Page 21 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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mass is stored in a multi-channel analyser and output to a microcomputer for data manipulation.

1.3 Basic Theory

A summary of the basic theory underlying the main atomic spectrometric techniques is given below.
For a more thorough discussion, refer to the individual chapters.

1.3.1 Atomic Absorption

Atomic absorption is the absorption of light by atoms. An atom has several energy levels. Under normal
circumstances, most atoms will be in the ground (unexcited) state. For the energy levels E (ground
0
state) and E (excited state), a transition from E fi E represents an absorption of radiation (Fig. 1.1).
j
0
j
For atomic absorption to occur, light of the correct wavelength (energy) is absorbed by ground-state
electrons, promoting them to a higher, excited state. The intensity of the light leaving the analytes is
therefore diminished. The amount by which it is diminished is proportional to the number of atoms that
were absorbing it. A situation analogous to the Beer-Lambert law is therefore obtained. This law is
expressed as

is the
0
where A is absorbance, I is the incident light intensity, I the transmitted light intensity, k v
absorption coefficient and l is the path length.
It can be shown that k and hence A, are proportional to atom concentration, and the plot of absorbance
v
against atom concentration is a straight line.

1.3.2 Atomic Emission
The intensity I of a spontaneous emission of radiation by an atom is given by the equation
em

where A ji is the transition probability for spontaneous emission, h is Planck's constant, v ji is the
frequency of radiation and N j the number of atoms in the excited state.

It can be shown (see Chapter 4) that N j, and hence I , are proportional to the atom concentration, and
em
for low concentrations the plot of emission intensity against atom concentration is a straight line.