Abstract
Operating from one to six silver ion-high-performance liquid chromatography (Ag+-HPLC) columns in series progressively improved the resolution of the methyl esters of conjugated linoleic acid (CLA) isomeric mixtures from natural and commercial products. In natural products, the 8 trans, 10 cis-octadecadienoic (18∶2) acid was resolved from the more abundant 7 trans, 9 cis-18∶2, and the 10 trans, 12 cis-18∶2 was separated from the major 9 cis, 11 trans-18∶2 peak. In addition, both 11 trans, 13 cis-18∶2 and 11 cis, 13 trans-18∶2 isomers were found in natural products and were separated; the presence of the latter, 11 cis, 13 trans-18∶2, was established in commercial CLA preparations. Three Ag+-HPLC columns in series appeared to be the best compromise to obtain satisfactory resolution of most CLA isomers found in natural products. A single Ag+-HPLC column in series with one of several normal-phase columns did not improve the resolution of CLA isomers as compared to that of the former alone. The 20∶2 conjugated fatty acid isomers 11 cis, 13 trans-20∶2 and 12 trans, 14 cis-20∶2, which were synthesized by alkali isomerization from 11 cis, 14 cis-20∶2, eluted in the same region of the Ag+-HPLC chromatogram just before the corresponding geometric CLA isomers. Therefore, CLA isomers will require isolation based on chain length prior to Ag+-HPLC separation. The positions of conjugated double bonds in 20∶2 and 18∶2 isomers were established by gas chromatography-electron ionization mass spectrometry as their 4,4-dimethyloxazoline derivatives. The double-bond geometry was determined by gas chromatography-direct deposition-Fourier transform infrared spectroscopy and by the Ag+-HPLC relative elution order.
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Abbreviations
- CFA:
-
conjugated fatty acids
- cis/trans :
-
refers to all the CLA isomers having either a cis, trans or a trans, cis configuration
- CLA:
-
conjugated linoleic acid
- DMOX:
-
4,4-dimethyloxazoline
- FAME:
-
fatty acid methyl esters
- GC-DD-FTIR:
-
gas chromatography-electron ionization mass spectrometry
- Ag+-HPLC:
-
silver ion-high-performance liquid chromatography
- PUFA:
-
polynsaturated fatty acids
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Sehat, N., Rickert, R., Mossoba, M.M. et al. Improved separation of conjugated fatty acid methyl esters by silver ion-high-performance liquid chromatography. Lipids 34, 407–413 (1999). https://doi.org/10.1007/s11745-999-0379-3
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DOI: https://doi.org/10.1007/s11745-999-0379-3