Regions of the calibration plot:

The standard calibration plot is commonly linear over wide range but a deviation is commonly observed at both low and high concentration ends. Now let us have a close look at a typical calibration plot as given in Figure. As you could see, three different regions of the calibration plot are distinctly visible. The explanation for these regions is given below.

i) The region marked 'a' within the figure is depicted at extremely low concentration range. In this extremely low concentration range, emission falls below expected value due to ionisation as some atoms are converted to ions. For instance, for potassium the ionisation is as given below.

K → K⁺   + e ^-

Such ionisation is therefore, insignificant at higher concentration.

ii) The region marked 'b' is the linear region that could be used for quantitative measurements.

iii) The region is marked 'c' depicted at a high concentration. Within this concentration range negative deviation is observed because of self absorption. Photons emitted through the excited atoms are partly absorbed through ground state atoms in flame.

Therefore, the calibration curve method helps in searching the concentration of unknown samples. Therefore, for some samples, it is hard to prepare standards that are sufficiently same. This might occur while the samples hold high and variable concentrations of matrix materials, or while the samples contain solids whose effect on absorption is hard to duplicate. Within such cases we required to resort to any of the following two methods.

• Standard addition method
• Internal standard method