Page 9 - AAS & AES & FES 01082016_Neat

I.2.1.2 Pressure broadening

Pressure broadening (also known as collisional broadening) occurs when the pressure of the
atomic vapor is sufficient to cause a relatively large number of collisions that involve the

emitting atoms. Usually the collisions are with molecules or atoms that are not identical to the
emitting atoms. As an example, in flames the collisions can be between the emitting atoms and

the molecules or atoms from the gases of the flame. Collisions that involve an emitting atom

result in a shortened lifetime of the atom in an excited electron level. Because the frequency of
the emission is a function of the time spent in the excited state, a change in the frequency of the

emitted radiation with a consequent broadening of the spectral line occurs.
Pressure broadening that is caused by collision between identical atoms is Resonant

broadening. Resonant broadening increases as the concentration of the assayed element
increases because the probability of collisions between the atoms increases.

At the temperature typically encountered during analysis (about 2000 to 3000 K) and at a

pressure of about 1 atm, the combined effect of Doppler and Pressure broadening is an
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increase in the width of a spectral line from about 10 to about 10 nm. The usual range of
observed line widths under those conditions of temperature and pressure is from 0.001 to 0.01

nm.

I.3. Instrumentation for AAS
A schematic block diagram of the instrumentation used for AAS is shown in Fig. The

components are similar to those used in other spectroscopic absorption methods.

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