CN114720610A - Analysis method of volatile compounds in walnut oil - Google Patents
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Abstract
The invention provides an analysis method of volatile compounds in walnut oil, which comprises the following steps: (1) adsorbing volatile compounds in the walnut oil by adopting a dynamic headspace sample preparation mode, wherein the adsorbing material is an electrostatic spinning fiber membrane, and the electrostatic spinning fiber membrane is selected from a polystyrene electrostatic spinning fiber membrane, an acrylic resin electrostatic spinning fiber membrane or a polystyrene-acrylic resin blending electrostatic spinning fiber membrane; (2) carrying out thermal desorption on the adsorbed volatile compounds to obtain enriched volatile compounds; (3) the enriched volatile compounds were analyzed using a gas chromatography-sniffing-mass spectrometer. The analysis method of the invention adopts the electrostatic spinning fiber membrane as the adsorption material, effectively extracts volatile compounds, especially flavor compounds, in the walnut oil, and has the advantages of simple operation and high adsorption efficiency.
Description
Technical Field
The invention relates to the technical field of food analysis, in particular to an analysis method of volatile compounds in walnut oil.
Background
Walnut, also known as Qiang Tao and Juglans, belongs to the genus Juglans of the family Juglandaceae. The walnut has rich resources, various varieties, and high cultivation area and yield. Because the history of walnut cultivation and utilization is long and the popularization is wide, people have considerable understanding and attention to the nutrition and health care functions and the economic and ecological benefits of walnuts. Along with the continuous improvement of the cultural living standard of human materials, besides the walnut is used as an instant food, the demand of the walnut finished products is also increasing, wherein the walnut oil is one of the most representative products.
The walnut oil is an oil product with high added value extracted from walnut kernels, and is widely applied to the fields of food and health care. The walnut oil has high nutritive value, the fatty acid composition of the walnut oil mainly comprises unsaturated fatty acids such as linoleic acid, oleic acid, linolenic acid and the like, the content of the fatty acids is as high as about 90 percent, and the walnut oil can effectively reduce cholesterol and prevent and treat coronary heart disease, arteriosclerosis and myocardial infarction. In addition to good fatty acid content, the minor components in walnut oil, such as polyphenols, tocopherols, squalene and phytosterols, also have important nutritional values. The walnut polyphenol can inhibit the activity of low-density lipoprotein oxidation; the tocopherol has antiproliferative and anti-inflammatory effects on cancer; squalene is used as a skin emollient; phytosterols may interfere with intestinal cholesterol absorption and reduce the risk of coronary heart disease. In addition, walnut oil is used in the production of cosmetics, sesame oil and soaps. Walnut oil is also commonly used as a high-grade lubricating oil and a stock oil in industry.
The aroma quality is the most important sensory quality of the vegetable oil, and is not only an important evaluation index of the walnut oil quality, but also a primary standard for selecting products by consumers, so that the research on the inherent smell of the walnut oil is very important. Due to the complexity of food matrix, different separation and extraction technologies are selected for different samples, which is crucial to the accuracy of analysis results, so different aroma extraction methods are required to extract volatile compounds in food to the maximum extent, and then the next step of analysis is performed, and the extraction methods of common volatile flavor substances are mainly divided into two categories: solvent extraction techniques and headspace adsorption techniques. Solvent-assisted flavor evaporation (SAFE) is the most common solvent extraction technology, has high volatile recovery rate and low temperature and high boiling point, realizes the separation of volatile compounds, and is widely applied at present.
The headspace adsorption technology is generally divided into two sample preparation methods, namely static headspace and dynamic headspace, and the solid-phase micro-extraction (SPME) technology is the most common static headspace adsorption technology and is a solvent-free extraction technology with simultaneous sampling, concentration and sample injection. Because of the advantages of easy operation, portability, strong adsorbability, high extraction rate, less required samples, low detection limit, better repeatability under certain strict conditions and the like, the method becomes one of the most effective and most used methods for extracting the aroma substances at present. Dynamic Headspace Sampling (DHS) is performed by purging the surface of a sample in a closed container with an inert gas (such as nitrogen), enriching volatile compounds in the headspace of the sample with a trap, volatilizing the compounds by a thermal desorption technique, and performing component analysis in a gas chromatograph or gas chromatograph-mass spectrometer. The common trap is a Tenax TA adsorption column, and the filler is a polymer adsorbent which can effectively adsorb odor volatile matters. In the last decade, many new materials, especially nanostructured materials, have been studied as adsorbents for organic molecules in gases, such as a method of concentrating volatile substances by using thin films as adsorbing materials, and the main difference of the technology compared with the traditional adsorption method is that an extraction phase with a larger volume and a larger specific surface area is used, so that the adsorption sensitivity can be improved without sacrificing sampling time. In addition, it is proved by research that the adsorbent based on the electrospun nanofiber can also be used for extracting organic matters, and the felt-like and fiber-like structures of the nanofiber provide better mechanical properties, larger specific surface area and pore structures for the adsorbent.
The extraction mode to volatile materials is more traditional and single in the present stage, along with the progress of science and technology, the crisscross development of multidisciplinary, and novel adsorption material is urgently needed to be researched and developed at present to can obtain more effective, quick, accurate analytical technique to volatile materials's extraction.
Disclosure of Invention
In order to solve the technical problems, the invention aims to provide an analysis method of volatile compounds in walnut oil, which adopts an electrostatic spinning fiber membrane as an adsorbing material to effectively adsorb and extract the volatile compounds in the walnut oil and analyze the volatile compounds.
In order to achieve the above object, the present invention provides a method for analyzing volatile compounds in walnut oil, which comprises the following steps:
(1) adsorbing volatile compounds in the walnut oil by adopting a dynamic headspace sample preparation mode, wherein the adsorbing material is an electrostatic spinning fiber membrane, and the electrostatic spinning fiber membrane is selected from a polystyrene electrostatic spinning fiber membrane, an acrylic resin electrostatic spinning fiber membrane or a polystyrene-acrylic resin blending electrostatic spinning fiber membrane;
(2) carrying out thermal desorption on the adsorbed volatile compounds to obtain enriched volatile compounds;
(3) analyzing the enriched volatile compounds by using a gas chromatography-sniffing-mass spectrometer.
According to a specific embodiment of the present invention, preferably, the electrospun fiber membrane is used in an amount of 0.15-0.25% based on 100% by mass of the walnut oil.
According to a specific embodiment of the present invention, preferably, the acrylic resin electrospun fiber film is an acrylic resin II electrospun fiber film.
According to a specific embodiment of the present invention, preferably, the fiber diameter of the polystyrene electrospun fiber membrane is 300-800 nm.
According to the specific embodiment of the present invention, preferably, the fiber diameter of the acrylic resin electrospun fiber membrane is 200-400 nm.
According to the specific embodiment of the invention, the fiber diameter of the polystyrene-acrylic resin blended electrostatic spinning fiber membrane is preferably 200-900 nm.
According to a specific embodiment of the present invention, preferably, an internal standard substance is further added to the walnut oil during the dynamic headspace sampling, and the internal standard substance is selected from 2-methyl-3-heptanone. The 2-methyl-3-heptanone is not present in the walnut oil sample, and the peak-out time does not mask the volatile compounds in the sample.
According to a particular embodiment of the present invention, preferably, the dynamic headspace sampling comprises an incubation procedure and an adsorption procedure, the adsorption temperature being between 50 and 60 ℃.
According to a particular embodiment of the present invention, preferably, the incubation temperature is 50-55 ℃ and the equilibration time is 10-15 min.
According to a specific embodiment of the present invention, it is preferable that the nitrogen flow rate at the time of adsorption is 20 to 25mL/min and that after completion of adsorption is 10 to 12 mL/min.
According to the specific embodiment of the present invention, the thermal desorption temperature is preferably 240-250 ℃, and the thermal desorption time is preferably 10-12 min.
According to a specific embodiment of the present invention, preferably, the step (2) further comprises: prior to the thermal desorption, the adsorbed volatile compounds are subjected to cryo-focusing in a solvent blow-down mode.
According to a particular embodiment of the invention, the cold trap temperature is preferably between-78 ℃ and-80 ℃ and the equilibration time is between 0.5 and 1 min.
According to a particular embodiment of the invention, preferably, the gas chromatography conditions are as follows:
a chromatographic column: j & W DB-WAX quartz capillary column, 30 mm × 0.25 μm;
temperature rising procedure: the initial column temperature is 40 ℃, the temperature is raised to 75 ℃ at the speed of 5 ℃/min after the column temperature is maintained for 2min, the temperature is raised to 150 ℃ at the speed of 2 ℃/min, and finally the temperature is raised to 230 ℃ at the speed of 7 ℃/min and maintained for 2 min; the temperature of the sample inlet is 230 ℃, and the temperature of the detector is 280 ℃; the carrier gas is nitrogen, and the flow rate is 3 mL/min; no flow split.
According to a particular embodiment of the invention, preferably, the mass spectrometry conditions are as follows:
electron bombardment with electron energy of 70eV, transmission line temperature of 280 deg.C, ion source temperature of 230 deg.C, quadrupole rod temperature of 150 deg.C, scanning mode of full scan, mass scanning range of 50-350m/z, and solvent delay of 5 min.
According to a particular embodiment of the invention, the analysis is preferably carried out in a semi-quantitative manner using an internal standard.
According to the specific embodiment of the invention, the walnut oil is preferably obtained by squeezing walnut kernels at the high temperature of 200-250 ℃.
In the analytical method of the present invention, volatile compounds refer to compounds in walnut oil which can be detected only by gas chromatography-mass spectrometry, and flavor compounds refer to compounds in volatile compounds which can be detected by an olfactory detector.
The technical scheme provided by the invention has the following beneficial effects:
(1) the analysis method of the invention adopts the electrostatic spinning fiber membrane as the adsorption material, effectively extracts volatile compounds in the walnut oil, has higher adsorption capacity, high adsorption efficiency and simple and convenient operation;
(2) the analysis method of the invention adopts the electrostatic spinning fiber membrane as the adsorption material, expands the detection range of the volatile compounds, and particularly has wider smell detection range for the flavor compounds in the volatile compounds;
(3) the electrostatic spinning fiber membrane is simple to prepare and low in price.
Drawings
FIG. 1 shows the comparison of the amounts of volatile compounds adsorbed by each of the four materials;
FIG. 2 shows the comparison of the amounts of the odorant flavor compounds adsorbed by the four materials;
fig. 3 is a comparison of the total concentration of each of the four odorable flavor compounds adsorbed by the four materials.
Detailed Description
The technical solutions of the present invention will be described in detail below in order to clearly understand the technical features, objects, and advantages of the present invention, but the present invention is not limited to the practical scope of the present invention.
The experimental materials and instruments used in the examples of the invention were as follows:
materials: hot-pressed walnut oil, 2-methyl-3 heptanone, a polystyrene electrostatic spinning fiber membrane (PS) (the fiber diameter is 300-;
the instrument comprises the following steps: 20mL of headspace glass sample bottle, a thermal desorption tube, a multipurpose sampler (MPS), a dynamic headspace sample preparation system (DHS), a cooling injection-thermal desorption coupling system (CIS-TDU) and a gas chromatography-sniffing-mass spectrometer (GC-O-MS).
The hot-pressed walnut oil used in the examples of the present invention was prepared by: selecting walnut kernels which are normally developed and have full fruits, chopping the walnut kernels, and then putting the chopped walnut kernels into a preheated oil press for high-temperature pressing at the temperature of 200-fold and 250 ℃; centrifuging the obtained hot squeezed walnut oil at 12000r/min for 30min, placing the supernatant in a glass bottle, sealing, and storing at 4 deg.C in dark place.
In the embodiment of the present invention, the actual retention index (actual RI) is obtained by using a logarithmic interpolation method based on the retention time of adjacent normal paraffins on both sides of the substance to be detected on the chromatogram.
Example 1
The embodiment provides an analysis method of volatile compounds in walnut oil, which specifically comprises the following steps:
1. the volatile compounds of the hot-pressed walnut oil are enriched
Accurately weighing 5.0g of hot-pressed walnut oil into a 20mL headspace glass sample bottle, adding 1 μ L of 2-methyl-3 heptanone (0.816 μ g/μ L) as an internal standard, capping, sealing with a sealing film to obtain an upper machine sample, and sampling with a multi-purpose sampler.
Incubating the sample in a dynamic headspace sample preparation system for 10min at the temperature of 55 ℃ and the stirring speed of 500 rpm; at 25 ℃, a sample is purged by 800mL of nitrogen at the nitrogen flow rate of 20mL/min, and volatile compounds are captured by three thermal desorption tubes provided with PS membranes; after the capture is finished, performing dewatering and drying treatment on the thermal desorption pipe by using 100mL of nitrogen at the flow rate of 10 mL/min; placing the thermal desorption tube in a cooling injection-thermal desorption coupling system (equipped with an inactivated glass wool lining), freezing and focusing in a solvent emptying mode, wherein the temperature of a cold trap is-80 ℃, balancing for 0.5min and maintaining for 0.1 min; then, heating to 250 ℃ at the speed of 10 ℃/min for thermal desorption, maintaining for 10min to obtain sample gas enriched with volatile compounds, and transferring to a gas chromatography-smelling-mass spectrometer.
2. Performing gas chromatography-sniffing-mass spectrometry
2.1 gas chromatography conditions
A chromatographic column: j & W DB-WAX quartz capillary column (30m × 0.25mm × 0.25 μm);
temperature rising procedure: the initial column temperature is 40 ℃, the temperature is raised to 75 ℃ at 5 ℃/min after the column temperature is maintained for 2min, the temperature is raised to 150 ℃ at 2 ℃/min, and finally the temperature is raised to 230 ℃ at the speed of 7 ℃/min, and the column temperature is maintained for 2 min. The temperature of the injection port is 230 ℃, and the temperature of the detector is 280 ℃; the carrier gas is nitrogen, and the flow rate is 3 mL/min; the split ratio is no split.
2.2 Mass spectrometer conditions
Electron Impact (EI), electron energy is 70eV, transmission line temperature is 280 ℃, ion source temperature is maintained at 230 ℃, quadrupole rod temperature is set at 150 ℃, scanning mode is full scanning, mass scanning range is 50-350m/z, and solvent delay is 5 min.
2.3 sniff Detector Condition
Nitrogen gas was introduced into a small bottle containing distilled water at a rate of 20mL/min to supply water vapor to the sniffing port, maintaining the nasal cavity in a wet state during sniffing by the test person.
3. Quantitative analysis of volatile compounds in hot-pressed walnut oil
The quantitative analysis of the volatile compounds of the hot-pressed walnut oil adopts an internal standard semi-quantitative mode. 2-methyl-3-heptanone (0.816 mu g/mu L) is taken as an internal standard substance, namely the mass concentration and the peak area of the internal standard substance are calculated according to the peak area ratio of an unknown substance, and the formula is as follows:
in the formula, f representsA correction factor, the value of which is 1; a. the1、AxRespectively represent the peak area of the internal standard substance and the peak area of the unknown substance, rho1、ρxRespectively representing the internal standard mass concentration and the unknown mass concentration.
Example 2
This example provides a method for analyzing volatile compounds in walnut oil, which is the same as example 1 except that: the adsorption material in the thermal desorption tube of this embodiment is an AR II film.
Example 3
This example provides a method for analyzing volatile compounds in walnut oil, which is the same as example 1 except that: the adsorbent in the thermal desorption tube of this example was a PS-AR film.
Comparative example 1
The comparative example provides an analysis method of volatile compounds in walnut oil, which adopts a conventional adsorption material Tenax TA (2, 6-diphenyl furan porous polymer), and comprises the following specific steps:
1. aging the adsorbent
The Tenax TA adsorption column is uniformly inserted into a tubular temperature controller (Tube Conditioner, TC2, Gerstel) in advance for aging treatment, the aging temperature is 250 ℃, the nitrogen purge flow rate is 700mL/min, and the aging treatment lasts for 2 h.
2. The volatile compounds of the hot-pressed walnut oil are enriched
Accurately weighing 5.0g of hot-pressed walnut oil into a 20mL headspace glass sample bottle, adding 1 μ L of 2-methyl-3 heptanone (0.816 μ g/μ L) as an internal standard, capping, sealing with a sealing film to obtain an upper machine sample, and sampling with a multi-purpose sampler.
Incubating the sample in a dynamic headspace sample preparation system at 55 ℃ and a stirring speed of 500rpm for 10 min; at 25 ℃, a sample is purged by 800mL of nitrogen at the nitrogen flow rate of 20mL/min, and volatile compounds are captured by a Tenax TA adsorption column; after the capture is finished, performing dewatering and drying treatment on the Tenax TA adsorption column by using 100mL of nitrogen at the flow rate of 10 mL/min; placing Tenax TA adsorption column in cooling injection-thermal desorption coupling system (equipped with deactivated glass wool lining), freezing and focusing in solvent emptying mode, keeping cold trap temperature at-80 deg.C, balancing for 0.5min and maintaining for 0.1 min; then, heating to 250 ℃ at the speed of 10 ℃/min for thermal desorption, maintaining for 10min to obtain sample gas enriched with volatile compounds, and transferring to a gas chromatography-sniffing-mass spectrometry combined instrument.
3. Performing gas chromatography-sniffing-mass spectrometry
3.1 gas chromatography conditions
A chromatographic column: j & W DB-WAX quartz capillary column (30m × 0.25mm × 0.25 μm);
temperature rising procedure: the initial column temperature is 40 ℃, the temperature is raised to 75 ℃ at 5 ℃/min after the column temperature is maintained for 2min, the temperature is raised to 150 ℃ at 2 ℃/min, and finally the temperature is raised to 230 ℃ at the speed of 7 ℃/min, and the column temperature is maintained for 2 min. The temperature of the sample inlet is 230 ℃, and the temperature of the detector is 280 ℃; the carrier gas is nitrogen, and the flow rate is 3 mL/min; the split ratio is no split.
3.2 Mass Spectrometry Detector Condition
Electron Impact (EI), electron energy is 70eV, transmission line temperature is 280 ℃, ion source temperature is maintained at 230 ℃, quadrupole rod temperature is set at 150 ℃, scanning mode is full scanning, mass scanning range is 50-350m/z, and solvent delay is 5 min.
3.3 sniff detector conditions
Nitrogen gas was introduced into a small bottle containing distilled water at a rate of 20mL/min to supply water vapor to the sniffing port and maintain the nasal cavity in a moist state during sniffing by the examiner.
4. Quantitative analysis of volatile compounds in hot-pressed walnut oil
The quantitative analysis of the volatile compounds of the hot-pressed walnut oil adopts an internal standard semi-quantitative mode. 2-methyl-3-heptanone (0.816 mu g/mu L) is taken as an internal standard substance, namely the mass concentration and the peak area of the internal standard substance are calculated according to the peak area ratio of an unknown substance, and the formula is as follows:
in the formula, f represents a correction factor, and the value is 1; a. the1、AxRespectively represent the peak area of the internal standard substance and the peak area of the unknown substance, rho1、ρxRespectively representing the internal standard substance concentration and the unknown substance concentration.
Table 1 adsorption results of volatile compounds by four materials
TABLE 2 smelling results of the four materials adsorbed flavor compounds
The results of measuring the volatile compound concentration and the smelling results of the hot-pressed walnut oils of examples 1 to 3 and comparative example 1 are shown in tables 1 and 2, respectively, and S, M, W in table 2 corresponds to three grades of strong, medium and weak smelling intensity, respectively. As can be seen from Table 1, for the same hot-pressed walnut oil sample, 93 volatile compounds were identified by Tenax TA, 50 were identified by PS membrane, 73 were identified by AR-II membrane, and 68 were identified by PS-AR membrane. While Tenax TA is advantageous in terms of the amount of volatile compounds identified, it is not so from the standpoint of the type of volatile compound. The volatile compounds identified in table 1 can be classified as: alcohols, aldehydes, acids, ketones, esters, ethers, aromatics, hydrocarbons and others, in total 9 classes. Fig. 1 shows the amounts of volatile compounds adsorbed by Tenax TA and the three electrospun fiber membranes of examples 1 to 3, and it can be seen from fig. 1 that Tenax TA has a better effect of extracting aldehydes and ketones, and the electrospun fiber membranes of the present invention also have a better effect of enriching volatile compounds in hot-pressed walnut oil, and it can also be seen from table 1 that the concentration of volatile compounds identified by the electrospun fiber membrane adsorbing material of the present invention is higher, and the concentration of some volatile compounds can even reach about 10 times of the adsorption amount of Tenax TA.
The AR II membrane and the PS-AR membrane can be enriched with more acids, ethers and hydrocarbons, such as 2-butyric acid (baking taste), (E) -2-nonenoic acid (fat and oil taste), ethylene glycol phenyl ether (balsam and cinnamon taste) and styrene (balsam and oil taste), and the like, and the flavor compounds have good oil flavor attributes and contribute to rich flavor components of the hot-pressed walnut oil, but are not detected when being adsorbed by Tenax TA. Therefore, the electrostatic spinning fibrous membrane adsorbing material has certain utilization value for identifying and analyzing the flavor profile composition of the hot-pressed walnut oil.
More importantly, the aroma quality is the most important sensory quality of the vegetable oil, not only is an important evaluation index of the walnut oil quality, but also is a primary standard for consumers to select products, and not all volatile compounds have outstanding contribution to the aroma composition of the hot-pressed walnut oil, so the smelling result is also an important judgment standard for identifying whether an extraction method is good or not. According to the invention, part of volatile compounds capable of being smelled and identified are taken as flavor compounds, 16 flavor compounds are smelled by a Tenax TA adsorption column, 21 flavor compounds are smelled by a PS membrane, 35 flavor compounds are smelled by an AR II membrane, and 28 flavor compounds are smelled by a PS-AR membrane according to the smelling results shown in Table 2. Combining the amounts of various odorable flavor compounds adsorbed by the four materials shown in fig. 2, the three electrospun fiber membranes of examples 1-3 have more excellent flavor compound adsorption performance. And as can also be seen from table 2, the compounds detected by the common adsorption of Tenax TA, PS membrane, AR II membrane or PS-AR membrane are enriched in Tenax TA at a concentration much lower than that of the electrospun fiber membrane of the present invention, which is also a part of the important reason for the weak smell of Tenax TA. In addition, according to the extracted 9 types of volatile compounds, the total concentration of each substance in each type of flavor compounds identified by smelling is compared, and as shown in fig. 3, it is obvious that the electrostatic spinning fiber membrane adsorbing material has certain advantages on the adsorption efficiency of the flavor compounds. In conclusion, based on the smell results, the electrostatic spinning fiber membrane of the invention is better than the common Tenax TA material when being applied to the extraction of the flavor compounds of the hot-pressed walnut oil.
It should be understood that the above-described embodiments of the present invention are examples for clearly illustrating the invention, and are not to be construed as limiting the embodiments of the present invention, and it will be obvious to those skilled in the art that various changes and modifications can be made on the basis of the above description, and it is not intended to exhaust all embodiments, and obvious changes and modifications can be made on the basis of the technical solutions of the present invention.
Claims (10)
1. A method for analyzing volatile compounds in walnut oil comprises the following steps:
(1) adsorbing volatile compounds in the walnut oil by adopting a dynamic headspace sample preparation mode, wherein the adsorbing material is an electrostatic spinning fiber membrane, and the electrostatic spinning fiber membrane is selected from a polystyrene electrostatic spinning fiber membrane, an acrylic resin electrostatic spinning fiber membrane or a polystyrene-acrylic resin blending electrostatic spinning fiber membrane;
(2) carrying out thermal desorption on the adsorbed volatile compounds to obtain enriched volatile compounds;
(3) the enriched volatile compounds were analyzed using a gas chromatography-sniffing-mass spectrometer.
2. The analysis method according to claim 1, wherein the electrospun fiber membrane is used in an amount of 0.15-0.25% based on 100% by mass of the walnut oil;
preferably, the acrylic resin electrospun fiber membrane is an acrylic resin II electrospun fiber membrane.
3. The analysis method as claimed in claim 1, wherein the fiber diameter of the polystyrene electrospun fiber membrane is 300-800 nm;
preferably, the fiber diameter of the acrylic resin electrospun fiber membrane is 200-400 nm;
preferably, the fiber diameter of the polystyrene-acrylic resin blended electrostatic spinning fiber membrane is 200-900 nm.
4. The assay of claim 1, wherein an internal standard selected from the group consisting of 2-methyl-3-heptanone is added to the walnut oil during dynamic headspace sampling.
5. The assay of claim 1, wherein the dynamic headspace sampling comprises an incubation procedure and an adsorption procedure, the adsorption temperature being 50-60 ℃;
preferably, the incubation temperature is 50-55 deg.C, and the equilibration time is 10-15 min;
preferably, the nitrogen flow rate during the adsorption is 20-25mL/min, and the nitrogen flow rate after the adsorption is finished is 10-12 mL/min.
6. The analytical method as defined in claim 1, wherein the thermal desorption temperature is 240 ℃ and 250 ℃ and the thermal desorption time is 10-12 min.
7. The analytical method of claim 1, wherein step (2) further comprises: cryo-focusing the adsorbed volatile compounds in a solvent vent mode prior to the thermal desorption;
preferably, the cold trap temperature is-78 ℃ to-80 ℃, and the equilibration time is 0.5-1 min.
8. The analytical method of claim 1, wherein the gas chromatography conditions are as follows:
and (3) chromatographic column: j & W DB-WAX quartz capillary column, 30 mm × 0.25 μm;
temperature rising procedure: starting the column temperature of 40 ℃, keeping for 2min, heating to 75 ℃ at a speed of 5 ℃/min, heating to 150 ℃ at a speed of 2 ℃/min, and finally heating to 230 ℃ at a speed of 7 ℃/min, and keeping for 2 min; the temperature of the sample inlet is 230 ℃, and the temperature of the detector is 280 ℃; the carrier gas is nitrogen, and the flow rate is 3 mL/min; no flow split.
9. The analytical method of claim 1, wherein the mass spectrometry conditions are as follows:
electron bombardment with electron energy of 70eV, transmission line temperature of 280 deg.C, ion source temperature of 230 deg.C, quadrupole rod temperature of 150 deg.C, scanning mode of full scan, mass scanning range of 50-350m/z, and solvent delay of 5 min.
10. The assay of claim 1, wherein the assay is performed in a semi-quantitative manner using an internal standard;
preferably, the walnut oil is obtained by squeezing walnut kernels at the high temperature of 200-250 ℃.
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