NORMAL-PHASE HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY OF TOCOPHEROLS ONPOLAR PHASES

Five plant tocopherols were separated by normal phase high performance liquid chromatography (HPLC) on aminopropylsilica or diol-bonded sili ca with fluorescence detection. HPLC characteristics of these compound s were studied under various mobile phase conditions. Mobile phases em ployed binary solvent systems comprising a hydrocarbon and an alcohol, an ether or an ester. Separation factors (alpha) of adjacent ring-met hylated tocol components were determined. Employment of aprotic weakly polar modifiers in the mobile phases increased the or values for the Fair of 5,8- and 7,8-dimethyltocol, but decreased those for the 5,7-an d 5,8-dimethyltocol pair. Both component resolution and detection sens itivity were adversely affected by the use of a diol-bonded silica pha se of large particle size. HPLC with cyclohexane-binary eluents invari ably led to lower values of analyte capacity factors (k') than with he xane counterparts. Further, regardless of the type of column evaluated , the tocol isomers eluted in an ethereal mobile phase in the order of increasing k' values: k'(tetrahydrofuran) <k'(dioxane) < k'(t-butyl m ethyl ether < k'(diisopropyl ether). In HPLC with protic co-solvents, the observed differences in k' values among the three dimetyltocols we re interpreted in terms of methyl substitution effects.