Joseph L. Glajch

Optimization of solvent strength and selectivity for reversed-phase liquid chromatography using an interactive mixture-design statistical technique

Abstract A general scheme combines the Snyder solvent selectivity-triangle concept with a mixture-design statistical technique to optimize the strength and selectivity of mobile phase solvents for reversed-phase liquid chromatography (LC) separations. In particular, a new method of data analysis called overlapping resolution mapping (ORM) shows advantages over previous chromatographic optimization schemes. The approach can be used to achieve a minimum resolution of all components of a mixture or, alternatively, a single pair or several different pairs of compounds within the mixture. In reversed-phase separations of nine naphthalene compounds substituted with different functional groups, tests with mixtures of methanol—, acetonitrile- and tetrahydrofuran—water that have significant selectivity diff erences revealed that no single organic modifier in water could separate all components. However, when data from seven separation experiments were analyzed with the interactive computer routine, an optimum solvent mixture predicted that subsequently gave complete isocratic separation of all components. While anticipated selectivity changes were found with aqueous mixtures of single organic solvents, aqueous binary or ternary mixtures of these organic solvents exhibited amonalous behavior toward certain solutes. In addition, individual solvent strengths were sometimes different from those predicted by previous studies. Tests on literature LC data set using simulated solvent mixtures with fifteen compounds, some exhibiting peak crossovers with diferent solvent mobile phases, clearly demonstrated the advantages of the mixture-design ORM method over other chromatographic optimization techniques.

Retention in reversed-phase liquid chromatography as a function of mobile-phase composition

The dependence of solute retention (k′) on mobile phase composition (%B for binary-solvent mixtures A–B) is reviewed and compared with various empirical and theoretical equations that have been proposed for this relationship. Because the functional dependence of k′ on organic modifier composition varies from one system to another, it is not possible from these data to draw any overall conclusions as to the nature of the retention process in reversed-phase chromatography. Likewise, there is probably no one best equation for extrapolating all retention data to 0 %B for purposes of predicting log Po/w values from chromatographic data. The relative change in k′ with change in %B can be described in terms of the parameter S = −d(log k′)/dϕ (ϕ = 0.01 %B). Values of S as a function of solute structure, mobile-phase composition, column type and experimental conditions are of interest for several reasons: insight into the retention process or “mechanism”, mobile phase optimization, etc. Previous work relating to this question is reviewed here and some conclusions are presented.

Simultaneous selectivity optimization of mobile and stationary phases in reversed-phased liquid chromatography for isocratic separations of phenylthiohydantoin amino acid derivatives☆

A mixture-design statistical technique has been used to optimize simultaneously the selectivity of both mobile and the stationary phases for the isocratic high-performance liquid chromatographic separation of phenylthiohydantoin derivatives of the 20 common amino acids. This approach permits the fine tuning of selectivity to achieve the rapid separation of this relatively complex mixture with maximum resolution between the various components. An optimum isocratic reversed-phase separation has been achieved in 13 min with a minimum resolution of 1.1 for all component peak pairs. The system uses an optimum combination of four mobile phase solvents (aqueous acids, methanol, acetonitrile and tetrahydrofuran) and three stationary-phase packings (C8- CN-, phenethyl-modified silica) to obtain various separation goals.

Practical optimization of solvent selectivity in liquid-solid chromatography using a mixture-design statistical technique

A systematic approach is described for the optimization of solvent selectivity in liquid-solid chromatography (LSC), with emphasis on changes in selectivity as a result of variation of mobile phase composition. Major contributions to selectivity are provided by solvent-solute localization and solvent-specific localization. Exploitation of these effects is achieved by the use of a mixture-design statistical technique to minimize the number of experiments to find an optimum solvent mixture for LSC separation. Quaternary-solvent mobile phases are required for difficult separations to invoke the full range of selectivity effects possible for LSC separation. The four preferred solvents for LSC optimization based on localization effects are hexane, methylene chloride, methyl tert.-butyl ether and acetonitrile. In the optimization process retention data are required for only seven mobile-phase systems, and an overlapping resolution mapping (ORM) technique of data analysis is used to establish the optimum solvent mixture for the highest resolution of all adjacent bands in the chromatogram.

Theoretical basis for systematic optimization of mobile phase selectivity in liquid-solid chromatography : Solvent-solute localization effects

Abstract The optimization of retention in liquid-solid chromatography (LSC) is explored in the present paper. Previously it was shown possible to calculate solvent strength (e0 values) for multi-component mobile phases, and specifically for quaternary solvent mixtures ABCD. With e0 held optimum and constant for a particular sample, the composition of ABCD can be further varied for optimization of separation factors α (solvent selectivity) for various solute-pairs in the sample of interest. The selection of optimum pure solvents AD for this purpose and the systematic variation in the proportions of these solvents for optimum separation are approached here in terms of a fundamental description of how solvent selectivity arises in LSC. In this paper we discuss two major contributions to solvent selectivity: solvent/solute localization and solvent-specific localization. In a later paper we apply these findings for the development of a systematic approach to the optimization of retention in LSC separation.

Optimization of mobile phases for multisolvent gradient elution liquid chromatography

Abstract Optimization procedures which have been previously developed for isocratic separations in liquid chromatography (LC) have been extended to include gradient elution systems. A multisolvent classification system is used which defines LC solvent systems based upon both solvent strength and selectivity considerations. A systematic experimental design is employed to gather basic retention data on the compounds in a mixture of interest. The data can then be fitted to a second-order polynominal surface and an overlapping resolution mapping technique is used to predict the optimum solvent system for selectivity purposes. Optimization of isoselective multisolvent gradient elution systems is the easiest and should be the most useful technique. A more powerful, but somewhat more complex, selective multisolvent gradient elution system is also described.

Solvent strength of multicomponent mobile phases in liquid—solid chromatography : Binary-solvent mixtures and solvent localization

Abstract The displacement model for the prediction of solvent strength in liquid—solid chromatography (LSC) has been summarized for the case of binary-solvent mobile phases. It is confirmed that a number of prior anomalies can be explained by the phenomenon of “solvent localization”. Taking solvent localization into account leads to a simple, non-empirical procedure for the more accurate calculation of solvent strength. Application of this approach to literature data for silica and alumina as adsorbent shows agreement between calculated and experimental solvent strength values (e 0 ) of 0.017 (1 standard deviation) for 93 binary-solvent mobile phases covering a range in solvent strength of 0.02 0 r = 0.994). This means that more accurate predictions of solvent strength are now possible for binary-solvent LSC systems, for use in optimizing the separation of various samples. The only experimental system showing lack of correlation with the present model is isopropanol—pentane as mobile phase with alumina as adsorbent. It is believed that the behavior of alcohol solutions is more complex than can be described by the simple displacement model. Apart from the alcohols as mobile phases, however, the present study provides further verification of the physical correctness of the displacement model for mobile phases containing polar solvents.

Solvent strength of multicomponent mobile phases in liquid—solid chromatography : Mixtures of three or more solvents

Abstract The recently improved theory for the calculation of solvent strength in binary-solvent liquid—solid chromatography (LSC) systems has been extended to ternary and higher order solvent systems. It has been shown that these new approaches result in greatly improved predictions of solvent strength in ternary- and quaternary-solvent systems, especially those involving strong polar solvents which can localize. This current method is more general and exact than previous approximations of solvent strength and facilitates development of solvent optimization schemes in LSC which use multiple solvents.

Method development in high-performance liquid chromatography using retention mapping and experimental design techniques

Abstract Method development in high-performance liquid chromatography (HPLC) using retention mapping and experimental design techniques is reviewed. The general strategy of overlapping resolution mapping is overviewed. A summary of various applications is examined for reversed-phase, normal-phase, ion-pair, and gradient elution HPLC, as well as stationary phase selectivity. In addition, numerical criteria for separation and optimization are detailed and a discussion of peak tracking and software is included.

Computer-assisted method development for high-performance liquid chromatography

Introduction Chapter: Computer-assisted method development for HPLC (J.L. Glajch & L.R. Snyder). Foreword (G.L. Glajch & L.R. Snyder). Simplex optimization of HPLC separations (J.C. Berridge). Computer-assisted optimization in HPLC method development (S.N. Deming et al.). Selection of mobile phase parameters and their optimization in reversed-phase LC (H.A.H. Billiet & L. de Galan). Method development in HPLC using retention mapping and experimental design techniques (J.L. Glajch & J.J. Kirkland). Isocratic elution (L.R. Snyder et al.). Drylab computer simulation for HPLC method development. I. Isocratic elution (L.R. Snyder et al.). II. Gradient elution (J.W. Dolan et al.). Predictive calculation methods for optimization of gradient elution using binary and ternary solvent gradients (P. Jandera). Computer-assisted retention prediction for HPLC in the ion-exchange mode (Y. Baba). Multivariate calibration strategy for reversed-phase chromatographic systems based on the characterization of stationary-mobile phase combinations with markers (A.K. Smilde et al.). Computer-aided optimization of HPLC in the pharmaceutical industry (E.P. Lankmayr et al.). Comparison of optimization methods in reversed-phase HPLC using mixture designs and multi-criteria decision making (P.M.J. Coenegracht et al.). Explanations and advice provided by an expert system for system optimization in HPLC (P.J. Schoenmakers & N. Dunand). Expert system for the selection of HPLC methods for the analysis of drugs (M. De Smet et al.). Expert system for the selection of initial HPLC conditions for the analysis of pharmaceuticals (R. Hindriks et al.). Expert system program for assistance in HPLC method development (S.S. Williams et al.). Expert system for method validation in chromatography (M. Mulholland et al.). Knowledge-based expert system for troubleshooting HPLC assay methods (K. Tsuji & K.M. Jenkins). Uniform shell designs for optimization in reversed-phase LC (Y. Hu & D.L. Massart). Retention prediction of analytes in reversed-phase HPLC based on molecular structure (R.M. Smith & C.M. Burr). Cathie: expert interpretation of chromatographic data (R. Milne). Prediction of retention of metabolites in HPLC by an expert system approach (K. Valko et al.). Reversed-phase chromatographic method development for peptide separations using the computer simulation program ProDigest-LC (C.T. Mant et al.). Rule-based approach for the determination of solute types in unknown sample mixtures as a first step of optimization parameter selection in reversed-phase ion-pair chromatography (A. Bartha & G. Vigh). Rationalization of the selection of the type of the organic modifier(s) for selectivity optimization in reversed-phase ion-pair chromatography (A. Bartha et al.). Predicting reversed-phase gradient elution separations by computer simulation (J. Schmidt). Computer-assisted optimization with NEMROD software (G. Mazerolles et al.).