Characterisation and prediction of retention in isocratic and gradient-elution normal-phase high-performance liquid chromatography on polar bonded stationary phases with binary and ternary solvent systems

Liquid chromatography at the university of pardubice: a tribute to Professor Pavel Jandera

Pavel Jandera was a world-leading analytical chemist who devoted his entire professional life to research in the field of high-performance liquid chromatography. During all his scientific career, he worked at the Department of Analytical Chemistry at the University of Pardubice, Czech Republic. His greatest contribution to the field of liquid chromatography was the introduction of a comprehensive theory of liquid chromatography with programmed elution conditions. He was also involved in the research of gradient elution techniques in preparative chromatography, modeling of retention and selectivity in various phase systems, preparation of organic monolithic microcolumns and, last but not least, in the development of theory and practical applications of two-dimensional liquid chromatography, mainly in the comprehensive form. In this review article, we have tried to capture the highlights of his scientific career and provide the readers with a detailed overview of Pavel Jandera's contribution to the evolution of separation sciences. This article is protected by copyright. All rights reserved.

Simple models for the effect of aliphatic alcohol additives on the retention in reversed-phase liquid chromatography

Four retention models for the effect of aliphatic alcohol additives on the retention of analytes in reversed-phase liquid chromatography have been developed following either a semi-thermodynamic treatment or an empirical approach. Their performance was tested using the experimental retention times of six non-polar analytes (alkylbenzenes) and ten o-phthalaldehyde derivatives of amino acids under different isocratic chromatographic runs when a small amount of ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol or 1-heptanol was added to methanol/water mixtures containing a constant amount of methanol. It was shown that for the structurally simple alkylbenzenes all the models can be adopted for retention prediction with good results. In contrast, just one out of four models, that with the fewest approximations, predicts satisfactorily the retention properties of amino acids derivatives. However, the most interesting feature is that this model can predict the effect of an alcohol-additive on the retention properties of solutes, even if this additive was not used in chromatographic runs done for the fitting procedure, provided that it belongs to the same homologous series of alkanols. This feature is also observed in all models described the retention of alkylbenzenes.

Retention prediction in reversed-phase liquid chromatography systems with methanol/water mobile phases containing different alkanols as additives

In an effort to gain enhancement of selectivity in reversed-phase liquid chromatography, retention was tuned in this study by introducing short and medium straight-chained-length alkanol additives (methanol (MeOH), ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol or 1-heptanol) at low concentrations in mobile phases containing MeOH as the main organic solvent. A six-parameter retention model considering simultaneously the contents of the main organic modifier and of the alcohol additive as well as of the number of alkyl chain of additive was developed by a direct combination of equations expressing separately a linear dependence of the retention upon each of these factors. The effectiveness of the above model was tested in the retention prediction of a mixture of six alkylbenzenes under isocratic conditions with mobile phases containing as an additive any member of the homologues series of alkanols (with 1-7 carbon atoms) at different low concentrations in a wide range of MeOH-water mixtures. The prediction was excellent in all cases even when the alkanol additives used in experiments for the fitting procedure are different than those used in chromatographic runs done for testing the prediction ability of the proposed model.

Structure-retention and mobile phase-retention relationships for reversed-phase high-performance liquid chromatography of several hydroxythioxanthone derivatives in binary acetonitrile-water mixtures

The reversed-phase high-performance liquid chromatographic (RP-HPLC) behavior of some newly synthesized hydroxythioxanthone derivatives using binary acetonitrile-water mixtures as mobile phase has been examined. First, the variation in the retention time of each molecule as a function of mobile phase properties was studied by Kamlet-Taft solvatochromic equations. Then, the influences of molecular structure of the hydroxythioxanthone derivatives on their retention time in various mobile phase mixtures were investigated by quantitative structure-property relationship (QSPR) analysis. Finally, a unified model containing both the molecular structure parameters and mobile phase properties was developed to describe the chromatographic behavior of the systems studied. Among the solvent properties, polarity/polarizability parameter (pi*) and hydrogen-bond basicity (beta), and among the solute properties, the most positive local charge (MPC), the sum of positive charges on hydrogen atoms contributing in hydrogen bonding (SPCH) and lipophilicity index (logP) were identified as controlling factors in the RP-HPLC behavior of hydroxythioxanthone derivatives in actonitrile-water binary solvents.

Can the theory of gradient liquid chromatography be useful in solving practical problems?

Advances in the theory of gradient liquid chromatography and their practical impacts are reviewed. Theoretical models describing retention in reversed-phase, normal-phase and ion-exchange modes are compared. Main attention is focused on practically useful models described by two- or three-parameter equations fitting the experimental data in the range of mobile phase composition utilized for sample migration during gradient elution. The applications of theory for gradient method development, optimization and transfer are addressed. The origins and possibilities for overcoming possible pitfalls are discussed, including the effects of the instrumental dwell volume, uptake of mobile phase components on the column and size of the sample molecules. Special attention is focused on gradient separations of large molecules.

Retention modelling in ternary solvent-strength gradient elution reversed-phase chromatography using 30mm columns

An optimization strategy for ternary solvent-strength gradient elution RP chromatography is described in which a two-dimensional model of gradient time (2 levels) against ternary proportions of organic modifiers (4 levels) was constructed. From the resolution surface the optimum ratio of organic modifiers could be selected. Excellent retention time and acceptable peak width and resolution simulations were obtained. The separation could be further optimized from the same input data by using a standard one-dimensional model in order to optimize for gradient slope, duration and shape. Excellent retention time and acceptable peak width and resolution simulations were obtained (< 1, 2 and 6% error, respectively).

Column equilibration effects in gradient elution in reversed-phase liquid chromatography

The fundamental equations and conditions for linear and stepwise gradient elution in reversed-phase liquid chromatography are applied to a mixture of amino acids in their underivatized form in aqueous mobile phases modified by 2-propanol, acetonitrile or methanol for examining column equilibration effects. It was found in all cases systematic deviations between experimental and calculated retention times, which are prominent in 2-propanol, reduced in acetonitrile and practically negligible in methanol. These deviations appear within a chromatogram just after the first change in the composition of the mobile phase reaches the detector and last ca. 5 min, where the magnitude of errors reduces exponentially with time. Based on these observations we propose a simple way to correct the calculated from the gradient elution theory retention times of sample solutes. The origin of the discrepancies between theory and experiment as well as their impact on the resolution is also discussed.

Adsorption model for retention in normal-phase liquid chromatography with ternary mobile phases

The review gives a link between the theory of adsorption from multicomponent solutions and liquid chromatography. The article surveys the methods developed to describe the retention in normal-phase chromatography with ternary mobile phases with emphasis on the results of the authors. In the model used the driving force for the separation is the difference in adsorption of a solute and all solvents onto the solid surface. The general equation generates a series of simple linear relationships to predict the retention factor in ternary mobile phase for which certain parameters remain fixed. Theoretical concepts are tested by comparison with experimental data. The correlations between parameters characterizing retention in ternary, binary and pure solvents are discussed.

Influence of mobile phase composition and thermodynamics on the normal phase chromatography of echinocandins

In the normal phase preparative HPLC of fermentation derived echinocandins, resolution of key impurities from the product of interest, pneumocandin B(o), is accomplished using a ternary ethyl acetate/methanol/water mobile phase with silica gel as the sorbent. In this work, previous characterization of the system is extended to define the impact and role of water content on the separation efficiency and retention of pneumocandin B(o). Experimental results indicate that column efficiency, measured using both the product of interest and small molecule tracers (compounds used for pulse tests), is good despite the use of an irregular silica and unusually high levels (greater than 6%) of water in the mobile phase. In contrast to column efficiency measurements using small molecules (MEK and toluene), measurements performed with the product itself indicate improved efficiency with increasing water content of the mobile phase. Building on these results, a scale-up/scale-down protocol was developed based on measurements of column efficiency using theoretical plate counts determined with pneumocandin B(o). Since the solubility of pneumocandin B(o) in the ternary mobile phase is relatively low, a higher strength solvent with higher levels of methanol and water is employed for dissolution of the crude product at concentrations of up to 40g/L. The mismatch between the high strength solvent used for the feed introduction and the mobile phase has the potential to affect column performance. The impact of this mismatch using plate count measurements with the product at both analytical and semi-preparative scales was found not to be significant. Finally, a van't Hoff analysis was performed to characterize the thermodynamics of adsorption of pneumocandin B(o) on silica. The analysis shows that the adsorption process for pneumocandin B(o) on silica in the ternary solvent system is endothermic (DeltaH(ads)>0), implying that the adsorption is entropically driven. Results from an overall water balance across the column indicate significant enrichment of adsorbed water on the silica surface. These results further emphasize the importance of selective partitioning of water between the bulk mobile phase and the silica as a dominant factor in controlling retention.

Retention prediction in ternary solvent reversed-phase liquid chromatography systems based on the variation of retention with binary mobile phase composition

An extension of the treatment adopted in a recent paper [P. Nikitas, A. Pappa-Louisi, P. Agrafiotou, J. Chromatogr. A 946 (2002) 33] was used to derive expressions describing the variation of solute retention k with composition in ternary reversed phase liquid chromatography, RP-LC, solvent systems. The equation of the partition model obtained in this way for a ternary mobile phase was identical to that previously derived using the solubility parameter concept. This equation as well as two new expressions of In k versus organic modifiers content were tested in a variety of ternary solvent systems in order to examine the possibility of predicting retention behavior of solutes under ternary solvent mixture elution conditions from known retention characteristics in binary mobile phases. It was demonstrated the superiority of both new equations derived in this paper to that previously proposed and applied to date in ternary solvent mixtures.

Gradient elution in normal-phase high-performance liquid chromatographic systems

Gradient elution is widely used for separation of complex samples in reversed-phase HPLC systems, but is less frequently applied in normal-phase HPLC, where it has a notoriously bad reputation for poor reproducibility and unpredictable retention. This behaviour is caused by preferential adsorption of polar solvents used in mixed mobile phases, which may cause significant deviations of the actual gradient profile from the pre-set program. Another important source of irreproducible retention behaviour is gradual deactivation of the adsorbent by adsorption of even traces of water during normal-phase gradient elution. To avoid this phenomenon, carefully dried solvents should be used. Finally, column temperature should be carefully controlled during normal-phase gradient elution if reproducible results are to be obtained. Working with dry solvents at a controlled constant temperature and using a sophisticated gradient-elution chromatograph, reproducibility of the retention data in normal-phase gradient elution better than 2% may be achieved even over several months of column use. The retention data in gradient elution can be calculated accurately if appropriate corrections are adopted for the gradient dwell volume and for the preferential adsorption of the polar solvents using experimental adsorption isotherms. The average error of prediction for the corrected calculated gradient retention data was lower than 2% for a silica gel column and lower than 3% for a bonded nitrile column, which may be suitable for the optimization of separation. Further, a simple approach is suggested for rapid estimation of changes in the retention induced by a change in the gradient profile in normal-phase HPLC. For such a rough estimation, it is not necessary to know the parameters of the dependence of the solute retention factors on the composition of the mobile phase.

Predicting the behaviour of polydisperse polymers in liquid chromatography under isocratic and gradient conditions

In this paper we describe how the existing theories to describe retention and peak width in isocratic and gradient-elution liquid chromatography can be expanded to describe the retention behaviour of natural and synthetic repetitive polymers, which feature distributions of molecules with different masses (and often different structures) rather than unambiguous molecular formulas. For polydisperse samples, it is vital that the model accommodates (isocratic) elution of sample components before the onset of a gradient, elution during the gradient, and elution after the completion of the gradient. The expanded models can readily be implemented in standard spreadsheet software, such as Excel. We have created such spreadsheets based on the conventional model for retention in reversed-phase liquid chromatography (RPLC) and on two different models for retention in normal-phase liquid chromatography. The implementation allows an easy visualization of the theoretical concept. Up to three different polymeric series can be entered, with a total of up to 100 peaks being computed and displayed in isocratic or gradient-elution chromatograms. Also visualized are "retention models" (diagrams of isocratic retention vs. composition) and "calibration curves" (retention or elution composition vs. molecular mass or degree of polymerization). The coefficients in the isocratic retention model may be correlated, as has often been observed in RPLC. It is shown that under certain conditions such a correlation corresponds to the existence of so-called critical (isocratic) conditions, at which all the members of a given polymeric series (same composition and end groups, different number of repeat units) show co-elution.

High-performance liquid chromatography separation methods for the analysis of peptide nucleic acids

An analytical analysis of peptide nucleic acids (PNAs) was carried out by reversed-phase HPLC using a solvent system comprised of aqueous trifluoroacetic acid and acetonitrile. A regression equation was obtained which represents the relationship of the molecular mass, sequence composition and retention time. This equation can be used to estimate the retention time of a known PNA under certain HPLC conditions. In addition to this equation, new HPLC conditions were also optimized which can be used for separation of pure PNAs.