Isotopic specification

Nuclear reaction analysis (NRA) is used to determine the concentration and depth distribution of light elements in the near sur ce (the first few lm) of solids. Because this method relies on nuclear reactions, it is insensitive to solid state matrix effects. Hence, it is easily made quantitative without reference to standard samples. NRA is isotope specific, making it ideal for isotopic tracer experiments. This characteristic also makes NRA less vulnerable than some other methods to interference effects that may overwhelm signals from low abundance elements. In addition, measurements are rapid and nondestructive. 

The migration rate of a groundwater constituent, relative to the groundwater flow rate, is controlled by the retardation factor, where Ri = 1 + Ki. Where Ki 1 (e.g., for Th and Ra), Ri Ki, and Iads + Iw = IwRi- Note that ki and k-i are element-specific but not isotope-specific. All isotopes that decay slower than desorption, so that k-i have a value of Ki that is equal to that of a stable isotope (Eqn. 3). The value of Ki may be lower for the shortest-lived nuclides (see Fig. 2b), and so a series of equations derived from Equation (3) applied to different isotopes of the same element may be used to obtain absolute values for the separate rate constants. 

As nuclear reactions are isotope specific, NRA may be used, for example, to distinguish the distribution of binary blends of polymers in a polymer film, where one of the polymers is labelled with deuterium. The depth distribution of the deuterium atoms can be established and hence that of the labelled polymers. 

NRA X-rays, gamma rays, ions A few microns A few ppm For Z < 9 isotope specific No Yes (No need for standards)... 

Of all of the machines used for microanalysis LAMMA seems to be the most problematic. A laser beam is used to disintegrate a spot in the sample, and the material emitted is then analyzed in a mass spectrometer. It has similar lateral resolution to PIXE, and like SIMS can be used to distinguish between isotopes of the same element. It has, however, proved very difficult to quantify, and is destructive to the specimen. One recent investigation (13) ofthe distribution of stable isotopes of calcium, magnesium, and potassium in Norway spruce used three microprobes EDAX at 0.3 pm lateral resolution isotope specific point analysis, using LAMMA at 1.5 pm lateral resolution and isotope specific imaging using SIMS at 1-3 pm lateral resolution. 

Some of the methods commonly used for the determination of thorium in biological materials are given in Table 6-1. The colorimetric methods are not capable of isotope-specific determination of thorium isotopes. Alpha spectrometric and neutron activation analysis are useful in the quantification of isotope-specific thorium and thorium-232, respectively, and have better sensitivities than colorimetric methods. Alpha spectrometry is the commonly used isotope-specific analysis for the determination of thorium-232 and the thorium-230 derived from the decay of uranium-238 (Wrenn et al. 1981). Standard reference materials (SRMs) containing thorium in human liver (SRM-4352) and human lung (SRM-4351) necessary for the determination of absolute recovery in a given sample are available from the National Institute of Standards and Technology (Inn 1987). 

Most often the transmission mode is found to be the most convenient in Mossbauer spectroscopy, i.e., the y radiation passes from the source through the absorber, and the attenuation of the primary beam is measured at the various Doppler velocities. However, there are a number of cases where a "scattering geometry may be advantageous (SO). The basis for this geometry lies in those processes that take place after resonant absorption of y radiation by the Mossbauer isotope. Specifically, after excitation the Mossbauer isotope may reemit the y ray, or it may decay by emission of internal conversion electrons and X rays [with the probability of internal conversion equal to a/(l + a)]. 

The results for the three HgO-forming substrates, Tf.O, 02, N20, were, reviewed by Gunning (27) in 1958. The water reaction was found to give an enrichment with both 198llg and 202Hg lamps. The isotopically specific route in the overall primary process was estimated from the observed enrichment, in the HgO product to be 8%. Pertel and Gunning (2fi) found a very marked increase in the enrichment in the HgO... 

On the basis of this simplified mechanism, which neglects the minor side reactions involving butadiene, a quantum yield [dq.mca)] of 0.3b was calculated for the isotopically specific primary step but a more valid statement would he that the quantum yields for the isotopically specific and nonspecific primary stops (9a) and (9b) are in the ratio, 0.3.1/0.6b = 0.56. 

In order to examine the effect of chain length on the isotopically-specific reaction, a brief investigation of n-propyl chloride and ti-amyl... 

Isopropyl chloride and (erf-butyl chloride were studied to determine the influence, on the primary process, of increasing alkyl substitution on the a-carbon atom. The data listed in Table V show that the efficiency of the isotopically specific primary step gradually decreases as the alkyl substitution on the a-carbon atom increases (assuming that the primary quantum yield for C-Cl bond scission remains at unity). 

Vinyl chloride is the only unsaturated chloride which has been studied by monoisotopic photosensitization. The abundance of 20211 g in the calomel product, as a function of substrate pressure, is shown in Figure 19. The highest value obtained was 0.5 2. Tire measured value of the quantum yield for calomel formation was 0.19 and, for the isotopically specific step, 0.053. In order to explain the low efficiency in the calomel-forming primary step a third primary route was postulated ... 

In spite of this very small range of absorption there is a preferential absorption by a single isotope when a mercury resonance line from a single isotope is used. The quantum yield of the isotopically specific step (50) with methyl chloride is 0.28 and of course part of the nonspecific step (49) will also be brought about by the isotope used to make the resonance lamp. 

Other features Surface-sensitive isotope-specific can distinguish molecular weight, molecular structure some capability for depth profiling and imaging... 

Isotope Isotopic specification is indicated by prefixing the atomic symbol with a number equal to the integral isotopic mass—for example, [2H] for deuterium and [13C] for carbon-13. 

Reaction (VIIl-2b) explains why the quantum yield of isotopically specific 202HgCI formation is much less than unity, although process (VIII-1), the selective production of 202Hg from the isotopic mixtures, is near unity. Reactions (VIII-3) and (VI11-4) increase the nonspecific formation of HgCl and consequently decrease enrichment. The addition of butadience is found to increase enrichment by scavenging H atoms... 

Introduction to Derivatization For many years derivatization by alkylation -especially as ethylation, but also as propylation - has been applied to transform Hg species into volatile Hg species before measurement with hyphenated techniques [2, 50, 52]. Sodium tetraphenylborate (NaBPh4) was also used for derivatization prior to measurement with GC-MIP-AED [53], Studies of possible species transformation, for example, during the analytical procedure, have been carried out with isotope-specific determination methods. The results showed that a direct ethylation of Me-Hg in an atmospheric precipitation sample by NaBEt4 produced no significant amount of artifactual Me-Hg [54]. Others investigated the species transformation processes in synthetic solutions to simulate environmental matrices. From the experiments it could be concluded that the species conversion, for example, of Me-Hg into zerovalent Hg, depends on the concentration levels of the halide [2]. Furthermore, the procedural order is of great importance, for example, ethylation should be done after addition of the organic phase to avoid species transformation [55]. 

Atoms without implied hydrogens or with any specific properties, such as charge, atomic mass for isotope specification, and so on, are given in square brackets. Thus, C is methane and [C] is elemental carbon C[13C](=0)0 is acetic acid with 13C atom in the carboxyl group, and CC(=0)[0-] is acetate anion. 

Not every actinide element has known or well-developed organometallic chemistry. By far the most research has been done on thorium and uranium compounds, a consequence of favorable isotope-specific nuclear properties and, at least until recently, the commercial availability of key starting materials such as Th metal, anhydrous ThCLi, U metal, and anhydrous UCL. Thorium chemistry is dominated by the -F4 oxidation state and has some similarities to the chemistry of the heavier group 4 metals. For uranium, one can access oxidation states from d-3 to 4-6 in organic media. Although there are some similarities to the chemistry of the heavier group 6 elements, for example, tungsten, there are also some remarkable differences made possible by the availability of the 5f valence orbitals. 

Arsenic is present mainly in ionic forms, which are usually separated by high-performance ion-exchange chromatography coupled to an atom- or isotope-specific... 

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