Determination of the Stable Carbon Isotopic Composition of Glycerol in Vegetable Oils

A method of gas chromatography-combustion-stable isotope ratio mass spectrometry (GC-C-IRMS) with ester exchange technology was established for the determination stable carbon isotope composition (δ¹³C) of glycerol in vegetable oils. After the sample was dissolved in isooctane, triglyceride produces glycerol and fatty acid methyl ester under the existence of KOH methanol solution. δ¹³C of glycerol was determined by GC-C-IRMS after the removal of fatty acid methyl ester. Through the optimization of reaction time, the best time is 4 h after addition of KOH methanol solution. Results show that the standard deviation is 0.18‰, and the ester exchange process does not lead to glycerol carbon isotope fractionation. By analysing the differences of δ¹³C value of glycerol in corn oil and soybean oil, δ¹³C values are -14.71‰ and -31.30‰, respectively, δ¹³C value of glycerol has a significant negative correlation (R²=0.98) with content of soybean oil in corn germ oil. The method has an advantage of easy and rapid operation, and which is applicable for δ¹³C analysis of adulteration of vegetable oil. This study provides a theoretical basis and technical support for the identification of varieties of vegetable oils.