Determination of 15 3-chloro-1,2-propanediol fatty acid esters in vegetable oils and fritters by ultra performance convergence chromatography-tandem mass spectrometry

Abstract

The presence of 3-chloro-1,2-propanediol fatty acid esters (3-MCPDE) in food and processed materials has recently become a topic of concern because of the toxicity of their metabolites. 3-MCPDE structurally similar to glyceride, which makes it difficult to separate or extract them from oils and fritters. A method based on ultra performance convergence chromatography-tandem mass spectrometry (UPC2-MS/MS) was established for the determination of 15 3-MCPDE in vegetable oils and fritters. Amino-packed columns were used to purify the samples. The analytical conditions were optimized, and the matrix effect was investigated. The sample was treated by column chromatography to remove glyceride and free fatty acids, which induce strong matrix effects. The amino-packed column was eluted with hexane and hexane-ethyl acetate (6:4, v/v). Every 1 mL of the eluent was analyzed using a UPC2 and ACQUITY QDa detector. Elution curves were drawn based on the testing data and used to determine the collection volume. The collection volumes for 3-chloro-1,2-propanediol diesters and monoesters according to the elution curves were 7-14 mL and 3-9 mL. The collected eluent was mixed and dried under nitrogen flow at a temperature of 60℃. A hexane-isopropanol (98:2, v/v, 1 mL) mixture was used to dissolve the residue. The resulting solution was separated on a Viridis HSS C18 SB column (150 mm×2.1 mm, 1.8 μm) under gradient elution. Supercritical carbon dioxide and methanol (containing 40% acetonitrile and 0.1% formic acid) were used as the mobile phases, and the flow rate was 1 mL/min. The separated compounds were analyzed by tandem MS with an electrospray ionization (ESI) source in positive and multiple reaction monitoring modes. Water (containing 97% isopropanol and 0.2% ammonia water) was used as the auxiliary pump mobile phase, and the flow rate was 0.2 mL/min. The method showed good linear relationships in the range of 0.5-100 μg/L (r2 ≥ 0.9973). The limits of detection (LODs) and limits of quantification (LOQs) were 0.01-0.68 μg/L (S/N=3) and 0.04-1.74 μg/L (S/N=10), respectively. The average recoveries (n=9) at the three spiked levels were in the range of 81.6%-98.5%. The relative standard deviations were in the range of 1.8%-6.4%. The matrix effects in the case of the oils and fritters were weak. The developed method was used to detect 44 oil samples and eight fritter samples. Meanwhile, some suspect 3-MCPDE compounds outside the scope of the investigation were analyzed based on their primary and secondary mass spectra. The detection rates of 3-MCPDE in oils and fritters were 84.1% and 87.5%, and their amounts were in the range of 0.024-4.481 mg/kg and 0.018-1.144 mg/kg, respectively. The detection rates of 3-MCPDE in rapeseed oil were higher compared to those for other kinds of oil. The method is specific, fast, simple, accurate, reliable, and environmentally friendly, in addition to being more sensitive than other methods and showing better matrix compatibility for oils. This method has been successfully used to determine the types and amounts of 3-MCPDE in vegetable oils and fritters. The research findings provided accurate data to assess the exposure risk of 3-MCPDE. The results of our experiment also provided valuable information for elucidating the formation mechanism of 3-MCPDE. The proposed method can be used to analyze waste edible oil based on large amounts of analysis data. However, this method has some limitations. The resolution ratio of the mass spectrometer used in this method is too low for the qualitative analysis of unknown compounds. The qualitative results for the suspect 3-MCPDE compounds are not particularly accurate, and a large variety of monomer standards are required for the quantitative determination of 3-MCPDE. The 3-MCPDE standards are expensive, and there is limited choice of these standards; moreover, they are difficult to synthesize. The poor ionization yield of 3-chloro-1,2-propanediol monoesters under the ESI conditions resulted in high LODs. Hence, it is necessary to develop a method for increasing the ionization of monoesters, for example, via derivatization.