Determination of cadmium and lead in tea leaves by graphite furnace atomic absorption method

Abstract: Tea leaf is a traditional health drink in China. It contains a large number of nutrients such as phosphorus, potassium, calcium, sulfur and beneficial trace elements such as copper, zinc and boron. It also contains a small amount of harmful lead and cadmium. As for trace metal elements, people are paying more and more attention to the nutrient content of tea leaves and also beginning to pay attention to the status of trace harmful metals in tea leaves. In this test, the content of trace harmful metals cadmium and lead in tea leaf composition analysis standard was determined by graphite furnace atomic absorption method. This experiment used nitric acid-perchloric acid for sample digestion. The measurement result standard curve Cd correlation coefficient r = 0.99995, Pb standard curve correlation coefficient r = 0.99756 or more. The standard substance analysis results of tea leaf composition analysis showed that the Cd content was 0.0202μg / g and the Pb content was 1.0805μg / g, (the standard values ​​of lead and cadmium were 0.023 ± 0.004μg / g, 1.00 ± 0.05μg / g). The measurement result is in accordance with the content range of the national standard value.
1. Experimental part
1.1.1 Instruments
SP-3520AAPC atomic absorption spectrophotometer (Shanghai Spectral Instrument Co., Ltd.)
SP-3500GA graphite furnace (Shanghai Spectrum Instrument Co., Ltd.) Lead and cadmium hollow cathode lamp (Hengshui Ningqiang Light Source Co., Ltd.), constant temperature heating plate
1.1.2 Reagents nitric acid (superior purity), perchloric acid (analytical purity), standard solution of deionized water Cd, Pb (purchased from Shanghai Institute of Measurement and Testing Technology) Matrix improver: â‘  Weigh 1g ammonium dihydrogen phosphate and 0.353g of magnesium nitrate is fixed in a 100ml volumetric flask.
â‘¡ No. Weigh 0.0834g palladium chloride and 0.0515g magnesium nitrate in a 50ml volumetric flask. When dissolving palladium chloride, add a few drops of hydrochloric acid and 5% 2% nitric acid, and heat until the palladium chloride is completely dissolved.
No. â‘¢ Mix 10ml No. â‘  solution and 10ml No. â‘¡ solution as a matrix improver.
Preparation of cadmium standard solution: prepare Cd stock solution of 0.4μg / ml, and dilute to 0.32, 1.6, 2.4ng / ml concentration in the analysis, and use 2% nitric acid to make up to the mark.
Preparation of lead standard solution: prepare 1 μg / ml of Pb stock solution, and dilute it step by step to 5, 20, 40, 60, 80 ng / ml concentration during analysis, and use 2% nitric acid to dilute to the mark.
1.2.1 Sample pretreatment Weigh 1g of tea leaf fine powder in a Teflon container, add 8ml of nitric acid and soak overnight, put 2ml of perchloric acid, cover with a watch glass, and place on a constant temperature heating plate at a temperature of 130 ~ 150 ℃ Under the heating digestion, continue heating until the solution is clear, white smoke. Cool and transfer to a 25ml volumetric flask, make up to the mark with 2% nitric acid. (Sample-2 weighed 0.5g, digested with 4ml nitric acid, 1ml perchloric acid)
1.3 Test conditions
Cd instrument conditions: the test wavelength is 228.8nm, the spectral bandwidth is 0.7nm, the lamp current is 3mA, the negative high voltage is 235.8v, the time constant is 0.04s, and the calculation method is called the peak area. A = × σ × h = 2.507σh = 1.064Wh / 2h> Peak area, background mode is deuterium lamp background, total injection volume is 20μL, including sample or reagent 15μL, matrix modifier (No. ①) 5μL .
Pb instrument conditions: the test wavelength is 283.30nm, the spectral bandwidth is 0.7nm, the lamp current is 4mA, the negative high voltage is 236.2v, the time constant is 0.04s, the calculation method is the peak area, the background method is the deuterium lamp background, and the total injection volume is 20μL, including 15μL of sample or reagent, 5μL of matrix modifier (No. ③).

Table 1 Cd, Pb atomization heating program (atomic heating method: light-controlled heating)
Stage start temperature (℃) End temperature (℃) Slope heating time (s) Holding time (s)

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