Concentration Dependence of Absorption:
You have learnt previously that according to Boltzmann distribution law, the population of the ground state that is the number of species in the ground state is highest and it keeps on decreasing as we go to higher energy levels. It could be display that for most elements at moderate temperatures prevailing in a flame; nearly all atoms are within ground state leaving only a few atoms in excited state. Absorption follows Lambert-Beer's law so in which the concentration of an analyte element within the vapours in the flame might be determined.
According to Lambert-Beer's law, an extent of radiation absorbed through the absorbing species is a function of the path length and the attentiveness of the absorbing species.
Mathematically, log P0/P = εbc
where,
Po = radiant power of incident light,
P = radiant power of transmitted light,
b = thickness of the absorbing medium,
ε = absorption coefficient, and
c = concentration of absorbing analyte atoms.
The term, log Po /P is called absorbance and is represented as 'A'. Thus we can write it as follows.
log P0/P = A = εbc
Therefore, absorbance of the sample is straight proportional to the concentration of the analyte. Thus, a calibration plot of concentration of analyte component versus absorbance is drawn from the standard solutions and the concentration of element in unknown solution is read straight from the graph. Thus, such a linear relationship among the absorption and the concentration could be observed only if all radiation passing by the sample is absorbed to the similar extent through the analyte atoms. Therefore, the experimental concentration versus intensity calibration curve is observed to be deviating from the linearity as a result of the presence of nonabsorbed radiation and other interferences. Hence, appropriate measures need to be taken so since to minimise the interferences and acquired the linearity within the calibration curves. We would elaborate about these interferences. Let us learn about the methodology used in quantitative determinations using atomic absorption spectrophotometry.