Determination of Double-Bond Position in Fatty Acid Methyl Esters by Acetonitrile Chemical Ionization Mass Spectrometry.

  The location of unsaturation in a fatty acid strongly influences its biological function and metabolic fate.  However, most methods for determining double bond position rely on laborious off-line derivitizations that occasionally result in ambiguous assignments, especially for novel or trace fatty acids.  

  We have developed a method using positive chemical ionization (CI) by a reagent ion of acetonitrile ((1-methyleneimino)-1-ethenylium (MIE)) that reacts across analyte double bonds.  Using a GC-ion trap, we achieve characteristic MS/MS spectra showing fragmentation on either side of the unsaturated bond.  This method was originally developed to study cis- and trans- monounsaturates, and homoallylic (i.e. methylene-interrupted) polyunsaturates. 

  We have recently extended the method to structural characterization of conjugated linoleic acids (CLAs).  This topic is particularly relevant because there is evidence that certain CLA isomers may be anti-carcinogenic, while others may lower triglyceride levels.

Chromatogram showing fatty acid methyl esters from golden algae sample.  Peaks were identified using acetonitrile chemical ionization on a GC-ion trap system