Information on EDS
EDS is a technique used in conjunction with field emission scanning electronmicroscopy (FE-SEM). When the electron beam from the FE-SEM hits the sample, it ejects an e- from an inner shell (e.g., K shell, see below) that is replaced with an e- from a higher-energy outer shell. The difference in energy levels between these electrons is given off as a characteristic x-ray. The x-rays are then displayed graphically as an EDS spectrum (see figure, below). Each element and its corresponding shells have their own diagnostic x-ray spectral peaks that are easily visualized. Taking the integral of the peaks in the spectrum gives the relative abundance of each element present in the scanned area, providing both the atomic and weight percentages for each element. However, the voltage of the electron beam limits how far the beam can penetrate into the specimen. For example, a 20kV beam penetrates 4.10 μm of pure carbon but only 0.5 μm of pure gold. A normally coated specimen for FE-SEM is covered by only several Å of gold, meaning that an electron beam will penetrate both the gold coating and approximately 4 μm of specimen.
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