Method for detecting four polycyclic aromatic hydrocarbons in vegetable oil by gas chromatography-tandem mass spectrometry

Abstract

The invention belongs to the technical field of analysis and detection, and discloses a method for detecting four polycyclic aromatic hydrocarbons in vegetable oil through gas chromatography-tandem mass spectrometry. According to the method, a saponification and degreasing method and dispersive liquid-liquid microextraction (DLLME) are organically combined, so that sample pretreatment is effectively simplified, and the advantages of high selectivity and stable isotope internal standard quantification of gas chromatography-tandem mass spectrometry (GC-MS / MS) are combined; a novel method for detecting four important polycyclic aromatic hydrocarbons (PAH4) including benzo (a) pyrene (BAP), benzo (a) anthracene (BAA), benzo (b) fluoranthene (BBF) and chrysene (CHR) in common edible vegetable oil is established. According to the method, the sample pretreatment is simple and rapid, the personnel environment is relatively friendly, and the detection limit and the accuracy can meet the requirements of monitoring PAH4 in common edible vegetable oil.

Description

Technical Field

[0001] The invention relates to the technical field of edible vegetable oil pollution analysis and detection, and in particular to a method for detecting four polycyclic aromatic hydrocarbons in vegetable oil by gas chromatography-tandem mass spectrometry. Background Art

[0002] Polycyclic aromatic hydrocarbons (PAH) are a class of highly lipophilic environmental and food contaminants. PAH contamination in edible vegetable oils poses a high food safety risk and is the main source of human intake. At present, although only the limit of benzo[a]pyrene (BAP) (10μg / kg) is stipulated in edible vegetable oils in my country, four EU-limited PAHs, namely BAP, benzo[a]anthracene (BAA), benzo[b]fluoranthene (BBF) and chrysene (CHR), have been listed as important food safety hazards and included in the food safety risk monitoring catalog. Therefore, the study of simple and effective PAH4 detection methods is of practical significance for improving the efficiency of food safety monitoring.

[0003] At present, high performance liquid chromatography-fluorescence detection and gas chromatography-mass spectrometry are the most widely used methods for detecting PAH in food; GC-MS / MS, due to its higher selectivity, provides favorable conditions for further simplifying sample pretreatment. As far as edible vegetable oil samples are concerned, removing the interference of oil and fat is the key to detecting PAH. Usually, solvent extraction is required, and then purification is carried out using C18 solid phase extraction columns, neutral alumina columns, molecular imprinting columns, dispersed solid phase extraction methods, etc. The method is relatively cumbersome and the cost is relatively high. Most of the existing technologies use dispersed liquid-liquid extraction methods to extract polycyclic aromatic hydrocarbons from aqueous solutions. The main body of edible oil is oil and polycyclic aromatic hydrocarbons are fat-soluble. Therefore, it is very difficult to extract polycyclic aromatic hydrocarbons from oil. The method process is long and the cost is high. Summary of the invention

[0004] The present invention provides a method for detecting four polycyclic aromatic hydrocarbons in vegetable oil by gas chromatography-tandem mass spectrometry, which effectively solves the technical problems in the prior art that it is difficult to extract fat-soluble polycyclic aromatic hydrocarbons from vegetable oil, and the existing technical problems that the pretreatment of PAH4 detection samples in edible vegetable oil is complex, time-consuming, costly, and requires a large amount of toxic reagents.

[0005] The present invention provides a method for detecting four polycyclic aromatic hydrocarbons in vegetable oil by gas chromatography-tandem mass spectrometry, comprising the following steps:

[0006] Weigh the vegetable oil sample, add the dichloromethane solution of BAP-d12 and the dichloromethane solution of CHR-d12 as internal standard substances, add the methanol solution of potassium hydroxide as a saponification reagent, vortex, perform saponification treatment to obtain a saponified solution, dilute with water, add sodium bicarbonate, dissolve, inject an extractant, perform ultrasonic oscillation, perform dispersed liquid-liquid microextraction, and centrifuge to obtain an organic extraction phase;

[0007] The organic extract phase is dehydrated and filtered through a filter membrane containing a dehydrating agent to obtain a test liquid; the test liquid is detected by gas chromatography-tandem mass spectrometry.

[0008] As a preferred embodiment, the concentrations of the dichloromethane solution of BAP-d12 and the dichloromethane solution of CHR-d12 are both 1 mg / L, the concentration of the methanol solution of potassium hydroxide is 5 mol / L, and the dosage ratio of the vegetable oil, internal standard substance and saponified solution is 0.5 g:20-25 μL:3-5 mL.

[0009] As a preferred embodiment, the saponification treatment is carried out at a temperature of 80-85° C. and for a time of 0.5 h.

[0010] As a preferred embodiment, the volume ratio of the water to the saponified liquid is 5-6:1, and the mass ratio of the sodium bicarbonate to water is 1:8-10.

[0011] As a preferred embodiment, the extractant is dichloromethane, which is injected vigorously using a syringe, and the added volume of the extractant is 1 / 25 to 1 / 20 of the total volume of the saponified liquid after dilution with water.

[0012] As a preferred implementation manner, the ultrasonic power is 500 W, and the ultrasonic time is 5 min; the centrifugal speed is 4000 r / min, and the centrifugal time is 5 min.

[0013] As a preferred embodiment, the dehydration and filtration are carried out through a filter membrane filled with a dehydrating agent, the dehydrating agent is anhydrous sodium sulfate, and the ratio of the organic extract phase to the dehydrating agent is 200 μL: 0.2-0.3 g.

[0014] As a preferred embodiment, the conditions of the gas chromatography are specifically as follows: the chromatographic column is a HP-5MS chromatographic column with a specification of 30m×0.25mm×0.25μm; the injection port temperature is 280°C, non-split injection, and the injection volume is 1μL; the carrier gas is helium with a purity of ≥99.999% and a flow rate of 1.0mL / min; the heating program is: 50°C for 1min, increased to 220°C at 20°C / min, and then increased to 320°C at 5°C / min and maintained for 2min.

[0015] As a preferred embodiment, the conditions of the tandem mass spectrometry are specifically as follows: ionization mode: EI; ionization energy 70 eV; ion source temperature: 230°C, quadrupole temperature: 150°C; interface temperature: 280°C; solvent delay: 3 min; and detection in multiple reaction monitoring mode.

[0016] As a preferred embodiment, the process from weighing the vegetable oil sample to obtaining the organic extraction phase is carried out in a 50 mL plastic centrifuge tube with a screw cap.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] The method for detecting four polycyclic aromatic hydrocarbons in vegetable oil by gas chromatography-tandem mass spectrometry provided by the present invention has simple sample pretreatment, using dichloromethane solution of BAP-d12 and dichloromethane solution of CHR-d12 as internal standard substances, using methanol solution of potassium hydroxide as saponification reagent, vortexing, saponification treatment, obtaining saponified liquid, diluting with water, adding sodium bicarbonate, dissolving, injecting extractant, ultrasonic oscillation, dispersing liquid-liquid microextraction, centrifugation, obtaining organic extraction phase; dehydrating and filtering the organic extraction phase through a filter membrane equipped with a dehydrating agent to obtain a liquid to be tested; the liquid to be tested is detected by gas chromatography-tandem mass spectrometry. The present invention is completed in the same centrifuge tube from weighing the sample to obtaining the organic extraction phase, with low cost, small amount of toxic reagents, relatively more green and environmentally friendly, lower detection limit and quantitative limit, good accuracy and precision, reliable quantification, and can significantly improve the detection efficiency of PAH4. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a diagram showing the influence of different addition amounts of sodium bicarbonate on the response intensity of the present invention.

[0020] Figure 2 This is a diagram showing the effects of different oscillation modes on dispersed liquid phase microextraction (DLLME) of the present invention.

[0021] Figure 3 This is a diagram showing the effect of the amount of the extractant used on the extraction effect of the present invention.

[0022] Figure 4 It is the MRM chromatogram of four polycyclic aromatic hydrocarbons (PAH4) and two internal standard substances in the samples of the present invention. DETAILED DESCRIPTION

[0023] In order to enable those skilled in the art to better understand the technical solution of the present invention and implement it, the present invention is further described below in conjunction with specific examples, but the examples are not intended to limit the present invention. The following test methods and detection methods, unless otherwise specified, are conventional methods; the reagents and raw materials, unless otherwise specified, are commercially available.

[0024] In view of what is mentioned in the background technology of the present invention: First, since removing the oil interference of vegetable oil is the key to detecting PAHs, the solvent extraction is adopted, and then purification is carried out using C18 solid phase extraction column, neutral alumina column, molecular imprinting column, dispersed solid phase extraction method, etc. The method is relatively cumbersome, but the cost is also relatively high; second, most of the existing technologies use dispersed liquid-liquid extraction methods to extract PAHs from aqueous solutions, while edible oil is mainly oil and PAHs are fat-soluble, so it is very difficult to extract PAHs from oil, the method process is long, and the cost is high. In view of the above technical problems, the present invention provides a method for detecting four PAHs in vegetable oil by gas chromatography-tandem mass spectrometry.

[0025] The technical effects of the present invention are described in detail below in conjunction with embodiments.

[0026] Example 1

[0027] A method for detecting four polycyclic aromatic hydrocarbons in vegetable oil by gas chromatography-tandem mass spectrometry comprises the following steps:

[0028] S1: Sample pretreatment: weigh 0.5g edible oil into a 50mL centrifuge tube, add 25μL of 1mg / L CHR-d12, BAP-d12 internal standard solution, let stand for 5min, add 5.0mL of 5moL / L potassium hydroxide methanol solution, vortex at 1500rpm for 30 seconds, saponify in an oven at 85℃ for 0.5h, add 30mL of water while hot to dilute, and then add 3.0g of sodium bicarbonate to fully dissolve; use a syringe to vigorously inject 1.5mL of dichloromethane, ultrasonically oscillate for 5min, centrifuge at 4000rpm for 5min, and discard most of the upper aqueous phase; use a pipette to take out 200μL of the lower organic phase, filter and collect the test solution in a sample bottle with a 100μL inner insert for testing through an organic needle filter pre-installed with 0.2g of anhydrous sodium sulfate in the column tube.

[0029] S2: Sample determination: The sample solution described in S1 is determined by gas chromatography-tandem mass spectrometry.

[0030] The analysis conditions of the gas chromatography are:

[0031] Chromatographic column: HP-5MS (30m×0.25mm×0.25μm) chromatographic column, non-split injection, injection volume 1μL; carrier gas is He, constant flow mode, flow rate is 1mL / min; temperature program: 50℃ for 1min, increase to 220℃ at 20℃ / min, and then increase to 320℃ at 5℃ / min and maintain for 2min.

[0032] The tandem mass spectrometry analysis conditions are:

[0033] Electron impact electron source (EI); ionization energy: 70 eV; MRM detection mode, parameters see Table 1.

[0034] Table 1 Gas chromatography-tandem mass spectrometry detection parameters for PAHs and internal standard substances

[0035]

[0036]

[0037] Note: * is the quantitative ion

[0038] Example 2

[0039] A method for detecting four polycyclic aromatic hydrocarbons in vegetable oil by gas chromatography-tandem mass spectrometry comprises the following steps:

[0040] S1: Sample pretreatment: weigh 0.5g edible oil into a 50mL centrifuge tube, add 20μL of 1mg / L CHR-d12, BAP-d12 internal standard solution, let stand for 5min, add 3.0mL of 5moL / L potassium hydroxide methanol solution, vortex at 1500rpm for 30 seconds, saponify in an 80℃ oven for 0.5h, add 30mL of water while hot to dilute, and then add 3.0g of sodium bicarbonate to fully dissolve; use a syringe to vigorously inject 1.5mL of dichloromethane, ultrasonically oscillate for 5min, centrifuge at 4000rpm for 5min, and discard most of the upper aqueous phase; use a pipette to take out 200μL of the lower organic phase, filter and collect the test solution through an organic needle filter pre-installed with 0.3g of anhydrous sodium sulfate in the column tube, and put it in a sample injection bottle with a 100μL inner insert for testing.

[0041] S2: Sample determination: The sample solution described in S1 is determined by gas chromatography-tandem mass spectrometry.

[0042] The analysis conditions of the gas chromatography are:

[0043] Chromatographic column: HP-5MS (30m×0.25mm×0.25μm) chromatographic column, non-split injection, injection volume 1μL; carrier gas is He, constant flow mode, flow rate is 1mL / min; temperature program: 50℃ for 1min, increase to 220℃ at 20℃ / min, and then increase to 320℃ at 5℃ / min and maintain for 2min.

[0044] The tandem mass spectrometry analysis conditions are:

[0045] Electron impact electron source (EI); ionization energy: 70 eV; MRM detection mode, parameters see Table 2.

[0046] Table 2 Gas chromatography-tandem mass spectrometry detection parameters for PAHs and internal standard substances

[0047]

[0048]

[0049] Note: * is the quantitative ion

[0050] Example 3

[0051] A method for detecting four polycyclic aromatic hydrocarbons in vegetable oil by gas chromatography-tandem mass spectrometry comprises the following steps:

[0052] S1: Sample pretreatment: weigh 0.5g edible oil into a 50mL centrifuge tube, add 22μL of 1mg / L CHR-d12, BAP-d12 internal standard solution, let stand for 5min, add 4.0mL of 5moL / L potassium hydroxide methanol solution, vortex at 1500rpm for 30 seconds, saponify in an 82℃ oven for 0.5h, add 30mL of water while hot to dilute, and then add 3.0g of sodium bicarbonate to fully dissolve; use a syringe to vigorously inject 1.5mL of dichloromethane, ultrasonically oscillate for 5min, centrifuge at 4000rpm for 5min, and discard most of the upper aqueous phase; use a pipette to take out 200μL of the lower organic phase, filter and collect the test solution through an organic needle filter pre-installed with 0.25g of anhydrous sodium sulfate in the column tube, and put it in a sample injection bottle with a 100μL inner insert for testing.

[0053] S2: Sample determination: The sample solution described in S1 is determined by gas chromatography-tandem mass spectrometry.

[0054] The analysis conditions of the gas chromatography are:

[0055] Chromatographic column: HP-5MS (30m×0.25mm×0.25μm) chromatographic column, non-split injection, injection volume 1μL; carrier gas is He, constant flow mode, flow rate is 1mL / min; temperature program: 50℃ for 1min, increase to 220℃ at 20℃ / min, and then increase to 320℃ at 5℃ / min and maintain for 2min.

[0056] The tandem mass spectrometry analysis conditions are:

[0057] Electron impact electron source (EI); ionization energy: 70 eV; MRM detection, parameters see Table 3.

[0058] Table 3 Gas chromatography-tandem mass spectrometry detection parameters for PAHs and internal standard substances

[0059]

[0060]

[0061] Note: * is the quantitative ion

[0062] Example 4

[0063] A method for detecting four polycyclic aromatic hydrocarbons in vegetable oil by gas chromatography-tandem mass spectrometry comprises the following steps:

[0064] S1: Sample pretreatment: weigh 0.5g edible oil into a 50mL centrifuge tube, add 25μL of 1mg / L CHR-d12, BAP-d12 internal standard solution, let stand for 5min, add 3.0mL of 5moL / L potassium hydroxide methanol solution, vortex at 1500rpm for 30 seconds, saponify in an 80℃ oven for 0.5h, add 30mL of water while hot to dilute, and then add 3.0g of sodium bicarbonate to fully dissolve; use a syringe to vigorously inject 1.5mL of dichloromethane, ultrasonically oscillate for 5min, centrifuge at 4000rpm for 5min, and discard most of the upper aqueous phase; use a pipette to take out 200μL of the lower organic phase, filter and collect the test solution through an organic needle filter pre-installed with 0.2g of anhydrous sodium sulfate in the column tube, and put it in a sample injection bottle with a 100μL inner insert for testing.

[0065] S2: Sample determination: The sample solution described in S1 is determined by gas chromatography-tandem mass spectrometry.

[0066] The analysis conditions of the gas chromatography are:

[0067] Chromatographic column: HP-5MS (30m×0.25mm×0.25μm) chromatographic column, non-split injection, injection volume 1μL; carrier gas is He, constant flow mode, flow rate is 1mL / min; temperature program: 50℃ for 1min, increase to 220℃ at 20℃ / min, and then increase to 320℃ at 5℃ / min and maintain for 2min.

[0068] The tandem mass spectrometry analysis conditions are:

[0069] Electron impact electron source (EI); ionization energy: 70 eV; MRM detection mode, parameters see Table 4.

[0070] Table 4 Gas chromatography-tandem mass spectrometry detection parameters for PAHs and internal standard substances

[0071]

[0072]

[0073] Note: * is the quantitative ion

[0074] The edible vegetable oil samples were tested by using the method for detecting four polycyclic aromatic hydrocarbons in vegetable oil by gas chromatography-tandem mass spectrometry provided in Examples 1 to 4 of the present invention. The sample pretreatment and test results are as follows:

[0075] Selection of saponification conditions

[0076] Potassium hydroxide or sodium hydroxide in methanol water or ethanol water solution can be used for the saponification of edible vegetable oil. The present invention compares the saponification of 5 mol / L sodium hydroxide and potassium hydroxide in methanol water (9+1) and ethanol water (9+1). The experiment found that during the saponification process, the methanol solution and ethanol solution of sodium hydroxide are easy to form lumps with the vegetable oil, and the dispersion effect is poor, while the corresponding potassium hydroxide solution is uniformly dispersed and the effect is better; methanol solution and ethanol solution have no effect on saponification, but the use of ethanol solution will make the subsequent DLLME stratification difficult. Therefore, this experiment uses a 5 mol / L methanol solution of potassium hydroxide. Considering the tolerance of the centrifuge tube, the saponification temperature is 85°C, and the sample solution is converted into a uniform solution when the saponification time is 0.5h, and the saponification reaction is completed.

[0077] Optimization of Dispersive Liquid Phase Microextraction (DLLME) Conditions

[0078] Salt selection and dosage

[0079] DLLME usually uses NaCl and Na2SO4 as auxiliary salts to improve the extraction effect of DLLME. After the vegetable oil sample of the present invention is saponified, a certain amount of sodium bicarbonate is added. The method is relatively convenient, which can neutralize the strongly alkaline sample solution to form HCO3 - -CO3 2- The buffer system can keep the pH stable and play a role in salting out, thus improving the extraction effect of DLLME. The present invention compares the effects of different sodium bicarbonate dosages on polycyclic aromatic hydrocarbons, such as Figure 1 As shown, the results show that when the amount of sodium bicarbonate is 3g, the stability is the best and the extraction effect of DLLME is the best.

[0080] Effects of different oscillation modes on DLLME

[0081] During DLLME, the dispersed droplets tend to aggregate quickly. To delay the aggregation of the droplets, the experiment compared the effects of three treatment methods on the target response value: no treatment, manual shaking for 5 minutes, and ultrasonic shaking for 5 minutes after injecting dichloromethane. Figure 2 The results showed that ultrasound was more effective.

[0082] The influence of centrifugal speed

[0083] After DLLME is completed, centrifugation is performed, and the sample solution is divided into three layers, the bottom layer is dichloromethane, the middle layer is a saponification product layer, and the upper layer is an aqueous solution. However, as the centrifugal speed increases, the dichloromethane layer and the saponification product layer merge, resulting in a decrease in the volume of dichloromethane precipitation, which is not conducive to the removal of dichloromethane. The present invention determines that the optimal centrifugal speed is 4000rpm.

[0084] Selection of extraction agent and dosage

[0085] Commonly used extractants for DLLME include dichloromethane, chloroform, and carbon tetrachloride, among which dichloromethane is relatively less toxic and has a relatively good extraction effect on PAH4. The present invention compares the effect of the volume of the extractant on the experimental results. When the volume of the extractant is 1.0mL or less, the precipitate volume after centrifugation is too small and difficult to collect. When the volume of the extractant is 1.5-2.5mL, the precipitate volume can be obtained from 250μL-1000μL. However, as the volume of the extractant increases, the extraction effect is not significantly improved. On the contrary, the concentration of the target is reduced due to the dilution of the target by the increase in the extraction volume. Figure 3 To ensure the detection capability of the method, the volume of the extractant was determined to be 1.5 mL in this experiment.

[0086] Effects of co-extractants on chromatographic separation and mass spectrometry detection

[0087] Since DLLME is essentially a liquid-liquid extraction, co-extractants are inevitable. Due to the high selectivity of GC-MS / MS, under the chromatographic mass spectrometry detection conditions of this experiment, except for the separation degree of 1.2 between BBF and its isomer benzo(K)fluoranthene (BKF), no interference from co-extractants was observed for the target compounds. The MRM chromatogram of PAH4 added at a concentration of 4 ng / kg and an internal standard concentration of 50 ng / kg is shown in Figure 3 .

[0088] Methodological validation

[0089] Linear equation, detection limit, quantitation limit

[0090] The PAH concentration range was 1-100 ng / mL, and all targets showed good linearity. 2 The detection limit and quantification limit of the method calculated by 3 times and 10 times the signal-to-noise ratio were 0.09-0.20 μg / kg and 0.30-0.66 μg / kg, respectively, as shown in Table 5. The detection limit and quantification limit of this method are lower than the current national standard detection method for polycyclic aromatic hydrocarbons in edible oils GB 5009.27-2016 and GB 5009.265-2016.

[0091] Table 5 Linear equation, correlation coefficient, detection limit and quantification limit

[0092]

[0093] Note: y is the peak area ratio of the analyte to the corresponding internal standard; x is the mass concentration ratio of the analyte to the corresponding internal standard.

[0094] Recovery and precision

[0095] Add 4, 10, and 50 μg / kg of PAHs to the control vegetable oil (PAH4 background is high, and the quantitative limit concentration level sample cannot be obtained by spiking), and repeat 6 times for each level, mix well, and let it stand for 5 minutes to make the components to be tested evenly distributed in the sample. Treat the sample according to the method in Section 1.3 and test; repeat the above experiment again one day after another. The recovery rate and relative standard deviation are shown in Table 6. The results show that the yield is 94.09% to 120.09%, and the total relative standard deviation is 1.7% to 18.29%, which are all within the range specified in GB-T35655-2017.

[0096] Table 6 Recovery and relative standard deviation (RSD) of spiked PAH4 in edible vegetable oils (n=6)

[0097]

[0098] Quality control sample testing

[0099] To further verify the accuracy of the method, this method was applied to test the standard edible oil quality control sample (certificate number GY-QCM-07-1002A) with a BAP characteristic value of 11.5μg / kg (characteristic value range of 9.5-13.5μg / kg). The test result was 11.76±0.52μg / kg, which was consistent with the standard value.

[0100] Actual sample testing

[0101] This method was used to test 29 edible oil samples of different types and brands on the market, including 5 sunflower oils, 3 soybean oils, 6 peanut oils, 2 sesame oils, 3 corn oils, 3 blended oils, 2 rice oils, 4 olive oils, and 1 rapeseed oil; each sample was tested twice. The test results are shown in Table 7, where the BAP content ranged from 3.07 to 11.32 μg / kg; the PAH4 content ranged from 13.10 to 64.98 μg / kg.

[0102] Table 7 Detection results of 4 PAH in different types of edible oils (μg / kg)

[0103]

[0104]

[0105] In summary, the present invention has established a new method for GC-MS / MS detection of PAH4 in edible oils by combining saponification treatment with DLLME for sample pretreatment. The method is simple in sample pretreatment, low in cost, relatively green and environmentally friendly, has sufficiently low limits of detection and quantification, is accurate and reliable, significantly improves detection efficiency, and can meet the needs of PAH4 risk monitoring in common edible vegetable oils.

Claims

1. A method for detecting four polycyclic aromatic hydrocarbons in vegetable oil by gas chromatography-tandem mass spectrometry, characterized in that: The following steps are involved: Weigh the vegetable oil sample, add the dichloromethane solution of BAP-d12 and the dichloromethane solution of CHR-d12 as internal standard substances, add the methanol solution of potassium hydroxide as a saponification reagent, vortex, perform saponification treatment to obtain a saponified solution, dilute with water, add sodium bicarbonate, dissolve, inject an extractant, perform ultrasonic oscillation, perform dispersed liquid-liquid microextraction, and centrifuge to obtain an organic extraction phase; Dehydrating and filtering the organic extract phase to obtain a test solution; The liquid to be tested is detected by gas chromatography-tandem mass spectrometry.

2. The method for detecting four polycyclic aromatic hydrocarbons in vegetable oil by gas chromatography-tandem mass spectrometry according to claim 1, characterized in that: The concentrations of the dichloromethane solutions of BAP-d12 and CHR-d12 are both 1 mg / L, the concentration of the methanol solution of potassium hydroxide is 5 mol / L, and the dosage ratio of the vegetable oil, internal standard substance and saponification solution is 0.5 g: 20-25 μL: 3-5 mL.

3. The method for detecting four polycyclic aromatic hydrocarbons in vegetable oil by gas chromatography-tandem mass spectrometry according to claim 1, characterized in that: The temperature of the saponification treatment is 80-85° C. and the time is 0.5 h.

4. The method for detecting four polycyclic aromatic hydrocarbons in vegetable oil by gas chromatography-tandem mass spectrometry according to claim 1, characterized in that: The volume ratio of the water to the saponified liquid is 5-6:1, and the mass ratio of the sodium bicarbonate to water is 1:8-10.

5. The method for detecting four polycyclic aromatic hydrocarbons in vegetable oil by gas chromatography-tandem mass spectrometry according to claim 1, characterized in that: The extractant is dichloromethane, and the added volume of the extractant is 1 / 25 to 1 / 20 of the total volume of the saponified liquid after dilution with water.

6. The method for detecting four polycyclic aromatic hydrocarbons in vegetable oil by gas chromatography-tandem mass spectrometry according to claim 1, characterized in that: The dehydration and filtration are performed through a filter membrane filled with a dehydrating agent, the dehydrating agent is anhydrous sodium sulfate, and the ratio of the organic extract phase to the dehydrating agent is 200 μL: 0.2-0.3 g.

7. The method for detecting four polycyclic aromatic hydrocarbons in vegetable oil by gas chromatography-tandem mass spectrometry according to claim 1, characterized in that: The gas chromatography conditions are specifically as follows: the chromatographic column is a HP-5MS chromatographic column with a specification of 30m×0.25mm×0.25μm; the injection port temperature is 280°C, non-split injection, and the injection volume is 1μL; the carrier gas is helium with a purity of ≥99.999% and a flow rate of 1.0mL / min; the heating program is: 50°C for 1min, increased to 220°C at 20°C / min, and then increased to 320°C at 5°C / min and maintained for 2min.

8. The method for detecting four polycyclic aromatic hydrocarbons in vegetable oil by gas chromatography-tandem mass spectrometry according to claim 1, characterized in that: The conditions of the tandem mass spectrometry are specifically as follows: ionization mode: EI; ionization energy: 70 eV; ion source temperature: 230° C., quadrupole temperature: 150° C.; interface temperature: 280° C.; solvent delay: 3 min; and detection in multiple reaction monitoring mode.

9. The method for detecting four polycyclic aromatic hydrocarbons in vegetable oil by gas chromatography-tandem mass spectrometry according to claim 1, characterized in that: The ultrasonic power is 500W, and the ultrasonic time is 5 minutes; the centrifugal speed is 4000r / min, and the centrifugal time is 5 minutes.

10. The method for detecting four polycyclic aromatic hydrocarbons in vegetable oil by gas chromatography-tandem mass spectrometry according to claim 1, characterized in that: The process from weighing the vegetable oil sample to obtaining the organic extract phase was carried out in a plastic centrifuge tube with a screw cap.

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