Performance and selected applications of a new range of chemically bonded packing materials in high-performance liquid chromatography

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.

A simple method for measuring excess adsorption isotherms of organic eluent components on reversed-phase packing materials

A simple frontal analysis method has been developed for the reliable measurement of excess adsorption isotherms of an organic component on reversed-phase adsorbents in a series of programmed concentration steps. In the present method, a peak, which is produced by refractive index change in column eluate, is detected at 589 nm; it represents the elution volume of the boundary. The method is applied to the measurement of the excess adsorption isotherms of organic eluent components from water on commercially available reversed-phase stationary phases. The results are in good agreement with the previously reported isotherms. We also measure the excess adsorption isotherms of organic eluent components from solutions containing electrolytes. There are not any interference peaks on the elution traces. The method is thus reliably applicable to the evaluation of the excess adsorption of organic eluent components in practical systems.

Hydrophilic interaction liquid chromatography (HILIC)--a powerful separation technique

Hydrophilic interaction liquid chromatography (HILIC) provides an alternative approach to effectively separate small polar compounds on polar stationary phases. The purpose of this work was to review the options for the characterization of HILIC stationary phases and their applications for separations of polar compounds in complex matrices. The characteristics of the hydrophilic stationary phase may affect and in some cases limit the choices of mobile phase composition, ion strength or buffer pH value available, since mechanisms other than hydrophilic partitioning could potentially occur. Enhancing our understanding of retention behavior in HILIC increases the scope of possible applications of liquid chromatography. One interesting option may also be to use HILIC in orthogonal and/or two-dimensional separations. Bioapplications of HILIC systems are also presented.

Chain conformation and solvent partitioning in reversed-phase liquid chromatography: Monte Carlo simulations for various water/methanol concentrations

Many structural models for the stationary phase in reversed-phase liquid chromatography (RPLC) systems have been suggested from thermodynamic and spectroscopic measurements and theoretical considerations. To provide a molecular picture of chain conformation and solvent partitioning in a typical RPLC system, a particle-based Monte Carlo simulation study is undertaken for a dimethyl octadecyl (C(18)) bonded stationary phase on a model siliceous substrate in contact with mobile phases having different methanol/water concentrations. Following upon previous simulations for gas-liquid chromatography and liquid-liquid phase equilibria, the simulations are conducted using the configurational-bias Monte Carlo method in the Gibbs ensemble and the transferable potentials for phase equilibria force field. The simulations are performed for a chain surface density of 2.9 micromol/m(2), which is a typical bonded-phase coverage for mono-functional alkyl silanes. The solvent concentrations used here are pure water, approximately 33 and 67% mole fraction of methanol and pure methanol. The simulations show that the chain conformation depends only weakly on the solvent composition. Most chains are conformationally disordered and tilt away from the substrate normal. The interfacial width increases with increasing methanol content and, for mixtures, the solvent shows an enhancement of the methanol concentration in a 10 Angstrom region outside the Gibbs dividing surface. Residual surface silanol groups are found to provide hydrogen bonding sites that lead to the formation of substrate bound water and methanol clusters, including bridging clusters that penetrate from the solvent/chain interfacial region all the way to the silica surface.

High-performance liquid chromatography retention mechanisms and their mathematical descriptions

Different mechanisms of analyte behavior in HPLC columns are considered from the point of view of analyte distribution between the adsorbent and the bulk liquid phase. Expressions describing the analyte retention volume are derived for each selected retention mechanism. Methodology of the incorporation of multi-equilibrium processes in the analyte retention expression is suggested for ideal linear chromatography conditions. The description of eluent components' adsorption is discussed on the basis of the excess adsorption process. The applicability of the excess adsorption description without introduction of the adsorbed layer model or any other convention to the chromatographic process is shown.

Influence of the degree of coverage of C18-bonded stationary phases on the mass transfer mechanism and its kinetics

The influence of the degree of coverage of a silica surface with bonded C18 alkyl chains on the mass transfer mechanism in RPLC was investigated. Five packing materials were used, prepared with the same batch of silica particles (5 microm diameter, 90 A average pore size): one column was packed with the silica derivatized by trimethylchlorosilane (TMS) (C1, 3.92 micromol/m2), and the other four with the silica first derivatized with octadecyl-dimethyl-chlorosilane (C18, 0.42, 1.01, 2.03, and 3.15 micromol/m2), and then endcapped with TMS. A solution of methanol and water (25/75, v/v) was used as the mobile phase. The experimental HETP curves were acquired for each column by measuring the first moment and the second central moment of phenol and correcting them for the influence of the temperature increase due to the heat generated by the friction of the stream against the bed. The different kinetic parameters of the mass transfer in these packed chromatographic columns were identified (longitudinal diffusion, eddy diffusion, film mass transfer, and transparticle mass transfer) and quantified by fitting the experimental data to a new general HETP equation recently derived [F. Gritti, G. Guiochon, Anal. Chem., in press (AC-060203R).]. The agreement was excellent and allowed the comparison of the kinetic parameters among the six columns used. The highest column efficiency measured at conventional or fast flow rates (>0.5 ml/min) is obtained for the most retentive column, which has a surface coverage of 2.03 micromol/m2. The smallest HETP measured is as low as 10 microm, only twice the average particle diameter dp, due to the large contribution of surface diffusion (90%) to the particle effective diffusivity. However, no significant difference was observed between the efficiencies of the columns packed with C1 and C18 derivatized silica.

Part II. Chromatography using ultra-stable metal oxide-based stationary phases for HPLC

In this part of the review authors discuss methods used for modification of metal oxide surfaces. On the basis of literature data it is shown, that silanization of the surfaces do not form stable supports for chromatography. On the other hand, the success of polymer modified surfaces such as polybutadiene (PBD) and polystyrene (PS) is emphasized. Permanent modification of metal oxide surfaces with Lewis bases is also widely discussed. Chromatographic properties of polymer modified surfaces of zirconia are discussed in details. The perspectives of carbon-coated metal oxide surfaces in HPLC and high temperature separations are described.

Characteristics and mechanism of surface diffusion in reversed-phase liquid chromatography using various alkyl ligand bonded silica gels

Kinetic and thermodynamic parameters concerning surface diffusion were derived in reversed-phase liquid chromatography (RPLC) systems using a series of silica gel packing materials bonded with C1, C4, C8, and C18 alkyl ligands. In this paper, a more detailed study on some characteristics and the mechanism of surface diffusion in the RPLC systems using the different alkyl ligand bonded silica gels was made from a thermodynamic point of view by analyzing the surface diffusion data with consideration of the correlation between the parameters concerning surface diffusion and those relating to the retention equilibrium. The mechanism of surface diffusion seems to be the same irrespective of the length of the alkyl ligands because of the presence of the extrathermodynamic relationships, i.e., the enthalpy-entropy compensation and the linear free energy relationship. It was indicated that the surface diffusion coefficient (Ds) of weakly retained compounds was of the same order of magnitude with corresponding molecular diffusivity and that Ds decreased with increasing retention. The activation energy of surface diffusion (Es) was formulated as a linear function of the isosteric heat of adsorption (Qst). A modified Arrhenius equation, which is derived by taking into account the linear correlation between the two thermodynamic parameters Es and Qst, appropriately explains the intrinsic characteristics and mechanism of surface diffusion.

Modified aluminas as chromatographic supports for high-performance liquid chromatography

This review begins by describing the relevant properties of alumina as a support material for chemically bonded stationary phases in HPLC. The most common chemical modification processes are summarized as well as the advantages and disadvantages of each method. In order to more fully understand the chemically modified alumina surface, some spectroscopic approaches are outlined for characterization of the bonded phases. Finally, a number of successful applications are described for a variety of chemically modified aluminas in order to illustrate their potential usefulness and to compare their chromatographic behavior to the more conventional silica-based materials.

Interpretation of the excess adsorption isotherms of organic eluent components on the surface of reversed-phase adsorbents

The excess adsorption isotherms of acetonitrile, methanol and tetrahydrofuran from water on reversed-phase packings were studied, using 10 different columns packed with C1-C6, C8, C10, C12, and C18 monomeric phases, bonded on the same type of silica. The interpretation of isotherms on the basis of the theory of excess adsorption shows significant accumulation of the organic eluent component on the adsorbent surface on the top of "collapsed" bonded layer. The accumulated amount was shown to be practically independent of the length of alkyl chains bonded to the silica surface. A model that describes analyte retention on a reversed-phase column from a binary mobile phase is developed. The retention mechanism involves a combination of analyte distribution between the eluent and organic adsorbed layer, followed by analyte adsorption on the surface of the bonded phase. A general retention equation for the model is derived and methods for independent measurements of the involved parameters are suggested. The theory was tested by direct measurement of analyte retention from the eluents of varied composition and comparison of the values obtained with those theoretically calculated values. Experimental and theoretically calculated values are in good agreement.

Influence of the modification conditions of alkyl bonded ligands on the characteristics of reversed-phase liquid chromatography

The parameters characterizing retention and the mass transfer mechanisms in reversed-phase liquid chromatography were derived from data acquired for a series of compounds on a silica gel bonded to alkyl ligands of various lengths and ligand densities. The adsorption equilibrium constant, the absolute value of the isosteric heat of adsorption, and the activation energy of surface diffusion increase while the surface diffusion coefficient decreases with increasing length of the alkyl chain and density of the C18 ligand. These results are consistent with increasing restriction of the mobility of an increasingly adsorbable molecule. There is a critical level of carbon content above which the four parameters just listed no longer change with increasing alkyl ligand density. This level is likely to depend on the sizes of the alkyl ligand and the sample molecule. Even at high densities of C18 chains, there may be a limitation in the contact area of the sample molecule and the alkyl ligand. The retention behavior on the stationary phases studied is explained by assuming that the sample molecules penetrate into the layer of alkyl ligands. Enthalpy-entropy compensation was demonstrated for surface diffusion. Its mechanism is probably similar on the different stationary phases studied, irrespective of the length and density of the bonded ligand.

Application of mixed mobile phases and a step gradient method in capillary electrochromatography for the separation of isomeric polycyclic aromatic hydrocarbon-deoxyribonucleoside adduct mixtures prepared in vitro

Capillary electrochromatography (CEC) was used for the analysis of mixtures of neutral isomeric compounds derived from the reaction of carcinogenic hydrocarbon (benzo[g]chrysene and 5,6-dimethylchrysene) dihydrodiol epoxides with calf thymus deoxyribonucleic acid (DNA). The CEC analysis demonstrated higher resolution, greater speed and lower analyte consumption than high-performance liquid chromatography (HPLC) in the analysis of the same samples using the same type of stationary phase. Proper selection of the mixed mobile phases was critical for the separation of these complex mixtures with enhanced speed and selectivity. The use of a step gradient further improved the speed of the CEC analysis resulting in electrochromatograms that required only 25-70% of the corresponding HPLC analysis times.