Isotopes, 69-70, Table isotopic enrichment

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). 

The only respect in which the hot atom chemistry of organometallic compounds has so far been applied to other fields of study is in the area of isotope enrichment. Much of this has been done for isolation of radioactive nuclides from other radioactive species for the purpose of nuclear chemical study, or for the preparation of high specific activity radioactive tracers. Some examples of these applications have been given in Table II. The most serious difficulty with preparation of carrier-free tracers by this method is that of radiolysis of the target compound, which can be severe under conditions suited to commercial isotope production, so that the radiolysis products dilute the enriched isotopes. A balance can be struck in some cases, however, between high yield and high specific activity (19, 7J),... 

TABLE 6. The composition of isotopically enriched samples of tin(IV) oxide... 

Because the " Ca isotopic enrichments in whole rocks are relatively small, Marshall and DePaolo (1982,1989) used the Sca value as defined in Table 1, which refers the Ca/ Ca value of rocks to the value expected in the Earth s mantle. Strictly speaking, the Ca/ Ca value of the mantle changes with time. However, taking the approximate value of K/Ca of the mantle to be 0.01, the " K/ -Ca ratio is about 0.0002, and the total change of " CaT Ca over the age of the Earth is 0.002 or about 0.12 units of Eca-... 

By comparing these constants with the equilibrium constants of anion-radical reactions (entries 7-10 in Table 2.1), one can conclude that in case of dianions, the equilibrium is definitely shifted to the right. The equilibrium constants of ca. 0.35 are increased almost twofold, up to approximately 0.7, but remain less than unity. This means that even with the formation of dianions, the possibility of isotopic enrichment still remains qnite feasible, although the process becomes less favorable than that in the case of nitrobenzene. 

Couplings between N and C are difficult to measure without isotopic enrichment. Values of 7( RN, RC) are generally less than 20 Hz. The sign can be positive or negative. If the value of 7 is close to zero, it can be exceeded by 7 or 7 couplings. Some examples of known "7( RN, RC) couplings are given in Table 16. 

We conclude this survey of applications of solid-state Mg NMR to studies of inorganic materials given in Table 2, a compilation of the experimentally derived solid-state Mg NMR parameters (Cq, 7q and iso) reported to date for a large number of non-metallic Mg-containing inorganic compounds. Also mentioned in the table are any points of note such as the special methods used to improve sensitivity or resolution in the performed experiments (e.g. isotopic enrichment, QCPMG, methods based on population transfers, MQMAS, STM AS, etc.) and the eventual discrepancies observed between results achieved by different researchers. 

Since the level of tritium in the atmosphere is presently greater than 10 TU, it is possible to study many physical and chemical processes using this equipment for sample analysis. Since isotopic enrichment is unnecessary for most samples, direct rapid analysis is possible. The equipment is being used presently to analyze water samples from Nevada in a hydrology project. Table IV shows the analytic data. No attempt has yet been made to evaluate these data. What is apparent qualitatively... 

Typical results for the overall isotopic separation for Linde X exchanged essentially completely with praseodymium are shown in Table I. Generally, 50-60% of the neutron capture product, 142Pr, was eluted from the zeolite, along with about 1% of the unreacted 141Pr target. In most cases, the isotopic enrichment factor (the ratio of specific activity of metal in the eluate to the average specific activity of all the metal) was in the... 

Table 4 Bias Corrected Isotope Ratio Measurements in the Unspiked Sample, Isotopically Enriched Spike, and Spiked Sample for Three Oil Leachate Samples as Determined by Glow Discharge Mass Spectroscopy Solution Residue Method...

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]. 

Table 2 summarizes the a factors for various substituted DBK s. Furthermore, this table shows, as is expected, that the application of an external magnetic field reduces the amount of isotope enrichment. Figure 8 and Fig. 9 illustrate the effect of the application ofan external magnetic field on the extent of the cage effect and on the efficiency of isotope enrichment. The results can be understood in terms of a change in the relative contributions in the intersystem crossing mechanism of the hyperfine and Zeeman mechanisms14). 

Table 2. The Isotope Enrichment Factor, a, for Various Dibenzyl Ketones... 

Table 3. Single Stage 13C Isotope Enrichment Parameters for Photolysis of Dibenzyl Ketone Under Various Conditions...

Phenyladamantyl ketone, when irradiated to 90 % conversion, yields an enrichment in the 13C content in both the products and recovered starting material. The recovered starting material is 200% enriched in 13C content benzaldehyde is 160% enriched, and adamantane in 180 % enriched (calculated assuming selective enrichment at the benzaldehyde CO carbon and the 1-carbon of adamantane)23). The role of the micelles in this reaction provides reduced dimentionality allowing recombination of the radical pairs to occur over diffusional processes. By comparison, viscosity has only a slight effect on the isotope enrichment (Table 5). 

Precision The precision of the absorption value depends upon tfe precision of P, and E measurements This method for isotopic enrichment measurements by mass spectrometry has a precision of 2% as does the measurement of F by atomic absorption This precision is adequate for absorption and bioavailability studies with zinc and copper (Table I) since zinc and copper absorption are in the range of 30-70% Only fairly large changes in iron absorption can be discerned because non-heme iron absorption is typically less than 10% This may not be a serious problem in bioavailability studies since it is doubtful that very small changes in iron absorption from single foods are biologically significant ... 

A typical set of results for the products of the reaction, i.e., CO2 and C3O2 is shown in Table I, and for isotopic enrichment in Table II. A separation between C and -C occurs with a substantial enrichment factor. A separation between and occurs. The amount of heavy isotopes produced is small owing to the very low absorption coefficient (0.0076) (i3) of CO at 2062.4 A. See Figure 4). 

Table 12.5 shows expected isotopic uranium contents which might be expected as product from cascade type (gaseous diffusion or gas centrifuge) enrichment facilities. While the highly enriched U is actually the target of these efforts one should also be able to verify the production of low enriched uranium. Columns 5 and 6 of Table 12.5 show the isotopic enrichments which would result from a 1000-1 mixture of namral U and low or high enriched U. Table 12.6 shows the ratios of the isotopes U and U relative to the in each mixture. The 0.6% isotopic shift in the ratio in the case of LEU-MIX should be detectable by today s technology. The 0.13% in ratio... 

It is clear from Table III that all transition metal ions other than Cu + have a well-defined six-fold coordination, and on the basis of these results and those from NMR, QENS, optical spectroscopy, and kinetic studies with isotopically enriched species, it can be concluded that they form stable hydration complexes. 

One factor influencing the usefulness of an NMR nucleus is its natural abundance, which can range from 100 percent down to the vanishingly small. In the latter case, it may only be possible to acquire an NMR spectrum if the sample has been artificially isotopically enriched, as is normally done for N and 0 NMR. The natural abundances of the most useful spin I = V2 nuclei are listed in Table 1.1 and those of the quadmpolar nuclei in Table 1.2. The standard substances against which the chemical shifts of the various nuclides are quoted are listed for the spin I = V2 nuclei in Table 1.1 and for the quadmpolar nuclei in Table 1.3. 

Although under the definition of low-y nuclei adopted in this chapter only one of the Ba nuclei ( Ba) falls into this category, both will be discussed here. Both nuclei are spin I = 2 and have small gyromagnetic ratios (see Chapter 1, Table 1.2). Although both nuclei have been used in NMR studies of isotopically-enriched samples, the slightly greater natural abundance of - Ba (11.32% by comparison with 6.6% for Ba) makes it more favourable for studying unenriched samples. 

In addition, fewer chemically distinct types of nitrogen moieties, compared to the number of carbon moieties, in polymers should result in simpler spectra, and easier identification of resonances and their quantitative analysis. These properties of15N NMR spectra should also enable the detection of byproducts and minor products which would probably be undetected in the relatively complicated 13C spectra. However, the small gyromagnetic ratio (about 40% of that of 13C) and low natural abundance of the 15N isotope (Table 1) result in about 30 times less sensitivity compared to 13CNMR. Therefore, isotopic enrichment is usually required, particularly for solid state experiments. 

Published MTBE carbon and hydrogen isotope enrichment factors detected under oxic [37,40,41] and anoxic conditions [23,38,39,66] in microcosm experiments up to 2007 are summarised in Table 1. Isotope fractionation studies of other ether oxygenates are still very scarce. For example, TAME carbon fractionation has been studied under methanogenic conditions [38] and recently carbon and hydrogen fractionation of ETBE under oxic conditions has been reported [40]. 

Table 1 Comparison of carbon and hydrogen isotopic enrichment factors (e) with 95% confidence intervals (95% Cl) for aerobic and anaerobic biodegradation of fuel oxygenates... 

A wide range of nuclei may be observed by NMR spectroscopy, but the inherent properties of some nuclei (e.g. a large quadrupole moment) may make their observation difficult. The main criterion is that the nucleus possesses a value of the nuclear spin quantum number (Table 2.3). Secondly, it is advantageous (but not essential) for the nucleus to occur in signihcant abundance. Carbon-13 is an example of a low abundant isotope which is, nevertheless, extensively used for NMR spectroscopy isotopic enrichment may be used to improve signal noise ratios. A third requirement is that the nucleus possesses a relatively short spin-relaxation time (tj) this property depends not only on the nucleus itself but also on its molecular environment. Some elements exhibit more than one NMR active nucleus and the choice for experimental observation may depend upon the relative inherent values of Tj. For example, Li and Li are NMR active, but whereas t values for Li are typically <3 s, those for Li lie in the range 10-80s Li is thus more appropriate... 

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