Figure 8.41
Schematic layout of an electron microscope – electron
probe analyser.target resulting in electronic excitations. The generated X-rays pass through a window made of a light
material such as beryllium or an organic polymer.
Electron probe microanalysis functions by direct examination of the primary X-rays produced when the
specimen is used as a target for an electron beam. Focused electron beams allow a 'spot analysis' of a 1
μm^3 section of the specimen. One current development employs the electron beam within a scanning
electron microscope to provide both a visual picture of the surface of the sample and an elemental
analysis of the section being viewed. Spectra obtained from primary X-rays always have the
characteristic emission peaks superimposed on a continuum of background radiation (Figure 8.32). This
feature limits the precision, sensitivity and resolution of electron probe microanalysis.
For the majority of X-ray fluorescence methods primary X-rays with a wide spectrum of energies are
directed on to the specimen. The secondary radiation emitted is viewed through a collimator at an angle
of 90° to the incident beam. X-ray emission may also be stimulated by bombardment with nuclear
radiation from a suitable radioactive source (e.g.^55 Fe,^241 Am,^238 Pu). The range of energies available is
more limited in this case and curtails the flexibility of the method. However, a number of specific
element analysers have been constructed in which radioactive excitation is used. One variable energy
device currently available uses^241 Am and a series of six interchangeable targets (Cu, Rb, Mo, Ag, Ba,
Tb). In this way a number of excitation lines can be produced. The advantages of these radioactive
sources lie in their simplicity and small size as well as their independence of electrical power. Thus they
are suited to portable instruments. Spectra obtained from fluorescent emission show much better
resolution than primary X-ray spectra as they lack the continuum background (Figure 8.32).
Consequently sensitivity and precision are rather better for fluorescence methods than they are for
electron excitation techniques.