The MS2 mass spectrum of the [M – H + 2Li]+ ion of m/z 728 (a), its MS3 spectra of the ions of m/z 605 (728 →605) (b) and the MS4 spectra of the ions of m/z 293 (728 →605 →293) (c), of m/z 295 (728 →605 →295) (d) and of m/z 309 (728 →605 →309) (e). The MS2 mass spectrum of the [M – H + 2Li]+ ion of 9,10-methyleneoctadecanoic acid standard at m/z 309 is shown in Panel f, which is similar to Panel e and gives the structural assignment. Shown in insets of Panels a, c, d and f are the proposed fragmentation pathways leading to locate the positions of double bond and/or of cycopropane group.