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Colin F. Poole


The solvation parameter model provides a new approach for method development in reversed-phase chromatography. The retention factors for a varied group of solutes were determined for five binary and ten ternary mobile phase compositions containing methanol, acetonitrile and water on cyanopropylsiloxane-bonded silica layers. A retention model was formulated at each composition using the solvation parameter model. A mixture-design approach was then used to model the individual system constants as a continuous function of mobile phase composition producing system surfaces from which retention surfaces could be calculated for method development. The predictive ability of the mixture-design models is demonstrated for the estimation of retention factors for solutes and mobile phase compositions not used to construct the original models. The solvation parameter model is used to characterize the retention properties of a varied group of solutes on silica gel thin-layer chromatography (TLC) and silica and alumina column liquid chromatography. The solvation parameter model was unable to describe retention on silica gel TLC plates with the RM value as the dependent variable for five single solvent systems. An improvement in the results was obtained by fitting the retention data to the simple competition model and separating the solute contribution (indicated as the S parameter) and solvent contribution (indicated as epsilon°AS) to the free energy of adsorption on the inorganic oxide. Separate models were then constructed to estimate values of the S parameter and AS from structure.These models were successful in describe retention in column liquid chromatography on silica gel with mixtures of methyl t-butyl ether and hexane as a mobile phase. The approach described here should be considered preliminary and thought of as a stepping stone in the direction towards a comprehensive model for structure-driven method development in normal-phase separations, which at present are less developed than models available for reversed-phase separations. The solvation parameter model is used to characterize the sorption properties of the porous polymer OasisRTM HLB for solid-phase extraction with water and water-methanol mixtures as a sample solvent. Increasing solute size and electron lone pair interactions favor retention from water. Oasis RTM HLB is not competitive with water for dipole-type and hydrogen-bond interactions, which result in lower analyte retention. Linear solvent strength gradients in conjunction with the solvation parameter model are explored as a rapid and convenient method for the construction of system maps for method development in isocratic reversed-phase chromatography. It is shown that the solvation parameter model provides an approximate model for gradient retention times.

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