|
Glossary
EVISA is providing a list of terms used in the area of speciation and fractionation analysis. Since speciation analysis is a field of analytical chemistry that is specified by a pronounced interdisciplinary cooperation between different sciences such as biochemistry, medicine, biology, environmental sciences, nutritional sciences and material sciences its terminology is a complex mixture of terms used in all these.
You may search for a term or browse the glossary alphabetically.
(In case that you cannot find the term you may consult more special glossaries or handbooks about nomenclature. For more details please consult EVISA's Link pages related to terminology,
|
|
X-ray Absorption Fine Structure (XAFS) spectroscopy is a structural probe for the local atomic environment around selected chemical species.
X-ray absorption spectroscopy shows a steep rise at the core-level X-ray energy of X-ray-absorbing atoms and attenuates gradually with the X-ray energy. An observed fine structure near the absorption edges called X-ray absorption near-edge structure (XANES) corresponds to the transition from a core-level to an unoccupied orbital or band and mainly reflects the electronic state of the absorbing atom. In contrast, an oscillatory structure extending for hundreds of electron volts past the edges, called extended X-ray absorption fine structure (EXAFS) results from the interference effect between an emitted electron from an X-ray-absorbing atom and scattered electrons by surrounding atoms and provides information on the local structures around an X-ray absorbing atom.
Nearly all elements can be studied with XAFS, though the emphasis has traditionally been on the heavier elements.
|
| |
|
|
Methods that use X-rays to investigate the physical and chemical properties of materials on an atomic scale. XAS includes X-ray adsorption fine structure (XAFS) spectroscopy and its X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra.
|
| |
|
A technique for the semiquantitative mineralogical analysis of a sample
of rock by measuring the diffraction peaks in X-rays diffracted by the
sample. The position of the diffraction peaks is a measure of the
distance between discrete crystallographic diffracting planes within
minerals, while their intensity indicates the quantity of the mineral.
The technique is only semiquantitative because the size and shape of
the diffraction peak are strongly influenced by the geometry of the
measurement, for example orientation of the minerals, and sample
preparation. Fine particles such as clays must be separated from larger
particles and measured separately if they are to be detected properly.
To reduce errors associated with preferred orientation of minerals,
samples are most commonly ground to a powder before analysis, a
technique known as powder X-ray diffraction.
|
| |
|
XRF is a technique for elemental analysis of samples based on the
characteristic fluorescence given off by different elements subjected
to X-rays. In geological analysis, X-ray fluorescence often is used to
help determine mineral content. The elemental volumes are inverted to
mineral volumes by assuming certain standard formulae for mineral
composition.
|
| |
|
|
X-ray photoelectron spectroscopy (XPS), also known as electron spectroscopy for chemical analysis (ESCA), is a highly specialized surface analytical method reaching the first 5-10 nm of surface layer. Samples are excited with a very intense beam of low-energy X-rays. The photoelectric effect causes electrons to be ejected from outer orbitals of target atoms. These electrons have a characteristic energy, equivalent to the energy of the incident photon less the ionization energy of the appropriate electron orbital. From the data, it is possible to determine the binding energy that varies from compound to compound and is a function of both oxidation state and coordination number of that element.
|
| |
|
X-ray absorption spectroscopy (XAS) can probe intact tissues, in principle non-destructively, and obtain biochemical information about a target element wherever it occurs within complex systems. Examples of samples which might be studied include cell cultures, tissue fragments, or even intact (small) organisms.
The technique has previously lacked the sensitivity to answer most
questions in this area, but recent developments in synchrotron light
technology mean that physiologically relevant levels are now feasible
in many cases.
Two fundamentally different types of measurement can be made - bulk XAS where a relatively large X-ray beam interrogates the whole sample, and XAS imaging where a microscopic beam is used to build up images or maps of the different chemical species of a particular element. Bulk XAS has the best sensitivity, but XAS imaging has obvious advantages in developing a biochemical understanding of biometals in whole tissues.
|
| |
|
