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Atomic Absorption Spectroscopy (AAS)

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.

SampleConcentration (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?

Step 1: Draw a calibration curve using the data:

  1. Plot the calibration curve using the concentrations and absorbances of the standard solutions (shown as red x's on the graph)

  2. Draw a line of best fit through the plotted points (shown as a red line on the graph)

Step 2: Use the calibration curve to find the concentration of lead in the sample:

  1. Mark the position of the 0.58 absorbance of the sample being investigated (shown on the graph as a blue x)
    (Draw a horizontal line, parallel to the x axis, from absorbance 0.58 until it meets the line drawn on the graph. Shown on the graph as a dotted blue line.)

  2. Read off the concentration of lead in the sample from the graph, 3.50 ppm
    (From the blue x on the line in the graph, draw a line vertically down to meet the x axis. Shown on the graph as a dotted blue line.)

The concentration of lead in the sample was 3.50 ppm.
(Check that your answer is sensible. The absorbance of the sample lies between the absorbance for standards 3 and 4, therefore the concentration of lead in the sample must be between 3.00 and 4.00 ppm)

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