建立了一种气相色谱-质谱同时测定植物油中3-氯丙二醇酯、2-氯丙二醇酯和缩水甘油酯的方法。称取0.25 g样品,依次加入内标工作液、四氢呋喃和酸性溴化钠溶液,50 ℃水浴反应15 min,加入6 g/L碳酸氢钠溶液终止反应,使用正己烷提取,上层液经氮气吹干后用四氢呋喃溶解。随后加入1.8%(v/v)硫酸-甲醇溶液于40 ℃恒温水浴中反应16 h,加入饱和碳酸氢钠溶液终止反应。样液再经过净化、衍生、提取、氮吹后,以1 mL正己烷定容,过膜,进样测定。采用毛细管气相色谱柱DB-5MS(30 m×0.25 mm×1 μm)分离,程序升温,电子轰击电离(EI)源检测,在选择离子扫描模式下,以保留时间和特征离子信息进行定性分析,内标法定量。结果表明,3-氯丙二醇酯、2-氯丙二醇酯和缩水甘油酯在0.01~0.80 mg/L范围内线性关系良好,相关系数( r 2)均在0.999以上,方法的检出限( S/N=3)和定量限( S/N=10)分别为25、25、20 μg/kg和75、75、60 μg/kg。选取4种不同基质类型的样品,在低、中、高3个不同添加水平下的平均回收率为89.0%~98.7%,相对标准偏差(RSD)在2.05%~7.81%之间。采用该方法测定了市售112份植物油样本,其中有84份样本检出3-氯丙二醇酯、2-氯丙二醇酯和缩水甘油酯。与已建立的国家标准方法(GB 5009.191-2016)和行业标准方法(SN/T 5220-2019)相比,该方法所采用的酸性酯交换法可避免副反应(碱性条件下3-氯丙二醇、2-氯丙二醇及3-溴丙二醇向游离态缩水甘油转化)的发生,同时该方法也弥补了国家标准和行业标准无法对3-氯丙二醇酯、2-氯丙二醇酯和缩水甘油酯同时进行测定的缺失。该方法实验操作更高效,结果更准确、重复性更好,对我国植物油中3-氯丙二醇酯、2-氯丙二醇酯和缩水甘油酯污染水平的控制、检测标准的制定和生产工艺的优化具有一定的理论和现实意义。
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.
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