Resolution Behavior of cis- and trans-Octadecenoic Acid Isomers by AOCS Official Method Using SP-2560 Column

The gas chromatography flame ionization detector equipped with a higher polarity column (i.e., SP-2560) has often been used for the quantification of trans-fatty acids in food. In particular, AOCS Ce 1h-05, the official method of the American Oil Chemists' Society (AOCS), is a highly effective method to separate the isomers of trans-fatty acids. In this study, the resolution behavior and the response factors of cis- and trans-octadecenoic acid methyl ester (C18:1-ME) isomers separated by the AOCS Ce 1h-05 method were investigated, and the contents of each cis- and trans-C18:1-ME isomer in partially hydrogenated vegetable oil (PHVO) and milk fat were quantified by using the calibration curves obtained for the respective isomers. The relative response factors for the trans- and cis-C18:1-ME isomers against the internal standard heneicosanoic acid methyl ester (C21:0-ME) were 1.031 +/- 0.040 (mean +/- SD) and 0.990 +/- 0.032, respectively. The relative response factors of trans-isomers tend to be higher than those of cis-C18:1-ME isomers. The peaks of cis-4-C18:1-ME, cis-5-C18:1-ME, cis-6-C18:1-ME, cis-7-C18:1-ME, cis-8-C18:1-ME, and cis-9-C18:1-ME isomers overlapped with those of trans-C18:1-ME isomers. Both PHVO and milk fat contained many types of cis- and trans-C18:1 isomers, and the total contents of the trans-C18:1 isomer in PHVO and milk fat were 28.01 g and 3.62 g per 100 g oil, respectively. When the trans-C18:1-ME isomer was separated from the cis-C18:1-ME by using a silver-ion cartridge column before the analyses, the total contents of the trans-C18:1 isomer in PHVO and milk fat were 23.03 g and 2.78 g per 100 g oil, respectively. The difference in the trans-C18:1 isomer content between the two methods was ascribed to the partial overlapping of cis-isomer peaks with the peaks of trans-C18:1-ME isomers, in the chromatogram.