Methyl Substituted Long Chain 1,4-O-Bridged-1,3-Dienes—Novel Suitable Internal Standards for the GC/MS Analysis of Furan Fatty Acids in Fish Oil

Furan fatty acids (FuFAs) are excellent antioxidants but rather unexplored because of their high instability and low contributions to the total lipids of food and biota samples. Therefore, the use of internal standards (IS) is essential for the correct determination of FuFAs in food. However, suitable IS, such as isotope-labeled FuFAs, are currently unavailable, and alternative solutions have to be considered. Since degradation of FuFAs occurs in the furan moiety, the IS must not necessarily carry a carboxyl group. Therefore, one monomethylated (M-11-5) and one dimethylated (D-11-5) 1,4-O-bridged-1,3-diene (oxaDE) were synthesized and applied in the GC/MS analysis of β-methyl substituted (M-FuFAs) and β,β'-dimethyl substituted FuFAs (D-FuFAs). GC/MS measurements verified a similar fragmentation as for FuFA methyl esters (FuFA-MEs), along with favorable low quantification limits (0.8–0.9 pg). Transesterification of two FuFA-EE (9M5, 11D5) along with two oxaDEs (M-11-5, D-11-5) enabled monitoring the recovery of FuFAs and verified the lower stability of D-FuFAs compared to M-FuFAs. Problems caused by unstable reagents in the transesterification step could be identified and solved with the newly introduced IS. The final application of the method to six fish oils enabled the detection of eight FuFAs with amounts up to 1.2 g/100 g fish oil.