ANALYSIS OF CIS-FATTY-ACID AND TRANS-FATTY-ACID ISOMERS IN HYDROGENATED AND REFINED VEGETABLE-OILS BY CAPILLARY-GAS-LIQUID-CHROMATOGRAPHY

For quantitation of cis- and trans-fatty acid isomers, infrared (IR) s pectroscopy, gas-liquid chromatography (GLC) on highly polar stationar y phases or the combination (GLC-IR) may be used. IR offers the advant age of simplicity and speed, but the lower determination limit of 5% a nd the lack of detailed information limit its use. Detailed fatty acid information, required for, e.g., food-labeling purposes, can only be obtained with GLC methods. Most of the GLC methods are optimized for p artially hydrogenated samples. AOCS Official Method Ce 1c-89 prescribe s a single, highly polar stationary phase, SP2340, but underestimates the amount of trans isomers due to 18.1 positional isomer overlap. The combined GLC-IR method may circumvent this problem but at the cost of time, effort, and precision. Trans isomers in refined (deodorized or stripped) oils are different in type and levels from isomers in partia lly hydrogenated oils; their trans isomers are mono-trans trienoic and dienoic isomers, occurring at levels up to about 1-3%. GLC conditions for hydrogenated samples are often not suitable for refined oils beca use of overlap problems, but this time in the 18:3 region. Through car eful selection of stationary phase and temperature program optimizatio n (Drylab(R)GC), we have developed a single method that is suitable fo r hydrogenated, as well as refined, processed oils. The accuracy was c hecked with cis and trans fatty acid fractions isolated by silver-ion exchange high-performance liquid chromatography. The trans values obta ined with the optimized method are in good agreement with the results obtained for the isolated fractions. We propose that recommended metho ds describe GLC conditions in terms of separation criteria rather than recommending only a fixed combination of stationary phase and tempera ture program.