Radioactive labeling tracers

The isotopic dilution radioimmunoassay is based on the principle of saturation analysis, which is outlined in Figure 1. A test compound (in this case insulin, represented by X) is quantified by its ability to progressively saturate a suitable reagent (in this case an antibody specific for insulin, represented by Y). In the first step, a known quantity of radioactively labeled tracer insulin (X ) is added to the sample. 

Because dideoxynucleotides lack 3 -OH groups, these nucleotides cannot serve as acceptors for 5 -nucleotide addition in the polymerization reaction, and thus the chain is terminated where they become incorporated. The concentrations of the four deoxynucleotides and the single dideoxynucleotide in each reaction mixture are adjusted so that the dideoxynucleotide is incorporated infrequently. Therefore, base-specific premature chain termination is only a random, occasional event, and a population of new strands of varying length is synthesized. Four reactions are run, one for each dideoxynucleotide, so that termination, although random, can occur everywhere in the sequence. In each mixture, each newly synthesized strand has a dideoxynucleotide at its 3 -end, and its presence at that position demonstrates that a base of that particular kind was specified by the template. A radioactively labeled dNTP is included in each reaction mixture to provide a tracer for the products of the polymerization process. 

The isotopic dilution method can be extended to non-radioactive tracers by using mass spectrometry or NMR to determine the variation in isotopic ratios. This method can be used for the measurement of molecules or elemental species (about 60 elements have stable isotopes). This approach allows ultra-trace analysis because, contrary to radioactive labelling where the measurement relies on detecting atoms that decompose during the period of measurement, all labelled atoms are measured. Isotopic mass spectrometers are well suited for these measurements. 

Canna mdica -for radioactive labeling [RADIOACTIVE TRACERS] (Vol 20)... 

At the Mellon Institute he applied l4C tracers to examine the behavior of intermediates in Fischer-Tropsch synthesis over iron catalysts. By adding small amounts of radioactively labeled compounds to the CO/H2 synthesis gas mixtures, he was able to prove that some of these compounds (e.g., small alcohols) are involved in the initiation step of the chain growth process that leads to larger hydrocarbon products. It was during this era that his associates first placed a catalytic reactor into the carrier gas stream of a gas chromatograph and developed the microcatalytic pulse reactor, which is now a standard piece of equipment for mechanistic studies with labeled molecules. While at Mellon Institute Emmett began editing his comprehensive set of seven volumes called Catalysis, which he continued at Hopkins. 

To a solution of known titre antiserum is added a known concentration of radioactive or tracer hapten (antigen) and an aliquot of the plant extract. There will be a competition of the labelled hapten (added) and unlabelled (plant extract) hapten for the fixed number of antibody sites (antiserum of known titre) and this results in some of the labelled hapten being bound while the remainder remains free. The distribution of the radioactive hapten between the free and bound state will be a function of the amount of unlabelled hapten present in the assay tube. 

Flux measurements (Schafer et al. 1974). The collection rate (Vc, nl/min) can be measured by the constant bore collection pipette by timed collections. Radioactive tracers can be added to the lumen or bath fluid. For instance, radioactively labelled inulin can be added to the perfusate (Inp) and can be used to measure volume absorption DV = perfusion rate (Vj-Vc). Unidirectional fluxes, bath to lumen and lumen to bath, for any given substance can be quantified, and permeabilities (Px) can be determined ... 

There are a number of tracers that have been used to help understand chemical reactions and interactions. Historically, development of modem tracer methods began with the pioneering work of the Hungarian physical chemist, George Charles de Hevesy, in the early 1900s. De Hevesy s work focused on the use of radioactive tracers to study chemical processes, for which he was awarded the Nobel Prize in Chemistry in 1943. Radioactive tracers, also known as radioactive labels, are based on the use of a given radioisotope. However, it is important to note that there are also isotopic tracers (or isotopic labels). Isotopes are forms of a chemical element with different atomic mass, which have nuclei with the same atomic number (i.e. number of protons) but different numbers of neutrons. Examples include H, " C, and which are radioactive forms of stable elements... 

The concept of using a tracer species to measure the mixing characteristics is not limited to chemical reactors. In the area of pharmacokinetics, the time course of renal excretion of species originating from intravenous injections in mans wa s resembles the input of a pulse of tracer into a chemical reactor. Normally, a radioactive labeled ( H, etc.) version of a drug is used to follow the pharmacokinetics of the drug... 

The uptake of trace metals from the soil by plants and animals can be studied with high sensitivity by radiotracer techniques. In these applications, it is important that the chemical form of the radiotracer is identical with that of the trace element to be studied. For example, in agriculture, the uptake of trace elements necessary for plant growth can be investigated. Essential trace elements, such as Se, are of special interest. By use of radioactively labelled selenium compounds the transfer of this element from soil to plants and animals can be measured. For the investigation of the transfer of radionuclides (radioecology), addition of tracers is, in general, not needed. 

No radioactive isotopes. Since the assay is noncompetitive, DNA adducts are detected without the use of highly labeled tracer molecules, and immunocomplexes can be detected enzymatically using a colorimetric assay. 

The earliest immunoassays made use of radioactively labeled antigens or antibodies. These analytes are referred to as radiolabeled immunoassays. Antibody binding sites are extremely specific and to retain this specifity the best option would be to replace a non-radioactive isotope in the tracer molecule by its radioisotope (e.g., replace hydrogen by H). However when the substitution is made in a part of the molecule away from the antibody binding site, the choice of radioisotope can be governed by other considerations, such as half-life, availability, high activity, and radiochemical purity. 

There are also several methods to determine patterns of fate and transport of pollutants in the environment. In some cases, microcosms and me-socosms are used to study fate, biodegradability, bioavailability, and transport within compartments. Field surveys may also be used to study fate and transport of pollutants in contaminated environments. Such studies involve collection and analysis of biota, water, air, soil, or sediment. In some cases, radioactively labeled contaminants ( tracers ) may be introduced in mesocosms or noncontaminated environments in order to determine their fate and patterns of transport. Finally, mathematical models are often used to produce computer simulations to... 

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