On the formation of conjugated linoleic acid diagnostic ions with acetonitrile chemical ionization tandem mass spectrometry

Abstract

Acetonitrile chemical ionization tandem mass spectrometry has recently been shown to be a rapid method for the identification of double-bond position and geometry in methyl esters of conjugated linoleic acids (CLAs); however, the structures of intermediate and diagnostic ions and their mechanisms of formation are not known. A mechanism is proposed here in which the m/z 54 ion, (1-methyleneimino)-1-ethenylium (MIE), undergoes nucleophilic attack preferentially by the cis double bond in CLAs with mixed geometry (cis/trans, trans/cis), favoring the observed C--C cleavage vinylic to the trans double bond. The [M+54](+) addition product intermediate is consistent with a heterocyclic six-membered ring resulting from the two-step addition of MIE to the CLA. Experiments with isotopically labeled CLAs and acetonitrile, and from MS/MS/MS experiments, yield data consistent with this proposal. The proposed mechanism is also consistent with known ion-molecule chemistry in smaller compounds, and explains most phenomena associated with MIE-CLA ion chemistry.

Both Dietary Fatty Acids and Those Present in the Cecotrophs Contribute to the Distinctive Chemical Characteristics of New Zealand Rabbit Milk Fat

“…Chromatographic conditions were similar to those reported for GC with flame-ionization detection, except that helium was the carrier gas. CACI-MS and MS/MS conditions were as previously reported (Michaud et al, 2005;Van Pelt and Brenna, 1999).…”

Identification of double bond position in lipids: From GC to OzID

“…Initial covalent adduct chemical ionization (CACI) experiments focused on the determination of double bond position in fatty acid methyl esters (FAMEs) [80][81][82][83][84]. Fragmentation of the [FAME + 54] + ions produces two series of fragment ions from the cleavage of the acyl chain on the carboxylate (␣) or methyl () side of the double bonds.…”

“…that might be considered as gas-phase derivatization [80][81][82][83][84][85]. In this method, chemical ionization of unsaturated lipids in the presence of acetonitrile results in the production of (1-methyleneimino)-1-ethylium ions [86] that add covalently across lipid double bonds to produce an [M + 54] + ion [87] (Scheme 3) [80]. Collision-induced dissociation of these adduct ions yield products that are indicative of the position of the double bond(s) within the molecule (see Scheme 3).…”

Identification of cis‐12,cis‐15 octadecadienoic acid and other minor polyenoic fatty acids in ruminant fat

“…Therefore, locating double bond positions directly on FAME seems to be a good and rapid alternative to the other methods. Covalent adduct chemical ionization tandem mass spectrometry (CACI-MS/MS) using acetonitrile as reagent of chemical ionization has been studied and applied as a method to locate double bonds in polyunsaturated FAME [4][5][6][7][8][9]. According to theory, a product of acetonitrile self reaction, (1methyleneimino)-1-ethenylium (CH 2 =C=N 1 =CH 2 ), generated by CACI-MS conditions, reacts with the analyte double bond to yield molecular ions with 54 mass units above the parent analyte.…”