Isotopes radiotracer

Reaction or exchange with stable isotopic tracers and quantitative identification of all products by mass spectrometry provides indications for molecular interactions on the surface. Reactions can be studied at steady state or by following the transient distribution of isotopic products. Langer and co-workers (25,26) presented the first steady-state mechanistic analysis for the electrocatalytic hydrogenation of ethylene on Pt in deuterated electrolytes. Proton abstraction in electroorganic synthesis has also been verified using deuterated solvents (374, 375). On-line mass spectrometry permitted indirect identification of adsorbed radicals in benzene and propylene fuel cell reactions (755,795,194). Isotopic radiotracers provided some notion on adsorption isotherms (376, 377) and surface species on electrocatalysts (208, 378, 379). 

Isotope Dilution Assay. An isotope dilution assay for biotin, based on the high affinity of avidin for the ureido group of biotin, compares the binding of radioactive biotin and nonradio active biotin with avidin. This method is sensitive to a level of 1—10 ng biotin (82—84), and the radiotracers typically used are p C]biotin (83), [3H]biotin (84,85) or an I-labeled biotin derivative (86). A variation of this approach uses I-labeled avidin (87) for the assay. 

A most important technique which has been developed as an extension of the isotope dilution principle is that of radioimmunoassay (RIA). Analyses by this method employ substoichiometric amounts of specific binding immuno-chemical reagents for the determination of a wide range of materials (immunogens) which can be made to produce immunological responses in animals such as sheep or rabbits. It is possible to combine the specificity of an immunochemical reaction with the extreme sensitivity of radiotracer detection. Analytical methods based upon these principles have achieved wide applicability in the determination of organic compounds at trace levels. 

The chemical reactions involving positron emitters have to be specially designed to take into account the short half life of the radionuclide, the limited number of labelled precursors and the sub-micromolar amounts of these precursors. Moreover, the reactions must be possible without any addition of the stable isotope (especially when ligands of receptors are synthesized). Several practical considerations that influence the design of positron-emitter radiotracers with a high specific radioactivity and their experimental handling have been reviewed [4,8,14-19]. 

Fitzgerald et al,54 studied the monitoring of calcium metabolism in patients in the final stages of renal disease using serum by AMS after isotopic labelling with a 41 Ca radiotracer. The authors hypothesized that bone resorption can be studied directly by serial measurements of the 41Ca/Ca,otal ratio in serum after in vivo labelling of the calcium pool with 41 Ca.54... 

The use of the isotope effect to study rate-determining steps in a sequence of chemical reactions represents an additional advantage of radiotracer methodology. The term isotope effect (to be discussed more fully later) refers to the influence on a reaction rate of the difference in the masses of isotopes. This effect may create significant problems in the use of radioisotopes as tracers but can, nevertheless, be used to advantage in a limited number of cases in order to understand the kinetics of certain chemical reactions. 

The use of radiotracers is dependent on certain basic assumptions being fulfilled. The first assumption, mentioned above, is that the radioactive isotopes of a given element behave identically as the stable isotopes of the same element. Actually, this assumption is not exactly true. The difference in masses between radiotracer nuclei and stable nuclei can cause a shift in the reaction rate or equilibria (the isotope effect). It is true, however, that in most cases the isotope effect does not significantly affect the utility of the radioisotope method. Since the degree of chemical bond stability due to vibrational motion is directly related to the square root of the masses of the isotopes involved, it is apparent that an isotope effect will be of significance only for elements of low atomic weight (at wt <25). 

Most importantly, the radionuclide and the stable nuclide must undergo isotopic exchange. In practice, this means that the tracer and the stable atom must be in the same redox state. By heating or using redox cycles, the experimenter must assure this to be true. Anomalous experimental results have frequently been traceable to the chemical form of the administered radiotracer. Since reactor production of radionuclides often results in side reactions (see Chapter 10), various oxidation states may be present when the sample is produced. In one case involving phosphate-32P uptake in plants, the unexpected experimental results were explained by the fact that a large percentage of the tracer dose was actually in the form of phosphite-32P. 

The use of radiotracers is an excellent technique for measuring the solubility product constant of sparingly soluble salts or for making other studies of substances present in low concentrations. Another very important and classic example of the use of radiotracers is that of studying the occurrence and properties of isotopic exchange reactions—reactions of the type... 

Up to now in our discussion of radiotracers, we have assumed that all isotopes of a given element, stable or radioactive, would behave alike chemically and physically. We will now examine this point more critically to see how different isotopes behave and how this difference in behavior (the isotope effect) can be detected and used to... 

The isotope 40K can be analyzed in natural water samples with the Cherenkov counting technique.2 3 Because of the lack of a suitable radiotracer for K and the similarity between the chemistries of rubidium and potassium, 86Rb can be used as a tracer for K.4 Also, thermal ionization mass spectrometry (TIMS) has been used to determine 40K in environmental samples. The interference of mass 40 can be solved by double spiking with 43Ca/48Ca the procedure for the routine high-precision isotope analysis of the K-Ca system will then be free of Ca fractionations.5... 

The radioisotope technetium-99 is often used as a radiotracer to detect disorders of the body. It has a half-life of 6.01 hours. If a patient received a 25.0-mg dose of this isotope during a medical procedure, how much would remain 48.0 hours after the dose was given ... 

The adsorption of Cd(II) and oxalate or citrate ions was calculated from an uptake of their concentration from solution. The ion concentration in solution was determined by the radiotracer method. For labelling the solution, 14C or 115Cd isotopes were used. Radioactivity of the solutions before and after adsorption was measured using a liquid scintillation counter (LS5000 TD Beckmann). Co-adsoprtion of Cd(II) and carboxylate ions was determined simultaneously. The adsorption of measured ions was conducted in a pH range from 3.0 to 10.0. 

Gel electrophoresis (GE) is a common separation technique in protein analysis and it has also been used for the speciation of metals bound to proteins [86]. In most applications, metals have been detected by autoradiography, limiting the studies to those elements for which a relatively stable radionuclide exists [87]. As an example, 75Se radiotracer allowed Se to be detected after two-dimensional GE (2-DE) separation [88]. Owing to the high sensitivity and isotopic capability of ICP-MS, this technique has been proposed as the detection tool of choice for elements in gel. The efbcient transport of the sample from the protein spot on gel to plasma has been achieved by laser ablation (LA) [89, 90] and electrothermal (ET) atomization [62, 91] techniques. The... 

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