Analysis of Heavy Metal Lead and Chromium Elements in Food Packaging Bags

Xiaoxiang QIU, Kaipo WANG

Abstract [Objectives] This study was conducted to detect the contents of heavy metal lead and chromium in food packaging bags.

[Methods] The contents of heavy metal lead and chromium in food packaging bags were determined by microwave digestion-flame atomic absorption spectrophotometer. With concentrated nitric acid and 30% hydrogen peroxide solution as the digestion system， food packaging bags of different materials， plastic packaging bags and paper packaging bags， were ultrasonically digested and then determined for the contents of heavy metal lead and chromium by flame atomic absorption spectrophotometry.

[Results] The determination results showed that the linear correlation coefficient of lead was 0.996 7， and the linear correlation coefficient of chromium was 0.997 7. The method has the characteristics of simplicity， high analysis speed and high sensitivity.

[Conclusions] This study provides a theoretical basis for the safety of food packaging bags.

Key words Food packaging bag; Heavy metals; Lead; Chromium; Atomic absorption

Received： May 7， 2022  Accepted： July 8， 2022

Supported by Special Scientific Research Project of Shaanxi Provincial Department of Education （16JK1275）.

Xiaoxiang QIU （1974-）， female， P. R. China， associate professor， master， devoted to research about analytical chemistry and modern instrumental analysis methods.

*Corresponding author. Kaipo WANG （1990-）， male， technician， devoted to research about environmental analysis and monitoring. E-mail： wnqiuxx@126.com.

With the continuous improvement of the quality of life， people not only pay attention to the safety of food itself， but also pay more and more attention to the safety of food packaging materials. Food packaging bags are film containers that are in direct contact with food and are used to contain and protect food， and are a type of packaging design. In order to facilitate the preservation and storage of food， people have produced food packaging bags. Food packaging bags are generally more than two-layer composite material packaging bags， and should be produced with food-grade plastic films.

In order to increase the plasticity， strength and transparency of food packaging bags， it is necessary to add various auxiliaries. While optimizing the quality of materials， these auxiliaries will also lead to the residues of toxic and harmful substances such as lead， chromium and heavy metals. Harmful substances of these heavy metal elements will dissolve out under certain conditions， and then migrate to the food in direct contact with them. When all heavy metals exceed a certain concentration， they will cause harm to human health. The first is to harm the liver. Heavy metals will combine with hematoporphyrin in the blood to damage the liver， leading to liver cirrhosis and liver cancer. Then， it will damage the blood circulatory system. After heavy metal poisoning， the viscosity of the blood will increase， and the low oxygen content will lead to stuffiness， and in severe cases， symptoms such as shock will occur. The second is to harm， inhibit and interfere with the function of the nervous system. For children and the elderly， the harm of heavy metals will be even greater. Because the immunity of children is low and the metabolism of the elderly is slow， it is relatively difficult for these heavy metal harmful substances to be excreted from the body. Therefore， such people should pay more attention to their diet and daily necessities that are likely to cause us to eat heavy metals.

Since food packaging bags are two kinds of samples， paper and plastic packaging bags， in order to accurately analyze the heavy metal elements in them， the pretreatment of the samples is particularly important[1-2]. There are three commonly used digestion methods. ① The ashing method （also known as the dry method）： It uses high temperature to remove organic matter in samples， and dissolves the remaining ash with acids to give sample solutions to be tested. However， under high temperature conditions， these heavy metal elements are easily lost by volatilization， so this method is not applicable. ② The wet digestion method： In this method， acids or alkaline solutions are used to destroy organic or reducing substances in samples under heating conditions. Wet digestion uses a large amount of acids and it is difficult to make the solutions clear and translucent， which is easy to cause environmental pollution and loss of elements， so this method is not used. ③ Microwave digestion[3-4]： Generally， microwave is used to heat digestion solutions and samples in airtight containers. It can quickly and effectively decompose samples and shorten the time for dissolving samples. This method consumes less amount of reagents， usually a few milliliters; the loss of samples during the digestion process and the possibility of cross-contamination are greatly reduced; it consumes less energy， has no need for personnel to be on duty in shifts， and can realize automatic operation easily and also reduce environmental pollution caused by routine digestion of acid mist; and the method avoids the loss of volatile trace elements， and the sample determination is reproducible. Therefore， the microwave digestion method was adopted in this study to digest food packaging bags. Flame atomic absorption spectrophotometry is used to detect heavy metal contents in food packaging bags. This method is simple to operate， fast in analysis， time-saving and efficient， has less cross-interference of spectral lines， high precision， low detection limit， good baseline stability， and can be used to determine the contents of macro and trace metal elements in a variety of different materials. At present， the most frequently used methods for the determination of heavy metal elements include inductively coupled plasma-optical emission spectrometry[5-6]， flame atomic absorption spectrometry （AAS）[7]， and graphite furnace-atomic absorption spectrometry （CF-AAS）[8-9]. Therefore， microwave digestion method-atomic absorption spectrophotometer was used in this study to analyze and determine the contents of heavy metals in food packaging bags[10-11].

Materials and Methods

Main instruments and reagents

Main instruments

Flame atomic absorption spectrophotometer： WFX-120， Beijing Ruili Analytical Instrument Company; microwave digestion apparatus： APL （APL） Instrument Co.， Ltd.; electronic analytical balance： XT220A， Shanghai Precision Instrument Co.， Ltd.

Main reagents

Concentrated nitric acid， 30% hydrogen peroxide solution： guaranteed reagent， commercially available; lead standard stock solution and chromium standard stock solution： the mass concentration was 0.1 g/L， and it were diluted step by step during use; sample： food packaging bags： packaging bags of Hongsifang turkey crispy noodles， packaging bags of Chinese hamburger; experimental water： double distilled water， self-made.

Sample collection

The paper packaging bags of Chinese hamburger were purchased from the canteen of Weinan Normal University， and the plastic packaging bags of Hongsifang turkey crispy noodles were purchased from a supermarket. The packaging bags had printed patterns on their surfaces.

Sample preparation

The plastic food packaging bags were washed with ultrapure water for three times and then air-dried naturally， and cut into 5 mm×5 mm. The cut pieces were mixed well， and a 0.2 g of sample was accurately weighed （accurate to 0.1 mg）， and stored in a desiccator for later use.

Sample digestion

A 0.200 0 g of food packaging bag sample was accurately weighed， then put into a high pressure digestion tank （polytetrafluoroethylene）， and added with 3 ml of concentrated nitric acid to soak overnight. After soaking overnight， 2 ml of 30% hydrogen peroxide solution was added， and then the high-pressure digestion tank was put into a microwave digestion apparatus to start digestion. The working parameters of the microwave digestion apparatus are shown in Table 1. After the digestion was completed， the digestion tank was taken out from the microwave digestion apparatus after 3-5 min， cooled for 20 min and uncovered in a fume hood. After the digestion solution was naturally cooled， it was transferred to a 50 ml volumetric flask with ultrapure water， and diluted to constant weight. The obtained solution was shaken well and filtered or stood for clarification， for later use. A blank experiment was also done at the same time.

Sample determination

The absorbance of each liquid to be tested was measured with a flame atomic absorption spectrophotometer， and the heavy metal contents in the food packaging bag was calculated in combination with the regression equation of the standard curve. The working condition parameters are shown in Table 2.

Results and Analysis

Selection of sample digestion system

When using a microwave digestion apparatus for digestion， different digestion systems and digestion conditions will have a greater impact on the determination results. Concentrated nitric acid and perchloric acid both have strong acidity and strong oxidizing properties. Because nitric acid has less interference in the analysis of heavy metal elements， it was selected as an ideal microwave digestion solution. And because hydrogen peroxide solution can accelerate the digestion of residual organic matter in liquids， thereby reducing the chromaticity and turbidity of digestion solutions and reducing the impact， we chose a mixed acid system of nitric acid-30% hydrogen peroxide solution （volume ratio 3∶2） to digest the samples. The results of the digestion under different conditions were shown in Table 3.

Too much or too little digestion amount of sample will cause a large error， generally not less than 0.1g， not more than 0.5 g， and the sample digestion amount of 0.2 g was selected in this study.   When the nitric acid was dripped， the paper packaging bags made a squeaking sound， but the plastic packaging bags did not make a sound.

Drawing of standard curves

Certain amounts of lead nitrate and potassium dichromate were accurately weighed after drying to prepare a 0.1 g/L lead-chromium standard stock solution. Then， with 1% nitric acid （volume fraction） as the solvent， the lead-chromium standard stock solution was prepared into a series of standard working solutions with mass concentrations of 0.0， 0.2， 0.4， 0.6， 0.8 and 1.0 g/L， respectively， and the volumes were adjusted to a 25 ml volumetric flask. The absorbance of a series of standard working solutions was measured with a flame atomic absorption spectrophotometer. The relationship between absorbance and concentration was plotted， and the results are shown in Fig. 1 and Fig. 2.

Linear regression equations for heavy metals lead and chromium

The linear regression equations and correlation coefficients are shown in Table 4.

Recovery test

First， 0.200 0 g of plastic food packaging bag was accurately weighed into a small beaker and digested according to "Sample digestion"， and the obtained solution was diluted with 1% nitric acid solution in a 25ml volumetric flask. Then， 3.00 ml of sample was pipetted， respectively， into three colorimetric tubes， which were then added with 10.00， 15.00， and 20.00 ml of the lead ion standard solution with a mass concentration of 0.1 g/L， respectively， and the obtained solution was measured according to "Sample determination". Specifically， with the original sample amount of 2.6 mg/L and the amount of standard added at 3.0 mg/L， the measured value was 8.8 mg/L， and the recovery was 123.6%.

Sample Determination

Determination results of heavy metals lead and chromium in food packaging bag samples

According to the working conditions of the above instrument， the absorbance of each sample was measured， and then the contents of metal elements in different samples were calculated according to the linear regression equations. The measured contents were multiplied by the dilution factor and divided by the mass of corresponding sample to obtain the metal content per kilogram of the sample. The contents of the two metals in the samples are shown in Table 5 and Table 6.

Conclusions and Discussion

A digestion system was established with a mixed acid of nitric acid-30% hydrogen peroxide solution （volume ratio of 3∶2）， and the contents of lead and chromium in food packaging bags were determined by a flame atomic spectrophotometer. The determination results showed that the contents of heavy metal lead and chromium in the food packaging bags were in line with Chinas food packaging hygiene standards. Meanwhile， the effective disposal of domestic waste can not only change the appearance of a city， but also create a cleaner and more comfortable living and working environment for citizens， which is also of great significance to urban construction.

References

[1] WANG CX， HUANG J. Determination of heavy metals in soil and sediment by microwave digestion–flame atomic absorption spectrometry[J]. Chemical Analysis and Meterage， 2018， 27（6）： 64-67. （in Chinese）.

[2] ZHANG J， ZHANG YB. Determination of lead and chromium in composite plastic films for food by ICP-MS and their uncertainty evaluation[J]. Plastics Science and Technology， 2013， 41（5）： 100-101. （in Chinese）.

[3] KAI JR， WANG CY， LI CH. Comparative study on two pretreatment methods for atomic absorptive spectrophotometry determination of Cu， Zn and Cd in rice[J]. Food Science and Technology， 2018， 43（2）： 322-323. （in Chinese）.

[4] TANG XW， ZHOU DW， LU TL， et al. Determination of zinc， plumbum and chromium contents in Akebia trifoliate peels by flame atomic absorption spectrophotometry[J]. Journal of Jishou University， 2014， 35（5）： 60-61. （in Chinese）.

[5] WANG CX， HUANG J. Simultaneous determination of seven heavy metal elements in municipal sludge by microwave digestion-inductively coupled plasma mass spectrometry （ICP-MS）[J]. Environmental Protection and Technology， 2019， 25（1）： 29. （in Chinese）.

[6] WANG TL， LIU BJ， GUO J， et al. Determination of 8 kinds of heavy metal elements in wheat grains by inductively coupled plasma mass spectrometry method[J]. Journal of Henan Agricultural Sciences， 2018， 47（11）： 148-149. （in Chinese）.

[7] WU ST. Evaluation of uncertainty in determination of copper in water by flame atomic absorption spectrometry[J]. China Resources Comprehensive Utilization， 2018， 36（1）： 51-52. （in Chinese）.

[8] YE YP， YE WP， ZHOU H. Detection and evaluation on Pb and Cd release amount of Longquan celadon by graphite furnace by atomic absorption spectrometry[J]. China Ceramics， 2016， 52（8）： 48. （in Chinese）.

[9] WU YF， ZHANG SJ， XU HD. Determination of lead in apple juice concentrate[J]. Modern Food， 2019（1）： 143-146. （in Chinese）.

[10] ERICO M， FLORES M， SAIDELLES PF， et al. Determination of Cd and Pb in medicinal plants using solid sampling flame atomic absorption spectrometry[J]. International Journal of Environmental Analytical Chemistry， 2009， 89（2）： 129-140.

[11] ALSHAWABKEH AN. Coupling electrokinetics with permeable reactive barriers of zero-valent iron for treating a chromium contaminated soil[J]. Separation science & technology， 2009， 44（10）： 2188-2202.