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Atomic Absorption Spectroscopy (AAS) relates to the study of the absorption of radiant energy commonly within the ultraviolet or possibly in the visible region of the electromagnetic spectrum by isolated atoms in the gaseous phase. Considering that, in Atomic Absorption Spectroscopy, the analyte is introduced to the optical beam of the instrument as free atoms, all the likely rotational and vibrational energy levels are degenerate (of the same energy). Contrary to the absorption spectra of polyatomic chemical species (ions or molecules) in which there is often a multiplicity of feasible transitions corresponding to several rotational and vibrational energy levels superimposed on distinct electronic energy levels, the spectra of free atoms are characterized by merely a reasonably very few sharp absorbances (line spectra) which are often correlated with changes in electronic energy levels. The multitude of possible different energy levels accessible to polyatomic species leads to almost a continuum of possible transitions. As a result the spectra of ions (molecules) are comprised of somewhat broad bands which are caused by the partial resolution of several individual transitions. Hence, one feature of atomic spectra is their simpleness compared to the spectra of polyatomic species.


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