Co-elution of Cis-NMIFAs in the 18:3t region: analytical approach in trans fatty acid identification.

Accurate identification of trans-18:3 fatty acid isomers remain challenging due to the co-elution of structurally diverse cis isomers, particularly non-methylene-interrupted fatty acids (NMIFAs). GC-FID analysis of pine nut using an SP™-2560 column revealed unusually high peak areas (> 13%) in the 18:3t region. However, a peak reduction in the 18:3t region after nitric acid-induced isomerization reaction, suggested the presence of cis isomers in native pine nut. GC-MS data identified the disappearing peak as pinolenic acid (5Z,9Z,12Z-octadecatrienoic acid; Δ5, 9, 12-18:3), while FT-IR and Raman spectroscopy confirmed hardly detectable trans fatty acid content prior to isomerization. Further, comparative analysis with perilla oil, rich in α-linolenic acid, under identical conditions aided in clarifying trans isomer positions in the 18:3t region. These findings demonstrate the limitation of retention-time-based identification alone to distinguish geometric isomers within polyunsaturated fatty acid subclasses.