Capillary zone electrophoretic determination of C 2C 18 linear saturated free fatty acids with indirect absorbance detection

Abstract

The potential of capillary zone electrophoresis for the separation of linear saturated fatty acids in their free form was evaluated. With increasing chain length, difficulties arise from decreasing analyte solubility in aqueous media, increasing occurrence of analyte aggregation and decreasing separation selectivity between successive homologues. A solution was to use electrolytes containing cyclodextrins (CDs) and methanol. The separation of C 2C 14 fatty acid homologues differing in only one carbon atom was achieved in less than 10 min using a purely aqueous electrolyte with trimethyl-β-CD as an additive. The separation of C 7C 18 homologues was completed in under 20 min with electrolytes containing up to 60% methanol, in addition to the aforementioned CD. In the presence of the CD, analyte solubility is enhanced through the inclusion of the alkyl chain of the acid in the CD cavity, while separation selectivity is improved because the stability constants of the inclusion complexes increase with increasing chain length of the acid. The lack of a suitable chromophore moiety was circumvented through the optimization of indirect absorbance detection conditions. p-Anisate was selected as the chromogenic species. The minimum detectable concentrations are of the order of (1–2)·10 −6 mol l −1 (0.2–0.5 ppm) and detection linearity was established over at least three orders of magnitude of concentration. The quantitative analysis of a coco oil extract sample is presented, showing results almost identical with those obtained by gas chromatography. Owing to the close values for response factors resulting from the indirect detection principles, a rapid percentage composition can be obtained by corrected peak area normalization.