Salwa K. Poole

Classification of stationary phases and other materials by gas chromatography

Abstract The origin and evolution of solute descriptors for use in the solvation parameter model applied to the classification of stationary phases and other materials by gas chromatography are described. The model system constants provide a breakdown of solute–stationary phase interactions in terms of the contribution to retention of cavity formation and dispersion interactions, lone-pair electron interactions, interactions of a dipole-type, and hydrogen-bonding interactions. The solvation properties of additional stationary phases with useful complementary selectivity to existing phases for method development in gas chromatography are identified. The influence of temperature on system selectivity and stationary phase classification is discussed. The contribution of interfacial adsorption to the estimation of retention in method development in gas chromatography is outlined. In addition, for materials characterization, it is shown that the solvation parameter model provides a conceptual mechanism for the evaluation of the sorption properties of a wide range of materials compatible with the operation characteristics of gas chromatography.

HYDROGEN BONDING. 42. CHARACTERIZATION OF REVERSED-PHASE HIGH-PERFORMANCE LIQUID CHROMATOGRAPHIC C18 STATIONARY PHASES

The linear free energy equation logk′ = c + rR2 + sπ2H + a∑α2H + b∑β2 + vVx was applied to the capacity factors for various sets of solutes on C18 stationary phases with aqueous methanol and acetonitrile eluents. Here, k′ are the capacity factors for a series of solutes with a given C18 phase and a given eluent, and R2, π2H, ∑α2H, ∑β2, Vx are parameters or descriptors of the solutes as follows: R2 is an excess molar refraction, π2H is the solute polarizability/dipolarity, ∑α2H and ∑β2 are the solute hydrogen–bond acidity and basicity and Vx is the solute volume. It is shown that although the regression coefficients r, s, a, b and v vary widely within the C18 column and mobile phase used, the ratios r/v, s/v, a/v and b/v are remarkably constant. Thus, for the retention of 25 series of solutes on six different C18 columns with 30-90% aqueous methanol as the eluent, all the 25 LFER equations can be combined into one general equation: logk′ = c + v(0.13 R2 − 0.32 π2H − 0.22 ∑α2H − 0.90 ∑β2O + 1.00 Vx) where only c and v vary from system to system. For 11 other phases for which data are available, the ratios v/A and (v + c)/A are constant, where A is the quantity of stationary phase per unit surface area. Similar results were found with C18 phases and aqueous acetonitrile as eluents. Although a first examination of equations based on the first equation above suggests that various C18 phases behave differently, for example the v coefficient, that is related to the observed hydrophobicity of a stationary phase relative to the mobile phase, varies considerably from phase to phase with the same eluent, a detailed analysis led to the conclusion that all the C18 phases examined have roughly the same hydrophobicity, when the v coefficients are corrected for the quantity of stationary phase per unit surface area. It is suggested that these corrected v coefficients, v/A and (v + c)/A, can be regarded as the ‘intrinsic’ phase hydrophobicity.

Determination of solute descriptors by chromatographic methods

The solvation parameter model is now well established as a useful tool for obtaining quantitative structure-property relationships for chemical, biomedical and environmental processes. The model correlates a free-energy related property of a system to six free-energy derived descriptors describing molecular properties. These molecular descriptors are defined as L (gas-liquid partition coefficient on hexadecane at 298K), V (McGowans characteristic volume), E (excess molar refraction), S (dipolarity/polarizability), A (hydrogen-bond acidity), and B (hydrogen-bond basicity). McGowans characteristic volume is trivially calculated from structure and the excess molar refraction can be calculated for liquids from their refractive index and easily estimated for solids. The remaining four descriptors are derived by experiment using (largely) two-phase partitioning, chromatography, and solubility measurements. In this article, the use of gas chromatography, reversed-phase liquid chromatography, micellar electrokinetic chromatography, and two-phase partitioning for determining solute descriptors is described. A large database of experimental retention factors and partition coefficients is constructed after first applying selection tools to remove unreliable experimental values and an optimized collection of varied compounds with descriptor values suitable for calibrating chromatographic systems is presented. These optimized descriptors are demonstrated to be robust and more suitable than other groups of descriptors characterizing the separation properties of chromatographic systems.

Sample preparation for chromatographic separations: an overview

Abstract Chromatographic methods dominate the field of organic analysis. However, many samples are too dilute, too complex or incompatible with the chromatographic system to render separation or detection possible without preliminary matrix simplification and/or preconcentration. A broad selection of the most common, recent and emerging isolation, clean-up and concentration techniques used to prepare samples for chromatographic separations are reviewed.

Recommendations for the determination of selectivity in micellar electrokinetic chromatography

Abstract A general approach for characterizing selectivity in micellar electrokinetic chromatography based on the solvation parameter model is recommended. Individual surfactants are characterized by their cohesion and capacity for polar interactions indicated as lone pair–lone pair electron attraction, dipole-type interactions, and hydrogen bond acidity and basicity. The statistical and chemical validity of the solvation parameter model requires that retention data are determined for a collection of 20–40 varied solutes with a wide range of retention factors and that clustering of values and cross-correlation among the solute descriptors are absent. Since micelles are interfacial solvents with properties that can vary with changes in their external environment (buffer composition, concentration, pH, temperature, etc.) a generic set of experimental conditions are recommended for the measurement of anionic surfactant selectivity under standard conditions. The system constants used to characterize surfactant selectivity are calculated for 12 common surfactants used in micellar electrokinetic chromatography. Of these surfactants sodium dodecyl sulfate, sodium cholate, lithium perfluorooctanesulfonate, sodium N-dodecanoyl-N-methyltaurine and tetradecyltrimethylammonium bromide are identified as providing a useful range of selectivity differences for methods development in micellar electrokinetic chromatography.

Recent advances in solvation models for stationary phase characterization and the prediction of retention in gas chromatography

SummaryRapid progress has been made in the last few years in the development of solvation models for gas chromatography. These models are based on the free energy of transfer of a solute from the gas phase to solution consisting of the additive processes of cavity formation and solute-solvent interactions expressed as dispersion, orientation, induction and hydrogen-bond formation. The adaptation of these models to characterize solvent selectivity for a wide range of common stationary phases and to the prediction of retention is critically discussed. The need to use high quality retention data in the further elaboration of solvation models is also demonstrated.

Chromatographic models for the sorption of neutral organic compounds by soil from water and air.

The solvation parameter model is used to construct models for the estimation of the soil-water and soil-air distribution constants and to characterize the contribution of fundamental intermolecular interactions to the underlying sorption processes. Wet soil is shown to be quite cohesive and polar but relatively non-selective for dipole-type, lone-pair electron and hydrogen-bond interactions. Using a comparison of system constant ratios chromatographic systems employing reversed-phase liquid chromatography on polar bonded phases are shown to provide suitable models for estimating soil-water distribution constants. No suitable gas chromatographic models were found for the soil-air distribution constant but the requirements for such a system are indicated. Models are also provided for adsorption at the air-water interface. Estimation methods based on either the solvation parameter model or chromatographic model reproduce experimental distribution constants for a wide variety of compounds with a similar error (0.2-0.3 log units) to that expected in the experimental data.

Chromatographic and spectroscopic studies of the solvent properties of a new series of room-temperature liquid tetraalkylammonium sulfonates

Abstract Fifteen tetraalkylammonium salts of the functionalized sulfonic acids N,N -bis(2-hydroxy- ethyl)-2-aminoethanesulfonic acid, 2-(cyclohexylamino)ethanesulfonic acid and 2-hydroxy-4- morpholinopropanesulfonic acid were synthesized and the eight organic salts that were liquid at room temperature were characterized by physical, spectroscopic and chromatographic methods. All liquid salts were viscous at room temperature but after dilution with a suitable cosolvent could be used as mobile phases in liquid chromatography. For this application, the rapid deterioration of silica-based column packings was more of a deterrent than the physical properties of the salts. The tetraalkylammonium sulfonates had wide liquid ranges and were stable as column packings up to 150–180°C, making them suitable for use in gas chromatography. Their solvent properties were characterized by solvatochromic parameters [ E T(30) , π*, α and β] and by thermodynamic parameters determined by gas chromatography. Good agreement was found between the predictions of the two methods. The liquid salts showed strong orientation and solvent hydrogen bond acceptor capacity with weak hydrogen bond donor capacity in spite of the functionalization of the anions. The most probable reason for the weak hydrogen bond donor capacity of the anions is the unavailability of these functional groups for solute interactions owing to their preferential involvement in the formation of anion association complexes. The chemically defined structure, unique solvent properties and wide usable temperature ranges make the tetraalkylammonium sulfonates useful stationary phases for gas chromatography.

Multidimensionality in planar chromatography

Multidimensionality in planar chromatography takes on a broader context than is common for column separations. Multidimensional methods used to improve the separation capacity in planar chromatography include unidimensional multiple development and two-dimensional development practiced under conditions of capillary controlled or forced flow of the mobile phase, mobile phase gradients and system automation. Theoretical considerations, the correct approach to methods development, instrumental requirements and contemporary applications of these approaches are outlined in this review. Interfacing column chromatographic methods, such as gas, liquid or supercritical fluid as one dimension and planar chromatography as the second, are not commonly mentioned in the literature but offer some unique possibilities for enhancing both separations and detection. These approaches are outlined here with a view to increasing the awareness of their potential for complex mixture analysis, component identification and trace analysis.

Rapid method for estimating the octanol–water partition coefficient (log Pow) by microemulsion electrokinetic chromatography

Several surfactant systems were evaluated based on their system constants determined by the solvation parameter model for the design of a surrogate chromatographic model for the rapid estimation of octanol-water partition coefficient (log Pow) by microemulsion electrokinetic chromatography. The system constant ratios responsible for the log Pow partition system are (nearly) the same as those for the microemulsion system containing sodium dodecyl sulfate (1.4% w/v), butan-1-ol (8% v/v) and heptane (1.2% v/v). Neutral and basic compounds are analyzed using a fused-silica capillary column with a 50 mM sodium phosphate-sodium borate (3:2) buffer at pH 10. Weakly acid compounds require the use of sulfonated silica capillary column and a 50 mM sodium phosphate buffer at pH 3. For 29 varied neutral and weakly basic compounds the average error between log Pow estimated using MEEKC and literature values was +/-0.12 over a log Pow range from 0.3 to 5.8.