Dispersive X-ray spectroscopy (WDS or WDX) separates the X-rays by diffracting them with crystals, collecting one wavelength, or energy, at a time. In contrast, its sister technique, energy- dispersive X-ray spectroscopy (EDS or EDX), collects X-rays of all energies simultaneously. The two methods are almost always used in combination. Energy-Dispersive X-Ray Spectroscopy. Energy-dispersive X-ray spectroscopy (EDX) is a surface analytical technique where an electron beam hits the sample, exciting an electron in an inner shell, causing its ejection and the formation of an electron hole in the electronic structure of the element.
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X-ray photoelectron spectroscopy (XPS) is a surface-sensitive quantitative spectroscopic technique based on the photoelectric effect that can identify the elements that exist within a material (elemental composition) or are covering its surface, as well as their chemical state, and the overall electronic structure and density of the electronic states in the material.
Description of Technique
Energy Dispersive X-Ray Spectroscopy (EDS or EDX) is a chemical microanalysis technique used in conjunction with scanning electron microscopy (SEM). (See Handbook section on SEM.) The EDS technique detects x-rays emitted from the sample during bombardment by an electron beam to characterize the elemental composition of the analyzed volume. Features or phases as small as 1 µm or less can be analyzed.
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.
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The EDS x-ray detector measures the relative abundance of emitted x-rays versus their energy. The detector is typically a lithium-drifted silicon, solid-state device. When an incident x-ray strikes the detector, it creates a charge pulse that is proportional to the energy of the x-ray. The charge pulse is converted to a voltage pulse (which remains proportional to the x-ray energy) by a charge-sensitive preamplifier. The signal is then sent to a multichannel analyzer where the pulses are sorted by voltage. The energy, as determined from the voltage measurement, for each incident x-ray is sent to a computer for display and further data evaluation. The spectrum of x-ray energy versus counts is evaluated to determine the elemental composition of the sampled volume.
Analytical Information
Qualitative Analysis - The sample x-ray energy values from the EDS spectrum are compared with known characteristic x-ray energy values to determine the presence of an element in the sample. Elements with atomic numbers ranging from that of beryllium to uranium can be detected. The minimum detection limits vary from approximately 0.1 to a few atom percent, depending on the element and the sample matrix.
Quantitative Analysis - Quantitative results can be obtained from the relative x-ray counts at the characteristic energy levels for the sample constituents. Semi-quantitative results are readily available without standards by using mathematical corrections based on the analysis parameters and the sample composition. 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.
Quantitative Analysis - Quantitative results can be obtained from the relative x-ray counts at the characteristic energy levels for the sample constituents. Semi-quantitative results are readily available without standards by using mathematical corrections based on the analysis parameters and the sample composition. 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.
Elemental Mapping - Characteristic x-ray intensity is measured relative to lateral position on the sample. Variations in x-ray intensity at any characteristic energy value indicate the relative concentration for the applicable element across the surface. One or more maps are recorded simultaneously using image brightness intensity as a function of the local relative concentration of the element(s) present. About 1 µm lateral resolution is possible.
Line Profile Analysis - The SEM electron beam is scanned along a preselected line across the sample while x-rays are detected for discrete positions along the line. Analysis of the x-ray energy spectrum at each position provides plots of the relative elemental concentration for each element versus position along the line.
Typical Applications
- Foreign material analysis
- Corrosion evaluation
- Coating composition analysis
- Rapid material alloy identification
- Small component material analysis
- Phase identification and distribution
Sample Requirements
Samples up to 8 in. (200 mm) in diameter can be readily analyzed in the SEM. Larger samples, up to approximately 12 in. (300 mm) in diameter, can be loaded with limited stage movement. A maximum sample height of approximately 2 in. (50 mm) can be accommodated. Samples must also be compatible with a moderate vacuum atmosphere (pressures of 2 Torr or less).
EDS spectrum of the mineral crust of the vent shrimp Rimicaris exoculata[1] Most of these peaks are X-rays given off as electrons return to the K electron shell.(K-alpha and K-beta lines) One peak is from the L shell of iron.
Energy-dispersive X-ray spectroscopy (EDS, EDX, EDXS or XEDS), sometimes called energy dispersive X-ray analysis (EDXA) or energy dispersive X-ray microanalysis (EDXMA), is an analytical technique used for the elemental analysis or chemical characterization of a sample. It relies on an interaction of some source of X-ray excitation and a sample. Its characterization capabilities are due in large part to the fundamental principle that each element has a unique atomic structure allowing a unique set of peaks on its electromagnetic emission spectrum[2] (which is the main principle of spectroscopy). The peak positions are predicted by the Moseley's law with accuracy much better than experimental resolution of a typical EDX instrument.
To stimulate the emission of characteristic X-rays from a specimen a beam of X-rays is focused into the sample being studied. At rest, an atom within the sample contains ground state (or unexcited) electrons in discrete energy levels or electron shells bound to the nucleus. The incident beam may excite an electron in an inner shell, ejecting it from the shell while creating an electron hole where the electron was. An electron from an outer, higher-energy shell then fills the hole, and the difference in energy between the higher-energy shell and the lower energy shell may be released in the form of an X-ray. The number and energy of the X-rays emitted from a specimen can be measured by an energy-dispersive spectrometer. As the energies of the X-rays are characteristic of the difference in energy between the two shells and of the atomic structure of the emitting element, EDS allows the elemental composition of the specimen to be measured.[2]
Equipment[edit]
Four primary components of the EDS setup are
- the excitation source (electron beam or x-ray beam)
- the X-ray detector
- the pulse processor
- the analyzer.[citation needed]
Electron beam excitation is used in electron microscopes, scanning electron microscopes (SEM) and scanning transmission electron microscopes (STEM). X-ray beam excitation is used in X-ray fluorescence (XRF) spectrometers. A detector is used to convert X-ray energy into voltage signals; this information is sent to a pulse processor, which measures the signals and passes them onto an analyzer for data display and analysis.[citation needed] The most common detector used to be Si(Li) detector cooled to cryogenic temperatures with liquid nitrogen. Now, newer systems are often equipped with silicon drift detectors (SDD) with Peltier cooling systems.
Technological variants[edit]
Principle of EDS
The excess energy of the electron that migrates to an inner shell to fill the newly created hole can do more than emit an X-ray.[3] Often, instead of X-ray emission, the excess energy is transferred to a third electron from a further outer shell, prompting its ejection. This ejected species is called an Auger electron, and the method for its analysis is known as Auger electron spectroscopy (AES).[3]
X-ray photoelectron spectroscopy (XPS) is another close relative of EDS, utilizing ejected electrons in a manner similar to that of AES. Information on the quantity and kinetic energy of ejected electrons is used to determine the binding energy of these now-liberated electrons, which is element-specific and allows chemical characterization of a sample.[citation needed]
EDS is often contrasted with its spectroscopic counterpart, WDS (wavelength dispersive X-ray spectroscopy). WDS differs from EDS in that it uses the diffraction of X-rays on special crystals to separate its raw data into spectral components (wavelengths). WDS has a much finer spectral resolution than EDS. WDS also avoids the problems associated with artifacts in EDS (false peaks, noise from the amplifiers, and microphonics).
A high-energy beam of charged particles such as electrons or protons can be used to excite a sample rather than X-rays. This is called Particle-induced X-ray Emission) or PIXE.
Accuracy of EDS[edit]
EDS can be used to determine which chemical elements are present in a sample, and can be used to estimate their relative abundance. EDS also helps to measure multi-layer coating thickness of metallic coatings and analysis of various alloys. The accuracy of this quantitative analysis of sample composition is affected by various factors. Many elements will have overlapping X-ray emission peaks (e.g., Ti Kβ and V Kα, Mn Kβ and Fe Kα). The accuracy of the measured composition is also affected by the nature of the sample. X-rays are generated by any atom in the sample that is sufficiently excited by the incoming beam. These X-rays are emitted in all directions (isotropically), and so they may not all escape the sample. The likelihood of an X-ray escaping the specimen, and thus being available to detect and measure, depends on the energy of the X-ray and the composition, amount, and density of material it has to pass through to reach the detector. Because of this X-ray absorption effect and similar effects, accurate estimation of the sample composition from the measured X-ray emission spectrum requires the application of quantitative correction procedures, which are sometimes referred to as matrix corrections.[2]
Emerging technology[edit]
Energy Dispersive X Ray Spectrometry
There is a trend towards a newer EDS detector, called the silicon drift detector (SDD). The SDD consists of a high-resistivity silicon chip where electrons are driven to a small collecting anode. The advantage lies in the extremely low capacitance of this anode, thereby utilizing shorter processing times and allowing very high throughput. Benefits of the SDD include:[citation needed]
- High count rates and processing,
- Better resolution than traditional Si(Li) detectors at high count rates,
- Lower dead time (time spent on processing X-ray event),
- Faster analytical capabilities and more precise X-ray maps or particle data collected in seconds,
- Ability to be stored and operated at relatively high temperatures, eliminating the need for liquid nitrogen cooling.
Because the capacitance of the SDD chip is independent of the active area of the detector, much larger SDD chips can be utilized (40 mm2 or more). This allows for even higher count rate collection. Further benefits of large area chips include:[citation needed]
- Minimizing SEM beam current allowing for optimization of imaging under analytical conditions,
- Reduced sample damage and
- Smaller beam interaction and improved spatial resolution for high speed maps.
Where the X-ray energies of interest are in excess of ~ 30 keV, traditional silicon-based technologies suffer from poor quantum efficiency due to a reduction in the detector stopping power. Detectors produced from high density semiconductors such as cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) have improved efficiency at higher X-ray energies and are capable of room temperature operation. Single element systems, and more recently pixelated imaging detectors such as the HEXITEC system, are capable of achieving energy resolutions of the order of 1% at 100 keV.
In recent years, a different type of EDS detector, based upon a superconducting microcalorimeter, has also become commercially available. This new technology combines the simultaneous detection capabilities of EDS with the high spectral resolution of WDS. The EDS microcalorimeter consists of two components: an absorber, and a superconducting transition-edge sensor (TES) thermometer. The former absorbs X-rays emitted from the sample and converts this energy into heat; the latter measures the subsequent change in temperature due to the influx of heat. The EDS microcalorimeter has historically suffered from a number of drawbacks, including low count rates and small detector areas. The count rate is hampered by its reliance on the time constant of the calorimeter's electrical circuit. The detector area must be small in order to keep the heat capacity small and maximize thermal sensitivity (resolution). However, the count rate and detector area have been improved by the implementation of arrays of hundreds of superconducting EDS microcalorimeters, and the importance of this technology is growing.
See also[edit]
Energy Dispersive X Ray Spectroscopy Pdf Software
References[edit]
Energy Dispersive X Ray Spectroscopy Pdf Download
- ^Corbari, L; et al. (2008). 'Iron oxide deposits associated with the ectosymbiotic bacteria in the hydrothermal vent shrimp Rimicaris exoculata'(PDF). Biogeosciences. 5 (5): 1295–1310. doi:10.5194/bg-5-1295-2008.
- ^ abcJoseph Goldstein (2003). Scanning Electron Microscopy and X-Ray Microanalysis. Springer. ISBN978-0-306-47292-3. Retrieved 26 May 2012.
- ^ abJenkins, R. A.; De Vries, J. L. (1982). Practical X-Ray Spectrometry. Springer. ISBN978-1-468-46282-1.
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External links[edit]
- MICROANALYST.NET – Information portal with X-ray microanalysis and EDX contents
- [1] -EDS on the SEM: Primer discussing principles, capabilities and limitations of EDS with the SEM
- Learn how to do EDS in an SEM – an interactive learning environment provided by Microscopy Australia
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