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Mainland tea is mainly exported to the United States, Japan, the European Union and other countries and regions, with the improvement of tea requirements of these importing countries, a variety of extensive and strict standards have been introduced, resulting in mainland tea exports encountered unprecedented "green barriers", seriously affecting mainland tea exports. The quality and safety inspection of tea involves the production and processing of tea raw materials, in which harmful microorganisms, non-tea foreign substances, and pesticide residues belong to raw material pollution, and harmful heavy metals and powder layers belong to the pollution factors existing in processing.
The problem of harmful heavy metal pollution in tea is mainly reflected in the excessive standards of Cu and Pb elements, and the national standard (GB 9579) has formulated strict limit standards for heavy metal elements such as Cu and Pb. The Tea Research Institute of China Academy of Agriculture measured the heavy metal content in tea in 20 provinces and cities in mainland China, and the results showed that black tea had a high Pb content, while the Pb content in green tea and oolong tea was relatively low.
For the detection of heavy metal content in tea, FS atomic fluorescence method, FAAS, GAAS atomic absorption method, electrochemical method, ICP, ICP_MS atomic emission spectrometry are more traditional heavy metal element determination methods, and the results are accurate, but the relative detection cost is high. In order to improve the speed and accuracy of detection, microwave digestion technology has been widely used in sample pretreatment, and acid extraction technology has gradually received attention, and extensive research has been carried out among scholars at home and abroad.
In order to seek the best determination conditions for acid extraction technology, the extraction reagent concentration, reagent temperature, tea and reagent reaction time and other factors were analyzed in the tea extraction process, and the optimal reaction conditions were obtained to achieve the purpose of improving extraction efficiency and rapid detection.
Sample and extraction reagent selection
Experimental sample: ordinary tea. Source: Market random purchase. Extraction reagent: hydrochloric acid (0.5~2.0mol/L, where the step size is 0.5). Source: High concentration of pure hydrochloric acid plus pure water. Standard copper solution: 1100μg/ml, National Iron and Steel Materials Testing Center, Iron and Steel Research Institute, 50, 90, standard copper solution prepared with high-purity water. Standard lead solution: 1100μg/ml, National Iron and Steel Materials Testing Center of the General Institute of Iron and Steel Research, prepared with high-purity water to prepare 50, 90, standard lead solution.
Graphite furnace atomization instrument, with flame atomization, 85-185 atomic absorption spectrophotometer, Zeeman effect clasp background method.
Table 1
The tea leaves are configured into solvents according to the ratio of raw materials and acids 1:9, and they are fully reacted under specific temperature and time conditions, separated by centrifugal precipitation technology, and the supernatant is obtained, at which time the heavy metal elements in the tea are basically dissolved in the supernatant, and finally the content of heavy metals in the supernatant is quantitatively determined by graphite furnace atomic spectrophotometer.
Two-way statistical analysis experiments
Change the two main factors to be studied, keep other external conditions unchanged, and analyze whether these two factors affect each other. (1) Analyze the influence of different crushing fineness. Weigh an appropriate amount of tea leaves in two parts (A, B), both 2.50g, of which A remains as it is, B is ground into powder, 25.00ml hydrochloric acid solution is added respectively, the reaction is the same time, and then precipitated, centrifuged, and the supernatant is extracted for later use. (2) Examine the influence of different temperature conditions. The concentrations of heavy metals contained in A and B solutions were determined by placing them at 30 °C and 70 °C respectively, and then changing the reaction time while changing the concentration of hydrochloric acid reagent.
Table 2
This method uses statistical knowledge to investigate their respective effects on extraction efficiency from three factors: time, temperature and reagent concentration. The tea sample was prepared into a sample solution with a material-liquid ratio of 1:9, hydrochloric acid solution was added to centrifuge, the supernatant was extracted, and purified water was added for reasonable dilution, and then its metal content was determined. Weigh an appropriate amount of tea leaves 2.50g, add 25.00ml of acid solution to soak, and then precipitate, centrifuge, obtain a volumetric dilution of the supernatant and set aside. The absorbance velocity of the sample solution was determined by graphite furnace atomizer, and the standard value was compared for quantitative analysis. Another set of blank tests was done as a control.
Analysis of the results obtained from each trial
In this paper, the contents of Pb and Cu in tea samples were determined separately, and the extraction efficiency was recorded, and the results obtained were shown in Table 3.
Table 3
At 30°C, 1.5mol/L hydrochloric acid solution was used to extract the contents of Cu and Pb metals in the crushed tea samples and the original tea samples, and the metal contents extracted at different reaction times were recorded. The experimental results obtained are obtained as shown in Figure 1 and Figure 2.
Figure 1
Figure 2
It is obvious from Figure 1 that the extraction rate of Cu element and direct extraction after crushing is basically the same, and there is no obvious difference, which can be considered that the crushing of tea samples can be ignored when Cu element is extracted. It can be seen from Figure 2 that for the metal element lead, the extraction efficiency after crushing is obviously higher than that of direct extraction, that is, the measurement result after crushing is significantly higher, which has a significant impact on the final detection result.
Two-factor experimental results
Various concentrations of hydrochloric acid reagent solution were added to tea samples of the same weight, reacted at the same temperature (30 °C), centrifuged according to different reaction times, extracted supernatant for analysis, and the content results of Cu and Pb were shown in Figures 3 and 4 below.
Figure 3
The figure above shows that at the same temperature, the extraction efficiency of using clean water as the extraction reagent is much smaller than that of hydrochloric acid solution, and the extraction efficiency of copper and lead is less than 30%. Figure 3 shows that the extraction time has almost no significant effect on copper, but when selecting 0.5mol/L hydrochloric acid solution extractant, the extraction efficiency and extraction time show a negative correlation, that is, the longer the reaction time, the extraction efficiency becomes lower, but the extraction efficiency of copper element is positively correlated with the concentration of hydrochloric acid solution, that is, the higher the reagent concentration, the faster the extraction speed and the higher the extraction rate.
The extraction efficiency of Pb elements also showed a simple positive correlation with the concentration of hydrochloric acid at 30°C. In general, when using an extraction temperature of 30 °C, the extraction rate of copper under various concentrations of hydrochloric acid reagents is less than 85%, and the extraction efficiency of lead is lower.
Figure 4
Weigh tea samples of the same weight, add different concentrations of hydrochloric acid solution at the same temperature (70 °C), centrifuge to extract the supernatant for analysis according to different reaction times, and Figures 5 and 6 are the results of the content of Cu and Pb.
Figure 5
Figure 6
From conclusion 5, using (0.5~2) mol/L concentration of hydrochloric acid solution as the extraction solution, the extraction efficiency of copper can reach more than 90% at a temperature of 70 °C, and with the increase of the concentration of hydrochloric acid solution, the extraction efficiency also increases accordingly. However, the extraction efficiency of lead element is not so obvious, compared with copper slightly reduced, its extraction efficiency is the same as copper, is also positively correlated with the concentration of hydrochloric acid reagent, but when the concentration of hydrochloric acid is less than 0.5mol/L, the extraction efficiency is less than 90%, and the use of 2mol/L HCl solution, at a temperature of 70 °C, the extraction rate of lead in tea is greater than 90%.
Taking Cu as an example, the obtained content determination results show that time factors have little influence on extraction efficiency, and the concentration of hydrochloric acid solution and reaction temperature during extraction are the main factors affecting extraction efficiency, among which hydrochloric acid concentration has the greatest influence, followed by the temperature used, and the reaction time has a weak effect. Therefore, the orthogonal experimental results show that the extraction of 2mol/L hydrochloric acid at 70 °C for one hour is the best condition for extracting copper elements from tea.
By extracting different tea leaves by acid extraction method, and comparing the measurement results with the ashing method, the precision and accuracy of the determination results were judged by the T-test method. Six extraction experiments were carried out on three kinds of tea, and the results showed that the maximum coefficient of variation of acid extraction method was 3.75%, all of which were less than 5%, indicating that the method had good precision. In addition, all T-test values were greater than 0.05 and less than -0.05, that is, the determination results of copper extracted by acid extraction technology were not significantly different from the traditional ashing method at the significance level of α=0.05, which further proved the accuracy of the method.
However, it can still be seen from the chart that although the extraction rate of acid extraction can reach more than 80% at a time, the measurement results of lead are relatively lower than those of the traditional ashing method. Therefore, when determining the content of lead in tea, the proportion of feed to liquid can be increased or secondary extraction can be carried out to meet the measurement requirements, obtain more accurate measurement results, and make more rigorous analysis.
Pre-treatment of tea leaves. Whether tea is pre-crushed has a significant impact on the determination of lead, and crushing helps to improve the efficiency of the determination, while the effect on the determination of copper content is little and can be ignored. (2) The control of clean water solution shows that water as an extraction agent has no obvious effect, and should not be used as an extraction reagent for determining the content of metal elements in tea, and also proves that drinking tea with water, the heavy metal elements in tea can rarely be dissolved in water, and the heavy metal content ingested by people is also minimal, which will not seriously affect health.
(3) Two-factor experiments show that since the extraction efficiency of copper and lead is positively correlated with the concentration of hydrochloric acid, and the time has little effect on the extraction of lead, copper and lead in tea can be extracted at the same time, reducing the number of experiments and greatly improving the experimental efficiency. (4) The orthogonal experimental results show that the influence of three factors on the extraction rate is as follows: concentration> temperature> time, that is, hydrochloric acid concentration and temperature are the main influencing factors. Using 2mol/L hydrochloric acid and extracting it at 70 °C for one hour, it is the best condition for extracting copper elements from tea.
(5) The comparison of acid extraction technology and ashing method shows that acid extraction is carried out for sample pretreatment, and the extraction rate of copper and lead reaches 90%, of which the extraction rate of copper is higher than 96%. The results show that the method is simple, convenient and has ideal effect. In order to obtain more precise measurement results, it can be achieved by increasing the ratio of extractant to liquid and combining it with secondary extraction.