How does Eds work? What is the spatial resolution?
Schematic diagram of excitation volumes in SEM. These volumes determine the overall spatial resolution for imaging and EDS analysis.
Energy Dispersive Spectroscopy (EDS) is a method for analyzing the chemical composition of most inorganic and inorganic solids. When the sample in the SEM is hit with high energy electrons, x-rays are produced from the sample. Some of these x-rays are known as "characteristic x-rays" because their energies are specific to the difference in energies between electron shells in the atom. An EDS detector can efficiently collect these x-rays and provide a quantitative or semi-quantitative measurement of the sample's composition. For most elements, we can detect levels down to 0.1 wt%. EDS is a great screening tool for quality control, alloy identification, foreign object identification and for determining species present in corrosion products. It is used in many industries including biomedical, pharmaceuticals, chemical engineering, materials science, biological/life science, geology and forensics. Our system capabilities include the following:
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The spatial resolution and depth of analysis for EDS depends on two major factors: the accelerating voltage of the incident electron beam and the density of the material being analyzed. In general, the higher the accelerating voltage, the deeper into your sample x-rays will be generated which will lead to more x-ray signal, but poorer spatial resolution. Low density materials such as carbon, oxygen and nitrogen will result in x-rays being generated much deeper into the surface compared to heavier elements such as gold or platinum. The highest spatial resolutions for EDS are therefore obtained at low accelerating voltages on higher density materials. However, low voltage is not always ideal for EDS as we need higher accelerating voltages to excite x-ray peaks (especially for heavier elements). There is no "perfect" solution for resolution vs. signal in EDS, these are known physical limitations for all bulk samples. The figure below shows a schematic diagram of the backscattered electron, secondary electron and characteristic x-ray excitation volumes.
EDS area analysis of corrosion pit region on a penny. Note the presence of Cl as the anion contributing to corrosion of the Cu and Zn in the penny.
EDS Chemical Mapping
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