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

Abstract

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 12mM borate buffer (3mM sodium tetraborate), 60mM sodium cholate, and 12mM sodium hydroxide in methanol as optimal. The other conditions used were hydrodynamic injection (30mbar for 3s), separation temperature of 50°C, and separation voltage of +20kV for the first 12.5min and +22kV 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 50.0μgmL−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 μgmL−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.