SEPARATIONS OF TOCOPHEROLS AND METHYLATED TOCOLS ON CYCLODEXTRIN-BONDED SILICA

alpha-, beta-, gamma-, and delta-tocopherols (methyl-substituted tocols) and 5,7-dimethyltocol were separated by normal-phase HPLC on beta- or gamma-cyclodextrin-bonded silica (CDS) with fluorescence detection. The HPLC behavior of the tocol components was studied under various mobile phase conditions. Hexane or cyclohexane was used in combination with oxygen-containing solvents (alcohol, ether, and esters) in binary and ternary mobile phases. Capacity factors (k') and separation factors (alpha) for adjacent tocol components were determined. Incorporation of non-polar ethers in the hydrocarbon (hexane or cyclohexane) mobile phases favored the separation of beta- and gamma-tocopherols with improved alpha values, which enabled trace analysis of the beta-isomer present in soybean oil. Analyte solutes tended to be more strongly adsorbed in mobile phases containing branched-chain alcohols and ethers than in those containing the corresponding straight-chain solvents. Generally, the k' values obtained with hexane mobile phases or with the beta-CDS phase were greater than those observed in HPLC with cyclohexane mobile phases or with the gamma-CDS phase.