Preparation method of solid standard substance for micro-plastic quantitative analysis

Abstract

The invention provides a preparation method of a solid standard substance for micro-plastic quantitative analysis, and relates to the technical field of micro-plastic quantitative analysis. According to the method, silicon dioxide is used as a diluent, and a vortex mixing mode is adopted to prepare the uniformly mixed solid standard substance. The method does not need to use any solvent, so that the treatment step of preparing the solid-state standard substance is greatly simplified, and the expenditure cost of equipment and reagents is greatly reduced. The solid standard substance for micro-plastic quantitative analysis can be achieved only through simple vortex physical mixing, the preparation steps of the solid standard substance are simple, convenient and efficient, the solid standard substance is suitable for various micro-plastics, and the application range of quantitative analysis is effectively expanded.

Description

Technical Field

[0001] The present invention relates to the technical field of microplastic quantitative analysis, and in particular to a method for preparing a solid standard substance for microplastic quantitative analysis. Background Art

[0002] Microplastics (MPs) usually refer to plastic particles with a diameter of less than 5mm. They come from the decomposition of plastics into solid polymers such as particles and fragments with a size of less than 5mm due to physical, photochemical or biological degradation in the environment. Once MPs enter the human body, they will accumulate in different tissues and organs, which may trigger a series of toxic effects. Due to the brittle nature of plastics that are easily aged in the environment, accidental breakage of particles will change the number of particles and affect the calculation of number concentration. This fact means that the number of particles cannot be regarded as a conserved basic quantity, and the mass will not be affected by the physical and chemical processes that occur when MPs are exposed to the environment. Therefore, measuring the mass concentration of MPs can better quantify the pollution status of MPs in the environment.

[0003] Common methods for mass concentration analysis include pyrolysis-gas chromatography-mass spectrometry, thermal desorption-gas chromatography-mass spectrometry, and thermogravimetric analysis-gas chromatography-mass spectrometry. These methods all require the establishment of an accurate standard curve, which in turn requires the preparation of standard solutions with an appropriate mass range. MPs standard curves often require relatively low mass gradients between 0 and 100 μg, a range far exceeding the minimum weighing accuracy of 0.1 mg on a typical laboratory scale. Therefore, appropriate methods are required to dilute a certain mass of MPs standard to obtain a lower-mass solid calibration solution.

[0004] Currently, MPs standard solutions are mostly prepared using a high-temperature solvent dissolution and dilution method, which utilizes a rapid solvent extractor. The main steps are as follows: 0.5 mg of a mixed powder of various MPs (e.g., PE / PP / PS) is accurately weighed and placed in a 5 mL high-pressure extraction vessel. Static extraction is performed at 180°C and 1500 psi for 90 minutes. Three extraction cycles are performed using dichloromethane (DCM) as the solvent, with each extraction followed by a DCM rinse of 80% of the vessel volume. All extracts are combined in a 10 mL volumetric flask, brought to the mark with DCM, and ultrasonically dispersed for 15 minutes before storage in a brown bottle at -20°C to produce a 50 mg / L MPs mixed standard stock solution. This process requires not only expensive equipment but also high temperature and high pressure, resulting in high costs and complex experimental procedures. Furthermore, the dissolution process must ensure that the MPs are well dissolved in the solvent and that the solution does not contain noticeable cotton-like particles. However, achieving this ideal dissolution state is difficult in actual experiments. Flocculent solids are clearly visible in the solution, and the particles vary in size. Therefore, a large injection volume is required to mitigate the deviation caused by uneven particle distribution. Furthermore, MPs like polytetrafluoroethylene, which contain extremely strong carbon-fluorine bonds and are insoluble in all solvents, cannot be prepared using this method for standard solutions.

[0005] In view of the defects of the above-mentioned method for preparing MPs standard solution, a liquid nitrogen freezing ball milling mixed dilution method is proposed in the field to prepare MPs standard solution. The liquid nitrogen freezing ball milling method selects inert materials as diluents, and uses liquid nitrogen to freeze the mixture of microplastics and diluents below the glass transition temperature t g Under these conditions, the microplastics become brittle, making them easier to break during the subsequent ball milling process. To ensure a uniform mixture of the diluent and microplastics, the liquid nitrogen freezing and ball milling process needs to be repeated more than four times. However, liquid nitrogen freezing is not only demanding and complex, but also whether the MPs below the glass transition temperature will affect the subsequent quantitative results of thermal cracking remains to be investigated. In addition, during the experiment, it was found that ball milling causes the MPs to carbonize and turn black, which directly affects the accuracy of qualitative and quantitative analysis.

[0006] In view of this, it is necessary to design an improved method for preparing solid-state standard substances for quantitative analysis of microplastics to solve the above problems. Summary of the Invention

[0007] The purpose of the present invention is to provide a method for preparing a solid standard substance for quantitative analysis of microplastics.

[0008] To achieve the above-mentioned object of the invention, in a first aspect, the present invention provides a method for preparing a solid standard substance for quantitative analysis of microplastics, comprising the following steps:

[0009] Silicon dioxide is used as a diluent, and the diluent and microplastics are evenly mixed by vortex mixing to prepare a solid standard substance. The mass ratio of the diluent to the microplastics is (100:1)-(9:1).

[0010] Preferably, the vortex mixing time is 1-10 min.

[0011] Preferably, the microplastic is at least one of polypropylene, polyethylene, polystyrene, polyvinyl chloride, and polyurethane.

[0012] Preferably, the mass ratio of the diluent to the microplastic is 100:1 or 50:1 or 25:1 or 17:1 or 11:1 or 9:1.

[0013] Preferably, the vortex mixing time is 5 minutes.

[0014] Preferably, before the diluent and the microplastic are mixed, the diluent needs to be burned at 700° C. for 0.5 h.

[0015] In a second aspect, the present invention provides a solid standard substance for quantitative analysis of microplastics.

[0016] The beneficial effects of the present invention are:

[0017] 1. The preparation method provided by the present invention only requires simple vortex physical mixing, and has low physical and chemical requirements for microplastic standards. Therefore, it is also applicable to polytetrafluoroethylene, which cannot be quantified by conventional high-temperature solvent methods, thereby expanding the application range of quantitative analysis.

[0018] 2. The preparation method provided by the present invention uses silicon dioxide as a diluent. Silica has a relatively high melting point (1713°C) and is made of the same composition as the quartz filter membrane used in conventional sample processing. Microplastic mass concentration analysis must be performed at relatively high temperatures, reaching a maximum temperature of 700°C. The stability of silicon dioxide at high temperatures prevents chemical contamination during the measurement process, which could affect the accurate determination of microplastics.

[0019] 3. The preparation method provided by the present invention uses vortex mixing, does not require chemical solvents, and has simple experimental steps. It is economical and efficient, environmentally friendly and green, and can reduce the chemical pollution caused by traditional treatment methods and the changes in the physical and chemical properties of microplastics that may be caused during the treatment process. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is the single-standard quantitative standard curve for polyvinyl chloride (PVC) established in Example 1 of the present invention. DETAILED DESCRIPTION

[0021] In order to make the objectives, technical solutions and advantages of the present invention more clear, the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0022] It should also be noted here that, in order to avoid obscuring the present invention due to unnecessary details, only structures and / or processing steps closely related to the solutions of the present invention are shown in the drawings, while other details that are not closely related to the present invention are omitted.

[0023] In addition, it should be noted that the terms "comprises", "includes" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or apparatus that includes a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such process, method, article or apparatus.

[0024] The present invention provides a method for preparing a solid standard substance for quantitative analysis of microplastics, comprising the following steps:

[0025] Using silicon dioxide as a diluent, silicon dioxide and microplastics (MPs) are uniformly mixed in different mass ratios by vortex mixing to prepare solid standard substances with different mass concentrations. The concentration of the above-mentioned solid standard substance is 10-100 μg / mg, specifically 10 μg / mg, 20 μg / mg, 40 μg / mg, 60 μg / mg, 80 μg / mg, and 100 μg / mg, corresponding to the mass ratio of silicon dioxide to microplastics of 100:1, 50:1, 25:1, 17:1, 11:1, and 9:1, respectively. In other embodiments, the solid standard substance can also be other concentrations included in the above ranges.

[0026] In the above technical solution, by using silicon dioxide as a diluent, its chemical inertness can be fully utilized to avoid the occurrence of thermal decomposition and other adverse reactions during the subsequent mixing process, which may affect the experiment and interfere with the experiment. By using vortex mixing for mixing, not only can the silicon dioxide and microplastics be fully mixed, but the impact of the mixing process on the microplastics can also be reduced, avoiding the mixing process affecting the accuracy of qualitative and quantitative analysis. This is because: vortex mixing is a mechanical mixing method that achieves mixing by generating vortex flow through rapid rotation and high-frequency oscillation. Its core mechanism is to use centrifugal force to make the substances to be mixed spiral up along the container wall, and then return to the center under the action of gravity, thereby achieving full-space flow mixing without dead ends. For micron-sized particles, such as microplastics with a diameter of 6μm, the high-frequency oscillation characteristics of vortex mixing subject the powdered particles to periodic acceleration impacts, greatly reducing the friction between the particles and the van der Waals force, electrostatic force, etc. that cause agglomeration. In addition, in the microscopic state, the particles collide at high speed to produce a shear dispersion layer, directly destroying the agglomerates and achieving microscopic shear dispersion. Compared with the traditional ball mill that relies on medium collision, the mixing efficiency is low and it is very easy to introduce impurities. The vortex method directly transfers the contact layer powder through the container wall. This direct method enables energy to be transferred efficiently, avoiding the step-by-step loss of traditional stirring energy through the blades to the powder, achieving uniform mixing in a shorter time. At the same time, the container wall energy transfer method can avoid cross-contamination caused by the intervention of various stirring media. For traditional propeller agitators, most of the shear force is concentrated in the blade part, which will cause local uneven distribution and poor dispersion effect on micron-sized particles (<10μm). Secondly, an unavoidable problem in the above-mentioned traditional mixing method is the difference in density of different particles, which will cause the particles to sink and produce stratification. The vortex oscillation mixing method can generate an extremely strong acceleration field. Usually, the vortex radial acceleration can reach more than 10 times the acceleration of gravity, which makes the sedimentation speed of particles of different densities tend to be consistent, thereby suppressing stratification. In addition, the extremely strong radial acceleration can make the particle powders in different areas flow rapidly, forming large-scale vortex flow, improving the mixing effect and speeding up the mixing time.

[0027] In some embodiments, the mass ratio of silica to MPs is 100:1, 50:1, 25:1, 17:1, 11:1, or 9:1.

[0028] In some embodiments, microplastics include polypropylene, polyethylene, polystyrene, polyvinyl chloride, polyurethane, etc.

[0029] In some embodiments, the vortex mixing time is 30 s-30 min, preferably 5 min.

[0030] The preparation method of the calibration standard for quantitative analysis of microplastics proposed by the present invention is further described below with reference to specific examples:

[0031] Example 1

[0032] In this example, a polyvinyl chloride (PVC) single-standard calibration standard substance was prepared, and the preparation method thereof included the following preparation steps:

[0033] (1) Sample preparation:

[0034] To avoid interference from impurities and contact with any plastics throughout the process, avoid using any plastic containers for sampling. Use glassware, use volumetric flasks for sampling, use glass bottles for vortex samples, and seal them with tinfoil. All glassware needs to be ultrasonically cleaned for 6 minutes and rinsed with ultrapure water twice. Silica and quartz wool need to be sintered and sent to a muffle furnace for sintering at 700°C for 0.5 hours. Silica is analytical grade.

[0035] (2) Uniformity verification:

[0036] For the convenience of calculation, all proportions in this experiment are fixed at 1g of silica sampling, and then the corresponding mass of PVC is weighed according to the concentration of the required solid standard substance for mixing.

[0037] Thermogravimetric analysis is performed using a programmed temperature ramp. Because silica is heat-resistant, its mass remains unchanged during the heating process. However, the PVC mixed in it begins to crack at 280°C and is complete at 380°C. The mass loss of the mixture is therefore solely due to the loss of PVC. The DTG curve is used to determine the start and end points of the thermal weight loss, and the thermal weight loss is calculated. The thermal weight loss reveals the proportion of PVC in the sample, allowing for a quantitative characterization of sample homogeneity. Five replicate measurements are performed, and the relative standard deviation (RSD) is calculated. A homogeneous mixture is considered if the RSD is less than 5%.

[0038] Two ratios of 9:1 and 100:1 were taken for silica and PVC, respectively, and the vortex time uniformity was verified by thermogravimetric analysis. Mixing times of 1 min, 5 min, and 10 min were selected for analysis and discussion. The thermogravimetric analysis parameters were as follows: a heating rate of 30 ° C / min from 30 ° C to 700 ° C, nitrogen as the protective gas, and a flow rate of 30 ml / min. The experimental data analysis is shown in Table 1 below. The results show that among the three vortex times, when the vortex time is 5 min, the relative standard deviation (RSD) of the thermogravimetric data of the mixing ratios of 9:1 and 100:1 are 0.43% and 1.25% respectively. The results are smaller than the RSD values ​​of 1 min and 10 min, indicating that the sample uniformity of the 5 min vortex time is better than that of other times, and the data is more concentrated. In view of the principle of simplifying the experiment, it can be considered that the 5 min vortex time is the optimal vortex time.

[0039] For comparison, a common ball milling method was used: a mixture of silica and PVC at a mass ratio of 10:1 was prepared and subjected to thermogravimetric analysis for quantified homogeneity after vortexing for 5 minutes. The results of five replicated experiments revealed the following thermogravimetric losses: 8.22%, 7.13%, 5.90%, 8.10%, and 6.74%. The RSD for these five data sets was 13.43%, far exceeding the requirement of less than 5%, indicating uneven sample mixing. The RSD for vortexing after 5 minutes was 1.25%, further demonstrating the advantages of vortex mixing.

[0040] At the same time, two ratios, the maximum and the minimum, were selected for research, covering the mass concentration range of the quantitative standard curve. The purpose was to further optimize the experimental process. At a large ratio of 9:1, a good mixing effect can be obtained in 5 minutes. Common sense generally believes that if a large ratio can be mixed in 5 minutes, then a small ratio can also be mixed in 5 minutes. Therefore, the experiment found that the RSD of 5 minutes was 1.25% at a small ratio of 100:1, which proves that a 5-minute vortex time can also achieve a good mixing effect at a small ratio. In this way, when faced with various different ratios required for subsequent quantification, if the ratio is between this ratio (9:1-100:1), the vortex time of 5 minutes can be directly used for mixing, thereby further optimizing and simplifying the experimental process.

[0041] Table 1 Thermogravimetric analysis results of PVC at different vortex times

[0042] Ratio / vortex time Sample No. 1 Sample No. 2 Sample No. 3 Sample No. 4 Sample No. 5 RSD 9:1 / 1min 5.09％ 5.27％ 5.11％ 5.04％ 5.25％ 1.98％ 9:1 / 5min 5.91％ 5.93％ 5.94％ 5.98％ 5.94％ 0.43％ 9:1 / 10min 5.98％ 6.22％ 5.97％ 5.80％ 5.87％ 2.67％ 100:1 / 1min 0.85％ 0.91％ 0.81％ 0.72％ 0.81％ 8.45％ 100:1 / 5min 0.68％ 0.67％ 0.66％ 0.67％ 0.68％ 1.25％ 100:1 / 10min 0.68％ 0.68％ 0.66％ 0.69％ 0.66％ 1.99％

[0043] In addition, this example also explores the correctness and reliability of the optimal vortex time of 5 minutes, including the following three aspects: the uniformity of silica and PVC when mixed in different proportions, the uniformity of the sample at the optimal vortex time, and the linearity of the single PVC standard curve.

[0044] Silica and PVC were vortex mixed at mass ratios of 100:1, 67:1, and 50:1 for 5 minutes. Three replicates were prepared for each ratio, and the results shown in Table 2 were calculated. The RSDs of the gravimetric values ​​for the 100:1, 67:1, and 50:1 ratios were 3.48%, 1.47%, and 1.53%, respectively. The RSDs were small, indicating high precision. These results indicate that even at different ratios, vortex mixing for 5 minutes can achieve good mixing results when silica and PVC are mixed at low ratios.

[0045] Table 2 Thermogravimetric analysis results at different mixing ratios

[0046]

[0047] Five parallel experiments were conducted on a 50:1 sample (silica:PVC = 50:1) with a mixing time of 5 minutes. The calculated thermal weight loss values ​​are plotted in Table 3 below. The calculated RSD of the thermal weight loss was 2.48%, indicating high precision and low dispersion. The data collection demonstrates that the samples processed with a 5-minute vortex time have good uniformity.

[0048] Table 3 Thermogravimetric analysis results of PVC parallel samples

[0049] sample Thermal weight loss (%) PVC Sample No. 1 1.08 PVC Sample No. 2 1.13 PVC Sample No. 3 1.15 PVC sample No. 4 1.13 PVC No. 5 sample 1.10

[0050] Based on the above content, a single-standard PVC standard substance was prepared and tested by pyrolysis-gas analysis. The linear correlation coefficient R was calculated by establishing a standard curve. 2 This can be used to determine the feasibility of the proposed solution and indirectly verify sample homogeneity. Due to the limitations of the pyrolysis-GC / MS method, a maximum of 1 mg can be injected per pyrolysis run. Therefore, obtaining the mass corresponding to the standard curve requires preparing mixed samples of varying proportions. The pyrolysis instrument parameters are as follows: pyrolysis probe initial temperature 0°C, rate 20.00 ms / °C, 700°C hold for 40 seconds, transfer line and valve box temperature in the isothermal region both at 290°C, and programmable interface operating time of 3.5 minutes. The gas chromatograph-mass spectrometer analysis parameters are as follows: column oven initial temperature 50°C for 5 minutes, then ramp to 280°C at 5°C / min and hold for 5 minutes. Inlet temperature 280°C, pressure 7.655 psi, total flow rate 14 mL / min, and bulkhead purge flow rate 3 mL / min. A 10:1 split ratio was used. The mass spectrometer was in full scan mode, sampling ions with masses ranging from 15 to 450.

[0051] The mass of PVC (10μg, 20μg, 40μg, 60μg, 80μg, 100μg) was used as the horizontal axis, and the mass ratio of silica to PVC in the corresponding standard was (100:1, 50:1, 25:1, 17:1, 11:1, 9:1). The literature was investigated to determine that the characteristic product of PVC thermal decomposition was naphthalene. 128m / z was used as the quantitative ion, and a quantitative standard curve was drawn according to the quantitative ion peak area. The linear correlation coefficient R was calculated. 2 , the experimental results are as follows Figure 1 As shown, the linearity R 2 The linearity is 0.9906, which is good. Therefore, it can be proved that the injection volume maintains a linear growth. This result indirectly verifies the good uniformity of the sample, and then proves the feasibility of the vortex mixing method and the reliability of the optimal vortex time.

[0052] The above experimental results show that vortexing for 5 minutes can ensure that PVC samples of different proportions are mixed evenly. The linearity R of the single-standard standard material made under this time is 2The value is greater than 0.99, which indicates the feasibility and reliability of the vortex method for preparing calibration standard materials.

[0053] In summary, the method for preparing solid-state standard substances for quantitative analysis of microplastics proposed in the present invention is based on vortex mixing technology. Silica and MPs are mixed, and the uniformity of the mixed sample obtained by vortex mixing is quantitatively characterized by thermogravimetric analysis to determine the feasibility and reliability of the vortex scheme.

[0054] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a solid standard substance for quantitative analysis of microplastics, characterized in that: The steps include: Silicon dioxide is used as a diluent, and the diluent and microplastics are evenly mixed by vortex mixing to prepare a solid standard substance. The mass ratio of the diluent to the microplastics is (100:1)-(9:1).

2. The preparation method according to claim 1, characterized in that The vortex mixing time is 1-10 minutes.

3. The preparation method according to claim 1, characterized in that The microplastic is at least one of polypropylene, polyethylene, polystyrene, polyvinyl chloride, and polyurethane.

4. The preparation method according to claim 1, characterized in that The mass ratio of the diluent to the microplastic is 100:1 or 50:1 or 25:1 or 17:1 or 11:1 or 9:

1.

5. The preparation method according to claim 2, characterized in that The vortex mixing time was 5 min.

6. The preparation method according to claim 1, characterized in that Before the diluent is mixed with the microplastic, the diluent needs to be burned at 700° C. for 0.5 h.

7. A solid standard substance for quantitative analysis of microplastics obtained by the preparation method according to any one of claims 1 to 6.

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