Screening method for unknown volatile substances in food and edible agricultural products

Abstract

The invention discloses a method for screening unknown volatile substances in food and edible agricultural products. The method comprises the following steps: pre-treating a to-be-detected sample to obtain a test sample; a gas chromatograph is connected with a comprehensive two-dimensional time-of-flight mass spectrometer in series to carry out data acquisition on the sample to be tested, and a total ion chromatogram is obtained; and performing spectrogram analysis on the total ion chromatogram to realize screening of unknown volatile substances in the to-be-detected sample. The method is simple, rapid and reliable, is suitable for screening unknown volatile substances in large-batch foods and edible agricultural products, and provides a scientific basis for quality control of the foods and the edible agricultural products and simultaneous rapid analysis of volatile exogenous hazardous substances.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of food analysis, and particularly relates to a screening method for unknown substances in food and edible agricultural products. BACKGROUND

[0002] Food is one of the basic needs in people's life, and food safety issues have always been concerned. Food contamination and the resulting foodborne diseases are major public health issues of global concern. Some chemical hazards can affect public health at trace and ultra-trace levels, and foodborne disease burden and traceability early warning are the scientific basis for evaluating the effectiveness of food safety monitoring measures. Strengthening food hazard exposure identification and health risk control is the top priority of food safety. Food safety risk monitoring is an important support for ensuring food safety.

[0003] Food safety hazards refer to factors or elements that potentially damage or endanger food safety and quality, including biological, chemical and physical hazards. In the food production process, food safety hazards mainly come from two aspects: one is that the food itself carries or the substance of the food itself has undergone spoilage change; the other is that the food has been contaminated by the outside world. For food production enterprises, no matter which hazard comes from which aspect, if not controlled, it will cause product quality unqualified, leading to economic losses, or even food safety accidents, endangering human health and even life safety, so food enterprises must control food safety hazards.

[0004] Food hazard analysis and identification is the basis of food safety hazard control. Detecting food hazards is an important means to ensure food safety, and rapid, efficient, economical and sensitive food hazard detection methods have become a research hotspot in the field of food. In the prior art, known risk factors are analyzed and researched according to national standards, and there is a lack of proactive response and prevention of potential risk factors of hazards. SUMMARY

[0005] The present application is to avoid the shortcomings of the prior art, and provides a screening method for unknown volatile substances in food and edible agricultural products. The application aims to provide a simple, rapid, high-throughput, low-cost and accurate qualitative method for screening unknown volatile substances in large quantities of food and edible agricultural products, and to provide a scientific method for food and edible agricultural product quality control and simultaneous rapid analysis of volatile exogenous hazards.

[0006] The present application is to avoid the shortcomings of the prior art, and provides a screening method for unknown volatile substances in food and edible agricultural products. The application aims to provide a simple, rapid, high-throughput, low-cost and accurate qualitative method for screening unknown volatile substances in large quantities of food and edible agricultural products, and to provide a scientific method for food and edible agricultural product quality control and simultaneous rapid analysis of volatile exogenous hazards. ​The screening method for unknown volatile substances in food and edible agricultural products has the characteristics that: a test sample is pretreated to obtain a test sample; a gas chromatograph is connected with a full two-dimensional time-of-flight mass spectrometer to collect data of the test sample, and a total ion current map is obtained; and the screening of unknown volatile substances in the test sample is realized by spectrum analysis of the total ion current map.

[0007] The screening method for unknown volatile substances in food and edible agricultural products also has the characteristics that the following steps are performed: Step 1, the test sample is sequentially crushed, extracted with ethanol, centrifuged and filtered to obtain a test sample; Step 2, the test sample is collected by a gas chromatograph connected with a full two-dimensional time-of-flight mass spectrometer, the data collection is performed in a Scan mode on volatile substances with a molecular weight of 50-5000, and a total ion current map is obtained; Step 3, unknown volatile substances are inferred by spectrum analysis, which means that, according to the total ion current map, chromatographic peaks with a parent ion response of more than 50000 and a matching degree of more than 80% are analyzed, the molecular weight of the substance is inferred from the parent ion, the specific substance is inferred according to the bond breaking position and structure characteristics of each substance, and the substance is recorded as substance A; the substance A is classified as a phenolic acid substance, an aldehyde ketone substance and an ester substance; the method for inferring the molecular weight of the substance includes: determining the molecular weight from the M+ peak; identifying chlorine and bromine from the M+2 and M+4 peaks; inferring nitrogen atoms according to nitrogen rules and fragmentation forms; and comparing fragment ions with spectrum data; Step 4, qualitative analysis of unknown volatile substances: standard substances of the substance A are taken, a standard substance solution of 100 ppb is prepared, the standard substance solution is detected by a triple quadrupole gas chromatograph-mass spectrometer, the parent ion and fragment ions of the standard substance are confirmed in positive and negative ion modes, and it is confirmed that the test sample contains the substance A, so that the name of the substance A is determined and recorded as substance B; Step 5, quantitative analysis of unknown volatile substances: the standard substance solution of the substance B is taken, 10 µL of the sample is injected into the triple quadrupole gas chromatograph-mass spectrometer for detection, the peak response value of the substance B is linearly regressed with respect to the corresponding mass concentration, a linear regression equation of the substance B is obtained, and the curve corresponding to each linear regression equation is a standard curve of the substance B, and the quantitative analysis of unknown substances is completed.

[0008] The method for screening unknown volatile substances in food and edible agricultural products of the present invention is characterized by the following: Step 1, the process of processing the sample to be tested, is as follows: Take 10-100g of the crushed sample to be tested into a 250mL centrifuge tube, add 200mL of 70% ethanol, vortex mix for 5min, centrifuge at 10000 rpm for 5min, take the supernatant and concentrate it to 1-2mL, then make up to 5mL with 70% ethanol, filter through a 0.22μm filter membrane to obtain the test sample.

[0009] The method for screening unknown volatile substances in food and edible agricultural products of this invention is also characterized by the following: the chromatographic conditions for connecting the gas chromatograph in series with the two-dimensional time-of-flight mass spectrometer in step 2 are as follows: The first-dimensional chromatographic column was a DB-5MS 30m x 0.25mm x 0.25um quartz capillary column; The second-dimensional chromatographic column is a DB-17HT 2m x 0.25mm x 0.15um quartz capillary column; Constant flow mode, column flow rate 1 mL / min, injection volume 0.1 μL, split injection split ratio 50:1, injection port 280℃, purge flow rate 2 mL / min; modulation period 4 s; thermal modulation time 0.3 s; The first-dimensional column oven heating program is as follows: start at 70℃ and hold for 2 min, then increase the temperature at 4℃ / min to 110℃ and hold for 1 min, then increase the temperature at 5℃ / min to 220℃ and hold for 5 min. The second-dimensional column oven heating program is as follows: start at 60℃ and hold for 2 min, then increase the temperature at 4℃ / min to 100℃ and hold for 1 min, then increase the temperature at 5℃ / min to 210℃ and hold for 5 min. Mass spectrometry conditions for a gas chromatograph connected in series with a two-dimensional time-of-flight mass spectrometer: injection volume: 10 μL; EI ion source; voltage: 70 eV; ion source temperature: 220 ℃; transfer line temperature: 250 ℃; detector voltage: 1840 V; mass spectrometry detection solvent delay: 2 min; mass scan range: 36-600 amu; acquisition frequency: 100 spectrum / s.

[0010] The method for screening unknown volatile substances in food and edible agricultural products of this invention is also characterized by setting the detection conditions of the triple quadrupole gas chromatography-mass spectrometry (GC-MS) instrument in step 4 as follows: Chromatographic columns: DB-5MS 30m x 0.25mm x 0.25um and DB-17HT 2m x 0.25mm x 0.15um; Flow rate: 1 mL / min, injection volume: 1.0 μL, split injection split ratio: 20:1; Column temperature: Initial column temperature 40℃, hold for 1 min; increase to 120℃ at 40℃ / min; then increase to 240℃ at 5℃ / min; then increase to 300℃ at 12℃ / min, hold for 6 min; Inlet temperature: 320℃; Ion source temperature: 320℃; The triple quadrupole mass spectrometer is equipped with an EI source.

[0011] The method for screening unknown volatile substances in food and edible agricultural products of this invention is also characterized by setting the detection conditions of the triple quadrupole mass spectrometer in step 5 as follows: Chromatographic columns: DB-5MS 30m x 0.25mm x 0.25um and DB-17HT 2m x 0.25mm x 0.15um; Flow rate: 1 mL / min, injection volume: 1.0 μL, split injection split ratio: 20:1; Column temperature: Initial column temperature 30℃, hold for 3 min; increase to 150℃ at 30℃ / min; increase to 250℃ at 5℃ / min; increase to 350℃ at 15℃ / min, hold for 10 min; Inlet temperature: 280℃; Ion source temperature: 280℃; The triple quadrupole mass spectrometer is equipped with an EI source; The screening method for unknown volatile substances in food and edible agricultural products of this invention is also characterized by the following indicators obtained by analyzing the chromatographic peaks in step 3: Number of peaks: The number of peaks indicates the number of different components separated in the sample. Each peak represents a substance or a group of isomers that are difficult to separate completely. Peak position: i.e. retention time. By comparing the retention time of an unknown peak with that of a known standard compound, a preliminary qualitative analysis of the unknown compound can be performed. Peak shape: Normal chromatographic peaks have a symmetrical Gaussian distribution. Abnormal peak shapes such as tailing, leading edge, double peaks, or broad peaks are judged to be due to poor chromatographic conditions, sample overload, column aging, injection problems, or the presence of co-elutants. Peak height and peak area: Peak height or peak area is directly proportional to the content of the corresponding component in the sample and is used for quantitative analysis.

[0012] Overlapping peaks: When the retention times of two or more components are close, resulting in peak overlap, the following methods can be used for analysis: By changing chromatographic conditions, including adjusting column temperature, flow rate, and column type, better separation can be achieved; or by using mass spectrometry information, the different components can be distinguished based on the differences in ion fragments by analyzing the mass spectra at different positions of the overlapping peaks. Shoulder peak: When a weaker component and a major component have similar retention times, a shoulder peak appears because they are not completely separated. By changing the chromatographic conditions, the resolution can be improved, enabling qualitative and quantitative analysis of the two corresponding components. Mass spectrum: For the total ion chromatogram, the corresponding mass spectrum is extracted from the apex of each peak; the mass spectrum provides information on ion fragments eluting from the column at the corresponding time, including ions with different mass-to-charge ratios and their relative intensities; Mass spectrometry retrieval: The mass spectrum is compared with the standard mass spectrum of known compounds, and a similarity score is calculated. Structural analysis: By analyzing the ion fragmentation information in the mass spectrum, the fragmentation pattern of the compound is obtained, and the molecular structure is inferred based on the fragmentation pattern.

[0013] Compared with existing technologies, the beneficial effects of this invention are reflected in: 1. This invention uses a gas chromatograph connected in series with a two-dimensional time-of-flight mass spectrometer to achieve rapid qualitative screening without standards; 2. The method of this invention is fast, accurate, has high throughput, strong specificity, and can analyze a wide variety of substances. It can provide a scientific method for the quality control of food and edible agricultural products and the simultaneous rapid analysis of unknown substances. 3. When collecting data, this invention can collect all volatile substances with molecular weights between 50 and 5000, and achieve qualitative analysis of all volatile substances with molecular weights between 50 and 5000. 4. The spectral analysis method in this invention is simple and reliable, and it uses standard substances to achieve qualitative and quantitative analysis of unknown volatile substances. The process is simple and the results are reliable. 5. Based on the method of this invention, we can identify new risk factors for food safety and establish a database of basic risk factors, thereby providing risk warnings and proactive response measures for food safety. Detailed Implementation

[0014] In this embodiment, the screening method for unknown volatile substances in food and edible agricultural products is to obtain a test sample by pretreatment; to collect data on the test sample using a gas chromatograph connected in series with a two-dimensional time-of-flight mass spectrometer to obtain a total ion chromatogram; and to screen for unknown volatile substances in the test sample by analyzing the total ion chromatogram.

[0015] The screening method for unknown volatile substances in food and edible agricultural products in this embodiment is carried out according to the following steps: Step 1: The sample to be tested is obtained by sequentially crushing, extracting with ethanol, centrifuging, and filtration. The process is as follows: Take 10-100g of the crushed sample to be tested into a 250mL centrifuge tube, add 200mL of 70% ethanol (water:ethanol=3:7), vortex mix for 5min, centrifuge at 10000 rpm for 5min, take the supernatant and concentrate it to 1-2mL, then make up to 5mL with 70% ethanol (water:ethanol=3:7), filter through a 0.22μm filter membrane to obtain the sample.

[0016] Step 2: Data acquisition of the test sample is performed using a gas chromatograph connected in series with a two-dimensional time-of-flight mass spectrometer. Data acquisition is conducted in Scan mode for volatile substances with molecular weights between 50 and 5000, obtaining a total ion chromatogram for spectral analysis. The chromatographic conditions for the gas chromatograph connected in series with the two-dimensional time-of-flight mass spectrometer are set as follows: The first-dimensional chromatographic column was a DB-5MS 30m x 0.25mm x 0.25um quartz capillary column; The second-dimensional chromatographic column is a DB-17HT 2m x 0.25mm x 0.15um quartz capillary column; Constant flow mode, column flow rate 1 mL / min, injection volume 0.1 μL, split injection split ratio 50:1, injection port 280℃, purge flow rate 2 mL / min; modulation period 4 s; thermal modulation time 0.3 s; The first-dimensional column oven heating program is as follows: start at 70℃ and hold for 2 min, then increase the temperature at 4℃ / min to 110℃ and hold for 1 min, then increase the temperature at 5℃ / min to 220℃ and hold for 5 min. The second-dimensional column oven heating program is as follows: start at 60℃ and hold for 2 min, then increase the temperature at 4℃ / min to 100℃ and hold for 1 min, then increase the temperature at 5℃ / min to 210℃ and hold for 5 min. Mass spectrometry conditions for a gas chromatograph connected in series with a two-dimensional time-of-flight mass spectrometer: Injection volume: 10 μL; EI ion source; voltage: 70 eV; ion source temperature: 220 ℃; transfer line temperature: 250 ℃; detector voltage: 1840 V; mass spectrometry detection solvent delay: 2 min; mass scan range: 36-600 amu; acquisition frequency: 100 spectrum / s.

[0017] Step 3: Spectral analysis: This refers to deriving the molecular formula based on the total ion chromatogram, calculating the degree of unsaturation, determining the structural units, identifying the remaining structural units not detected by the five spectra, using the identified structural units to form several possible structures of the compound, and then selecting the most likely structure for qualitative and quantitative analysis.

[0018] Inferring unknown volatile substances through spectral analysis specifically refers to analyzing chromatographic peaks with a precursor ion response above 50,000 and a matching degree above 80% based on the total ion chromatogram. The molecular weight of the substance is inferred from the precursor ion, and the specific substance is identified based on the bond break position and structural characteristics of each substance, and is denoted as substance A. Substance A is classified into phenolic acids, aldehydes and ketones, and esters.

[0019] The methods used in this step to infer the molecular weight of a substance include: determining the molecular weight from the M+· peak; identifying chlorine and bromine from the M+2 and M+4 peaks; inferring nitrogen-containing atoms based on nitrogen rules and fragmentation patterns; and comparing fragment ions with spectral data.

[0020] This step involves analyzing chromatographic peaks to obtain various indicators, including: peak quantity, peak position, peak shape, peak height and peak area, overlapping peaks, shoulder peaks, mass spectrum, mass spectrometry search, and structural analysis. Specifically: the number of peaks indicates the number of different components separated from the sample; each peak represents a substance or a group of isomers that are difficult to completely separate. Peak position refers to retention time; comparing the retention time of an unknown peak with that of a known standard compound allows for preliminary qualitative analysis of the unknown compound. Peak shape refers to the symmetrical Gaussian distribution of a normal chromatographic peak. Abnormal peak shapes such as tailing, leading edges, double peaks, or broad peaks indicate poor chromatographic conditions, sample overload, column aging, injection problems, or the presence of co-elutants. Peak height and peak area are proportional to the content of the corresponding component in the sample and are used for quantitative analysis. Overlapping peaks refer to peaks caused by the close retention times of two or more components, which are identified as follows. Analysis involves several methods: First, modifying chromatographic conditions, including column temperature, flow rate, and column type, to achieve better separation. Second, utilizing mass spectrometry information, analyzing mass spectra at different positions of overlapping peaks to distinguish different components based on differences in ion fragments. Third, identifying shoulder peaks, where a weaker component is close to a major component in retention time and not fully separated, allows for qualitative and quantitative analysis of the two components by improving resolution. Fourth, extracting the corresponding mass spectrum from the apex of each peak in the total ion chromatogram. Fifth, obtaining ion fragment information of the component eluting from the column at the corresponding time from the mass spectrum, including ions with different mass-to-charge ratios and their relative intensities. Sixth, comparing the mass spectrum with standard mass spectra of known compounds and calculating a similarity score. Seventh, analyzing ion fragment information in the mass spectrum to determine the fragmentation patterns of the compound and inferring the molecular structure based on these patterns.

[0021] Step 4: Qualitative analysis of unknown volatile substances: Prepare a 100 ppb standard solution using a standard substance A. Analyze the standard solution using a triple quadrupole gas chromatography-mass spectrometry (GC-MS) system in both positive and negative ion modes to identify the parent ion and fragment ions of the standard substance. Then, confirm the presence of substance A in the sample to be tested. Based on this, determine the name of substance A, denoted as substance B. Set the detection conditions of the triple quadrupole GC-MS as follows: Chromatographic columns: DB-5MS 30m x 0.25mm x 0.25um and DB-17HT 2m x 0.25mm x 0.15um; Flow rate: 1 mL / min, injection volume: 1.0 μL, split injection split ratio: 20:1; Column temperature: Initial column temperature 40℃, hold for 1 min; increase to 120℃ at 40℃ / min; then increase to 240℃ at 5℃ / min; then increase to 300℃ at 12℃ / min, hold for 6 min; Inlet temperature: 320℃; Ion source temperature: 320℃; The triple quadrupole GC-MS is equipped with an EI source.

[0022] Step 5: Quantitative analysis of unknown volatile substances: Take a standard solution of substance B and inject 10 µL into a triple quadrupole gas chromatography-mass spectrometry (GC-MS) instrument for detection. Perform linear regression on the peak response value of substance B against the corresponding mass concentration to obtain the linear regression equation for substance B. The curve corresponding to each linear regression equation is the standard curve for substance B, thus completing the quantification of the unknown substance. Set the detection conditions of the triple quadrupole GC-MS instrument as follows: Chromatographic columns: DB-5MS 30m x 0.25mm x 0.25um and DB-17HT 2m x 0.25mm x 0.15um; Flow rate: 1 mL / min, injection volume: 1.0 μL, split injection split ratio: 20:1; Column temperature: Initial column temperature 30℃, hold for 3 min; increase to 150℃ at 30℃ / min; increase to 250℃ at 5℃ / min; increase to 350℃ at 15℃ / min, hold for 10 min; Inlet temperature: 280℃; Ion source temperature: 280℃; The triple quadrupole GC-MS is equipped with an EI source.

[0023] Step 6: Analysis of Screening Results for Unknown Substances After completing steps 1 to 5 to confirm the unknown volatile substances, a database of unknown volatile substances in food and edible agricultural products is established to support subsequent screening of risk substances.

[0024] This embodiment describes a method for screening unknown volatile substances in puffed foods, which is carried out according to the following steps: Step a: Take 10-100g of the crushed sample to be tested into a 250mL centrifuge tube, add 200mL of 70% ethanol (water:ethanol=3:7), vortex to mix for 5min, centrifuge at 10000 rpm for 5min, take the supernatant and concentrate it to 1-2mL, then make up to 5mL with 70% ethanol (water:ethanol=3:7), filter through a 0.22μm filter membrane to obtain the test sample.

[0025] Step b: Use a gas chromatograph connected in series with a two-dimensional time-of-flight mass spectrometer to acquire data of the test sample. Data acquisition is performed using Scan mode to acquire data of volatile substances with molecular weights between 50 and 5000, and obtain a total ion chromatogram for spectral analysis. Set the chromatographic conditions of the gas chromatograph connected in series with the two-dimensional time-of-flight mass spectrometer as in step 2, and set the mass spectrometric conditions of the gas chromatograph connected in series with the two-dimensional time-of-flight mass spectrometer as in step 2.

[0026] Step c, Spectrum analysis: In the total ion current, substance A with a mass-to-charge ratio of 88.093 was found to have a high response, having lost one hydrogen atom, suggesting that the molecular weight of substance A is 89.093; then, using secondary mass spectrometry to examine its characteristic ions, the characteristic ions 44, 62, 74, and 89 were found, suggesting that substance A is ethyl carbamate.

[0027] Step d, Qualitative analysis of unknown volatile substances: Take the standard substance of substance A (ethyl carbamate) and prepare a 100 ppb standard substance solution. Use a triple quadrupole gas chromatography-mass spectrometry (GC-MS) instrument to detect the standard substance solution. Collect data in both positive and negative ion modes to identify the parent ion and fragment ions of the standard substance. Then, detect and confirm that the sample to be tested contains substance A (ethyl carbamate). Determine the name of substance A accordingly and record it as ethyl carbamate. Set the detection conditions of the triple quadrupole GC-MS instrument according to step 4.

[0028] Step e, Quantitative analysis of unknown volatile substances: Take a standard solution of ethyl carbamate and inject it in 10 µL into a triple quadrupole gas chromatography-mass spectrometry (GC-MS) instrument for detection. Perform linear regression on the peak response value of ethyl carbamate against the corresponding mass concentration to obtain the linear regression equation of ethyl carbamate. The curve corresponding to each linear regression equation is the standard curve of ethyl carbamate, thus completing the quantification of unknown substances. Set the detection conditions of the triple quadrupole GC-MS instrument according to step 5.

[0029] Step f: Analysis of screening results for unknown substances After confirming the unknown volatile substances in steps 1 to 5, a database of unknown volatile substances in food and edible agricultural products is established to support subsequent screening for risk substances. This embodiment demonstrates that the method of the present invention can accurately screen for unknown substances in food and edible agricultural products.

Claims

1. A method for screening unknown volatile substances in food and foodstuffs, characterized by: A test sample is pretreated to obtain a test sample; a gas chromatograph connected with a full two-dimensional time-of-flight mass spectrometer is used to collect data of the test sample, and a total ion current chromatogram is obtained; unknown volatile substances in the test sample are screened by spectrum analysis of the total ion current chromatogram.

2. The screening method for unknown volatile substances in food and edible agricultural products according to claim 1, characterized in that: The following steps are performed: Step 1: The test sample is sequentially subjected to crushing, ethanol extraction, centrifugation and membrane filtration to obtain a test sample; Step 2: The test sample is subjected to data collection by using a gas chromatograph connected with a full two-dimensional time-of-flight mass spectrometer, and the data collection is performed by using a Scan mode to collect data of volatile substances with a molecular weight of 50-5000, and a total ion current chromatogram is obtained; Step 3: Unknown volatile substances are inferred by spectrum analysis, which means that, according to the total ion current chromatogram, chromatographic peaks with a parent ion response of more than 50000 and a matching degree of more than 80% are analyzed, the molecular weight of the substances is inferred from the parent ions, the specific substances are inferred according to the bond breaking position and structural characteristics of each substance, and the substances are recorded as substance A; the substance A is classified as a phenolic acid substance, an aldehyde ketone substance and an ester substance; The method for inferring the molecular weight of the substances includes: determining the molecular weight from M+· peaks; identifying chlorine and bromine from M+2 and M+4 peaks; and inferring nitrogen-containing atoms according to nitrogen rules and breaking forms; The fragment ions are compared with spectrum data; Step 4: Qualitative analysis of unknown volatile substances: standard substances of the substance A are prepared into a 100 ppb standard substance solution, the standard substance solution is detected by using a triple quadrupole gas chromatograph-mass spectrometer, the parent ions and fragment ions of the standard substance are confirmed in positive and negative ion modes, and it is confirmed that the test sample contains the substance A, so that the name of the substance A is determined and recorded as substance B; Step 5: Quantitative analysis of unknown volatile substances: the standard substance solution of the substance B is detected by using a triple quadrupole gas chromatograph-mass spectrometer with a 10 µL injection, the peak response value of the substance B is linearly regressed with respect to the corresponding mass concentration, a linear regression equation of the substance B is obtained, and a curve corresponding to each linear regression equation is a standard curve of the substance B, and the quantitative analysis of unknown substances is completed.

3. The method of claim 2, wherein the food and agricultural products are selected from the group consisting of fruits, vegetables, grains, and nuts. The treatment process of the test sample in step 1 is as follows: 10-100 g of the crushed test sample is taken into a 250 mL centrifuge tube, 200 mL of 70% ethanol is added, vortexed and mixed for 5 min, centrifuged at 10000 r / min for 5 min, the supernatant is taken and concentrated to 1-2 mL, 70% ethanol is added to 5 mL, and then filtered through a 0.22 μm filter membrane to obtain a test sample.

4. The method of claim 2, wherein the food and agricultural products are selected from the group consisting of fruits, vegetables, grains, and meats. The chromatographic conditions of the gas chromatograph connected with the full two-dimensional time-of-flight mass spectrometer in step 2 are as follows: The first-dimensional chromatographic column is a DB-5MS 30mx0.25mmx0.25um quartz capillary column; The second-dimensional chromatographic column is a DB-17HT 2mx0.25mmx0.15um quartz capillary column; The constant flow mode is used, the column flow rate is 1 mL / min, the injection amount is 0.1 μL, the split injection split ratio is 50:1, the injection port temperature is 280℃, and the purge flow rate is 2 mL / min; The modulation period is 4 s; Thermal modulation time 0.3s; The first dimension oven temperature program: initial temperature 70℃, hold for 2 min, then increase to 110℃ at 4℃ / min, hold for 1 min, then increase to 220℃ at 5℃ / min, hold for 5 min; The second dimension oven temperature program: initial temperature 60℃, hold for 2 min, then increase to 100℃ at 4℃ / min, hold for 1 min, then increase to 210℃ at 5℃ / min, hold for 5 min; The mass spectrometry condition of the gas chromatograph coupled with the full two-dimensional time-of-flight mass spectrometer: injection volume: 10 μL; EI ion source; voltage: 70 eV; ion source temperature: 220℃; transfer line temperature: 250℃; detector voltage: 1840 V; solvent delay for mass detection: 2 min; mass scan range: 36-600 amu; acquisition frequency: 100 spectrum / s.

5. The method of claim 2, wherein the food and agricultural products are selected from the group consisting of fruits, vegetables, grains, and nuts. The detection condition of the triple quadrupole gas chromatograph-mass spectrometer in step 4 is set as follows: Chromatographic column: DB-5MS 30mx0.25mmx0.25um and DB-17HT 2mx0.25mmx0.15um; Flow rate: 1 mL / min, injection volume: 1.0 μL, split injection split ratio: 20:1; Column temperature: initial column temperature 40℃, hold for 1 min, then increase to 120℃ at 40℃ / min, then increase to 240℃ at 5℃ / min, then increase to 300℃ at 12℃ / min, hold for 6 min; Injection port temperature: 320℃; ion source temperature: 320℃; The triple quadrupole gas chromatograph-mass spectrometer is equipped with an EI source.

6. The method of claim 2, wherein the food and agricultural products are selected from the group consisting of fruits, vegetables, grains, and nuts. The detection condition of the triple quadrupole gas chromatograph-mass spectrometer in step 5 is set as follows: Chromatographic column: DB-5MS 30mx0.25mmx0.25um and DB-17HT 2mx0.25mmx0.15um; Flow rate: 1 mL / min, injection volume: 1.0 μL, split injection split ratio: 20:1; Column temperature: initial column temperature 30℃, hold for 3 min, then increase to 150℃ at 30℃ / min, then increase to 250℃ at 5℃ / min, then increase to 350℃ at 15℃ / min, hold for 10 min; Injection port temperature: 280℃; ion source temperature: 280℃; The triple quadrupole gas chromatograph-mass spectrometer is equipped with an EI source.

7. The screening method of unknown volatile substances in food and edible agricultural products according to claim 2, characterized in that: The following indexes are obtained by analyzing the chromatographic peaks in step 3: The number of peaks: the number of peaks represents the number of different components separated from the sample, and each peak represents a substance or a group of isomers that are difficult to completely separate; Peak position: that is, the retention time, the retention time of the unknown peak is compared with the retention time of the known standard compound to preliminarily identify the unknown compound; Peak shape: normal chromatographic peaks are symmetrical Gaussian distribution shape, and abnormal conditions such as peak shape tailing, front, doublet or wide peak are caused by poor chromatographic conditions, sample overload, column aging, injection problems or the presence of co-elution problems; Peak height and peak area: the peak height or peak area is proportional to the content of the corresponding component in the sample, which is used for quantitative analysis. Overlapping peaks: when the retention time of two or more components is close, resulting in peak overlap, the following methods are used for resolution: by changing the chromatographic conditions, including adjusting the column temperature, flow rate and column type, to achieve better separation; or using mass spectrometry information, by analyzing the mass spectra at different positions of the overlapping peak, according to the difference of ion fragments to distinguish different components; Shoulder peak: when a weak component is close to a major component in retention time, it presents a shoulder peak because it is not completely separated, by changing the chromatographic conditions to improve the separation degree, to achieve qualitative and quantitative analysis of the two corresponding components; Mass spectrum: for the total ion current chromatogram, the corresponding mass spectrum is extracted at the top of each peak; from the mass spectrum, the ion fragment information of the component flowing out of the chromatographic column at the corresponding time is obtained, including ions of different mass-to-charge ratios and their relative intensities; Mass spectrum search: compare the mass spectrum with the standard mass spectrum of known compounds, and obtain a similarity score by calculation; Structure analysis: by analyzing the ion fragment information in the mass spectrum, the fragmentation rule of the compound is obtained, and the molecular structure is inferred according to the fragmentation rule.