Mass-independent (isotope) fractionation 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 Mayeda, and Lawrence Grossman in 1973, in the oxygen isotopic composition of refractory calcium-aluminium-rich inclusions in the Allende meteorite. The inclusions, thought to be among the oldest solid materials in the Solar System, show a pattern of low 18O/16O and 17O/16O relative to samples from the Earth and Moon. Both ratios vary by the same amount in the inclusions, although the mass difference between 18O and 16O is almost twice as large as the difference between 17O and 16O. Originally this was interpreted as evidence of incomplete mixing of 16O-rich material (created and distributed by a large star in a supernova) into the Solar nebula. However, recent measurement of the oxygen-isotope composition of the Solar wind, using samples collected by the Genesis spacecraft, shows that the most 16O-rich inclusions are close to the bulk composition of the solar system. This implies that Earth, the Moon, Mars and asteroids all formed from 18O- and 17O-enriched material. Photochemical dissociation of carbon monoxide in the Solar nebula has been proposed to explain this isotope fractionation.
Mass-independent fractionation also has been observed in ozone. Large, 1:1 enrichments of 18O/16O and 17O/16O in ozone were discovered in laboratory synthesis experiments by John Heidenreich and Mark Thiemens in 1983, and later found in stratospheric air samples measured by Konrad Mauersberger. These enrichments were eventually traced to the three-body ozone formation reaction.
- O + O2 → O3* + M → O3 + M*
Theoretical calculations by Rudolph Marcus and others suggest that the enrichments are the result of a combination of mass-dependent and mass-independent kinetic isotope effects (KIE) involving the excited state O3* intermediate related to some unusual symmetry properties. For formation of ozone substituted with a heavy oxygen atom at the terminal position of the molecule, a highly zero-point energy difference sensitive KIE creates large enrichments for the differences in mass between 18O, 17O, and 16O. For formation of ozone substituted with a heavy oxygen atom at the central position of the molecule (or unsubstituted), the relatively short lifetime the O3* intermediate does not allow a statistical distribution of energy throughout all the degrees of freedom, resulting in a mass-independent distribution of isotopes.
Mass-independent sulfur fractionation
Mass-independent fractionation of sulfur can be observed in ancient sediments, where it preserves a signal of the prevailing environmental conditions. The creation and transfer of the mass-independent signature into minerals would be unlikely in an atmosphere containing abundant oxygen, constraining the Great Oxygenation Event to some time after . Prior to this time, the MIS record implies that sulfate-reducing bacteria did not play a significant role in the global sulfur cycle, and that the MIS signal is due primarily to changes in volcanic activity.
- ^ Clayton, R. N.; Grossman, L.; Mayeda, T. K. (1973). "A Component of Primitive Nuclear Composition in Carbonaceous Meteorites". Science 182 (4111): 485–488. Bibcode 1973Sci...182..485C. doi:10.1126/science.182.4111.485. PMID 17832468.
- ^ Heidenreich, J. E.; Thiemens, M. H. (1986). "A non-mass-dependent oxygen isotope effect in the production of ozone from molecular oxygen: the role of molecular symmetry in isotope chemistry". The Journal of Chemical Physics 84 (4): 2129. Bibcode 1986JChPh..84.2129H. doi:10.1063/1.450373.
- ^ Mauersberger, K. (1987). "Ozone isotope measurements in the stratosphere." Geophysical Research Letters 14: 80-83
- ^ Morton, J. .; Barnes, J. .; Schueler, B. .; Mauersberger, K. . (1990). "Laboratory Studies of Heavy Ozone". Journal of Geophysical Research 95: 901. Bibcode 1990JGR....95..901M. doi:10.1029/JD095iD01p00901.
- ^ Gao, Y.; Marcus, R. (2001). "Strange and unconventional isotope effects in ozone formation". Science 293 (5528): 259–263. Bibcode 2001Sci...293..259G. doi:10.1126/science.1058528. PMID 11387441.
- ^ Farquhar, J.; Bao, H.; Thiemens, M. (2000). "Atmospheric Influence of Earth's Earliest Sulfur Cycle". Science 289 (5480): 756–758. Bibcode 2000Sci...289..756F. doi:10.1126/science.289.5480.756.
- ^ Halevy, I.; Johnston, D.; Schrag, D. (2010). "Explaining the structure of the Archean mass-independent sulfur isotope record". Science 329 (5988): 204–207. Bibcode 2010Sci...329..204H. doi:10.1126/science.1190298. PMID 20508089.
Wikimedia Foundation. 2010.
Look at other dictionaries:
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… … Wikipedia
Kinetic fractionation — is a process that separates stable isotopes from each other by their mass during unidirectional processes.One naturally occurring example of kinetic fractionation is the evaporation of seawater to form clouds. In this instance, isotopically… … Wikipedia
Isotope fractionation — There are three types of isotope fractionation: * equilibrium fractionation * kinetic fractionation * mass independent fractionation … Wikipedia
Stable isotope — Graph of isotopes/nuclides by type of decay. Orange and blue nuclides are unstable, with the black squares between these regions representing stable nuclides. The unbroken line passing below many of the nuclides represents the theoretical… … Wikipedia
Natural abundance — In chemistry, 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) of these isotopes is the atomic weight listed for the element in the… … Wikipedia
δ18O — Foraminifera samples … Wikipedia
Great Oxygenation Event — O2 build up in the Earth s atmosphere. Red and green lines represent the range of the estimates while time is measured in billions of years ago (Ga). Stage 1 (3.85–2.45 Ga): Practically no O2 in the atmosphere. Stage 2 (2.45–1.85 Ga): O2 produced … Wikipedia
MIF — The abbreviation MIF may have many meanings: In medicine Macrophage migration inhibitory factor, a protein believed to be involved in immune response. Müllerian inhibiting factor, a hormone that plays a role in the sexual differentiation of… … Wikipedia
rare-earth element — /rair errth /, Chem. any of a group of closely related metallic elements, comprising the lanthanides, scandium, and yttrium, that are chemically similar by virtue of having the same number of valence electrons. Also called rare earth metal. [1955 … Universalium
Kinetic isotope effect — The kinetic isotope effect (KIE) is the ratio of reaction rates of two different isotopically labeled molecules in a chemical reaction. It is also called isotope fractionation, although this term is somewhat broader in meaning. A KIE involving… … Wikipedia