What methods are used to detect heavy metals in tea leaves

1 Overview of pre-treatment methods for samples

The pre-treatment for the detection of heavy metal elements in tea is generally to remove the organic components in tea and retain the inorganic components including the heavy metal elements to be detected.

1.1 Traditional methods

The traditional methods are generally divided into two types: ashing and digestion. The ashing method uses high-temperature burning to destroy the organic matter in the sample, and finally uses dilute nitric acid to dissolve the heavy metals in the ash. Digestion method using concentrated nitric acid and concentrated sulfuric acid or nitric acid and perchloric acid and other strong oxidizing agents, and heating digestion so that the sample in the organic material is completely decomposed, oxidized, gaseous escape, to be measured components are converted to inorganic state exists in the digestive solution for testing. These two methods are national standards for sample processing methods, but in the detection process found that these two methods have some unfavorable factors:

The ashing method is too long, generally need 6 ~ 8 h, sometimes even need to spend dozens of hours, but also may cause the loss of volatile elements or crucible suction retention to reduce the value of the determination and the recovery rate; digesting method is also digested for a long period of time, the steps are cumbersome, the digestive process is prone to produce The digestion method also has a long digestion period, cumbersome steps, easy to produce a lot of harmful gases in the digestion process, and the amount of reagents, easy to make the blank value is high.

1.2 Microwave digestion and high-pressure digestion

Microwave heating is a direct "body heating" mode, its energy can be through the packaging materials, directly into the test solution. Many scientists have conducted research in this area. Fu Ming et al. used microwave digestion and inductively coupled plasma atomic emission spectrometry (ICP-AES) to determine the lead, arsenic, copper, cadmium, manganese, iron, zinc, selenium and other 12 elements in the tea, the RSD is less than 9%, the recovery rate of 84.5% to 115%. Lin Jie et al. determined the Cu and Pb elements in tea by using microwave digestion of samples with recoveries ranging from 93.1% to 105.9%, which has the advantages of fast, efficient, simple, reagent-saving and low blank value.

High-pressure digestion is to place tea in a high-pressure digestion tank, the use of high-temperature, high-pressure, closed strong acid or alkali environment to achieve the purpose of rapid digestion of insoluble substances, which can shorten the process of digestion, and minimize the volatilization loss of the measured components, which is conducive to the control of the accuracy of the determination. At present, it has been widely used in various analytical fields and recognized as a standard method. The degree of dissolution of this method is relatively good and the cost is not high, but the risk factor is relatively high compared with other methods. Lu Yang et al. established a method for the simultaneous determination of selenium and tin in tea by sealed digestion atomic fluorescence spectrometry, with the detection limit of selenium being 0.32 μg/L, the relative standard deviation (RSD) of 1.8%, and the recoveries of sample spiking being 96.4%~98.8%, and the detection limit of tin being 0.30 μg/L, the RSD of 3.5%, and the recoveries of sample being 92.7%~102.0%. Peng Yukui with pressurized dissolved samples and other heavy metal detection in tea research overview

1.3 Acid extraction method

Acid extraction method as a sample rapid determination of the pretreatment technology, but also by domestic and foreign scholars are widely studied. Leaching method is to leach out the measured components in it with appropriate leaching agent, which is easy and fast to operate, but sometimes not all the measured components can be extracted completely, and attention must be paid to check the degree of extraction. Dachun Li used HCl leaching and atomic absorption to determine lead and copper in tea, and the precision and accuracy of the results were basically the same as those of the tea specimen using the national standard NHO3-H2SO4-HClO4 method. Yuan Jian used 2 mol/L hydrochloric acid at 70 ℃ for 60 min to extract the heavy metal elements in tea, and the extraction rate of Cu was 96.7%, and the extraction rate of Pb was 93.2%.

1.4 Other methods

The traditional sample pretreatment methods have the problems of cumbersome and time-consuming operation, low recovery, high experimental blank value, and the pollution of the environment by reagents, etc. Finding simple and effective sample pretreatment methods has always been a research topic for analysts. For example, Wang Jiangjie et al. applied the suspension injection technique and flame atomic spectrometry to determine copper in tea. Weng Di et al. used ultrasonic stirring suspension sampling technique and flame atomic absorption spectrometry to successfully determine the content of copper, iron, zinc, lead and cadmium in tea. Electrothermal evaporation (ETV), as a feeding technique for the combined use of ICP-AES and ICP-MS, has the advantages of high feeding efficiency, low sample requirement, low detection limit, and the ability to directly analyze solid specimens. Chen Shizhong used polytetrafluoroethylene (PTFE) suspension as a chemical modifier for the direct and simultaneous determination of the evaporation behaviors of trace elements La, Yb, Y, Cu, and Cr in tea by the ETV-ICP-AES method using suspension samples, and the main influencing factors were investigated.

2 Overview of sample detection methods

2.1 Atomic spectrometry

Atomic Absorption Spectrometry (AAS) [2][6][10][20] is currently one of the most commonly used methods for the detection of heavy metal elements in tea, and it is useful for analyzing the Pb, Cd, Zn, Cu and other heavy metal elements in tea with high sensitivity. The method is based on the absorption of ultraviolet light and visible light by the outer electrons of the base atom of the measured element in the gaseous state for quantitative analysis of the element. According to the different ways of atomization, it can be divided into flame atomic absorption spectrometry (FAAS) and graphite furnace atomic absorption spectrometry (GF-AAS).FAAS is a mature analytical technique, with the advantages of simple operation, fast analysis speed, low interference in the determination of high-concentration elements, and stable signals, etc. GF-AAS is a commonly used trace analytical technique, which has a very high sensitivity, and has the advantages of a small amount of sampling, GF-AAS is a commonly used trace analysis technique with high sensitivity, small sample size, simple chemical pretreatment, and the ability to directly analyze solid and high-viscosity liquid samples. The disadvantages of these two methods are that FAAS is not suitable for the determination of heat-resistant elements that cannot be completely decomposed in the flame (e.g., B, V, Ta, W, Mo) and alkaline-earth metal elements as well as *** vibration absorption lines in the far-ultraviolet region of the elements (e.g., P, S, halogens); the GF-AAS method of the matrix of the interference is more serious, and it is not suitable for multi-elemental analysis. Margo et al[21] studied the determination of lead and tin in tea by transversely heated graphite furnace atomic absorption spectrometry, using ferric dihydrogen phosphate and magnesium nitrate as a mixed matrix improver to eliminate matrix interference, and the detection limits of lead and tin reached 0.0078 μg/g and 0.0015 μg/g, respectively.Atomic Emission Spectrometry (AES) is a technique using gaseous emission spectroscopy (GF-AAS) to determine lead and tin in tea. Atomic Emission Spectrome-try (AES) is a method of detecting gaseous atoms (or ions) that emit characteristic ultraviolet and visible radiation when they are thermally or electrically excited. The method has high sensitivity, good selectivity, and can analyze a variety of elements at the same time, it is a commonly used analytical method, especially ICP-AES has higher sensitivity and a wide linear range (0~105), which has been researched more in recent years.

Atomic Fluorescence Spec-trometry (AFS) is a method of determining the content of an element to be measured by measuring the fluorescence intensity of the atomic vapor of the element to be measured under the excitation of radiation energy. The method has less matrix interference and high sensitivity, but the disadvantage is that the application area is narrow, and the measurement is seriously affected by scattered light.In 1969, Holak combined the classical arsenide generation reaction with AFS, and created the joint technique of hydride generation-atomic spectrometry (HG-AAS).In 1974, Tsjiu and Kuga combined the hydride generation feeding technique with dispersionless AFS, and realized the combined technique of hydride generation-atomic spectrometry (HG-AFS). In 1974, Tsjiu and Kuga combined the hydride injection technique with the dispersionless atomic fluorescence analysis to realize the joint analysis of hydride generation - dispersionless atomic fluorescence spectroscopy (HG-AFS), and then the HG-AFS analytical technique was rapidly developed and applied, and now it has become an important means of analysis of metal elements. In recent years there have been used this method to determine the tea

2.2 Electrochemical method

The detection of trace elements and heavy metals in tea by electrochemical methods has also been reported more, it is represented by polarographic analysis, based on which it is Issue 1 Hou Fang Overview of the research on the detection of heavy metals in tea 15

Derived from the voltammetric analysis and ion-selective electrodes and other methods. Wang Hui et al. determined the lead content in tea using differential pulse dissolution voltammetry (DPRV) with a silver-based mercury film electrode as the working electrode in 0.1 mol/L HCl substrate, and the peak currents of the lead and its mass concentration showed a good linear relationship within the range of 0.1-15 μg/mL, with the lowest limit of detection (LOD) of 0.01 μg/mL. The electrochemical analysis method has the advantages of being independent of the color and turbidity of the sample liquid, wide measurement range, high sensitivity, simple and rapid analytical steps, no use of large-scale instruments, and economic applicability. Its disadvantage is that the conditions are harsh and the reproducibility of the measurement results is poor. With the development of various biosensors, catalytic systems and complexation systems, as well as the development of enzyme electrodes, microelectrodes and modified electrodes, the application of electrochemistry in the heavy metal analysis of tea has a broad prospect. Guangying Zhao et al [30] utilized screen-printed carbon electrodes modified by isotopic mercury plating and electrochemical square-wave dissolution voltammetry for the rapid detection of lead in tea, and the sensitivity, linear range and detection limit of the method were 22.7 nA-μg-1-L-1, 10-225 μg-L-1 (r=0.9986) and 0.74 μg-L-1 (S/N=3), respectively.

2.3 ICP/MS method

ICP/MS (Inductively Coupled Plasma Mass Spec-trometry) is a mass spectrometry method using inductively coupled plasma (ICP) as the ionization source. Many people at home and abroad have used this method to determine heavy metals in tea, and the linear detection range of the present ICP-MS instruments can reach 9 orders of magnitude. Huang Zhiyong et al [31] used ICP-MS to simultaneously determine the content of trace elements in five kinds of tea, namely Biluochun, Oolong tea, Maofeng, Jasmine tea and Yunnan Dianhong, and the recoveries of the method were mostly between 95% and 110%, with the relative standard deviation less than 5%.

2.4 Other methods

Ultraviolet-visible spectrophotometric method has high sensitivity, simple equipment, low cost of determination, good quantification, and is suitable for use in the laboratory. The disadvantage of this method is that the detection limit of low content of heavy metals can not meet the requirements of the determination of certain elements of organic solvents need to be extracted several times, the operation is more cumbersome and so on. Wan Yiqun et al. used wax phase spectrophotometry to determine trace manganese in tea with a limit of detection of 1.6×10-9 g/mL, and the sensitivity of the method was 10 times higher than that of liquid phase spectrophotometry. Pan Zhongwei et al. used ion exchange resin photometry to determine trace copper in three kinds of tea, and the sensitivity of the method was nearly 7 times higher than that of the solution photometric method.

In addition, there are fluorescence quenching method, liquid chromatography, chemiluminescence, neutron activation analysis method. Ion chromatography was used to determine the content of copper, lead and other seven metal elements in tea. Shu Youqin et al. determined the zinc, manganese, copper, lead and cadmium in tea by capillary ionization analysis (CIA) with an average recovery of 96.4%~104.2%, and the detection limit of the method was 0.02~0.2 μg/ml. Zhou Yuehua et al. determined the selenium content of tea by molecular fluorescence method.