Wavelength-dispersive X-ray spectroscopy (WDXS or WDS) is a non-destructive analysis technique used to obtain elemental information about a range of materials by measuring characteristic x-rays within a small wavelength range. The technique generates a

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...

in which the peaks correspond to specific x-ray lines and elements can be easily identified. WDS is primarily used in chemical analysis, wavelength dispersive

X-ray fluorescence X-ray fluorescence (XRF) is the emission of characteristic "secondary" (or fluorescent) X-rays from a material that has been excited by being bombarded with high-energy X-rays or gamma rays. The phenomenon is widely used for elemental analysis ...

(WDXRF) spectrometry,

electron microprobe An electron microprobe (EMP), also known as an electron probe microanalyzer (EPMA) or electron micro probe analyzer (EMPA), is an analytical tool used to non-destructively determine the chemical composition of small volumes of solid materials. It ...

s, scanning electron microscopes, and high precision experiments for testing atomic and plasma physics.

Theory

Wavelength-dispersive X-ray spectroscopy is based on known principles of how the characteristic x-rays are generated by a sample and how the x-rays are measured.

X-ray generation

X-rays are generated when an electron beam of high enough energy dislodges an electron from an inner orbital within an atom or ion, creating a void. This void is filled when an electron from a higher orbital releases energy and drops down to replace the dislodged electron. The energy difference between the two orbitals is characteristic of the electron configuration of the atom or ion and can be used to identify the atom or ion.

X-ray measurement

According to

Bragg's law In physics and chemistry , Bragg's law, Wulff–Bragg's condition or Laue–Bragg interference, a special case of Laue diffraction, gives the angles for coherent scattering of waves from a crystal lattice. It encompasses the superposition of wave ...

, when an X-ray beam of wavelength "λ" strikes the surface of a crystal at an angle "Θ" and the crystal has atomic lattice planes a distance "d" apart, then

constructive interference In physics, interference is a phenomenon in which two waves combine by adding their displacement together at every single point in space and time, to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructive ...

will result in a beam of diffracted x-rays that will be emitted from the crystal at angle "Θ" if ::nλ = 2d sin Θ, where n is an

integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign (−1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language ...

. This means that a crystal with a known lattice size will deflect a beam of x-rays from a specific type of sample at a pre-determined angle. The x-ray beam can be measured by placing a detector (usually a

scintillation counter A scintillation counter is an instrument for detecting and measuring ionizing radiation by using the excitation effect of incident radiation on a scintillating material, and detecting the resultant light pulses. It consists of a scintillator w ...

or a

proportional counter The proportional counter is a type of gaseous ionization detector device used to measure particles of ionizing radiation. The key feature is its ability to measure the energy of incident radiation, by producing a detector output pulse that is ''prop ...

) in the path of the deflected beam and, since each element has a distinctive x-ray wavelength, multiple elements can be determined by having multiple crystals and multiple detectors. To improve accuracy the x-ray beams are usually

collimated A collimated beam of light or other electromagnetic radiation has parallel rays, and therefore will spread minimally as it propagates. A perfectly collimated light beam, with no divergence, would not disperse with distance. However, diffraction p ...

by parallel copper blades called a Söller collimator. The single crystal, the specimen, and the detector are mounted precisely on a goniometer with the distance between the specimen and the crystal equal to the distance between the crystal and 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 is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...

and

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

). The technique generates a spectrum with peaks corresponding to x-ray lines. This is compared with reference spectra to determine the elemental composition of the sample. As the atomic number of the element increases so there are more possible electrons at different energy levels that can be ejected resulting in x-rays with different wavelengths. This creates spectra with multiple lines, one for each energy level. The largest peak in the spectrum is labelled K_α, the next K_β, and so on.

Applications

Applications include analysis of catalysts, cement, food, metals, mining and mineral samples, petroleum, plastics, semiconductors, and wood.

Limitations

* Analysis is generally limited to a very small area of the sample, although modern automated equipment often use grid patterns for larger analysis areas. * The technique cannot distinguish between

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...

s of elements as the electron configuration of isotopes of an element are identical. * It cannot measure the valence state of the element, for example Fe²⁺ vs Fe³⁺. * In certain elements, the K_α line might overlap the K_β of another element and hence if the first element is present, the second element cannot be reliably detected (for example VK_α overlaps TiK_β)

References

{{reflist, refs= {{cite web , title=BraggsLaw , website=Geochemical Instrumentation and Analysis , date=10 November 2016 , url=https://serc.carleton.edu/research_education/geochemsheets/BraggsLaw.html , access-date=14 September 2020 {{cite web , title=EDXRF - XRF - Elemental Analysis , website=Applied Rigaku Technologies Inc , url=https://www.rigakuedxrf.com/index.php , access-date=14 September 2020 {{cite web , title=Wavelength-dispersive spectroscopy (WDS) , website=Geochemical Instrumentation and Analysis , date=10 November 2016 , url=https://serc.carleton.edu/research_education/geochemsheets/wds.html {{cite web , title=An Introduction to Energy-Dispersive and Wavelength-Dispersive X-Ray Microanalysis , website=Wiley Analytical Science , date=14 September 2020 , url=https://analyticalscience.wiley.com/do/10.1002/micro.565 , access-date=14 September 2020 Emission spectroscopy X-ray spectroscopy

Theory

X-ray generation

X-ray measurement

Applications

Limitations

See also

References