Method for detecting heavy metal elements such as arsenic, mercury, lead, cadmium, iron and zinc in tea
Detection of heavy metal elements such as arsenic, mercury, lead, cadmium, iron and zinc in tea
First, the method summary
After the sample is processed, the test solution is introduced into an atomic fluorescence spectrometer (AFS) to determine arsenic, mercury, lead, and cadmium in a certain acid medium, and is introduced into an atomic absorption spectrometer (AAS) to measure iron, zinc, nickel, copper, and work. The signal response values ​​corresponding to the elements in the curve are compared to obtain the content of each element.
Second, reagents and materials
Unless otherwise specified, the reagents used are of high grade purity. The water used is ultrapure water treated by ultrapure water machine. The resistivity is not less than 18.2M Ω•cm. All experimental glassware passes before use. Soak for 5% HNO 3 (v/v) for 24 h, then rinse with deionized water and set aside.
2.1 Hydrochloric acid.
2.2 Nitric acid.
2.3 hydrogen peroxide (30%).
2.4 Perchloric acid.
2.5Cd1 and Cd2 signal enhancer: special signal enhancer for measuring cadmium.
2.6 Hydrochloric acid (1+1): Take a portion of hydrochloric acid, one part of water, and mix well.
2.7 Nitric acid (1+1): Take a portion of nitric acid, one part of water, and mix well.
2.8 Mixed acid of nitric acid and perchloric acid (10+1): Take ten parts of nitric acid, one part of perchloric acid, and mix well.
2.9 Mixture of thiourea and ascorbic acid (100 g/L): Weigh 10.0 g of thiourea and 10.0 g of ascorbic acid in 100 mL of water and continue to stir until completely dissolved.
2.10 Oxalic acid (40 g/L): Weigh 4.0 g of oxalic acid in 100 mL of water and stir until completely dissolved.
2.11 Potassium dichromate (5 g / L): Weigh 5.0 g of potassium dichromate in 1 L of water and continue to stir until completely dissolved.
2.12 Cd2 (1g / L) special cadmium sensitizer: Weigh 0.1g Cd2 special cadmium sensitizer in 100mL water, and continue to stir until completely dissolved.
2.13 Potassium Hydroxide (5 g/L): Weigh 5.0 g of potassium hydroxide in 1000 mL of water and stir until completely dissolved.
2.14 Boron hydrogen potassium (20 g / L): Weigh 20.0g of potassium borosilicate in 1000 mL of 5 g / L potassium hydroxide solution, and continue to stir until completely dissolved, the reducing agent solution for the determination of arsenic.
2.15 borohydride haul potassium (10 g/L): Weigh 10.0 g of borohydride in 1000 mL of 5 g/L potassium hydroxide solution and continue to stir until completely dissolved. This reducing agent solution is used for mercury measurement.
2.16 Peng potassium hydride (20 g / L) and iron potassium potassium (20 g / L) mixture: weigh 20.0g of borohydride and 20.0g of potassium hydride in 1000mL 5g / L of potassium hydroxide solution, And continuously stirring until completely dissolved, the reducing agent solution is used for measuring lead;
2.17 Cd1 special cadmium sensitizer (30 g / L): Weigh 30.0g Cd1 special cadmium sensitizer in 1000 mL 5 g / L potassium hydroxide solution, and continue to stir until completely dissolved, this reducing agent The solution is used to measure cadmium.
2.18 standard solution: standard solution of arsenic, mercury, lead, cadmium, iron, zinc, nickel and copper. The standard stock solution of arsenic, mercury, lead and cadmium with a concentration of 1000μg/mL is gradually diluted into 10.00μg/mL arsenic, mercury, lead, cadmium standard intermediate solution and 1.00μg/mL arsenic and lead standard use solution; 0.10 Gg/mL standard solution of mercury and cadmium; diluted with iron, zinc, nickel and copper standard stock solution at a concentration of 1000μg/mL into a standard use solution of iron, zinc, nickel and copper at 100.00μg/mL.
2.18 high purity argon, purity ≥ 99.99%.
2.19 Acetylene gas - 99.9% or more purity.
Third, instruments and equipment
3.1 Atomic Fluorescence Spectrometer.
3.2 Atomic absorption spectrophotometer.
3.3 Air compressor.
3.4 Microwave digestion instrument.
3.5 Hollow cathode lamps with arsenic, mercury, lead, cadmium, iron, zinc, nickel and copper.
3.6 Analytical balance (accuracy 0.1mg).
3.7 Ultrapure water preparation system.
3.8 Adjustable hot plate.
3.9 Plant shredder.
3.10 oven.
Fourth, the analysis steps
4.1 Preparation of samples
Each tea sample was rinsed four times with tap water, and then rinsed four times with distilled water. The washed leaves were placed in a tray, placed in an oven, kept at 105 ° C for 30 minutes, and then continuously baked at 65 ° C for 24 hours. The baked sample is pulverized by a pulverizer, passed through a 200 mesh sieve, and then placed in a sealed sample bag and stored in a desiccator for use. Care must be taken during the preparation to prevent contamination of the sample.
4.2 Preparation of the test solution
4.2.1 Wet digestion
Accurately weigh 0.5 g of the pulverized tea sample (accurate to 0.0001 g), place it in a 100 ml flask, add 20 mL of mixed acid (nitric acid: perchloric acid = 10:1), cover the watch glass, and let it stand overnight. The next day, the adjustable electric heating plate is heated and digested. If the digestion is not complete, a small amount of mixed acid is added until the white smoke is emitted. The solution is colorless transparent or slightly yellow and the residual amount does not exceed 1 mL, and is cooled. Wash into a 50mL volumetric flask with a small amount of ultrapure water and dilute to the mark, mix and test, and take the same amount of mixed acid digestive solution as the digested sample as the reagent blank test according to the above operation. If the solution cannot be measured in time, transfer it to a 50 ml polyethylene bottle and store at 4 ° C for testing.
4.2.2 Microwave digestion
Accurately weigh 0.5 g (accurate to 0.0001 g) of the comminuted tea sample into a clean digestion tank. Then add 6mL of nitric acid, let stand for 30min., add 2mL of hydrogen peroxide, let stand for 2min., put the inner plug of the digestion tank cover, tighten the outer cover, and put it into the digestion turntable in turn. The position of the digestion tank is as symmetric as possible. The digestion is completed and the natural cooling is completed. The digestion solution is transferred to a 40 mL small beaker with a small amount of ultra-pure water. The acid is transferred to the adjustable hot plate until the solution is colorless, transparent or slightly yellow and remains. Do not exceed 1 mL and cool. Wash into a 50mL volumetric flask with a small amount of ultrapure water and dilute to the mark, mix and test, and take the same amount of mixed acid digestive solution as the digested sample as the reagent blank test according to the above operation. If the solution cannot be measured in time, transfer it to a 50 mL polyethylene bottle and store at 4 °C for testing.
4.2.3 Preparation of test solutions for each element
Arsenic: Accurately transfer 2 mL of the test solution into a 10 mL colorimetric tube, add 1 mL (1+1) hydrochloric acid, 1 mL (thiourea + ascorbic acid) (100 g / L) mixture, and bring up to volume with water. Tested after 30 minutes.
Mercury: Accurately remove 5 mL of the test solution into a 10 mL colorimetric tube, add 1 mL (1+1) nitric acid, 1 mL potassium dichromate (5 g/L), and dilute to volume with water.
Lead: Accurately transfer 2 mL of the test solution to a 10 mL colorimetric tube, add 0.4 mL (1+1) hydrochloric acid, 1 mL of oxalic acid (40 g/L), and dilute to volume with water.
Cadmium: Accurately remove 5 mL of the test solution into a 10 mL colorimetric tube, add 0.4 mL (1+1) hydrochloric acid, 1 mL Cd2 (1g/L) special cadmium sensitizer, and measure with water.
Iron, zinc, nickel, copper: Pipette 1 mL (1+1) of nitric acid into a 25 mL colorimetric tube, and dilute to the mark with the solution to be tested. That is, the volume of the liquid to be tested is 24 mL.
4.3 Determination of test solution
4.3.1 Preparation of standard series of each element
Arsenic: Accurately absorb 1.00μg/mL arsenic standard solution 0.00mL, 0.10 mL, 0.30 mL, 0.50 mL, 0.70 mL, 1.00 mL, respectively, in a 100 mL volumetric flask, add 10 mL (1+1) hydrochloric acid respectively. Mix with 10 mL (thiourea + ascorbic acid) (100 g / L) and dilute to the mark with water, ie the standard series of arsenic is 0.00, 1.00, 3.00, 5.00, 7.00, 10.00 ng / mL. Shake well for testing.
Mercury: Accurately absorb 0.10μg/mL of mercury standard solution using 0.00 mL, 0.10 mL, 0.30 mL, 0.50 mL, 0.70 mL, and 1.00 mL, respectively, in a 100 mL volumetric flask, and add 10 mL (1+1) nitric acid. With 10 mL of potassium dichromate (5 g/L), dilute to the mark with water, ie, the standard series of mercury is 0.00, 0.10, 0.30, 0.50, 0.70, 1.00 ng/mL. Shake well for testing.
Lead: Accurately absorb 1.00μg/mL lead standard solution 0.00mL, 0.10 mL, 0.30 mL, 0.50 mL, 0.70 mL, 1.00 mL, respectively, in a 100 mL volumetric flask, and add 4 mL (1+1) hydrochloric acid respectively. Dilute to the mark with 10 mL of oxalic acid (40 g/L) with water, ie lead standard series of 0.00, 1.00, 3.00, 5.00, 7.00, 10.00 ng/mL. Shake well for testing.
Cadmium: Accurately absorb 0.10μg/mL of cadmium standard solution using 0.00 mL, 0.10 mL, 0.30 mL, 0.50 mL, 0.70 mL, and 1.00 mL, respectively, in a 100 mL volumetric flask, and add 4 mL (1+1) hydrochloric acid. With 10 mL Cd2 (1g / L) special cadmium sensitizer, diluted with water to the mark, the standard series of cadmium is 0.00, 0.10, 0.30, 0.50, 0.70, 1.00 ng / mL. Shake well for testing.
Iron: Accurately absorb 100.00μg/mL of iron standard solution 0.00mL, 0.10 mL, 0.50 mL, 1.00 mL, 5.00 mL, 10.00 mL, respectively, in a 100 mL volumetric flask, and add 4 mL (1+1) nitric acid respectively. Dilute to the mark with water, that is, the iron standard series is 0.00, 0.10, 0.50, 1.00, 5.00, 10.00 μg / mL. Shake well for testing.
Zinc: Accurately absorb 100.00μg/mL zinc standard solution 0.00mL, 0.10 mL, 0.50 mL, 1.00 mL, 5.00 mL, 10.00 mL, respectively, in a 100 mL volumetric flask, and add 4 mL (1+1) nitric acid respectively. Diluted to the mark with water, ie, the zinc standard series is 0.00, 0.10, 0.50, 1.00, 5.00, 10.00 μg / mL. Shake well for testing.
Nickel: Accurately absorb 100.00μg/mL of nickel standard solution 0.00mL, 0.10 mL, 0.50 mL, 1.00 mL, 5.00 mL, 10.00 mL, respectively, in a 100 mL volumetric flask, and add 4 mL (1+1) nitric acid respectively. Diluted to the mark with water, ie, the standard series of nickel is 0.00, 0.10, 0.50, 1.00, 5.00, 10.00 μg / mL. Shake well for testing.
Copper: Accurately absorb 100.00μg/mL of copper standard solution 0.00mL, 0.10 mL, 0.50 mL, 1.00 mL, 5.00 mL, 10.00 mL, respectively, in a 100 mL volumetric flask, and add 4 mL (1+1) nitric acid respectively. Diluted to the mark with water, ie, the standard series of copper is 0.00, 0.10, 0.50, 1.00, 5.00, 10.00 μg / mL. Shake well for testing.
4.3.2 Determination of samples
Set all parameters after power on, and measure after the instrument is stable. During the measurement, the standard series solution, reagent blank solution and test solution are introduced into the instrument for measurement. Among them, arsenic, mercury, lead and cadmium are tested by atomic fluorescence spectrometer, and iron, zinc, nickel and copper are tested by atomic absorption spectrometer. Determine the content of the element to be tested in the test solution.
Fifth, the calculation of the analysis results.
Calculate the content of arsenic, mercury, lead and cadmium according to the following formula:
ï·X A /(μg /g)=
In the formula:
X A - the content of the measured element, where A refers to the measured element, μg / g;
——Check the mass concentration of the measured elements in the test solution from the working curve, ng/mL;
——Check the mass concentration of the measured elements in the blank test solution from the working curve, ng/mL;
- the total volume of the sample solution, mL;
- take the volume of the sample solution, mL;
—— After taking the sample solution, prepare the volume of the test solution, mL;
- Weigh the mass of the sample, g.
Calculate the content of iron, zinc, nickel and copper according to the following formula:
ï·ï·X B /(μg /g)=
In the formula:
X B - the content of the measured element, where B refers to the measured element, μg / g;
——Check the mass concentration of the measured elements in the test solution from the working curve, μg /mL;
——Check the mass concentration of the measured elements in the blank test solution from the working curve, μg /mL;
- the total volume of the sample solution, mL;
- take the volume of the sample solution, mL;
—— After taking the sample solution, prepare the volume of the test solution, mL;
- Weigh the mass of the sample, g.
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