Elucidation of double‐bond positions of polyunsaturated alkenes through gas chromatography/mass spectrometry analysis of mono‐dimethyl disulfide derivatives

Rationale Derivatization with dimethyl disulfide (DMDS) followed by gas chromatography/mass spectrometry (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 min. 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.