Isotopes selecting

Quantitative measurement of light elements (particularly hydrogen) in solid materials, without standards has isotope selectivity... 

NRA is an effective technique for measuring depth profiles of light elements in solids. Its sensitivity and isotope-selective character make it ideal for isotopic tracer experiments. NRA is also capable of profiling hydrogen, which can be characterized by only a few other analytical techniques. Future prospects include further application of the technique in a wider range of fields, three-dimensional mapping with microbeams, and development of an easily accessible and comprehensive compilation of reaction cross sections. 

Complementary to other methods that constimte a basis for the investigation of molecular dynamics (Raman scattering, infrared absorption, and neutron scattering), NIS is a site- and isotope-selective technique. It yields the partial density of vibrational states (PDOS). The word partial refers to the selection of molecular vibrations in which the Mossbauer isotope takes part. The first NIS measurements were performed in 1995 to constitute the method and to investigate the PDOS of... 

In summary, NIS provides an excellent tool for the study of the vibrational properties of iron centers in proteins. In spectroscopies like Resonance Raman and IR, the vibrational states of the iron centers are masked by those of the protein backbone. A specific feature of NIS is that it is an isotope-selective technique (e.g., for Fe). Its focus is on the metal-ligand bond stretching and bending vibrations which exhibit the most prominent contributions to the mean square displacement of the metal atom. 

The direct determination of sulfur in bisphenol A at trace levels by SS-ETV-ICP-MS has been described using 34S as the isotope selected [206]. Table 8.67... 

The single filter elements (as shown in Figs. 17.4a-c) are often called X-halffilters since each of them acts only in one dimension of a 2D experiment, to be distinguished from Xfilters that select (or suppress) 1H-X pairs in both dimensions of a 2D experiment [17, 20, 21]. Of course, X-half filters can be employed twice in a 2D experiment, to yield isotope selection in both dimensions (see Sect. 17.3.3). 

Braden B, Haisch M, Duan LP et al (1994) Clinically feasible stable-isotope technique at a reasonable price - analysis of 13C02/12C02-abundance in breath samples with a new isotope selective nondispersive infrared spectrometer. Zeitschrift fur Gastroenterologie 32(12) 675-678... 

Radionuclides. Isotope selection was based primarily on three experimental requirements (1) both alpha and... 

The monochromaticity of laser beams from a tunable source permits selective photochemistry, even isotopically selective ones. For example,... 

Photolysis is carried out on IR laser irradiation and is isotope selective reaction, as the molecules containing 238U turn to be more sensitive to its action and the samples become finally 235U-enriched. 

The magnet of an on-line mass separator will generally assure the selectivity in respect to different isotopes. Hence, the isobaric selectivity of laser ion sources is of primary interest, although isotopic selectivity might also be achieved by RIS using the HFS and IS, which differ from isotope to isotope. 

Whereas isotope selectivity during bacterial oxidation of CH has been measured (8,9), there are few data relevant to TSR. Consequently, laboratory experiments were conducted to measure carbon isotope fractionation accompanying the oxidation of methane, ethane, and propane. 

The oxidation of methane was very slow under the experimental conditions employed The slowest rates are those with anhydrite as oxidant. Because the ratio of the rate constants, a, is dependent upon the oxidant, it is difficult to estimate the carbon isotope selectivity during sulfate reduction at temperatures relevant to TSR in sour gas occurrences. However, the effects are substantial with the cupric oxide-manganese dioxide and hematite-anhydrite trends in Figure 2 giving extrapolated a-values of about 1.02 and 1.04 respectively at 200°C. 

Nuttall et al. [231] reported the use of ICP-MS for routine determination of As, Bi, Cd, Pb, Sb, Te, and T1 in a clinical laboratory. Blood was deproteinized, centrifuged, and passed through a 2-jjim filter before analysis. Other specimens were diluted or digested with dilute mineral acids. Se, Zn, and As suffered from interferences that could be overcome by proper isotope selection or mathematical correction. The authors [231] concluded that Cr and Fe were too prone to interferences (due to spectral overlaps) to be practically measured by ICP-MS in clinical samples. 

Some refractory elements cannot be determined by ET-AAS at the levels usually present in waters. That is the case with M. El Himri et al. [28] developed a fast and accurate procedure, without any prior treatment, to analyze tap and mineral waters from Spain and Morocco for this highly toxic element. ICP-MS was employed. The analytical isotope selected was 238U, with Rh as internal standard. An LoD of 2ngl 1 was obtained. The estimated repeatability was 3 percent at the concentration level of 73 ng l-1. The method was validated by comparison with a radiochemical procedure devised for natural samples and by analysis of a Certified Reference Material (CRM). Multi-element capabilities of ICP-AES have also been employed for surveys of trace elements. Al-Saleh and Al-Doush [29] reported the concentrations of dissolved Be, Cd, Cr, Cu, Fe, Mg, Mn, Hg, Ni, Se, Sr, V, and Zn in 21 samples of retail bottled waters from Riyadh, Saudi Arabia. It was found that Cd, Fe, Hg, Ni, and Zn were present at concentrations higher than the limits recommended by the EU and World Health Organization (WHO) guidelines. 

Since the dissociation of the abundant H2 and CO molecules result from photoabsorption at discrete wavelengths, isotopically selective photodissociation based on self-shielding is possible. Two conditions are required for this (1) dissociation via line absorption for each isotopically substituted molecule, and (2) differential photolysis that depends upon the isotopic abundances. Self-shielding occurs when the spectral lines leading to dissociation of the major isotopic species optically saturate, while the other residual lines relevant for dissociation of the minor isotopes remain transparent. As a consequence, such photolysis depends on nucleic abundance rather than the mass of a molecule see Fig. 4.4 (Langer 1977 Thiemens Heidenreich 1983). 

The absorption cross-sections for the isotopically substituted species are the same and thus for a given photon flux and path length, the photon absorption scales with the abundance of the isotopolog. Under conditions of the interstellar medium and solar nebula, isotope-selective chemistry may occur for abundant CO molecules (e.g. Dalgarno Stephens 1970 Thiemens Heidenreich 1983 Draine Bertoldi 1996 Lee et al. 1996 van Dishoeck Black 1988). 

Other mass spectrometric techniques such as RIMS and AMS possess high isotope selectivity for extreme ultratrace and isotope analysis of, in particular, radiotoxic isotopes ( C, "Ca, Sr, Tc, Pb, U and plutonium isotopes) in the environment, in cosmochemistry, radiodating, nutrition and biomedical research. RIMS has become as an nltrasensitive and selective analytical technique for the determination of extremely low isotope abundances. In spite of the excellent analytical features of RIMS (detection limit for isotopes 10 atoms per sample) and exciting applications for the determination of extremely low abundances and isotope ratios of long-lived radionuclides, such as all plutonium isotopes (including Pu), U or " Ca, but no commercial instrument is available on the analytical market. 

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