Compositional analysis with energy discriminated x-rays (EDX)

EDX Image

Materials can be studied or identified on the basis of elemental composition with an SEM.  By employing the SEM's EDX x-ray detector, variations in compositions of materials can be mapped over the scale of millimeters to micrometers (microns). This is done by bombarding a sample surface with electrons and measuring the energies and intensities of x-rays emitted. X-rays are produced as a result of electrons ionizing core shells of the atoms present in the sample, and because the shell transitions, and resulting x-rays, are characteristic of each chemical element, the resulting x-rays are very useful. That is, the x-ray energies are characteristic of the specimen's elemental composition, and the intensity of the x-rays are directly associated with weight fraction of each of those chemical elements.

This type of compositional analysis is unique because the spatial resolution for the x-ray measurement of a scanning electron microscope is only several microns. SEM/EDX allows petrologists to analyze very small regions of mineral grains; for example, to determine the chemical and physical environment of crystallization.

Qualitative elemental analysis can be acquired from a specific location, or in the form of an elemental profile relative to a line or an area of interest (i.e., as elemental linescans or elemental maps). The technique lends itself easily to qualitative analysis when the researcher only needs to know relative elemental distributions. However, quantitative analysis is also possible, with detection limits near 1000ppm and accuracies near 2 percent relative (however, EDX does not provide for error analysis).

The FEI Quanta 400 is equipped with a Bruker 4th generation XFlash SDD X-ray detector, while the FEI MLA 650F is equipped with dual Bruker 5th generation XFlash SDD X-ray detectors that are capable of x-ray acquisition speeds nearly 5 times that of the 4th gen SDD and 10 times the speed of traditional x-ray detectors.



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