Convenient Determination of Trace Free Fatty Acids in Edible, Expired and Gutter Oils through Catalytic Esterification with Trimethylsilyl Diazomethane and Gas Chromatography–Mass Spectrometry Analysis

The content of free fatty acids (FAs) is a critical safety indicator for edible oils. However, the current GC–MS analysis of FAs is not satisfactory due to their inefficient derivatization under mild conditions. In this study, we achieved efficient methylation of fatty acids within 10 min by derivatization with trimethylsilyldiazomethane (TMSD) catalyzed trimethyl chlorosilane (TMSC). Then, an efficient methylation of FAs was achieved within 10 min through derivatization with trimethylsilyldiazomethane under the catalysis of trimethylchlorosilane, and a convenient method for the determination of 33 free fatty acids in oil samples has been developed by methylation in combination with extraction with methanol and GC–MS analysis. The method exhibited good reproducibility, sensitivity, and recovery. The determined content range of several free FAs (e.g. palmitoleic acid, α-linolenic acid, linoleic acid, elaidic acid, and arachidic acid) in edible oils was significantly lower than that in expired edible oils (or gutter oil). Furthermore, the content of γ-linolenic acid, which is prone to oxidation, was below the LOQ (0.40 ng/g) in gutter oil, suggesting it as an indicator of severe oxidative degradation. This method was characterized by its ease of operation and good reproducibility, presenting significant potential for oil quality control.