Highly Sensitive Chemical Ionization Tandem Mass Spectrometry Method for the Identification of Unsaturated Fatty Acids Derivatized by Dimethyl Disulfide

Derivatization of unsaturated fatty acids with dimethyl disulfide (DMDS) and analysis by electron ionization mass spectrometry (EIMS) represent a convenient offline method for the identification of double bond positions. However, the presence of overlapping mass spectra from multiple compounds poses significant challenges for spectral interpretation and library matching, leading to ambiguous molecular information and low sensitivity. To overcome the issue, we developed a novel chemical ionization (CI) tandem mass spectrometry method involving the pre-derivatization with DMDS and collisional activation of [M+47]⁺ ions generated in the chemical ion source. The method provides better specificity to the analysis of targeted fatty acids and does not require any customized devices. Further, a multiple reaction monitoring (MRM) version of the method was designed by screening all the diagnostic ions of possible double bond positional isomers, which significantly boosts the sensitivity. Compared to the traditional EIMS method, the new method exhibits a lower limit of detection (LLOD) that is one-tenth or lower. Employing the new method, unusual isomer 18:2(5<i>Z</i>,8<i>Z</i>) was co-analyzed with 18:2(9<i>Z</i>,12<i>Z</i>), and a novel 20:2(7<i>Z</i>,10<i>Z</i>) was characterized in human sebum. Additionally, 16:2(9<i>Z</i>,12<i>Z</i>), an odd-chain omega-3 polyunsaturated fatty acid (21:5n-3) and polymethylene-interrupted isomers, i.e. 22:2(7<i>Z</i>,13<i>Z</i>) and 22:2(7<i>Z</i>,15<i>Z</i>) were identified in seafood and related products. Our method can be readily applied to any GC instrument equipped with tandem MS and is expected to facilitate the discovery and identification of unknown fatty acids from food, clinical, and environmental sources.