Structural Determination and Occurrence in Ahiflower Oil of Stearidonic Acid Trans Fatty Acids

Several marine oils and seed oils on the market contain relevant quantities of stearidonic acid (18:4n‐3, SDA). The formation of 18:4n‐3 trans fatty acids ( t FA) during the refining of these oils necessitates the development of a method for their quantification. In this study, 18:4n‐3 was isolated from Ahiflower and isomerized to obtain its 16 geometric isomers. The geometric isomers of 18:4n‐3 were isolated by silver ion HPLC (Ag + ‐HPLC) and characterized by partial reduction with hydrazine followed by gas chromatography analysis. The elution order of all 16 isomers was established using a 100 m × 0.25 mm 100% poly(biscyanopropyl siloxane) capillary column and at the elution temperature of 180 °C. The 4 mono‐ trans ‐18:4n‐3 isomers produced during the refining of oils rich in 18:4n‐3 were chromatographically resolved from each other, but c 6, t 9, c 12, c 15‐18:4 coeluted with the tetra‐ cis isomer. These 2 fatty acids (FA) were resolved by reducing the separation temperature to 150 °C, but this change caused tetra‐ cis ‐18:4n‐3 to coelute with t 6, c 9, c 12, c 15–18:4. Combining the results from 2 isothermal separations (180 and 150 °C) was necessary to quantify the 4 mono‐ trans 18:4n‐3 FA in Ahiflower oil.