Identification and quantitation of cis/trans C16:1 and C17:1 fatty acid positional isomers in German human milk lipids by thin-layer chromatography and gas chromatography/mass spectrometry

The intake of trans C18:1 as well as of trans hexadecenoic acids (trans C16:1) is believed to be related with numerous physiological disadvantages, such as the risk of coronary heart disease. Since most of the existing data on trans C16:1 contents in human milk fat have been determined without a pre-separation by thin-layer chromatography (TLC), the gas chromatographically determined contents of trans C16:1 frequently are too high due to overlaps with C17 fatty acids. Using a highly polar column with a length of 100 m after AgNO3-TLC allowed to establish an average content of total trans C16:1 of 0.15 +/- 0.04% from 39 samples of human milk fat. Moreover, the C16:1 positional isomers trans Delta 4, Delta 5, Delta 6/7, Delta 8, Delta 9, Delta 10, Delta 11, Delta 12, Delta 13 and Delta 14 could be quantified from 15 samples exhibiting mean relative contents of 2.6, 3.5, 7.6, 7.2, 24.7, 10.4, 10.1, 14.3, 8.4 and 11.3% related to the total trans C16:1 content, respectively. Also, the C16:1 isomer trans Delta 3 could be identified occurring in traces with a mean absolute content of 2 mg/100 g fatty acids. A baseline separation of almost all trans isomers could be achieved for the first time. Further, mass spectrometric analyses of FAME and DMOX derivatives allowed to identify the isomer trans Delta 4. Among the C16:1 isomers cis Delta 7 to cis Delta 14 the isomer cis Delta 9 predominated with a relative proportion of 68.3% and an absolute content of 1.88% of all fatty acids. Correspondingly, among the C17:1 isomers cis Delta 7 to cis Delta 11 the isomer cis Delta 9 with 82.6% had the highest relative content.