Charge-remote metastable ion decomposition of free fatty acids under FAB MS: evidence for biradical ion structures

Classical charge-remote fragmentation (CRF) of a series of long-chain saturated and monounsaturated fatty acid anions, a well-known phenomenon under collisional activation conditions, is observed for the first time during fast atom bombardment of the analyte-matrix mixture without collisional activation. The process is efficient enough to allow collision-induced dissociation and metastable ion decomposition MS/MS spectra of any charge-remote [M-H2-(CH2)n]- fragments as well as spectra of neutral losses to be recorded. The results obtained are in contradiction to the generally accepted theory that CRF results exclusively in terminally unsaturated carboxylate anions. The new results indicate that a multistep radical mechanism is involved in CRF ion formation. The first step of the process appears to be accompanied by hydrogen elimination that occurs randomly throughout the molecule. The primary fragment radical ions formed can decompose further with the formation of the next generation of CRF ions.