Structural Identification of Monounsaturated Branched Chain Fatty Acid Methyl Esters by Combination of Electron Ionization and Covalent Adduct Chemical Ionization Tandem Mass Spectrometry

Monounsaturated normal fatty acids (n-MUFA) and saturated branched chain fatty acids (BCFA) are structurally characterized by separate tandem mass spectrometry methods for double bond localization and for chain branching in their respective fatty acid methyl ester (FAME) derivatives; however, these methods have never been applied to branched monounsaturated FAME. Here, we report application of electron ionization (EI)-MS/MS and solvent-mediated covalent adduct chemical ionization (CACI)-MS/MS of monounsaturated BCFA methyl esters (MUBCFAME) of a chain length of 15–20 carbons. A novel system was used to implement CI with low vapor pressure reagents in a tabletop triple quadrupole MS. Anteiso-MUBCFA EI-MS/MS of the molecular ion (M) yields a characteristic diagnostic ion [M-29]⁺. iso-MUBCFA can be distinguished from n-MUFA by an ion intensity ratio of [M-32]⁺/[M-43]⁺, with iso-MUBCFA yielding a ratio greater than 1.7, while n-MUFA yields a ratio less than 1.0. Chain branching at the iso and anteiso positions, terminal isopropyl and sec-butyl, respectively, do not alter CACI-MS/MS diagnostic ions compared to normal BCFA, enabling double bond positions of MUBCFA to be determined with the analogous α and ω diagnostic ions from cleavage on both sides of the erstwhile double bond. Taken together, this straightforward FAME-based technique via combination of EI-MS/MS and CACI-MS/MS enables fundamental structural identification of MUBCFA without standards.