Resolution of Linolenic Acid Geometrical Isomers by Gas--Liquid Chromatography on a Capillary Column Coated with a 100% Cyanopropyl Polysiloxane Film (CP Sil 88

Fatty acid methyl or isopropyl esters (FAMEs and FAIPEs, respectively) of linolenic acid (9, 12, 15-18:3) geometrical isomers (LAGIs) are prepared with elaidinized linseed oil. FAMEs and FAIPEs are then analyzed by gas—liquid chromatography (GLC) on a fused-silica capillary column coated with a 100% cyanopropyl polysiloxane film (CP™Sil 88). FAMEs are further fractionated by thin-layer chromatography on silica gel plates impregnated with AgNO3 (AgNO3-TLC) and each fraction is analyzed by GLC. Fractionated FAMEs are also transformed into FAIPEs. Six out of the eight LAGI (analyzed as FAMEs) are almost base-line resolved when the column is operated at relatively low temperature (150°C) and pressure (0.8 kg/cm2 He). There is no interference between LAGI and either 20:0 or 20:1. Identification of LAGI is made by combining Rf values determined after AgNO3-TLC, using esters prepared with deodorized rapeseed or soybean oils, and literature data. The elution order of LAGI (either as FAME or as FAIPE) on the CP™Sil 88 is: t,t,t < c,t,t < t,c,t < c,c,t < t,t,c < c,t,c < t,c,c < c,c,c. When FAIPEs of LAGI are analyzed on the same column operated at 165°C with a carrier gas pressure of 1.05 kg/cm2, all eight isomers are separated. The time of analysis (20 vs. 45 min) is significantly shortened. However, peak resolution of FAIPE is not as good as that obtained with FAME.