Acetonitrile chemical ionization tandem mass spectrometry for the structural characterization of minor homoallylic and conjugated fatty acid methyl esters

The m/z 54 ion, (1-methyleneimino)-1-ethenylium (MIE), produced from acetonitrile under chemical ionization (CI) conditions covalently adds across carbon-carbon double bonds to yield an [M+54]+ ion. Collisionally activated dissociation (CAD) of this ion has been shown to yield products characteristic of the positions of double bonds in parent fatty acid methyl esters (FAME). Two of these ions, one containing the ester moiety and the other containing the terminal methyl group, are referred to as the alpha and o diagnostic ions respectively, and are used for identification of the original double bond positions. The task remained to evaluate the effectiveness of the method for the identification of minor components of real FAME samples and to evaluate it for the analysis of other classes of fatty acids.;Chapter 1 provides a general introduction to ion trap mass spectrometry and chemical ionization. The results of analyses of the minor FAME components of samples originating from algae (schizochitrium sp.), primate brain white matter, and transgenic mouse liver, are presented in chapter 2. These show that double bond positions in FAME present at concentrations well below 1% of the total fatty acids can be identified. Even for peaks barely discernable from the background it is possible to identify the chain length and the number of double bonds.;Chapters 3 and 4 investigate the use of acetonitrile CI-MS and CI tandem MS (MS/MS) for the analysis of conjugated linoleic acid (CLA) methyl esters. The diagnostic ions are formed by C-C cleavage vinylic to the conjugated diene unit. In CLA methyl esters with mixed double bond geometry cleavage occurs preferentially vinylic to the trans double bond. The ratios of diagnostic ion abundances are 4.8 for trans/cis, and 0.7--3.2 for cis/cis and trans/trans isomers, thus distinguishing these classes of CLAs. The mechanism of formation of the [M+54]+ and the diagnostic ions is investigated in chapter 4 by use of labeled CLAs, labeled acetonitrile, and MS/MS/MS experiments. The proposed mechanism explains observations from these experiments and is consistent with previous studies.