Analysis of cis- 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) spectroscopy, gas‐liquid chromatography (GLC) on highly polar stationary phases or the combination (GLC‐IR) may be used. IR offers the advantage of simplicity and speed, but the lower determination limit of 5% and 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 partially hydrogenated samples. AOCS Official Method Ce 1c‐89 prescribes 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 partially 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 because of overlap problems, but this time in the 18∶3 region. Through careful selection of stationary phase and temperature program optimization (Drylab ® GC), we have developed a single method that is suitable for hydrogenated, as well as refined, processed oils. The accuracy was checked with cis and trans fatty acid fractions isolated by silverion exchange high‐performance liquid chromatography. The trans values obtained with the optimized method are in good agreement with the results obtained for the isolated fractions. We propose that recommended methods describe GLC conditions in terms of separation criteria rather than recommending only a fixed combination of stationary phase and temperature program.