Page 94 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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Dv = (V x/c)v 0. To this is applied the distribution of velocities. After evaluation of constants, this
simplifies to the Doppler half-width:

where M = relative atomic mass. The Doppler effect acts primarily on the centre of the profile.

Lorentz (collisional) broadening arises from collisions of atoms with atoms or molecules of a
different kind. It has been shown experimentally that these collisions shift, broaden and cause
asymmetry in the line. Different gases have different effects. Collisional theory offers the best fit
equations to describe these events at the line centre, and statistical theory describes the events at the
wings. Lorentz broadening increases with pressure (P) and temperature (T), and is generally regarded as
being proportional to P and . Thus, Dv increases with increasing T and P. It is accepted that Lorentz
broadening affects the wings of the profile.

The profile of the line can be summarized by the Voigt profile:

where

Dv is the Lorentz half-width and d is the frequency displacement v - v .
L 0
Stark broadening occurs in the presence of an electric field, whereby the emission line is split into
several less intense lines. At electron densities above 1013 the field is relatively inhomogeneous, the
splitting is different for different atoms and the result is a single broadened line.

Other broadening processes also exist. Holtsmark (resonance) broadening arises from collisions
between atoms of the same kind and is therefore negligible when compared with other collisions.

For resonance lines, self-absorption broadening may be very important, because it is applied to the sum
of all the factors described above. As the maximum absorption occurs at the centre of the line,
proportionally more intensity is lost on self-absorption here than at the wings. Thus, as the
concentration of atoms in the atom cell increases, not only the intensity of the line but also its profile
changes (Fig. 4.2b) High levels of self-absorption can actually result in self-reversal, i.e. a minimum at
the centre of the line. This can be very significant for emission lines in flames but is far less pronounced
in sources such as the inductively coupled plasma, which is a major advantage of this source.