Colin F. Poole

New trends in solid-phase extraction

Solid-phase extraction using cartridge and disc devices is a widely used sample-preparation technique for isolation, concentration, clean-up and medium exchange. To meet the varied needs of contemporary applications, there is available an ever-increasing range of sorbent chemistries based on inorganic oxides, low-specificity (chemically bonded, porous polymer and carbon) and compound and group-selective (ion exchange, mixed mode, macrocyclic, restricted access, immunoaffinity and molecularly imprinted polymer) materials. Advanced device formats facilitate processing of problem samples combined with a high level of automation. Approaches to computer-aided method development promise to replace tedious trial-and-error procedures with fast simulations based on suitable kinetic and retention models.

Extraction of organic compounds with room temperature ionic liquids

Room temperature ionic liquids are novel solvents with a rather specific blend of physical and solution properties that makes them of interest for applications in separation science. They are good solvents for a wide range of compounds in which they behave as polar solvents. Their physical properties of note that distinguish them from conventional organic solvents are a negligible vapor pressure, high thermal stability, and relatively high viscosity. They can form biphasic systems with water or low polarity organic solvents and gases suitable for use in liquid-liquid and gas-liquid partition systems. An analysis of partition coefficients for varied compounds in these systems allows characterization of solvent selectivity using the solvation parameter model, which together with spectroscopic studies of solvent effects on probe substances, results in a detailed picture of solvent behavior. These studies indicate that the solution properties of ionic liquids are similar to those of polar organic solvents. Practical applications of ionic liquids in sample preparation include extractive distillation, aqueous biphasic systems, liquid-liquid extraction, liquid-phase microextraction, supported liquid membrane extraction, matrix solvents for headspace analysis, and micellar extraction. The specific advantages and limitations of ionic liquids in these studies is discussed with a view to defining future uses and the need not to neglect the identification of new room temperature ionic liquids with physical and solution properties tailored to the needs of specific sample preparation techniques. The defining feature of the special nature of ionic liquids is not their solution or physical properties viewed separately but their unique combinations when taken together compared with traditional organic solvents.

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.

Explanation of the matrix-induced chromatographic response enhancement of organophosphorus pesticides during open tubular column gas chromatography with splitless or hot on-column injection and flame photometric detection

Abstract The observed chromatographic response for organophosphorus pesticides in extracts from milk and butterfat is shown to be matrix dependent. The matrix protects the organophosphorus compounds from adsorption and/or decomposition in hot vaporizing injectors ensuring a more complete transfer from injector to column compared to the results observed when standards dissolved in matrix-free solvent are used. This results in recoveries in excess of 100% for residue-free extracts spiked with organophosphorus pesticides when standards prepared in residue-free solvents are used for calibration. The chromatographic response enhancement is minimized by using hot on-column injection at an optimized injection temperature, but not completely eliminated. The preferred method of calibration is to use matrix standard solutions prepared by adding known amounts of organophosphorus pesticides to residue-free sample matrix of the same character and in similar concentration to the samples to be analyzed.

Column selectivity from the perspective of the solvation parameter model

The solvation parameter model is a useful tool for delineating the contribution of defined intermolecular interactions to retention of neutral molecules in separation systems based on a solute equilibrium between a gas, liquid or fluid mobile phase and a liquid or solid stationary phase. The free energy for this process is decomposed into contributions for cavity formation and the set up of intermolecular interactions identified as dispersion, electron lone pair, dipole-type and hydrogen bonding. The relative contribution of these interactions is indicated by a series of system constants determined by the difference of the defined interaction in the two phases. The interpretation of these system constants as a function of experimental factors that affect retention in the chromatographic system provides the connection between relative retention (selectivity) and the control variables for the separation system. To aid in the understanding of these processes we perform an analysis of system constants for gas chromatography, liquid chromatography, supercritical fluid chromatography and micellar electrokinetic chromatography as a function of different experimental variables as a step towards gaining a theoretical understanding of selectivity optimization for method development.

Contributions of theory to method development in solid-phase extraction

The kinetic and retention properties of solid-phase extraction devices are reviewed from the perspective of method development strategies. Models based on frontal analysis are used to correct retention properties of solid-phase extraction devices to account for the fact that too few theoretical plates are provided for retention to be independent of kinetic factors. The available pressure drop for the sampling device largely dictates the choice of useful particle sizes and maximum bed length. The use of octanol--water partition coefficients and extrapolated values of the retention factor obtained by liquid chromatography are poor empirical models for the estimation of breakthrough volumes with water as the sample solvent. The solvation parameter model provides an adequate description of sorbent retention for the estimation of breakthrough volumes, rinse solvent volume and composition, and elution solvent volume and composition. Combining the frontal analysis and solvation parameter models offers a comprehensive approach to computer-aided method development in solid-phase extraction. This is the first step in the development of a structure-driven approach to method development in solid-phase extraction that should be more reliable and less tedious than traditional trial and error approaches.

Ionic liquid stationary phases for gas chromatography

This article provides a summary of the development of ionic liquids as stationary phases for gas chromatography beginning with early work on packed columns that established details of the retention mechanism and established working methods to characterize selectivity differences compared with molecular stationary phases through the modern development of multi-centered cation and cross-linked ionic liquids for high-temperature applications in capillary gas chromatography. Since there are many reviews on ionic liquids dealing with all aspects of their chemical and physical properties, the emphasis in this article is placed on the role of gas chromatography played in the design of ionic liquids of low melting point, high thermal stability, high viscosity, and variable selectivity for separations. Ionic liquids provide unprecedented opportunities for extending the selectivity range and temperature-operating range of columns for gas chromatography, an area of separation science that has otherwise been almost stagnant for over a decade.

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.

Thin-layer chromatography: challenges and opportunities

The purpose of this article is to identify core technologies with the potential to influence the development of thin-layer chromatography over the next decade or so. Core technologies are identified as: (i) methods to provide a constant and optimum mobile phase velocity (forced flow and electroosmotically-driven flow), (ii) video densitometry for recording multidimensional chromatograms, (iii) in situ scanning mass spectrometry, and (iv) bioactivity monitoring for selective detection. In combination with two-dimensional, multiple development and coupled column-layer separation techniques these core technologies could dramatically increase the use of thin-layer chromatography for the characterization of complex mixtures. It is also demonstrated that thin-layer chromatography has strong potential as a surrogate chromatographic model for estimating biopartitioning properties. To convert these opportunities into practice the current state-of-the-art of the core technologies is described and the principle obstacles to progress identified.

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.