Collisional activation of a series of homoconjugated octadecadienoic acids with fast atom bombardment and tandem mass spectrometry.

High-energy collisional activation (CA) of long-chain fatty acid ions induces decompositions that occur remote from the charge site. These charge-remote fragmentations (CRFs) have been shown to provide much structural information. In this report, the CRF of a continuous series of 12 homoconjugated octadecadienoic acids was studied with fast atom bombardment and tandem mass spectrometry. Each fatty acid was desorbed as the carboxylate anion, [M - H](-), the dilithiated species, [M - H+2Li](+), or the bariated species, [M - H+Ba](+), giving three ways of localizing the charge. A characteristic pattern is generated for CRF of the 1,4-diene functional group, and this allows for the rapid identification of the functional group and its location on the chain. Minor variations of this pattern are observed for the different ionic precursors and for different locations of the double bonds. Furthermore, there are a few complications from different types of charge-proximate reactions, especially of the fatty acid carboxylates.