Prediction of retention in gradient-elution normal-phase high-performance liquid chromatography with binary solvent gradients

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.

Application of retention modeling in chiral method development. I. Selection of isocratic composition for preparative separation with SFC

Preparative chiral separations are carried out using chiral stationary-phases (CSP) employing isocratic composition mode to take advantage of stacking multiple injections within a single continuous operation. Development of the separation method, however, is not conducted directly in the preparative systems. Chromatographic systems at analytical scale are set up to screen multiple CSPs with various mobile-phases (MP) to detect a suitable CSP-MP combination. For faster method screening, solvent-gradients are implemented - operating from low to higher modifier composition, e.g. 5 to 70%. Once the right CSP-MP pair is detected, the isocratic method for preparative separation is developed through further experimental trials in the analytical system. The scope of the trial steps is generally limited to detecting a "good-enough" separation condition through one or two isocratic experiments. Ideally, the analyst should scout all possible isocratic conditions to detect the most suitable method; which, however, is not possible in high-throughput separation laboratories. In this report we demonstrate the utility of a simple set of algebraic equations, supported by an experimental protocol, in generating complete isocratic method options based on minimum number of experimental trials. The approach presented here was developed for chiral separation with supercritical-fluid chromatography. We also suggest an approach to identify an isocratic composition for the purification step. The process proposed in this report should be useful in developing better preparative separation methods in high-throughput laboratories.

Recent applications of retention modelling in liquid chromatography

Recent applications of retention modelling in liquid chromatography (2015–2020) are comprehensively reviewed. The fundamentals of the field, which date back much longer, are summarized. Retention modeling is used in retention‐mechanism studies, for determining physical parameters, such as lipophilicity, and for various more‐practical purposes, including method development and optimization, method transfer, and stationary‐phase characterization and comparison. The review focusses on the effects of mobile‐phase composition on retention, but other variables and novel models to describe their effects are also considered. The five most‐common models are addressed in detail, i.e. the log‐linear (linear‐solvent‐strength) model, the quadratic model, the log–log (adsorption) model, the mixed‐mode model, and the Neue–Kuss model. Isocratic and gradient‐elution methods are considered for determining model parameters and the evaluation and validation of fitted models is discussed. Strategies in which retention models are applied for developing and optimizing one‐ and two‐dimensional liquid chromatographic separations are discussed. The review culminates in some overall conclusions and several concrete recommendations.

Mobile phase effects in reversed-phase and hydrophilic interaction liquid chromatography revisited

Correct adjustment of the mobile phase is equally important as the selection of the appropriate column for the separation of polar compounds in LC. Both solvophobic and selective polar interactions control the retention in the Reversed Phase and Hydrophilic Interaction modes. The retention models describing the effects of the volume fraction of the strong eluent component in binary mobile phases on the sample retention factors apply in a limited mobile phase composition range. We introduced a three-parameter retention model, which provides improved prediction of retention over a broad mobile phase range, under isocratic and gradient elution conditions. The model does not imply any assumptions concerning either adsorption or partition distribution mechanism, but allows estimating retention in pure strong and in pure weak mobile phase components. The experimental retention data for phenolic acids and flavones on several core-shell columns with different types of stationary phases agree with the theory. Many polar columns with important structural hydrophobic moieties show dual retention mechanism, (Reversed Phase in water rich mobile phases and Hydrophilic Interaction at high acetonitrile concentrations). It is possible to select the mobile phase compositions in each of the two modes for separations of samples containing compounds largely differing in polarity. The three-parameter model describes the retention in each mode, with separately determined best-fit parameters. We applied the two-mode model to the retention data of sulfonamides and benzoic acid related compounds on a new polymethacrylate zwitterionic monolithic micro-column.

Retention and bandwidths prediction in fast gradient liquid chromatography. Part 2-Core-shell columns

Recently, we confirmed that the well-established theory of gradient elution can be employed for prediction of retention in gradient elution from the isocratic data, method development and optimization in fast gradient chromatography employing short packed fully porous and monolithic columns and gradient times in between 1 and 2min, or even less. In the present work, we extended this study to short core-shell reversed-phase columns. We investigated the effects of the specification of the stationary phase in the core-shell structure on the prediction of gradient retention data. Two simple retention models describing the effects of the mobile phase on the retention by two-parameter equations yield comparable accuracy and can be used for prediction of elution times. The log-log model provides improved prediction of gradient bandwidths, especially for less retained compounds. A more sophisticated three-parameter model did not offer significant improvement of prediction. We compared the efficiency, selectivity and peak capacity of fast gradient separations of alkylbenzenes, phenolic acids and flavones on seven core shell columns with different lengths and chemistry of bonded shell stationary phase. Within the limits dictated by a fixed short separation time, appropriate adjustment of the range of the composition of mobile phase during gradient elution is the most efficient means to optimize the gradient separation. The gradient range affects sample bandwidths equally or even more significantly than the column length. Both 5-cm and 3-cm core-shell columns may provide comparable peak capacity in a fixed short gradient time.

Comprehensive two-dimensional liquid chromatography — practical impacts of theoretical considerations. A review

A theory of comprehensive two-dimensional separations by liquid chromatographic techniques is overviewed. It includes heart-cutting and comprehensive two-dimensional separation modes, with attention to basic concepts of two-dimensional separations: resolution, peak capacity, efficiency, orthogonality and selectivity. Particular attention is paid to the effects of sample structure on the retention and advantages of a multi-dimensional HPLC for separation of complex samples according to structural correlations. Optimization of 2D separation systems, including correct selection of columns, flow-rate, fraction volumes and mobile phase, is discussed. Benefits of simultaneous programmed elution in both dimensions of LCxLC comprehensive separations are shown.

Experimental setup, modulation of the fraction collection and transfer from the first to the second dimension, compatibility of mobile phases in comprehensive LCxLC, 2D asymmetry and shifts in retention under changing second-dimension elution conditions, are addressed. Illustrative practical examples of comprehensive LCxLC separations are shown.

Stationary and mobile phases in hydrophilic interaction chromatography: a review

Hydrophilic interaction chromatography (HILIC) is valuable alternative to reversed-phase liquid chromatography separations of polar, weakly acidic or basic samples. In principle, this separation mode can be characterized as normal-phase chromatography on polar columns in aqueous-organic mobile phases rich in organic solvents (usually acetonitrile). Highly organic HILIC mobile phases usually enhance ionization in the electrospray ion source of a mass spectrometer, in comparison to mobile phases with higher concentrations of water generally used in reversed-phase (RP) LC separations of polar or ionic compounds, which is another reason for increasing popularity of this technique. Various columns can be used in the HILIC mode for separations of peptides, proteins, oligosaccharides, drugs, metabolites and various natural compounds: bare silica gel, silica-based amino-, amido-, cyano-, carbamate-, diol-, polyol-, zwitterionic sulfobetaine, or poly(2-sulphoethyl aspartamide) and other polar stationary phases chemically bonded on silica gel support, but also ion exchangers or zwitterionic materials showing combined HILIC-ion interaction retention mechanism. Some stationary phases are designed to enhance the mixed-mode retention character. Many polar columns show some contributions of reversed phase (hydrophobic) separation mechanism, depending on the composition of the mobile phase, which can be tuned to suit specific separation problems. Because the separation selectivity in the HILIC mode is complementary to that in reversed-phase and other modes, combinations of the HILIC, RP and other systems are attractive for two-dimensional applications. This review deals with recent advances in the development of HILIC phase separation systems with special attention to the properties of stationary phases. The effects of the mobile phase, of sample structure and of temperature on separation are addressed, too.

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.

Deformation of gradient shape as a result of preferential adsorption of solvents in mixed mobile phases

Gradient elution has been studied in typical normal and reversed-phase systems. Deformations of gradient profiles have been evidenced as a result of preferential adsorption of modifiers of the mobile phase. This phenomenon was pronounced in the normal-phase system, for which gradient profiles deviated significantly from those programmed. This influenced the retention and shapes of band profiles of the eluting solute. Hence, in order to predict gradient propagation correctly the adsorption equilibrium of modifiers has been quantified. Moreover, at low modifier content, deformations of band profiles of the solute has been registered as a result of the competitive adsorption in the system solute-modifier. This effect has been predicted by a competitive adsorption model. For the reversed-phase systems the influence of the modifier adsorption on gradient propagation was insignificant for typical mobile phases investigated. Therefore, the work has been focused on gradient predictions in the normal-phase system.

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.

A practical approach to transferring linear gradient elution methods

Attempts to theoretically address the problems involved in transferring linear gradient elution methods have been somewhat ad hoc due to the simplifying assumptions usually made in conventional gradient elution theory. Until now, all equations based on the k* parameter of linear gradient elution theory used as the basis for predicting the separation selectivity have not explicitly included the effect of the dwell volume (VD). Using an exact equation for predicting k*, that is, one which fully accounts in an a priori fashion for VD, we find a set of simple yet exact equations which unequivocally must be satisfied to transfer an optimized linear gradient elution method from one system (column or instrument or both) to another. These relationships absolutely mandate that a change in the instrument dwell volume requires a proportional change in the column volume; in turn, a change in the column volume requires a proportional change in the flow rate and/or gradient time to maintain a constant gradient steepness. Although we are not the first to suggest these guidelines, this work provides a complete theoretical foundation for these exact guidelines for the maintenance of gradient selectivity for the case of transferring a linear gradient elution method between different columns packed with the same particles and/or between different instruments.

Controlling the retention in capillary LC with solvents, temperature, and electric fields

Once a suitable stationary phase and column dimensions have been selected, the retention in liquid chromatography (LC) is traditionally adjusted by controlling the mobile phase composition. Solvent gradients enable achievement of good separation selectivity while decreasing the separation time as compared to isocratic elution. Capillary columns allow use of other programming parameters, i.e. temperature and applied electric fields, in addition to solvent gradient elution. This paper presents a review of programmed separation techniques in miniaturized LC, including retention modeling and method transfer from the conventional to micro- and capillary scales. The impact of miniaturized instrumentation on retention and the limitations of capillary LC are discussed. Special attention is focused on the gradient dwell volume effects, which are more important in micro-LC techniques than in conventional analytical LC and may cause significant increase in the time of analysis, unless special instrumentation and (or) pre-column flow-splitting is used. The influence of temperature upon retention is also discussed, and applications where the temperature has been actively used for retention control in capillary LC are included together with the instrumentation utilized. Finally the possibilities of additional selectivity control by applying an electric field over a packed capillary LC column are discussed.

Impact of normal-phase gradient elution in chiral chromatography: a novel, robust, efficient and rapid chiral screening procedure

Novel normal-phase gradient systems have been employed for fast high-throughput chiral analyses of Discovery compounds in our research laboratories in Eli Lilly and Company. In this report, we describe an automated screening approach based on gradient elution, in order to achieve accurate enantiomeric excess determinations, and chiral separations when needed, in the shortest possible timeframe. Baseline resolution of enantiomers has been obtained for over 85% of the samples so tested. For the remaining cases, complete enantioseparation by isocratic optimisation is generally achieved in a single shot. This technique has been proven to be robust and is now standard operating procedure at our analytical research laboratories.

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.

Optimization methods in chromatography and capillary electrophoresis

Many methods have been developed in order to optimize the parameters of interest in either chromatography or capillary electrophoresis. In chemometric approaches experimental measurements are performed in such a way that all factors vary together. An objective function is utilized in which the analyst introduces the desired criteria (selectivity, resolution, time of analysis). Simplex methods and overlapping resolution maps are declining. Factorial designs and central composite designs are more and more popular in electrodriven capillary separations since the number of parameters to master is much larger than in either GC or LC. The use of artificial neural networks is increasing. The advantage of chemometrics tools is that no explicit models are required, conversely the number of experiments to perform may be high and boundaries of the domain are not straightforward to draw and the approach does more than is required. When models are available optimization is easier to perform by regression methods. Computer assisted methods in RPLC are readily available and work well but are still in infancy in CE. Linear solvation energy relationships seem a very valuable tool but estimates of coefficients still require many experiments.