Abstract:

A comprehensive analytical method based on gas chromatography-mass spectrometry (GC-MS) was developed for the determination of 3-monochloropropanediol esters, 2-monochloropropanediol esters, and glycidyl esters in vegetable oils. Different parameters, such as bromination reaction temperature, bromination reaction time, derivatization reagent dosage, and derivative reaction time, were studied. The optimal conditions were as follows: 0.25 g of oil was weighed in a 10-mL glass tube, followed by the addition of 2 mL tetrahydrofuran, 25 μL of internal working standard solutions, and 30 μL of acid aqueous solution of NaBr, homogenized, and the mixture was incubated at 50 ℃ for 15 min. The reaction was stopped by the addition of 3 mL of an aqueous solution of sodium hydrogen carbonate. To separate the oil from the water phase, n-heptane was added, and the upper layer was transferred to an empty test tube and evaporated to dryness under a nitrogen stream. The residue was dissolved in 1 mL of tetrahydrofuran. 1.8 mL of sulfuric acid solution in methanol was added to the sample, and the resulting mixture was incubated at 40 ℃ for 16 h. The reaction was stopped by the addition of 0.5 mL of an aqueous solution of sodium hydrogen carbonate. After purification by n-hexane and derivatization of phenylboric acid, the derivatives were extracted with n-hexane. After nitrogen blowing, the residue was dissolved in 1 mL of n-hexane, and then filtered through a 0.45-μm membrane filter unit prior to GC-MS analysis. Temperature programming was applied at an initial temperature of 80 ℃. After 0.5 min, the temperature was raised to 180 ℃ at a rate of 20 ℃/min, held for 0.5 min, raised to 200 ℃ at a rate of 5 ℃/min for 4 min, and finally raised to 300 ℃ at a rate of 40 ℃/min for 4 min. The target compounds were separated on a DB-5MS column (30 m×0.25 mm×1 μm). Identification and quantification were achieved using an electron impact (EI) ion source in the positive ion mode with the selected ion monitoring mode. The internal standard method was used to quantify the 3-chloropropanediol esters, 2-chloropropanediol esters, and glycidyl esters. Under the optimal conditions, the correlation coefficients of the standard calibration curves were greater than 0.999 in the mass concentration range of 0.01-0.80 mg/L. The limits of detection were 25, 25, and 20 μg/kg (S/N=3), and the limits of quantification were 75, 75, 60 μg/kg (S/N=10). Four samples of different matrix types were selected for scaling experiments. At spiked levels of 250, 500, and 750 μg/kg, the recoveries of 3-chloropropanediol esters, 2-chloropropanediol esters, and glycidyl esters in spiked samples ranged from 89.0% to 98.7%, with relative standard deviations between 2.05% and 7.81% (n=6). This method was used to determine 112 commercially available vegetable oil samples, among which 84 samples were detected with 3-chloropropanediol esters, 2-chloropropanediol esters, or glycidyl esters.
The method developed in this study was remarkably different from the standard method, which are mentioned in the national standard method (GB 5009.191-2016) and industry standard method (SN/T 5220-2019), especially in the pretreatment step that involved acidic transesterification. Use of the acidic transesterification method can avoid side reactions, such as the conversion of 3-chloropropanediol, 2-chloropropanediol, and 3-bromopropanediol to free glycidol under alkaline conditions. The method developed in this study was more efficient, and the results were more accurate and reproducible. It has theoretical and practical significance for the control of 3-chloropropanediol esters, 2-chloropropanediol esters, and glycidyl esters residues in vegetable oils, establishment of detection standards, and optimization of the production process.

Key words: acidic transesterification, gas chromatography-mass spectrometry(GC-MS), monochloropropanediol esters, glycidyl esters, vegetable oils