Instrumentation
The detailed design and construction of instruments used in the various branches of spectrometry are
very different and at first sight there may seem to be little in common between an X-ray fluorescence
analyser and a flame photometer. On closer examination, however, certain common features emerge
and parallels between the functions of the various parts of the different instruments are observed. The
basic functions of any spectrometer are threefold (Figure 7.4):
(a) the production of radiation of frequencies appropriate to energy changes within the sample
(b) spectral examination of this radiation to facilitate qualitative analysis of the sample
(c) measurements of the intensity of radiation at frequencies selected from the information obtained in
(b) to facilitate quantitative analysis of the sample.
Figure 7.4
Schematic layout of a spectrometer.Depending on the technique, the sample may itself fulfil the function of (a) or it may be positioned
between (a) and (b) or between (b) and (c).
Many instruments utilize a double beam principle in that radiation absorbed or emitted by the sample is
automatically compared with that associated with a blank or standard. This facilitates the recording of
data and corrects for matrix effects and instrumental noise and drift. Instrumentation for the generation
of radiation is varied and often peculiar to one particular technique. It will be discussed separately in the
relevant sections. Components (b) and (c), however, are broadly similar for most techniques and will be
discussed more fully below.
The Analyser
The function of this subunit is to present so-called monochromatic radiation to the detector, i.e. to
separate or disperse the radiation so that selected frequencies corresponding to particular energy
transitions within the sample may be individually examined. For instruments designed to operate in the
ultraviolet, visible and infrared regions of the spectrum, there are two approaches to this problem.
A filter may be employed which selectively absorbs all except the required range of frequencies – a
technique known as filter photometry. The absorption characteristics of some standard filters suitable
for use in the visible region are given in Figure 7.5. This is a simple but inflexible approach with a poor
selectivity as it is difficult to make filters with narrow