Page 53 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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Figure 2.13
Double-beam atomic absorption instrumentation: a = rotating
half-silvered mirror; b = front surface mirror.

2.2.5.2 Background Correction.

Considerably more advantage can be derived from the use of a second beam of continuum radiation to
correct for non-atomic absorption. Figure 2.14 schematically summarizes how this operates. When
using a line source such as a hollow-cathode lamp, we observe atomic absorption in the flame,
absorption from molecular species and scattering from particulates. The latter, known as non-specific
absorption, is a particular problem at shorter wavelengths and can lead to positive errors. When using a
continuum source (e.g. a deuterium arc or hydrogen hollow-cathode lamp), the amount of atomic
absorption observed, as we have already seen (Section 2.1), is negligible, but the same amount of non-
specific absorption is seen. Thus, if the signal observed with the continuum source is subtracted from
that observed with the line source, the error is removed.

Figure 2.15 shows an instrument capable of doing this simultaneously and automatically. Lead is
particularly prone to this problem, and Fig. 2.16 shows how background correction can be used to
remove the interference of non-specific absorption when determining lead in chromium. Notice that the
precision is also improved, mainly because the effects which give rise to the background are not very
reproducible.

Other types of background correction have also been developed. The Zeeman effect background
correction system started gaining popularity in the early 1980s. An atomic spectral line when generated
in the presence of a strong magnetic field can be split into a number of components