Effect of pressure on the selectivity of polymeric C18 and C30stationary phases in reversed-phase liquid chromatography.Increased separation of isomeric fatty acid methyl esters,triacylglycerols, and tocopherols at high pressure

摘要

A high-density, polymeric C18 stationary phase (Inertsil ODS-P) or a polymeric C30 phase (Inertsil C30)provided improved resolution of the isomeric fatty acids (FAs), FA methyl esters (FAMEs), triacylglycerols(TAGs), and tocopherols with an increase in pressure of 20–70 MPa in reversed-phase HPLC. With respectto isomeric C18 FAMEs with one cis-double bond, ODS-P phase was effective for recognizing the positionof a double bond among petroselinic (methyl 6Z-octadecenoate), oleic (methyl 9Z-octadecenoate), andcis-vaccenic (methyl 11Z-octadecenoate), especially at high pressure, but the differentiation betweenoleic and cis-vaccenic was not achieved by C30 phase regardless of the pressure. A monomeric C18 phase(InertSustain C18) was not effective for recognizing the position of the double bond in monounsaturatedFAME, while the separation of cis- and trans-isomers was achieved by any of the stationary phases.The ODS-P and C30 phases provided increased separation for TAGs and β- and γ-tocopherols at highpressure. The transfer of FA, FAME, or TAG molecules from the mobile phase to the ODS-P stationaryphase was accompanied by large volume reduction (?30 ~ ?90 mL/mol) resulting in a large increasein retention (up to 100% for an increase of 50 MPa) and improved isomer separation at high pressure.For some isomer pairs, the ODS-P and C30 provided the opposite elution order, and in each case higherpressure improved the separation. The two stationary phases showed selectivity for the isomers havingrigid structures, but only the ODS-P was effective for differentiating the position of a double bond inmonounsaturated FAMEs. The results indicate that the improved isomer separation was provided by theincreased dispersion interactions between the solute and the binding site of the stationary phase at highpressure.