Isotopic enrichement

Jcc and have been measured in isotopically enriched molecules, but in this case again the experimental values are in poor accordance with those calculated using the CNDO/2 approximation (Table 1-40) (113). 

In this equation the red O signifies oxygen enriched in Its mass 18 isotope analysis of isotopic enrichment was performed by mass spectrometry... 

Isotopically enriched ion. Any ion containing an abundance of a particular isotope above the level at which it occurs in nature. 

Principal ion. A molecular or fragment ion that is made up of the most abundant isotopes of each of its atomic constituents. In the case of compounds that have been artificially isotopically enriched in one or more positions (such as or CH2D2), the principal ion can be... 

The phenomenon of multiphoton dissociation finds a possible application in the separation of isotopes. For this purpose it is not only the high power of the laser that is important but the fact that it is highly monochromatic. This latter property makes it possible, in favourable circumstances, for the laser radiation to be absorbed selectively by a single isotopic molecular species. This species is then selectively dissociated resulting in isotopic enrichment both in the dissociation products and in the undissociated material. 

Figure 9.32 illustrates the isotopic enrichment of SFe following irradiation with a pulsed CO2 laser in the 3g vibrational band, at 945 cm, of SFe, V3 being a strongly infrared active bending vibration. The natural abundances of the isotopes of sulphur are (95.0 per cent), (4.24 per cent), (0.74 per cent) and (0.017 per cent). The figure shows that depletion of SFg has been achieved to such an extent that equal quantities of SFg and SFa remain. 

Figure 9.32 Isotopic enrichment of SFg by multiphoton dissociation following irradiation in the vibrational band of SFg. The absorption spectrum is shown (a) before and (b) after irradiation. (Reproduced, with permission, from Letokhov, V S., Nature, Land., Ill, 605, 1979 Copyright 1979 Macmillan Journals Limited)...

In TBP extraction, the yeUowcake is dissolved ia nitric acid and extracted with tributyl phosphate ia a kerosene or hexane diluent. The uranyl ion forms the mixed complex U02(N02)2(TBP)2 which is extracted iato the diluent. The purified uranium is then back-extracted iato nitric acid or water, and concentrated. The uranyl nitrate solution is evaporated to uranyl nitrate hexahydrate [13520-83-7], U02(N02)2 6H20. The uranyl nitrate hexahydrate is dehydrated and denitrated duting a pyrolysis step to form uranium trioxide [1344-58-7], UO, as shown ia equation 10. The pyrolysis is most often carried out ia either a batch reactor (Fig. 2) or a fluidized-bed denitrator (Fig. 3). The UO is reduced with hydrogen to uranium dioxide [1344-57-6], UO2 (eq. 11), and converted to uranium tetrafluoride [10049-14-6], UF, with HF at elevated temperatures (eq. 12). The UF can be either reduced to uranium metal or fluotinated to uranium hexafluoride [7783-81-5], UF, for isotope enrichment. The chemistry and operating conditions of the TBP refining process, and conversion to UO, UO2, and ultimately UF have been discussed ia detail (40). 

The most important role of UO3 is in the production of UF4 [10049-14-6] and UF [7783-81-5], which are used in the isotopic enrichment of uranium for use in nuclear fuels (119—121). The trioxide also plays a part in the production of UO2 for fuel peUets (122). In addition to these important synthetic appHcations, microspheres of UO3 can themselves be used as nuclear fuel. Fabrication of UO3 microspheres has been accompHshed using sol-gel or internal gelation processes (19,123—125). FinaHy, UO3 is also a support for destmctive oxidation catalysts of organics (126,127). 

Uranium hexafluoride [7783-81-5], UF, is an extremely corrosive, colorless, crystalline soHd, which sublimes with ease at room temperature and atmospheric pressure. The complex can be obtained by multiple routes, ie, fluorination of UF [10049-14-6] with F2, oxidation of UF with O2, or fluorination of UO [1344-58-7] by F2. The hexafluoride is monomeric in nature having an octahedral geometry. UF is soluble in H2O, CCl and other chlorinated hydrocarbons, is insoluble in CS2, and decomposes in alcohols and ethers. The importance of UF in isotopic enrichment and the subsequent apphcations of uranium metal cannot be overstated. The U.S. government has approximately 500,000 t of UF stockpiled for enrichment or quick conversion into nuclear weapons had the need arisen (57). With the change in pohtical tides and the downsizing of the nation s nuclear arsenal, debates over releasing the stockpiles for use in the production of fuel for civiUan nuclear reactors continue. 

Various borate esters are chemostetilants for house flies (51). Tributyl borate, available from Eagle-Picher, Miami, Oklahoma, which is isotopically enriched in boron-10, is being used as a chemical precursor in the synthesis of pharmacologically active boron compounds suitable for boron neutron capture therapy. 

The only large-scale use of deuterium in industry is as a moderator, in the form of D2O, for nuclear reactors. Because of its favorable slowing-down properties and its small capture cross section for neutrons, deuterium moderation permits the use of uranium containing the natural abundance of uranium-235, thus avoiding an isotope enrichment step in the preparation of reactor fuel. Heavy water-moderated thermal neutron reactors fueled with uranium-233 and surrounded with a natural thorium blanket offer the prospect of successful fuel breeding, ie, production of greater amounts of (by neutron capture in thorium) than are consumed by nuclear fission in the operation of the reactor. The advantages of heavy water-moderated reactors are difficult to assess. 

Proceedings of the Fourth International Conference on the Peaceful Cses of Atomic Energy, Geneva, Sept. 6—16,1971, United Nations and the International Atomic Energy Agency, 1972, particulady Vol. 9, Isotope Enrichment, Fuel Cycles and Safeguards. 

Cu/ Zn0/Si02 catalyst reduced at 700 K [3.147]. These LEIS spectra were obtained at three different ion doses - 3 x 3.41 x 10 and 8.67 x 10 Ne" cm . Because of the use of isotopically enriched Cu and Zn, and of Ne" ions as projectiles, Cu and Zn can clearly be separated in the LEIS spectrum. Strong dose-dependence is apparent. Eig. 3.60b shows the dose-dependent surface concentrations of Cu and Zn. At low doses (<1.5 x lO " Ne cm ) the Zn concentration remains constant whereas the Cu concentration increases. At these low doses a hydroxyl layer on top of the catalyst is sputtered. The Zn signal stays constant despite removal of the adsorbate, indicating that at the virgin surface the Zn concentration was even higher. 

In the laboratory NH3 is usually obtained from cylinders unless isotopically enriched species such as NH3 or NDj are required. Pure dry can be prepared by treating an enriched NH salt with an excess of KOH and drying the product gas over metallic Na. Reduction of NOs or with Devarda s alloy (50%... 

The imbalance between and NMR studies in the solid state (Section VI,F) partly reflects the fact that it is easier to introduce N than into heterocyclic compounds, particularly azoles (DNMR in the solid state usually requires isotopic enrichment). Compared to solution studies, solid-state intermolecular proton transfer between tautomers has the enormous advantage that the structure of the species involved is precisely defined. 

Pu-238 is available in various isotopic enrichments ranging from 99+% to 78%. Uses are for small thermal and electric power generators. 

Tieszen 1991). Water stress, for instance, lowers photo synthetic discrimination against C, resulting in isotopic enrichment. C4 plants are not affected isotopically by environmental parameters although their distributions are (Ehleringer et al. 1997). Under conditions of increasing aridity, the climate variable of most likely importance in African sites, C3 plants should become slightly more enriched on average. Therefore one should be cautious about use... 

A second kind of model is briefly treated, based on isotopic mass balance arguments, and it is shown that large isotopic discrimination during methano-genesis in ruminants may account for data trends when comparing herbivores and carnivores. A third class of model is sketched at the level of biochemical flows, where some fundamental points are made concerning points where the isotopic composition of metabolites may be altered. The relevance of this to nitrogen isotopic enrichment is considered. 

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