Radioisotope-labeled substances

There are two principal methods for determining the susceptibility of skin to penetration by toxicants. The first of these is measurement of the dose of the substance received by the organism using chemical analysis, radiochemical analysis of radioisotope-labeled substances, or observation of clinical symptoms. Secondly, the amount of substance remaining at the site of administration may be measured. This latter approach requires control of nonabsorptive losses of the substance, such as those that occur by evaporation. 

The essential criteria for a useful analytical technique are specificity, sensitivity, accuracy, precision, simplicity, rapidity, economy, wide applicability, and freedom from hazard. It is well known that radioimmunoassay (RIA) was developed in 1959 by Yalow and Berson (Yl). Since then the radioimmunoassay method has been widely used in the field of clinical chemistry. Radioimmunoassay has inherent in it the advantages listed above. However, this method always requires special facilities for use and disposal of radioisotopes and consideration must be given to the fact that the labeled substances have short half-lives. Immunoassay methods are explosively increasing in use and development as an analytic technique in basic science as well as in clinical laboratory medicine (L1-L3, VI). With these points as background, efforts have been made to develop nonisotopic immunoassay methods or alternative immunoassay methods that are based on antigen-antibody reactions but do not involve use of a radioisotope. 

The basic information in the study of sorption processes is the quantity of substances on the interfaces. In order to measure the sorbed quantity accurately, very sensitive analytical methods have to be applied because the typical amount of particles (atoms, ions, and molecules) on the interfaces is about I0-5 mol/m2. In the case of monolayer sorption, the sorbed quantity is within this range. As the sorbed quantity is defined as the difference between quantities of a given substance in the solution and/or in the solid before and after sorption processes (surface excess concentration, Chapter 1, Section 1.3.1), all methods suitable for the analysis of solid and liquid phases can be applied here, too. These methods have been discussed in Sections 4.1 and 4.2. In addition, radioisotopic tracer method can also be applied for the accurate measurement of the sorbed quantities. On the basis of the radiation properties of the available isotopes, gamma and beta spectroscopy can be used as an analytical method. Alpha spectroscopy may also be used, if needed however, it necessitates more complicated techniques and sample preparation due to the significant absorption of alpha radiation. The sensitivity of radioisotopic labeling depends on the half-life of the isotopes. With isotopes having medium half-time (days-years), 10 14-10-10 mol can be measured easily. 

The reaction is not as simple as it appears though, because a labelled substance (tracer) is needed to facilitate the detection of the Ab-Ag binding event. These labels can be radioisotopes (radio immunoassay (RIA), not used in environmental analysis), fluorophores (fluoro immunoassay (FIA)) or enzymes (enzyme immunoassay (EIA)). Therefore a competition of the Ag with the label (L) is involved, and is defined by the association and dissociation rate constants ka2 and kd2 (Equation (3.3.2)) ... 

The rate of incorporation and discharge of radioactively labeled substances in the body provides a measure of the metabolism of healthy and of sick tissues. On medical patients this information is obtained by external measurements referred to as radioisotope scanning (RIS). Such scanning can yield information about a medical disorder much before it is observed by other means. Since the amount of radioactive tracer is very small, this technique is referred to as non-invasive. In hospitals the department of nuclear medicine is normally responsible for these investigations. 

Whole Cell The conversion of a radioisotopically labeled substrate can be used to assess the potential activity of microbial populations in field samples. The technique depends on bacterial growth for detection, but it generates results in about two days and is specific to SRBs. The sample is incubated with a known trace amount of radioactive-labeled sulfate. (SRBs reduce sulfate to sulfide.) After incubation, the reaction is terminated by adding an acid to kill the cells and the radioactive sulfide is fixed with zinc acetate for evaluation. This is a highly specialized technique, involving expensive laboratory equipment and the handling of radioactive substances [19]. 

C-labelled active plant protection substances for 13 years (17, 18). The results indicate that intensive cooperation between plant protection chemists, phytopathologists, phytophysiologists and specialists in the radioisotope techniques is necessary to fully exploit the application possibilities and to interpret the results. The special experimental facilities at the JUlich Nuclear Research Center which include practically oriented field tests supplemented by detailed studies under defined climatic conditions enable practical and relevant results to be obtained (17-19). The aim of this contribution is to provide new insights and information on the system effectiveness and residue behavior of azole fungicides. 

Radiochemical purity determinations consist of separating the different chemical substances containing the radionuclide. The radiochemical purity of labeled pharmaceuticals is typically determined by paper chromatography (paper impregnated with silica gel or silicic acid). The most frequently used radioisotope is technetium-99m obtained by daily elution with saline... 

Radiochemical methods of analysis can be grouped according to whether one measures radioactivity present in the sample or employs some means of introducing radioactivity into an otherwise nonradioactive sample in order to analyze for some component. An example of the first type is the determination of radioactive in rock samples. The second type is exemplified by using labeled KPO3 (I denoting a radioisotope of iodine) to determine the concentration of SO2 in air by the radiorelease method. This chapter will deal with the use of radioactivity to analyze otherwise nonradioactive substances. 

It is often necessary to detect on the same chromatogram, substances which contain different radioisotopes or which are doubly labelled. This is easily possible through autoradiography when the half-life and/or the hardness of the jS-radiation of the two isotopes differ widely. 

The desired compound in labelled form of known specific activity is added to a measured quantity of the mixture to be analysed. After equilibration, the substance is isolated in pure form and the specific activity determined. The degree of dilution obtained enables the amount of compound originally present in the mixture to be calculated. There are many situations where direct dilution analysis is unsuitable. The method requires an accurate determination of the specific activity and when the amount recovered falls to a fraction of a microgramme the accuracy falls off sharply because of the difficulty in measuring these amounts. However, the determination of radioactivity is both easy and accurate and such determinations serve, therefore, as a measure of the recovery of the purified compound. In any separation procedures, technical losses occur and the use of labelled compounds as internal standards represents one of the most widespread and important uses of radioisotopes. 

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