Physical Principles of Electron Microscopy: An Introduction to TEM ...

170 Chapter 6
to be analyzed, whereas the XWDS analyzing crystal needs room to move
and subtends a smaller (solid) angle, so that much less than 1% of the x-ray
photons are collected. Although a commercial XEDS system costs $50,000
or more, this is considerably less expensive than an EPMA machine or
XWDS
attachment to an SEM.
The main advantage of the XWDS system is that it provides narrow xray
peaks (width � 5 eV, rather than > 100 eV for XEDS system) that stand
out clearly from the background; see Fig. 6-7. This is particularly important
when analyzing low-Z elements (whose K-peaks are more narrowly spaced)
or elements present at low concentration, whose XEDS peaks would have a
low signal/background ratio, resulting in a poor accuracy of measurement.
Consequently, XWDS analysis is the preferred method for measuring low
concentrations: 200 parts per million (ppm) is fairly routine and 1 ppm is
detectable in special cases. At higher concentrations, the narrow XWDS
peak allows a measurement accuracy of better than 1% with the aid of ZAF
corrections. In contrast, the accuracy of XEDS analysis is usually not better
than 10%.
Figure 6-7. X-ray intensity as a function of photon energy (in the range 2.1 to 2.7 keV),
recorded from a molybdenum disulfide (MoS2) specimen using (a) XWDS and (b) XEDS.
The wavelength-dispersed spectrum displays peaks due to Mo and S, whereas these peaks are
not separately resolved in the energy-dispersed spectrum due to the inadequate energy
resolution Courtesy of S. Matveev, Electron Microprobe Laboratory, University of Alberta.