Pim G. Muijselaar

Micelles as pseudo-stationary phases in micellar electrokinetic chromatography.

This review article describes some general comments on micellar electrokinetic chromatography (MEKC) from the viewpoint of pseudo-stationary phases and presents a compiled list of surfactants used for MEKC, prepared from published papers. We tried to give comments on some typical surfactants from the practical point of view.

On-line coupling of partial-filling micellar electrokinetic chromatography with mass spectrometry

Abstract The on-line coupling of micellar electrokinetic chromatography (MEKC) with mass spectrometry (MS) is hampered by the presence of high concentrations of surfactant in the electrolyte system. Partial-filling MEKC (PF-MEKC) provides a possible way to overcome this problem. In PF-MEKC only a part of the capillary is filled with an electrolyte solution containing micelles, thus allowing an MEKC separation without allowing surfactant to enter the mass spectrometer. The migration behaviour of the micellar zone was investigated applying indirect UV detection. It was demonstrated that micelles gradually break down under PF-MEKC conditions and migrate as surfactant monomers at a concentration at or below the critical micelle concentration. The influence of several experimental parameters on the separation performance is studied and the possibilities as well as the limitations of PF-MEKC are discussed. On-line PF-MEKC–MS with sodium dodecyl sulphate micelles is demonstrated for some pharmaceuticals applying an atmospheric pressure chemical ionisation interface.

Determination of structurally related phenothiazines by capillary zone electrophoresis and micellar electrokinetic chromatography

Fourteen structurally related basic phenothiazines were determined by capillary zone electrophoresis (CZE) and micellar electrokinetic chromatography (MEKC), applying electrolyte systems containing the fluorinated cationic surfactant FC 135. Effective mobilities of the phenothiazines were determined as a function of pH by CZE in the anionic mode with a reversed electroosmotic flow (EOF). Larger differences in overall effective mobilities were obtained in MEKC, applying the cationic surfactant cetyltrimethylammonium bromide as micelle forming agent. The quantitative abilities and matrix effects of CZE with reversed EOF were studied, and as an example the concentrations of thioridazine and promethazine in pharmaceutical preparations were determined.

Parameters controlling the elution window and retention factors in micellar electrokinetic capillary chromatography

Abstract The resolution of uncharged compounds in micellar electrokinetic capillary chromatography (MECC) is influenced by both the elution window and the retention factors of the compounds. The influence of the electroosmotic mobility and the effective mobility of the micelles on the elution window is treated theoretically and the effect of different experimental conditions on the elution window and the retention factors is determined. Although these variables cannot be controlled independently in many cases, the resolution in MECC can be improved by adjusting the composition of the applied electrolyte system. This is demonstrated for several electrolyte systems with different pH values, ionic strengths, surfactant concentrations and organic modifier contents. Further, the influence of the applied field strength, capillary surface modifications and the alkyl chain length of the surfactant are evaluated.

Separation of hydrophobic compounds by electrokinetic chromatography with tetraalkylammonium ions

Abstract Tetraalkylammonium ions were evaluated as potential pseudo-stationary phases for the separation of highly hydrophobic compounds with electrokinetic chromatography (EKC) in aqueous–organic media. The direction of the electroosmotic flow and, as a consequence, the migration behaviour of the hydrophobic compounds, is shown to be strongly dependent on the tetraalkylammonium concentration and the organic modifier content of the applied electrolyte system. The potential of tetraalkylammonium pseudo-stationary phases in EKC is illustrated by the separation of several geometric isomers of polycyclic aromatic hydrocarbons.

Migration behaviour of monovalent weak acids in micellar electrokinetic chromatography; mobility model versus retention model

Migration of charged compounds in micellar electrokinetic chromatography (MEKC) is based on micellar solubilization as well as electrophoretic migration. Consequently, the migration behaviour of these compounds can be described with a retention model or a mobility model. Both models are discussed and evaluated for the migration behaviour of monovalent weak acids in a sodium dodecyl sulphate micellar system. We have shown that the calculation of retention factors may be troublesome for hydrophobic compounds, due to interaction with surfactant molecules in the aqueous phase. Therefore, the mobility model is preferable. The simultaneous determination of dissociation constants by spectroscopic and electrophoretic methods is discussed and apparent dissociation constants in micellar media are determined, showing that pKa shifts may occur in MEKC analyses, due to micellar solubilization. Furthermore, a sample stacking procedure is described, based on differences in overall effective mobilities between aqueous and micellar electrolyte systems.

Retention indices in micellar electrokinetic chromatography

The use of retention indices in micellar electrokinetic chromatography (MEKC) is evaluated both from a theoretical and a practical point of view. Fundamental equations for the determination of retention indices in MEKC are described, showing that retention indices are independent of the surfactant concentration. Possibilities as well as limitations of different homologous series as reference standards are described. In addition, the practical application of retention indices for identification, investigation of solute-micelle interactions, characterization and classification of pseudo-stationary phases and determination of solute lipophilicity are discussed.

Determination of diffusion coefficients and separation numbers in micellar electrokinetic chromatography

Diffusion coefficients in micellar electrokinetic chromatography (MEKC) were determined for a homologous series of alkylaryl ketones. Overall diffusion coefficients in micellar electrolyte systems were calculated from measured spatial peak variances, using the stopped migration method. From overall diffusion coefficients at two different surfactant concentrations, diffusion coefficients in the aqueous phase and in the micellar phase, respectively, were determined. Diffusion in the micellar phase was found to be approximately one order of magnitude lower than diffusion in the aqueous phase for the alkylaryl ketones. The influence of micellar diffusion on efficiency is treated theoretically. It is demonstrated that efficiency in MEKC strongly depends on the retention factor, especially for small values of the elution window. Under practical MEKC conditions, however, the influence of micellar diffusion is of minor importance. The determination of separation numbers in MEKC is evaluated and their use as parameter for the separation performance is discussed.

Migration behaviour of micelle counterions in micellar electrokinetic chromatography: Influence on micelle mobility, efficiency and selectivity

The migration behaviour of micelle counterions in micellar electrokinetic chromatography (MEKC) is treated theoretically. It is demonstrated that, due to a high degree of counterion binding, micelle counterions in commonly used surfactant systems possess negative transport numbers under MEKC conditions. This phenomenon is illustrated for a sodium dodecyl sulphate micellar system in which sodium ions possess a negative effective mobility. In addition to that, the influence of sodium and tris(hydroxymethyl)aminomethane as dodecyl sulphate counterions on the effective mobility of micelles, efficiency and selectivity in MEKC analyses is evaluated.

Evaluation and characterization of pseudo-stationary phases in Micellar Electrokinetic Chromatography

The separation mechanism of neutral species in Micellar Electrokinetic Chromatography (MEKC) is based on differences in the partitioning between the aqueous mobile phase and the pseudo-stationary micellar phase [1, 2]. An important feature of MEKC is its flexibility. The composition of the electrolyte system can easily be changed in order to control migration behaviour and optimize selectivity. In this respect the pseudo-stationary phase plays a key role, since its chemical nature has a major influence on the separation process. Various surfactant systems can be used as well as mixed micelles, possessing different solubilization characteristics. Despite the ease of varying the experimental conditions, Proper selection of a suitable surfactant system in MEKC is still a difficult task. In gas chromatography stationary phases can be chosen on the basis of specific selective chemical interactions using the Rohrschneider-McReynolds scale [3], whereas in liquid chromatography mobile phases can be selected based on Snyders selectivity triangle [4]. At present, however, the influence of the chemical nature of the pseudo-stationary phase and the structural properties of solutes on solute-micelle interactions in MEKC is still not well understood. Terabe and Okada studied the solubilization characteristics of several surfactant systems by the determination of thermodynamic quantities [5]. Recently, Yang and Khaledi applied Linear Solvation Energy Relationship (LSER) modelling for the characterization of solute-micelle interactions [6, 7]. LSER studies are also based on a thermodynamic approach and provide quantitative information about different solute-micelle interaction phenomena. They can be used to elucidate which mechanisms play a dominant role in