Gas-phase ion decompositions occurring remote to a charge site

Several classes of closed-shell, gas-phase ions have been found to decompose upon collisional activation in a manner which does not rely on charge initiation. The reactions provide specific and highly informative structural information. This type of fragmentation, which was observed first in a study of carboxylate anions of unsaturated fatty acids, is now identified as parallel losses of the elements of C_nH_2n+2 initiated from the alkyl terminus. A homologous series of saturated fatty acid anions ranging in chain length from 5 to 18 carbons has been used as a model system to characterize the phenomenon. The fragmentation occurs also for collisionally activated sulfate and sulfonate anions and long-chained amine and phosphonium cations. Deuterium labeling was used to determine that the C_nH_2n+2 losses are from the alkyl terminus and that a probable mechanism is a process which involves a 1,4 hydrogen elimination with subsequent formation of terminally unsaturated carboxylate anions and neutral olefins.