Isotopic oxygen, uses

The data in Example 5 tell us that there are two kinds of nitrogen atoms differing in mass. The atoms of the same element having different masses are called ISOTOPES. Oxygen possesses three isotopes. To compare the masses of the atoms of the elements, a standard must be established the presently accepted standard is the isotope of carbon, carbon-12, = exactly 12. 12.0000 g of contains as many atoms as there are atoms of oxygen in 15.9994 g of oxygen obtained from air it contains the Avogadro number of atoms. 

Let us fix attention on a particular H20 molecule A in the interior of water (if we wish to identify this molecule we can suppose that it contains a nucleus of the oxygen isotope 01S) and let us consider the water molecules which happen to be nearest neighbors of this molecule at the moment. These molecules have been in contact with A for different lengths of time. Since all the molecules in the liquid wander about, there was a time when none of these molecules was in contact with A. Further, if we could now begin to watch these molecules, we should find that, after the lapse of different periods of time, they become separated from A and each is replaced by another molecule. Similar remarks can be made about the molecules which come into contact with any chosen molecule. We can now raise the question—-What is the rate of turnover of this process The rate depends on the degree of local order and disorder, which in turn depends on the strength and character of the forces between adjacent molecules. 

When the number of carbon atoms is calculated on the basis of the M + 1 peak one should return to the M + 2 peak, to exclude contributions of 13C isotopes and estimate the number of oxygen atoms with greater precision. Let us consider as an example the mass spectmm represented in Fig. 5.21. 

Such a measurement can tell us about the chemical evolution of oxygen, such as whether the isotopes differentiated via a thermal cycle in which lighter leO fractionates from the heavier lsO, much as Vostok ice-core oxygen ratios reveal the Earth s prehistoric climate. From this fixed point of the Sun s oxygen ratios, we can then trace the history of water in other planetary bodies since their birth in the solar nebulae through the subsequent cometary bombardment [13]. In NASA s search for water on the Moon, important for the establishment of a future Moon base, such isotopic ratios will determine whether the water is a vast mother lode or just a recent cometary impact residue. 

Fortier SM, Cole DR, Wesolowski DJ, Riciputi LR, Paterson BA, Valley JW, Horita J (1995) Determination of the magnetite-water equilibrium oxygen isotope fractionation factor at 350°C a comparison of ion microprobe and laser fluorination techniques. Geochim Cosmochim Acta 59 3871-3875 Friedman I, O Neil JR (1977) Compilation of Stable Isotope Fractionation Factors of Geochemical Interest. US Geol Surv Prof Paper 440-KK... 

Figure 5A shows experimentally derived profiles of pH vs rate for reactions in H2O and D2O [30, 50, 71]. The magnitude of the apparent isotope effect (ratio of rate constants in H2O and D2O) is 4.4 and the profiles appear to support the possibility that a proton is transferred from (Mg -bound) water molecules. However, careful analysis led us to conclude that a metal ion binds directly to the 5 -oxygen. Since the concentration of the deproto-nated 2 -oxygen in H2O should be higher than that in D2O at a fixed pH, we must take into account this difference in pKa, namely ApKa (=pKa °-pKa ), when we analyze the solvent isotope effect of D2O [30, 50, 68, 71]. We can estimate the pKa in D2O from the pKa in H2O using the linear relationship shown in Fig. 5B [30, 68, 73-75]. If the pKa for a Mg -bound water molecule in H2O is 11.4, the ApKa is calculated to be 0.65 (solid line in Fig. 5B). Then, the pKa in D2O should be 12.0. Demonstrating the absence of an intrinsic isotope effect (kH2o/kD20=l)> the resultant theoretical curves closely fit the experimental data, with an approximate 4-fold difference in...

Let us consider, for instance, the decay process of the unstable oxygen isotope... 

The oxygen isotope ratio of ice cores is mostly controlled by a different influence, however the temperature of the clouds from which the snow fell. When water vapour condenses to water or ice, isotope sifting occurs just as it does during evaporation - but in reverse the lighter isotope stays behind. The last precipitation to leave a cloud - the snow that falls over the poles - is therefore enriched in O. The amount of enrichment turns out to depend on how cold it is over the ice sheet. So ice-core isotope records show us how atmospheric temperatures have changed over time. 

Fig. 9 Deuterium isotope effect on the kinetics of oxidation of G by 2AP(-H) radicals in the [2AP]T2GGTio duplex in oxygenated H2O/D2O buffer solutions (pH 7.0) [13]. The kinetic profiles (resolution of 0.5 us/point) of the 2AP(-H) decay (365 nm) and the G(-H) formation (320 nm) were linearized according to a semilogarithmic form of Eq. 7. The solid lines are the best linear fits to the experimental data. Reprinted with permission from the J Phys Chem, Copyright (2001) American Ghemical Society...

The oxygen isotopic composition of the solar system is a key constraint for understanding the environment in which the Sun formed, the environment in the early solar system, and the processes that operated in the early solar system. Oxygen isotopic composition is also a key property by which we classify meteorites. Yet until very recently, we have not been able to make any measurement that could unambiguously tell us what the oxygen isotopic composition of the Sun, and by extension the solar system, really is. 

Recently Peterson and Wiley discussed the non-stoichiometry ofCuj- O, based on their measurement of the diffusion coefficient of Cu (radioactive isotope of Cu) in Cu2 5O as a function of temperature and oxygen partial pressure, Here let us review the results in some detail. 

The structural information at an atomic level is essential for understanding the various properties of supercooled and glassy solutions. X-ray and neutron diffraction enables us to obtain direct structure information (bond distance and coordination number) of ionic solutions in terms of the radial distribution function. In the case of aqueous lithium halide solutions. X-ray diffraction data are dominated by halide-oxygen, halide-oxygen, and oxygen-oxygen interactions. On the contrary, neutron isotopic substitution... 

Coplen T. B. and Kendall C. (2000) Stable Hydrogen and Oxygen Isotope Ratios for Selected Sites of the US Geological Survey s Nasqan and Benchmark Surface-water Networks. US Geol. Surv., Open-file Report. 00-160, 424. 

Taylor B. E. and Wheeler M. C. (1994) Sulfur- and oxygen-isotope geochemistry of acid mine drainage in the western US Field and experimental studies revisited. In Environmental Geochemistry of Sulfide Oxidation, ACS Symposium Series 550 (eds. C. N. Alpers and D. W. Blowes). American Chemical Society, Washington, DC, pp. 481-514. 

The study of the catalysis of isotopic exchange in molecular oxygen has begun rather lately and covers a limited number of catalysts and a rather narrow region of variation of conditions for carrying out reactions. Nevertheless, the results obtained enable us to hope that this type of reaction will be a useful method for the study of the mechanism of reactions of oxidation, and especially for the elucidation of nature of intermediate forms of oxygen interaction with solid catalysts. 

Plant remains and mollusk shells from marine Holocene layers in the central depression (layers containing Cerastoderma, station 15) have been dated by radiocarbon methods [16]. The results provided evidence of marine sedimentation being confined to the second half of the Holocene and permitted us to date some large regressive phases in the history of the Aral. Other studies have been performed in the central basin, including oxygen isotope analysis of carbonates in mollusk shells... 

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