Determination of cadmium in tea by AA-1800 flame/graphite furnace atomic absorption spectrometry - Master's thesis - Dissertation

Determination of Cadmium in Tea by AA-1800 Flame/Graphite Furnace Atomic Absorption Spectrometry

Key words: flame/graphite furnace atomic absorption spectrometry; cadmium in tea; analysis instrument ; AA-1800H 1, flame/graphite furnace atomic absorption spectrophotometer test conditions: 1.1 instruments and reagents 1.1.1 instruments ADE-V type self-control electric heat digestion device; AA-1800H atomic absorption spectrophotometer (America China), WNA-1 type metal cover glass high efficiency atomizer, hollow cathode lamp, H-1 micro-mixer; glass instrument Soak for 24h with nitric acid (1+3) before use, rinse with water, dry and set aside. 1.1.2 Reagent and sample digestion agent: HNO3 + HClO4 (2 + 1); matrix modifier: l20g.L-1 (NH4) 2HPO4, dilution: 0.5 mol / L of nitric acid. Add 32 ml of nitric acid to a 1000 ml volumetric flask, add water and mix until the mark. The reagents used were all of the AR grade and the water was secondary deionized water. Sample: Daxinganling North Qishen Health Products Co., Ltd. produces Beibei Shencha. Main ingredients: Astragalus (root, leaf, flower), Ganoderma lucidum, alfalfa, tea and so on. Cadmium standard stock solution (concentration 1 mg.ml-1): 0.1354 g of dried CdCl2 was weighed, dissolved in a diluent and diluted to 100 ml. Mixed, spare. Cadmium standard use solution (concentration 100ng.ml-1): Pipette 1.0ml of cadmium standard stock solution into a 100ml volumetric flask, dilute to the mark with the diluent; draw 1.0ml of the solution into a 100ml volumetric flask, use the diluent Dilute to the mark. Before the measurement, add 5.0 ml of a standard cadmium solution having a concentration of 100 ng/ml to a 100 ml volumetric flask, dilute to the mark with a diluent, and mix. The solution concentration was 5.0 ng/ml. Use the diluent as the standard blank. 1.2 Test method 1.2.1 Working condition Digester temperature control program: 15 ° C, 2 h; 30 ° C, 2 h; 65 ° C, 2 h; 165 ° C, 8 h; See the digestion diagram. Heating rate: automatic. Cd cathode lamp: current 5.0 mA, wavelength 253.7 nm, spectral passband width 0.5 nm. PSD injector: sample injection volume 10ml. Matrix improver: The injection volume is 5ml, and the injection method is common. 4520A graphite furnace: drying 120 ° C, 20 s; ashing 350 ° C, 20 s; atomization: 1800 ~ 2100 ° C, 3 s. 1.2.2 Establishment of digestion method Digestion: Whether the digestion effect is completely the key to the success or failure of a method. The digestion effect can be judged by whether or not the incomplete substance is dissolved in the digestion solution and the degree of clarification of the digestion solution is used. We have tested HNO3+HCLO4(6+1), HNO3+HCLO4(4+1), HNO3+HCLO4(2+1), HNO3+HCLO4+H2SO4(6+2+1), HNO3+H2SO4(2+ 1), H2SO4 + H202 (2+10) and HNO3 and other digesting agents, according to the best effect of digestion, selected HNO3 + HCLO4 (2 + 1) is the digesting agent of this test. Sample dosage: Since the ADE-V type self-controlled electrothermal digestion device uses a φ28mm tube-type digestion tube, the digestion space is smaller than the triangular bottle specified by the national standard. If the amount of the sample is large, the foaming and blistering substances produced during the digestion process are liable to cause overflow. After experiments, this phenomenon is prone to occur when the sample amount exceeds 1.5 g. Therefore, this experiment uses a sample amount of 1.0g. The amount of the digesting agent is tested by using a digesting agent added in an amount of 5, 8, 10, 15, or 20 (ml). When the amount of the digesting agent is 8 ml or more, the sample can be completely digested. Considering the comprehensive factors such as digestion speed, the choice of 10ml addition digestion program was chosen: the domestic and foreign literatures on temperature detection and digestion procedures applied to tea detection are rare. We designed different digestion programs for experiments. 130 ° C, 1 h; 65 ° C, 2 h; 165 ° C, 6 h; Result: The reaction was intense, and there were blistering and sputum appearances. 265 ° C, 2 h; 165 ° C, 5 h; RESULTS: The reaction was intense, with blistering and sputum appearing severe. 330 ° C, 4 h; 165 ° C, 2 h; Result: The reaction was intense, there was suspended matter, and the solution was cloudy. 420 ° C, 2 h; 30 ° C, 2 h; 65 ° C, 2 h; 165 ° C, 4 h; Result: There was a suspension and the solution was cloudy. 515 ° C, 2 h; 30 ° C, 2 h; 65 ° C, 2 h; 165 ° C, 8 h; Results: The reaction was gentle and the solution was clear. 620 ° C, 3 h; 30 ° C, 3 h; 65 ° C, 4 h; 165 ° C, 10 h; Results: The reaction was gentle, the solution was clear, and the total digestion time was longer. 720 ° C, 4 h; 30 ° C, 2 h; 65 ° C, 2 h; 165 ° C, 8 h; Results: The reaction was gentle and the solution was clear. After comprehensive consideration of factors such as reaction rate, foaming, scavenging, and total digestion time at the beginning of sample digestion, Procedure 5 was selected as the digestion procedure for this experiment. 1.2.3 Establishment of working curve and sample setting The concentration of standard samples is 0.0, 1.0, 3.0, 5.0 ng/ml, and the calibration curve of 3-point method is established (curve fitting equation: new reasonable fitting) 2. (graphite furnace Atomic Absorption Spectrophotometer) Results and Discussion: 2.1 Sample Digestion According to the sample content, 1.00g of the sample which has been pulverized and passed through a 120 mesh sieve is placed in the digestion tube, and a glass frosted glass bead and a digesting agent 10 ml are added. Connect the condensing return pipe and start the temperature control program of the temperature control digester; when the program runs to the 6th hour, remove the condensing return pipe to drive away the nitrogen oxides; when the program is finished, remove the digestion tube and cool. The digestion solution was filtered and transferred to a 25 ml volumetric flask, and the volume was adjusted with water and mixed for testing. At the same time make a blank. Shake on the mixer for 20 s before the measurement. The standard use solution with a concentration of 5.0 ng/ml (PSD is automatically set at a concentration of 0.0, 1.0, 3.0, 5.0 ng/ml), the diluent, the matrix modifier, the sample blank, and the sample digestion solution are placed separately. Enter the corresponding position of the PSD, input the corresponding data according to the prompts of the instrument, and make the measurement. 2.2 Recovery rate A standard solution was added to the sample to carry out a recovery experiment. The results (Table 1) show that the recovery rates are 85.5% to 113.6%, respectively. 2.3 Atomic Absorption Determination Precision A sample was taken 11 times and the concentrations were determined to be 30.62, 30.68, 30.60, 30.66, 30.59, 30.60, 30.61, 30.58, 30.63, 30.62, 30.64 (ng.ml-1). The highest value was 30.68 ng.ml-1, the lowest value was 30.58 ng.ml-1, the average value was 30.62 ng.ml-1, and the RSD was 3.0%. Meet the analytical requirements. 2.4 Method Precision 11 samples were determined according to 2.1 sample digestion and measurement methods. The precision of the calculation method was: average value 0.063, RSD 0.6%. 2.5 Comparative experiment This method and GBW 07605 tea component analysis standard substance (Cd content is 57ng.ml-1) for comparative experiments, the content of GBW 07605 content is determined to be 58.3, 56.9, 57.4, 57.1, 56.9 (ng.ml-1) The results were tested by t test, indicating that there was no significant difference in the results of the two methods. 3, flame / graphite furnace atomic absorption spectrophotometer conclusion This method is simple, accurate, rapid, high degree of automation, after the program is set, the digestion process does not need to look after; the condensation return tube recovers a large amount of digestion agent, and automatically on the digestion tube The residue was washed; the amount of the digesting agent was small, and the reagent blank was significantly reduced; no blistering or sputum appeared; after comparison with the standard substance, the results were tested without significant difference. At the same time, because the digestor can simultaneously dissolve 40 to 60 samples, it is especially suitable for the determination of cadmium in large-scale tea samples.