crystal. Limitations on the size of the instrument and the low radiation intensities at long distances
mean, however, that the best resolution is about 8 × 10 -^18 J (50 eV). By contrast the solid state detector
and its associated analyser has no mechanical movement but can provide a resolution of about 2.4 × 10 -
(^16) J (150 eV). Improvement in semiconductor technology is steadily increasing the resolving power but
Figure 8.44 shows the advantage still enjoyed by dispersive analysis. An earlier disadvantage of solid
state detectors was their relatively low efficiency, but improvement in crystal design and the fabrication
of larger crystals (50–100 cm^3 ) have reduced this problem.
The random nature of the ionizing events recorded by the detector must also be borne in mind. To
achieve measurements with standard deviations of 1% it is necessary to record at least 10^4 counts. For
signals of low intensity this may take several hours to accumulate. This point is discussed further in
Chapter 10.
Applications of X-ray Emission Spectrometry
Electron probe and X-ray fluorescence methods of analysis are used for rather different but
complementary purposes. The ability to provide an elemental 'spot analysis' is the important
characteristic of electron probe methods, which thus find use in analytical problems where the
composition of the specimen changes over short distances. The examination of the distribution of heavy
metals within the cellular structure of biological specimens, the distribution of metal crystallites on the
surface of heterogeneous catalysts, or the differences in composition in the region of surface
irregularities and faults in alloys, are all important examples of this application. Figure 8.45 illustrates
the analysis of parts of a biological cell just 1 μm apart. Combination of electron probe analysis with
electron microscopy enables visual examination to be used to identify the areas of interest prior to the
analytical measurement.
X-ray fluorescence measurements have a wide application in routine industrial analysis particularly in
mineral processing and metallurgy. The simplicity of operation, and the elimination of time consuming
wet chemical steps can have a dramatic effect on the time taken to complete an analysis. Niobium-
tantalum ores for instance present a special problem as a result of the chemical similarity of the two
elements. The analysis of such samples by wet chemical procedures has typically taken 10 days for
completion. X-ray fluorescence, however, provides a result in less than an hour because it readily
distinguishes between elements with widely differing atomic numbers (Figure 8.43). A further
important characteristic of this method of analysis is its non-destructive nature, which together with the
previously mentioned factors produces an attractive basis for automated 'on stream' analysis in a
chemical plant. Lead or bromine containing petroleum additives are monitored in this way as are the
major constituents of cement, e.g. CaO, SiO 2 , Al 2 O 3 and Fe 2 O 3.