Identification of cis-12,cis-15 octadecadienoic acid and other minor polyenoic fatty acids in ruminant fat

The fatty acid pattern of ruminant edible fats is very complex and its diversity is mostly determined by extensive interactions between dietary factors and rumen metabolism. Characterization of intermediates of polyunsaturated fatty acid (PUFA) biohydrogenation in lipids from ruminants, particularly when supplemented with oils, can be a rather difficult task. In this study, we characterized some minor PUFA that occur in muscle samples from linseed oil-fed lambs, by covalent adduct chemical ionization tandem mass spectrometry (CACI-MS/MS) using acetonitrile as reagent of chemical ionization. Two minor metabolites, cis-12, cis-15 18:2 and cis-9,trans-11,cis-15 18:3 were identified for the first time in lamb meat through CACI-MS/MS. However, the cis-9, trans11, cis-15 18:3 isomer was incompletely resolved from 20:3n-9 under our GC/MS conditions. Thus, in studies reporting biohydrogenation intermediates in meat, caution must be taken in order to ensure proper resolution of these two fatty acid methyl esters (FAME) and therefore avoid quantitative overestimations. CACl-MS/MS was a powerful and rapid method for the structural characterization of minor polyenoic FAME present in ruminant fat, and it seems to be a good alternative to classic 4,4-dimethyloxazoline and picolinyl derivatization methods.