Micellar electrokinetic chromatography of long chain saturated and unsaturated free fatty acids with neutral micelles: Considerations regarding selectivity and resolution optimization

Abstract

The separation of saturated and unsaturated long chain (C14–C20) free fatty acids (FFAs) was investigated by micellar electrokinetic chromatography (MEKC) in the presence of polyoxyethylene-23-dodecylether (Brij 35) micelles and methanol containing electrolytes and under the conditions of a cathodic electroosmotic flow (EOF). The detection was performed by indirect absorbance detection using p-methoxybenzoate (p-anisate) as absorbing co-ion in the electrolyte. This type of separation, featuring anionic analytes and neutral micelles, can be conveniently modelled by adapting the physico-chemical framework initially developed by Terabe et al. [Anal. Chem. 57 (1985) 834] for the case of neutral analytes and anionic micelles. It has been established that optimal resolution is obtained when the anionic analytes spend more time in the bulk hydroorganic electrolyte than in the neutral micellar pseudo-phase. This approach was implemented for optimizing the Brij 35 concentration and the methanol content in the electrolyte. It has also allowed us to determine chromatographic-type selectivity, which is a major issue for the assessment of alternate separation methods. Accordingly, the selectivity of Brij 35 micelles for the C16–C18 FFA pair was quite similar to that currently obtained in gas chromatography. Finally, this work demonstrates that MEKC is a cost and time effective method for the separation of critical pairs of saturated and unsaturated FFAs of identical equivalent chain length.