measure kν at the centre of the absorption line, where it reaches a maximum value, using a sharp line
radiation source (vide infra) characteristic of the element of interest. Assuming that Doppler broadening
is the only significant line broadening effect, then the maximum value of kν is given by
where ∆λ is the Doppler linewidth at wavelength λ. Thus kmax is directly proportional to Nν, the sample
concentration. The profiles of an absorption line, an emission line from a sharp line source and the
bandpass of a monochromator are shown in Figure 8.25. It will be seen that radiation from the sharp
line source is absorbed at the centre of the absorption line and that the amount absorbed represents a
substantial proportion of the total radiated intensity. By contrast, radiation passed by a monochromator
(bandwidth 1–20 nm) is absorbed over the entire width of the absorption line, which invalidates
equation (8.5). Furthermore the fraction absorbed is extremely small, leading to poor sensitivity.
Figure 8.25
Profiles of an absorption line, an emission line from
a sharp line source and the bandpass of
a monochromator.Instrumentation
A diagram of a single beam atomic absorption spectrometer is shown in Figure 8.26. It consists of a
sharp-line radiation source characteristic of the element of interest, usually a hollow-cathode lamp
(HCL), a solution nebulizer and burner, or an electrically heated furnace, and a monochromator,
photomultiplier and recording system. A deuterium continuum radiation source used for background
correction (see below) is also shown. The purpose of the monochromator is to isolate a particular
emission line from a number of characteristic lines emitted by the hollow-cathode lamp. The
components are aligned so as to allow radiation from the lamp to pass directly through the flame and
then via the monochromator to the detector. An absorption measurement is made by comparing the
intensities of radiation reaching the detector with and without the sample solution being aspirated into
the flame (Iν and Io in equation (8.3)). In practice, the reading for the latter is set to zero absorbance
whilst a blank solution is aspirated, the reading for the sample then being given directly in absorbance
units.