A test method for simultaneously quantitatively testing the content of multiple microplastics

Abstract

This invention belongs to the field of microplastic content detection technology, specifically relating to a method for simultaneously and quantitatively testing the content of multiple microplastics. The method provided by this invention utilizes a thermogravimetric-infrared spectroscopy (TGA-IR) analyzer to simultaneously and quantitatively analyze multiple microplastics in a mixed sample, enabling rapid and accurate determination of the content of each microplastic.

Description

Technical Field

[0001] This invention belongs to the field of microplastic content detection technology, specifically relating to a test method for simultaneously and quantitatively testing the content of multiple microplastics. Background Technology

[0002] Microplastics typically refer to solid polymer particles smaller than 5 mm in size, with most of their raw materials derived from petroleum and natural gas. Due to their excellent corrosion resistance, microplastics are not easily degraded in a short period and can migrate with environmental changes. Furthermore, microplastics are characterized by their small size and large specific surface area, enabling them to adsorb various pollutants in the environment, such as heavy metals, polycyclic aromatic hydrocarbons (PAHs), and polychlorinated biphenyls (PCBs), thus posing a threat to ecosystems. Microplastic pollution has become a global environmental problem, and the development of analytical and identification methods is of great significance for the research and remediation of microplastic pollution.

[0003] Although the literature provides extensive analytical techniques for identifying microplastics in environmental samples, research on the quantitative detection of microplastics remains insufficient. Currently, quantitative characterization of microplastics mainly relies on imaging, spectroscopic, and thermal analysis techniques. Visual counting of microplastics is the most commonly used method for quantitative detection. After extracting microplastic fragments from the environmental system, they are observed with the naked eye or under a microscope, and classified and counted according to their size, shape, and color. However, the number of microplastic fragments in the environment is enormous, and relying solely on individual identification methods makes it difficult to obtain reliable statistical data and is prone to misjudgment. Fourier transform infrared spectroscopy and Raman spectroscopy are the most commonly used microplastic identification techniques, mainly identifying microplastics in environmental samples through specific absorption spectra. Combining spectroscopic techniques with imaging techniques can obtain information about particle size, shape, and number, but this method requires complex and time-consuming sample preparation. Furthermore, the above methods can only identify the overall content of microplastics, not the content of different microplastics.

[0004] The present invention aims to establish an analytical method for rapidly and simultaneously quantitatively testing the content of multiple microplastics. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the purpose of this invention is to provide a method for simultaneously and quantitatively testing the content of multiple microplastics.

[0006] To achieve the above objectives, the solution adopted by the present invention is as follows:

[0007] This invention provides a method for simultaneously and quantitatively testing the content of multiple microplastics, comprising the following steps:

[0008] Step (1): The mixed sample containing multiple microplastics is placed in the thermogravimetric analyzer of the thermogravimetric-infrared analyzer, and nitrogen is used as the carrier gas and protective gas respectively to raise the temperature of the thermogravimetric analyzer from 30°C to 600°C.

[0009] Step (2): While the heating program of the thermogravimetric analyzer is started, the infrared spectrometer in the thermogravimetric-infrared analyzer samples and analyzes the pyrolysis gas in the mixed sample to obtain the second derivative infrared spectrum. Based on the second derivative infrared spectrum, the peak area corresponding to the infrared characteristic absorption peak of each microplastic is obtained.

[0010] Step (3): Substitute the peak area corresponding to the infrared characteristic absorption peak obtained in step (2) into the linear equation of the standard curve corresponding to each microplastic to obtain the content of each microplastic in the mixed sample.

[0011] Preferably, in step (1), the mixed sample includes at least two of polystyrene, polyethylene, or polypropylene; the characteristic infrared absorption peak of the polystyrene is 773 cm⁻¹. -1 The infrared characteristic absorption peak of the polyethylene and polypropylene is 2946 cm⁻¹. -1 .

[0012] Preferably, the mixed sample is polystyrene, polyethylene, and polypropylene.

[0013] Preferably, in step (2), obtaining the peak area corresponding to the infrared characteristic absorption peak of each microplastic includes: fitting the peak area of ​​2946 cm⁻¹ with a Gaussian function. -1 The infrared characteristic absorption peaks at the point are separated to obtain the peak areas of the infrared characteristic absorption peaks corresponding to polyethylene and polypropylene, respectively.

[0014] Preferably, in step (1), the mixed sample is composed of polystyrene, polyethylene, and polypropylene; in step (2), obtaining the peak area corresponding to the infrared characteristic absorption peak of each microplastic includes obtaining 773 cm⁻¹. -1 The peak area of ​​the infrared characteristic absorption peak corresponding to polystyrene was calculated, and Gaussian function fitting was used to fit the peak area at 2946 cm⁻¹. -1 The infrared characteristic absorption peaks at the point are separated to obtain the peak areas of the infrared characteristic absorption peaks corresponding to polyethylene and polypropylene, respectively.

[0015] Preferably, in step (1), the heating rate of the thermogravimetric analyzer is 10℃ / min.

[0016] Preferably, in step (1), the flow rates of the carrier gas and the protective gas are 20 mL / min and 50 mL / min, respectively.

[0017] Preferably, in step (2), the temperature of the pyrolysis gas transmission line in the thermogravimetric-infrared analyzer is 270°C.

[0018] Preferably, in step (1), the content of each microplastic in the mixed sample is no more than 6 mg and no less than 0.5 mg.

[0019] According to the present invention, a method for simultaneously and quantitatively testing the content of multiple microplastics is provided. The mixed sample of multiple microplastics is composed of polystyrene, polyethylene, and polypropylene. The mixed sample of multiple microplastics is placed in the thermogravimetric analyzer of a thermogravimetric-infrared (TGA) system, using nitrogen as both the carrier gas and the protective gas. The temperature of the TGA system is raised from 30°C to 600°C. Simultaneously with the start of the TGA system's temperature rise program, the infrared spectrometer in the TGA system samples and analyzes the pyrolysis gases in the mixed sample. The characteristic infrared absorption peak of polystyrene is 773 cm⁻¹. -1 The infrared characteristic absorption peak of the polyethylene and polypropylene is 2946 cm⁻¹. -1 The second derivative infrared spectrum was obtained, and based on the second derivative infrared spectrum, the peak area corresponding to the infrared characteristic absorption peak of each microplastic was obtained, i.e., the 773 cm⁻¹ peak area was obtained. -1 The peak area of ​​the infrared characteristic absorption peak corresponding to polystyrene was calculated, and Gaussian function fitting was used to fit the peak area at 2946 cm⁻¹. -1 The infrared characteristic absorption peaks at the point are separated to obtain the peak areas of the infrared characteristic absorption peaks corresponding to polyethylene and polypropylene respectively. Finally, the peak areas corresponding to the infrared characteristic absorption peaks obtained above are substituted into the linear equation of the standard curve corresponding to each microplastic to obtain the content of each microplastic in the mixed sample of microplastics composed of polystyrene, polyethylene and polypropylene.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] This invention uses a thermogravimetric-infrared spectroscopy analyzer to simultaneously perform quantitative analysis of multiple microplastics in a mixed sample, and can quickly and accurately obtain the content of each microplastic. Attached Figure Description

[0022] Figure 1 The graphs are TG(a) and DTG(b) of PS, PP, and PE at a heating rate of 10℃ / min in Example 1 of the present invention.

[0023] Figure 2 The three-dimensional infrared spectra of the FTIR spectra of the PS(a), PP(b) and PE(c) pyrolysis products in Example 1 of the present invention as a function of temperature, and the FTIR spectrum (d) of the pyrolysis products at the maximum weight loss rate.

[0024] Figure 3The second derivative spectra of PS, PP, and PE in Embodiment 1 of the present invention are shown.

[0025] Figure 4 The TG / DTG curve (a) of the PS, PP, and PE mixture in Example 1 of the present invention and the pyrolysis products at 773 cm⁻¹ are shown. -1 And the curve of infrared absorption intensity at 2946 cm⁻¹ as a function of temperature (b);

[0026] Figure 5 In Embodiment 1 of the present invention, 2946 cm -1 Gaussian function fitting decomposition plot of spectral peaks at the band;

[0027] Figure 6 This is a standard curve diagram of PS(a), PP(b), and PE(c) in Embodiment 2 of the present invention. Detailed Implementation

[0028] The present invention will be further described below with reference to the embodiments. All intermediate alloys used in the present invention are commercially available products. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any way. Those skilled in the art can make various modifications and improvements without departing from the concept of the present invention. All equivalent transformations made using the present invention specification, or direct or indirect applications in other related technical fields, are included within the protection scope of this patent.

[0029] Example 1:

[0030] The pyrolysis process of polystyrene (PS), polyethylene (PE), or polypropylene (PP) was studied using thermogravimetric-infrared spectroscopy (TGA). 5 mg of each of PS, PP, and PE was placed in the crucible of the TGA analyzer. Nitrogen was used as both the carrier gas and protective gas. The temperature of the TGA analyzer was increased from 30 °C to 600 °C, and the TG and DTG curves of the PS, PP, and PE pyrolysis were obtained. Figure 1 As shown, the TG and DTG curves reveal that PS, PP, and PE exhibit similar weight loss patterns, all representing a one-stage pyrolysis process. Extrapolation was used to determine the initial and final pyrolysis temperatures of the three microplastics. PS, PP, and PE began decomposing at 392, 408, and 438℃, respectively, and completed decomposition at 430, 459, and 484℃, with maximum weight loss temperatures occurring at 416, 442, and 470℃. This indicates that PE exhibits the strongest thermal stability, followed by PP, and then PS. At 600℃, all microplastic polymers completely decomposed. Due to significant overlap in decomposition temperatures among the three microplastics, quantification cannot be achieved solely based on TGA data. Therefore, to accurately characterize and quantify microplastics, it is necessary to detect the thermal decomposition products to enhance quantitative criteria.

[0031] As the sample decomposes, the three-dimensional FTIR spectra of the pyrolysis products of PS, PP, and PE at different temperatures are as follows: Figure 2 As shown in (a) to (c). To obtain the optimal signal-to-noise ratio, Figure 2 (d) shows the FTIR spectra of the pyrolysis products of each microplastic at the maximum weight loss rate. Typical gas infrared characteristic absorption peaks and their assignments are shown in Table 1.

[0032] Table 1. Infrared peak analysis of pyrolysis products of PS, PP, and PE

[0033]

[0034] Infrared analysis results for PS, PP, and PE show significant overlap in their characteristic infrared absorption peaks. To enhance the characteristic peak signals and reduce background interference, the infrared spectra of the three microplastics at the maximum weight loss rate were converted into second derivative (SD) spectra. Figure 3 As shown, the characteristic peaks of PS, PP and PE in the SD spectrum exhibit higher recognizability.

[0035] By locally magnifying the SD spectrum, such as Figure 3 As shown, only PS is at 773cm. -1 A spike appears at this point, where the characteristic infrared absorption signal generated by the PS does not interfere with the infrared absorption signals of the PE and PP. Figure 2 This is also reflected in d, so 773cm can be selected. -1 This is a characteristic quantitative peak for PS. Additionally, at 2946 cm⁻¹... -1 At the 2946 cm⁻¹ band, PP and PE exhibit strong absorption peaks, while PS does not produce characteristic absorption peaks. To separate PE and PP, it is necessary to distinguish the overlapping infrared absorption peaks of the two microplastics. In this invention, Gaussian function fitting can be used to differentiate the infrared absorption peaks of PP and PE at 2946 cm⁻¹. -1 The spectral peaks at the specified locations are separated. The Gaussian function fitting for peak separation can employ existing methods and is not limited here.

[0036] In summary, the applicant's research has shown that a thermogravimetric-infrared spectroscopy (TGA-IR) analyzer can be used at a resolution of 773 cm⁻¹. -1 As a characteristic quantitative peak of PS, 2946 cm⁻¹ -1 As characteristic quantitative peaks of PP and PE, PS, PP, and PE can be quantitatively analyzed.

[0037] To further verify the rationality of this method, PS, PP, and PE were mixed and placed in the crucible of a thermogravimetric analyzer. Nitrogen was used as both the carrier gas and the protective gas. The thermogravimetric analyzer's temperature program was started, and the temperature was increased from 30℃ to 600℃. The TG and DTG curves of the pyrolysis of PS, PP, and PE were obtained, as shown below. Figure 4 As shown in the figure, the weight loss process of the mixed sample is divided into two stages: the first stage mainly involves the pyrolysis of PS and PP, and the second stage involves the pyrolysis of PE. Figure 4 b represents the mixed sample at 773 cm⁻¹ -1 and 2946cm -1 The correlation curve between infrared absorption and temperature change at a given location. Figure 4 b shows that 773cm -1 The infrared absorption exhibits only one sharp peak during the pyrolysis process, which is formed by the pyrolysis of PS. At 2946 cm⁻¹ -1 The infrared absorption peak is wider at 2946 cm⁻¹. Based on the above, this indicates that the absorption peak is at 2946 cm⁻¹. -1 The infrared absorption at 2946 cm⁻¹ can be considered as the result of PP and PE at this point. -1 The absorption peaks at 2946 cm⁻¹ are formed by superposition and fitted using a Gaussian function. Table 2 shows the absorption peaks at 2946 cm⁻¹. -1 The fitting decomposition diagram of the two Gaussian curve parameters obtained at the band is shown below. Figure 5 As shown. It can be seen that the original data ( Figure 5 The line L in the middle) and the fitted data ( Figure 5 The lines M in the model have a high degree of overlap, and the R-values ​​of the overlapping peak fitting decomposition are high. 2 The value is 0.998. The two spectral peaks obtained after fitting ( Figure 5 The midline (N) is located at 439℃ (corresponding to PP) and 468℃ (corresponding to PE), with peak areas of 0.85 and 3.13, respectively. Therefore, this method can distinguish between PS, PP, and PE, and a 773cm² peak can be selected. -1 and 2946cm -1 As a quantitative infrared characteristic absorption peak.

[0038] Table 2 2946cm -1 Gaussian peak parameters at the band

[0039]

[0040] Example 2:

[0041] Six PS, six PP, and six PE samples of different masses with a mass range of 0.5 mg to 6 mg were weighed and mixed to obtain six sets of standard samples.

[0042] The obtained standard sample was placed in the thermogravimetric analyzer of the thermogravimetric-infrared analyzer, with nitrogen as both the carrier gas and the protective gas. The heating program of the thermogravimetric analyzer was started, and the temperature of the thermogravimetric analyzer was raised from 30°C to 600°C.

[0043] Simultaneously with the start of the heating program in the thermogravimetric analyzer, the infrared spectrometer in the thermogravimetric-infrared coupled analyzer samples and analyzes the pyrolysis gas in the mixed sample 1, obtaining an infrared second derivative spectrum. Based on the infrared second derivative spectrum, a Gaussian function is used to fit the values ​​of PP and PE at 2946 cm⁻¹. -1 The spectral peaks at the specified locations were separated to obtain the peak areas corresponding to the infrared characteristic absorption peaks of PS, PP, and PE, respectively.

[0044] See Figure 6 The peak areas of the infrared characteristic peaks of PS, PP, and PE were correlated with their corresponding mass values. Quantitative analysis standard curves for PS, PP, and PE were plotted, yielding the linear equations for each: PS: y1 = 0.235x1 + 0.115; PP: y2 = 0.215x2 - 0.035; PE: y3 = 0.817x3 - 0.741. Figure 6 As shown, the correlation coefficient R of the standard curves for PS, PP, and PE is... 2 All values ​​are greater than 0.99, proving that within the mass range of 0.5–6 mg, there is a good linear relationship between the peak area of ​​the corresponding infrared characteristic absorption peak and the mass.

[0045] Example 3:

[0046] Weigh 3.91 mg PP, 2.37 mg PE and 2.51 mg PS and mix them evenly to obtain mixed sample 1. Place mixed sample 1 in the thermogravimetric analyzer of a thermogravimetric-infrared analyzer, use nitrogen as carrier gas and protective gas respectively, start the heating program of the thermogravimetric analyzer, and raise the temperature of the thermogravimetric analyzer from 30℃ to 600℃.

[0047] Simultaneously with the start of the heating program in the thermogravimetric analyzer, the infrared spectrometer in the thermogravimetric-infrared coupled analyzer samples and analyzes the pyrolysis gas in the mixed sample 1, obtaining an infrared second derivative spectrum. Based on the infrared second derivative spectrum, a Gaussian function is used to fit the values ​​of PP and PE at 2946 cm⁻¹. -1 The peaks at the specified locations were separated, and the peak areas corresponding to the infrared characteristic absorption peaks of PS, PP, and PE were obtained as 0.712, 0.804, and 1.248, respectively.

[0048] Substituting the peak areas corresponding to the infrared characteristic absorption peaks of PS, PP, and PE into the linear equation of the standard curve in Example 2, the masses of PS, PP, and PE can be calculated to be 3.90 mg, 2.44 mg, and 2.55 mg, respectively. The errors between the measured values ​​and the weighed values ​​are 0.26%, 2.90%, and 1.59%, respectively.

[0049] Example 4:

[0050] The difference between Example 4 and Example 3 is that the masses of PS, PP, and PE weighed are 1.64 mg, 3.02 mg, and 0.56 mg, respectively, and the test method is the same as in Example 3.

[0051] The calculated masses of PS, PP, and PE were 1.63 mg, 3.04 mg, and 0.55 mg, respectively. The errors between the measured values ​​and the weighed values ​​were 0.61%, 0.66%, and 1.79%, respectively.

[0052] As can be seen from Examples 3 and 4 above, the content of each microplastic in the mixed sample calculated using the testing method of the present invention is very close to the actual weighed mass, with an error of less than 3%. Therefore, the testing method of the present invention can simultaneously and accurately obtain the content of each microplastic in the mixed sample, especially the content of microplastics with similar structures such as PS, PP, and PE in the mixed sample.

[0053] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for simultaneously and quantitatively testing the content of multiple microplastics, characterized in that, Includes the following steps: Step (1): The mixed sample containing multiple microplastics is placed in the thermogravimetric analyzer of a thermogravimetric-infrared (TGA) system, using nitrogen as both the carrier gas and protective gas, and the temperature of the TGA system is raised from 30°C to 600°C; the mixed sample includes polystyrene, polyethylene, and polypropylene, and the characteristic infrared absorption peak of the polystyrene is 773 cm⁻¹. -1 The infrared characteristic absorption peak of the polyethylene and polypropylene is 2946 cm⁻¹. -1 ; Step (2): Simultaneously with the start of the heating program of the thermogravimetric analyzer, the infrared spectrometer in the thermogravimetric-infrared coupled analyzer samples and analyzes the pyrolysis gas in the mixed sample to obtain the second derivative infrared spectrum. The characteristic infrared absorption peak of the polystyrene is 773 cm⁻¹. -1 The infrared characteristic absorption peak of the polyethylene and polypropylene is 2946 cm⁻¹. -1 Based on the infrared second derivative spectrum, a Gaussian function was used to fit the 2946 cm⁻¹ spectrum. -1 The infrared characteristic absorption peaks at the location are separated to obtain the peak areas of the infrared characteristic absorption peaks corresponding to polyethylene and polypropylene, respectively. Step (3): Substitute the peak area corresponding to the infrared characteristic absorption peak obtained in step (2) into the linear equation of the standard curve corresponding to each microplastic to obtain the content of each microplastic in the mixed sample.

2. The method according to claim 1, characterized in that, In step (1), the heating rate of the thermogravimetric analyzer is 10°C / min.

3. The method according to claim 1, characterized in that, In step (1), the flow rates of the carrier gas and the protective gas are 20 mL / min and 50 mL / min, respectively.

4. The method according to claim 1, characterized in that, In step (2), the temperature of the pyrolysis gas transmission line in the thermogravimetric-infrared analyzer is 270°C.

5. The method according to claim 1, characterized in that, In step (1), the content of each microplastic in the mixed sample shall not exceed 6 mg and shall not be less than 0.5 mg.

Images