Eicosapentaenoic acid geometrical isomer artifacts in heated fish oil esters

Gas liquid chromatography (GLC) on polar capillary columns showed that the ester of eicosapentaenoic acid (EPA) in distilled fish oil methyl and ethyl esters sometimes is accompanied by several artifacts. The same EPA artifacts did not arise from saponification and/or esterification but were formed in a significantly high yield during prolonged heating of the authentic acid. Physicochemical studies, isolation and partial degradation showed that these artifacts are mainly geometrical ( cis‐trans ) isomers of the natural cis ethylenic bonds of eicosapentaenoic acid. Several C 20 mono‐and diethylenic geometrical isomers were prepared and the GLC equivalent chain length values determined. On the basis of measured and calculated equivalent chain length values on two different GLC phases, some of the EPA artifacts were identified as 20:5‐Δ rans ‐5, cis ‐8, cis ‐11, cis ‐14, cis ‐17; 20:5‐Δ cis ‐5, trans ‐8, cis ‐11, cis ‐14, cis ‐17; and 20:5‐Δ cis ‐5, cis 8, cis ‐11, cis ‐14, trans ‐17. No evidence was found for any positional isomers.