Method for detecting content of N, N-dimethylformamide
By using trichloroethylene or tetrachloroethylene as the extraction agent, the deviation of the measurement result caused by the strong volatility of the organic solvent in the prior art is solved, and a higher detection accuracy is achieved.
Patent Information
- Application Number
- CN202411715702.8
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-11-27
- Publication Date
- 2025-02-28
AI Technical Summary
In the existing gas chromatography-mass spectrometry analysis methods, when N,N-dimethylformamide is extracted using organic solvents such as acetone or methanol, the volatile nature of these solvents leads to a deviation from the actual results.
Trichloroethylene or tetrachloroethylene is used as the extraction agent, and the detection accuracy of N,N-dimethylformamide is improved by dissolution and ultrasonic extraction.
Because trichloroethylene or tetrachloroethylene has a high boiling point and good stability, it can effectively reduce losses during the extraction process and improve the accuracy of determining the content of N,N-dimethylformamide.
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Abstract
Description
Technical Field
[0001] The present application relates to the field of detection of N,N-dimethylformamide, and specifically to a method for detecting the content of N,N-dimethylformamide. Background Art
[0002] 1. N-dimethylformamide is an organic compound, a colorless transparent liquid. It is not only a widely used chemical raw material, but also an excellent solvent with a wide range of uses. It can be mixed with water and most organic solvents at will, and has good solubility for a variety of organic and inorganic compounds.
[0003] In the production of synthetic fibers, N,N-dimethylformamide plays an important role as a spinning solvent, such as in the wet spinning process of polyacrylonitrile fibers. However, due to the strong irritation of N,N-dimethylformamide, long-term contact may cause skin burns, erythema, blisters and other traumas, and even induce painful skin diseases such as dermatitis and eczema. Therefore, after N,N-dimethylformamide is used as a spinning solvent to prepare synthetic fibers, it is necessary to remove the residual N,N-dimethylformamide in the synthetic fibers. After removal, the synthetic fibers need to be tested to see if the residual N,N-dimethylformamide meets the standard.
[0004] Gas chromatography-mass spectrometry is an effective method for determining trace N, N-dimethylformamide. This method uses an inert gas as a mobile phase and utilizes the different distribution coefficients between the gas phase and the stationary phase of each component in the sample to be tested in the chromatogram, so that each component in the sample to be tested presents different chromatographic peaks in the chromatogram. According to the peak position and peak area of the sample to be tested, the content of N, N-dimethylformamide in the sample to be tested can be determined. However, during conventional gas chromatography-mass spectrometry, an extractant such as acetone or methanol is used. Acetone or methanol has a certain volatility at room temperature, so that when preparing a low-concentration N, N-dimethylformamide solution (such as 10 μg / mL), the actual concentration does not conform to the prepared concentration. And when extracting the sample, due to volatilization, the extraction effect is insufficient. In this case, the result of the final determination is caused to have a certain deviation from the actual N, N-dimethylformamide content. Summary of the invention
[0005] In order to solve the problem that when N,N-dimethylformamide is extracted with organic solvents such as acetone or methanol for gas chromatography-mass spectrometry analysis, due to its volatility at room temperature, the measurement result has a certain deviation from the actual result, the present application provides a method for detecting the content of N,N-dimethylformamide, which uses trichloroethylene or tetrachloroethylene to extract N,N-dimethylformamide. Since trichloroethylene or tetrachloroethylene has a higher boiling point and better stability, the final detection accuracy of N,N-dimethylformamide is higher.
[0006] In the first aspect, the present application provides a method for detecting the content of N,N-dimethylformamide, using the following technical solution: A method for detecting the content of N,N-dimethylformamide comprises the following steps: Prepare standard solution: weigh N,N-dimethylformamide, dissolve it in vinyl chloride and make up to volume to obtain standard solution; Dilution: dilute the standard solution step by step, and then analyze the diluted solution by gas chromatography-mass spectrometry to obtain the test working curve; Pretreatment: adding the vinyl chloride to the sample, heating and ultrasonic extraction, and obtaining an extract; Filtration analysis: filtering the extract, performing gas chromatography-mass spectrometry analysis, and comparing with the test working curve to obtain the N,N-dimethylformamide content in the sample; The chloroethylene includes at least one of trichloroethylene and tetrachloroethylene.
[0007] By adopting the above technical solution, if organic solvents such as acetone and methanol are used to dissolve N,N-dimethylformamide, due to the strong volatility of acetone and methanol, when diluting step by step, the actual concentration of low-concentration N,N-dimethylformamide is more likely to be higher than the prepared concentration, so that the accuracy of the test working curve finally obtained is reduced. In addition, when extracting N,N-dimethylformamide from the sample, due to the strong volatility of acetone and methanol, under the condition of heating and ultrasound, acetone and methanol are more likely to be in a volatile state, which may cause insufficient extraction of N,N-dimethylformamide in the sample, thereby causing a certain deviation between the test content and the actual content.
[0008] Chlorovinyl, whether trichloroethylene or tetrachloroethylene, has a high boiling point and is relatively stable at room temperature and under heating and ultrasonic conditions. When dissolving low concentrations of N,N-dimethylformamide, the actual concentration is closer to the prepared concentration. The N,N-dimethylformamide in the extracted sample can also be extracted as fully as possible. As a result, the final determination of the N,N-dimethylformamide content is more accurate.
[0009] The present application formulates a test working curve by dissolving N,N-dimethylformamide by vinyl chloride, extracts N,N-dimethylformamide in the sample by vinyl chloride, and then uses gas chromatography-mass spectrometry analysis. Compared with organic solvents such as acetone and methanol, the accuracy of determining the content of N,N-dimethylformamide in the sample can be improved.
[0010] Preferably, the chloroethylene is trichloroethylene, and in the pretreatment step, the volume ratio of the sample mass to the trichloroethylene is 1 g: (10-15) mL.
[0011] Preferably, the chloroethylene is tetrachloroethylene, and in the pretreatment step, the volume ratio of the sample mass to the tetrachloroethylene is 1 g: (8-12) mL.
[0012] By adopting the above technical solution, the volume ratio of sample mass to trichloroethylene or tetrachloroethylene is based on experimental data and practical experience, aiming to ensure that N,N-dimethylformamide in the sample can be fully extracted. The amount of solvent added needs to be large enough to cover the sample and fully contact it, so as to extract as much N,N-dimethylformamide in the sample as possible.
[0013] The boiling point of trichloroethylene is 87°C, while that of tetrachloroethylene is 121°C. This means that tetrachloroethylene will begin to boil at a higher temperature, so it is more stable. Since tetrachloroethylene is more stable, it can remain liquid at higher temperatures, so it has a longer contact time with the sample and a higher extraction efficiency. In contrast, trichloroethylene may begin to volatilize at lower temperatures, which may result in a reduced contact time with the sample, thereby affecting the extraction efficiency. To compensate for this deficiency, when using trichloroethylene, its addition amount needs to be increased to ensure sufficient contact time and extraction efficiency.
[0014] Preferably, the vinyl chloride is trichloroethylene and tetrachloroethylene, and in the pretreatment step, the ratio of the mass of the sample to the volume of the vinyl chloride is 1 g: (6-10) mL.
[0015] Preferably, the volume ratio of trichloroethylene to tetrachloroethylene is 1:2-4.
[0016] By adopting the above technical solution, the boiling point of trichloroethylene is 87°C, which is relatively low. The boiling point of tetrachloroethylene is 121°C, which is relatively high, showing better stability. Due to its lower boiling point, trichloroethylene can quickly reach the working state during the ultrasonic extraction process, making the extraction reaction more intense and helping to speed up the extraction speed. Due to its high boiling point, tetrachloroethylene shows better stability and can maintain a relatively constant concentration and state throughout the extraction process, ensuring the continuity and consistency of the extraction.
[0017] When trichloroethylene and tetrachloroethylene are mixed in a certain proportion, they can play their respective advantages and form a synergistic effect. The strong extraction effect of trichloroethylene can quickly separate N, N-dimethylformamide from the sample, while the stability of tetrachloroethylene can ensure the overall concentration and extraction effect of the extractant. The trichloroethylene in the mixture can accelerate the extraction process and improve the extraction efficiency. At the same time, the stability of tetrachloroethylene helps to reduce losses during the extraction process and ensure the effective use of the extractant. Since the mixture can play the advantages of trichloroethylene and tetrachloroethylene at the same time, the mixture can more accurately extract the N, N-dimethylformamide content in the sample compared to using either solvent alone. This helps to reduce experimental errors and improve the accuracy of test results.
[0018] After experimental testing and verification, it was found that when the volume ratio of trichloroethylene to tetrachloroethylene was 1:2-4, the mixture showed the best effect in extracting N,N-dimethylformamide. This ratio can ensure that the synergistic effect between the two can be fully exerted while avoiding the adverse effects caused by excessive use of one of the solvents.
[0019] Preferably, in the pre-treatment step, the ultrasonic conditions of the heating ultrasound are an operating frequency of 20-40 kHz and a time of 20-40 min.
[0020] By adopting the above technical solution, the higher the frequency of ultrasound, the weaker its penetration in biological tissues or solutions, but the effect may be more concentrated. On the contrary, the lower the frequency, the stronger the penetration, but the effect may be relatively dispersed. In the frequency range of 20-40kHz, ultrasound can better balance penetration and effect, and can not only act deeply inside the sample, but also maintain a certain intensity of action. In this frequency range, ultrasound can produce moderate cavitation, that is, tiny bubbles in the liquid vibrate, expand and collapse under the action of ultrasound, thereby releasing energy and promoting the dissolution and dispersion of the sample.
[0021] The ultrasonic treatment time needs to be long enough to ensure that the ultrasonic wave can fully act on the sample and promote its dissolution, dispersion and extraction. Within the time range of 20-40 minutes, the ultrasonic wave can continuously act on the sample to achieve the expected extraction effect. Long-term or high-intensity ultrasonic treatment may damage the sample or destroy its structure, thereby affecting the extraction effect and subsequent analysis.
[0022] Preferably, in the pre-treatment step, the heating condition of the heating ultrasound is a temperature of 55-65°C.
[0023] By adopting the above technical solution, the selected solvent (such as a mixture of trichloroethylene and tetrachloroethylene) has a specific boiling point. Heating within this temperature range can prevent the solvent from reaching the boiling point and volatilizing prematurely, thereby ensuring that the solvent maintains an effective concentration during the extraction process. The increase in temperature can increase the solubility of the solvent, allowing it to better dissolve the target compound (such as N,N-dimethylformamide) in the sample. In the temperature range of 55-65°C, the solubility of the solvent can usually be optimized, thereby improving the extraction efficiency.
[0024] Appropriate temperature conditions help maintain the stability of the sample and prevent unnecessary chemical reactions or structural changes during the extraction process. Increased temperature can promote the movement and interaction between molecules, thereby accelerating the transfer of target compounds from the sample to the solvent. This promotion effect is most significant in the temperature range of 55-65°C.
[0025] Preferably, the conditions of gas chromatography in the gas chromatography-mass spectrometry analysis are: Chromatographic column: DB-1701, 30m×0.25mm×0.25μm; carrier gas: helium (purity ≥99.999%), flow rate 0.5-1.5mL / min; injection port temperature: 220-250℃; column flow rate: 20-34mL / min; Column box heating program: initial column temperature 40-60℃, hold for 6 minutes, then increase the temperature to 100-140℃ at a rate of 25℃ / min, hold for 2 minutes, then increase the temperature to 200-240℃ at a rate of 15℃ / min, hold for 6 minutes.
[0026] Preferably, the injection method in the gas chromatography adopts splitless injection.
[0027] Preferably, the mass spectrometry conditions in the gas chromatography-mass spectrometry analysis are: Ion source temperature: 230°C; quadrupole temperature: 150°C; EI source bombardment energy: 70 eV; mass scanning range: 10-200 m / z; Solvent delay time: 5 min; Mass spectrometry running time: 15 min; Scan mode: full scan and selected ion scan; Ions 44, 87, 43, and 72 m / z were monitored, and 44 m / z was selected as the quantitative ion.
[0028] By adopting the above technical solution, gas chromatography-mass spectrometry analysis can be effectively performed, thereby effectively determining the content of N,N-dimethylformamide in the sample.
[0029] In summary, this application has the following beneficial effects: The present application formulates a test working curve by dissolving N,N-dimethylformamide by vinyl chloride, extracts N,N-dimethylformamide in the sample by vinyl chloride, and then uses gas chromatography-mass spectrometry analysis. Compared with organic solvents such as acetone and methanol, the accuracy of determining the content of N,N-dimethylformamide in the sample can be improved. DETAILED DESCRIPTION
[0030] The raw materials in this application include the following parts: 1. N-dimethylformamide: a commercially available product with CAS number 68-12-2; Trichloroethylene: a commercial product with CAS No. 79-01-6; Perchloroethylene: a commercial product with CAS number 127-18-4 is used; Dimethylacetamide: a commercially available product with CAS number 127-19-5; N-methylformamide: a commercially available product with CAS number 123-39-7; Deionized water: Use commercially available product with CAS number 7732-18-5; The above commercially available products are all chromatography grade products.
[0031] The present application is further described in detail below with reference to examples and comparative examples.
[0032] Example 1 Prepare standard solution: weigh 0.25 mg of N,N-dimethylformamide, dissolve it in 15 mL of trichloroethylene and dilute to 25 mL to obtain a 10 μg / mL standard solution; Dilution: dilute the 10μg / mL standard solution step by step to prepare a series of solutions of 2μg / mL, 4μg / mL, 6μg / mL, 8μg / mL, and 10μg / mL, and then perform gas chromatography-mass spectrometry analysis on the diluted solutions to obtain the test working curve; Pretreatment: Weigh 0.5 g N, N-dimethylformamide, 1 g dimethylacetamide and 1.5 g N-methylformamide and 997 g deionized water respectively, mix thoroughly to obtain a sample; add 1 g of the sample into 15 mL trichloroethylene, perform heating ultrasonic extraction, the heating temperature is 60°C, the working frequency is 30 kHz, and the working time is 30 min to obtain an extract; Filtration analysis: The extract was filtered through a 0.45 μm organic phase filter and then analyzed by gas chromatography-mass spectrometry. The N,N-dimethylformamide content in the sample was obtained by comparing it with the test working curve.
[0033] The conditions for gas chromatography-mass spectrometry analysis were: 1. Gas chromatography conditions are: Chromatographic column: DB-1701, 30m×0.25mm×0.25μm; carrier gas: helium (purity ≥99.999%), flow rate 1mL / min (0.5-1.5mL / min is possible, only 1mL / min is used as an example); injection port temperature: 250℃ (220-250℃ is possible, only 250℃ is used as an example); column flow rate: 34mL / min (20-34mL / min is possible, only 34mL / min is used as an example); Column box heating program: initial column temperature (40-60℃, 50℃ is used as an example), keep it for 6 minutes, then heat it to 120℃ (100-140℃, 120℃ is used as an example) at a rate of 25℃ / min, keep it for 2 minutes, then heat it to 220℃ (200-240℃, 220℃ is used as an example) at a rate of 15℃ / min, keep it for 6 minutes; Splitless injection was used.
[0034] 2. Mass spectrometry conditions are: Ion source temperature: 230°C; quadrupole temperature: 150°C; EI source bombardment energy: 70 eV; mass scanning range: 10-200 m / z; Solvent delay time: 5 min; Mass spectrometry running time: 15 min; Scan mode: full scan and selected ion scan; Ions 44, 87, 43, and 72 m / z were monitored, and 44 m / z was selected as the quantitative ion.
[0035] Embodiment 2-5 Example 2-5 is based on the preparation method of Example 1, and the amount of trichloroethylene added in the pretreatment step is adjusted. The specific adjustment is shown in Table 1.
[0036] Comparative Example 1-2 In Comparative Example 1, trichloroethylene was replaced with acetone in the preparation of the standard solution and the pretreatment step, and other conditions remained unchanged.
[0037] Comparative Example 2 In Comparative Example 2, trichloroethylene was replaced with methanol in the preparation of the standard solution and the pretreatment step, and other conditions remained unchanged.
[0038] Table 1. Amount of trichloroethylene added and performance test in the pretreatment steps of Examples 1-5 and Comparative Examples 1-2 project Example 1 Example 2 Example 3 Example 4 Example 5 Comparative Example 1 Comparative Example 2 Trichloroethylene / mL 15 8 10 12 17 / / Accuracy / % 95.6 90.4 92.8 93.3 95.6 84.7 83.2 Performance testing The samples of Examples 1-5 and Comparative Examples 1-2 were subjected to the following performance tests, and the test results are shown in Table 1.
[0039] The measured value of the N,N-dimethylformamide content in the sample is compared with the actual value of the N,N-dimethylformamide content in the sample, 0.5‰, and the accuracy of the measurement method is calculated as (measured value / 0.5‰)*100%.
[0040] Referring to Table 1, it can be seen from the comparison between Examples 1-5 and Comparative Examples 1-2 that the accuracy of Examples 1-5 is higher than that of Comparative Examples 1-2, indicating that trichloroethylene is more effective in extracting N,N-dimethylformamide than acetone or methanol. This may be because trichloroethylene has a higher boiling point and is more stable than acetone and methanol, resulting in a better extraction effect.
[0041] As the amount of trichloroethylene added continued to increase, the accuracy rate showed a trend of rising and then tending to a stable level. This may be because as the amount of trichloroethylene added continued to increase, it was able to cover the sample and fully contact it, thereby extracting as much N, N-dimethylformamide as possible from the sample and improving the accuracy rate.
[0042] Comparison of Examples 1-5 shows that Example 1 has the highest accuracy and a relatively low dosage. Therefore, Example 1 is preferred.
[0043] Embodiment 6-10 Example 6 Based on the preparation method of Example 1, the trichloroethylene in the preparation of the standard solution is replaced by tetrachloroethylene, and the 15 mL of trichloroethylene in the pretreatment step is replaced by 12 mL of tetrachloroethylene, and the other conditions remain unchanged.
[0044] In Examples 7-10, based on the preparation method of Example 6, the amount of tetrachloroethylene added in the pretreatment step was adjusted. The specific adjustments are shown in Table 2.
[0045] The samples of Examples 6-10 were subjected to the above performance tests, and the test results are shown in Table 2.
[0046] Table 2 The amount of perchloroethylene added and the performance test table of Example 1 and Examples 6-10 project Example 1 Example 6 Example 7 Example 8 Example 9 Example 10 Tetrachloroethylene / mL / 12 6 8 10 15 Accuracy / % 95.6 96.8 90.7 93.1 94.5 96.8 Referring to Table 2, it can be seen from the comparison between Example 1 and Examples 6-10 that tetrachloroethylene can also be used in the present application.
[0047] As the amount of tetrachloroethylene added continued to increase, the accuracy rate showed a trend of rising and then tending to a stable level. This may be because as the amount of tetrachloroethylene added continued to increase, it was able to cover the sample and fully contact it, thereby extracting as much N, N-dimethylformamide as possible from the sample and improving the accuracy rate.
[0048] Examples 11-15 Example 11 Based on the preparation method of Example 1, the trichloroethylene in the prepared standard solution is replaced with a mixture of trichloroethylene and tetrachloroethylene, the volume ratio of trichloroethylene to tetrachloroethylene in the mixture is 1:3, and the 15mL trichloroethylene in the pretreatment step is replaced with 10mL of the mixture of trichloroethylene and tetrachloroethylene, and the other conditions remain unchanged.
[0049] Examples 12-15 are based on the preparation method of Example 11, and the volume ratio of trichloroethylene and tetrachloroethylene in the mixture is adjusted. The specific adjustments are shown in Table 3.
[0050] The samples of Examples 11-15 were subjected to the above performance tests, and the test results are shown in Table 3 respectively.
[0051] Table 3 Volume ratio of trichloroethylene to tetrachloroethylene and performance test table of Example 1 and Examples 11-15 project Example 1 Embodiment 11 Example 12 Embodiment 13 Embodiment 14 Embodiment 15 Volume ratio / 1:3 1:1 1:2 1:4 1:5 Accuracy / % 95.6 98.1 97.0 97.6 97.8 97.2 Referring to Table 3, it can be seen from the comparison between Example 1 and Examples 11-15 that as the volume ratio of trichloroethylene to tetrachloroethylene in the mixture continues to decrease, the accuracy rate shows a trend of first increasing and then decreasing. This may be because as the volume ratio of trichloroethylene to tetrachloroethylene in the mixture continues to decrease, the content of tetrachloroethylene in the mixture continues to increase, which helps to reduce the loss in the extraction process and ensure the effective use of the extractant, so that the accuracy rate continues to increase. When it exceeds a certain range, the content of trichloroethylene accounts for a low proportion in the mixture, and trichloroethylene accelerates the extraction process and improves the extraction efficiency. The effect is low, which causes the accuracy rate to decrease.
[0052] Examples 16-19 In Examples 16-19, based on the preparation method of Example 11, the addition amount of the mixture of trichloroethylene and tetrachloroethylene was adjusted. The specific adjustments are shown in Table 4.
[0053] The samples of Examples 16-19 were subjected to the above performance tests, and the test results are shown in Table 4.
[0054] Table 4 Addition amount and performance test table of trichloroethylene and tetrachloroethylene mixture of Example 11 and Examples 16-19 project Embodiment 11 Example 16 Embodiment 17 Embodiment 18 Embodiment 19 Mixture / mL 10 4 6 8 12 Accuracy / % 98.1 96.1 97.2 97.8 98.1 Referring to Table 4, it can be seen from the comparison between Example 11 and Examples 16-19 that a mixture of trichloroethylene and tetrachloroethylene can also be used in the present application.
[0055] Compared with Example 11, Example 16 has a lower accuracy, and compared with Examples 17-19, the accuracy is higher, indicating that the mixture of trichloroethylene and tetrachloroethylene has a better effect, and a certain amount is also required to better extract N, N-dimethylformamide in the sample.
[0056] Examples 20-23 In Examples 20-23, based on the preparation method of Example 1, the conditions of heating ultrasound were adjusted. The specific adjustments are shown in Table 5.
[0057] The samples of Examples 20-23 were subjected to the above performance tests, and the test results are shown in Table 5 respectively.
[0058] Table 5 Conditions and performance test table of heating ultrasound of Example 1 and Examples 20-23 Referring to Table 5, it can be seen from the comparison between Example 1 and Examples 20-23 that within the frequency range of 20-40kHz, ultrasound can better balance penetration and effect, and can not only act deeply inside the sample, but also maintain a certain intensity of action. In this frequency range, ultrasound can produce moderate cavitation, that is, the tiny bubbles in the liquid vibrate, expand and collapse under the action of ultrasound, thereby releasing energy and promoting the dissolution and dispersion of the sample. Within the time range of 20-40min, ultrasound can continuously act on the sample to achieve the expected extraction effect.
[0059] Appropriate temperature conditions help maintain the stability of the sample and prevent it from undergoing unwanted chemical reactions or structural changes during the extraction process. Increased temperature can promote the movement and interaction between molecules, thereby accelerating the transfer of target compounds from the sample to the solvent.
[0060] This specific embodiment is merely an explanation of the present application and is not a limitation of the present application. After reading this specification, those skilled in the art may make modifications to the present embodiment without any creative contribution as needed, but such modifications are protected by the patent law as long as they are within the scope of the claims of the present application.
Claims
1. A method for detecting the content of N,N-dimethylformamide, characterized in that: The following steps are involved: Prepare standard solution: weigh N,N-dimethylformamide, dissolve it in vinyl chloride and make up to volume to obtain standard solution; Dilution: dilute the standard solution step by step, and then analyze the diluted solution by gas chromatography-mass spectrometry to obtain the test working curve; Pretreatment: adding the vinyl chloride to the sample, heating and ultrasonic extraction, and obtaining an extract; Filtration analysis: filtering the extract, performing gas chromatography-mass spectrometry analysis, and comparing with the test working curve to obtain the N,N-dimethylformamide content in the sample; The chloroethylene includes at least one of trichloroethylene and tetrachloroethylene.
2. The method for detecting the content of N,N-dimethylformamide according to claim 1, characterized in that: The chloroethylene is trichloroethylene. In the pretreatment step, the mass ratio of the sample to the volume of the trichloroethylene is 1 g: (10-15) mL.
3. The method for detecting the content of N,N-dimethylformamide according to claim 1, characterized in that: The chloroethylene is tetrachloroethylene. In the pretreatment step, the mass ratio of the sample to the volume of the tetrachloroethylene is 1 g: (8-12) mL.
4. The method for detecting the content of N,N-dimethylformamide according to claim 1, characterized in that: The chloroethylenes are trichloroethylene and tetrachloroethylene. In the pretreatment step, the ratio of the mass of the sample to the volume of the chloroethylene is 1 g: (6-10) mL.
5. The method for detecting the content of N,N-dimethylformamide according to claim 4, characterized in that: The volume ratio of trichloroethylene to tetrachloroethylene is 1:2-4.
6. The method for detecting the content of N,N-dimethylformamide according to claim 1, characterized in that: In the pre-treatment step, the ultrasonic conditions of the heating ultrasound are a working frequency of 20-40 kHz and a time of 20-40 min.
7. The method for detecting the content of N,N-dimethylformamide according to claim 6, characterized in that: In the pre-treatment step, the heating condition of the heating ultrasound is a temperature of 55-65°C.
8. The method for detecting the content of N,N-dimethylformamide according to claim 1, characterized in that: The conditions of gas chromatography in the gas chromatography-mass spectrometry analysis are: Chromatographic column: DB-1701, 30m×0.25mm×0.25μm; Carrier gas: helium (purity ≥ 99.999%), flow rate 0.5-1.5 mL / min; Inlet temperature: 220-250°C; Column flow: 20-34 mL / min; Column box heating program: initial column temperature 40-60℃, hold for 6 minutes, then increase the temperature to 100-140℃ at a rate of 25℃ / min, hold for 2 minutes, then increase the temperature to 200-240℃ at a rate of 15℃ / min, hold for 6 minutes.
9. The method for detecting the content of N,N-dimethylformamide according to claim 8, characterized in that: The injection method in the gas chromatography adopts splitless injection.
10. The method for detecting the content of N,N-dimethylformamide according to claim 8, characterized in that: The conditions of mass spectrometry in the gas chromatography-mass spectrometry analysis are: Ion source temperature: 230°C; quadrupole temperature: 150°C; EI source bombardment energy: 70 eV; mass scanning range: 10-200 m / z; Solvent delay time: 5 min; Mass spectrometry running time: 15 min; Scan mode: full scan and selected ion scan; Ions 44, 87, 43, and 72 m / z were monitored, and 44 m / z was selected as the quantitative ion.