Page 96 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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4.2.3 Flames VS Plasmas

The effect of temperature and E, the energy difference between the excited and ground states, is best
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illustrated by the practical examples in Table 4.1. It can clearly be seen that the number of excited
atoms, and hence the intensity of emission, increase very rapidly with increasing temperature, and that
the number of excited atoms is greater the lower is the energy level.

The number of excited atoms at typical flame temperatures (ca 2200-3200 K) is very low indeed
compared with the number of ground-state atoms, even for easily excited lines. For difficult-to-excite
lines (e.g. Zn 213.9 nm), it can be shown that only about one excited atom will exist at any given time
in an air-propane flame when aspirating a 1 mg 1 zinc solution. This is one reason why flames are
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poor sources for atomic emission spectrometry, but are well suited to atomic absorption spectrometry,
i.e. most of the atoms are in the ground state. As will be seen, the typical temperatures obtainable in
plasma sources are of the order of 8000 K, at which there is a much high ratio of excited-to ground-state
atoms, and hence a much greater intensity of atomic emission.

4.2.3.1 Self-absorption

Figure 4.3 shows a Grotrian diagram, or partial energy level diagram, for sodium emission. If the
oscillator strengths of the lines were equal (which, of course, they are not), we would expect to see
maximum emission from lines where the upper excited states lie closest to the ground state, that is,
where the excitation energy E is small and the Boltzmann distribution predicts a greater population of
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excited atoms than at other states. The 3P-3S transition for sodium is very intense—the famous D
lines—and hence it is an element very favourable for analysis by atomic emission spectrometry.

In a flame, as the concentration of atoms increases, the possibility increases that photons emitted by
excited atoms in the hot region in the centre will collide with atoms in the cooler outer region of the
flame, and thus be absorbed. This self-absorption effect contributes to the characteristic curvature of
atomic emission calibration curves towards the concentration axis, as illustrated in Fig. 4.4. The
inductively coupled plasma (ICP) tends to be hotter in the outer regions compared with the centre—a
property known as optical thinness—so very little self-absorption occurs, even at high atom
concentrations. For this reason, curvature of the calibration curve does not occur until very high atom
concentrations are reached, which results in a much greater linear dynamic range.