When an element is bombarded with a particle beam, in this case, an electron beam, the specimen will release some of the absorbed energy as x-rays. Much of the time, the energy is the result of changes in the speed of an electron, which is random; however, when this interaction removes an electron from a specimen's atom, frequently an electron from an outer shell (or orbital) occupies the vacancy. When an outer electron occupies a vacancy, it must lose a specific amount of energy to occupy the closer shell. This amount is readily predicted by the Laws of Quantum Mechanics and usually much of the energy is emitted in the form of X-rays. Two methods are used to determine the x-rays that are produced: (1) energy-dispersive analysis separates and detects x-rays of specific energy and displays them as histograms, whereas (2) wavelength-dispersive analysis uses the reflection of x-rays off of a crystal at a characteristic angle to detect x-rays of specific wavelength. The SRNML is equipped for energy-dispersive spectroscopy on all of the microscopes capable of a scanning mode.
Electron microscopic analysis of elements provides some advantages compared to wet chemical analysis:
Imaging of different elements can be used to locate the distribution of elements with energy-dispersive x-ray analysis data. Representative methods of analysis include: (1) linescans of relative element concentrations along a scanned line passing through a selected object, (2) presence/absence analysis ("dot mapping") at a specific x-ray energy level to detect a specific element, or (3) cumulative computer maps that can image up to 15 low resolution maps of different elements at the same time and can recursively collect data until the required resolution is obtained. Spectra can be photographed or plotted using a color plotter, inkjet printer or dot-matrix printer.
Wavelength dispersive X-ray spectroscopy (WDS) may extend the threshhold of detection by at least an order of magnitude. However, WDS requires optically flat, stable specimens and is limited to bulk analysis modes, limiting spatial resolution to more than 0.5 micrometer. WDS analysis is available at the Electron Microprobe Facility in the Energy Center. Contact Dr. George Morgan, Electron Microprobe Operator at 325-2642 for further information or email (gmorgan@ou.edu).
Electron diffraction provides information about position of atoms in crystalline structures. Thin sections of metals, ceramics and geologicals are made using an ion thinning mill. Proteins are prepared by crystalization from solution. A double tilt stage is available to orient crystals in the TEM. Available modes include: