Chromatographic characteristics of surfactant-mediated separations: micellar liquid chromatography vs ion pair chromatography.

The effects of concentrations of organic solvent and surfactant on elution strength and selectivity in MLC and IPC are studied. It is observed that selectivity between most pairs of solutes used in this study increases in MLC and either decreases or passes through a minimum in IPC, with the volume fraction of organic modifier. In both MLC and IPC, selectivity varies with surfactant concentration; however, the overall variation in selectivity and elution order are more pronounced in MLC. The solvent strength decreases in IPC and increases in MLC as a result of an increase in surfactant concentration. An iterative regression design is used to predict the optimum mobile-phase compositions in terms of solvent strength and selectivity. The correlation between the predicted and measured chromatograms is excellent in MLC and poor in IPC. This is due to a more regular and reproducible retention behavior in MLC which greatly facilitates the development of robust methodologies. For a mixture of amino acids and peptides, a large retention gap between the first and the last eluting solutes is observed in IPC, which makes the use of organic solvent gradient inevitable. However, a better separation for the same mixture of solutes can be achieved in MLC isocratically. Apparently, the general elution problem can be alleviated in MLC by using an optimum eluent composition. It is observed that the efficiencies of MLC and IPC are comparable. The above observations indicate that MLC can be a powerful alternative to IPC in order to achieve optimized separations in shorter analysis time.