Basic Principles of MLC

Micellar liquid chromatography (MLC) is an efficient alternative to conventional reversed-phase liquid chromatography with hydro-organic mobile phases. Almost three decades of experience have resulted in an increasing production of analytical applications. Current concern about the environment also reveals MLC as an interesting technique for “green” chemistry because it uses mobile phases containing 90% or more water. These micellar mobile phases have a low toxicity and are not producing hazardous wastes. The stationary phase is modified with an approximately constant amount of surfactant monomers, and the solubilising capability of the mobile phase is altered by the presence of micelles, giving rise to a great variety of interactions (hydrophobic, ionic, and steric) with major implications in retention and selectivity. From its beginnings in 1980, the technique has evolved up to becoming in a real alternative in some instances (and a complement in others) to classical RPLC with aqueous-organic mixtures, owing to its peculiar features and unique advantages. The addition of an organic solvent to the mobile phase was, however, soon suggested in order to enhance the low efficiencies and weak elution strength associated with the mobile phases that contained only micelles. 1. Introduction Micellar liquid chromatography (MLC), which uses mobile phases containing a surfactant above its critical micellar concentration (CMC), is an alternative to conventional reversed-phase liquid chromatography and provides a solution to the direct injection of physiological or food samples by solubilising proteins (that are eluted together or shortly after the solvent front) [1–3]. The possibility of directly injecting samples into the chromatograph simplifies and expedites treatment, which confers analytical procedures greater accuracy and a lower cost. The versatility of MLC is due to the wide variety of interactions that are established among the eluted solutes, the stationary phase, the aqueous phase and micelles. Their eluent characteristics allow the analysis of compounds with a wide range of polarities. The presence of a surfactant not only modifies the interactions established inside the column but also reduces the necessary amount of organic solvent in the mobile phase, which can be recycled due to low evaporation. These characteristics are genuinely interesting given current concerns about reducing organic contaminant residues in laboratories. MLC shares the basic components of reversed-phase liquid chromatographic (RPLC) systems, that is, a non-polar stationary