Isotope enrichments fractionation

The great advantage of the described condensation resin with [2b.2.2] anchor groups lies in the very favourable isotopic separation dependent on the eluted amount of substance (see Fig. 16) and by the fact that a pure solvent can be used as eluant which simplifies the isolation of the isotopic-enriched fractions. Now, additional experiments with larger columns and the application of the cascade principle (Chap. 2.5.2) will have to prove whether or not an enrichment of the heavy isotopes on a technical scale is possible with this exchange system. 

One of the most significant sources of change in isotope ratios is caused by the small mass differences between isotopes and their effects on the physical properties of elements and compounds. For example, ordinary water (mostly Ej O) has a lower density, lower boiling point, and higher vapor pressure than does heavy water (mostly H2 0). Other major changes can occur through exchange processes. Such physical and kinetic differences lead to natural local fractionation of isotopes. Artificial fractionation (enrichment or depletion) of uranium isotopes is the basis for construction of atomic bombs, nuclear power reactors, and depleted uranium weapons. 

In contrast, Meckenstock et al. [280] reported larger isotopic enrichments in residual toluene, 3-6%o and up to 10%o during anaerobic and aerobic biodegradation experiments, respectively. These results indicated that isotopic fractionation effects may be different for different compounds, terminal electron-accepting processes (TEAP), degradative metabolic pathways, or microbial populations. 

The primed or unprimed symbols refer to isotope fractions. The single stage separation factor for infinitesimal product removal (yi/zj 0), (rj)0, is given the symbol a . The i s index the stage number. Most often a is close to unity and it is convenient to define the isotope enrichment factor (the single stage enrichment factor), e, between the i th and (i + l) th stage... 

Silicon. Normal inclusions are spread on a mass dependent fractionation line over a few %o/amu around the solar system average. FUN inclusions display a heavy isotope enrichment from 5 to 15 %o/amu in a similar way to Mg. Non-linear effects are small and indicate an excess of Si smaller than 0.5 %o (Clayton et al. 1984). 

Regardless of the ultimate sources of these compositions, these results clearly show that strongly isotopically fractionated Li from crustal sources plays a role in the mantle. Processes active in subduction zones appear to be cardinal in the control of the Li isotopic composition of different parts of the mantle. The results to date imply that both isotopically enriched (8 Li > MORE) and depleted (5T i < MORE) material are available for deep subduction, and that areas of the continental lithosphere may retain these records on long time scales. 

This model qualitatively explains the deviation of isotopic compositions away from the Meteoric Water Line because molecular diffusion adds a non-equilibrium fractionation term and the limited isotopic enrichment occurs as a consequence of molecular exchange with atmospheric vapor. It is mainly the humidity which controls the degree of isotope enrichment. Only under very arid conditions, and only in small water bodies, really large emichments in D and are observed. For example, Gonfiantini (1986) reported a 5 0-value of +31.3%c and a 8D-value of +129%c for a small, shallow lake in the western Sahara. 

Evaporation and Evaporative Fractionation of Water. Evaporation from standing water bodies is the principal fractionation mechanism in most hydrological systems. Evaporative isotopic enrichment is a function of numerous factors (e.g., temperature, salinity, and relative humidity) that cause considerable variation in the lsO/ ieO and D/H ratios of natural surface waters. Craig and Gordon (22) evaluated isotopic effects on precipitation and evaporation in the ocean-atmosphere system. Much of what was developed in that work is directly applicable to the freshwater systems discussed here. 

Fthyl chloride, in 2(12Hg photosensitization, behaves in a manner very similar to methyl chloride. The quantum yield of calomel formation was 0.91 and the maximum fractional abundance of 2flow rate on the isotopic enrichment is shown in Figure 10. 

The humic constituents of SOM are usually regarded as the primary resistant compounds (Stout et al., 1981). In spite of the fact that the accumulation of C in soil is not indefinite even in natural ecosystems, it is certainly true that HS have been accumulating on the surface of the earth since the appearance of life. They now make up a considerable fraction of the soil organic C pool The amount of C stored as HS is 60 x 1017g, and it exceeds that which occurs in living organisms (Stevenson, 1994). 14C dating combined with isotope enrichment techniques have been used to... 

Isotopic enrichment has also been found by monoisotopic photosensitization for mixtures of natural mercury and alkyl chlorides and vinyl chloride by similar processes. Isotopic enrichment is dependent on such factors as lamp temperatures, flow rates, and substrate pressures. Enrichment increases with decreasing lamp temperature and increasing flow rate, since process (VIII-1) is more ellicient at low temperatures and Cl atoms react with natural mercury containing higher fractions of 202Hg in (VIII-3) at higher flow rates of HC1 or under intermittent illumination. The intermittent illumination results in higher enrichment than the steady illumination. 

Phonon modes in cubic GaN on GaAs (001) have been identified in Raman and infrared absorption (TABLE 2) [12,13], Relative intensities have been used to identify fractions of cubic or hexagonal inclusions [14]. Phonon modes of GaN with isotopically enriched 15N have been studied very recently [15]. 

These experimental results for the first time demonstrate that a heavy-isotope-enriched and anomalously fractionated O-reservoir within the solar nebula (or... 

However, the mechanism through which this anomaly is sequestered inside the solids is not clear. It is postulated that, as the nebular disk evolved, this isotopically enriched ice migrates inward (and mixes with the isotopically normal water) and volatilizes in the inner nebula, driving gas-phase oxidation processes to form solids, during which the isotopic anomaly is passed to condensates (Yurimoto Kuramoto 2004). A major issue is that the silicate materials do not show the required level of heavy-oxygen fractionation, so another mass-independent mechanism is required. 

Carbon isotopes are fractionated in biological cycles and in inorganic oxidation-reduction reactions. In equilibrium systems of various oxidation states, 13C enrichment up to 500 °K occurs in the following sequence ... 

FIGURE 2.3 Heavy oxygen isotope enriched mass independent fractionation, experiment [59] and theory [13]. 

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