Quantitative determination of epoxy stearic acids derived from oxidized frying oil based on solid-phase extraction and gas chromatography

An improved preparation method based on solid phase extraction (SPE) for determination of epoxy fatty acids derived from oleic acid in frying oils was developed. A three-step separation of fatty acid methyl esters (FAMEs) by SPE on a silica gel column was proposed, and quantification by gas chromatograph with a fiame ionization detector (GC-FID) was performed. The limits of detection (LODs) of trans- and cis-epoxy stearic acid (ESA) were 6.32 and 13.09 μg/mL, respectively. And the limits of quantification (LOQs) ranged from 20.87 to 43.19 μg/mL with recoveries ranging from 89.32% to 107.08%. Good reproducibility was observed, with an intraday relative standard deviations of 0.93-2.35%, and an interday relative standard deviations of 2.44-11.68%. Results showed total levels of ESAs in different oil samples were in the range of 235.65-5916.05 mg/kg, and the contents of ESAs increased with the increase of frying time. Thus, combining the improved sample preparation method for ESAs analysis with the separation on a fused-silica capillary column using GC is highly useful for determination of ESAs in frying oils.