Comparative Study on Correction Methods of Carbon Isotope Measured Values of the Fatty Acid Methyl Esters

Methyl esterification of fatty acids is required prior to gas chromatography-combustion-isotope ratio mass spectrometry(GC-C-IRMS) analysis,but each fatty acid methyl ester(FAME) contains a methyl group from derivatization that is not present in the original fatty acid.So it is necessary to correct the δ13C value of each FAME for the δ13C value of the extra methyl carbon.In order to obtain the accurate δ13C value of the methanol used to methylate the fatty acids,and then improve the accuracy of the corrected δ13C values of fatty acids,this paper analyse the δ13C values of the methanol as determined by three different methods: 1) The δ13C values of sodium methoxide solid from chemical reaction between the methanol and sodium are determined by off-line method;2) The δ13C values of methanol crystal as prepared by a special method that liquid nitrogen cools down quartz tube are determined by off-line method;3) The δ13C values of the methanol are determined by GC-C-IRMS with headspace sampling.Besides,the δ13C values of the fatty acids and FAMEs are also determined by off-line method,and the δ13C values of the methanol as determined by three different methods correct the δ13C values of each FAME respectively,and then this paper compare the calculated δ13C values of the fatty acids with that of the fatty acids as determined by off-line method.The results show that the average δ13C value of the methanol crystals as determined by off-line method is-50.17‰,and the standard deviation is 0.086‰(n=5).Reproducibility of these values is the best,and the calculated δ13C values that the δ13C values of the FAMEs are corrected by the δ13C values of the methanol are the most similar with the δ13C values of the fatty acids as determined by off-line method.So it is a reliable method that methanol crystals are determined by off-line method,and then its accuracy is improved that the δ13C value of the methanol correct carbon isotope measured values of FAMEs.