Determination of dioctyl terephthalate content in nylon plastics

Abstract

The invention discloses a method for detecting the content of dioctyl terephthalate in nylon plastic. The method comprises the following steps: firstly, dissolving the nylon plastic with N,N-dimethylformamide (DMF) to completely extract the nylon and dioctyl terephthalate into a solvent; then, dropping an excess of methanol as a main precipitant to precipitate nylon polymer molecules; then, adding acetonitrile as an auxiliary precipitant to perform secondary precipitation to more thoroughly precipitate small polymer molecules in a supernatant; filtering the supernatant with a 0.22-μm organic filter membrane to remove the polymer molecules; and finally, determining the content of dioctyl terephthalate in the nylon plastic by GC-MS. The method solves the problem of inaccurate detection results caused by solvent extraction in the prior art, solves the problem of excessively long processing time due to multiple purifications required in the solvent extraction method, improves the responsiveness of the target object on a chromatogram, and protects the chromatographic system.

Description

Technical field:

[0001] The present invention relates to the field of plastic product detection, in particular to a method for detecting the content of dioctyl terephthalate in nylon plastic. Background technology:

[0002] Polyamide, commonly known as nylon, is an important thermoplastic engineering plastic. It boasts excellent properties such as high mechanical strength, low density, good self-lubrication, oil resistance, wear resistance, and ease of molding and processing. It is widely used in the automotive, electrical and electronic, household appliances, and mechanical parts industries. However, ordinary nylon resin suffers from poor impact resistance, which limits its application. To meet the needs of industrial development, nylon requires toughening and modification. Plasticization is one method of improving nylon's toughness. Plasticizers weaken the adhesion between polymer molecules and reduce the resistance to the movement of molecular chains, thereby acting as a lubricant and improving macroscopic fluidity.

[0003] Dioctyl terephthalate (DOTP) is a new plasticizer. Compared to the commonly used di(2-ethyl) phthalate (DEHP), DOTP offers advantages such as heat resistance, low volatility, flexibility, and good insulation properties. In finished products, DOTP exhibits excellent durability, soap and water resistance, and low-temperature flexibility. It is widely used in various industrial materials. Furthermore, DOTP is not restricted by the European Union and other countries for phthalate plasticizers, making it an increasingly popular alternative to phthalates. However, DOTP has potential chronic toxicity, teratogenicity, mutagenicity, and strong endocrine disrupting properties, necessitating the use of detection technologies to determine its content in food contact materials.

[0004] Gas chromatography-tandem mass spectrometry and liquid chromatography-tandem mass spectrometry are currently the common methods for detecting the content of dioctyl terephthalate in nylon in China. The pre-treatment of samples before detection using these methods is mostly to crush the sample and extract the target substance by ultrasonic extraction using organic solvents such as methanol or ethanol. However, these extraction methods are difficult to guarantee the extraction effect, and the extraction time is too short, the target substance is not completely extracted, and the dioctyl terephthalate inside the nylon plastic cannot be extracted, that is, the requirements for accurate quantitative determination cannot be met, resulting in inaccurate test results. In addition, the solution extracted using solvent extraction will subsequently have polymer molecules precipitated, and it needs to be processed multiple times by appropriate purification means, which has long processing time, low efficiency, and is also prone to contamination of the chromatography-mass spectrometry system during detection.

[0005] There is an urgent need for a method that can efficiently, quickly and accurately detect the content of dioctyl terephthalate in nylon. Summary of the invention:

[0006] The invention provides a method for detecting the content of dioctyl terephthalate in nylon plastic. The invention establishes a "dissolution-primary precipitation-auxiliary precipitation" method based on a "dissolution-precipitation" method, wherein N,N-dimethylformamide (DMF) is first added to dissolve the nylon plastic, then an excess of methanol (primary precipitant) is added dropwise to precipitate nylon polymer molecules, and then acetonitrile (auxiliary precipitant) is used to completely remove the polymer molecules. Finally, a GC-MS method is used to detect the content of dioctyl terephthalate in the nylon plastic. The method solves the problem of inaccurate detection results caused by a solvent extraction method in the prior art, and solves the problem that the solvent extraction method in the prior art requires multiple purification treatments after extraction, resulting in long processing time and low efficiency, and that residual nylon polymer molecules during detection are prone to contaminating a chromatography-mass spectrometry system.

[0007] The present invention is achieved through the following technical solutions:

[0008] A method for detecting the content of dioctyl terephthalate in nylon plastic comprises the following steps: respectively taking a sample to be tested and a standard working solution for GC-MS analysis, wherein the preparation of the sample to be tested comprises the following steps: crushing the nylon plastic, adding N,N-dimethylformamide (DMF), shaking until the nylon plastic is completely dissolved, adding methanol dropwise while shaking until no polymer continues to precipitate, diluting the volume to a calibration mark with methanol, thoroughly shaking the solution in the bottle and letting it stand, taking the supernatant and filtering it through a 0.22 μm filter membrane to obtain a filtrate, adding acetonitrile dropwise to the filtrate until precipitation is complete, diluting the volume to a calibration mark with acetonitrile, thoroughly shaking the solution in the bottle and letting it stand, filtering it through a 0.22 μm filter membrane, and concentrating the solution to 1 mL with nitrogen blow-through for testing;

[0009] GC conditions were as follows: chromatographic column: HP-5MS UI capillary column; column flow rate: 1.2-1.8 mL / min; injection mode: splitless mode; temperature program: 120°C for 1 min, then heating to 280-320°C at 28-32°C / min and holding for 5 min; injection volume: 0.8-1.2 μL; injection port temperature: 300°C; carrier gas: helium;

[0010] The mass spectrometry conditions were as follows: solvent delay: 5 min; ion source: EI source at 70 eV; ion source temperature: 230°C; quadrupole temperature: 150°C; mass spectrometry interface temperature: 300 m / z; detection mode: selected ion mode (SIM); qualitative ions: 167 m / z, 261 m / z, 221 m / z; quantitative ion: 149 m / z.

[0011] Preferably, the nylon plastic is crushed into small pieces of (4-6) mm x (4-6) mm.

[0012] Preferably, the shaking is performed using a vortex shaker.

[0013] Preferably, the capillary column has a specification of 30 m×0.25 mm and a diameter of 0.25 μm.

[0014] The preparation of standard working solution includes the following steps:

[0015] 1) Preparation of a 1000 mg / L dioctyl terephthalate stock solution: Accurately weigh 10 mg (accurate to 0.1 mg) of dioctyl terephthalate into a 10 mL volumetric flask and dilute to the mark with N,N-dimethylformamide (DMF) to obtain a 1000 mg / L dioctyl terephthalate stock solution.

[0016] 2) Preparation of a 10 mg / L dioctyl terephthalate standard intermediate solution: Pipette 0.1 mL of the dioctyl terephthalate stock solution into a 10 mL volumetric flask and dilute to volume with N,N-dimethylformamide (DMF) to obtain a 10 mg / L dioctyl terephthalate standard intermediate solution.

[0017] 3) Preparation of standard working solution: Accurately pipette 0 μL, 100 μL, 200 μL, 400 μL, 1000 μL, 2000 μL, 3000 μL, and 4000 μL of a 10 mg / L dioctyl terephthalate standard intermediate solution into a 10 mL volumetric flask and dilute to the mark with N,N-dimethylformamide (DMF) to obtain standard working solutions with concentrations of 0 mg / L, 0.1 mg / L, 0.2 mg / L, 0.4 mg / L, 1.0 mg / L, 2.0 mg / L, 3.0 mg / L, and 4.0 mg / L, respectively.

[0018] The present invention adopts the "dissolution-precipitation method", first adding N,N-dimethylformamide (DMF) to dissolve the nylon plastic so that the nylon and dioctyl terephthalate are extracted into the solvent, and then removing the nylon polymer molecules by precipitation by dropwise adding excess methanol (main precipitant) to obtain a visible supernatant.

[0019] However, the inventors found that tiny polymer molecules invisible to the naked eye still existed in the supernatant, interfering with the chromatographic response of dioctyl terephthalate during detection. At the same time, these polymer molecules gradually accumulated after long-term sampling and analysis, and may also contaminate the chromatographic system.

[0020] Therefore, based on the "dissolution-precipitation method", the "dissolution-primary precipitation-auxiliary precipitation" method was established, acetonitrile was selected as the auxiliary precipitant, and deep purification was carried out through secondary precipitation to more thoroughly precipitate the small polymer molecules in the supernatant. The polymer molecules were removed by filtration using a 0.22μm organic filter membrane, further reducing the interference of the nylon plastic matrix on the detection of dioctyl terephthalate, improving the chromatographic response of the target object, and protecting the chromatographic system.

[0021] The beneficial effects of the present invention are as follows:

[0022] 1) The present invention utilizes a "dissolution-primary precipitation-auxiliary precipitation" method, first dissolving nylon plastic with N,N-dimethylformamide (DMF) so that nylon polymer molecules and dioctyl terephthalate are completely extracted into the N,N-dimethylformamide (DMF) solvent, then precipitating the nylon polymer molecules by dropwise adding an excess of methanol (primary precipitant), and then adding acetonitrile (auxiliary precipitant) for secondary precipitation, so that small polymer molecules in the supernatant are more thoroughly precipitated, and dioctyl terephthalate in the polynylon plastic can be more effectively extracted. The polymer molecules are then removed by filtration using a 0.22 μm organic filter membrane, and the solution is concentrated to 1 mL for testing, thereby further reducing the interference of the nylon plastic matrix on the detection of dioctyl terephthalate, improving the chromatographic response of the target object, significantly reducing the detection limit, and protecting the chromatographic system.

[0023] 2) This method saves solvent, is accurate, has low interference, reduces noise in the mass spectrometry system, and further reduces the detection limit by 1-2 orders of magnitude. It has higher accuracy and sensitivity, high analysis efficiency, and can meet higher detection requirements.

[0024] 3) The present invention utilizes GC-MS to separate dioctyl terephthalate in polyamide using an HP-5MS UI capillary column (30 m × 0.25 mm, 0.25 μm), employing an EI ion source and scanning in selected ion mode (SIM). The results demonstrate that dioctyl terephthalate elutes within 10 minutes with a good peak shape, exhibits good linearity within the concentration range of 0.1 to 2.0 mg / L, achieves a detection limit of 0.05 mg / L, and a quantification limit of 0.1 mg / L. The method exhibits excellent recovery and precision, with spiked recoveries ranging from 80.0% to 110% and relative standard deviations below 10%, demonstrating broad application prospects. Description of the drawings:

[0025] Figure 1 It is a chromatogram of dioctyl terephthalate (2 mg / L) in a specific embodiment.

[0026] Figure 2 This is the linear relationship and correlation coefficient graph of dioctyl terephthalate. Specific implementation method:

[0027] In order to make the purpose, technical solution and beneficial technical effects of the present invention clearer, the present invention is further described in detail below with reference to the embodiments. It should be understood that the embodiments described in this specification are only for the purpose of explaining the present invention and are not intended to limit the present invention. The parameters, proportions, etc. of the embodiments can be selected according to local conditions without substantial impact on the results.

[0028] Unless otherwise specified, the test materials used in the following examples can be purchased through conventional commercial channels.

[0029] 1. Reagents and Materials

[0030] 1.1 Reagents: N,N-dimethylformamide (DMF): analytical grade; methanol, acetonitrile: chromatographic grade

[0031] 1.2 Standard material: Dioctyl terephthalate, CAS No. 6422-86-2; purity ≥ 98.5%, or a standard material certified by the state and issued with a standard material certificate.

[0032] 1.3 Preparation of standard solution

[0033] 1) Preparation of a 1000 mg / L dioctyl terephthalate stock solution: Accurately weigh 10 mg (accurate to 0.1 mg) of dioctyl terephthalate into a 10 mL volumetric flask and dilute to the mark with N,N-dimethylformamide (DMF). This is the dioctyl terephthalate stock solution (1000 mg / L). Store in a sealed container at 4°C in the dark. The shelf life is 12 months.

[0034] 2) Preparation of a 10 mg / L dioctyl terephthalate standard intermediate solution: Pipette 0.1 mL of the dioctyl terephthalate stock solution into a 10 mL volumetric flask and dilute to the mark with N,N-dimethylformamide (DMF). This is the dioctyl terephthalate standard intermediate solution (10 mg / L). Store in a sealed container at 4°C in the dark. The shelf life is 3 months.

[0035] 3) Preparation of standard working solution: Accurately pipette 0 μL, 100 μL, 200 μL, 400 μL, 1000 μL, 2000 μL, 3000 μL, and 4000 μL of the standard intermediate solution with a concentration of 10 mg / L into a 10 mL volumetric flask and dilute to the mark with N,N-dimethylformamide (DMF) to obtain standard working solutions with concentrations of 0 mg / L, 0.1 mg / L, 0.2 mg / L, 0.4 mg / L, 1.0 mg / L, 2.0 mg / L, 3.0 mg / L, and 4.0 mg / L, respectively.

[0036] 2. Sample Pretreatment

[0037] 2.1 Sample pretreatment

[0038] Weigh 4.00 g of nylon plastic sample, cut the sample into 5 mm × 5 mm small pieces with scissors, and mix well.

[0039] Specimen preparation

[0040] Weigh 1.0 g (accurate to 0.1 mg) of the nylon plastic crushed sample obtained in step 2.1 and place it in a clean 20 mL volumetric flask. Add 10 mL of N, N-dimethylformamide (DMF) and shake it in a vortex shaker until the sample is completely dissolved. Add methanol dropwise to the volumetric flask, shaking the volumetric flask while adding. After the polymer is completely precipitated, use methanol (primary precipitant) to make up the volume to the calibration mark. Shake the solution in the bottle thoroughly and let it stand. Take the supernatant and filter it through a 0.22 μm filter membrane in a 20 mL volumetric flask to obtain 10 mL of sample solution. Then add acetonitrile (precipitant) dropwise. After precipitation is complete, use acetonitrile to make up the volume to the mark, and filter all the obtained solutions with a 0.22 μm filter membrane into a test tube. Concentrate to 1 mL in a nitrogen blower and set aside.

[0041] 3. Preparation of blank samples

[0042] Prepare the method blank sample according to the above operation process.

[0043] 4. Setting of instrument conditions

[0044] 4.1 Gas chromatography conditions:

[0045] Chromatographic column: HP-5MS UI capillary column, 30m×0.25mm, 0.25μm; flow rate: 1.5mL / min; injection mode: splitless mode; heating program: 120℃ for 1min, then increase the temperature to 300℃ at 30℃ / min and hold for 5min; injection volume: 1μL; injection port temperature: 300℃; carrier gas: helium.

[0046] 4.2 Mass spectrometry conditions

[0047] Solvent delay: 5 min; ion source: EI source (70 eV); ion source temperature: 230°C; quadrupole temperature: 150°C; mass spectrometry interface temperature: 300 m / z; selected ion mode (SIM); qualitative ions: 167 m / z, 261 m / z, 221 m / z; quantitative ion: 149 m / z.

[0048] 5. Qualitative Determination

[0049] Determine the standard working solution and the sample. If the mass chromatographic peak retention time of the sample solution is within ±2.5% of that of the standard solution; the relative abundance of the qualitative ion pair is consistent with that of the mixed matrix standard solution of equivalent concentration, and the relative abundance deviation does not exceed the requirements of Table 1, then it can be determined that the corresponding analyte is present in the sample. Figure 1 .

[0050] Table 1 Maximum allowable deviation of relative ion abundance in qualitative determination

[0051] Relative ion abundance, K / % K≥50 20＜K＜50 10＜K≤20 K≤10 Allowable relative deviation / % ±20 ±25 ±30 ±50

[0052] 6. Quantitative Determination

[0053] The sample solution and the blank sample solution were injected into a gas chromatography tandem mass spectrometer to obtain the peak area, and the concentration of dioctyl terephthalate in the test solution was obtained according to the standard curve.

[0054] Example 1: Selection of pre-treatment test conditions

[0055] 1. Selection of sample solvent

[0056] Weigh 1.0 g (accurate to 0.1 mg) of the nylon plastic crushed sample obtained in step 2.1 and place it in a clean 20 mL volumetric flask. Add 10 mL of N,N-dimethylformamide (DMF), formic acid, concentrated sulfuric acid, hexafluoroisopropanol, and m-cresol respectively to dissolve it. It was found that dimethylaminoformamide has a shorter dissolution time and better effect.

[0057] 2. Selection of precipitation solvent

[0058] After dissolving the spiked nylon plastic sample in N,N-dimethylformamide (DMF), the inventors tested methanol, ethanol, acetonitrile, butanone, acetone, ether, and hydrocarbons as precipitants. Methanol, ether, and acetonitrile were found to be most effective, with methanol being the most effective. However, the inventors discovered that the supernatant still contained tiny, invisible polymer molecules, which interfered with the chromatographic response of dioctyl terephthalate during detection. Furthermore, these small polymer molecules gradually accumulated after prolonged sample injection and analysis, potentially contaminating the chromatographic system.

[0059] Therefore, when methanol is used as the primary precipitant and acetonitrile as the auxiliary precipitant, the precipitation effect is more obvious and more uniform. Deep purification is performed to more thoroughly precipitate the small polymer molecules in the supernatant. The polymer molecules are then removed by filtration using a 0.22μm organic filter membrane, further reducing the interference of the nylon plastic matrix on the detection of dioctyl terephthalate, improving the chromatographic response of the target, and protecting the chromatographic system.

[0060] Example 2: Linearity of standard working curve, detection limit, quantification limit and recovery rate

[0061] According to the optimized determination conditions, the standard working solution (0.1mg / L~2.0mg / L) was tested, with the concentration of dioctyl terephthalate in the standard solution as the horizontal axis (in mg / L), and the corresponding peak area average as the vertical axis, to draw the standard working curve and obtain the linear equation and correlation coefficient. The test results are as follows Figure 2 The results showed a good linear relationship between the concentration of dioctyl terephthalate and the response value. The standard curve was y = 116.3245 × x + 88.2334, with a linear relationship of 0.9987. As shown in Table 2, a pretreated sample solution of known concentration was tested six times and compared with the blank measurement signal. The concentration corresponding to a signal-to-noise ratio (S / N) ≥ 3 was set as the limit of detection (LOD), and the concentration corresponding to a signal-to-noise ratio (S / N) ≥ 10 was set as the limit of quantification (LOQ). The experimental results showed that the limit of detection was 0.050 mg / kg and the limit of quantification was 0.10 mg / kg, which well met the testing requirements.

[0062] Table 2 Signal-to-noise ratio of detection limit and quantification limit of dioctyl terephthalate

[0063]

[0064] A blank sample spiked with dioctyl terephthalate (DTT) recovery test was conducted. Blank samples containing DTT without the target substance were pretreated according to the preferred method described in the Examples. Three target concentration levels, 0.10 mg / L, 0.50 mg / L, and 1.5 mg / L, were prepared for measurement. Six separate measurements were performed at each level, and recovery and precision tests were performed. The test results (Table 3) show that the spiked recovery of DTT in polyamide using this method ranged from 80% to 110%, with relative standard deviations (RSDs) below 10%, demonstrating good recovery and precision.

[0065] Table 3 Test data of recovery rate of dioctyl terephthalate

[0066]

[0067] .

Claims

1. A method for detecting the content of dioctyl terephthalate in nylon plastic, characterized in that: The method comprises the following steps: respectively taking a sample to be tested and a standard working solution for GC-MS analysis, wherein the preparation of the sample to be tested comprises the following steps: weighing 4.00 g of a nylon plastic sample, cutting the sample into small pieces of 5 mm×5 mm, and mixing them evenly; weighing 1.0 g of the sample into a clean 20 mL volumetric flask, adding 10 mL of N,N-dimethylformamide, and shaking the sample in a vortex shaker until the sample is completely dissolved; adding methanol dropwise into the volumetric flask while shaking the volumetric flask; after the polymer is completely precipitated, the volume is adjusted to the calibration mark with methanol; the solution in the flask is fully shaken and then allowed to stand; taking the supernatant and filtering it through a 0.22 μm filter membrane to obtain 10 mL of sample solution; then adding acetonitrile dropwise; after the precipitation is completed, the volume is adjusted to the scale with acetonitrile; filtering the solution through a 0.22 μm filter membrane into a test tube; and concentrating the solution to 1 mL in a nitrogen blower for standby use.

2. The method according to claim 1, characterized in that GC conditions were as follows: chromatographic column: HP-5MS UI capillary column; column flow rate: 1.2-1.8 mL / min; injection mode: splitless mode; temperature program: 120°C for 1 min, then heating to 280-320°C at 28-32°C / min and holding for 5 min; injection volume: 0.8-1.2 μL; injection port temperature: 300°C; carrier gas: helium; mass spectrometry conditions: solvent delay: 5 min; Ion source: 70 eV EI source; ion source temperature: 230°C; quadrupole temperature: 150°C; detection mode: selected ion mode; qualitative ions: 167 m / z, 261 m / z, 221 m / z; Quantitative ion: 149 m / z.

3. The method according to claim 2, characterized in that The capillary column specifications are 30m×0.25mm, 0.25μm.

4. The method according to claim 1, wherein The preparation of standard working solution includes the following steps: 1) Preparation of a 1000 mg / L dioctyl terephthalate stock solution: Accurately weigh 10 mg of dioctyl terephthalate into a 10 mL volumetric flask and dilute to the mark with N,N-dimethylformamide to obtain a 1000 mg / L dioctyl terephthalate stock solution. 2) Preparation of a 10 mg / L dioctyl terephthalate standard intermediate solution: Pipette 0.1 mL of the dioctyl terephthalate stock solution into a 10 mL volumetric flask and dilute to the mark with N,N-dimethylformamide to obtain a 10 mg / L dioctyl terephthalate standard intermediate solution. 3) Preparation of standard working solutions: Accurately pipette 0 μL, 100 μL, 200 μL, 400 μL, 1000 μL, 2000 μL, 3000 μL, and 4000 μL of a 10 mg / L dioctyl terephthalate standard intermediate solution into a 10 mL volumetric flask and dilute to the mark with N,N-dimethylformamide to obtain standard working solutions with concentrations of 0 mg / L, 0.1 mg / L, 0.2 mg / L, 0.4 mg / L, 1.0 mg / L, 2.0 mg / L, 3.0 mg / L, and 4.0 mg / L, respectively.

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