Page 33 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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From classical dispersion theory we can show that k v is in practical terms proportional to the number of
atoms per cubic centimetre in the flame, i.e. A is proportional to analyte concentration.

Atomic absorption corresponds to transitions from low to higher energy states. Therefore, the degree of
absorption depends on the population of the lower level. When thermodynamic equilibrium prevails,
the population of a given level is determined by Boltzmann's law. As the population of the excited
levels is generally very small compared with that of the ground state (that is, the lowest energy state
peculiar to the atom), absorption is greatest in lines resulting from transitions from the ground state;
these lines are called resonance lines.

Although the phenomenon of atomic absorption has been known since early last century, its analytical
potential was not exploited until the mid-1950s. The reason for this is simple. Monochromators capable
of isolating spectral regions narrower than 0.1 nm are excessively expensive, yet typical atomic
absorption lines may often be narrower than 0.002 nm. Figure 2.1 illustrates this, but not to scale! The
amount of radiation isolated by the conventional monochromator, and thus viewed by the detector, is
not significantly reduced by the very narrow atomic absorption signal, even with high concentrations
of analyte. Thus, the amount of atomic absorption seen using a continuum source, such as is used in
molecular absorption spectroscopy, is negligible.

The contribution of Walsh was to replace the continuum source with an atomic spectral source (Fig.
2.1). In this case, the monochromator only has to isolate the line of interest from other lines in the lamp
(mainly lamp filler gas lines). In Fig. 2.1, we see that the atomic absorption signal exactly overlaps the
atomic emission signal from the source and very large reductions in radiation are observed.

Of course, this exact overlap is no accident, as atomic absorption and atomic emission lines have the
same wavelength. The very narrowness of atomic lines now becomes a positive advantage. The lines
being so narrow, the chance of an accidental overlap of an atomic absorption line of one element with
an atomic emission line of another is almost negligible. The uniqueness of overlaps in the Walsh
method is often known as the 'lock and key effect' and is responsible for the very high selectivity
enjoyed by atomic absorption spectroscopy.

The best sensitivity is obtained in this method when the source line is narrower than the absorption
profile of the atoms in the flame. Obviously, the other situation tends towards Fig. 2.1.

In recent years, it has been shown that the construction of atomic absorption spectrometers using
continuum sources is possible, if somewhat expensive and complicated. So far, commercial
manufacturers have not yet produced instruments of this type, which have remained the creations of a