脂肪酸甲酯碳同位素测定值的校正方法对比研究

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

气相色谱-燃烧-同位素比值质谱(GC-C-IRMS)分析脂肪酸单体化合物碳同位素时,首先对脂肪酸甲酯化,但是测得的δ13 C值为脂肪酸甲酯的δ13 C值,因此需要对其进行数学校正,才能得到脂肪酸δ13 C值。为了得到准确的甲酯化使用的甲醇δ13 C值,提高校正后的脂肪酸δ13 C值的精准度,本研究利用3种不同方法测定甲醇δ13 C值:1)甲醇与钠反应制备甲醇钠固体,离线测定甲醇钠固体δ13 C值;2)液氮冷却石英管注样法制备甲醇结晶体,离线测定甲醇结晶体δ13 C值;3)GC-C-IRMS顶空进样测定甲醇δ13 C值。并通过离线测定得到脂肪酸单体化合物及其甲酯化后的脂肪酸甲酯δ13 C值。然后用此3种不同方法测定的甲醇δ13 C值分别对脂肪酸甲酯δ13 C值进行数学校正,将计算得到的脂肪酸δ13 C值与脂肪酸单体化合物δ13 C值进行误差分析。研究结果表明:离线测定的甲醇结晶体δ13 C值为-50.17‰,标准偏差为0.086‰(n=5),重现性优于其他两种方法,用其校正后的脂肪酸δ13 C值与离线测定的脂肪酸δ13 C值误差最小。因此,离线测定甲醇结晶体δ13 C值是一种相对可靠的测定甲醇δ13 C值的方法,可提高校正脂肪酸甲酯碳同位素测定值的精准度。

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 δ^13 Cvalue of each FAME for the δ^13 Cvalue of the extra methyl carbon.In order to obtain the accurate δ^13 Cvalue of the methanol used to methylate the fatty acids,and then improve the accuracy of the corrected δ^13 Cvalues of fatty acids,this paper analyse the δ^13 Cvalues of the methanol as determined by three different methods： 1） The δ^13 Cvalues of sodium methoxide solid from chemical reaction between the methanol and sodium are determined by off-line method;2） The δ^13 Cvalues of methanol crystal as prepared by a special method that liquid nitrogen cools down quartz tube are determined by off-line method;3） The δ^13 Cvalues of the methanol are determined by GC-C-IRMS with headspace sampling.Besides,the δ^13 Cvalues of the fatty acids and FAMEs are also determined by off-line method,and the δ^13 Cvalues of the methanol as determined by three different methods correct the δ^13 Cvalues of each FAME respectively,and then this paper compare the calculated δ^13 Cvalues of the fatty acids with that of the fatty acids as determined by off-line method.The results show that the average δ^13 Cvalue 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 δ^13 Cvalues that the δ^13 Cvalues of the FAMEs are corrected by the δ^13 Cvalues of the methanol are the most similar with the δ^13 Cvalues 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 δ^13 Cvalue of the methanol correct carbon isotope measured values of FAMEs.