Isotopic related problems

A related problem is the conspicuous xenon isotope anomalies in various meteorites. The heavy fraction [79-87] enriched in Xe was originally [79, 80] ascribed to y-ray or neutron-induced fission of Pu (half-life 83 million years, only 0.08 percent probability of spontaneous fission otherwise, He emitter) and a controversy maintained about plutonium(IV) occupying sites in (quite scarce mineral fractions) and forming xenon or the noble gas directly incorporated from the protoplanetary cloud. Some of the most remarkable components of the... 

A related problem in isotope geology is the correlation between the ratio Sr/ Sr and Sr concentrations. In this case (provided there is no analytical error) the correlation is most likely to result from the random mixing of two uniform sources of Sr with different isotopic compositions) although there is also the possibility that the observed correlations have arisen as a consequence of the common denominator effect resulting from ratio correlation. 

Preparing isotopically enriched carbonyls and related species is not always a trivial problem. We have recently developed (20) a method which looks particularly promising in some cases. CW CO2 laser irradiation of a gas phase mixture containing SF5 as an energy transfer agent can promote thermal chemistry without complications due to wall reactions, e.g. 

In Section III.D various methods were mentioned for determination of the 15N to 14N isotope ratio. Some applications to amines that appeared in the recent literature are presented here. Isotope dilution with a known aliquot of labelled compound allows solving some of the problems related to nonquantitative recovery yields of analyte in the analytical processing of a sample. However, the possibility of isotopic fractionation has to be taken into consideration. 

Stable isotope analysis of Earth, Moon, and meteorite samples provides important information concerning the origin of the solar system. 8lsO values of terrestrial and lunar materials support the old idea that earth and moon are closely related. On the other hand three isotope plots for oxygen fractionation in certain meteoric inclusions are anomalous. They show unexpected isotope fractionations which are approximately mass independent. This observation, difficult to understand and initially thought to have important cosmological implications, has been resolved in a series of careful experimental and theoretical studies of isotope fractionation in unimolecular kinetic processes. This important geochemical problem is treated in some detail in Chapter 14. 

Until now, the isotopic effect was discnssed only in relation to the reactants. In electron-transfer reactions, the solvent plays an eqnally important role. As mentioned, different solvate forms are possible for reactants, transition states, and products. Therefore, it seems important to find a reaction where the kinetic effect resulting from the introduction of an isotope would be present for solvents, but absent for reactants. For a published work concerning this problem, refer Yusupov and Hairutdinov (1987). In this work, the authors studied photoinduced electron transfer from magnesium ethioporphyrin to chloroform followed by a dark recombination of ion-radicals in frozen alcohol solutions. It was determined that the deuteration of chloroform does not affect the rate of transfer, whereas deuteration of the solvent reduces it. The authors correlate these results with the participation of solvent vibrational modes in the manner of energy diffraction during electron transfer. 

Burgoyne and Hayes (1998) and Sessions et al. (1999) introduced the continuously flow technique for the D/H measurement of individnal organic compounds. The precise measurement of D/H ratios in a He carrier poses a number of analytical problems, related to the tailing from the abundant " He+ onto the minor HD+ peak as well as on reactions occurring in the ion source that produce H3+. However, these problems have been overcome and precise hydrogen isotope measurements of individnal organic compounds are possible. 

By comparison with many other silicate minerals, isotope studies of natural clays are complicated by a number of special problems related to their small particle size and, hence, much larger specific surface area and the presence of interlayer water in certain clays. Surfaces of clays are characterized by 1 or 2 layers of adsorbed water. Savin and Epstein (1970a) demonstrated that adsorbed and interlayer water can exchange its isotopes with atmospheric water vapor in hours. Complete removal of interlayer water for analysis with the total absence of isotopic exchange between it and the hydroxyl group, may not be possible in all instances (Lawrence and Taylor 1971). 

As a nuclear reaction, the s process is relatively well understood, but the problem lies in identifying an astrophysical site for it and determining the relevant physical parameters, such as neutron flux, mean time separating two neutron captures, and temperature. It has been shown that the most propitious temperatures are those of helium fusion. Added to the fact that the surfaces of certain red giants are rich in s isotopes, such as radioactive technetium and barium, this observation confirms the idea that the s process may be related to helium fusion regions in stars. 

A specific example of applications in the second category is the dating of rocks. Age determination is an inverse problem of radioactive decay, which is a first-order reaction (described later). Because radioactive decay follows a specific law relating concentration and time, and the decay rate is independent of temperature and pressure, the extent of decay is a measure of time passed since the radioactive element is entrapped in a crystal, hence its age. In addition to the age, the initial conditions (such as initial isotopic ratios) may also be inferred, which is another example of inverse problems. 

A key problem in the present natural analogue study is the distinction between chemical variations related to trace element migration during basalt alteration and variations due to magmatic fractionation and other syn-intrusive processes. The detailed evaluation of the available data has shown that the chemical and isotopic composition of the HC1 residues is largely controlled by fractional crystallization and syn-intrusive assimilation of salt. In contrast, the chemical composition of the leachates is strongly modified by post-intrusive alteration (Steinmann et al. 1999). 

Despite these strengths, ICP-MS has also some important drawbacks, many of them related to the spectral isotopic and/or chemical interferences, which affect analyte signal intensities and, therefore, the applicability of the technique. The complexity of the optimisation of the methodological and operating conditions, the differences in the ionisation rates of the various elements, the sequential isotopic measurements and the limited speed of signal acquisition (a serious drawback in multielemental analysis of fast transient signals) are some other problems to be considered. 

If severe nonlinearities might be present, the linear inner relation can be modified to a quadratic or cubic one. This strong nonlinear situation might arise whenever problems occur on the detector or monochromator, malfunction of the automatic sampler in ETAAS, strong influence of the concomitants on the signal, when the linear range for the analyte is too short or when LIBS, LA-ICP-MS measurements or isotope dilution are carried out (see Chapter 1 for more possibilities). 

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