Energy-dispersive X-ray spectroscopy

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Energy dispersive X-ray spectroscopy (EDS, EDX or EDXRF) is an analytical technique used for the elemental analysis or chemical characterization of a sample. It is one of the variants of XRF. 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 from 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.

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. 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).

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 (i.e. EDS detectors cannot detect presence of elements with atomic number less than 5, meaning that EDS cannot detect H, He, Li, or Be)^[1]. 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). 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 inhomogeneous and rough samples.