Wavelength dispersive X-ray spectroscopy
Encyclopedia
The Wavelength dispersive X-ray spectroscopy (WDXRF or WDS) is a method used to count the number of X-ray
s of a specific wavelength diffracted
by a crystal. The wavelength of the impinging x-ray and the crystal's lattice spacings are related by Bragg's law
and produce constructive interference if they fit the criteria of Bragg's law. Unlike the related technique of Energy dispersive X-ray spectroscopy (EDS) WDS reads or counts only the x-rays of a single wavelength at time, not producing a broad spectrum of wavelengths or energies simultaneously. WDS is mainly used in chemical analysis, in an X-ray fluorescence
spectrometer
, in an electron microprobe
, and may be used in a scanning electron microscope
.
at a precise angle. The single crystal diffracts
the photons (Bragg's law
) which are collected by a detector, usually a scintillation counter
or a proportional counter.
The single crystal, the specimen, and the detector are mounted precisely on a goniometer
with the distance from the source of x-rays (the specimen) and the crystal equal to the distance from the crystal to the detector. It is usually operated under vacuum to reduce the absorption of soft radiation (low-energy photons) by the air and thus increase the sensitivity for the detection and quantification of light elements (between boron
and oxygen
).
Modern systems contain a small number of crystals of known but differing properties, with automated changing of the crystal depending on the energy being analysed, enabling elements from the entire periodic table to be analyzed, with the exception of H. He and Li.
It is a convenient and sensitive method for determining the chemical constituents and composition of phases on the microscale.
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s of a specific wavelength diffracted
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
by a crystal. The wavelength of the impinging x-ray and the crystal's lattice spacings are related by Bragg's law
Bragg's law
In physics, Bragg's law gives the angles for coherent and incoherent scattering from a crystal lattice. When X-rays are incident on an atom, they make the electronic cloud move as does any electromagnetic wave...
and produce constructive interference if they fit the criteria of Bragg's law. Unlike the related technique of Energy dispersive X-ray spectroscopy (EDS) WDS reads or counts only the x-rays of a single wavelength at time, not producing a broad spectrum of wavelengths or energies simultaneously. WDS is mainly used in chemical analysis, in an X-ray fluorescence
X-ray fluorescence
X-ray fluorescence is the emission of characteristic "secondary" X-rays from a material that has been excited by bombarding with high-energy X-rays or gamma rays...
spectrometer
Spectrometer
A spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...
, in an electron microprobe
Electron microprobe
An electron microprobe , also known as an electron probe microanalyzer or electron micro probe analyzer , is an analytical tool used to non-destructively determine the chemical composition of small volumes of solid materials...
, and may be used in a scanning electron microscope
Scanning electron microscope
A scanning electron microscope is a type of electron microscope that images a sample by scanning it with a high-energy beam of electrons in a raster scan pattern...
.
Explanation
The X-rays emitted by the sample being analyzed are collimated by parallel copper blades (called collimator or Soller slits), and irradiate a known single crystalCrystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
at a precise angle. The single crystal diffracts
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
the photons (Bragg's law
Bragg's law
In physics, Bragg's law gives the angles for coherent and incoherent scattering from a crystal lattice. When X-rays are incident on an atom, they make the electronic cloud move as does any electromagnetic wave...
) which are collected by a detector, usually a scintillation counter
Scintillation counter
A scintillation counter measures ionizing radiation. The sensor, called a scintillator, consists of a transparent crystal, usually phosphor, plastic , or organic liquid that fluoresces when struck by ionizing radiation. A sensitive photomultiplier tube measures the light from the crystal...
or a proportional counter.
The single crystal, the specimen, and the detector are mounted precisely on a goniometer
Goniometer
A goniometer is an instrument that either measures an angle or allows an object to be rotated to a precise angular position. The term goniometry is derived from two Greek words, gōnia, meaning angle, and metron, meaning measure....
with the distance from the source of x-rays (the specimen) and the crystal equal to the distance from the crystal to the detector. It is usually operated under vacuum to reduce the absorption of soft radiation (low-energy photons) by the air and thus increase the sensitivity for the detection and quantification of light elements (between boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
).
Modern systems contain a small number of crystals of known but differing properties, with automated changing of the crystal depending on the energy being analysed, enabling elements from the entire periodic table to be analyzed, with the exception of H. He and Li.
It is a convenient and sensitive method for determining the chemical constituents and composition of phases on the microscale.
See also
- X-ray fluorescenceX-ray fluorescenceX-ray fluorescence is the emission of characteristic "secondary" X-rays from a material that has been excited by bombarding with high-energy X-rays or gamma rays...
- X-ray spectroscopyX-ray spectroscopyX-ray spectroscopy is a gathering name for several spectroscopic techniques for characterization of materials by using x-ray excitation.-Characteristic X-ray Spectroscopy:...
- Energy dispersive X-ray spectroscopy