile phase. One route for the evaluation of the value of Φ uses the measurements of retention factors k on a specific column and mobile phase for two or more hydrocarbons for which the octanol/water partition coefficients log K_ow are known. This procedure has been applied in the present study for the evaluation of Φ for three commercially available C18 columns and two mobile phase compositions water/acetonitrile, in the temperature range 20 °C to 50 °C. It was found that phase ratio does change depending on the temperature, its "effective value" decreasing as the temperature increases which is in accordance with the decrease of retention times in reversed phase HPLC when the temperature increases. Besides other factors that may affect the correct calculation of thermodynamic functions, the change of phase ratio with temperature has implications regarding the possibility to calculate the enthalpy and entropy values from van't Hoff plots of the variation of log k as a function of 1/T, even when the retention process is dominated by unique hydrophobic type interactions.

Sources of Nonlinear van't Hoff Temperature Dependence in High-Performance Liquid Chromatography

In HPLC, the nonlinear behavior of the retention factor k' with temperature (dependence of ln k' on 1/T) can be attributed to the multiple interactions of a unique analyte in the separation process and/or to the existence in solution of multiple forms of the analyte (also leading to different free enthalpies of interaction). In this study, several examples of nonlinear retention-temperature dependence are evaluated for both reversed-phase (RP) and hydrophilic interaction chromatography (HILIC) separations. The potential explanation for nonlinear retention-temperature behavior is evaluated for each example, some caused by multiple interactions in the separation system of a unique analyte and others by multiple forms of the analyte. In cases where the analyte does not have more forms and the separation is based predominantly on one type of interaction (e.g., hydrophobic interaction in RP-HPLC), the dependence is linear, as expected. By studying the changes in the chemical structure of a compound as a function of pH it is possible to decide, in many cases, if a unique form or multiple forms of a compound are present in the solution. The use of this information allows us to determine when the lack of linearity (when present) is caused by multiple interactions in the separation system (for one form of the compound) and when more forms are causing the lack of linearity. The approximation with a quadratic form for the nonlinear dependence has been verified in most cases to be good, and only minor improvements were obtained when using higher polynomial dependencies.

Liquid Chromatographic Separation and Thermodynamic Investigation of Ethyl Nipecotate Enantiomers on Immobilized Amylose-Based Chiral Stationary Phase

Ethyl nipecotate enantiomers are widely used as chiral building blocks in the synthesis of drug substances. An efficient and economic chiral high-performance liquid chromatographic method for determination of enantiomeric purity of ethyl nipecotate is developed and validated. Chiral separation was achieved on immobilized amylose-based stationary phase using a mixture of n-hexane: ethanol: diethylamine (80:20:0.1, v/v/v) as mobile phase. Resolution between R-(-)-ethyl nipecotate (REN) and S-(+)-ethyl nipecotate (SEN) peaks was found to be 3.59. The detector response of SEN exhibited an excellent linearity over the concentration range of 4.5–120 μg mL−1. The limit of detection and limit of quantification for SEN were 0.016 and 0.045 μg and REN were 0.015 and 0.043 μg, respectively. The influence of column oven temperatures on chiral retention and separation was studied in the range of 25°C to 50°C; the thermodynamic parameters ΔH°, ΔS° and ΔG° were evaluated from van’t Hoff plots and used to explain the strength of interaction between analyte and stationary phase.

High-Temperature Liquid Chromatography and the Hyphenation with Mass Spectrometry Using High-Pressure Electrospray Ionization

Increasing the operating temperature of the liquid chromatography (LC) column has the same effect as reducing the diameter of the packing particles on minimizing the contribution of C-term in the van Deemter equation, flattening the curve of plate height vs. linear velocity in the high-speed region, thus allowing a fast LC analysis without the loss of plate count. While the use of smaller particles requires a higher pumping pressure, operating the column at higher temperature reduces the pressure due to lower liquid viscosity. At present, the adoption of high-temperature LC lags behind the ultra-high-pressure LC. Nevertheless, the availability of thermally stable columns has steadily improved and new innovations in this area have continued to emerge. This paper gives a brief review and updates on the recent advances in high-temperature liquid chromatography (HTLC). Recent efforts of hyphenating the capillary HTLC with mass spectrometry via a super-atmospheric pressure electrospray ionization is also reported.

High-performance liquid chromatographic and subcritical fluid chromatographic separation of α-arylated ß-carboline, N-alkylated tetrahydroisoquinolines and their bioisosteres on polysaccharide-based chiral stationary phases

New, pharmacologically interesting chiral amino compounds, namely, stereoisomers of α-hydroxynaphthyl-ß-carboline, benz[d]azepine and benz[c]azepine analogs as well as N-α-hydroxynaphthylbenzyl-substituted isoquinolines were enantioseparated by high-performance liquid chromatographic and subcritical fluid chromatographic methods on polysaccharide-based chiral stationary phases. Separation of the stereoisomers was optimized in both subcritical fluid chromatography and normal phase liquid chromatographic modes by investigating the effects of the composition of the bulk solvent, temperature, and the structures of the analytes and selectors. Both normal phase liquid chromatography and subcritical fluid chromatography exhibited satisfactory performance, albeit with somewhat different effectiveness in the separation of the stereoisomers studied. The optimized methods offer the possibility to apply preparative-scale separations thereby enabling further pharmacological investigations of the enantiomers.

Efficient preparative separation of 6-(4-aminophenyl)-5-methyl-4, 5-dihydro-3(2H)-pyridazinone enantiomers on polysaccharide-based stationary phases in polar organic solvent chromatography and supercritical fluid chromatography

6-(4-Aminophenyl)-5-methyl-4,5-dihydro-3(2H)-pyridazinone is a key synthetic intermediate for cardiotonic agent levosimendan. Very few studies address the use of chiral stationary phases in chromatography for the enantioseparation of this intermediate. This study presents two efficient preparative methods for the isolation of (R)(-)-6-(4-aminophenyl)-5-methyl-4,5-dihydro-3(2H)-pyridazinone in polar organic solvent chromatography and supercritical fluid chromatography using polysaccharide-based chiral stationary phases and volatile organic mobile phases without additives in isocratic mode. Under optimum conditions, Chiralcel OJ column showed the best performance (α = 1.71, Rs = 5.47) in polar organic solvent chromatography, while Chiralpak AS column exhibited remarkable separations (α = 1.81 and Rs = 6.51) in supercritical fluid chromatography with an opposite enantiomer elution order. Considering the sample solubility, runtime and solvent cost, the preparations were carried out on Chiralcel OJ column and Chiralpak AS column (250 × 20 mm i.d.; 10 µm) in polar organic mode and supercritical fluid chromatography mode with methanol and CO₂ /methanol as mobile phases, respectively. By utilizing the advantages of chromatographic techniques and polysaccharide-based chiral stationary phases, this work provides two methods for the fast and economic preparation of (R)(-)-6-(4-aminophenyl)-5-methyl-4,5-dihydro-3(2H)-pyridazinone, which are suitable for the pharmaceutical industry.

Chiral high-performance liquid and supercritical fluid chromatographic enantioseparations of limonene-based bicyclic aminoalcohols and aminodiols on polysaccharide-based chiral stationary phases

Enantioseparation of limonene-based bicyclic 1,3-aminoalcohols and 1,3,5- and 1,3,6-aminodiols was performed by normal-phase high-performance liquid chromatographic and supercritical fluid chromatographic (SFC) methods on polysaccharide-based chiral stationary phases. The effects of the composition of the mobile phase, the column temperature and the structures of the analytes and chiral selectors on retention and selectivity were investigated by normal-phase LC and SFC technique. Thermodynamic parameters derived from selectivity-temperature-dependence studies were found to be dependent on the chromatographic method applied, the nature of the chiral selector and the structural details of the analytes. Enantiorecognition in most cases was enthalpically driven but an unusual temperature behavior was also observed: decreased retention times were accompanied by improved separation factors with increasing temperature, i.e. some entropically driven separations were also observed. The elution sequence was determined in all cases. The separation of the stereoisomers was optimized in both chromatographic modalities.

Thermodynamic Insights into the Separation of Carotenoids in Reversed-Phase Liquid Chromatography

The retention mechanism of four major carotenoids, two xanthophylls (i.e., lutein and zeaxanthin) and two carotenes (i.e., lycopene and β-carotene), was investigated in reversed-phase liquid chromatography with the aim of thermodynamic analysis. The experimental variables considered in this study were the composition of mobile phase (MP) and the temperature. Chromatographic elutions were undertaken under linear, isocratic conditions by using a C18 stationary phase, four different MP compositions (by varying the ratio methanol/acetonitrile from 66.5/28.5 to 47.5/47.5 v/v), and column temperatures in the range 283–313 K. Traditional Van't Hoff analysis has been used to estimate changes of standard enthalpy (ΔH°) and Gibbs free energy (ΔG°) associated with the solute transfer from the mobile to the stationary phase at each mobile phase composition. The thermodynamic quantities have been correlated to the structure of investigated carotenoids and their interaction with the octadecyl silica stationary phase.

Column Characterization and Selection Systems in Reversed-Phase High-Performance Liquid Chromatography

Reversed-phase high-performance liquid chromatography (RP-HPLC) is the most popular chromatographic mode, accounting for more than 90% of all separations. HPLC itself owes its immense popularity to it being relatively simple and inexpensive, with the equipment being reliable and easy to operate. Due to extensive automation, it can be run virtually unattended with multiple samples at various separation conditions, even by relatively low-skilled personnel. Currently, there are >600 RP-HPLC columns available to end users for purchase, some of which exhibit very large differences in selectivity and production quality. Often, two similar RP-HPLC columns are not equally suitable for the requisite separation, and to date, there is no universal RP-HPLC column covering a variety of analytes. This forces analytical laboratories to keep a multitude of diverse columns. Therefore, column selection is a crucial segment of RP-HPLC method development, especially since sample complexity is constantly increasing. Rationally choosing an appropriate column is complicated. In addition to the differences in the primary intermolecular interactions with analytes of the dispersive (London) type, individual columns can also exhibit a unique character owing to specific polar, hydrogen bond, and electron pair donor-acceptor interactions. They can also vary depending on the type of packing, amount and type of residual silanols, "end-capping", bonding density of ligands, and pore size, among others. Consequently, the chromatographic performance of RP-HPLC systems is often considerably altered depending on the selected column. Although a wide spectrum of knowledge is available on this important subject, there is still a lack of a comprehensive review for an objective comparison and/or selection of chromatographic columns. We aim for this review to be a comprehensive, authoritative, critical, and easily readable monograph of the most relevant publications regarding column selection and characterization in RP-HPLC covering the past four decades. Future perspectives, which involve the integration of state-of-the-art molecular simulations (molecular dynamics or Monte Carlo) with minimal experiments, aimed at nearly "experiment-free" column selection methodology, are proposed.

Enantiomeric quality control of R-Tofisopam by HPLC using polysaccharide-type chiral stationary phases in polar organic mode

A novel, fast and economic chiral HPLC method was developed and validated for the resolution of the four isomers of tofisopam. The separation capacity of eleven different chiral columns: six polysaccharide-type including three amylose-based (Chiralpak AD, Chiralpak AD-RH and Chiralpak AS) and three cellulose-based (Chiralcel OD, Chiralcel OJ and Lux Cellulose-4); three cyclodextrin- (Quest-BC, Quest-C2 and Quest-CM) and two macrocyclic glycopeptide antibiotic-type (Chirobiotic T and Chirobiotic TAG) were screened using polar organic or reversed-phase mode. Chiralpak AD, based on amylose tris(3,5-dimethylphenylcarbamate) as chiral selector with neat methanol was identified as the most promising system. In order to improve resolution, an orthogonal experimental design was employed, altering the concentration of 2-propanol, column temperature, and flow rate in a multivariate manner. Using the optimized method (85/15 v/v methanol/2-propanol, 40°C, flow rate: 0.7 mL/min) we were not only able to separate the four isomers but also detect 0.1% S-enantiomer as chiral impurity in R-tofisopam. This is important since the latter is under development as a single enantiomeric agent. Thermodynamic investigation revealed an unusual entropy and enthalpy-entropy co-driven controlled enantioseparation on Chiralcel OJ and on Chiralpak AD column, respectively. Our newly developed HPLC method was validated according to the ICH guidelines and its application was tested on a pharmaceutical formulation containing the racemic mixture of the drug. As a further novelty, a separate circular dichroism method was applied for the investigation of the interconversion kinetics of tofisopam conformers, which proved to be crucial for sample preparation and method validation.

Liquid chromatographic enantiomer separations applying chiral ion-exchangers based on Cinchona alkaloids

As the understanding of the various biological actions of compounds with different stereochemistry has grown, the necessity to develop methods for the analytical qualification and quantification of chiral products has become particularly important. The last quarter of the century has seen a vast growth of diverse chiral technologies, including stereocontrolled synthesis and enantioselective separation and analysis concepts. By the introduction of covalently bonded silica-based chiral stationary phases (CSPs), the so-called direct liquid chromatographic (LC) methods of enantiomer separation became the state-of-the-art methodology. Although a large number of CSPs is available nowadays, the design and development of new chiral selectors and CSPs are still needed since it is obvious that in practice one needs a good portfolio of different CSPs and focused "chiral columns" to tackle the challenging tasks. This review discusses and summarizes direct enantiomer separations of chiral acids and ampholytes applying anionic and zwitterionic ion-exchangers derived from Cinchona alkaloids with emphasis on literature data published in the last 10 years. Our aim is to provide an overview of practical solutions, while focusing on the integration of molecular recognition and methodological variables.

Functionalities tuned enantioselectivity of phenylcarbamate cyclodextrin clicked chiral stationary phases in HPLC

The mixed chloro- and methyl- functionalities can greatly modulate the enantioselectivities of phenylcarbamate cyclodextrin (CD) clicked chiral stationary phases (CSPs). A comparison study is herein reported for per(4-chloro-3-methyl)phenylcarbamate and per(2-chloro-5-methyl)phenylcarbamate β-CD clicked CSPs (i.e., CCC4M3-CSP and CCC2M5-CSP). The enantioselectivity dependence on column temperature was studied in both normal-phase and reversed-phase mode high performance liquid chromatography (HPLC). The thermodynamic study revealed that the stronger intermolecular interactions can be formed between CCC4M3-CSP and chiral solutes to drive the chiral separation. The higher enantioselectivities of CCC4M3-CSP were further demonstrated with the enantioseparation of 17 model racemates in HPLC.

Liquid chromatographic enantioseparation of carbocyclic β-amino acids possessing limonene skeleton on macrocyclic glycopeptide-based chiral stationary phases

Polar-ionic and reversed-phase high-performance liquid chromatographic separations of limonene-based cyclic β-amino acid enantiomers were carried out by using macrocyclic glycopeptide-based chiral selectors applying Chirobiotic T, TAG and R columns. The effects of additives, concentration of the co- and counter-ions and the temperature in polar-ionic mobile phase systems were studied. The influence of pH, MeOH content and alcohol additives were investigated in the reversed-phase mode. The difference in the change in standard enthalpy Δ(ΔH°), entropy Δ(ΔS°), and free energy Δ(ΔG°) was calculated from the linear van't Hoff plots derived from the ln α vs 1/T curves in the temperature range 5-40°C. Unusual temperature behavior was observed on Chirobiotic TAG for most of the analytes: decreased retention times were accompanied with increased separation factors with increasing temperature, and separation was entropically-driven. For two of the studied analytes enthalpically-driven enantioseparations were observed. The elution sequence was determined in all cases, but no general rule could be established.