energy dispersive X-ray spectroscopy, EDS

An energy spectroscopy method for characteristic X-rays emitted from a specimen (EDS). The method measures the energy spectra of the characteristic X-rays using a semiconductor detector, and then identifies the constituent elements of the specimen (qualitative analysis) from the spectral energy values and determines the concentrations of the identified constituent elements from the spectral intensities (quantitative analysis). EDS can simply and rapidly (in a few minutes) perform qualitative analysis and quantitative analysis of the elements ranging from beryllium (Be) to uranium (U). The energy resolution of EDS is 130 to 140 eV (for Mn Kα 5.9 keV). The analysis region (volume) is several 100 nm to several mm in both horizontal and depth directions. The region is large compared to the diameter of the incident electron probe because the incident electrons scatter in the specimen.
Comparing to wavelength dispersive X-ray spectroscopy (WDS), the advantages of EDS are as follows: 1) Measurement time is short for one specimen, 2) Damage to the specimen is small because of the use of a small incident probe current (several nA to several 10 nA). 3) No pre-treatment of the specimen (surface polishing, etc.) is required, and 4) The size of the spectrometer is compact and its design is not complex.
However, its energy resolution is one order of magnitude lower than that of WDS (approximately 20 eV). As a result, it can happen in EDS analysis that the L line of an element and the K line of another element cannot be observed separately because their energy difference is smaller than the energy resolution of EDS. For example in the case of barium titanate (BaTiO3), the spectrum of TiKα overlaps with that of BaLα, and TiKβ overlaps with BaLβ, making it difficult to identify the elements. In addition EDS has a low peak-to-background (P/B) ratio because of the high background due to continuous X-rays created by bremsstrahlung. As a result, the detection limit (ability to detect trace elements) of EDS is approximately two orders of magnitude lower than that of WDS. EDS is difficult in analyzing trace elements whose concentration is one place of decimals or lower.
 
In quantitative analysis using EDS, two methods are available. One is a method which uses standard specimens, and another is a method which does not use standard specimens, called the standard-less method. The standard-less method is mainly used in EDS quantitative analysis. The reason why the standard-less method requires no standard specimens is that a great number of X-ray spectral data pre-acquired from many standard specimens are built into the analyzing systems created by EDS makers. In this method, once a spectrum is acquired from an unknown specimen, the concentrations of all of the constituent elements are automatically determined by the computer. The computer also automatically executes the ZAF correction, which takes account of the differences in 1) the scattering features of the incident electrons and 2) the absorption and fluorescence excitation effects between the standard specimen and the unknown specimen.
Since the standard-less method rapidly provides the concentrations of all of the constituent elements, it is very suitable for simultaneously acquiring rough concentrations of all the constituent elements in the specimen. However, the quantitative accuracy of the standard-less method is inferior to that of the method using the standard specimens, in which the spectral intensity of each constituent element is measured from both an unknown specimen and a corresponding standard specimen under the same acquisition condition.
In WDS, the standard specimen method is applied to quantitative analysis.
The table below lists the advantages and disadvantages of EDS and WDS.
Table. Comparison of the performance of EDS and WDS

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