Isotopic labeling tracer techniques

The ZLC method offers advantages of speed and simplicity and requires only a very small adsorbent sample thus making it useful for characterization of new materials. The basic experiment using an inert carrier (usually He) measures the limiting transport difiiisivity (Do) at low concentration. A variant of the technique using isotopically labeled tracers (TZLC) yields the tracer diffiisivity and counter diffusion in a binary system may also be studied by this method. To obtain reliable results a number of preliminary experiments are needed, e.g. varying sample quality, nature of the purge gas, the flow rate and, if possible, particle size to confirm intracrystalline diffusion control. 

Many of the macroscopic techniques can be apphed to the measurement of self-diffusion by using isotopically labeled tracers. Such methods, first introduced by Barrer and Fender [97], have been widely applied in order to obtain data which should be directly comparable with microscopic self-diffusion measurements. Such comparisons are presented in several of the chapters within the present volume. 

ICP-MS offers not only unequalled sensitivity and selectivity for a majority of elements in the Periodic Table, but is also a robust detector that can (more or less easily) be coupled to any of the aforementioned chromatographic separation techniques. Moreover, the multi-element and multi-isotope abilities offer invaluable potential for environmental studies. For example, for mercury, isotopically labelled tracers have been introduced into the environment or used in laboratory experiments, which has shed light on extremely complex reaction pathways. ... 

Work on the biosynthesis of cholesterol began in earnest after Rudolf Schoenheimer and David Rittenberg, at Columbia University, developed isotopic tracer techniques for the analysis of biochemical pathways. In 1941, Rittenberg and Konrad Bloch were able to show that deuterium-labeled acetate (C2H, COO ) was a precursor of cholesterol in rats and mice. In 1949, James Bonner and Barbarin Arreguin postulated that three acetates could combine to form a single five-carbon unit called isoprene. 

The methods described so far for studying self-diffusion are essentially based on an observation of the diffusion paths, i.e. on the application of Einstein s relation (eq 3). Alternatively, molecular self-diffusion may also be studied on the basis of the Fick s laws by using iso-topically labeled molecules. As in the case of transport diffusion, the diffusivities are determined by comparing the measured curves of tracer exchange between the porous medium and the surroundings with the corresponding theoretical expressions. As a basic assumption of the isotopic tracer technique for studying self-diffusion, the isotopic forms are expected to have... 

Imai, T., and Terashima, N., 1991, Determination of the distribution and reaction of polysaccharides in wood cell-walls by the isotope tracer techniques. II. Selective radio-labeling of pectic substances in mitsumata Edgeworthiapapyrifera). Mokuzai Gakkaishi 37 733-740 (in English). 

Tracer techniques have revolutionized biochemistry and molecular biology. For example, the availability of isotopically labeled compounds made it possible to demonstrate that macromolecules such as proteins, nucleic acids, and complex lipids are synthesized from simple cellular metabolites and provided many insights into the mechanisms and control of the synthetic events. The utility of radiochemical techniques is afforded by (1) their great sensitivity compared to other analytical methods (Table 3-1) and (2) the fact that they label the atoms of molecules without significantly altering their chemical properties, thus allowing them to be traced or followed from one molecule to another. 

Steroid hormones by the application of isotope tracer techniques 3I8-321]. Lieberman and co-workers undertook urinary metabolic studies [304,325] by administration of isotopically labeled steroids followed by the determination of specific activities of urinary metabolites derived only from the administered substance. Mathematical analyses were performed by compartmentalization of the metabolism [385]. One- or two-compartment systems are most frequently used [304,306,384,386,387]. A compartment is defined as a particular species (e.g., dehydroepiandro-sterone sulfate) in a particular space (e.g., peripheral organ). It is assumed that a species entering a compartment mixes immediately with the whole compartment. Two factors were responsible for the institution of com-partmental analysis ... 

The mechanism of osazone formation has been studied by the isotopic-tracer technique, by using an N -labeled arylhydrazone, treating it with an unlabeled arylhydrazone, and making an isotopic assay of the reaction products. The results were characteristic of Weygand s mechanism involving the oxidation of the hydrazone to a l-imino-lV -2-keto derivative (40) and subsequent osazone formation with the elimination of ammonia-iV . 

As shown by Helfferich and Peterson [110], if a small pulse of one of the mixture components, isotopically labeled, is injected in a multicomponent solution, the labeled component moves at a velocity that is proportional to the slope of the corresponding chord of the isotherm. In the same time, as many system peaks as there are components in the mixtiure arise and move at velocities related to the slopes of the corresponding isotherms (see Figure 4.27a,c,e). This assumes that equilibrium has previously been reached throughout the column. In the tracer pulse technique, only the labeled component is detected. Successive injections of a sample of each labeled component (or the simultaneous injection of all of them if their separate identification is possible) permit the direct determination of the competitive isotherms of all the components. From a theoretical point of view, the presence of large concentrations of the other components does not complicate the measurement nor its evaluation, since the retention time of each isotopic pulse is a linear fimction of the slope of the corresponding chord Aq / AC, see Eq. 2.15). 

In the competitive technique, the enzyme reacts with a mixture of labeled and unlabeled substrate, yielding isotope effects on k t/Ku [29]. Competitive measurements, while limited to kcat/KM isotope effects, are substantially more precise than noncompetitive measurements. In addition, they allow the use of tracer-level radioactive labels, permitting tritium isotope effects at the primary and secondary positions (kH/kj or ko/kj) to be determined. General methods for determining competitive isotope effects have been published [17b]. One drawback is that multiple isotopic labels must often be used, leading to extensive synthetic efforts. 

In chemical reaction kinetics, isotope-labelled reactants are frequently employed to follow a reaction pathway and to determine the reaction mechanism (see Chapter 7.6). The isotopic tracer technique is a useful tool in catalyst surface analysis, because it enables determination of whether the adsorbed species present on the surface during the reaction are by-products or reaction intermediates. One of the adsorbed species is labelled by an isotope atom and its rate of disappearance is followed by surface spectroscopy. Simultaneously, its rate of appearance in the product molecule is followed by mass spectrometry. When both rates are identical, it can be concluded that the observed adsorbed species is the reaction intermediate. 

Quantitative Applications - The use of stable-isotope-labeled compounds as internal standards for the quantitation of drugs and metabolites in biological fluids offers a unique combination of sensitivity and selectivity of detection for pharmacokinetic studies. The principles of the technique have been outlined, and applications up to 1981 have been compiled. Specific aspects of the isotope dilution method, e.g. its utility as a reference technique and associated procedures for handling the data generated from the use of multiple isotope tracers simultaneously, have been discussed. Examples of isotope dilution methods used in clinical psychopharmacology have also been reviewed. ... 

Isotopic labeling of selected components of the system frequently provides an answer to such questions. The technique possesses many different aspects adapted to individual cases. As an example, let us cite the many investigations of Melvin Calvin and coUaborators over a period of twenty years. In these studies, systematic use was made of c u bon dioxide labeled with C to elucidate the early steps of photosynthesis and the network of reactions involved in the assimilation of carbon dioxide by green plants. An important variation of the technique consists in the use of tracers in a kinetic sense. Among the several possibilities of this kind proposed by Neiman, one deserves special attention. 

With radioisotopes now available for many elements, the tracer technique became generally applicable. New variants were developed, such as neutron activation analysis, which was introduced in 1936 for the determination of dysprosium in rare-earth samples (Hevesy and Levi 1936) and subsequently became a widely used technique for sensitive trace analyses, particularly when much larger neutron fluxes became available with the advent of nuclear reactors. Another frequently applied method for trace determination is isotope dilution the species to be determined in the sample is diluted by addition of a known amount of the same species labeled with known specific activity. From the specific activity then resulting and measured, the original quantity of the species is derived, even if only a fi action of the species is finally recovered. The impact on biosciences was revolutionary, when suitable isotopes of key elements in the biosphere were soon discovered (Ti/2 = 10 min) was one of I. Curie and... 

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