Determination of fluorine content in disposable degradable materials for food contact by oxygen bomb combustion-ion chromatography

Abstract

The application discloses an oxygen bomb combustion-ion chromatography method for determining fluorine content in food contact disposable degradable materials, and comprises the following steps: sampling, placing food contact disposable degradable material samples in a quartz crucible, adding benzoic acid, adding anhydrous ethanol dropwise, filling oxygen, high-temperature oxidation decomposition is generated through oxygen bomb combustion, fluoride is converted into free fluoride ions, and the fluoride ions are absorbed or dissolved in an ultrapure water absorption liquid, 5.0 mmol / L sodium bicarbonate and 1.0 mmol / L anhydrous sodium carbonate solution are used as a mobile phase for isocratic elution, the injection amount is 20 muL, the flow rate is 0.8 mL / min, a MetrosepA Supp 7 chromatographic column and a MetrosepA Supp 5 Guard protection column are used, the column temperature is 45 DEG C, a conductivity detector is used for determination, and an external standard method is used for quantification. The method is sensitive, reliable, good in separation effect and good in reproducibility.

Description

TECHNICAL FIELD

[0001] The present application relates to the determination of fluorine content in food contact disposable degradable materials by oxygen bomb combustion-ion chromatography. BACKGROUND

[0002] At present, with the increasing concern of people on environmental protection and sustainable development, degradable materials as a new type of material to replace traditional plastics have been widely used, such as polylactic acid (PLA), polybutylene succinate (PBS) and polybutylene adipate terephthalate (PBAT) etc. However, some disposable degradable materials contain fluorine element, which may cause potential risks to human health and environment. GB / T 18006.3-2020, GB / T 41010-2021, "Guidelines for Classification and Identification of Degradable Plastic Products" and "California Sustainable Packaging Act 2018" stipulate that the fluorine content in disposable degradable materials shall not exceed 100 mg / kg, and it is particularly important to accurately determine the fluorine content in food contact disposable degradable materials. The detection methods for fluorine content mainly include EDTA back titration method, fluorescence spectroscopy (AFS), atomic absorption spectrometry (AAS), nuclear magnetic resonance spectroscopy (NMR), ion selective electrode method and ion chromatography etc. These methods have their own limitations, such as complicated sample preparation, the need for complex pretreatment steps, low detection sensitivity, unsuitable for determination of trace fluorine, possible interference of sample matrix and expensive instruments etc. Ion chromatography has many advantages, making it an ideal choice for detecting fluorine ions. First, ion chromatography has high sensitivity and can determine the content of trace fluorine. Second, ion chromatography has good selectivity for fluorine ions, and appropriate chromatographic conditions can be selected through method optimization to avoid the interference of other ions, thereby ensuring the accuracy of the determination. The main pretreatment methods for determining fluorine by ion chromatography include immersion extraction method, ashing and alkali fusion method, oxygen bomb combustion method and tube furnace combustion method. In the immersion extraction method, fluorine in disposable degradable samples is difficult to be completely extracted. In the ashing and alkali fusion method, the sample is placed in a non-fully-closed nickel crucible, and fluorine is easily volatilized into the air, causing loss and low recovery rate, and the high concentration of cations in the absorption salt may affect the determination of the sample. The tube furnace combustion method has high combustion efficiency and is suitable for detecting trace anions, but the instrument equipment is expensive and may not be suitable for general promotion. The oxygen bomb combustion method burns the sample in a closed environment rich in oxygen, and the fluorine element in the sample is completely converted into inorganic fluorine ions, which can meet the detection requirements of fluorine in disposable degradable materials.

[0003] Oxygen bomb combustion-ion chromatography as a sensitive, accurate and reliable analysis method, with sample preparation simple, high sensitivity, accurate and reliable results and other advantages. Currently, there are few reports on the determination of fluorine content in food contact disposable degradable materials by oxygen bomb combustion-ion chromatography. EN 14582:2016 and DIN 51723:2002 respectively stipulate the halogen pretreatment test method of solid waste and solid fuel, but do not provide specific ion chromatography method. SUMMARY

[0004] The purpose of the present application is to provide an oxygen bomb combustion-ion chromatography method for determining the fluorine content in food contact disposable degradable materials, which solves the problem of inaccuracy and low reproducibility in the prior art.

[0005] The present application is realized by the following technical solutions:

[0006] The oxygen bomb combustion-ion chromatography method for determining the fluorine content in food contact disposable degradable materials comprises the following steps: placing the food contact disposable degradable material sample in a quartz crucible, adding benzoic acid, adding anhydrous ethanol dropwise, filling oxygen, and performing high-temperature oxidative decomposition by oxygen bomb combustion, converting the fluoride into free fluoride ions, absorbing or dissolving in the ultrapure water absorption liquid for testing, and the ion chromatography conditions are as follows: using MetrosepA Supp 7 chromatographic column and MetrosepASupp 5Guard guard column, column temperature 45℃, using 1.0-5.0mmol / L sodium bicarbonate and 1.0mmol / L anhydrous sodium carbonate solution as the mobile phase for isocratic elution, injection amount 20μL, flow rate 0.8-1mL / min, using conductivity detector for determination, chemical suppressor: MSN-A; suppressor regeneration liquid: mixture of 200mmol / L sulfuric acid and 5% acetone, external standard quantitative method.

[0007] The preparation of the standard working solution comprises the following steps: taking 1000mg / L of fluorine ion standard solution into a volumetric flask, preparing a standard stock solution with a mass concentration of 10mg / L, and storing at 4℃; taking 10mg / L of fluorine ion standard stock solution to prepare a fluorine ion standard intermediate solution with a mass concentration of 1.0mg / L, and storing at 4℃; respectively taking 1.0mg / L of fluorine ion standard stock solution 0.00, 0.20, 0.50, 1.00, 2.50, 3.50, 5.00mL into a 10mL volumetric flask, adding water to 10mL, and preparing standard working solutions with mass concentrations of 0, 0.02, 0.05, 1.0, 2.5, 3.5, 5.0mg / L.

[0008] The calculation formula of the fluorine content in food contact disposable degradable materials

[0009]

[0010] In the formula:

[0011] X - Fluorine content in the sample, mg / kg;

[0012] C - Corresponding machine-readable concentration of the sample in the standard curve, mg / L;

[0013] C0 - Corresponding machine-readable concentration of the sample blank in the standard curve, mg / L;

[0014] V - Constant volume, mL;

[0015] m - Sample weight, g.

[0016] In particular, the food contact disposable degradable material sample is pretreated, and the pretreatment comprises the following steps: the sample is cut into (1.5-2.5) mm x (1.5-2.5) mm, 0.15-0.25 g is weighed, placed in a quartz crucible, 0.1-0.22 g of benzoic acid in sheet form is added, preferably 0.15-0.22 g of benzoic acid, the ignition wire is installed, 0.5-2.5 mL of anhydrous ethanol is added, preferably 1-2 mL of anhydrous ethanol, 15 mL of ultrapure water is added as an absorption liquid in the oxygen bomb. The oxygen charging pressure is 2.5-3.0 Mpa, after oxygen charging, the oxygen is released, and after repeating three times, oxygen is charged again, the oxygen bomb is placed in a normal temperature water bath, the ignition combustion program of the oxygen bomb is started. Cool for more than 30 min, shake the oxygen bomb every 5-10 min. Slowly release the gas, open the oxygen bomb, remove the absorption liquid in the oxygen bomb to a volumetric flask, rinse the inner wall of the oxygen bomb, the electrode pipeline, the sample cup holder and the quartz crucible with ultrapure water, combine and transfer to the volumetric flask, make up to the mark, shake well, filter with a 0.22 μm polyether sulfone needle filter, and then measure.

[0017] In particular, the specifications of the Metrosep A Supp7 chromatographic column are 250 mm x 4.0 mm; the specifications of the Metrosep A Supp5 Guard protection column are 50 mm x 4.0 mm.

[0018] In the previous exploratory research, the inventors found that when the food contact disposable degradable material is combusted in the oxygen bomb and then analyzed by ion chromatography, the fluorine ion is interfered by small molecule organic acids such as acetate and formate.

[0019] In the previous sample test, it was found that fluoride ions were generally present in food contact disposable degradable material samples. In order to avoid the co-elution of fluoride ions, acetate and formate due to high mass concentration of fluoride ions, which affects the quantification of fluoride ions, different chromatographic columns, mobile phases and elution conditions were explored. Finally, the chromatographic conditions of Metrosep A Supp7 column with 5.0 mmol / L sodium bicarbonate + 1.0 mmol / L anhydrous sodium carbonate as the mobile phase were selected, with good peak shape, sensitivity and high response for fluoride content analysis. Such optimization makes the separation and detection of fluoride ions more accurate and reliable. Metrosep A Supp7 chromatographic column is selected, and 5.0 mmol / L sodium bicarbonate + 1.0 mmol / L anhydrous sodium carbonate solution is used for isocratic elution of fluoride ions.

[0020] When using benzoic acid as a combustion aid, a tablet press is needed to press benzoic acid into a sheet shape, which makes it difficult for benzoic acid to splash or overflow during the ignition process, avoiding unnecessary interference, and better controlling the dosage of the combustion aid, ensuring the stability of the ignition link and the reliability of the experiment. However, benzoic acid also has the problem of uneven accumulation of combustion aid, which will cause part of the sample to not contact enough combustion aid, affecting the ignition effect and the stability of combustion.

[0021] In order to overcome the problem of poor contact of benzoic acid, it is found that mixing pressed benzoic acid with anhydrous ethanol can make benzoic acid more evenly contact with the sample and ignition wire, avoiding the failure of ignition. Anhydrous ethanol also has good fluidity, which helps to ensure the full contact of the combustion aid with the ignition wire and the sample, and improves the ignition efficiency and the stability of combustion. Therefore, an appropriate amount of benzoic acid mixed with anhydrous ethanol is selected as the combustion aid.

[0022] The beneficial effects of the present application are as follows: the present application establishes a method for determining the fluorine content in food contact disposable degradable materials by oxygen bomb combustion-ion chromatography, and optimizes the chromatographic conditions or pretreatment conditions such as chromatographic column, mobile phase, flow rate, oxygenation pressure, combustion improver and absorption liquid, realizes good separation of fluorine ions and formate and acetate, the detection limit of fluorine ions is 0.01 mg / L, the quantification limit is 0.02 mg / L, when the sample amount is 0.2 g and the constant volume is 100 mL, the correlation coefficient of fluorine content in the sample is >0.995, the detection limit of fluorine is 5.0 mg / kg, the quantification limit is 10 mg / kg, the linear relationship is good, the standard addition recovery is 93.8-98.4%, and the precision is 0.5-7.0% (n=6). The method is sensitive, reliable, has good separation effect and good reproducibility, and is suitable for determining the fluorine content in food contact disposable degradable materials. Through quantitative analysis of the fluorine content in 119 actual samples, the overall detection rate is 94.11%, and the overall unqualified rate is 15.94%; the detection rates of PLA straws, PLA+PBAT straws and PLA+PBS straws are 100%, and the unqualified rates are 45.16%, 30.77% and 5.88% respectively. Through the test and analysis of the actual samples, it is found that the research results will provide technical and data support for the monitoring of the fluorine content in disposable degradable materials by market supervision agencies and food-related product manufacturers. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 is a schematic diagram of the separation effect of different chromatographic columns in the embodiment;

[0024] wherein a is a Metrosep A Supp 4 chromatographic column, b is a Metrosep A Supp 5 chromatographic column, and c is a Metrosep A Supp 7 chromatographic column.

[0025] Figure 2 is the separation of fluorine ions, acetate and formate under different mobile phases;

[0026] wherein a, 3.6 mmol / L Na2CO3; b, 1.0 mmol / L Na2CO3+1.0 mmol / L NaHCO3; c, 5.0 mmol / L Na2CO3+1.0 mmol / L NaHCO3; d, 1.0 mmol / L Na2CO3+5.0 mmol / L NaHCO3.

[0027] Figure 3 is the influence of oxygenation pressure on the fluorine release effect in the sample;

[0028] Figure 4 is the influence of absorption time on the fluorine release effect in the sample;

[0029] Figure 5This is an ion chromatogram of fluoride ions in an actual sample of a disposable biodegradable material (retention time approximately 6.8 min). Detailed implementation method:

[0030] The following is a further description of the invention, but not a limitation thereof.

[0031] 1. Instruments and reagents

[0032] 930 Compact IC Flex ion chromatograph, equipped with 863 Compact Autosampler, MSN-A Rotor anion suppressor, conductivity detector, and MagIC chromatography workstation (Metroxide AG, Switzerland); Sartorius Quintix analytical balance (accuracy 0.1 mg, Sartorius, USA); EDI, Millipore Elix3 Essential, Millipore Milli-Q Reference ultrapure water system (Merck Millipore, France); YCY-4 oxygenator, oxygen bomb combustion tank, SLSY ignition control device (Nanjing Sangli Electronic Equipment Factory, China); GM-0.33A vacuum filtration pump (Tianjin Jinteng Experimental Equipment Co., Ltd.); microporous filter membrane (50mm diameter / 0.45μm pore size, Tianjin Jinteng Experimental Equipment Co., Ltd.); high-purity oxygen (purity ≥99.999%, Qingyuan Liansheng Air Liquefaction Co., Ltd.); polyethersulfone needle filter membrane (13mm diameter / 0.22μm pore size, Shanghai Anpu Experimental Technology Co., Ltd.); quartz crucible (Guangzhou Sibote Biotechnology Co., Ltd.);

[0033] Fluoride ion standard solution (1000 mg / L, Shanghai Anpu Experimental Technology Co., Ltd.); anhydrous ethanol, acetone (chromatographic grade, CNW Technologies, Germany), anhydrous sodium carbonate, sodium bicarbonate (chromatographic grade, Thermo Fisher Scientific, USA); benzoic acid (analytical grade, Guangzhou Chemical Reagent Factory); ignition wire (0.12 mm in diameter, Changsha Kaiyuan Instrument Co., Ltd.); experimental water was Milli-Q ultrapure water;

[0034] Samples: Disposable biodegradable materials for food contact: 31 PLA straws, 13 PLA+PBAT straws, 16 PLA+PBS straws, 10 PBAT plastic particles, 8 PLA plastic particles, 11 PLA+PBS plastic particles, 8 PBAT film bags, 8 PLA film bags, 7 PLA cutlery, and 6 PLA lunch boxes, purchased from an e-commerce platform.

[0035] 2. Experimental Methods

[0036] 2.1 Preparation of standard solutions

[0037] Take 1 mL of fluoride ion standard solution (1000 mg / L) into a 100 mL volumetric flask, dilute to the mark with ultrapure water, prepare a standard stock solution with a mass concentration of 10 mg / L, and store at 4°C. Take 10 mL of fluoride ion standard stock solution (10 mg / L) into a 100 mL volumetric flask, dilute to the mark with ultrapure water, obtain a fluoride ion standard intermediate solution with a mass concentration of 1.0 mg / L, and store at 4°C. Take 0.00, 0.20, 0.50, 1.00, 2.50, 3.50, 5.00 mL of fluoride ion standard stock solution (1.0 mg / L) into a 10 mL volumetric flask, add water to dilute to 10 mL, prepare standard working solutions with mass concentrations of 0, 0.02, 0.05, 1.0, 2.5, 3.5, 5.0 mg / L, and use them immediately after preparation.

[0038] 2.2 Preparation of mobile phase solution

[0039] Weigh 0.841 g of sodium bicarbonate and 0.212 g of anhydrous sodium carbonate into a 2 L volumetric flask, dilute to the mark with ultrapure water, mix well, and prepare a mobile phase solution with a molar concentration of 5.0 mmol / L sodium bicarbonate and 1.0 mmol / L anhydrous sodium carbonate, which is prepared immediately before use and degassed with a vacuum pump for more than 10 minutes before use.

[0040] 2.3 Preparation of suppressor regeneration solution

[0041] Measure 100 mL of acetone and 1800 mL of ultrapure water into a 2 L volumetric flask, slowly add 40 mL of sulfuric acid, dilute to the mark with ultrapure water, mix well, and prepare a suppressor regeneration solution of 200 mmol / L sulfuric acid + 5% acetone, which is prepared immediately before use.

[0042] 2.4 Pretreatment of samples

[0043] Before testing, cut the sample into about 2 mm x 2 mm. Weigh 0.2 g of the prepared sample (accurate to 0.0001 g) into a quartz crucible, add about 0.1 g of flaky benzoic acid, install the ignition wire, and drop 1.0 mL of anhydrous ethanol. Add 15 mL of ultrapure water as the absorbing liquid in the oxygen bomb. The oxygen charging pressure is 3.0 Mpa, and after oxygen charging, release the gas, repeat three times, then charge oxygen again, place the oxygen bomb in a normal temperature water bath, start the oxygen bomb ignition combustion program. Cool for 30 minutes or more, shake the oxygen bomb every 5-10 minutes. Slowly release the gas, open the oxygen bomb, remove the absorbing liquid in the oxygen bomb to a 100 mL volumetric flask, rinse the inner wall of the oxygen bomb, electrode pipeline, sample cup holder and quartz crucible with ultrapure water, combine and transfer to the volumetric flask, dilute to the mark, shake well, filter with a 0.22 μm polyether sulfone needle filter, and test. Determine in triplicate.

[0044] 2.5 Ion chromatography conditions

[0045] Chromatographic column: Metrosep A Supp7 chromatographic column (250 mm x 4.0 mm, 5.0 μm) + Metrosep A Supp5 guard column (50 mm x 4.0 mm, 5.0 μm); mobile phase: 5.0 mmol / L sodium bicarbonate + 1.0 mmol / L anhydrous sodium carbonate; column temperature: 45 °C; sample injection volume: 20 μL; flow rate: 0.8 mL / min; detector: conductivity detector; chemical suppressor: MSN-A; suppressor regeneration liquid: 200 mmol / L sulfuric acid + 5% acetone; isocratic elution.

[0046] 2.6 Calculation formula

[0047]

[0048] In the formula:

[0049] X - fluorine content in the sample, mg / kg;

[0050] C - the corresponding machine-readable concentration of the sample in the standard curve, mg / L;

[0051] C0 - the corresponding machine-readable concentration of the sample blank in the standard curve, mg / L;

[0052] V - constant volume, mL;

[0053] m - sample weight, g;

[0054] 2.7 Data processing

[0055] Excel was used to analyze data and make tables, and Origin software was used to draw graphs.

[0056] Before implementation, the implementation conditions need to be determined first:

[0057] I. Ion chromatography condition optimization

[0058] Selection of chromatographic column:

[0059] Metrosep A Supp 4 (250 mm x 4 mm, 5.0 μm), Metrosep A Supp 5 (250 mm x 4 mm, 5.0 μm) and Metrosep A Supp 7 250 / 4.0 (250 mm x 4 mm, 5.0 μm) were investigated respectively. The results showed that the retention time of fluoride ion, formate ion and acetate ion on Metrosep A Supp 4 column were 3.45 min, 3.61 min and 3.88 min respectively, and there was co-elution phenomenon, which affected the quantification; on Metrosep A Supp 5 column, the retention time of fluoride ion, formate ion and acetate ion were 5.84 min, 6.25 min and 6.78 min respectively, and the separation effect was general, when the content of fluoride ion in sample was high, the peak area increased, and there might be co-elution phenomenon, so it could not meet the determination; on Metrosep A Supp 7 column, the retention time of fluoride ion, formate ion and acetate ion were 3.48 min, 3.75 min and 3.95 min respectively, and the separation degree was good, therefore, Metrosep A Supp 7 column was selected as the chromatographic column, and the separation effect of three ion chromatographic columns was shown in Figure 1. Figure 1 .

[0060] Selection of mobile phase:

[0061] The addition of 4 different mass percentages (0%, 10%, 20% and 30%) of organic additives methanol, acetonitrile and acetone to the mobile phase was compared, and it was found that the addition of organic additives did not cause obvious change in the retention time of fluoride ion, formate ion and acetate ion, but the pressure of the chromatographic column increased with the increase of the organic additives added to the mobile phase, which was not conducive to the subsequent method optimization, therefore, methanol, acetonitrile and acetone were not selected as organic additives added to the mobile phase.

[0062] On the other hand, the mobile phase of carbonate salt: 3.6 mmol / L anhydrous sodium carbonate, 1.0 mmol / L sodium bicarbonate + 1.0 mmol / L anhydrous sodium carbonate, 1.0 mmol / L sodium bicarbonate + 5.0 mmol / L anhydrous sodium carbonate and 5.0 mmol / L sodium bicarbonate + 1.0 mmol / L anhydrous sodium carbonate was investigated. The results were shown in Figure 2. Figure 2The separation effects of fluoride, formate and acetate under different eluent concentrations were compared. Under 1.0 mmol / L sodium bicarbonate + 1.0 mmol / L anhydrous sodium carbonate, the separation effects of fluoride, formate and acetate were good; under 1.0 mmol / L sodium bicarbonate + 5.0 mmol / L anhydrous sodium carbonate, the retention time of fluoride, formate and acetate was significantly shortened, and the separation effect was worse; under 5.0 mmol / L sodium bicarbonate + 1.0 mmol / L anhydrous sodium carbonate, the separation effect of fluoride, formate and acetate was close to that under 1.0 mmol / L sodium bicarbonate + 1.0 mmol / L anhydrous sodium carbonate, and the retention time was about 1 min earlier. This shows that too high molar concentration of anhydrous sodium carbonate will make the separation effect of fluoride, formate and acetate worse, and reducing the concentration of anhydrous sodium carbonate and appropriately increasing the concentration of sodium bicarbonate can improve the detection efficiency while ensuring good separation effect.

[0063] In summary, after further optimization experiments, the chromatographic conditions of Metrosep A Supp7 column with 1.0-5.0 mmol / L sodium bicarbonate + 1.0 mmol / L anhydrous sodium carbonate as the mobile phase were selected, the peak shape was good, the sensitivity and response were high, and it was used for the analysis of fluorine content. Such optimization selection makes the separation and detection of fluoride ions more accurate and reliable, and the most preferred is: selecting Metrosep A Supp7 chromatographic column, and using 5.0 mmol / L sodium bicarbonate + 1.0 mmol / L anhydrous sodium carbonate solution for isocratic elution of fluoride ions.

[0064] Selection of flow rate

[0065] The separation of fluoride ions was investigated at flow rates of 0.6, 0.7, 0.8, 0.9, and 1.0 mL / min, respectively. The analysis time was longer at flow rates of 0.6 and 0.7 mL / min; at flow rates of 0.9 and 1.0 mL / min, increasing the flow rate could improve the detection efficiency, but it reduced the separation degree and caused baseline instability, and the column pressure was large. The flow rate was selected as 0.8 mL / min.

[0066] II. Optimization of pretreatment conditions

[0067] In the study, five representative disposable degradation samples were selected, and the oxygenation pressure, combustion aid, absorption liquid and absorption time were explored.

[0068] Selection of oxygenation pressure:

[0069] By setting the oxygenation pressure to 1.0, 1.5, 2.0, 2.5, 3.0, 3.5 and 4.0 MPa, respectively, the change of fluorine content in the sample was studied. The experimental results showed (see Figure 3), the fluorine content in the sample continued to rise with the increase of the oxygenation pressure. However, in the range of 1.0-2.0 Mpa, the standard deviation of the results was abnormally large, which was due to the insufficient oxygenation pressure leading to incomplete combustion of the sample, and the degree of oxidative decomposition was inconsistent between the parallel samples. In contrast, in the range of 2.5-4.0 Mpa, the fluorine content in the sample tended to be stable. This shows that with the increase of the oxygenation pressure, the fluorine in the sample is fully analyzed and released during the combustion process. This is because the sample will consume a certain amount of oxygen during combustion, and higher oxygenation pressure means that there is more oxygen available for sample combustion in the oxygen bomb, which is conducive to the complete combustion of the sample, so that the fluorine in the sample can be fully analyzed. In summary, the results under 2.5, 3.0, 3.5, and 4.0 Mpa are relatively close, and considering the safety of the oxygen bomb in use, the optimal choice is 3.0 Mpa as the oxygenation pressure of the method. This study provides a reliable method for effective detection of fluorine content in disposable degradation samples and a reference for the selection of oxygenation pressure.

[0070] Selection of combustion improver:

[0071] In the pre-treatment of oxygen bomb combustion, the addition of combustion improver can provide enough heat to promote the complete oxidation and release of fluorine elements in the sample. Usually, absorbent cotton or benzoic acid will be chosen as the combustion improver. Absorbent cotton itself is relatively fluffy, which may cause ignition failure due to incomplete contact between the ignition wire, absorbent cotton and the sample. In addition, general absorbent cotton contains halogens such as fluorine, chlorine, bromine, etc., which may interfere with the experimental results and affect the determination of fluorine content during the oxygen bomb combustion process. In order to solve the problem of ignition failure and halogen interference caused by absorbent cotton, benzoic acid has become an alternative choice.

[0072] When using benzoic acid as a combustion aid, it is necessary to use a tablet press to compress benzoic acid into tablets, which makes it not easy to splash or overflow during ignition, avoiding unnecessary interference, while also better controlling the dosage of the combustion aid, ensuring the stability of the ignition link and the reliability of the experiment. However, benzoic acid also has the problem of uneven accumulation of combustion aid, which will cause a part of the sample to not contact enough combustion aid, thereby affecting the effect and stability of combustion. In order to overcome these problems that may lead to poor contact of benzoic acid, the compressed benzoic acid is mixed with anhydrous ethanol (see Table 1), by mixing with anhydrous ethanol, benzoic acid can be more uniformly contacted with the sample and the ignition wire, avoiding the failure of ignition. Anhydrous ethanol also has good fluidity, which helps to ensure that the combustion aid is in full contact with the ignition wire and the sample, improving the ignition efficiency and the stability of combustion. Therefore, an appropriate amount of benzoic acid is mixed with anhydrous ethanol as a combustion aid. Preferably, the benzoic acid is 0.1-0.22g, and the anhydrous ethanol is 0.5-2.5mL, more preferably, the benzoic acid is 0.15-0.22g, and the anhydrous ethanol is 1-2mL.

[0073] Table 1 Selection of the amount of combustion aid benzoic acid and anhydrous ethanol

[0074]

[0075] Selection of absorption liquid:

[0076] The main function of the oxygen bomb combustion absorption liquid is to ensure that the sample is fully absorbed by the absorption liquid after complete combustion in the oxygen bomb combustion pot. In this study, 15mL of ultrapure water and 0.3mol / L NaOH were selected for comparison and determination. The results show that after a certain time of full absorption, both of these absorption liquids have good absorption effect on fluorine ions. EN 14852:2016 stipulates that when the amount of halide in the object to be measured reaches the order of 10g / kg, it is recommended to choose 0.3mol / L sodium hydroxide solution as the absorption liquid. This is mainly because when the concentration of halogen such as hydrogen fluoride is too high, the acidic sample solution will cause corrosion to the oxygen bomb combustion pot and the ion chromatography system, not only affecting the determination result, but also affecting the service life of the instrument equipment. However, using sodium hydroxide as an absorption liquid can adjust the pH value of the sample solution and enhance the absorption effect of high-concentration halogen. However, in this study, the fluorine content in the disposable degradation sample does not reach the order of 10g / kg. Therefore, ultrapure water solution is finally selected as the absorption liquid, which can meet the experimental requirements and effectively determine the fluorine content in the disposable degradation sample.

[0077] Selection of absorption time:

[0078] According to the experimental steps of sample pretreatment, the absorption liquid was used to shock absorption of the rectification residue after combustion, and the absorption time was selected as 10 min, 20 min, 30 min, 40 min, 50 min and 60 min respectively. After the absorption solution absorbed the chlorine and fluorine in the oxygen bomb according to the test time, the absorption liquid was transferred to a 250 mL volumetric flask, and the volume was adjusted with ultrapure water. After shaking, the solution was filtered, and the prepared clear solution was filtered through a 0.22 μm microporous filter membrane and analyzed by ion chromatography. The effect of absorption time on the absorption of fluorine ions is shown in Figure 4 When the absorption time was 40 min, the fluorine content detection result no longer increased significantly, so it was appropriate to set the extraction time as 40 min.

[0079] III. Methodology parameters

[0080] Linear range, detection limit and quantification limit:

[0081] The standard solution of fluorine ions was diluted with ultrapure water to prepare standard working solutions with concentrations of 0, 0.02, 0.05, 1.5, 2.5, 3.5 and 5.0 mg / L. The standard curve was drawn with the mass concentration of fluorine ions as the abscissa (x, mg / L) and the peak area of fluorine ions (y, [μs / cm]×min) as the ordinate according to the current optimized conditions. The linear regression equation was obtained, and the correlation coefficient was greater than 0.999. The detection limit (LOD) was calculated according to the signal-to-noise ratio S / N≥3, and the quantification limit (LOQ) was calculated according to the signal-to-noise ratio S / N≥10. The detection limit and quantification limit of fluorine ions in the sample solution were 0.01 mg / L and 0.02 mg / L respectively. When the sample weight was 0.2 g and the constant volume was 100 mL, the detection limit and quantification limit of fluorine in the sample were 5.0 mg / kg and 10 mg / kg respectively. The specific results are shown in Table 2.

[0082] Table 2 Linear equation, correlation coefficient, linear range, detection limit and quantification limit of fluorine ions

[0083]

[0084] The recovery rate and precision were tested in the pretreatment of the sample. Three concentration levels (10, 50 and 100 mg / L) of fluorine ion standard solution and a certain mass of external certified reference material were added to the quartz crucible containing the sample, so that the measured component and the sample matrix component were fully mixed. Six consecutive measurements were made, and the average recovery rate and the relative standard deviation (RSD) were calculated. The results (Table 3) showed that the average recovery rates of fluorine ions in disposable degradable samples were 93.8%, 96.2% and 98.4% respectively, and the RSDs were 6.8%, 4.8% and 2.9% respectively, which met the detection requirements.

[0085] Table 3 Recovery rate and precision of fluorine ions at different addition levels (n=6)

[0086]

[0087] Example 1: Analysis of actual samples

[0088] For the 119 purchased disposable degradable samples, the fluorine content was detected and analyzed according to the established and optimized method, and the results are shown in Table 4. Fluorine was detected in different degrees in 10 different types of disposable degradable samples. Among the collected samples, 31 PLA straws: the fluorine content was distributed in the range of 43.89-326.01 mg / kg, with an average of 94.39 mg / kg, and the unqualified rate was relatively high, reaching 45.16%; 13 PLA+PBAT straws: the fluorine content was distributed in the range of 24.25-139.15 mg / kg, with an average of 95.82 mg / kg, and the unqualified rate reached 30.77%; 17 PLA+PBS straws: the fluorine content was distributed in the range of 19.72-109.15 mg / kg, with an average of 55.81 mg / kg, and the unqualified rate was 5.88%; the remaining 7 types had detection or no detection, but no unqualified results were found. The results showed that, except for a small part of the disposable degradable materials in which fluorine was not detected, the detection of fluorine in most disposable degradable materials was relatively common, and a relatively high fluorine content was detected in some samples. The unqualified rate of PLA straws was the highest, followed by PLA+PBAT straws, and a small proportion of PLA+PBS straws also appeared unqualified.

[0089] Table 4 Results of fluorine content in 119 disposable degradable material samples (n=3)

[0090]

[0091]

[0092] ND: below the limit of quantification.

Claims

1. Determination of fluorine content in disposable degradable materials for food contact by bomb combustion-ion chromatography, characterized in that, The method comprises the following steps: pretreating the food contact disposable degradable material sample, converting the fluoride into free fluoride ions, and testing the fluoride ions absorbed or dissolved in the ultrapure water absorption solution; ion chromatography conditions are as follows: a Metrosep ASupp7 chromatographic column and a Metrosep A Supp 5 Guard protection column are used, the column temperature is 45 DEG C, 1.0-5.0 mmol / L sodium bicarbonate and 1.0 mmol / L anhydrous sodium carbonate solution are used as the mobile phase for isocratic elution, the injection amount is 20 mu L, the flow rate is 0.8-1 mL / min, a conductivity detector is used for testing, a chemical suppressor is MSN-A, a suppressor regeneration solution is a mixture of 200 mmol / L sulfuric acid and 5% acetone, an external standard method is used for quantification; the food contact disposable degradable material sample is pretreated, and the pretreatment comprises the following steps: the sample is cut into (1.5-2.5) mm * (1.5-2.5) mm, 0.15-0.25 g is weighed, and is placed in a quartz crucible, 0.1-0.22 g of flaky benzoic acid is added, a ignition wire is installed, 0.5-2.5 mL of anhydrous ethanol is added dropwise, 15 mL of ultrapure water is added in an oxygen bomb as an absorption solution, the oxygen charging pressure is 2.5-3.0 Mpa, the oxygen is discharged after being charged, the process is repeated three times, then oxygen is charged again, the oxygen bomb is placed in a normal temperature water bath, the ignition combustion program of the oxygen bomb is started, the oxygen bomb is shaken every 5-10 min for more than 30 min, the oxygen is slowly discharged, the absorption solution in the oxygen bomb is removed to a volumetric flask, the inner wall of the oxygen bomb, the electrode pipeline, the sample cup holder and the quartz crucible are washed with ultrapure water, and then are transferred to the volumetric flask, the volume is fixed to the scale, and after being shaken, the solution is filtered through a 0.22 mu m polyether sulfone needle filter for testing.

2. The method according to claim 1, wherein the method for determining the fluorine content in the food contact disposable degradable material by oxygen bomb combustion-ion chromatography is characterized in that, Preparation of the standard working solution comprises the following steps: 1000 mg / L of a fluoride ion standard solution is taken into a volumetric flask to prepare a 10 mg / L standard stock solution, which is stored at 4 DEG C; 10 mg / L of the fluoride ion standard stock solution is taken to prepare a 1.0 mg / L fluoride ion standard intermediate solution, which is stored at 4 DEG C; 0.00, 0.20, 0.50, 1.00, 2.50, 3.50 and 5.00 mL of the 1.0 mg / L fluoride ion standard stock solution are respectively taken into 10 mL volumetric flasks, and water is added to fix the volume to 10 mL, so as to prepare 0, 0.02, 0.05, 1.0, 2.5, 3.5 and 5.0 mg / L standard working solutions.

3. The method according to claim 1, wherein the method is for determining the amount of fluorine in a food contact disposable degradable material. The benzoic acid is 0.15-0.22 g, and the anhydrous ethanol is 1-2 mL.

4. The method for determining the fluorine content in food contact disposable degradable materials by oxygen bomb combustion-ion chromatography according to claim 1, characterized in that, The specification of the Metrosep A Supp7 chromatographic column is 250 mm * 4.0 mm; and the specification of the Metrosep A Supp5 Guard protection column is 50 mm * 4.0 mm.

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