The net action of ruminal bacteria and endogenous bovine enzymes are responsible for cow's milk having the most complex fatty acid profiles among common foods. About 40 monounsaturated fatty acids below 1.5% w/w are known. Analysis of trace and ultratrace fatty acids is a challenge to the highest resolution chromatography even with prior fractionation. We employ solvent-mediated covalent adduct chemical ionization (CACI) tandem mass spectrometry (MS/MS) to enable rapid, unambiguous identification of unsaturated fatty acid methyl esters (FAME) at high sensitivity. Fifty-four monounsaturated fatty acids (C10-24) were completely characterized, with the discovery of 15 novel fatty acids including nine at ultratrace levels 10-100 ppm, g/106 g fatty acids (lowest concentration 19:1n-6 (10 ± 11 ppm, w/w (0.001%, w/w))). Ultratrace monoenes were typically odd chain lengths and all analyzed in a single 20 min analysis. These data establish the abundance of 15 new monoene fatty acids in bovine milkfat and a strategy for rapid unambiguous analysis of ultratrace monounsaturated fatty acids.
Keywords: bovine milk; covalent adduct chemical ionization (CACI); double-bond isomers; mass spectrometry; monounsaturated fatty acid methyl esters.