Micelles as separation media in high-performance liquid chromatography and high-performance capillary electrophoresis: overview and perspective

The use of micelles in high-performance liquid chromatography and capillary electrophoresis is reviewed. In the first part, an overview of micellar liquid chromatography (MLC) is provided. Since the first introduction of the technique by Armstrong and Henry [D.W. Armstrong and S.J. Henry, J. Liq. Chromatogr., 3 (1980) 657; D.W. Armstrong, Sep. Purif. Methods 14 (1985) 213] in 1980, the technique has received much attention due to its numerous capabilities and advantages, such as simultaneous separation of charged and uncharged solutes, rapid gradient capability, direct on-column injection of physiological fluids, unique separation selectivity, high reproducibility, robustness, enhanced luminescence detection, low cost and safety. The main shortcoming of the technique is poor chromatographic efficiency. Nevertheless, MLC is superior to ion-pair LC and ion-exchange LC for the separation of charged molecules and mixtures of charged and uncharged solutes. The roles of micelles and organic modifiers in controlling retention and selectivity in MLC is described. The differences between MLC and reversed-phase LC in terms of chromatographic behavior and scope of application are examined. A main focus of this overview is on micellar electrokinetic chromatography (MEKC). In 1984, Terabe et al. [S. Terabe, K. Otsuka, K. Ichikawa, A. Tsuchiya and T. Ando, Anal. Chem. 56 (1984) 111; S. Terabe, K. Otsuka and T. Ando, Anal. Chem. 57 (1985) 834] reported the use of micelles in buffer solutions for capillary electrophoresis (CE). MEKC was primarily developed for the separation of uncharged solutes, but it has grown far beyond its initial intent. The scope of applications covers wide groups of organic, inorganic and biochemical compounds that are of interest in various disciplines, such as pharmaceutical, clinical, biotechnological, environmental sciences and others. This is due to its unique advantages, such as high efficiency, speed, ease of method development, feasibility of incorporating various chemistries to influence retention and selectivity, small sample size and low cost. The instrumental set-up in MEKC is the same as that for CE. However, charged organized media such as micelles are incorporated in the buffer solution and act as a pseudo-stationary phase. Unchanged solutes are separated on the basis of their differential partitioning into the micellar pseudo-stationary phase. The roles of various parameters on the overall chromatographic behavior are described. Special attention is given to the characterization of selectivity of pseudo-stationary phases on MEKC. (C) 1997 Elsevier Science B.V.