Characterization of diacylglycerol isomers in edible oils using gas chromatography–ion trap electron ionization mass spectrometry

Verifying the authenticity of edible oils is of international concern. A new quality control standard for olive oil has been proposed that relates the ratio of 1,2-diacylglycerol (DAG) to 1,3-DAG to sensory aspects of olive oil. DAGs and their isomers are difficult to quantitate and characterize by Flame Ionization Gas Chromatography (GC-FID) due to the lack of suitable standards. Mass detectors offer the advantage of providing structural detail to the eluding DAG(s), thus removing ambiguity to the identification of both resolved and unresolved DAGs in GC chromatograms. In this study, a GC Electron Ionization Mass Spectrometry (GC-EI-MS) method was developed to determine the fatty acid composition and molecular structure of trimethylsilyl (TMS) derivatized DAGs present in edible oils. Twenty-two species of DAG isomers were identified in refined coconut oil and unrefined olive oil utilizing signature fragment ions, [M−15]+, [M−89]+, [M−RCO2]+, [RCO2+58]+ and [M−RCO2CH2]+. The [M−RCO2CH2]+ ion is considered the key diagnostic ion to distinguish between DAG positional isomers. MS/MS spectra of [M−RCO2]+ and [M−15]+ ions obtained from commercial standards containing both 1,2- and 1,3-DAG isomers were used as a model system to confirm the identification of DAG isomers in natural products. Furthermore, a number of reaction mechanisms are proposed to explain the formation of the most abundant mass fragments of DAGs and their isomers.