GC Separation of cis ‐Eicosenoic Acid Positional Isomers on an Ionic Liquid SLB‐IL100 Stationary Phase

Gas chromatography (GC) of cis ‐eicosenoic acid (20:1) positional isomers has been investigated on a capillary column of ionic liquid 1,9‐di(3‐vinyl‐imidazolium)nonane bis(trifluoromethyl)sulfonylimidate stationary phase (SLB‐IL100). A test mixture of isomeric 20:1 methyl esters was prepared from flathead flounder flesh lipids. On a 60‐m column operated at 150–180 °C, six peaks appeared in the elution order of 20:ln‐15 → 20:ln‐13 → 20:ln‐11 → 20:ln‐9 → 20:ln‐7 → 20:ln‐5. These peaks were baseline resolved within 20 min at 180 °C. The 20:ln‐13 and 20:ln‐11 isomers, poorly resolved on conventional polar polysiloxane stationary phases, were completely separated from each other with separation factor α = 1.02 and peak resolution (Rs) ≥ 1.57. When equivalent chain length (ECL) values were compared between the SLB‐IL100 and CP‐Sil 88 (biscyanopropyl polysiloxane), those of 20:ln‐15 and 20:ln‐13 exceptionally tended to be lower on the SLB‐IL100. The excellent separation of 20:1 isomers seems due to less retention of 20:ln‐15 and 20:ln‐13 on SLB‐IL100 rather than simply due to its high polarity. Analysis of herring oil 20:1 revealed the occurrence of 20:ln‐13 in the Pacific herring but not in the Atlantic herring. The ionic liquid stationary phase, SLB‐IL100, is effective for analyzing 20:1 isomers occurring in fish and other natural oils.