Elucidation of double bond positions of polyunsaturated alkenes through GC-MS analysis of mono-dimethyl disulfide derivatives

RATIONALE Derivatization with dimethyl disulfide (DMDS) followed by GC-MS analysis is a well-established method for locating double bond position on the alkyl chain of mono-unsaturated compounds such as alkenes. For alkenes containing more than one double bond, however, the conventional DMDS derivatization approach forms poly- or cyclized DMDS adducts whose mass spectra are difficult to interpret in terms of double bond positions. In this study, we report an efficient experimental procedure to produce mono-DMDS adducts for polyunsaturated alkenes with two to six double bonds. GC-MS analyses of these mono-DMDS adducts yield highly characteristic mass fragments, allowing unambiguous assignments of double bond positions on the alkyl chain. We also apply our new approach (i.e., preferential formation of mono-DMDS adducts during derivatization with DMDS) to determine the double bond positions of unsaturated alkenes produced by laboratory cultured Isochrysis litoralis, a haptophyte algal species. METHODS Alkenes from different sources were derivatized with DMDS at 25 °C for 20 to 160 minutes. The mass spectra of mono-DMDS adducts were obtained by GC-EI-MS analysis of reaction products which contain chromatographically resolved mono-DMDS adducts. RESULTS Mass spectra of corresponding mono-DMDS adducts contain prominent diagnostic ions that allow a conclusive elucidation of double bond positions. In culture samples of Isochrysis litoralis, a series of novel mono- to tri-unsaturated C31 alkenes (9-C31:1 , 6, 9-C31:2 , 6, 22-C31:2 , 6, 25-C31:2 , 9, 22-C31:2 , 6, 9, 25-C31:3 ) were discovered for the first time. CONCLUSIONS A highly efficient DMDS derivatization approach is developed to yield abundant mono-DMDS adducts of polyunsaturated alkyl alkenes for elucidating double bond positions using GC-MS.