Micellar electrokinetic capillary chromatography (MECC) is suitable for the separation of mixtures of uncharged and charged solutes. In this paper, the migration behavior of acidic compounds in MECC is quantitatively described in terms of different models. These equations describe the relationships between the two migration parameters in MECC (retention factor and mobility) and the two important experimental parameters (pH and micelle concentration) that have a great influence on the migration behavior and selectivity. Interestingly, the mobility and retention factor of a given solute could behave differently with the variations in pH. This would raise a question of which parameter actually represents the migration behavior of a solute in MECC: retention factor (a chromatographic parameter) or mobility (an electrophoretic parameter). The consequences of micellar-mediated shifts of ionization constants on selectivity and optimization strategies in MECC are discussed. The mathematical models would allow the prediction of migration behavior of solutes based on a limited number of initial experiments. This would greatly facilitate the method development and optimization of separations of ionizable compounds by MECC and, in addition, important physical and chemical characteristics of solutes such as their apparent ionization constants in micellar media and their partition coefficients into micelles (over a wide range pH values) can be determined. The models were verified, as good agreements were observed between the predicted and the experimentally observed migration behavior. Based on the preliminary results, the pH and micelle concentration are likely to be interactive parameters in many situations. As a result, simultaneous optimization of these two parameters would be the most effective strategy to enhance the MECC separation of acidic solutes.
A phenomenological approach is presented to describe the migration of cationic solutes in micellar electrokinetic capillary chromatography (MECC). The migration behavior of an organic base is complicated by the presence of an acid-base equilibrium, the ion-pairing formation between the conjugated acid of the base and the monomer surfactants, and the interactions of both the base and its conjugated acid with the micellar pseudophase. An equation was derived that allows the calculation of the migration factor of a cationic solute in MECC with anionic micelles. Two limiting cases were considered: first the cationic solute completely associates with the anionic surfactant (ion-pair formation constant, KIP, approaches infinity), and therefore there is no free charged species in the solution; second, the KIP = 0 and the free conjugated acid, BH+ migrates in the aqueous bulk solvent at its own electrophoretic velocity. An estimate for the ion-pair formation constant between cationic solutes and free surfactant can be obtained by using the model.
Previously, the use of phenomenological models to describe the migration behavior of acidic solutes in micellar electrokinetic chromatography (MEKC) was reported. In this paper, the phenomenological approach is further extended by including both acidic and basic solutes and simultaneously taking two important experimental factors (pH and micelle concentration) into consideration. In addition, a general method is described to model the migration behavior of ionizable (both acidic and basic) solutes in MEKC with anionic and cationic micelles. The practical implication of the phenomenological approaches is that they will provide quantitative relationships between solute migration and experimental factors such that the migration behavior can be predicted on the basis of a few initial experiments and that physicochemical parameters of solutes can also be estimated from model fitting. Through computer-assisted modeling, migration behavior of several acidic and basic solutes over a pH-micelle concentration factor space was successfully predicted on the basis of only five experiments. Furthermore, this phenomenological approach was used to predict the separation of a group of aromatic amines in MEKC with anionic micelles, which resulted in a successful separation of 18 aromatic amines in less than 15 min.
The role of micelles and organic solvents as the modifiers of the aqueous mobile phase in reversed-phase liquid chromatography (RPLC) in controlling retention and selectivity is discussed. Elution strength increases in RPLC with an increase in organic solvent or micelle concentration. Simultaneous enhancement of separation selectivity with elution strength in the hybrid eluents of water-organic solvent-micelles was observed. This selectivity enhancement occurs systematically, i.e. peak separation increases monotonically with volume fraction of organic solvent added to micellar eluent, and is observed for a large number of ionic and nonionic compounds with different functional groups and for two surfactants (anionic and cationic). For two test mixtures, 13 amino acids/peptides and 15 phenols, it is shown that a better separation and shorter analysis time are observed at stronger hybrid eluents. This selectivity enhancement can be attributed to the competing partitioning equilibria in micellar LC systems and/or to the unique characteristics of micelles to compartmentalize solutes and organic solvents.
This paper reviews the use of lipid vesicles as model membranes in capillary electrophoresis (CE). The history and utility of CE in the characterization of microparticles is summarized, focusing on the application of colloidal electromigration theories to lipid vesicles. For instance, CE experiments have been used to characterize the size, surface properties, enclosed volumes, and electrophoretic mobilities of lipid vesicles and of lipoprotein particles. Several techniques involving small molecules or macromolecules separated in the presence of lipid vesicles are discussed. Interactions between the analytes and the lipid vesicles - acting as a pseudostationary phase or coated stationary phase in electrokinetic chromatography (EKC) - can be used to obtain additional information on the characteristics of the vesicles and analytes, and to study the biophysical properties of membrane-molecule interactions in lipid vesicles and lipoproteins. Different methods of determining binding constants by EKC are reviewed, along with the relevant binding constant calculations and a discussion of the application and limitations of these techniques as they apply to lipid vesicle systems.
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