Due to its wide range of raw materials, variety, easy processing and excellent performance, plastics have become an indispensable material in various industries. Plastic products have been widely used in food packaging, drinking water pipes, children's toys, medical equipment, etc. Due to contact with the human body, these products are required to be non-toxic or low-toxic. However, various additives in plastics such as plasticizers, stabilizers, fillers, colorants, antioxidants, etc., have varying degrees of toxicity, and they may be taken out by water or oily foods and then enter the body.
At present, China only has polyethylene (PE) as raw material for food utensils, packaging containers and food industrial appliances. The sanitary standard GB/T5009.60-1995, this method only stipulates that PE molded products are soaked in 4% acetic acid for 2h. The amount of lead dissolved should not exceed 1 mg/L, and there is no hygienic standard requirement for plastic raw materials. Since raw materials are the basis of all finished products, it is necessary to study the indicators of harmful substances involved in plastic raw materials. The authors are related to plastic materials such as PE, polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), (acrylonitrile/butadiene/styrene) copolymers which are closely related to daily life and safety and hygiene. A preliminary study was conducted on the determination methods of lead, cadmium and mercury in ABS). Microwave digestion was used as the pretreatment method, and ammonium dihydrogen phosphate was used as the matrix modifier to determine lead and cadmium by graphite furnace atomic absorption spectrometry. Mercury was determined by flow injection-hydride generation-atomic absorption method. The method was simple and accurate.
1 experimental part
1.1 main instruments and reagents
Atomic absorption spectrophotometer: PE3110 type, with AS-60 automatic sampler, HGA-600 graphite furnace, American PE company; lead, cadmium hollow cathode lamp, mercury high-intensity electrodeless discharge lamp: American PE company; PE platform Graphite tube: Beijing Libao Scientific Equipment Co., Ltd.; microwave dissolution system: MLS-1200, Italy Mileston; flow injection-hydride generator: WHG-102A2, Beijing Hanshi Scientific Instrument Factory; potassium borohydride ( KBH4) solution: 15g/L. Weigh KBH4 (purity about 96%) 1.5g, sodium hydroxide 0.3g, pour into PE plastic bottle together, add 100mL distilled water to dissolve, valid for one week, not available glass container; lead, cadmium, mercury standard solution: 1mg /mL, National Standards Research Center. Dilute to the appropriate concentration when used; nitric acid, hydrogen peroxide, sulfuric acid: all grade pure; ammonium dihydrogen phosphate solution: 50mg / mL. 5.0 g of ammonium dihydrogen phosphate was dissolved in water and dissolved to 100 mL; the experimental water was secondary deionized water.
1.2 Experimental methods
(1) Method 1 Accurately weigh 0.5g plastic sample (accurate to 0.001g), place it in a polytetrafluoroethylene pressure tank for microwave digestion, add 8.0mL concentrated nitric acid, 1.0mL hydrogen peroxide, and cover the pressure tank. The protective cover is then placed in a microwave digestion device for microwave digestion. After the digestion process (see Table 3) is completed and cooled, the in-tank digestion solution is transferred to a 50 mL volumetric flask and made up to volume with secondary deionized water. Determination of lead and cadmium by graphite furnace atomic absorption spectrometry and determination of mercury by flow injection-hydride generation-atomic absorption method. At the same time, do a blank test.
(2) Method 2 accurately weigh 0.5g plastic sample (accurate to 0.001g), place it in a PTFE pressure tank for microwave digestion, add 3.0mL concentrated sulfuric acid, and cover the pressure tank with a protective sleeve. Digestion was carried out for 5 min under 300 W conditions. After cooling, the digestion device was opened, 3.0 mL concentrated nitric acid and 1.0 mL hydrogen peroxide were added. After the digestion process (see Table 4) was completed and cooled, the in-tank digestion solution was transferred to a 50 mL volumetric flask. Reduce the volume with secondary deionized water. Determination of lead and cadmium by graphite furnace atomic absorption spectrometry and determination of mercury by flow injection-hydride generation-atomic absorption method. At the same time, do a blank test.
1.3 Drawing of standard working curves
1.3.1 Drawing of standard working curves of lead and cadmium
Prepare 50μg/L and 20μg/L lead and cadmium standard solutions, respectively, 20μL, 4μL 50mg/mL ammonium dihydrogen phosphate solution as matrix modifier, auto-dilution by autosampler, under selected instrument working conditions The absorption peak area was measured and a standard working curve was drawn.
1.3.2 Drawing of the standard working curve of mercury
Prepare 100μg/L mercury standard solution, and transfer 0, 1.00, 2.00, 5.00, 10.00, 20.00, 50.00mL in a 100mL volumetric flask, and make up to volume with 0.5% sulfuric acid, using flow injection-hydride generation-atomic absorption method. The absorption peak area was measured and a standard working curve was drawn.
2. Results and discussion
2.1 Selection of dissolution conditions
2.1.1 Power-time digestion program settings
The sample was completely dissolved, and the digestion solution was colorless, clear (or slightly yellow) for complete digestion, and the effect of digestion power and time on the digestion effect was investigated. Several experiments were carried out using different heating power-time digestion procedures, and it was found that the sample was difficult to digest completely at low power, and the digestion solution was a turbid yellow liquid; while heating at high power for a long time would cause acid gas leakage. The results show that for plastic samples such as PE, PP, PS, PVC, etc., using the digestion procedure shown in Table 3, a good digestion effect can be obtained. For plastic samples that are difficult to disintegrate, such as ABS, concentrated sulfuric acid should be added for preliminary digestion, and then further digested by the procedure shown in Table 4.
2.1.2 Effect of concentrated nitric acid on digestion
The microwave digestion device was used to digest PE, PP, PS, PVC and other plastic samples, and 5.0, 6.0, 7.0, 8.0, 9.0, 10.0 mL concentrated nitric acid and 1.0 mL hydrogen peroxide were added to digest the sample. table 5. It can be seen from Table 5 that a good digestion effect can be obtained by adding 7.0 mL of concentrated nitric acid; the amount of concentrated nitric acid is less than 7.0 mL, and the sample digestion is incomplete; if the amount of concentrated nitric acid is too large, the blank value is increased. In this experiment, 8.0mL concentrated nitric acid was used as a reagent for digesting plastic samples such as PE, PP, PS, PVC. For plastic samples that are difficult to dissolve, such as ABS, firstly add 3.0mL of concentrated sulfuric acid for preliminary digestion, then add 3.0mL of concentrated nitric acid and 1.0mL of hydrogen peroxide for digestion, which can obtain good digestion effect.
2.1.3 Effect of the amount of hydrogen peroxide on the digestion effect
In order to completely destroy the complex organic matrix of plastic, it is necessary to add a strong oxidant, select hydrogen peroxide and examine its dosage. The effect of the amount of hydrogen peroxide on the digestion of PE is shown in Table 6. As can be seen from Table 6, the PE sample was completely digested by the addition of 1.0 mL of hydrogen peroxide. The amount of hydrogen peroxide used in the other samples was also selected to be 1.0 mL.
2.2 Selection of determination conditions for lead and cadmium
2.2.1 Selection of matrix modifier
The ammonium dihydrogen phosphate, ammonium nitrate and magnesium nitrate were selected as matrix modifiers respectively for experimental comparison. The effect of ammonium dihydrogen phosphate was found to be the best. When 20 μL of 20 μL was injected, 4 μL of 50 mg/mL ammonium dihydrogen phosphate solution was added to achieve the best effect. The ashing temperature of lead was increased from 500 ° C to 800 ° C, and the ashing temperature of cadmium was increased from 300 ° C to 850 ° C.
2.2.2 Selection of ashing temperature and atomization temperature
With 50mg/mL ammonium dihydrogen phosphate solution as the matrix modifier, lead has the highest absorption value when the ashing temperature is 800 °C; cadmium has the highest absorption value when the ashing temperature is 850 °C. The optimum atomization temperatures of lead and cadmium are 1900 ° C and 1700 ° C, respectively.
2.3 Selection of mercury determination conditions
2.3.1 Determination of carrier gas flow rate
The gaseous compound formed by the reaction of the elemental element mercury with KBH4 is carried by the carrier gas to the atomizer, so the carrier gas flow rate plays an important role in the detection signal of the sample. A smaller carrier gas flow is beneficial to the enhancement of signal intensity. If the flow rate is too large, the concentration of mercury atoms will be diluted to reduce the sensitivity; however, the carrier gas flow rate is too small and the signal is unstable. The carrier gas flow rate at the time of mercury measurement was selected to be 100 mL/min.
2.3.2 The effect of KBH4 concentration
The atomic absorption intensity of mercury is relatively high at low concentration of KBH4 (0.5-3.0 g/L), and the signal intensity does not change much. The further increase of KBH4 concentration will lead to a decrease in the detection signal, because mercury does not form hydride. It only needs to be reduced to gaseous mercury, and is carried by the carrier gas to the atomizer for atomization, so no excessive KBH4 is needed. Excessive KBH4 will generate a large amount of hydrogen, which will dilute the concentration of mercury. The KBH4 concentration should be chosen to be 0.5 g/L when measuring mercury.
2.3.3 Acidity of the solution
The reaction for generating a gaseous compound needs to have a suitable acidity. When the concentration of hydrochloric acid is in the range of 4% to 20%, the detection signal of the test solution is not greatly affected. When the concentration of hydrochloric acid is less than 4%, the result is remarkably low. The concentration of hydrochloric acid was chosen to be 10% when measuring mercury.
2.4 standard
Working curve and detection limit Under the optimized working conditions, the lead, cadmium and mercury concentrations in the range of 0-50, 0-10, 0-50μg/L are in good linear relationship with the absorbance, and the linear regression equation and correlation coefficient They are: lead: y=0.0086x, r=0.9969 cadmium: y=0.0893x, r=0.9997 mercury: y=0.0044x, r=0.9998 The detection limit of lead is 4.5×10-11g, the detection of cadmium The limit is 3.7×10-12g, and the detection limit of mercury is 0.64μg/kg.
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