title: Improved separation of conjugated fatty acid methyl esters by silver ion-high-performance liquid chromatography.
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.
authors.affiliation: U.S. Food and Drug Administration, Center for Food Safety and Applied Nutrition, Washington, DC 20204, USA.
authors.forename: N
authors.initials: N
authors.lastname: Sehat
authors.forename: R
authors.initials: R
authors.lastname: Rickert
authors.forename: M M
authors.initials: MM
authors.lastname: Mossoba
authors.forename: J K
authors.initials: JK
authors.lastname: Kramer
authors.forename: M P
authors.initials: MP
authors.lastname: Yurawecz
authors.forename: J A
authors.initials: JA
authors.lastname: Roach
authors.forename: R O
authors.initials: RO
authors.lastname: Adlof
authors.forename: K M
authors.initials: KM
authors.lastname: Morehouse
authors.forename: J
authors.initials: J
authors.lastname: Fritsche
authors.forename: K D
authors.initials: KD
authors.lastname: Eulitz
authors.forename: H
authors.initials: H
authors.lastname: Steinhart
authors.forename: Y
authors.initials: Y
authors.lastname: Ku
mesh_terms: D002851:Chromatography, High Pressure Liquid / Q000379:methods*; D004952:Esters / Q000737:chemistry; D005227:Fatty Acids / Q000737:chemistry* / Q000302:isolation & purification; D007536:Isomerism; D012834:Silver
chemical_list: D004952:Esters; D005227:Fatty Acids; D012834:Silver