Monochromators:

a) In AES, the analyte signal is produced by the small number of excited atoms or ions whereas in AAS the signal is obtained from absorption by much larger number of unexcited species. Any small change in flame conditions influence the number of excited species. While such changes have insignificant effect on the number of unexcited species.

b) Monochromator plays an important role in the resolution and selectivity of ICP emission. Therefore a high resolution monochromator can isolate the analyte spectral line from other lines and background emission and reduce spectral interferences. In AAS, however, resolution comes primarily from specific line emitted by a hollow cathode lamp. The monochromator isolates only the emission line of the analyte element from lines of impurities and fill the gas where a much lower resolution is needed.

c) Temperature of inductively coupled plasma is quite high which favours the formation of atoms and ions. In addition sample residence times are long so that desolvation and vaporisation are complete. Further atoms and ions are formed in a nearly chemically inert atmosphere. Nearly constant electron concentration leads to fewer ionization interferences. Since excited state is not formed or it is less stable because of high temperature, it is not useful for AAS.