The relatively large mean free paths
of X-rays, particularly those of high-energy photons, allows signals to be
collected from a large volume of the sample probed by the electron
beam. EDS measurement becomes more efficient for heavier atoms (say Z > 30). However, X-rays cannot be deflected into an appropriate detector so that their collection is always inefficient (usually < 1%) and thus signal intensity is a problem from a thin specimen.
The main advantages of EDS are:
i) High speed of data collection in most cases. The EDS detector can be positioned very close to the specimen and thus a large solid angle is obtained for the collection of emitted X rays. The solid angle in a typical EDS configuration is about 10 times greater than that of a WDS. Therefore, for EDS measurement, many X-ray photons are collected per incident electron with either a small probe diameter or a low beam current.
ii) High detector's efficiency. Comparing with WDS detector, EDS detector has superior geometrical collection efficiency and allows parallel recording of the X-ray intensities of all lines/energies in the spectrum.
iii) Elemental coverage for almost all (carbon and above are detectable) but the lightest elements, e.g. boron, is problematic).
iv) Ease of use.
v) Able to scan areas (raster scanning) and single spots.
vi) Elemental spectra are linked to image maps generated by SEM or STEM.
vii) Map a large spatial range from ~1 mm2 to submicron2. Elemental maps can be generated from the data.
ix) A most widely used method in electron microscopes because of ease of interfacing to existing microscopes and of a higher speed of detection (comparing with WDS detector).
x) For SEM specimens, depth information is possible using various excitation voltages and modeling techniques such as Monte Carlo simulations.
xi) Surface sensitive: generate data from only the top couple of microns of SEM specimens.
The main disadvantages of EDS are:
i) Is destructive analysis in many cases.
ii) Poor energy resolution of the peaks.
iii) Low peak-to-background ratio. The bremsstrahlung background is caused by decelerating the primary electrons. This background continuum makes it unable to detect characteristic X-rays from components below the 0.1-0.05% level. Even though trace elements down to ~500 ppm of oxides can be detected by SEM-EDS, their quantification is not accurate. Note that collecting EDS data for longer time only slightly improves the detection limit.
iv) Limit on the input signal rate because of pulse processing requirements.
v) For most common detector designs, some elements, e.g. nitrogen, produce very weak responses, resulting in unreliable detection for most materials.
vi) Surface sensitivity (see above) complicates bulk analysis.
vii) Quantitative analysis of heterogeneous materials often provides inaccurate data.