Abstract: Cadmium is an excellent metal that absorbs neutrons. Making rods can slow down the nuclear chain reaction rate in an atomic reactor, and it is quite useful in zinc-cadmium batteries. Test conditions 1.1 Instruments and reagents 1.1.1 Instruments ADE-V type automatic control electrothermal digester; CAAM-2001 type graphite furnace detector atomic absorption spectrophotometer, WNA-1 type metal sleeve glass high-efficiency atomizer, hollow cathode lamp, H-1 type micro mixer; glass instrument use Before soaking with nitric acid (1 3) for 24h, rinse with water, dry, and set aside. 1.1.2 Reagents and samples Digestion agent: HNO3 HClO4 (21); matrix improver: 120g. L-1 (NH4) 2HPO4, diluent: 0.5mol / L nitric acid. Add 32ml of nitric acid to a 1000ml volumetric flask, add water and mix well to the mark. The reagents used are all AR grade, and the water is secondary deionized water. Sample: Beiqishen tea produced by Daxinganling Beiqishen Health Products Co., Ltd. Main ingredients: Astragalus (root, leaf, flower), Ganoderma lucidum, wolfberry, tea, etc. Cadmium standard stock solution (concentration 1mg.ml-1): Weigh 0.1354g of dried CdCl2, dissolve in diluent and dilute to 100ml. Mix well and set aside. Cadmium standard solution (concentration 100ng.ml-1): Draw 1.0ml of cadmium standard stock solution in a 100ml volumetric flask and dilute to the mark with diluent; draw 1.0ml of the solution in a 100ml volumetric flask and dilute Dilute to the mark. Before measurement, add 5.0ml of cadmium standard use solution with a concentration of 100ng / ml to a 100ml volumetric flask, dilute to the mark with diluent, and mix well. The concentration of this solution is 5.0ng / ml. Use the diluent as the standard blank. 1.2 Test method 1.2.1 Working conditions Digester temperature control program: 15 ° C, 2h; 30 ° C, 2h; 65 ° C, 2h; 165 ° C, 8h; -121. See the digestion diagram. Heating rate: automatic. Cd cathode lamp: current 5.0mA, wavelength 253.7nm, spectral passband width 0.5nm. PSD sampler: The sample injection volume is 10ml. Matrix improver: the injection volume is 5ml, and the injection mode is co-advancement. Graphite furnace: drying 120 ℃, 20s; ashing 350 ℃, 20s; atomizing: 1800 ～ 2100 ℃, 3s. 1.2.2 Establishment of digestion method Digestion agent: Whether the digestion effect is the key to the success or failure of a method. The digestion effect can be judged by whether there are incomplete digestion substances in the digestion solution and the degree of clarification of the digestion solution. 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), HNO3, etc. According to the best effect of digestion, HNO3 HCLO4 (21) was selected as the digestion agent for this experiment. Sample consumption: Since the ADE-V automatic control electric digester uses a φ28mm test tube type digestion tube, the digestion space is smaller than that of the triangular flask prescribed by the national standard. If the sample volume is large, the bubbly and foamy substances produced during the digestion process are prone to overflow. After the experiment, when the sample amount exceeds 1.5g, this phenomenon easily occurs. Therefore, a sample volume of 1.0 g was selected for this experiment. Amount of digestion agent: The digestion agent with the addition amount of 5, 8, 10, 15, 20 (ml) is used for the test. When the addition amount of the digestion agent is more than 8ml, the sample can be completely digested. Considering the comprehensive factors such as digestion speed, the amount of 10ml was selected. The choice of digestion program: The temperature-controlled digestion program used in tea detection is relatively rare at home and abroad. We designed different digestion procedures for experiments. ①30 ℃, 1h; 65 ℃, 2h; 165 ℃, 6h; end. Result: The reaction was intense with blistering and constriction. ②65 ℃, 2h; 165 ℃, 5h; end. Result: The reaction was intense, with blistering and severe appearance. ③30 ℃, 4h; 165 ℃, 2h; end. Result: The reaction was intense, there was suspended matter, and the solution was cloudy. ④20 ℃, 2h; 30 ℃, 2h; 65 ℃, 2h; 165 ℃, 4h; end. Result: There is suspended matter, and the solution is cloudy. ⑤ 15 ℃, 2h; 30 ℃, 2h; 65 ℃, 2h; 165 ℃, 8h; end. Result: The reaction was gentle and the solution was clear. ⑥20 ℃, 3h; 30 ℃, 3h; 65 ℃, 4h; 165 ℃, 10h; end. Results: The reaction was gentle, the solution was clear, and the total digestion time was longer. ⑦20 ℃, 4h; 30 ℃, 2h; 65 ℃, 2h; 165 ℃, 8h; end. Result: The reaction was gentle and the solution was clear. After comprehensively considering the factors such as reaction speed, bubbling, constriction, and total digestion time at the beginning of sample digestion, the procedure ⑤ was selected as the digestion procedure for this test. 1.2.3 Establishment of working curve and sample determination Set the standard sample concentration to 0.0, 1.0, 3.0, 5.0ng / ml, and establish a 3-point method calibration working curve (curve fitting equation: new reasonable fitting) 2 Results and discussion 2.1 Sample digestion According to the content of the sample, weigh 1.00g of the sample that has been crushed and passed through the 120 mesh sieve in the digestion tube, add one glass frosted glass bead and 10ml of digestion agent, connect the condensate reflux tube, and start the temperature control digestion device heating program; When the program runs to the 6th hour, remove the condensate reflux tube to drive off the nitrogen oxides; at the end of the program run, remove the digestion tube and cool. After filtering, the digestion solution was transferred to a 25ml volumetric flask, made up to volume with water, mixed well, to be tested. At the same time make blank samples. Shake on the mixer for 20s before measurement. Put the standard use solution with a concentration of 5.0ng / ml (PSD is automatically injected according to the concentration settings of 0.0, 1.0, 3.0, and 5.0ng / ml), the diluent, matrix improver, sample blank, and sample digestion solution, etc. Enter the corresponding position of the PSD, enter the corresponding data according to the prompt of the instrument, and perform the measurement. 2.2 Recovery rate Standard solutions were added to the samples to conduct the recovery rate experiment. The results (Table 1) show that the recovery rates are 85.5% to 113.6%, respectively. Table 1 Recovery rate experiment Table 1 Experiment of reclaim ratio Input value (ng / ml) Input value 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 Reclaim value (ng / ml) Reclaim value 8.5 18.8 30.8 40.6 56.8 57.6 66.4 76.4 Reclaim ratio (%) Reclaim ratio 85.0 94.0 102.7 101.5 113.5 96.0 94.8 95.02.3 Precision of atomic absorption determination Take a sample for 11 measurements, respectively, measured the concentration of 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 is 30.68ng. ml-1, the lowest value is 30.58ng.ml-1, the average value is 30.62ng.ml-1, and the RSD is 3.0%. Meet the analysis requirements. 2.4 Method precision According to 2.1 sample digestion and determination method, 11 samples were measured. The precision of the calculation method was: average value 0.063, RSD 0.6%. 2.5 Comparative experiment This method was compared with GBW 07605 tea ingredient analysis standard substance (Cd content was 57ng.ml-1), and the GBW 07605 content was determined to be 58.3, 56.9, 57.4, 57.1, 56.9 (ng.ml-1) The t test showed that there was no significant difference between the results of the two methods. 3 Conclusion The method is simple, accurate, rapid, and highly automated. After the program is set, the digestion process does not need to be taken care of; while the condensate reflux tube recovers a large amount of digestion agent, it automatically rinses the residue on the digestion tube; Significantly reduced the reagent blank; no blistering or entanglement; after comparing with the standard material, the result was no significant difference by t test. At the same time, the digester can digest 40 to 60 samples at the same time, so it is especially suitable for the determination of cadmium in tea samples in large quantities.
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