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On the other hand, sodium atoms are excited more easily than the atoms of most other

elements. Nevertheless, at 3000 K only 1 sodium atom is excited for every 1000 atoms in the

ground state. In a normal atom population there are very few atoms in states E1, E2, E3, and
higher.

The total amount of radiation absorbed depends, among other things, on how many
atoms are available in the lower-energy state to absorb radiation and become excited.

Consequently, the total amount of radiation absorbed is greatest for absorptions from the

ground state. Excited to excited state transitions are very rare, because there are so few excited
atoms; only the ground state resonance lines are useful analytically in AAS.

For practical purposes, all absorption in AAS is by atoms in the ground state. This greatly
restricts the number of absorption lines that can be observed and used for measurement in atomic

absorption. Quite frequently only three or four useful lines are available in the UV/VIS spectral
region for each element, and in some cases fewer than that.

AAS is useful for the analysis of approximately 70 elements, almost all of them metal or

metalloid elements. Grotrian diagrams correctly predict that the energy required to reach even
the first excited state of nonmetals is so great that they cannot be excited by normal UV radiation

(>190 nm). The resonance lines of nonmetals lie in the vacuum UV region.
Commercial AAS systems generally have air in the optical path, and the most common

atomizer, the flame, must operate in air. Consequently, using flame atomizers, atomic

absorption cannot be used for the direct determination of nonmetals. However, nonmetals
have been determined by indirect methods, as will be discussed in the applications section.

I.2.1 SPECTRAL LINE WIDTHS
In flames, absorptive bandwidths of atoms typically vary from about 0.001 to 0.01 nm.

The narrow bands of absorbed or emitted radiation that are observed with atoms are referred to
as spectral lines. For some elements, such as iron and uranium, hundreds of such electron

transitions are observed. Potassium was chosen for inclusion in because of the relatively small
number of observed transitions.

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