MICELLAR ELECTROKINETIC CHROMATOGRAPHY OF LONG-CHAIN SATURATED AND UNSATURATED FREE FATTY-ACIDS WITH NEUTRAL MICELLES - CONSIDERATIONS REGARDING SELECTIVITY AND RESOLUTION OPTIMIZATION

The separation of saturated and unsaturated long chain (C-14-C-20) fre e fatty acids (FFAs) was investigated by micellar electrokinetic chrom atography (MEKC) in the presence of polyoxyethylene-23-dodecylether (B rij 35) micelles and methanol containing electrolytes and under the co nditions of a cathodic electroosmotic flow (EOF). The detection was pe rformed by indirect absorbance detection using p-methoxybenzoate (p-an isate) 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 develop ed by Terabe et al. [Anal. Chem. 57 (1985) 834] for the case of neutra l analytes and anionic micelles. It has been established that optimal resolution is obtained when the anionic analytes spend more time in th e bulk hydroorganic electrolyte than in the neutral micellar pseudo-ph ase. This approach was implemented for optimizing the Brij 35 concentr ation and the methanol content in the electrolyte. It has also allowed us to determine chromatographic-type selectivity, which is a major is sue for the assessment of alternate separation methods. Accordingly, t he selectivity of Brij 35 micelles for the C-16-C-18 FFA pair was quit e similar to that currently obtained in gas chromatography. Finally, t his work demonstrates that MEKC is a cost and time effective method fo r the separation of critical pairs of saturated and unsaturated FFAs o f identical equivalent chain length. (C) 1997 Elsevier Science B.V.