Reversed-phase retention characteristics of tocotrienol antioxidants

Mixtures of alpha-, beta-, gamma- and delta-tocotrienols were separated by reversed-phase (RP) high-performance liquid chromatography (HPLC). Four sets of subcomponents (cis/cis-, cis/trans-, trans/cis- and trans/trans-geometrical isomers) were further resolved under various HPLC conditions. Retention characteristics of the beta- and gamma-isomers on octadecylsilica (ODS) were indistinguishable. However, complete separations of all sixteen tocotrienol components were achieved by RP-HPLC with a nonsilica-based octadecanoyl polyvinyl alcohol (ODPVA) column. In this system, beta-tocotrienol peaks were found to interpose alternatively with those of gamma-tocotrienols, which was attributable to apparent differences in intrinsic molecular polarity and hydrophobicity. HPLC with a pentafluorophenylsilica (PFPS) column led to partial separations of tocotrienols, of which two subclass species remained unresolved. Peak profiles obtained with PFPS columns of different packings were similar but not identical. Nonetheless, the component elution patterns on PFPS were markedly different from those on ODPVA. Furthermore, the use of a triacontylsilica column enabled separations of fifteen components and yielded an elution order parallel to that of an ODPVA column. In general, the elution sequence of tocotrienols appeared to depend largely on the type of organic modifier used, despite the subtle influence of stationary phases and the isomeric characteristics of alkenyl chains. While poorly resolved on silica, geometrical isomers were readily separated on the RP phases studied with acetonitrile-water as the preferred eluent. Factors affecting analyte retention and component resolution were delineated. (C) 1999 Published by Elsevier Science B.V. All rights reserved.