A method for quantitatively detecting trace polyethylene terephthalate microplastics

Abstract

The present application relates to the technical field of microplastic detection, in particular to a method for quantitatively detecting trace polyethylene terephthalate microplastics. In the current research on PET microplastics based on Py-GC / MS, different masses of PET are mostly weighed by a balance to draw a standard curve, and the lowest quantification limit can only reach about 0.6 mu g. The present application combines solvent dissolution method and thermal cracking-gas chromatography mass spectrometry (Py-GCMS) technology, discloses a high-sensitivity and high-precision trace PET microplastic detection technology, reduces the quantification limit of PET microplastics from 0.6 mu g to 10 ng, shows the excellent sensitivity and potential of Py-GC / MS method in PET detection, and provides an effective analysis method for environmental tracking and monitoring of microplastic pollution.

Description

Technical Field

[0001] This invention relates to the field of microplastic detection technology, and in particular to a method for quantitatively detecting trace amounts of polyethylene terephthalate microplastics. Background Technology

[0002] Polyethylene terephthalate (PET) is the most important type of thermoplastic polyester, possessing excellent stability and widely used in the production of containers such as polyester fibers, films, mineral water bottles, and various plastic beverage bottles, making it essential to our daily lives. With the rapid development of the PET industry, the amount of waste generated is enormous. Under conditions of mechanical stress, light exposure, and natural microorganisms, PET exposed to the environment can break down and release microplastics (smaller than 5 mm), which has attracted widespread attention. Microplastics smaller than 1 mm pose certain risks to human health, and their environmental impact increases with decreasing particle size. Through the mass production and washing of synthetic fiber clothing, the indiscriminate disposal of plastic bottles, and other means, PET microplastics continuously enter the environment, becoming ubiquitous environmental pollutants in soil, oceans, freshwater, sediments, and organisms.

[0003] To assess the potential impact of PET microplastic pollution and develop corresponding management mechanisms, reliable analytical techniques are essential for detecting the mass concentration of PET microplastics in environmental matrices. Due to the influence of environmental matrices, the analysis of PET microplastics presents numerous challenges, and a standardized method is currently lacking. Therefore, overcoming the influence of complex environmental matrices to conduct qualitative and quantitative analysis of PET microplastics is particularly important.

[0004] Pyrolysis-gas chromatography-mass spectrometry (Py-GC / MS) technology has high specificity for the qualitative and quantitative analysis of PET microplastics. Polymers undergo pyrolysis under high-temperature, oxygen-free conditions in a pyrolysis furnace, producing characteristic pyrolysis products. If suitable pyrolysis conditions are controlled, these products can be separated by gas chromatography and detected by mass spectrometry for qualitative and quantitative analysis of the polymer. However, the sample volume is limited by the capacity of the pyrolysis vessel, generally not exceeding 10 mg. In previous studies, balances were often used to weigh different masses of PET to plot standard curves, but the limits of detection and quantitation are limited by the lowest accurate weighing value of the balance used; the quantitation limit of the standard sample can only reach a minimum of about 0.6 μg. Since the PET content in environmental samples is low, extensive sample pretreatment is required to increase the concentration to reach the instrument's detection limit. Complex pretreatment methods for enriching and concentrating large amounts of sample often lead to errors in the detection results. Therefore, methods for detecting PET in environmental samples are still under development. Summary of the Invention

[0005] To address the aforementioned problems, the present invention aims to provide a method for the quantitative detection of trace polyethylene terephthalate (PET) microplastics. This invention combines a solvent dissolution method with thermal pyrolysis-gas chromatography-mass spectrometry (Py-GCMS) to disclose a highly sensitive and accurate trace PET microplastic detection technique, providing an effective analytical method for environmental tracking and monitoring of microplastic pollution.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] This invention provides a method for quantitative detection of trace amounts of polyethylene terephthalate microplastics, specifically comprising the following steps:

[0008] (S1) Mix the concentration gradient PET microplastics with the derivatizing agent and internal standard to obtain the concentration gradient PET microplastic standard.

[0009] (S2) The concentration gradient PET microplastic standards prepared above were detected by thermal pyrolysis-gas chromatography-mass spectrometry. The vaporized products after thermal pyrolysis entered the gas chromatograph-mass spectrometer. A standard curve was plotted with the mass of the PET microplastic standard as the X-axis and the peak area of ​​dimethyl terephthalate as the Y-axis.

[0010] (S3) After mixing the sample powder with the derivatizing agent and internal standard, the sample is detected by thermal pyrolysis-gas chromatography-mass spectrometry. The vaporized product after thermal pyrolysis enters the gas chromatography-mass spectrometer. The peak area of ​​the obtained dimethyl terephthalate is substituted into the standard curve of step (S2) to obtain the content of PET microplastics in the sample.

[0011] The detection limit for PET microplastics obtained by this method is 10 ng / mL.

[0012] In one embodiment of the present invention, the solvent dissolution method in step (S1) is specifically as follows:

[0013] Dissolve PET microplastics in a solvent and heat until the PET microplastics are completely dissolved to obtain a PET standard solution mother liquor;

[0014] Volume gradient PET standard solution mother liquor was taken, dried, and concentration gradient PET microplastics were obtained.

[0015] In one embodiment of the present invention, the ratio of PET microplastics to solvent is 100 mg: 10 mL.

[0016] In one embodiment of the present invention, the solvent is a mixture of 1,1,2,2-tetrachloroethane and phenol.

[0017] In one embodiment of the present invention, the volume ratio of 1,1,2,2-tetrachloroethane to phenol is 1:1.

[0018] In one embodiment of the present invention, the heating process is carried out at a temperature of 120°C for 30 minutes.

[0019] In one embodiment of the present invention, the temperature during the drying process is 70°C.

[0020] In one embodiment of the present invention, in step (S1), the amount of the derivatizing agent added is 1 to 5 μL;

[0021] The amount of internal standard added is 0.5–5 μL.

[0022] In one embodiment of the present invention, the derivatizing agent is THMA;

[0023] The internal standard is anthracene-D10.

[0024] In one embodiment of the present invention, the pyrolysis conditions are as follows: pyrolysis at 300°C; gas chromatography-mass spectrometry detection conditions are: DB-5MS (30m*0.25mm*0.25μm), column oven temperature program is 50°C held for 1 min, then increased to 300°C at 10°C / min and held for 10 min; carrier gas is helium, column flow rate is 1 mL / min, injection port temperature is 300°C, split ratio is set to 10:1, transfer line temperature is 300°C, solvent delay is 3 min, mass spectrometry detection range is 33–500 amu, ion source temperature is 230°C, quadrupole temperature is 150°C, scanning mode is simultaneous full scan mode and selected ion scan mode; the qualitative spectral library is the NIST20 standard library.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) PET Quantification Limit Reduced to 10 ng: Currently, most Py-GC / MS-based studies on PET microplastics use a balance to weigh different masses of PET to plot a standard curve, with the lowest quantification limit reaching only about 0.6 μg. This invention proposes an improved testing method, using a solvent heating dissolution method to overcome the limitation of the lowest accurate weighing value of the balance, reducing the quantification limit of PET microplastics from 0.6 μg to 10 ng. This demonstrates the superior sensitivity and potential of the Py-GC / MS method in PET detection, making it more suitable for the quantitative detection of trace PET in environmental samples.

[0027] (2) Simplified sample pretreatment: This invention incorporates THMA as a derivatizing agent. The quantitative product of PET is induced by esterification, transesterification, and methylation processes of terephthalic acid under thermal decomposition conditions. Through online thermochemical reaction, the generated dimethyl terephthalate significantly improves the tailing of chromatographic peaks. This greatly improves the detection sensitivity of PET microplastics, simplifies the pretreatment steps for environmental samples, eliminates the need for complex pretreatment, and enables direct loading and testing of environmental samples.

[0028] (3) Use of internal standard: Pyrolysis is a rather complex process because most pyrolysis products are almost non-volatile or completely non-volatile. Non-volatile compounds accumulate within the system and may interact with the pyrolysis products of subsequent samples; the degree of interaction is related to the organic matter content in the sample. On the other hand, after routine maintenance (such as replacing the pyrolysis quartz tube), the original surfaces inside the Py-GCMS system may also interact differently with the pyrolysis products, leading to inaccurate quantification. To balance these different surface properties and reduce other interactions, anthracene-D10 is introduced as an internal standard for internal calibration of the pyrolysis process, significantly reducing the interaction between relevant compounds and byproducts derived from residual organic matter in the sample.

[0029] (4) The qualitative and quantitative detection method for PET of the present invention is rapid, requires only trace substrates, is stable, and can be used to cover the detection of microplastics of different concentrations by establishing a standard curve in one step. It provides a convenient and fast method for environmental tracking and monitoring of different PET microplastic pollution. The establishment of this detection method can provide scientific data for environmental tracking and monitoring of microplastic pollution, and provide a new perspective and guidance for the remediation and treatment of PET waste pollution. Attached Figure Description

[0030] Figure 1 The following are chromatograms of PET thermal decomposition under different conditions: a. without derivatizing agent, split ratio 10:1; b. with catalyst, split ratio 10:1; c. without derivatizing agent, split ratio 100:1; d. with catalyst, split ratio 100:1.

[0031] Figure 2 a is the mass spectrum of dimethyl terephthalate; Figure 2 b is the mass spectrum of anthracene-D10.

[0032] Figure 3 The results of six parallel determinations of PET content were obtained from activated sludge samples from three wastewater treatment plants. Detailed Implementation

[0033] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0034] Unless otherwise specified, all reagents used in the following embodiments are commercially available reagents, and all detection methods and techniques used are conventional detection methods and techniques in the art.

[0035] Example 1

[0036] This embodiment provides the thermal decomposition characteristics of PET.

[0037] 200 μg of PET standard was weighed into a pyrolysis vessel. Pyrolysis temperatures were set at 200℃, 300℃, 400℃, 500℃, 600℃, 700℃, 800℃, and 900℃, respectively. The sample underwent thermal decomposition in the pyrolysis furnace. Gas chromatography-mass spectrometry (GC-MS) conditions were as follows: DB-5MS (30m*0.25mm*0.25μm), column oven temperature program: 50℃ held for 1 min, increased to 300℃ at 10℃ / min, held for 10 min. Helium was used as the carrier gas, column flow rate was 1 mL / min, injection port temperature was 300℃, split ratio was 100:1, transfer line temperature was 300℃, solvent delay was 3 min, mass spectrometry detection range was 33–500 amu, ion source temperature was 230℃, quadrupole temperature was 150℃, and full scan mode was used. The qualitative library was the NIST20 standard library.

[0038] As the pyrolysis temperature rises, PET begins to decompose gradually from 400℃. For various polyester polymers, pyrolysis produces benzoic acid fragments, a common characteristic. Table 1 shows that a characteristic compound with a relatively large peak area is present in all pyrolysis products; this compound is identified as benzoic acid. At 500℃, the relative content of benzoic acid is 88.55%, decreasing with increasing temperature, reaching 3.79% at 800℃, and below the detection limit at 900℃. This indicates that for the same mass of PET microplastics, at a pyrolysis temperature of 500℃, more stable benzoic acid is formed after pyrolysis, improving detection accuracy and reducing energy consumption.

[0039] Table 1 Summary of Thermal Decomposition Results

[0040]

[0041] Example 2

[0042] This embodiment provides an analysis of the changes in the thermal decomposition products of PET after the addition of a derivatizing agent.

[0043] 50 μg of PET was weighed and TMAH (dissolved in 25% methanol, v / v) was used as a derivatizing agent. It was found that at a temperature of 300 °C, under the derivatization of THMA, PET thermally decomposed to generate a large amount of dimethyl terephthalate. Therefore, the thermal decomposition temperature in the subsequent examples was 300 °C.

[0044] Compare the changes in products before and after adding the derivatizing agent under different split ratios:

[0045] It was found that under the condition of a split ratio set to 10:1 (e.g.) Figure 1 a) and 1b) both produce characteristic thermal decomposition products of PET before and after the addition of the derivatizing agent. The former is benzoic acid, while the latter induces esterification, transesterification and methylation processes under thermal decomposition conditions. Through online thermochemical reaction, the generated dimethyl terephthalate accounts for an absolute dominant proportion of the product.

[0046] When the split ratio is adjusted to 100:1 and no derivatizing agent is added (e.g.) Figure 1 c) Due to severe tailing of benzoic acid, quantitative analysis was not possible; when the split ratio was adjusted to 100:1 and TMAH was used as the derivatizing agent, significant peaks still appeared even under the current conditions (e.g., Figure 1 d) can better quantify PET.

[0047] Therefore, this embodiment found that using TMAH can significantly improve the detection sensitivity of PET.

[0048] Example 3

[0049] This embodiment provides a method for quantitatively detecting trace amounts of polyethylene terephthalate microplastics, specifically including the following steps:

[0050] (S1) Weigh 100 mg of PET microplastics into a glass bottle, add 10 mL of solvent (1,1,2,2-tetrachloroethane:phenol = 1:1), heat at 120 °C for 30 min until the PET is completely dissolved, and obtain the PET standard solution stock solution (concentration of 10 mg / mL).

[0051] (S2) Using a micro-sampling needle, 2-5 μL of the PET standard solution mother liquor of different concentrations were added to the thermal pyrolysis cup to prepare PET standards with masses of 2 ng, 10 ng, 20 ng, 50 ng, 100 ng, 200 ng, 500 ng, 1000 ng, and 2000 ng respectively. The solvent in the pyrolysis cup was dried at 70℃ to obtain PET standards with concentration gradients.

[0052] (S3) Before testing, add 5 μL of THMA (dissolved in 25% methanol, v / v) as a derivatizing agent and 1 μL of anthracene-D10 as an internal standard. The mass spectrum of dimethyl terephthalate (DMT) is then analyzed. Figure 2 a) It can be seen that at m / z 163, the fragment peak intensity is the strongest, and the mass number is relatively large, which can eliminate the interference of a large number of low molecular weight fragment peaks generated after the thermal decomposition of other compounds. Anthracene-D10 was selected with m / z 188 as the quantitative ion based on the same principle. Figure 2 b).

[0053] (S4) The concentration gradient PET standards prepared in step (S2) above were detected using a thermal pyrolysis-gas chromatography-mass spectrometry (GC-MS) system. The vaporized products after thermal pyrolysis entered the GC-MS system. A standard curve was obtained with the mass of the PET standards as the X-axis and the peak area of ​​dimethyl terephthalate as the Y-axis. The thermal pyrolysis conditions were: thermal pyrolysis at 300℃; the GC-MS detection conditions were: DB-5MS (30m*0.25mm*0.25μm), and the column oven temperature program was: 50℃ for 1 min, then increased to 300℃ at 10℃ / min and held for 10 min. Helium was used as the carrier gas, the column flow rate was 1 mL / min, the injection port temperature was 300℃, the split ratio was set to 10:1, the transfer line temperature was 300℃, the solvent delay was 3 min, the mass spectrometry detection range was 33-500 amu, the ion source temperature was 230℃, the quadrupole temperature was 150℃, and the scanning mode was simultaneous full scan and selected ion scan; the qualitative spectral library was the NIST20 standard library.

[0054] The results were integrated using peak area at m / z = 163, and quantification was performed using the external standard method. The working curve was plotted with the ratio of the peak area of ​​the target compound to that of the internal standard on the ordinate and the concentration on the abscissa, yielding y = 1.98 * 10⁻⁶. -3 x + 1.49 * 10 -1 Correlation coefficient R 2 It reached 0.997.

[0055] When the mass of PET is 10 ng, the signal-to-noise ratio (S / N, m / z 163) of dimethyl terephthalate is 37.4 > 10, which can be used as the limit of quantitation for detecting the mass of PET in this method. This reflects the advantage of solvent-dissolved PET in plotting standard curves, and the minimum detection limit is no longer limited by the minimum weighing value of the balance.

[0056] Comparative Example 1

[0057] Compared to Example 3, this comparative example was identical except that anthracene-D10 was replaced with naphthalene-D8. The results showed that using naphthalene-D8 as an internal standard resulted in poor peak area stability. Before pyrolysis, the sample was suspended above the pyrolysis furnace using a sample hook. The furnace temperature was a preset pyrolysis temperature. Although the temperature at the suspended sample position was lower than that of the furnace, it still caused some volatilization of low-boiling-point substances. Naphthalene-D8, with a boiling point of 217.9℃, was affected by this, leading to unstable peak areas during subsequent testing.

[0058] Example 4

[0059] This embodiment provides an application of a method for quantitative detection of trace polyethylene terephthalate microplastics (testing of activated sludge samples).

[0060] Activated sludge from three different wastewater treatment plants was collected. Figure 3 The samples were labeled A, B, and C respectively, and homogenized (dried at 70℃ until the moisture content was less than 1‰, then ground into a uniform powder for later use) to obtain activated sludge samples. Six parallel activated sludge samples (1 mg each) were weighed, and 5 μL of THMA and 1 μL of anthracene-D10 internal standard were added for pyrolysis testing. The standard curve established in Example 3 was used to quantitatively calculate the PET microplastics in the activated sludge samples from the wastewater treatment plant, and the results are as follows. Figure 3 As shown, the average contents were 261.22 μg / g ± 69.56 μg / g, 595.80 μg / g ± 77.72 μg / g, and 103.54 μg / g ± 17.26 μg / g, respectively (and the repeatability was good).

[0061] Furthermore, in order to determine whether there was any loss during the entire homogenization, drying and grinding process, as well as during the process of the sample entering the pyrolysis target, a known amount of PET was added to the actual soil sample, and the soil sample was tested using the method of Example 3.

[0062] Specifically, approximately 1 mg of soil sample was weighed, 20 ng of PET standard was added, along with 5 μL of THMA and 1 μL of internal standard anthracene-D10, and a pyrolysis test was performed. The PET microplastics in the soil sample were quantitatively calculated using the standard curve established in Example 3, yielding a content of 18.71 ng. A standard recovery experiment was also conducted. After three replicates, the recovery rate was 71.88%.

[0063] Therefore, it can be demonstrated that the method provided in this application can achieve accurate quantitative detection of trace PET.

[0064] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the interpretation of the present invention, without departing from the scope of the invention, should be within the protection scope of the present invention.

Claims

1. A method for quantitatively detecting trace amounts of polyethylene terephthalate microplastics, characterized by, Specifically, the following steps are included: (S1) Preparation of concentration gradient polyethylene terephthalate microplastics by solvent dissolution method: concentration gradient PET microplastics; Then it was mixed with a derivatizing agent and an internal standard to obtain a concentration gradient PET microplastic standard. (S2) The concentration gradient PET microplastic standards prepared above were detected by thermal pyrolysis-gas chromatography-mass spectrometry. The vaporized products after thermal pyrolysis entered the gas chromatograph-mass spectrometer. A standard curve was plotted with the mass of the PET microplastic standard as the X-axis and the peak area of ​​dimethyl terephthalate as the Y-axis. (S3) After mixing the sample powder with the derivatizing agent and internal standard, the sample is detected by thermal pyrolysis-gas chromatography-mass spectrometry. The vaporized product after thermal pyrolysis enters the gas chromatography-mass spectrometer. The peak area of ​​the obtained dimethyl terephthalate is substituted into the standard curve of step (S2) to obtain the content of PET microplastics in the sample. The detection limit for PET microplastics obtained by this method is 10 ng / mL.

2. The method for quantitatively detecting trace polyethylene terephthalate microplastics according to claim 1, characterized in that, In step (S1), the solvent dissolution method is as follows: Dissolve PET microplastics in a solvent and heat until the PET microplastics are completely dissolved to obtain a PET standard solution mother liquor; Volume gradient PET standard solution mother liquor was taken, dried, and concentration gradient PET microplastics were obtained.

3. The method for quantitative detection of trace amounts of polyethylene terephthalate microplastics according to claim 2, characterized in that, The ratio of PET microplastics to solvent is 100 mg: 10 mL.

4. The method for quantitative detection of trace amounts of polyethylene terephthalate microplastics according to claim 2, characterized in that, The solvent is a mixture of 1,1,2,2-tetrachloroethane and phenol.

5. The method for quantitative detection of trace amounts of polyethylene terephthalate microplastics according to claim 2, characterized in that, The volume ratio of 1,1,2,2-tetrachloroethane to phenol is 1:

1.

6. The method for quantitative detection of trace polyethylene terephthalate microplastics according to claim 2, characterized in that, During the heating process, the temperature is 120℃ and the time is 30 minutes.

7. The method for quantitative detection of trace amounts of polyethylene terephthalate microplastics according to claim 2, characterized in that, The drying process is carried out at a temperature of 70℃.

8. The method for quantitative detection of trace amounts of polyethylene terephthalate microplastics according to claim 1, characterized in that, In step (S1), the amount of the derivatizing agent added is 1 to 5 μL; The amount of internal standard added is 0.5–5 μL.

9. The method for quantitative detection of trace amounts of polyethylene terephthalate microplastics according to claim 7, characterized in that, The derivative is selected from THMA; The internal standard is selected from anthracene-D10.

10. The method for quantitative detection of trace amounts of polyethylene terephthalate microplastics according to claim 1, characterized in that, The specific pyrolysis conditions are as follows: pyrolysis at 300℃; gas chromatography-mass spectrometry detection conditions are: DB-5MS (30m*0.25mm*0.25μm), column oven temperature program: 50℃ held for 1 min, increased to 300℃ at 10℃ / min, held for 10 min; carrier gas is helium, column flow rate is 1 mL / min, injection port temperature is 300℃, split ratio is set to 10:1, transfer line temperature is 300℃, solvent delay is 3 min, mass spectrometry detection range is 33–500 amu, ion source temperature is 230℃, quadrupole temperature is 150℃, scanning mode is full scan mode and selected ion scan mode simultaneously; the qualitative spectral library is the NIST20 standard library.

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