2. Atomic Absorption Spectroscopy (AAS)
Introduction
• When an element is introduced into a flame, a few of its atoms
absorb thermal energy from the flame and get excited.
• The majority of the atoms remain in the ground stage.
• The excited atoms emit their characteristic radiation and return to the
ground state.
• The study of this emission is called flame emission spectroscopy.
• The atoms which remain in the ground state can absorb radiation
which is characteristic of the element under study. Of course, this
radiation is the same as that emitted by the excited atoms of the
element in flame photometry.
3. • Atomic Absorption spectroscopy (AAS) is the study of absorption of
radiation by the ground state atoms when a sample is introduced into
a flame.
• This absorption of characteristic radiation helps in the identification of
the absorbing element.
• The intensity of absorption depends on the amount of sample
introduced into the flame.
• Hence AAS is an invaluable tool for the qualitative and quantitative
analyses of a large number of elements.
• This elegant method of elemental analysis was discovered by Sir
Allan Walsh in 1955.
4. Principles of Atomic Absorption Spectroscopy
•When a metal salt solution is introduced into a flame, the solvent
evaporates, leaving behind tiny solid particles of the salt.
•The solid particles vapourise into gaseous molecules which are then
converted into gaseous atoms.
•MX (s) MX (g) M (g)+X (g)
•Most of the atoms remain in the ground state.
•These atoms are capable of absorbing their own characteristic
radiation.
•If light of this characteristic wavelength is allowed to pass through a
flame containing these atoms, absorption takes place.
5. • The amount of radiation absorbed is proportional to the number of
absorbing atoms present in the flame.
• This absorption depends on the number of unexcited atoms present
in the flame and hence is independent of the flame temperature.
• As against this, in flame emission spectroscopy, the emission
intensity depends on the number of excited atoms and hence on the
flame temperature.
