Energy Dispersive X-ray Spectroscopy (EDS). Wavelength Dispersive X-ray Spectroscopy.Micro-analysis 10x more sensitive than EDS.Detection limit 0.01%. Energy Dispersive X-Ray Spectroscopy (EDS) เป็นการวิเคราะห์องค์ประกอบทางเคมีด้วยสเปก โทรเมตรีรังสีเอกซ์แบบกระจายพลังงานที่ใช้ร่วมกับกล้อง. Sum Peaks in Energy-Dispersive X-ray Spectroscopy - Volume 14 Supplement - A Eades Please note, due to essential maintenance online purchasing will be unavailable between 7:00 and 11:00 (GMT) on 23rd November 2019.
Interaction of an electron beam with a sample target produces a variety of emissions, including x-rays. An energy-dispersive (EDS) detector is used to separate the characteristic x-rays of different elements into an energy spectrum, and EDS system software is used to analyze the energy spectrum in order to determine the abundance of specific elements. EDS can be used to find the chemical composition of materials down to a spot size of a few microns, and to create element composition maps over a much broader raster area. Together, these capabilities provide fundamental compositional information for a wide variety of materials.
How it Works - EDS
An EDS detector, showing liquid nitrogen dewar, cold arm and detector tip that is mounted in the sample chamber. Details
EDS systems are typically integrated into either an SEM or EPMA instrument. EDS systems include a sensitive x-ray detector, a liquid nitrogen dewar for cooling, and software to collect and analyze energy spectra. The detector is mounted in the sample chamber of the main instrument at the end of a long arm, which is itself cooled by liquid nitrogen. The most common detectors are made of Si(Li) crystals that operate at low voltages to improve sensitivity, but recent advances in detector technology make availabale so-called 'silicon drift detectors' that operate at higher count rates without liquid nitrogen cooling.
An EDS detector contains a crystal that absorbs the energy of incoming x-rays by ionization, yielding free electrons in the crystal that become conductive and produce an electrical charge bias. The x-ray absorption thus converts the energy of individual x-rays into electrical voltages of proportional size; the electrical pulses correspond to the characteristic x-rays of the element.
Strengths
- When used in 'spot' mode, a user can acquire a full elemental spectrum in only a few seconds. Supporting software makes it possible to readily identify peaks, which makes EDS a great survey tool to quickly identify unknown phases prior to quantitative analysis.
- EDS can be used in semi-quantitative mode to determine chemical composition by peak-height ratio relative to a standard.
Limitations
- There are energy peak overlaps among different elements, particularly those corresponding to x-rays generated by emission from different energy-level shells (K, L and M) in different elements. For example, there are close overlaps of Mn-Kα and Cr-Kβ, or Ti-Kα and various L lines in Ba. Particularly at higher energies, individual peaks may correspond to several different elements; in this case, the user can apply deconvolution methods to try peak separation, or simply consider which elements make 'most sense' given the known context of the sample.
- Because the wavelength-dispersive (WDS) method is more precise and capable of detecting lower elemental abundances, EDS is less commonly used for actual chemical analysis although improvements in detector resolution make EDS a reliable and precise alternative.
- EDS cannot detect the lightest elements, typically below the atomic number of Na for detectors equipped with a Be window. Polymer-based thin windows allow for detection of light elements, depending on the instrument and operating conditions.
Results
A typical EDS spectrum is portrayed as a plot of x-ray counts vs. energy (in keV). Energy peaks correspond to the various elements in the sample. Generally they are narrow and readily resolved, but many elements yield multiple peaks. For example, iron commonly shows strong Kα and Kβ peaks. Elements in low abundance will generate x-ray peaks that may not be resolvable from the background radiation.
EDS spectrum of multi-element glass (NIST K309) containing O, Al, Si, Ca, Ba and Fe (Goldstein et al., 2003). Details
EDS spectrum of biotite, containing detectable Mg, Al, Si, K, Ti and Fe (from Goodge, 2003). Details
References
- Severin, Kenneth P., 2004, Energy Dispersive Spectrometry of Common Rock Forming Minerals. Kluwer Academic Publishers, 225 p.--Highly recommended reference book of representative EDS spectra of the rock-forming minerals, as well as practical tips for spectral acquisition and interpretation.
- Goldstein, J. (2003) Scanning electron microscopy and x-ray microanalysis. Kluwer Adacemic/Plenum Pulbishers, 689 p.
- Reimer, L. (1998) Scanning electron microscopy : physics of image formation and microanalysis. Springer, 527 p.
- Egerton, R. F. (2005) Physical principles of electron microscopy : an introduction to TEM, SEM, and AEM. Springer, 202.
- Clarke, A. R. (2002) Microscopy techniques for materials science. CRC Press (electronic resource)
Related Links
- Petroglyph--An atlas of images using electron microscope, backscattered electron images, element maps, energy dispersive x-ray spectra, and petrographic microscope-- Eric Chrisensen, Brigham Young University
Teaching Activities
- Argast, Anne and Tennis, Clarence F., III, 2004, A web resource for the study of alkali feldspars and perthitic textures using light microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy, Journal of Geoscience Education 52, no. 3, p. 213-217.
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]
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]
Energy Dispersive X Ray
- 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]
References[edit]
- ^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.
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
Energy Dispersive X Ray Spectroscopy Pdf Converter
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Energy-dispersive_X-ray_spectroscopy&oldid=992456752'