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.