|
Kinetic Fractionation
Kinetic fractionation is an isotopic fractionation process that separates stable isotopes from each other by their mass during unidirectional processes. Biological processes are generally unidirectional and are very good examples of "kinetic" isotope reactions. All organisms preferentially use lighter isotopes, because "energy costs" are lower, resulting in a significant fractionation between the substrate (heavier) and the biologically mediated product (lighter). For example, photosynthesis preferentially takes up the light isotope of carbon C during assimilation of atmospheric CO. This kinetic isotope fractionation explains why plant material (and thus fossil fuels, which are derived from plants) is typically depleted in C by 25 per mil (2.5%) relative to most inorganic carbon on Earth. A naturally occurring example of non-biological kinetic fractionation occurs during the evaporation of seawater to form clouds under conditions in which some part of the transport is unidirection ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Isotopic Fractionation
Isotope fractionation describes fractionation processes that affect the relative abundance of isotopes, a phenomena that occurs (and so advantage is taken of it) in the study geochemistry, biochemistry, food science, and other fields. Normally, the focus is on stable isotopes of the same element. Isotopic fractionation can be measured by isotope analysis, using isotope-ratio mass spectrometry, nuclear magnetic resonance methods ( specialised techniques,) cavity ring-down spectroscopy, etc., to measure ratios of isotopes, important tools to understand geochemical and biological systems, past and present. For example, biochemical processes cause changes in ratios of stable carbon isotopes incorporated into biomass. Definition Stable isotopes partitioning between two substances ''A'' and ''B'' can be expressed by the use of the isotopic fractionation factor (alpha): : where ''R'' is the ratio of the heavy to light isotope (e.g., 2H/1H or 18O/16O). Values for alpha tend to be v ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Equilibrium Fractionation
Equilibrium isotope fractionation is the partial separation of isotopes between two or more substances in chemical equilibrium. Equilibrium fractionation is strongest at low temperatures, and (along with kinetic isotope effects) forms the basis of the most widely used isotopic paleothermometers (or climate proxies): D/H and 18O/16O records from ice cores, and 18O/16O records from calcium carbonate. It is thus important for the construction of geologic temperature records. Isotopic fractionations attributed to equilibrium processes have been observed in many elements, from hydrogen ( D/H) to uranium ( 238U/235U). In general, the light elements (especially hydrogen, boron, carbon, nitrogen, oxygen and sulfur) are most susceptible to fractionation, and their isotopes tend to be separated to a greater degree than heavier elements. Definition Most equilibrium fractionations are thought to result from the reduction in vibrational energy (especially zero-point energy) when a more ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hydrogen Isotope Biogeochemistry
Hydrogen isotope biogeochemistry (HIBGC) is the scientific study of biological, geological, and chemical processes in the environment using the distribution and relative abundance of hydrogen isotopes. Hydrogen has two stable isotopes, protium H and deuterium H, which vary in relative abundance on the order of hundreds of Per mille, permil. The ratio between these two species can be called the hydrogen isotopic signature of a substance. Understanding isotopic fingerprints and the sources of Isotope fractionation, fractionation that lead to variation between them can be applied to address a diverse array of questions ranging from ecology and hydrology to geochemistry and paleoclimate reconstructions. Since specialized techniques are required to measure natural hydrogen isotopic composition (HIC), HIBGC provides uniquely specialized tools to more traditional fields like ecology and geochemistry. History of hydrogen isotopes Earliest work The study of hydrogen stable isotopes began ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Kinetic Isotope Effect
In physical organic chemistry, a kinetic isotope effect (KIE) is the change in the reaction rate of a chemical reaction when one of the atoms in the reactants is replaced by one of its isotopes. Formally, it is the ratio of rate constants for the reactions involving the light (''k'') and the heavy (''k'') isotopically substituted reactants ( isotopologues): KIE = ''k/k''. This change in reaction rate is a quantum effect that occurs mainly because heavier isotopologues have lower vibrational frequencies than their lighter counterparts. In most cases, this implies a greater energy input needed for heavier isotopologues to reach the transition state (or, in rare cases, dissociation limit), and therefore, a slower reaction rate. The study of KIEs can help elucidate reaction mechanisms, and is occasionally exploited in drug development to improve unfavorable pharmacokinetics by protecting metabolically vulnerable C-H bonds. Background KIE is considered one of the most essential ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Isotopic Ratio
In physics, natural abundance (NA) refers to the abundance of isotopes of a chemical element as naturally found on a planet. The relative atomic mass (a weighted average, weighted by mole-fraction abundance figures) of these isotopes is the atomic weight listed for the element in the periodic table. The abundance of an isotope varies from planet to planet, and even from place to place on the Earth, but remains relatively constant in time (on a short-term scale). As an example, uranium has three naturally occurring isotopes: 238U, 235U, and 234U. Their respective natural mole-fraction abundances are 99.2739–99.2752%, 0.7198–0.7202%, and 0.0050–0.0059%. For example, if 100,000 uranium atoms were analyzed, one would expect to find approximately 99,274 238U atoms, approximately 720 235U atoms, and very few (most likely 5 or 6) 234U atoms. This is because 238U is much more stable than 235U or 234U, as the half-life of each isotope reveals: 4.468 billion years for 238U compared ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Isotopic Enrichment
Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes. The use of the nuclides produced is varied. The largest variety is used in research (e.g. in chemistry where atoms of "marker" nuclide are used to figure out reaction mechanisms). By tonnage, separating natural uranium into enriched uranium and depleted uranium is the largest application. In the following text, mainly uranium enrichment is considered. This process is crucial in the manufacture of uranium fuel for nuclear power plants and is also required for the creation of uranium-based nuclear weapons (unless uranium-233 is used). Plutonium-based weapons use plutonium produced in a nuclear reactor, which must be operated in such a way as to produce plutonium already of suitable isotopic mix or ''grade''. While chemical elements can be purified through chemical processes, isotopes of the same element have nearly identical chemical properties which makes this type ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Transient Kinetic Isotope Fractionation
Transient kinetic isotope effects (or fractionation) occur when the reaction leading to isotope fractionation does not follow pure first-order kinetics (FOK) and therefore isotopic effects cannot be described with the classical equilibrium fractionation equations or with steady-state kinetic fractionation equations (also known as the Rayleigh equation). In these instances, the general equations for biochemical isotope kinetics (GEBIK) and the general equations for biochemical isotope fractionation (GEBIF) can be used. The GEBIK and GEBIF equations are the most generalized approach to describe isotopic effects in any chemical, catalytic reaction and biochemical reactions because they can describe isotopic effects in equilibrium reactions, kinetic chemical reactions and kinetic biochemical reactions. In the latter two cases, they can describe both stationary and non-stationary fractionation (i.e., variable and inverse fractionation). In general, isotopic effects depend on the num ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Mass-independent Fractionation
Mass-independent isotope fractionation or Non-mass-dependent fractionation (NMD), refers to any chemical or physical process that acts to separate isotopes, where the amount of separation does not scale in proportion with the difference in the masses of the isotopes. Most isotopic fractionations (including typical kinetic fractionations and equilibrium fractionations) are caused by the effects of the mass of an isotope on atomic or molecular velocities, diffusivities or bond strengths. Mass-independent fractionation processes are less common, occurring mainly in photochemical and spin-forbidden reactions. Observation of mass-independently fractionated materials can therefore be used to trace these types of reactions in nature and in laboratory experiments. Mass-independent fractionation in nature The most notable examples of mass-independent fractionation in nature are found in the isotopes of oxygen and sulfur. The first example was discovered by Robert N. Clayton, Toshiko May ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Transient Kinetic Isotope Fractionation
Transient kinetic isotope effects (or fractionation) occur when the reaction leading to isotope fractionation does not follow pure first-order kinetics (FOK) and therefore isotopic effects cannot be described with the classical equilibrium fractionation equations or with steady-state kinetic fractionation equations (also known as the Rayleigh equation). In these instances, the general equations for biochemical isotope kinetics (GEBIK) and the general equations for biochemical isotope fractionation (GEBIF) can be used. The GEBIK and GEBIF equations are the most generalized approach to describe isotopic effects in any chemical, catalytic reaction and biochemical reactions because they can describe isotopic effects in equilibrium reactions, kinetic chemical reactions and kinetic biochemical reactions. In the latter two cases, they can describe both stationary and non-stationary fractionation (i.e., variable and inverse fractionation). In general, isotopic effects depend on the num ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Isotope
Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemical element), but different nucleon numbers (mass numbers) due to different numbers of neutrons in their nuclei. While all isotopes of a given element have similar chemical properties, they have different atomic masses and physical properties. The term isotope is derived from the Greek roots isos (wikt:ἴσος, ἴσος "equal") and topos (wikt:τόπος, τόπος "place"), meaning "the same place"; thus, the meaning behind the name is that different isotopes of a single element occupy the same position on the periodic table. It was coined by Scottish doctor and writer Margaret Todd (doctor), Margaret Todd in a 1913 suggestion to the British chemist Frederick Soddy, who popularized the term. The number of protons within the atomic nuc ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Fractionation
Fractionation is a separation process in which a certain quantity of a mixture (of gasses, solids, liquids, enzymes, or isotopes, or a suspension) is divided during a phase transition, into a number of smaller quantities (fractions) in which the composition varies according to a gradient. Fractions are collected based on differences in a specific property of the individual components. A common trait in fractionations is the need to find an optimum between the amount of fractions collected and the desired purity in each fraction. Fractionation makes it possible to isolate more than two components in a mixture in a single run. This property sets it apart from other separation techniques. Fractionation is widely employed in many branches of science and technology. Mixtures of liquids and gasses are separated by fractional distillation by difference in boiling point. Fractionation of components also takes place in column chromatography by a difference in affinity between statio ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), nonmetal, and a potent oxidizing agent that readily forms oxides with most elements as well as with other chemical compound, compounds. Oxygen is abundance of elements in Earth's crust, the most abundant element in Earth's crust, making up almost half of the Earth's crust in the form of various oxides such as water, carbon dioxide, iron oxides and silicates.Atkins, P.; Jones, L.; Laverman, L. (2016).''Chemical Principles'', 7th edition. Freeman. It is abundance of chemical elements, the third-most abundant element in the universe after hydrogen and helium. At standard temperature and pressure, two oxygen atoms will chemical bond, bind covalent bond, covalently to form dioxygen, a colorless and odorless diatomic gas with the chemical formula ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
