can be observed will vary with the density of the matrix and the extent to which the emitted rays are
attenuated. Similarly the depths to which the incident X-rays or electrons penetrate will be limited and
varied. In general X-ray fluorescence data will be characteristic of the surface layers of atoms only (5–
500 μm) and it is important to know whether or not these are truly representative of the specimen as a
whole. For electron bombardment penetration may be limited to 1– 2 μm.
In addition to absorption problems, measurements will be affected by secondary fluorescence and
scattered radiations which will enter the detector and increase the general background. Detection limits
under optimum conditions (a heavy element in a light matrix) may be as low as 10 ppm. Quantitative
analysis is however difficult below the 20–100 ppm region if a reasonable precision (5% or better) is to
be obtained.
Instrumentation
The instruments used in X-ray emission spectrometry reflect the principles set out in Chapter 7.
Radiation characteristic of the specimen is produced by electron or radiation bombardment.
Monochromatic radiation is then presented to the detector by a diffraction device or by use of a series of
narrow bandpass filters. Alternatively pulse height analysis (p. 465) can be applied to a series of pulses
which have been generated with a size proportional to the radiation energy. Typical X-ray spectrometry
arrangements are shown in Figures 8.40 and 8.41.
Figure 8.40
Schematic layout of a dispersive X-ray fluorescence spectrometer.Excitation
Primary X-rays are produced by the bombardment of a suitable target with a stream of accelerated
electrons. A typical X-ray generator uses an evacuated tube into which the target (e.g. tungsten) projects
as a cooled anode together with a tungsten filament cathode. At a potential of 20–50 keV electrons are
emitted from the cathode and bombard the