A review of the background, operating parameters and applications of microemulsion liquid chromatography (MELC)

Abstract

A review of the background, operating parameters and applications of microemulsion liquid chromatography (MELC) Microemulsions are dispersions of nanometre-sized droplets of an immiscible liquid within another liquid. Droplet formation is facilitated by the addition of surfactants and often also cosurfactants. Microemulsions are classified as either oil-in-water (O/W) (oil droplets such as octane dispersed throughout aqueous buffer) or water-in-oil (W/O) (aqueous droplets in oil such as hexane). Both microemulsion types have been used as mobile phases for separation in microemulsion HPLC (MELC). There has been a recent increase of interest in this area with new applications and developments such as gradient elution and optimisation of methods using experimental design. O/W microemulsions have been employed as eluents for RP-HPLC while W/O microemulsions have been used for normal phase chromatography. Separations can have superior speed and efficiency to conventional HPLC modes while offering a unique selectivity with excellent resolution. The capability for quantitative and stability-indicating analysis has also been demonstrated. Specific advantages include the ability to operate at low UV wavelengths and elimination of the need for an equilibration rinse between gradients. Operational issues associated with the use of MELC have been identified including the need to add salt to the gradient eluent, relatively high back-pressures and increased need for equipment cleaning compared to conventional RP eluent. This report details the different microemulsion types and compositions used and their reported applications. The use of gradient and isocratic elution is described. The effects on separations of varying operating parameters such as temperature, oil type and concentration, surfactant type and concentration, sample solvent, column type, and organic solvent addition will be discussed and illustrated.

Analysis of isoquinoline alkaloids using chitosan‐assisted liquid–solid extraction followed by microemulsion liquid chromatography employing a sub‐2‐micron particle stationary phase

“…Reports have shown that the oil concentration did not significantly affect MELC selectivity . In this study, a slight decrease in retention times of analytes was observed with increasing the oil content from 0.6 to 1.2%.…”

Development and validation of a generic stability‐indicating MEEKC method for five fluoroquinolone antibiotics

“…A significant negative multivariate interaction effect was found with IPA, thus combining a high amount of n ‐butanol and IPA as well as lowering the concentration of SDS resulted into a longer analysis time. Two experiments with this combination were removed from the model because of the instability of the ME at these extreme conditions . Keeping SDS in the ME was important to accumulate the overall negative charge which will make the FQ—micelle complex migrate to the cathode under the reversed polarity.…”

Physicochemical factors governing the partition of pramipexole and its five impurities in microemulsion liquid chromatographic systems

“…The evaluation of the operating parameters in order to assess its capabilities in pharmaceutical analysis was performed. [4][5][6][7][8][9][10] Also, the potential of MELC is proved by its expansion to the optimization of microemulsion mobile phase composition in order to model the drug penetration across blood-brain barrier. 11 Apropos the selected substances, they were previously investigated by the same authors employing RP-HPLC in order to determine their pKa values, as well as for the method optimization and robustness testing.…”