Wavelength-dispersive systems

Table 8.39 shows the main features of EDXRF. EDXRF is not able to detect the fine structure of the K, L, M, etc. lines. EDXRF is used for applications which require measurement of a limited number of elements, and where the resolution and ultralow detection limits of wavelength-dispersive systems are not necessary. For example, EDXRF has been used as a rapid screening technique for the determination of Br and Sb in plastic recyclate at a LOD of 5 ppm [230] the method was validated by means of NAA [231]. Conventional EDXRF systems and benchtop units have a limited detection capability for low-Z-elements and cannot directly measure fluorine in processing aids. 

FIGURE 10.7 The two instrument designs for x-ray fluorescence spectroscopy. Left, the energy-dispersive system. Right, the wavelength-dispersive system. 

Use of the analyzer crystal (wavelength-dispersive system) means better resolution, but it is slower and less sensitive. 

Extraction of the useful signal information from the whole information using a device such as a wavelength dispersive system, filter etc. 

Wavelength Dispersive Systems. . . 759 Energy Dispersive Systems.760... 

The X-ray analysis in most SEMs is the energy-dispersive analyster using a semiconductor detector, such as a lithium-drifted silicon, Si(Li), or lithium-drifted germanium, Ge(Li), detector, which were discussed in Section 12B-4. Wavelength-dispersive systems have also been used in electron microprobe analyses. 

Figure 21-16 is a schematic ot an electron microprobe system. The instrument employs three integrated sources of radiation an electron beam, a visible light beam, and an X-ray beam. In addition, a vacuum system is required that provides a pressure of less than 10" torr as is a wavelength- or an energy-dispersive X-ray spectrometer (a wavelength-dispersive system is shown in Figure 21 -16). The electron beam is produced by a heated tungsten cathode and an accelerating an ode (notshown). Two electromagnetic lenses focus the beam on the specimen the diameter of the beam is between 0.1 and 1 pm. An associated optical microscope is used to locate the area to be bombarded. Finallv. the...

---