Acetonitrile covalent adduct chemical ionization tandem mass spectrometry of non-methylene-interrupted pentaene fatty acid methyl esters

Acetonitrile covalent adduct chemical ionization tandem mass spectrometry (CACIMS/MS) has shown to be an efficient method for the identification of double‐bond position in homoallylic, conjugated and several polyene non‐methylene‐interrupted (NMI) fatty acid methyl esters. However, it has not been thoroughly evaluated for NMI highly unsaturated fatty acids (HUFA) with more than four double bonds. Docosahexaenoic acid (DHA)‐rich single cell oil (DHASCO^®; Martek Biosciences, Corp.) was partially hydrogenated (partially hydrogenated DHASCO; PHDO) producing ten novel 22:5 and 22:6 HUFA isomers. In single‐stage MS, the ratio of M+54]⁺/M+54‐32]⁺ for the 22:5 and 22:6 isomers indicated the presence of homoallylic or partially conjugated double‐bond systems. The CACIMS/MS spectra revealed six 22:5 isomers with diagnostic ions corresponding to the homoallylic 22:5n‐6 and 22:5n‐3 isomers, and four distinct NMI 22:5 isomers. Diagnostic ions for four 22:6 isomers were identical to the native DHA illustrating that CACIMS/MS is sensitive to double‐bond position but not geometry. Three gas chromatography (GC) peaks for partially conjugated 22:6 isomers were also detected and clearly distinguishable from homoallylic 22:6 isomers, but their CACIMS/MS spectra did not yield prominent ions indicative of double‐bond position, possibly due to co‐elution. Overall, CACIMS/MS was effective for determining double‐bond position in NMI 22:5 isomers. Further investigations are warranted to evaluate and determine fragmentation patterns for partially conjugated and NMI 22:6 HUFA. Copyright © 2011 John Wiley & Sons, Ltd.