Determination of tocopherols in vegetable oil samples by non-aqueous capillary electrophoresis (NACE) with fluorimetric detection

A non-aqueous capillary electrophoresis (NACE) method is presented for the assay of tocopherols in vegetable oils. The composition of the separation buffer was optimized using an experimental design and response-surface methodology, resulting in the selection of a solution made of 12 mM borate buffer (3 mM sodium tetraborate), 60 mM sodium cholate, and 12 mM sodium hydroxide in methanol as optimal. The other conditions used were hydrodynamic injection (30 mbar for 3 s), separation temperature of 50 °C, and separation voltage of +20 kV for the first 12.5 min and +22 kV subsequently. A capillary with two detection windows for serial on-line UV and fluorescence detection was used. The later showed improved sensitivity and selectivity. Linearity of the relationship between the corrected peak area (fluorescence detection) and the concentration was verified from 1.0 to 50.0 μg mL−1 of each tocopherol. The detection limits, calculated as 3 × SD where SD is the standard deviation of the most dilute sample, were of the order of a few μg mL−1. A cleaning stage of solid-phase extraction with a silica cartridge was applied prior to the assay of the samples, and the results were adequately validated by reversed-phase liquid chromatography.