CAPILLARY ZONE ELECTROPHORETIC DETERMINATION OF C-2-C-18 LINEAR SATURATED FREE FATTY-ACIDS WITH INDIRECT ABSORBENCY DETECTION

The potential of capillary zone electrophoresis for the separation of linear saturated fatty acids in their free form was evaluated. With in creasing chain length, difficulties arise from decreasing analyte solu bility 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 m ethanol. 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-beta-CD as an additive. The separat ion of C-7-C-18 homologues was completed in under 20 min with electrol ytes 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 s eparation selectivity is improved because the stability constants of t he inclusion complexes increase with increasing chain length of the ac id. The lack of a suitable chromophore moiety was circumvented through the optimization of indirect absorbance detection conditions. p-Anisa te was selected as the chromogenic species. The minimum detectable con centrations 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 ex tract sample is presented, showing results almost identical with those obtained by gas chromatography. Owing to the close values for respons e factors resulting from the indirect detection principles, a rapid pe rcentage composition can be obtained by corrected peak area normalizat ion.