qualitative analysis

Qualitative analysis means to identify constituent elements of a specimen from the energy or wavelength of characteristic X-rays emitted from the specimen. The method to measure the energy is termed EDS (Energy Dispersive X-ray Spectroscopy) and the method to measure the wavelength is termed WDS (Wavelength Dispersive X-ray Spectroscopy).

Analysis modes of qualitative analysis include 1) Point analysis that reveals constituent elements obtained from the acquired X-ray spectra from one point of the specimen, 2) Line analysis that displays the distribution of elements of interest on a specified line of the specimen, and 3) Area analysis (elemental mapping) that displays the distribution of elements of interest over a specified specimen area.

EDS provides fast and easy qualitative analysis. Its energy resolution is 128 to 140 eV (defined at manganese (Mn) Kα 5.9 keV). Then, spectral peaks sometimes overlap to each other, when the energies of the two peaks from different elements are close to the resolution of EDS. For example, the peak of strontium (Sr) Lα (1.806 keV) overlaps with that of silicon (Si) Kα (1.739 keV). Table below shows the energies of Kα lines from the major elements and those of characteristic X-rays from other elements close to them.

It should be noted that the use of WDS enables to avoid such peak overlap and to observe the peaks clearly separated because the energy resolution of WDS is as high as10 to 20 eV (wavelength is converted to energy). Another advantage of WDS is that trace element of very small amounts, down to 0.002 or 0.003% can be detected, though EDS cannot detect the trace elements less than a few tenths of a percent (e.g. 0.2%) as peaks (detection limit), so they cannot be identified by qualitative analysis (detection limit).

It is noted that in both EDS and WDS, the X-ray generation region reaches several 100 nm to several μm in size, even when the diameter of the incident electron beam is small.

Table. Energies of Kα lines of the major elements and those of characteristic X-rays from other elements close to them.