Energy-dispersive X-ray spectroscopy

Energy dispersive X-ray spectroscopy (EDS, EDX or EDXRF) is an analytical technique used for the elemental analysis or chemical characterization of a sample. As a type of spectroscopy, it relies on the investigation of a sample through interactions between electromagnetic radiation and matter, analyzing x-rays emitted by the matter in response to being hit with charged particles. Its characterization capabilities are due in large part to the fundamental principle that each element has a unique atomic structure allowing x-rays that are characteristic of an element's atomic structure to be identified uniquely from each other.

To stimulate the emission of characteristic X-rays from a specimen, a high energy beam of charged particles such as electrons or protons, or 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 energy of the X-rays are characteristic of the difference in energy between the two shells, and of the atomic structure of the element form which they were emitted, this allows the elemental composition of the specimen to be measured.

Equipment: the connection with SEM

There are four primary components of the EDS setup: the beam source; the X-ray detector; the pulse processor; and the analyzer. A number of free-standing EDS systems exist. However, EDS systems are most commonly found on scanning electron microscopes (SEM-EDX) and electron microprobes. Scanning electron microscopes are equipped with a cathode and magnetic lenses to create and focus a beam of electrons, and since the 1960s they have been equipped with elemental analysis capabilities. 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.

Technological Variants

(AES).
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.EDS is often contrasted with its spectroscopic counterpart, WDS (Wavelength-Dispersive X-ray Spectroscopy). WDS differs from EDS in that it uses the diffraction patterns created by light-matter interaction as its raw data. 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. In WDS only one element can be analyzed at a time, while EDS gathers a spectrum of all elements, within limits, of a sample.

Accuracy of EDS

Accuracy of EDS spectrum can be affected by many variants. Windows in front of the SiLi detector can absorb low-energy X-rays(a.k.a. EDS detectors cannot detect presence of oxygen, carbon, boron, etc.). Differing the over-voltage of the EDS will result in different peak sizes - Raising over-voltage on the SEM will shift the spectrum to the larger energies making higher-energy peaks larger while making lower energy peaks smaller. Also many elements will have overlapping peaks (ex. Ti Kβ and V Kα, Mn Kβ and Fe Kα). [http://www.cameca.fr/html/epma_eds.html Study of EDS vs. WDS accuracy in common labs] The accuracy of the spectrum can also be affected by the nature of the sample. X-rays can be generated by any atom in the sample that is sufficiently excited by the incoming beam. These X-rays are emitted in any direction, and so 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 amount and density of material it has to pass through. This can result in reduced accuracy in inhomogenous and rough samples.

Emerging Technology

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 1) High count rates and processing 2) Better resolution than traditional SiLi detectors at high count rates 3) Lower dead time (time spent on processing x-ray event) 4) Faster analytical capabilities and more precise X-ray maps or particle data collected in seconds and 5) Ability to be stored and operate 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 (40mm sq. or more). This allows for even higher count rate collection. Further benefits of large area chips include 1) Minimizing SEM beam current allowing for optimization of imaging under analytical conditions 2) Reduced sample damage and 3) Smaller beam interaction and improved spatial resolution for high speed maps.

In recent years a different type of EDS detector, based upon a microcalorimeter, has become commercially available. This new model allegedly has the simultaneous detection capabilities of EDS as well as the high spectral resolution of WDS. The EDS microcalorimeter relies highly on two components: an absorber, and a thermistor. 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 (in essence, a thermometer). The EDS microcalorimeter has suffered from a number of drawbacks; including low count rates, poor collection efficiencies and small detector areas. The count rate is hampered by its reliance on the time constant of the calorimeter’s electrical circuit. The collection efficiency is a function of the absorber material and remains to be optimized. The detector area must be small in order to keep the heat capacity as small as possible and maximize thermal sensitivity (resolution). Innovative engineering solutions are necessary for further improvement of spectroscopic microanalysis.

See also

* Wavelength dispersive X-ray spectroscopy

External links

* [http://www.microanalyst.net/index_e.phtml MICROANALYST.NET] - Information portal with X-ray microanalysis and EDX contents