Atomic Absorption Spectroscopy: Theory

THEORY

The underlying principle of atomic absorption spectroscopy (AAS) is the absorption of energy exclu-sively by ground state atoms while they are in the gaseous form.

It may be further expatiated as follows below :

A solution consisting of certain metallic species when aspirated into a flame, it will give rise to the corresponding vapours of metallic species. As it has already been discussed under flame emission spectroscopy (FES) : Some metal atoms would be raised directly to an energy level to such an extent as to emit the particular radiation of the metal. At this critical point, a sufficiently large quantum of the metal atoms of a particular element would still remain in the non-emitting ground-state, which in turn shall be receptive of light radiation having their own specific wavelength. Consequently, when a light of this wavelength is passed through a flame ; along the atoms of the metallic species, a portion of the same would be absorbed ; and the resulting absorption has been found to be directly proportional to the density of the atoms present in the flame at that material time. In AAS, one logically determines the amount of light absorbed. In other words, the concentration of the metallic element may be determined directly from the value of absorption.

The total amount of light absorbed may be provided by the following mathematical expression :

            ................................(a)

υ = Frequency of the light path,                          

e = Charge on the electron,                        

m = Mass of the electron,                          

c = Speed of light,                            

N = Total number of atoms which can absorb     at υ, and

f = Ability for each atom to absorb at υ (oscillator strength).

The components in Eq. (a), namely : π, e, m and c are constants, therefore, it can be further written in a simplified form as below :

Total amount of light absorbed = K × N × f                           ...(b)

Hence, from Eq. (b) it may be inferred that :

(a) it is independent of the wavelength, and

(b) it is independent of temperature,

More explicitly, the absorption by atom is independent of both the wavelength of absorption and the temperature of the atoms. And these two specific characteristic features give AAS a clear distinct and posi-tive edge over FES.

1. MERITS OF AAS OVER FES

The various points of merit of atomic absorption spectroscopy over flame spectroscopy are enumerated below :

2. DEMERITS OF AAS

The various points of demerit of atomic absorption spectroscopy are as follows :

(i) It essentially requires a separate lamp for each element to be determined ; and this serious lacuna is usually overcome either by using a line-source with the introduction of flame or by using a continuous source with the introduction of a very high resolution monochromator,

(ii) AAS cannot be employed very effectively for such elements that produce their corresponding oxides when exposed in the flame, for example : Al, Mo, Si, Ti, W, V. Nevertheless, these estima-tions may be performed under suitably modified experimental parameters, and

(iii) When the solutions of metal salts are made in an aqueous medium the predominant anion present affects the resulting signal to a negotiable extent.