The identities of these peaks were confirmed by the similar MS/MS spectra patterns of most of these peaks with those of WE standards (
Fig. 3). All the CID spectra of these unsaturated WEs showed the characteristic protonated unsaturated fatty acid peaks with one or two water molecule losses, similar to previous reports for unsaturated WEs with one or more double bonds in the fatty acid moiety.
11,19 Some peaks were the mixture of more than one isobaric species. For instance, the example shown in
Figure 3 suggests the peak
m/z 591.7 is composed of two major species with the corresponding fatty acid/fatty alcohol moieties being 18:1/22:0 and 16:1/24:0. The fatty acid moieties for the other WEs were found to be mainly C18:1 but with a considerable amount of C16:1, which are consistent with their high percentages of total fatty acids (57.39% and 11.66%, respectively) in WEs reported by Nicolaides et al.,
7 where the reported percentages of the fatty acid components were summed from those for different types of fatty acids including normal, iso, and anteiso with different double bond positions. However, the fatty acid C16:1-based WEs were not reported by Butovich et al.
11,16 On the other hand, it was found in this study that the unsaturated WEs with two double bonds were primarily composed of at least two different species, with one mainly containing C18:1 fatty acid moiety, and the other containing C18:2 fatty acid moiety at a much smaller percentage. The corresponding fatty alcohol moieties for the high-intensity unsaturated WEs include C26:0, C24:0, C25:0, C26:1, C28:1, and C30:1, again consistent with the reported high percentages of these fatty alcohols in WEs (account for 23.35%, 18.59%, 15.04%, 5.94%, 4.33%, 5.13%, respectively, calculated in a similar manner as discussed above for the percentage of C18:1 and C16:1 fatty acid moieties in WEs).
7 C32:1 fatty alcohol observed in this work was not reported by Nicolaides et al.,
7 possibly because of the difficulty in detecting it with GC-MS.