Compound-specific analysis of δ13C and δ2H of olive oil fatty acids

A method to determine δ13C, and for the first time δ2H of the main fatty acids extracted from olive oil triglycerides was developed. Triglycerides were subjected to base-catalysed transesterification with methanol to obtain the corresponding fatty acid methyl esters (FAMEs) in a one-step reaction. Then they were analysed through Gas Chromatography-Isotope Ratio Mass Spectrometry (GC-C\Py-IRMS). Here we present the validation data for the method, which were consistent with those of methods for the analysis of FAMEs in other matrices. As there are no reference materials available, a standard mixture of FAMEs replicating the composition of olive oil was prepared and analysed to test the accuracy of measurement. The resulting δ13C and δ2H values were compared with the 'true' isotopic values of pure single FAMEs obtained with EA-IRMS and TC/EA-IRMS. The difference was no more than ±0.2‰ and ±1.9‰ for δ13C and δ2H. To assess precision, the reference FAMEs mixture was analysed 10 times with GC-C\Py-IRMS, the obtained mean standard deviations were ±0.2‰ and ±2‰ for δ13C and δ2H. Precision over a long period of time was monitored using quality control charts. The uncertainty of the whole process, from preparation to analysis, obtained by analysing one olive oil 10 times, was on average ±0.3‰ and ±3‰ for δ13C and δ2H (1s). Finally the effects of adding carbon and hydrogen during methylation on δ13C and δ2H were evaluated. The results demonstrated that the method presented can be used for precise and accurate determination of δ13C and δ2H of FAMEs from olive oil.