What is MELC?

Figure 1

Water-in-oil (w/o) microemulsions are also possible where surfactant-coated droplets of water are suspended in oil. In Microemulsion liquid chromatography (MELC)^1–4 a microemulsion mobile phase is used with conventional reversed-phase (RP) columns. Generally o/w microemulsions are used in MELC.

The surfactant is present at concentrations well in excess of its CMC and the oil phase positions itself inside the resultant surfactant micelles. This reduces the interfacial tension between the oil and water. The co-surfactant molecules position themselves between the head groups of the surfactant molecules, which reduces the electrostatic repulsion between them and results in the overall ultra-low surface tension required for the spontaneous formation of the microemulsion.

O/w MELC using RP columns has been more widely documented since it first appeared in literature in 1992.^5,6 W/o MELC was first reported in 1986.⁷

Gradient elution can also be performed in MELC and this will be covered in a later section.

Readers interested in more theoretical background are referred to a recent review paper,² which provides a more comprehensive review of MELC background theory, applications and the effects of changing various operating and microemulsion parameters on MELC separations.

Microemulsions are also used in the capillary electrophoretic technique of microemulsion electrokinetic chromatography (MEEKC). Solutes separate because of partitioning and interacting with the droplets that move when a voltage is applied.⁸

Why use MELC?

Reduced sample preparation needs for complex samples: Microemulsions have great solubilizing power for both water-insoluble and -soluble compounds. They, therefore, offer the ability to directly solubilize hydrophobic samples and matrices, such as creams and waxes, without lengthy pre-extraction steps. The surfactant in the microemulsion solubilize/bind proteins, which reduces sample pre-treatment needs for analysis of biofluids such as plasma. This characteristic was widely exploited by several authors.^1,3,8–11 This saves time and cost as it avoids procedures such as solvent–solvent extraction used in advance of a conventional HPLC analysis. These extractions are needed to avoid column fouling as a result of in-column precipitation of sample components. However, in MELC the samples are generally dissolved/diluted in the microemulsion, which is also used as mobile phase.

Generic conditions for rapid method development: Separation using the standard microemulsion² is a good starting point when developing a method for a new separation where there is no previously published work. The standard microemulsion consists of 33 g SDS, 66 g butan-1-ol, 8 g n-octane in 1 L 0.05% trifluoroacetic acid (TFA). The sample for analysis is dissolved in the microemulsion and separation is performed on a C18 RP column, at ambient temperature, UV detection 254 nm and a flow-rate of 1 mL/min