In X-ray absorption spectroscopy , the K-edge is a sudden increase in x-ray absorption occurring when the energy of the X-rays is just above the binding energy of the innermost electron shell of the atoms interacting with the photons. The term is based on X-ray notation , where the innermost electron shell is known as the K-shell. Physically, this sudden increase in attenuation is caused by the photoelectric absorption of the photons. For this interaction to occur, the photons must have more energy than the binding energy of the K-shell electrons (K-edge). A photon having an energy just above the binding energy of the electron is therefore more likely to be absorbed than a photon having an energy just below this binding energy or significantly above it.
47-454: The energies near the K-edge are also objects of study, and provide other information. The two radiocontrast agents iodine and barium have ideal K-shell binding energies for absorption of X-rays: 33.2 keV and 37.4 keV respectively, which is close to the mean energy of most diagnostic X-ray beams. Similar sudden increases in attenuation may also be found for other inner shells than
94-414: A ligand-to-metal charge transfer to the excited state. This rising-edge transition can be fitted to a valence bond configuration (VBCI) model to obtain the composition of the ground state wavefunction and information on ground state covalency . The VBCI model describes the ground and excited state as a linear combination of the metal-based d-state and the ligand-based charge transfer state. The higher
141-433: A 500 – 1000 eV range beginning before an absorption edge of an element in the sample. The x-ray absorption coefficient is usually normalized to unit step height. This is done by regressing a line to the region before and after the absorption edge, subtracting the pre-edge line from the entire data set and dividing by the absorption step height, which is determined by the difference between the pre-edge and post-edge lines at
188-399: A characteristic absorption feature called the K-edge. Transitions at energies lower than the edge can occur, provided they lead to orbitals with some ligand p character; these features are called pre-edges. Pre-edge intensities ( D 0 ) are related to the amount of ligand (L) character in the unfilled orbital: where ψ ∗ {\displaystyle \psi ^{*}}
235-475: A contrast material because it is less radio-opaque than the tissues it is defining. In the picture it highlights the interior of the colon. An example of a technique using purely air for the contrast medium is an air arthrogram where the injection of air into a joint cavity allows the cartilage covering the ends of the bones to be visualized. Before the advent of modern neuroimaging techniques, air or other gases were used as contrast agents employed to displace
282-728: A hot feeling around the throat, and this hot sensation gradually moves down to the pelvic area. The documentation of adverse drug reactions to contrast media should be documented precisely so that the patient receives adequate prophylaxis if contrast medium is administered again. Iodinated contrast may be toxic to the kidneys , especially when given via the arteries prior to studies such as catheter coronary angiography. Non-ionic contrast agents, which are almost exclusively used in computed tomography studies, have not been shown to cause CIN when given intravenously at doses needed for CT studies. Iodinated radiocontrast can induce overactivity (hyperthyroidism) and underactivity (hypothyroidism) of
329-476: A molecular orbital. Radiocontrast Radiocontrast agents are substances used to enhance the visibility of internal structures in X-ray -based imaging techniques such as computed tomography ( contrast CT ), projectional radiography , and fluoroscopy . Radiocontrast agents are typically iodine, or more rarely barium sulfate . The contrast agents absorb external X-rays, resulting in decreased exposure on
376-422: Is a spectroscopic technique used to study the electronic structures of transition metal atoms and complexes . This method measures X-ray absorption caused by the excitation of a 1s electron to valence bound states localized on the metal, which creates a characteristic absorption peak called the K-edge. The K-edge can be divided into the pre-edge region (comprising the pre-edge and rising edge transitions) and
423-422: Is a subset of X-ray absorption spectroscopy ( XAS ). Like other absorption spectroscopies , XAS techniques follow Beer's law . The X-ray absorption coefficient of a material as a function of energy is obtained by directing X-rays of a narrow energy range at a sample, while recording the incident and transmitted x-ray intensity, as the incident x-ray energy is incremented. When the incident x-ray energy matches
470-405: Is analogous to eigenstates of the particle in a box toy model. The δ j {\displaystyle \delta _{j}} factor inside the sin {\displaystyle \sin } is an element dependent phase shift. Since EXAFS requires a tunable x-ray source, data are frequently collected at synchrotrons , often at beamlines which are especially optimized for
517-443: Is described by the EXAFS equation, first demonstrated by Sayers, Stern, and Lytle. The oscillatory part of the dipole matrix element is given by χ ( k ) {\displaystyle \chi (k)} , where the sum is over the j {\displaystyle j} sets of neighbors of the absorbing atom, N j {\displaystyle N_{j}}
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#1732780300314564-492: Is mainly used in the imaging of the digestive system. The substance exists as a water-insoluble white powder that is made into a slurry with water and administered directly into the gastrointestinal tract . Barium sulfate, an insoluble white powder, is typically used for enhancing contrast in the GI tract. Depending on how it is to be administered the compound is mixed with water, thickeners, de-clumping agents, and flavourings to make
611-411: Is now excited and the ejected photoelectron interacts with electrons in the surrounding non-excited atoms. If the ejected photoelectron is taken to have a wave -like nature and the surrounding atoms are described as point scatterers, it is possible to imagine the backscattered electron waves interfering with the forward-propagating waves. The resulting interference pattern shows up as a modulation of
658-545: Is the main type of radiocontrast used for intravenous administration . Iodine has a particular advantage as a contrast agent for radiography because its innermost electron ("k-shell") binding energy is 33.2 keV, similar to the average energy of x-rays used in diagnostic radiography. When the incident x-ray energy is closer to the k-edge of the atom it encounters, photoelectric absorption is more likely to occur. Its uses include: Organic iodine molecules used for contrast include iohexol , iodixanol and ioversol . Barium sulfate
705-417: Is the number of atoms at distance R j {\displaystyle R_{j}} , k {\displaystyle k} is the wavenumber and is proportional to energy, σ {\displaystyle \sigma } is the thermal vibration factor with σ j 2 {\displaystyle \sigma _{j}^{2}} being the mean square amplitude of
752-540: Is the wavefunction of the unfilled orbital, r is the transition dipole operator, and α 2 {\displaystyle \alpha ^{2}} is the "covalency" or ligand character in the orbital. Since ψ ∗ = 1 − α 2 | M d ⟩ − α | L n p ⟩ {\displaystyle \psi ^{*}={\sqrt {1-\alpha ^{2}}}\vert M_{d}\rangle -\alpha \vert L_{np}\rangle } ,
799-561: The X-ray detector . This is different from radiopharmaceuticals used in nuclear medicine which emit radiation. Magnetic resonance imaging (MRI) functions through different principles and thus MRI contrast agents have a different mode of action. These compounds work by altering the magnetic properties of nearby hydrogen nuclei. Radiocontrast agents used in X-ray examinations can be grouped in positive (iodinated agents, barium sulfate), and negative agents (air, carbon dioxide, methylcellulose). Iodinated contrast contains iodine . It
846-430: The binding energy of an electron of an atom within the sample, the number of x-rays absorbed by the sample increases dramatically, causing a drop in the transmitted x-ray intensity. This results in an absorption edge. Every element has a set of unique absorption edges corresponding to different binding energies of its electrons, giving XAS element selectivity. XAS spectra are most often collected at synchrotrons because
893-451: The cerebrospinal fluid in the brain while performing a pneumoencephalography . Sometimes called an "air study", this once common yet highly-unpleasant procedure was used to enhance the outline of structures in the brain, looking for shape distortions caused by the presence of lesions. Carbon dioxide also has a role in angioplasty. It is low-risk as it is a natural product with no risk of allergic potential. However, it can be used only below
940-441: The reverse Monte Carlo method can help in extracting more reliable and richer structural information. EXAFS is, like XANES , a highly sensitive technique with elemental specificity. As such, EXAFS is an extremely useful way to determine the chemical state of practically important species which occur in very low abundance or concentration. Frequent use of EXAFS occurs in environmental chemistry , where scientists try to understand
987-464: The 5 - 30 keV range are feasible for lab based EXAFS studies. XAS is an interdisciplinary technique and its unique properties, as compared to x-ray diffraction, have been exploited for understanding the details of local structure in: XAS provides complementary to diffraction information on peculiarities of local structural and thermal disorder in crystalline and multi-component materials. The use of atomistic simulations such as molecular dynamics or
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#17327803003141034-404: The K shell; the general term for the phenomenon is absorption edge . Dual-energy computed tomography techniques take advantage of the increased attenuation of iodinated radiocontrast at lower tube energies to heighten the degree of contrast between iodinated radiocontrast and other high attenuation biological material present in the body such as blood and hemorrhage. Metal K-edge spectroscopy
1081-415: The above expression relating intensity and quantum transition operators can be simplified to use experimental values: where n is the number of absorbing ligand atoms, h is the number of holes, and I s is the transition dipole integral which can be determined experimentally. Therefore, by measuring the intensity of pre-edges, it is possible to experimentally determine the amount of ligand character in
1128-405: The amount of each of the known standard spectra within an unknown sample. The dominant physical process in x-ray absorption is one where the absorbed photon ejects a core photoelectron from the absorbing atom, leaving behind a core hole. The ejected photoelectron's energy will be equal to that of the absorbed photon minus the binding energy of the initial core state. The atom with the core hole
1175-1016: The atom's relative displacements, λ ( k ) {\displaystyle \lambda (k)} is the mean free path of the photoelectron with momentum k {\displaystyle k} (this is related to coherence of the quantum state), and f j ( k ) {\displaystyle f_{j}(k)} is an element dependent scattering factor. χ ( k ) = ∑ j N j e − 2 k 2 σ j 2 e − 2 R j / λ k f j ( k ) k R j 2 sin [ 2 k R j + δ j ( k ) ] {\displaystyle \chi (k)=\sum _{j}{\frac {N_{j}e^{-2k^{2}\sigma _{j}^{2}}e^{-2R_{j}/\lambda _{k}}f_{j}(k)}{kR_{j}^{2}}}\sin[2kR_{j}+\delta _{j}(k)]} The origin of
1222-702: The body might sometimes lead to arachnoiditis , a potentially painful and debilitating lifelong disorder of the spine. Iofendylate's use ceased when water-soluble agents (such as metrizamide ) became available in the late 1970s. Also, with the advent of MRI , myelography became much less-commonly performed. Modern iodinated contrast agents – especially non-ionic compounds – are generally well tolerated. The adverse effects of radiocontrast can be subdivided into type A reactions (e.g. thyrotoxicosis), and type B reactions (hypersensitivity reactions: allergy and non-allergy reactions [formerly called anaphylactoid reactions]). Patients receiving contrast via IV typically experience
1269-408: The case of higher-oxidation-state copper atoms, the 1s→4p transition lies higher in energy, mixed in with the near-edge region. However, an intense transition in the rising-edge region is observed for Cu and some Cu complexes from a formally forbidden two electron 1s→4p+shakedown transition. This “shakedown” process arises from a 1s→4p transition that leads to relaxation of the excited state, followed by
1316-445: The contrast agent. As the barium sulfate doesn't dissolve, this type of contrast agent is an opaque white mixture. It is only used in the digestive tract; it is usually swallowed as a barium sulfate suspension or administered as an enema. After the examination, it leaves the body with the feces . As in the picture on the right where both air and barium are used together (hence the term "double-contrast" barium enema) air can be used as
1363-496: The contribution of the charge transfer state to the ground state, the higher is the ground state covalency indicating stronger metal-ligand bonding. The near-edge region is difficult to quantitatively analyze because it describes transitions to continuum levels that are still under the influence of the core potential. This region is analogous to the EXAFS region and contains structural information. Extraction of metrical parameters from
1410-412: The diaphragm as there is a risk of embolism in neurovascular procedures. It must be used carefully to avoid contamination with room air when injected. It is a negative contrast agent in that it displaces blood when injected intravascularly. Thorotrast was a contrast agent based on thorium dioxide , which is radioactive . It was first introduced in 1929. While it provided good image enhancement, its use
1457-533: The edge region can be obtained by using the multiple-scattering code implemented in the MXAN software. Ligand K-edge spectroscopy is a spectroscopic technique used to study the electronic structures of metal-ligand complexes . This method measures X-ray absorption caused by the excitation of ligand 1s electrons to unfilled p orbitals ( principal quantum number n ≤ 4 {\displaystyle n\leq 4} ) and continuum states, which creates
K-edge - Misplaced Pages Continue
1504-408: The high intensity of synchrotron X-ray sources allows the concentration of the absorbing element to reach as low as a few parts per million. Absorption would be undetectable if the source were too weak. Because X-rays are highly penetrating, XAS samples can be gases, solids or liquids. EXAFS spectra are displayed as plots of the absorption coefficient of a given material versus energy , typically in
1551-418: The measured absorption coefficient, thereby causing the oscillation in the EXAFS spectra. A simplified plane-wave single-scattering theory has been used for interpretation of EXAFS spectra for many years, although modern methods (like FEFF, GNXAS) have shown that curved-wave corrections and multiple-scattering effects can not be neglected. The photelectron scattering amplitude in the low energy range (5-200 eV) of
1598-421: The metal leads to greater stabilization of the 1s orbital with respect to the metal d orbitals, resulting in higher energy of the pre-edge. Bonding interactions with ligands also cause changes in the metal's effective nuclear charge (Z eff ), leading to changes in the energy of the pre-edge. The intensity under the pre-edge transition depends on the geometry around the absorbing metal and can be correlated to
1645-473: The near-edge region (comprising the intense edge transition and ~150 eV above it). The K-edge of an open shell transition metal ion displays a weak pre-edge 1s-to-valence-metal-d transition at a lower energy than the intense edge jump. This dipole-forbidden transition gains intensity through a quadrupole mechanism and/or through 4p mixing into the final state. The pre-edge contains information about ligand fields and oxidation state . Higher oxidation of
1692-521: The opposite effect, called Jod-Basedow phenomenon , where the iodine induces overproduction of thyroid hormone; this may be the result of underlying thyroid disease (such as nodules or Graves' disease ) or previous iodine deficiency. Children exposed to iodinated contrast during pregnancy may develop hypothyroidism after birth and monitoring of the thyroid function is recommended. EXAFS Extended X-ray absorption fine structure ( EXAFS ), along with X-ray absorption near edge structure ( XANES ),
1739-420: The oscillations in the absorption cross section are due to the sin {\displaystyle \sin } term which imposes the interference condition, leading to peaks in absorption when the wavelength of the photoelectron is equal to an integer fraction of 2 R j {\displaystyle 2R_{j}} (the round trip distance from the absorbing atom to the scattering atom). This
1786-461: The past, some non water-soluble contrast agents were used. One such substance was iofendylate (trade names: Pantopaque, Myodil) which was an iodinated oil-based substance that was commonly used in myelography . Due to it being oil-based, it was recommended that the physician remove it from the patient at the end of the procedure. This was a painful and difficult step and because complete removal could not always be achieved, iofendylate's persistence in
1833-474: The photoelectron kinetic energy become much larger so that multiple scattering events become dominant in the XANES (or NEXAFS) spectra. The wavelength of the photoelectron is dependent on the energy and phase of the backscattered wave which exists at the central atom. The wavelength changes as a function of the energy of the incoming photon. The phase and amplitude of the backscattered wave are dependent on
1880-399: The propagation of pollutants through an ecosystem . EXAFS can be used along with accelerator mass spectrometry in forensic examinations, particularly in nuclear non-proliferation applications. A very detailed, balanced and informative account about the history of EXAFS (originally called Kossel's structures) is given by R. Stumm von Bordwehr . A more modern and accurate account of
1927-544: The purpose. The utility of a particular synchrotron to study a particular solid depends on the brightness of the x-ray flux at the absorption edges of the relevant elements. Recent developments in the design and quality of crystal optics have allowed for some EXAFS measurements to take place in a lab setting, where the tunable x-ray source is achieved via a Rowland circle geometry. While experiments requiring high x-ray flux or specialized sample environments can still only be performed at synchrotron facilities, absorption edges in
K-edge - Misplaced Pages Continue
1974-494: The rising-edge contains information about the oxidation state of the metal. In the case of copper complexes, the rising-edge consists of intense transitions, which provide information about bonding. For Cu species, this transition is a distinct shoulder and arises from intense electric-dipole-allowed 1s→4p transitions. The normalized intensity and energy of the rising-edge transitions in these Cu complexes can be used to distinguish between two-, three- and four-coordinate Cu sites. In
2021-428: The structural symmetry in the molecule. Molecules with centrosymmetry have low pre-edge intensity, whereas the intensity increases as the molecule moves away from centrosymmetry. This change is due to the higher mixing of the 4p with the 3d orbitals as the molecule loses centrosymmetry. A rising-edge follows the pre-edge, and may consist of several overlapping transitions that are hard to resolve. The energy position of
2068-499: The thyroid gland. The risk of either condition developing after a single examination is 2–3 times that of those who have not undergone a scan with iodinated contrast. Thyroid underactivity is mediated by two phenomena called the Plummer and Wolff–Chaikoff effect , where iodine suppresses the production of thyroid hormones; this is usually temporary but there is an association with longer-term thyroid underactivity. Some other people show
2115-407: The type of atom doing the backscattering and the distance of the backscattering atom from the central atom. The dependence of the scattering on atomic species makes it possible to obtain information pertaining to the chemical coordination environment of the original absorbing (centrally excited) atom by analyzing these EXAFS data. The effect of the backscattered photoelectron on the absorption spectra
2162-574: The value of E0 (on the absorption edge). The normalized absorption spectra are often called XANES spectra. These spectra can be used to determine the average oxidation state of the element in the sample. The XANES spectra are also sensitive to the coordination environment of the absorbing atom in the sample. Finger printing methods have been used to match the XANES spectra of an unknown sample to those of known "standards". Linear combination fitting of several different standard spectra can give an estimate to
2209-449: Was abandoned in the late 1950s since it turned out to be carcinogenic . Given that the substance remained in the bodies of those to whom it was administered, it gave a continuous radiation exposure and was associated with a risk of cancers of the liver, bile ducts and bones, as well as higher rates of hematological malignancy (leukemia and lymphoma). Thorotrast may have been administered to millions of patients prior to being disused. In
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