Key Concepts
- Atomic Absorption Spectroscopy (AAS) was developed by CSIRO Scientist Dr Alan Walsh in the 1950s.
- Light with specific frequencies is absorbed by different metals when they vaporize in a flame.
The energy absorbed excites electrons, moving them from their ground state to a higher energy state.
- Atomic Absorption Spectroscopy uses hollow cathode lamps to emit light with these frequencies which is then absorbed by the sample containing the metal ion.
- The amount of light absorbed is proportional to the concentration of the metal ion in solution.
Concentrations are often expressed as mg/L or ppm.
- The amount of light absorbed by the sample is compared to the amount of light absorbed by a set of standards of known concentration.
Atomic Absorption Spectroscopy can be used to measure the concentration of metals in :
- mining operations and in the production of alloys as a test for purity
- contaminated water, especially heavy metal contamination in industrial waste water
- organisms, such as mercury in fish
- air, eg lead
- food
Example : Undiluted Sample
Atomic Absorption Spectroscopy (AAS) can be used to determine the lead concentration in soil collected from the side of a road.
A student prepared standard lead solutions for comparison and the aborbance of each solution was measured.
A road-side soil sample was also prepared.
The results are shown in the table below.
| Sample | Concentration (ppm) | Absorbance |
|---|
| Blank |
0.00 |
0.00 |
| Standard 1 |
1.00 |
0.17 |
| Standard 2 |
2.00 |
0.34 |
| Standard 3 |
3.00 |
0.48 |
| Standard 4 |
4.00 |
0.65 |
| Standard 5 |
5.00 |
0.83 |
| Sample |
? |
0.58 |
What was the concentration of lead in the soil sample?
- Plot the calibration curve using the concentrations and absorbances of the standard solutions (shown as red x's on the graph)
- Draw a line of best fit through the plotted points (shown as a red line on the graph)
- Mark the position of the 0.58 absorbance of the sample being investigated (shown on the graph as a blue x)
- Read off the concentration of lead in the sample from the graph, 3.50ppm
The concentration of lead in the sample was 3.50ppm.
Example : Diluted Sample
Samples are often diluted before being analysed.
When this occurs, you will need to take this into account when you calculate the concentration of the original sample.
A student prepared standard lead solutions for comparison and the aborbance of each solution was measured.
A road-side soil sample was also prepared.
10mL of this prepared soil sample was placed in a 100mL volumetric flask and enough water was added to make it up to the mark.
The absorbance of this diluted soil sample was recorded.
The results are shown in the table below.
| Sample | Concentration (ppm) | Absorbance |
|---|
| Blank |
0.00 |
0.00 |
| Standard 1 |
1.00 |
0.17 |
| Standard 2 |
2.00 |
0.34 |
| Standard 3 |
3.00 |
0.48 |
| Standard 4 |
4.00 |
0.65 |
| Standard 5 |
5.00 |
0.83 |
| Sample |
? |
0.26 |
What was the concentration of lead in the original (undiluted) soil sample?
- Plot the calibration curve using the concentrations and absorbances of the standard solutions (shown as red x's on the graph)
- Draw a line of best fit through the plotted points (shown as a red line on the graph)
- Mark the position of the 0.26 absorbance of the sample being investigated (shown on the graph as a blue x)
- Read off the concentration of lead in the diluted sample from the graph, 1.60ppm
- Calculate the concentration of lead in the original undiluted sample.
lead concentration in diluted sample = 1.60ppm = 1.60mg/L
mass lead in the 100mL (100mL/1000mL/L) diluted sample = 1.60mg/L x 100/1000L = 0.16mg
The original 10mL (10/1000L) of sample solution contained 0.16mg
Lead concentration of original sample = 0.16mg/(10/1000) = 16.00mg/L = 16.00ppm
Lead concentration in the sample of road-side soil was 16.00ppm.
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