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  • Author or Editor: Colin Poole x
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An interphase model for retention is appropriate for biphasic systems in which one phase (the mobile phase) modifies the properties of the other phase through absorption of mobile phase components. This is typical of liquid chromatography, where separation occurs by the distribution of sample components between a bulk mobile phase and an interphase region in intimate contact with the mobile phase. This has profound implications for the interpretation of retention mechanisms since the properties of the stationary phase are those of the interphase region, which can be quite different to those known or perceived for the nonsolvated stationary phase. For reversed-phase chromatography, it is shown that retention properties can be adequately described by the solvation parameter model and visualized as a function of the bulk mobile phase composition by system maps. For normal-phase chromatography, a modified approach is required for inorganic oxide adsorbents to accommodate site-specific interactions (localization of sample and/or mobile phase components) on high energy adsorption sites within the interphase region. This is achieved using a competition model approach to separate out the contributions of solvent and solute interactions with the adsorbent surface and the solvation parameter model to provide insight into the relative importance of various intermolecular interactions on retention and selectivity. The above discussion is set within the framework of thin-layer chromatography although the conclusions are general and equally applicable to column liquid chromatography.

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A new method of solvent classification is proposed based on the five system constants of the solvation parameter model for transfer of neutral compounds from the gas phase to solvent and hierarchical cluster analysis for identifying solvents with similar properties and for organizing them into selectivity groups. This method resulted in the classification of 36 common solvents used in chromatography into seven selectivity groups with four solvents (2,2,2-trifluoroethanol, N,N-dimethylformaide, dimethyl sulfoxide, and water) behaving independently. Except for water, the three organic solvents identified as independent are probably a consequence of the lack of other solvents with similar properties in the data set rather than a demonstration of unique solvent behavior. The classification scheme provides a logical approach for solvent selection as the first step in chromatographic method development. A similar approach using the solvation parameter model suitable for the transfer of neutral compounds between condensed phases and hierarchical cluster analysis was used to classify 19 aqueous and 17 totally organic biphasic partition systems for liquid-liquid extraction. The dynamic range of system constants for the aqueous biphasic systems was not great, and selectivity differences were dominated by the high cohesion and strong hydrogen-bond acidity of water. The aqueous biphasic partition systems were classified into three general groups and a fourth dispersed group characterized by high mutual solubility. In contrast, the totally organic biphasic partition systems exhibit an almost continuous range of properties with minimal group formation demonstrating a wider and complementary range of selectivity to the aqueous biphasic systems. The classification of the liquid-liquid partition systems provides a suitable method for the identification of systems for sample preparation based on liquid-liquid extraction and for the simulation of extractions for target compound isolation.

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The solvation parameter model has been used to characterize the retention properties of a varied group of solutes in silica gel thin-layer chromatography (TLC) and in silica gel and alumina column liquid chromatography. The model was unable to describe retention on silica gel TLC plates with the R M value as the dependent variable for five single-solvent mobile phases. The results were improved by fitting the retention data to the simple competition model and separating the solute and solvent contributions (denoted S and eºA S, respectively) to the free energy of adsorption on the inorganic oxide. Separate models were then constructed to enable estimation of values of S and A S from structure. These models were successful in describing retention in column liquid chromatography on silica gel with mixtures of methyl t-butyl ether and hexane as a mobile phase. This approach proved less reliable for calculating retention in TLC, probably because of non-equilibrium in the separation system. Evidence is presented that neither the solute adsorption parameter (S) nor the solute cross-section (A S) as used in these studies is unambiguously defined. Further refinements aimed at establishing clearly defined solute and solvent adsorption terms, and possibly including selective solute–solvent interactions in the mobile phase as a secondary contribution to retention, could result in improved model performance. 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 are currently less developed than models available for reversed-phase separations.

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Room temperature ionic liquids are a new class of solvents of potential interest for liquid chromatography. Ionic liquids possess a combination of physical and solvation properties that are complementary to conventional organic solvents. Applications in liquid chromatography are currently limited by their unfavorable viscosity and low-wavelength absorption in the ultraviolet (UV) region. In addition, for planar chromatography, the absence of a vapor pressure does not allow evaporation of ionic liquid solvents after development. The room temperature ionic liquids are good solvents for nonionic compounds with a different blend of intermolecular interactions compared with conventional organic solvents as indicated by solvatochromic measurements and the system constants of the solvation parameter model. Current applications in column and planar chromatography are reviewed to demonstrate the potential of room temperature ionic liquids as mobile phases or mobile phase additives in separation science. A real breakthrough in their use, however, requires the identification of new room temperature ionic liquids with viscosity closer to those of conventional organic solvents as well as addressing other minor issues described in the text.

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