RESOLUTION OF LINOLENIC ACID GEOMETRICAL-ISOMERS BY GAS-LIQUID-CHROMATOGRAPHY ON A CAPILLARY COLUMN COATED WITH A 100-PERCENT 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: f, f, f ` 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/cm2all 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. © Oxford University Press 1992.