Radioactivity detection principles

There are two versions of the sodium leak detection systems based on the radioactivity detection principle ... 

A radioactivity detector is used to measure radioactivity in the HPLC eluent, using a flow cell. The detection principle is based on liquid scintillation technology to detect phosphors caused by radiation, though a solid-state scintillator is often used around the flow cell [17,31]. This detector is very specific and can be extremely sensitive. It is often used for conducting experiments using tritium or C-14 radiolabeled compounds in toxicological, metabolic, or degradation studies. 

Other detection principles have been applied to HPLC, e.g. conductivity, radioactivity, infra-red, and photoconductivity detectors. Such detectors are not widely used in drug analysis but can find application in special circumstances (e.g. the identification of drug metabolites arising from a radiolabelled drug by radioactivity detection). 

Tritium is readily detectable because of its radioactivity. Under certain conditions concentrations as low as 370 )-lBq/mL (10 //Ci/mL) can be detected. Most detection devices and many analytical techniques exploit the ioni2ing effect of the tritium P-decay as a principle of operation (62,63). 

This example assumes that RIA was chosen. The principle behind RIA is the competition between the analyte A and a radioactively tagged control C (e.g., a /-marked ester of the species in question) for the binding site of an antibody specifically induced and harvested for this purpose. The calibration function takes on the shape of a logistic curve that extends over about three orders of magnitude. (Cf. Fig. 4.38a.) The limit of detection is near the B/Bo = 1 point (arrow ) in the upper left corner, where the antibody s binding sites are fully sequestered by C the nearly linear center portion is preferrably used for quantitation. 

The analytical detectability applying a CL method should, in principle, be comparable to that obtained using radioactive labels, without all the disadvantages related to the use of isotopic labeling. In fact, assuming reasonable values for the quantum efficiency of the chemiluminescent reaction (Cl 0.01), for the overall photon collection efficiency of the optical system-CCD camera assembly (T) 0.01%), and for the intensity of the lowest detectable CL signal (about... 

Tracer studies in which chemically similar species are studied on the basis of containing a radioisotope are discussed in Chapter 10. It is fairly obvious that, with detection techniques readily available for the measurement of non-radioactive isotopes, the principle can be extended to non-radioactive systems. Where in vivo studies are concerned there are clear safety reasons for so doing. Although some progress is being made in this direction, it is... 

Ionization and condensation nuclei detectors alarm at the presence of invisible combustion products. Most industrial ionization smoke detectors are of the dual chamber type. One chamber is a sample chamber the other is a reference chamber. Combustion products enter an outer chamber of an ionization detector and disturb the balance between the ionization chambers and trigger a highly sensitive cold cathode tube that causes the alarm. The ionization of the air in the chambers is caused by a radioactive source. Smoke particles impede the ionization process and trigger the alarm. Condensation nuclei detectors operate on the cloud chamber principle, which allows invisible particles to be detected by optical techniques. They are most effective on Class A fires (ordinary combustibles) and Class C fires (electrical). 

SPR biosensors are label-free detection devices - binding between the biomolecular recognition element and analyte can be observed directly without the use of radioactive or fluorescent labels. In addition, the binding event can be observed in real-time. SPR affinity biosensors can, in principle, detect any analyte for which an appropriate biomolecular recognition element is available. Moreover, analyte molecules do not have to exhibit any special properties such as fluorescence or characteristic absorption or scattering bands. 

Radioactivation Analysis. The principle of this technique is that a stable isorope when irradiated by neurrons, by charged particles such as protons or deuterons or by gamma rays, can undergo a nuclear reaction to produce a radioactive nuclide. After the radionuclide is formed, and its radiations have been characterized by radiation detection devices, calculations can be made of the elements contained in the sample before irradiation. 

Liquid Scintillators This detection mechanism is quite similar in principle to the preceding one. Here, however, the radioactive sample and the fluor are the solute in a liquid medium, usually a nonpolar solvent. The energy of nuclear radiation first excites the solvent molecules. This excitation energy eventually appears as photons emitted from the fluor following an intermediate transfer stage. The photons are detected by means of a photomultiplier arrangement. 

In principle, chemical information on a system can only be obtained with methods that do not alter the species present in solution. However, in order to get this information, an external perturbation must be applied to the system and its response must be analyzed. In the case of radioactive tracer, where the radioactivity measurement is the only way to detect the element (but it does not allow the identification of the form of the species), two types of external perturbation can be applied (i) by contacting the system with a second phase and subsequently observing the distribution of the radionuclides between the two phases (static or dynamic partition) or (ii) by applying an electrical potential or a chemical gradient (transport methods). So far, transport methods have not yet been used in one-atom-at-a-time... 

For the monitoring of personnel radiation exposures, measurement of radioactive contamination and surveying of laboratories and equipment, and for the detection of radionuclides incorporated in the human body, various detectors and instruments are used. The principles of operation of these detectors have been discussed in the previous sections of this chapter. 

The low detection limits of radioactive substances are very attractive for use in analytical chemistry. In principle, a single radioactive atom can be detected provided that it is measured at the moment of its decay. In practice, however, a greater number of radioactive atoms is necessary to measure their radioactivity with a sufficiently low statistical error. The mass m of a radionuclide and its activity A are correlated by the half-life ti/2-... 

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