Key words: constant temperature platform graphite furnace atomic absorption spectrometry lead-palladium solution filter for sewage discharge
Graphite furnace atomic absorption spectrometry (GFAAS) has been widely used in the determination of lead in source water and drinking water, but there are few reports on the determination of total lead in sewage discharged from backwashing of filter in water works. At present, nitric acid-hydrogen peroxide is used to pretreat the sewage discharged from backwashing filter, palladium is used as matrix modifier, and then the total lead in the sewage is determined by constant temperature platform -GFAAS.
1 determination method
1. 1 instruments and reagents
Varian AA-640 Atomic Absorption Spectrophotometer Pyrolytic Coated Graphite Tube and Fully Pyrolytic Graphite Platform Automatic Sampler (Varian Company)
2.00 g/L palladium solution: prepared with high-grade pure palladium nitrate.
Lead standard stock solution: lead 1.00mg/mL.
30% hydrogen peroxide solution
5% nitric acid solution (nitric acid purity is extremely high)
1% nitric acid solution
1.2 instrument determination conditions
Wavelength: 283.3 nm, slit: 0.5 nm, lamp current: 4 mA,
Deuterium lamp button background, Ar as protective gas, stop gas when atomization.
Temperature program of graphite furnace with constant temperature platform;
Drying: 200℃ (heating 65438±05s, keeping temperature for 45 s),
Ashing: 65438 0000℃ (heating 65438±00s, holding for 20 s),
Cooling: 200 ℃( 1 s cooling; Reserved 10 s),
Atomization: 2 000℃ (maximum power heating for 4 seconds),
Removal: 2 600℃ (ramp temperature rises for 2 seconds and remains for 2 seconds).
1.3 test method
Take a proper amount of sewage from the filter, add 4.00 mL of nitric acid and 1.00 mL of hydrogen peroxide solution, and put it on an electric heating plate to be heated and evaporated to dryness. If the residue is brown-black, put it into a high-temperature furnace at 450℃ for ashing (to white or light yellow), take it out, let it cool, add 5 mL of nitric acid solution (5%), slightly heat it to dissolve it, add 10 mL of high-purity water to fully soak all the inner walls, let it cool, then transfer it into a 25 mL volumetric flask (if there are sand grains, filter them out), and add 2.50 mL.
2 Results and discussion
2. 1 Selection of ashing and atomization temperature
The experiment was carried out with 20 μL of lead solution with the concentration of15.00μ g/L. In the presence of palladium solution, there was no obvious loss of lead when the ashing temperature was 1 000℃. When the atomization temperature is 1 800 ~ 2 200℃, the absorption signal is at a stable level.
At the same time, in order to produce isothermal state, the graphite furnace is cooled to 200℃ after ashing and before atomization, and then heated to atomization with maximum power. As a result, the atomization of lead was delayed by about 0.4 s, and the reproducibility of the determination results was good.
Therefore, the ashing temperature is 65438 0000℃, the cooling temperature is 200℃, and the atomization temperature is 2 000℃.
2.2 Function and dosage of matrix modifier
In order to investigate the effect of lead as matrix modifier to eliminate matrix interference, the slope ratio of standard addition curve and standard curve of lead in backwashing sewage samples of filter with and without matrix modifier was compared. Without matrix modifier, its slope ratio is 0.79; In the presence of matrix modifier, its slope ratio is 0.97. Because the slope ratio is closer to 1, the matrix interference is smaller, so it is proved that the matrix modifier can reduce the matrix interference in the sample.
The experiment under the above conditions shows that 2.50 mL palladium solution is enough to make the absorption signal of lead constant, so 2.50 mL palladium solution is selected.
2.3 Calibration curve, detection limit and relative standard deviation
Take the lead standard stock solution and dilute it with 1% nitric acid solution one by one to prepare 0.00, 30.00, 60.00 and 90.00 μg/L series standard solutions. According to the test method, the regression equation is a = 4.28×10-4c+1.8.
Determination of lead blank solution 1 1 time. According to CL(k=3)=3 SA/S (injection volume is 20μL), the detection limit is 0.55 μg/L and the relative standard deviation is 4.30%.
2.4 Recovery rate test
See table 1 for the results of standard addition recovery test.