Isotope tracer experiments using

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. 

Since NRA focuses on inducing specific nuclear reactions, it permits selective observation of certain isotopes. This makes it ideal for tracer experiments using stable isotopes. Generally, there are no overlap or interference effects because reactions have very different Qvalues, and thus different resultant particle energies. This permits the observation of species present at relatively low concentrations. A good example is oxygen and O can be resolved unambiguously, as they are... 

NO. The same parameters were then used to predict the responses for the isotopic tracer experiments reported in Figs. 9 and 10. 

To further test the model, calculations were performed to simulate the isotopic tracer experiments presented in Figs. 9 and 10. It should be noted that while the tracer experiments were performed at 438K, the rate coefficients used in the model were chosen to fit the experiments in which chemisorbed NO was reduced at 423 K. Figures 21 and 22 illustrate the nitrogen partial pressure and surface coverage responses predicted for an experiment in which 5 0 is substituted for l NO at the same time that H2 is added to the NO flow. Similar plots are shown in Figs. 23 and 24 for an experiment in which NO is substituted for during steady-state reduction. 

Figure 12. Extent of dissolution and re-precipitation between aqueous Fe(III) and hematite at 98°C calculated using Fe-enriched tracers. A. Percent Fe exchanged (F values) as calculated for the two enriched- Fe tracer experiments in parts B and C. Large diamonds reflect F values calculated from isotopic compositions of the solution. Small circles reflect F values calculated from isotopic compositions of hematite, which have larger errors due to the relatively small shifts in isotopic composition of the solid (see parts B and C). Curves show third-order rate laws that are fit to the data from the solutions. B. Tracer experiment using Fe-enriched hematite, and isotopically normal Fe(lll). C. Identical experiment as in part B, except that solution Fe(lll) is enriched in Te, and initial hematite had normal isotope compositions. Data from Skulan et al. (2002).

A multiple ion collector device is required for the simultaneous determination of separated ion beams in precise and accurate isotope ratio measurements in order to study, for example, isotope fine variation in Nature or during tracer experiments using enriched stable isotope tracers. In thermal ionization mass spectrometers or in ICP-MS, mostly a system of several Faraday cups (up to 16) and/or ion counters (electron multipliers) is applied. In the photographs in Figures 4.7 and 4.8 examples of multiple ion collector systems are shown from the mass spectrometers MC-ICP-MS... 

For application in the isotope dilution technique and for tracer experiments using isotope enriched spikes, Merck (Darmstadt, Germany) have launched the production of new sets of isotopically enriched materials, such as 53Cr or mCd enriched spike calibration solutions, which were characterized by the Institute for Reference Materials and Measurements (IRMM, Geel, Belgium).52... 

Tracer experiments using enriched isotopes tracers or long-lived radionuclides... 

Activation with 14 MeV neutrons has been used to determine the oxygen content of various metals such as beryllium 20>, Cl, F, O, Na, Si, and various rare earths in complex molten salt electrolytes 45>, the protein content of food products by means of the nitrogen content 46>, i60/180 and 14N/15N isotopic ratios in stable isotope tracer experiments 47,48), and in a wide variety of other applications. One application we... 

Another reactor equation useful in reaction kinetics analyses represents the transient CSTR. This situation is encountered in temperature-programmed desorption and isotope tracer experiments. The material balance for the transient CSTR operating at constant total pressure for an ideal gas is... 

The composition and reactivity of the carbon laid down during the initial stages of the propane dehydrogenation reaction was examined by transient isotope labelling experiments using [2-]3C]-C3HgandC3JHs as tracers in a series of reactions in a pulsed flow microcatalytic reactor. In these experiments alternate series of labelled and unlabelled propane pulses were passed over the catalyst sample and the products analysed by glc and mass spectrometry. 

The calculated values are those obtained by assuming the ratedetermining step to be abstraction of an allylic hydrogen with an isotope effect equal to the discrimination effect obtained from the retention of deuterium in the products. The observed kinetic isotope effects show clearly that this first abstraction is the slow step. The good agreement with the discrimination values gives excellent confirmation for the stepwise mechanistic scheme proposed. In the tracer experiments using deuterium it was found that appreciable propylene isomerization occurred unless ammonia was present. The oxidation rate was unaffected by the ammonia. This was taken to indicate that the initial oxidative abstraction reaction had no carbonium ion characteristic. However, this conclusion does not apply in the case of cuprous oxide, where ammonia very severely inhibited the oxidation reaction. 

We propose the biosynthetic pathway of the carbon framework of matrine as shown in Fig. 4. This scheme also indicates the pathway for the formation of sparteine and lupanine. The former part of this scheme was proposed by Wink et al. [63], with minor modification by Leete [64], from the in vitro experiments using isolated chloroplasts of leaves of Lupinus. They postulated the presence of 17-oxosparteine as the first key intermediate for the formation of lupanine and sparteine [63]. However, this hypothesis involving 17-oxosparteine synthase was not confirmed by the tracer experiments using and independently conducted by the groups of Spenser [65, 66] and Robins [67]. They, in turn, hypothesized the pathway involving the diiminium cation (73) as the tetracyclic intermediate [68, 69]. The postulation of the presence of this reactive intermediate is consistent with the results of isotope incorporation into lupanine and sparteine. The biosynthetic scheme of matrine can be also drawn by involving the electronically equivalent diiminium cation (76) preceded by additional 1,3-hydride shift or imine-enamine isomerization (74 75). All these reactions take... 

A quite different use of isotopes in mechanistic studies involves their use as labels for ascertaining the location of a given atom involved in a reaction. As in kinetic experiments, the isotopic substitution will not qualitatively affect the course of the reaction. The nuclei most commonly used for isotopic tracer experiments in organic chemistry are deuterium, tritium, and the and isotopes of carbon. There are several means of locating isotopic labels. Deuterium can frequently be located by analysis of NMR spectra. In contrast to the normal isotope, deuterium... 

Finally, in the context of elemental assay via isotope dilution and tracer experiments using stable isotopes, an isotopically enriched spike or tracer is added to a sample wherein the element of interest typically shows the natural isotopic composition. These isotopically enriched spikes are another example of human-made variations. 

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