Metallic salts (or metallic compounds) after dissolution in appropriate solvents when introduced into a flame (for instance : acetylene burning in oxygen at 3200°C), turns into its vapours that essentially contain mostly the atoms of the metal. Quite a few such gaseous metal atoms are usually raised to a particular high energy level that enables them to allow the emission of radiation characteristics features of the metal : for example-the characteristic flame colourations of metals frequently encountered in simple organic compounds such as : Na-yellow, Ca-brick-red ; Ba-apple-green. This forms the fundamental basis of initially called Flame Photometry, but more recently known as Flame Emission Spectroscopy (FES).
It is quite evident that a relatively large proportion of the gaseous metal atoms shall remain in the ground state i.e., in an unexcited form. It has been observed that such ground-state atoms shall absorb radiant energy pertaining to their own particular resource wavelength. Therefore, when a light having the same resonance wavelength is made to pass through a flame consisting of such atoms, a portion of the light shall be absorbed accordingly. Furthermore, the extent or degree of absorption would be directly proportional to the total number of ground-state present in the flame. And this is the basis of Atomic Absorption Spectroscopy (AAS).
The emission spectrum thus obtained is made up of a number of lines that actually originate from the resulting excited atoms or ions ; and these steps may be shown diagrammatically as represented in Figure 25.1.
The various steps (I to VII) in Figure 25.1, above are explained as under :
Step-I : The liquid sample containing a suitable compound of the metal (M+ A–) is aspirated into a flame, thereby converting it into its vapours or liquid droplets,
Step-II : The evaporation of vapours (or droplets) give rise to the corresponding solid residue,
Step-III : The vapourization of the solid residue into its gaseous state occurs,
Step-IV : The dissociation of the gaseous state into its constituent atoms, namely : M(gas)+ A(gas)
take place, that initially, is in ground state,
Step-V : The thermal excitation of some atoms into their respective higher energy levels will lead ultimately to a condition whereby they radiate energy (flame emission) measured by Flame Emission Spectroscopy (FES), and
Step-VI : The absorption of radiant energy by some atoms into their higher energy levels enable them to radiate energy (atomic absorption) measured by Atomic Absorption Spectroscopy (AAS).