Energy-Dispersive Spectroscopy (EDS, also called EDXA for Energy-Dispersive X-ray Analysis) is a technique that uses a scanning electron microscope for chemical microanalysis. As the SEM scans the surface of a sample with an electron beam, the sample emits x-rays. An x-ray detector collects the x-rays, measuring both their numbers and energies. Since each element produces characteristic x-ray energies,1 the user is able to determine the elemental composition of the sample. Also, the amount of each element present can be determined from the relative counts of the detected x-rays.
Depending on the sample, EDS can detect most of the naturally-occuring elements in concentrations as low as 0.1 weight percent. Although the technique does not require it, the accuracy of the results may be improved by using standards with structures and compositions close to that of the unknown sample. In addition, because SEM is a surface analysis technique, it will not necessarily match the bulk sample composition. Several small areas of the sample may be scanned to obtain an average surface composition.
Normal SEM operation requires the sample chamber to be evacuated (2 Torr or less), so samples must be solids. Usually the sample should be conductive, or coated with a thin layer of some conducting material (e.g. Au). EDS is often used for characterization of coatings and alloys, and evaluation of corrosion.
1. The SEM electron beam causes electrons to be emitted from the atoms of the sample surface. Another electron from a higher-energy shell of each atom will drop down to fill the vacancy and will emit an x-ray with energy equal to the energy difference between the two electrons. Every element has distinctly unique energy levels.
References:
Skoog, D. A.; Leary, J. J. Principles of Instrumental Analysis 4th ed.; Saunders: Fort Worth, TX, USA; 1992; Section 16B.
Handbook of Analytical Methods http:www.mee-inc.com/eds.html