Picolinyl esters for the structural determination of fatty acids by GC/MS

The position of unsaturation, chain branching, and other structural features of fatty acids are not often apparent from the mass spectra of common derivatives such as methyl esters because of factors such as charge location at the carboxy termiunus and migration of double bonds. The spectra of picolinyl esters, on the other hand, contain fragment ions that provide this information. The esters are synthesized by reaction of the acids with thionyl chloride to form the acid chloride that is reacted with 3-pyridylcarbinol to give the ester. Under electron impact conditions in the mass spectrometer, an electron is removed from the nitrogen of the pyridine ring and a hydrogen atom is abstracted from the alkyl chain to this electron-deficient site. This process produces a radical site in the chain that initiates chain cleavage. Hydrogen atoms can be removed from any position of the chain with varying probability, depending on the chain structure. Thus, diagnostic ions are produced from each type of fatty acid whose masses and relative abundances reflect the structure of the alkyl chain and any substituents. Patterns of fragmentation for straight-chain, branched-chain, unsaturated and cyclic fatty acids are described together with those containing hydroxy-, epoxy-, keto-, and ether groups.