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.
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