脂肪酸甲酯碳同位素测定值的校正方法对比研究

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δ¹³C值为脂肪酸甲酯的δ¹³C值，因此需要对其进行数学校正，才能得到脂肪酸δ¹³C值。为了得到准确的甲酯化使用的甲醇δ¹³C值，提高校正后的脂肪酸δ¹³C值的精准度，本研究利用3种不同方法测定甲醇δ¹³C值：1）甲醇与钠反应制备甲醇钠固体，离线测定甲醇钠固体δ¹³C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δ¹³C值；3）GC-C-IRMS顶空进样测定甲醇δ¹³C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δ¹³C值。然后用此3种不同方法测定的甲醇δ¹³C值分别对脂肪酸甲酯δ¹³C值进行数学校正，将计算得到的脂肪酸δ¹³C值与脂肪酸单体化合物δ¹³C值进行误差分析。研究结果表明：离线测定的甲醇结晶体δ¹³C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δ¹³C值与离线测定的脂肪酸δ¹³C值误差最小。因此，离线测定甲醇结晶体δ¹³C值是一种相对可靠的测定甲醇δ¹³C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。

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.