Collision-induced dissociations and B/E linked scans for structural determination of modified fatty acid esters

Abstract

Structures of modified fatty acid esters are determined by collisionally activating [M + Li] + ions desorptively ionized by fast atom bombardment (FAB). Collision-induced dissociations (CIDs) that occur in the first field-free region of a normal geometry mass spectrometer are observed by using B/E linked scans. The CID spectra provide immediate information about chain length, location and type of substituent in saturated, unsaturated, methyl-branched and hydroxy esters. New spectra of more unusual ethyl esters that contain longer alkyl branches and keto substituents also indicate the location and type of substituent. Novel charge-remote reactions that are directly analogous to thermolytic hydro-acyloxy eliminations and heteroatom analogs of the retro-ene reaction are suggested to explain the formation of some fragment ions from the alkyl and keto esters. Deuterium labeling provides evidence for mechanisms for structurally informative charge-mediated fragmentations of the hydroxy esters. Individual components of complex mixtures can be determined, and the detection limit for the structural determination of methyl palmitate is 25 ng. Methods also are described for optimizing the practical use of FAB in conjunction with B/E linked scans, which can be hampered by poor precursor ion resolution.