- Prepare solutions containing 5, 10, 15,20,and 25 ppm Pb by dilution of 1000ppm Pb.
- Prepare solutions containing 2.5, 5,10, 15 and 20 ppm Mg.
- Measure the Pb standard solution in 217nm 1nm slit width.
- Measure the Mg standard solution in 202.6nm with 1nm slit width.
Effect of slit width
- Set the wavelength to 217nm, using a slit width of 1nm.
- Set up AAS with an air-acetylene flame, and lamp current 5mA.
- Measure the absorbance of 5 solutions
- Change the slit width to 0.5nm, reset peak meter needle to normal position.
Matrix effects
- Prepare 10ppm Pb in 25% glycerol.
- Measure the absorbance of the 10ppm Pb and Pb in glycerol at 217nm and slit 1.0nm.
Background effects
- Measure the absorbance of the low-lead 1M NaCl at 217nm using a slit of 1.0nm.
- Measure the absorbance of the 10ppm Pb standard solution(prepared in preparation part) with the background off.
- Without changing the wavelength, slit or flame, changing the background corrector on.
Investigation of chemical interferences
- Measure the absorbance of each of the solution 2-14(table below)
Nitrous oxide-acetylene flame
- The nitrous oxide-acetylene burner replaces the are-acetylene burner.
- Adjusted to give a pressure of 25psi.
- Set the air flow rate to 260kpa
- Finally adjust the acetylene flow rate to produce a flame with a 1cm red interconal zone.
- Measure the absorbance of solution 2,3,4 and 9 and the reagent blanks 11, 12 and 13.
Results
Effect of slit width
The slit width determined the amount of radiation that enters the monochromator. If the slit is too wide, the light will be too high and the signal-to-noise ratio may be excellent. On the other hand, if the slit is too narrow, the resolution may be excellent but the signal-to-noise ratio may be poor due to the reduced light throughput. In this experiment, the absorbance in 1nm is higher than in 0.5nm. This proves that the slit width affects the absorbance.
Figure 2. Pb concentration with absorbance in 217nm and 1.0nm.
Figure 3. Pb concentration with absorbance in 217nm and 0.5 slit width
Effect of wavelength
Different wavelength will affect the absorbance. The recommended analytical wavelength is in the appendix. According to the appendix, the best wavelength for 20ppm is 217nm. 283.3nm is the recommended working range for 0.5-50ppm. 217nm should be the better one because it provides a clear absorbance.
Table 1. absorbance of 20 ppm Pb in different wavelength
Matrix effect
Solution contains glycerol give a lower absorbance. Matrix causes a different behavior in sample and calibration solutions. Matrix interferences due to differences between surface tension and viscosity of test solutions and standards.
Table 2. Matrix effects of Pb solution
Background effects
In AAS, background corrector is an important part. There will be a problem when the composition of standards and sample is different. Such problem can be caused by background absorption and ion interference. However, this problem can be solved by background corrector. In this case, when we turn on the background corrector, the absorbance of NaCl becomes zero. This means corrector corrects the background impact.
Table 3. 2M NaCl in 217nm, the effect of background on and off
Unknown solution of Mg
Figure 4. Mg concentration with absorbance.0.5 width
According the calibration curve, the concentration of unknown solution is 10.80ppm
Absolute standard deviation of slope:
=0.010
Chemical interference
Different ions have interfaces with magnesium in solution, thus, the absorbance will be effect by free ion in solution. In this experiment, Aluminum, Phosphorus, Strontium, potassium and EDTA solution effect on magnesium solution. The EDTA was added as its disodium salt, so it was possible that the effect was that of sodium. The absorbance will increase due to radiation overlapping that of the light source. Phosphorus in solution will cause low absorbance because Phosphorus acid forms compounds with Mg that do not dissociate in the flame. Sr in Mg solution will course higher absorbance, because Ionization of the analyte reduces the signal. Ionization of alkaline earth metals in flame prevents it from going to the excited state doesn't absorb light of appropriate wavelength.Interferences can be overcome by addition of EDTA.
Table 4. absorbance of different solution in 5ppm Mg solution.
Nitrous oxide-acetylene flame
In AAS, the flame convert the elements present in the solution into free atoms. There is an optimum flame and optimum temperature for each analyte. The flame used most frequently in AAS is the air-acetylene flame. There are fewer interfaces in solution when using Nitrous oxide-acetylene flame. In this case, all absorbance reduced in Nitrous oxide-acetylene flame condition. This might cause by different max flame speed and max temperature of this two flames. Some elements can only be converted to atoms at high temperatures. Thus, for some metal, need to check the data to choose particular flame for the experiment.
Table 5. Effect of Change flame
Calculations
According to figure 4, the line of Mg absorbance and concentration is as follow.
y = 0.0267x
Absorbance of unknown solution=0.2884
So, concentration of unknown solution is:
X=10.80ppm
Discussion
In ASS, it required a liquid sample to be aspirated. During combustion, atoms of the element of interest in the sample are reduced to free, unexcited ground state atoms, which absorb light at characteristic wavelengths. A computer data system converts the change in intensity into an absorbance. As concentration goes up, absorbance goes up.
Compared AAS with flame emission, the form shows as follow.
Table 6. compare of AAS and AES
Uncertainty of unknown Mg,
Source of error
In general, the systematic errors in AAS usually result from the sample preparation. In this experiment, there are some errors during the Mg preparation which causes the Mg calibrate curve is not good.
Conclusion
In this experiment, the unknown Mg solution is 10.08ppm. the error is caused by the preparation of standard solution. In addition, the interference has a great impact on absorbance of Mg solution. Change to Nitrous oxide-acetylene flame will also affect on absorbance.
References
1. Skoog, D.A., D.M. West, and F.J. Holler, Fundamentals of analytical chemistry, 5th edition. Other Information: From review by Thomas R. Gilbert, Northeastern Univ., in Analytical Chemistry, Vol. 61, No. 6 (15 Mar 1989). 1988. Medium: X; Size: Pages: (910 p).
2. Walter, S., Chapter 3 Graphite furnace AAS, in Techniques and Instrumentation in Analytical Chemistry, R.F.M. Herber and M. Stoeppler, Editors. 1994, Elsevier. p. 53-85.
3. Walter, S., Chapter 4 Atomic absorption spectrometry Flame AAS, in Techniques and Instrumentation in Analytical Chemistry, R.F.M. Herber and M. Stoeppler, Editors. 1994, Elsevier. p. 87-90.
Appendix
Table 7.different concentration in Pb solution in 217nm, slit width 1nm
Table 8. differen concentration in Pb solution in 217nm, width 0.5nm ()
Table 9. different concentration of Mg in 202.6nm,