Analysis of hydroxy fatty acids as pentafluorobenzyl ester, trimethylsilyl ether derivatives by electron ionization...

Electron ionization (EI) gas chromatography/mass spectrometry (GC/MS) analysis of pentafluorobenzyl ester-trimethyl sllyl ether (PFB-TMS) derivatives of hydroxy-subshtuted fatty acids provides structural information comparable to that obtained in analysis of methyl ester-trimethyl silyl ether (Me-TMS) derivatives. Use of this derivative eliminates the need to prepare two separate derivatives, the PFB-TMS derivative for molecular weight determination by electron capture ionization (negative ions) analysis and the Me-TMS derivative for structural determination by EI GC/MS analysis. The relative abundance of fragment ions observed during EI GC/MS analysis of these derivatized unsaturated fatty acids indicates the location of the -OTMS substituents relative to double bond positions in those cases studied. The most abundant fragment ions are observed when the compound contains an unsaturation two carbon atoms removed from the -OTMS ether carbon (the β-OTMS position). The "saddle effect" observed in the GC/MS analyses of some derivatized monohy- droxy unsaturated fatty acids is suggested to be due to a thermally allowed pericyclic double bond rearrangement and indicates the presence of a conjugated diene one carbon atom removed from the -OTMS ether carbon (the α-OTMS position). The saddle effect is most prominent for fatty acids that contain additional unsaturation separated by a single methylene unit from the conjugated diene moiety.