EDS - X-ray microanalysis and its possibilities

The so called EDS-system is the analytical accessory which is by far mostly utilized at SEMīs. The EDS makes use of the characteristic X-ray radiation generated by the electrons hitting the sample. By evaluation of this radiation one gets the chemical (elemental) analysis of the scanned area of the sample, sometimes just a few cubicmikrons. The analytical information we get from the EDS has been used in nearly all applications for SEMīs, e.g. in the investigation of materials (alloys, ceramics, high tech materials), evaluation of surfaces, failure analysis (investigation of fractures), forensic investigations (comparison of very small objects), biology and medicine (metabolism, distribution of substances), semiconductor technology (quality control of parts), and many more applications.

In comparison to other analytical methodes the EDS offers some special advantages:
spectrum of gold alloy
* Without much effort the EDS works microanalytical, that means very small volumina can be analysed while they are under microscopical observation and control.  
* In many cases the EDS will work without any destruction. The samples will not be changed in any way at all. Painstacking steps of sample preparation can be avoided.
* The EDS works quite fast. In many cases the single measurement with contemporary qualitative evaluation can be taken in less than a minute.
* The EDS works simultanous. All elements of the substance under investigation in a way will be seen by the EDS at the same time. Because of this, previous knowledge of what the substance consists of or what the substance could consist of is absolutely unnecessary. An experienced EDS operator will not overlook any component of the substance under investigation.
iron distribution
iron distribution - 122,0 kB
* After qualitative analysis quantitative analyses of individual measurment points can be taken within a few minutes. Weīve prepared a sample printout of EDS quantitative analysis on another page.
* The EDS is able to provide maps of element distribution within an area of the object (see image on the left) and will also show the concentration of elements following a profil across an area of the object. That means that in this way it is possible to correlate specific morphological details with chemical analytical informations.

In opposition to the advantages of the EDS some disadvantages also exist :

  * The EDS detection sensitivity is relatively poor. Because of this, it is not useful for trace analysis. In standard applications concentrations as low as 0,1 weight% are detectable. In some case detection sensitivity can be improved at least one order of magnitude using X-ray fluorescence within the SEM.  
* In order to achieve a precise quantitative measurement, in many cases a slide of the sample has to be prepared. This is necassery, because the x-ray microanalysis is very dependent on geometry. x-ray microanalysis is also very dependent on the matrix, so that precise analyses can only be taken from areas of the sample that show homogenious distribution of all elements.
In praxis, both rules very often will not be taken into account, but only an experienced EDS operator may try to evaluate and minimize the systematic errors that will be introduced into the analysis in these cases.
* For just a few elemenal combinations showing close spaced lines within the x-ray-spectrum, these lines cannot securily be divided, because of the limited energetical resolution of EDS systems.

Perhaps you are interested of how an EDS-system works. An EDS consists of:

  * An X-ray detector, which collects X-ray pulses, that are freed as electrons hit the atoms of the sample. Within the detector the X-ray pulses are converted into small current pulses proportional in energy to the energy of the individual X-ray-pulses,  
* a pulsprocessor, which amplifies the current pulses and then measures there size,
* a multichannel analyser, which collects all the measurement results sorted by size, that means the whole X-ray spectrum of the scanned area of the sample,
* a computer with software that is able to read the content of the multichannel analyser and do the qualitative and quantitative evaluation of the collected spectra as well as to control the graphical and numerical display and printout of the results.

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