Notes on Atomic Absorption Spectroscopy (AAS)

Introduction to Atomic Absorption Spectroscopy (AAS)

  • Discovered by Sir Allan Walse in 1955.
  • Related to Flame Emission Spectroscopy (uses a flame for atomization).

Principle of AAS

  • Aqueous solution of inorganic salts is aspirated into a hot flame, converting metallic constituents to atomic vapor.
  • Most atoms remain in the ground state, some are excited thermally and emit characteristic radiation.
  • Emission study is termed Flame Photometry; absorption study of ground state atoms is Atomic Absorption Spectroscopy (AAS).

Key Features of AAS

  • Absorption of radiation helps identify elements.
  • Intensity of absorption correlates with quantity of metal atoms present.
    • Useful for both Quantitative and Qualitative analysis of various elements.

Mechanism of AAS

  1. Sample Aspirated: Solvent evaporates, leaving dehydrated salt.
  2. Dissociation: Produces free gaseous ground state atoms.
  3. Thermal Excitation: Some atoms absorb energy from the flame and become excited.
  4. Characteristic Absorption: Ground state atoms absorb specific wavelengths characteristic of the element.

Energy Consideration in AAS

  • Radiation incident on flame allows metal atoms in ground state to absorb radiation, getting excited.
  • Energy difference between states helps identify the metal:
    • hγh\gamma is characteristic radiation.
  • Absorption is used for qualitative analysis; the amount of absorption indicates concentration.

Comparing AAS and Flame Emission Photometry

  • AAS: Independent of flame temperature.
  • Flame Emission Photometry: Depends on flame temperature.

Limitations of AAS

  • Requires a different source for each element, making the technique specific but somewhat limiting.