Collisional Activation Decomposition Mass Spectra for Locating Double Bonds in Polyunsaturated Fatty Acids

It was established previously that gas-phase carboxylate anions of saturated fatty acids undergo losses of the elements of CH₄, C₂H₆, C₃H₈ … by way of a highly specific 1,4-elimination of H₂- These C_nH_2n+2 losses begin at the alkyl terminus and progress along the entire alkyl chain. Introduction of a double bond causes a disruption In the C_nH_2n+2 loss pattern which can be Interpreted readily to locate the double bond position. However, locating double bonds In polyunsaturated fatty acids Is more difficult and becomes Impossible for acids containing four or more carbon-carbon double bonds. These highly unsaturafed fatty acid anions fragment upon collisional activation by eliminating 45 mass units which preempts the C_nH_2n+₂-loss process. It Is demonstrated In this paper that the highly specific mechanism for C_nH_2n+₂loss and a dllmlde (N₂D₂) reduction provide the opportunity to develop a method of determining double bond location In polyunsaturated fatty acids. Accordingly, a series of polyunsaturated fatty acids was reduced and the resulting deuterated saturated acids were desorbed as carboxylate anions. Interpreting the mass shifts caused by Incorporation of deuterium Into the departing C_nH_2n+2 neutral fragments permits the double bonds In the original acid to be located.