The Scanning Electron Microscope at Prevest Research Institute is equipped with an Energy dispersive X-ray spectrometer (EDS) for the analysis of bulk elemental composition of the surface, in a qualitative manner. The EDS is Liquid N2 Free (Peltier Cooled), Model: No: AZtecOne + Xplore Compact 30 from OXFORD Instruments NanoAnalysis (UK).
EDS is used for non destructive elemental analysis and can accommodate orders of magnitude for sample size from μm to cm. Elements down to 10 wt% from 4Be to 92U can be detected.
When the sample is bombarded by the SEM’s electron beam, electrons are ejected from the atoms comprising the sample’s surface. The resulting electron vacancies are filled by electrons from a higher state, and an x-ray is emitted to balance the energy difference between the two electrons’ states. The x-ray energy is characteristic of the element from which it was emitted. This involves a change in energy (∆E), which is released in the form of a photon (hν = ∆E).
∆E = Efinal orbit– Einitial orbit
∆E increases with Z (0.1 keV < ∆E < 10 keV), this energy of the emitted x-ray is measured by an energy dispersive spectrometer. The spectrum of x-ray energy versus counts is evaluated to determine the elemental composition of the sampled volume. The location of the peaks identifies the elements. Labelling of the peaks is based on previously published tables. The peak heights vary because each transition has a different probability of occurring and the detector’s efficiency is a function of energy. EDS is used for Qualitative analysis by comparing the energy values from the EDS spectrum with known characteristic x-ray energy values to determine the presence of an element in the sample. The minimum detection limits vary from approximately 0.1 to a few atom percent, depending on the element and the sample matrix. Quantitative results can be obtained from the relative x-ray counts at the characteristic energy levels for the sample constituents. The accuracy of standardless analysis depends on the sample composition. Greater accuracy is obtained using known standards with similar structure and composition to that of the unknown sample.