A method for determining phthalimide residues in tea and soil

Abstract

This invention discloses a method for determining phthalimide (PI) residues in tea and soil, belonging to the technical field of phthalimide residue determination. The key technical points are: gas chromatography-tandem mass spectrometry (GC-MS / MS) is used to detect PI residues in soil and tea, and the PI content of tea samples from multiple locations in Guizhou Province was also detected. Samples were extracted with 1% acetic acid acetonitrile, purified by PSA and GCB, quantified using the external standard method, and detected by GC-MS / MS. This invention establishes a rapid, sensitive, and accurate method for detecting PI residues in dried and fresh tea leaves to monitor phthalimide residue levels in tea and identify pollution sources. The PI content of dried and fresh tea leaves produced in major tea-producing areas of Guizhou Province was detected, providing technical means and theoretical basis for the quality and safety of Guizhou tea products and tea exports. This method is simple to operate, and its accuracy, sensitivity, and precision meet the requirements for pesticide residue analysis, ensuring the accuracy of the detection results.

Description

Technical Field

[0001] This invention relates to the field of phthalimide residue determination technology, and more specifically, to a method for determining phthalimide residues in tea and soil. Background Technology

[0002] Phthalimide (PI) is a synthetic intermediate and degradation product of chemical pesticides such as the fungicide captan, the insecticide phosmet, and the herbicide bentazon. It can also be used to produce high-efficiency ion exchange resins, surfactants, and heavy metal extractants. PI is not an organic pesticide directly used to control pests, diseases, and weeds, and there are currently no related pesticide product registration records in tea gardens. According to the China Pesticide Information Network database, captan, chlorpyrifos, and phosmet are not currently registered for use on tea trees, while bentazon is registered for use on tea trees as a herbicide. The discovery of this type of compound in tea trees may originate from pesticides or other chemical industrial products (such as dyes and fertilizers) that use it as a raw material. Summary of the Invention

[0003] To monitor the residual level of phthalimide in tea and locate pollution sources, this invention provides a method for determining phthalimide residues in tea and soil. A rapid, sensitive, and accurate method for detecting PI residues in dried and fresh tea leaves has been established. The PI content of dried and fresh tea leaves produced in major tea-producing areas of Guizhou Province has been tested, providing technical means and theoretical basis for the quality and safety of Guizhou tea products and the export of tea.

[0004] The above-mentioned technical objective of this invention is achieved through the following technical solution: a method for determining phthalimide residues in tea and soil, wherein the reagents used in the method include: phthalimide standard, ultrapure water, chromatographically pure hexane, chromatographically pure acetic acid, analytically pure acetonitrile, analytically pure sodium chloride, analytically pure anhydrous sodium sulfate, graphitized carbon black, N-propylethylenediamine, and a 0.22 μm nylon organic phase filter membrane; the instruments used in the method include: gas chromatography-mass spectrometry, HP-5MS column, FW100 micro tissue grinder, benchtop high-speed centrifuge, SK-1 vortex mixer, HGC-24A nitrogen blower, CP213 electronic balance, BSA224S-CW electronic balance, graduated cylinder, pipette, volumetric flask, and centrifuge tube;

[0005] The determination method includes the following steps:

[0006] S1: Preparation of the matrix standard working solution;

[0007] S2: Soil pretreatment and tea pretreatment;

[0008] S3: The phthalimide residue in the sample obtained by the S2 treatment was determined using gas chromatography-tandem mass spectrometry.

[0009] The present invention is further configured such that the solution in S1 is prepared by the following steps:

[0010] S11: Accurately transfer 1.0 mL of 1000 mg / L acetone phthalimide standard solution using a pipette, prepare a 100 mg / L standard stock solution with n-hexane-acetone solution at a volume ratio of 1:1, and store it in a refrigerator at 4°C for later use.

[0011] S12: Before use, accurately transfer an appropriate amount of the stock solution prepared in S11 and dilute it with n-hexane-acetone solution to prepare a series of standard solutions with mass concentrations of 0.005 mg / L, 0.01 mg / L, 0.05 mg / L, 0.1 mg / L, 0.5 mg / L, and 1 mg / L.

[0012] S13: Accurately transfer 1.0 mL of each standard solution obtained in S12 and dry the solvent with nitrogen gas;

[0013] S14: Accurately add 1.0 mL of soil and tea blank matrix extract to each standard solution obtained in S13 to obtain the corresponding series of matrix standard working solutions. The tea blank matrix extract is either dry tea blank matrix extract or fresh tea blank matrix extract.

[0014] S15: Transfer 0.5 mL of acetic acid to 500 mL of acetonitrile, mix thoroughly by sonication, and set aside for later use.

[0015] The present invention is further configured such that the pretreatment of the soil in S2 includes the following steps:

[0016] S2a1: Accurately weigh 5.00±0.02g of the soil sample after crushing, mixing and sieving into a 50mL glass centrifuge tube;

[0017] S2a2: Add 10 mL of cold water at 4℃ to the glass centrifuge tube in S2a1, then add 10 mL of 1% acetic acid acetonitrile solution, mechanically shake for 5 min, add 5 g of sodium chloride, continue shaking for 5 min, then centrifuge at 2000 r / min for 5 min, and take the supernatant for purification.

[0018] S2a3: Accurately transfer 2.0 mL of the supernatant obtained from S2a2 into a glass centrifuge tube containing 300 mg of anhydrous sodium sulfate and 50 mg of PSA, vortex for 30 s and then centrifuge at 2000 r / min for 5 min;

[0019] S2a4: Accurately transfer 1.8 mL of the supernatant from centrifugation of S2a3 and blow it dry with nitrogen. Make up to 0.9 mL with chromatographically pure hexane, filter through a 0.22 μm organic phase membrane, and then inject the sample.

[0020] The present invention is further configured such that the pretreatment of tea leaves in S2 includes the following steps:

[0021] S2b1: Accurately weigh 2.00±0.02g of the pulverized and mixed tea sample into a 50mL glass centrifuge tube;

[0022] S2b2: Add 5 mL of 4℃ cold water to the glass centrifuge tube in S2b1, then add 10 mL of 0.1% acetic acid acetonitrile solution, mechanically shake for 5 min, add 5 g of sodium chloride, continue shaking for 5 min, then centrifuge at 2000 r / min for 5 min, and take the supernatant for purification.

[0023] S2b3: Accurately transfer 2.5 mL of the supernatant obtained from S2b2 into a glass centrifuge tube containing 200 mg anhydrous sodium sulfate, 100 mg PSA, and 200 mg GCB dried tea leaves, or into a glass centrifuge tube containing 200 mg anhydrous sodium sulfate, 250 mg PSA, and 200 mg GCB fresh tea leaves. Vortex for 30 seconds and then centrifuge at 2000 r / min for 5 minutes.

[0024] S2b4: Take all the supernatant obtained by centrifuging S2b3 and blow it dry with nitrogen. Make up the volume to 1.0 mL with acetone-n-hexane solution with a volume ratio of 1:1. Filter the solution through a 0.22 μm organic phase membrane before injection.

[0025] The present invention is further configured such that the instrument conditions of the chromatography-mass spectrometer used in S3 are as follows;

[0026] Chromatographic conditions: 30m × 250μm × 0.25μm HP-5MS column; injection port temperature 260℃; injection mode: splitless injection; injection volume 1.0μL; temperature program: initial 60℃, hold for 1 min; increase to 170℃ at 40℃ / min; increase to 210℃ at 10℃ / min; increase to 280℃ at 5℃ / min; carrier gas: helium; flow rate 1.4mL / min; auxiliary heating temperature 260℃;

[0027] Mass spectrometry conditions: Ion source temperature 230℃; Quadrupole temperature 150℃; Mass spectrometry scan mode: Selected ion scan; Parent / daughter ion m / z: Phthalimide 76 / 104, 76 / 147, 76 * .

[0028] In summary, the present invention has the following beneficial effects:

[0029] In order to monitor the residual level of phthalimide in tea and find the source of pollution, this invention establishes a rapid, sensitive and accurate method for detecting PI residues in dried and fresh tea leaves. The PI content of dried and fresh tea leaves produced in the main tea-producing areas of Guizhou was tested, providing technical means and theoretical basis for the quality and safety of Guizhou tea products and the export of tea.

[0030] The analytical method of this invention meets the requirements for pesticide residue analysis in terms of accuracy, sensitivity, and precision, and can ensure the accuracy of the test results. Attached Figure Description

[0031] Figure 1a shows the effect of using n-hexane as an extractant on the PI peak shape in an embodiment of the present invention.

[0032] Figure 1b shows the effect of using acetonitrile as an extractant on the PI peak shape in an embodiment of the present invention;

[0033] Figure 1c shows the effect of using acetic acid acetonitrile as an extractant on the PI peak shape in an embodiment of the present invention;

[0034] Figure 2a is a chromatogram of phthalimide in a blank fresh tea leaf matrix in an embodiment of the present invention;

[0035] Figure 2b is a chromatogram of phthalimide in acetone:n-hexane (volume ratio 1:1) in an embodiment of the present invention. Detailed Implementation

[0036] To enable those skilled in the art to better understand the present invention, the technical solution of the present invention will be described in further detail below with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are merely some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort should fall within the scope of protection of the present invention.

[0037] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0038] Example:

[0039] A method for determining phthalimide residues in tea and soil, comprising the following reagents: phthalimide standard (1000 mg / L), purchased from Dr. Ehrenstorfer; ultrapure water; chromatographically pure n-hexane (MERCK); chromatographically pure acetic acid (Shanghai Shenbo Chemical Co., Ltd.); analytically pure acetonitrile (Shanghai Xingke); analytically pure sodium chloride and analytically pure anhydrous sodium sulfate (Sinopharm Chemical Reagent Co., Ltd.); graphitized carbon black (GCB) (Agela Technologies); N-propylethylenediamine (PSA) (Agela Technologies); and a 0.22 μm nylon organic phase filter membrane (Bona Agela Technologies Co., Ltd.).

[0040] The instruments used included gas chromatography-mass spectrometry (GC-MS), an HP-5MS column (30m×250μm×0.25μm); an FW100 micro tissue homogenizer (Tianjin Tester); a benchtop high-speed centrifuge (Changsha Maijia); an SK-1 vortex mixer (Jiangsu Zhongda); an HGC-24A nitrogen evaporator (Tianjin Hengao); a CP213 electronic balance (0.001~200g) (Ohaus); a BSA224S-CW electronic balance (0.1mg~220g) (Sartorius); and commonly used laboratory instruments and consumables such as graduated cylinders, pipettes, volumetric flasks, and centrifuge tubes.

[0041] The determination method includes the following steps:

[0042] S1: Preparation of the matrix standard working solution;

[0043] Accurately pipette 1.0 mL of phthalimide solution (1000 mg / L, purchased from Dr. Ehrenstorfer) in acetone and prepare a 100 mg / L standard stock solution with n-hexane-acetone solution (1:1 v / v). Store at 4°C until use. Immediately before use, accurately pipette an appropriate amount of the stock solution and dilute with n-hexane-acetone solution to prepare a series of standard solutions with mass concentrations of 0.005, 0.01, 0.05, 0.1, 0.5, and 1 mg / L. Accurately pipette 1.0 mL of each standard solution, dry the solvent with nitrogen gas, and then accurately add 1.0 mL of blank matrix extracts (soil, dried tea leaves, and fresh tea leaves) to obtain the corresponding series of matrix standard working solutions. 0.1% acetic acid-acetonitrile solution: Pipe 0.5 mL of acetic acid into 500 mL of acetonitrile, mix ultrasonically, and set aside.

[0044] S2: Soil pretreatment and tea pretreatment;

[0045] Soil pretreatment:

[0046] Accurately weigh (5.00±0.02) g of the pulverized, mixed, and sieved soil sample into a 50 mL glass centrifuge tube, add 10 mL of 4℃ cold water, then add 10 mL of 1% acetic acid acetonitrile solution, mechanically shake for 5 min, add 5 g of sodium chloride, continue shaking for 5 min, centrifuge at 2000 r / min for 5 min, and collect the supernatant for purification.

[0047] Accurately transfer 2.0 mL of the supernatant into a glass centrifuge tube containing 300 mg of anhydrous sodium sulfate and 50 mg of PSA. Vortex for 30 s, then centrifuge at 2000 r / min for 5 min. Accurately transfer 1.8 mL of the supernatant to dryness under nitrogen, and dilute to 0.9 mL with chromatographic grade n-hexane. Filter the solution through a 0.22 μm organic phase filter membrane before injection.

[0048] Tea pretreatment:

[0049] Accurately weigh (2.00±0.02) g of the pulverized and mixed tea sample into a 50 mL glass centrifuge tube, add 5 mL of cold water at 4℃, then add 10 mL of 0.1% acetic acid acetonitrile solution, mechanically shake for 5 min, add 5 g of sodium chloride, continue shaking for 5 min, then centrifuge at 2000 r / min for 5 min, and take the supernatant for purification.

[0050] Accurately transfer 2.5 mL of the supernatant into a glass centrifuge tube containing 200 mg anhydrous sodium sulfate, 100 mg PSA, and 200 mg GCB (dried tea leaves) or 200 mg anhydrous sodium sulfate, 250 mg PSA, and 200 mg GCB (fresh tea leaves). Vortex for 30 s, then centrifuge at 2000 r / min for 5 min. Take all the supernatant and blow it dry with nitrogen. Make up the volume to 1.0 mL with acetone-n-hexane (volume ratio = 1:1) solution. Filter the solution through a 0.22 μm organic phase filter membrane before injection.

[0051] S3: The phthalimide residue in the sample obtained after S2 treatment was determined using gas chromatography-tandem mass spectrometry. Instrument conditions were as follows:

[0052] Chromatographic conditions: HP-5MS column (30m × 250μm × 0.25μm); injection port temperature 260℃; injection mode: splitless injection; injection volume 1.0μL; temperature program: initial 60℃, hold for 1 min; increase to 170℃ at 40℃ / min; increase to 210℃ at 10℃ / min; increase to 280℃ at 5℃ / min; carrier gas: helium; flow rate 1.4mL / min; auxiliary heating temperature 260℃;

[0053] Mass spectrometry conditions: Ion source temperature 230℃; Quadrupole temperature 150℃; Mass spectrometry scan mode: Selected ion scan; Parent / daughter ion m / z: Phthalimide 76 / 104, 76 / 147, 76 * .

[0054] Analysis of test results

[0055] (1) Optimization of instrument conditions and pretreatment conditions

[0056] A 5 mg / L PI standard solution was prepared, and instrument conditions were optimized under full scan mode and a scan range (m / z) of 50–500. First, the precursor ion and retention time of PI were determined for full scan. Then, the target compound was qualitatively identified based on the standard library comparison search results and the abundance ratio of fragment ions. In selected ion mode, ion pairs with high signal-to-noise ratio, good peak shape, and low interference were selected as quantitative ions. PI has strong polarity; as shown in Figure 2, peak tailing occurred in the HP-5MS column. Peak shapes were compared using acetonitrile, n-hexane, acetone, and acetone:n-hexane (V:V=1:1) as solvents. Using acetone:n-hexane (V:V=1:1) as solvent effectively alleviated peak tailing, and the peak shape was sharper and more symmetrical.

[0057] The tea matrix has a complex composition. Adding deionized water during pretreatment ensures thorough wetting and mixing of the matrix, guaranteeing complete extraction of the target analyte. Regarding the extraction solvent, the recovery rate and peak shape of PI were compared using hexane, acetonitrile, and 0.1% acetonitrile acetate as the extraction solvent. The QuEChERS method based on an acetate buffer system using acetonitrile as the extraction solvent can improve pesticide recovery. Simultaneously, acidification of the extraction solvent ensures the stability of the target analyte, thus improving extraction efficiency. PI is soluble in glacial acetic acid and pyridine. Based on the principle of "like dissolves like," an appropriate amount of ionically strong acetate buffer is added to the extraction solvent to increase PI solubility. Figures 1a and 1b show that the PI peak shape is symmetrical and sharp under the 0.1% acetonitrile acetate extraction solvent, and both the recovery rate and RSD% meet the analytical requirements.

[0058] Regarding the selection of purifying agents, PSA mainly removes fatty acids, organic acids, pigments, and some sugars from the matrix, while GCB has a good removal effect on pigments and sterols, especially on colored vegetable and fruit matrices. Table 1 shows a comparison of the recovery rates of different ratios of the two agents. Combining the color of the purified solution with the recovery rate, it can be seen that increasing the PSA content does not significantly change the color of the extracted solution after adsorption and purification, but it effectively improves the recovery rate of PI. GCB has the best purification effect on the color of the tea matrix; when the GCB dosage reaches 200 mg or more, the matrix color turns pale yellow, and further increases in dosage do not significantly change the color, but the recovery rate begins to decrease significantly (71%). In summary, the purifying agent ratio for the dry tea pretreatment process was tested using 200 mg anhydrous sodium sulfate, 100 mg PSA, and 200 mg GCB; the fresh tea extract has a darker color, so the dosage of PSA was increased, and the ratio was tested using 200 mg anhydrous sodium sulfate, 250 mg PSA, and 200 mg GCB.

[0059] Table 1 Comparison of PI recovery rates with different purifying agent ratios

[0060]

[0061] (2) Standard curve, limit of quantitation, correlation coefficient

[0062] The solvent, blank soil, and blank tea matrix samples were analyzed using the method described above. Standard solutions of phthalimide in solvent and soil matrix with concentrations ranging from 0.01 to 5.0 mg / L were prepared, along with standard solutions of phthalimide in dried and fresh tea matrix with concentrations ranging from 0.005 to 5.0 mg / L. Linear regression was performed using the peak area y (for the quantification of the target pesticide) against the corresponding concentration x (mg / L) to obtain the linear equation for phthalimide in different matrices. The results showed that the correlation coefficient (R²) of phthalimide in soil and tea within the range of 0.005–5.0 mg / L was above 0.99. Matrix effect refers to the influence of other substances that significantly interfere with the analysis of the analyte during sample extraction, affecting the accuracy of the determination results. Matrix effects are typically evaluated using the ratio of the slopes of the matrix standard curve to the solvent standard curve. A ratio less than 0.8 indicates a matrix inhibition effect; a ratio between 0.8 and 1.2 indicates a weak matrix effect (i.e., no matrix effect); and a ratio greater than 1.2 indicates a matrix enhancement effect. Table 2 shows that the matrix effects of PI obtained using the current pretreatment method in soil, dried tea leaves, and fresh tea leaves are 0.9447, 0.4286, and 0.0404, respectively, falling between 0.8 and 1.2, indicating a weak matrix effect (which can be considered as no matrix effect). The matrix effects in dried tea leaves and fresh tea leaves are 0.4286 and 0.0404, respectively, indicating a matrix inhibition effect. This demonstrates that the above method can ensure the accuracy of the measurement results. To ensure the stability of the experimental process and the accuracy of the detection results, a matrix-matched standard curve is used to quantify PI to weaken the matrix inhibition effect.

[0063] Table 2. Linear range, linear equation, and correlation coefficient of phthalimide in tea leaves and soil matrix.

[0064]

[0065] (3) Recovery rate and precision

[0066] The accuracy of this method was examined through addition and recovery experiments on different substrates, including soil and tea. Precision was measured using the relative standard deviation (RSD) at each mass fraction level in the addition and recovery experiments. The recoveries and RSDs of PI in soil and different tea substrates are shown in the table below. The results showed that the average recovery rate of PI in soil was 77%–100%, with an RSD of 8.14%–8.96%; the average recovery rate of PI in dried tea leaves was 74%–86%, with an RSD of 7.74%–14.12%; and the average recovery rate of PI in fresh tea leaves was 73%–108%, with an RSD of 3.45%–10.13%. The limits of quantitation (LOQ) for PI in all three substrates was 0.01 mg / kg, which is lower than the maximum residue limit for captan in tea in the EU (the total amount of captan is the sum of twice the results of captan and PI, which is 0.1 mg / kg). The recovery rate and precision of this method meet the detection requirements of the "Guidelines for Pesticide Residue Testing in Crops".

[0067] Table 3. PI recovery rate and precision in different matrices (n=5)

[0068]

[0069] (4) Actual tea sample test results from various parts of Guizhou

[0070] A total of 121 tea samples were collected from various parts of Guizhou Province. The results of PI (potassium iodide) testing are shown in Table 4. All results were calculated according to the EU standard's limit for captan residue in tea, which is the sum of the detected captan and twice the PI (maximum residue limit is 0.1 mg / kg). Among the dry tea samples, PI was detected in 19 samples, with the PI content (twice the LOQ) ranging from <0.1 mg / kg to 0.37 mg / kg. The detection rate according to the captan residue standard was 63.33%, and 12 samples exceeded the EU maximum captan residue limit, accounting for 40.00%. Among the fresh tea samples, PI was detected in 46 samples, with the PI content (twice the LOQ) ranging from <0.1 mg / kg to 0.235 mg / kg. The detection rate according to the captan residue standard was 50.55%, and 16 samples exceeded the EU maximum captan residue limit, accounting for 17.58%. PI was detected to varying degrees in both dry and fresh tea leaves, but the content in the tea garden soil was below the detection limit.

[0071] Table 4. Detection results of phthalimide in Guizhou tea.

[0072]

[0073] Table 5 shows the distribution of tea content detection results across different regions. In Region 1, 12 out of 19 tea samples tested positive for PI (pigmented polyoxin), with content ranging from <LOQ to 0.226 mg / kg, a detection rate of 63.16%. Six samples exceeded the EU maximum limit for captan, accounting for 31.58%. In Region 2, 6 out of 15 tea samples tested positive for PI, with content ranging from <LOQ to 0.096 mg / kg, a detection rate of 40.00%. No samples exceeded the EU maximum limit for captan. In Region 3, 4 out of 10 tea samples tested positive for PI, with content ranging from <LOQ to 0.17 mg / kg, a detection rate of 40.00%. Three samples exceeded the EU maximum limit for captan, accounting for 30.00%. In Region 4, 5 tea samples tested positive for PI with content ranging from <LOQ to 0.095 mg / kg. Among the samples from different regions, none exceeded the EU maximum limit for captan. In region 5, 30 out of 56 tea samples tested positive for PI, with levels ranging from <LOQ to 0.37 mg / kg (detection rate 53.57%), and 17 samples exceeded the EU maximum limit for captan (30.36%). In region 6, 9 out of 16 tea samples tested positive for PI, with levels ranging from <LOQ to 0.202 mg / kg (detection rate 56.25%), and 2 samples exceeded the EU maximum limit for captan (12.5%). These results indicate that the residue levels of PI in tea vary across different regions of Guizhou, with an overall high detection rate.

[0074] Table 5 Distribution of Phthalimide Content in Tea from Various Regions of Guizhou

[0075]

[0076] This specific embodiment is merely an explanation of the present invention and is not intended to limit the invention. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they are within the scope of the claims of the present invention.

Claims

1. A method for determining phthalimide residues in tea and soil, characterized in that: The reagents used in the assay include: phthalimide standard, ultrapure water, chromatographic grade n-hexane, chromatographic grade acetic acid, analytical grade acetonitrile, analytical grade sodium chloride, analytical grade anhydrous sodium sulfate, graphitized carbon black, N-propylethylenediamine, and a 0.22 μm nylon organic phase filter membrane; the instruments used in the assay include: gas chromatography-mass spectrometry, HP-5MS column, FW100 micro tissue homogenizer, benchtop high-speed centrifuge, SK-1 vortex mixer, HGC-24A nitrogen evaporator, CP213 electronic balance, BSA224S-CW electronic balance, graduated cylinder, pipette, volumetric flask, and centrifuge tubes; The determination method includes the following steps: S1: Preparation of the matrix standard working solution; S2: Soil pretreatment and tea pretreatment: The soil pretreatment includes the following steps: S2a1: Accurately weigh 5.00±0.02g of the soil sample after crushing, mixing and sieving into a 50mL glass centrifuge tube; S2a2: Add 10 mL of cold water at 4℃ to the glass centrifuge tube in S2a1, then add 10 mL of 1% acetic acid acetonitrile solution, mechanically shake for 5 min, add 5 g of sodium chloride, continue shaking for 5 min, then centrifuge at 2000 r / min for 5 min, and take the supernatant for purification. S2a3: Accurately transfer 2.0 mL of the supernatant obtained from S2a2 into a glass centrifuge tube containing 300 mg of anhydrous sodium sulfate and 50 mg of PSA, vortex for 30 s and then centrifuge at 2000 r / min for 5 min; S2a4: Accurately transfer 1.8 mL of the supernatant after centrifugation of S2a3 and blow it dry with nitrogen, then dilute to 0.9 mL with chromatographically pure hexane, filter through a 0.22 μm organic phase membrane, and wait for injection. The pretreatment of the tea leaves includes the following steps: S2b1: Accurately weigh 2.00±0.02g of the pulverized and mixed tea sample into a 50mL glass centrifuge tube; S2b2: Add 5 mL of 4℃ cold water to the glass centrifuge tube in S2b1, then add 10 mL of 0.1% acetic acid acetonitrile solution, mechanically shake for 5 min, add 5 g of sodium chloride, continue shaking for 5 min, then centrifuge at 2000 r / min for 5 min, and take the supernatant for purification. S2b3: Accurately transfer 2.5 mL of the supernatant obtained from S2b2 into a glass centrifuge tube containing 200 mg anhydrous sodium sulfate, 100 mg PSA and 200 mg GCB dried tea leaves, or into a glass centrifuge tube containing 200 mg anhydrous sodium sulfate, 250 mg PSA and 200 mg GCB fresh tea leaves. Vortex for 30 s and then centrifuge at 2000 r / min for 5 min. S2b4: Take all the supernatant obtained by centrifuging S2b3 and blow it dry with nitrogen. Make up the volume to 1.0 mL with acetone-n-hexane solution with a volume ratio of 1:

1. Filter the solution through a 0.22 μm organic phase membrane before injection. S3: The phthalimide residue in the sample obtained by the S2 treatment was determined using gas chromatography-tandem mass spectrometry.

2. The method for determining phthalimide residues in tea and soil according to claim 1, characterized in that: The solution in S1 is prepared through the following steps: S11: Accurately transfer 1.0 mL of 1000 mg / L acetone phthalimide standard solution using a pipette, prepare a 100 mg / L standard stock solution with n-hexane-acetone solution at a volume ratio of 1:1, and store it in a refrigerator at 4°C for later use. S12: Before use, accurately transfer an appropriate amount of the stock solution prepared in S11 and dilute it with n-hexane-acetone solution to prepare a series of standard solutions with mass concentrations of 0.005 mg / L, 0.01 mg / L, 0.05 mg / L, 0.1 mg / L, 0.5 mg / L and 1 mg / L. S13: Accurately transfer 1.0 mL of each standard solution obtained in S12 and dry the solvent with nitrogen gas; S14: Accurately add 1.0 mL of soil or tea blank matrix extract to each standard solution obtained in S13 to obtain the corresponding series of matrix standard working solutions. The tea blank matrix extract is either dry tea blank matrix extract or fresh tea blank matrix extract. S15: Transfer 0.5 mL of acetic acid to 500 mL of acetonitrile, mix thoroughly by sonication, and set aside for later use.

3. The method for determining phthalimide residues in tea and soil according to claim 1, characterized in that: The instrument conditions for the chromatography-mass spectrometer used in S3 are as follows: Chromatographic conditions: 30m×250μm×0.25μm HP-5MS column; injection port temperature 260℃; injection mode: splitless injection; injection volume 1.0μL; The temperature program was as follows: initial temperature 60℃, held for 1 min; temperature increased to 170℃ at 40℃ / min; temperature increased to 210℃ at 10℃ / min; temperature increased to 280℃ at 5℃ / min; carrier gas: helium; flow rate 1.4 mL / min; auxiliary heating temperature 260℃. Mass spectrometry conditions: Ion source temperature 230℃; Quadrupole temperature 150℃; Mass spectrometry scan mode: Selected ion scan; Parent / daughter ion m / z: Phthalimide 76 / 104, 76 / 147, 76 * .

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