Preferential formation of mono‐dimethyl disulfide adducts for determining double bond positions of poly‐unsaturated fatty acids

Determination of double bond positions of unsaturated lipids is fundamental for understanding their biofunctions and biosynthetic pathways. Derivatizing unsaturated lipids with dimethyl disulfide (DMDS) represents a convenient method to yield characteristic ions for determining double bond positions. However, DMDS adduction of poly‐unsaturated lipids often leads to the formation of poly‐ and/or cyclized DMDS adducts, which yield complex mass spectra that are difficult to interpret in terms of double bond positions. Here, we report a room‐temperature, convenient experimental procedure to preferentially form mono‐DMDS adducts for poly‐unsaturated fatty acid methyl esters (FAMEs) with two to five double bonds (c9, c12‐C18:2, t9, t12‐C18:2, c5, c9, c12‐C18:3, c7, c10, c13, c16‐C22:4 and c5, c8, c11, c14, c17‐C20:5). Electron‐impact ionization mass spectra of these mono‐DMDS adducts provide highly diagnostic ions for determining positions of all double bonds. Our approach overcomes the limit of traditional DMDS derivatization methods which are generally limited to mono‐unsaturated molecules. In addition, we show gas chromatography and gas chromatography–mass spectrometry analyses of mono‐DMDS adducts can also provide useful information about double bond geometry, as cis double bonds are prone to isomerize to trans isomers.