Structural determination of modified fatty acids by collisional activation of cationized molecules

The nature and location of a variety of modifications of fatty acids are determined by collisional activation (CA) of M + 2Li ? H]⁺ ions. The sample molecules are cationized in situ on the probe tip, desorbed by fast atom bombardment and, upon CA, undergo charge-remote decompositions. This approach is a direct, totally instrumental method for structure elucidation. Advantages of CA of M + 2Li ? H]⁺ ions are that fatty acids with substituents in close proximity to the carboxylate terminus and modified short-chain acids are readily determined: decompositions of carboxylate anions of these fatty acids result in collision-activated dissociation (CAD) spectra that give incomplete structural information. However, the CAD spectra of some M ? H]^? ions, such as those from epoxy acids, are simpler to interpret than those of the M + 2Li ? H]⁺ ions. Thus, CA of fatty acid M + 2Li ? H]⁺ ions is a complementary approach to CA of M ? H]^? ions for determining the fatty acid structures investigated here. The use of this approach for analyzing complex mixtures of modified fatty acids is also evaluated.