Direct measurement of radioactivity

The Cs concentration in foods, plants, com samples, etc., can be determined by direct measurement of radioactivity with a y-ray spectrometer. This method requires a higher Cs concentration than in the radiochemical procedure, and radioactive impurities whose y-rays have energies similar to that of cesium y-rays (such as Zr) can be present only in negligible amounts. The spectrometric method can be used either directly on the original sample or after concentrating Cs, depending on the detection efficiency and on the Cs concentration in the sample. 

Direct measurement of radioactivity on adsorbent layers is insensitive because of the high seK-absorption of the radiation and the softness of the (glass) background it is unreliable as a quantitative procedure. 

C. Direct Measurement of Radioactivity Using Radiation Detectors... 

Fig. 8. Radiochromatogram (49) of the bile collected (0-8 hours) from a fistula rat given 8.8 >>g. of labeled 3,5,3 -triiodothyronine at the beginning of the experiment (solvent, aqueous collidine). TRITh, free TRITh A = glucuronic acid conjugate of TRITh, B = unknown x, detected by autography on Figure 3, is not detected by direct measurement of radioactivity.

Three common quantitative applications of radiochemical methods of analysis are considered in this section the direct analysis of radioactive isotopes by measuring their rate of disintegration, neutron activation, and the use of radioactive isotopes as tracers in isotope dilution. 

Radiochemical methods of analysis take advantage of the decay of radioactive isotopes. A direct measurement of the rate at which a radioactive isotope decays may be used to determine its concentration in a sample. For analytes that are not naturally radioactive, neutron activation often can be used to induce radioactivity. Isotope dilution, in which a radioactively labeled form of an analyte is spiked into the sample, can be used as an internal standard for quantitative work. 

Direct measurement of particle velocity and velocity fluctuations in fluidized beds or riser reactors is necessary for validating multiphase models. Dudukovic [14] and Roy and Dudukovic [28] have used computer-automated radioactive particle tracking (CARPT) to foUow particles in a riser reactor. From their measurements, it was possible to calculate axial and radial solids diffusion as well as the granular temperature from a multiphase KTGF model. Figure 15.10 shows one such measurement... 

The discovery of cosmic ray produced 81Kr in meteorites [l]1 introduced a new method of high sensitivity measurements of 81Kr concentrations and cosmic ray exposure dating. The method consists of a direct measurement of both radioactive 81Kr atoms (Ty2 = 2.13 x 105y, [2] and of stable spallation Kr atoms by a... 

Very few direct measurements of the reaction of surface silanol groups on quartz have been reported. This is apparently caused by the small effects due to the limited surface areas available. Adsorption of sodium ions on quartz was measured by radioactive tracer techniques by Gaudin et al. (293). Saturation was achieved at high pH (>10) and sodium ion concentrations above 0.07 Jlf. The calculated packing density of silanol groups was 4.25/100 A. Goates and Anderson (294) titrated quartz with aqueous sodium hydroxide and alcoholic sodium ethylate. The occurrence of two types of acidic groups was reported. 

What are the advantages of the radiochemical method compared with other in situ techniques It offers a direct relationship between surface radiation (N ) and surface concentration, which allows a direct measurement of the amount of adsorbed molecules on the electrode, a condition difficult to determine with other in situ techniques. The main limitation of the technique is the availability of radioactive forms of the compound the experimenter wants to study. In this respect, the type of radiation preferred is of the P-type, mainly because of the ease of detection and minimal safety hazards. Typical P-emitters used are H, C, S, Cl, and P, which as constituents of molecules, open a great variability of compounds for study. Figure 6.21 shows some experimental results obtained for the measurement of adsorption on single crystals using this radiochemical method. 

N may be expressed in any convenient units because whatever is used cancels out in the ratio NJN2. A common unit is "counts per minute" (cpm) because, through Equation 26-1, the observed cpm are a direct measure of the number of radioactive atoms present. 

Adsorption of surfactants "Loss from solution" method, direct measurement using radioactive tracers, ellipsometry, nephelometry... 

Synaptic neurotransmission in brain occurs mostly by exocytic release of vesicles filled with chemical substances (neurotransmitters) at presynaptic terminals. Thus, neurotransmitter release can be detected and studied by measuring efflux of neurotransmitters from synapses by biochemical methods. Various methods have been successfully employed to achieve that, including direct measurements of glutamate release by high-performance liquid chromatography of fluorescent derivatives or by enzyme-based continuous fluorescence assay, measurements of radioactive efflux from nerve terminals preloaded with radioactive neurotransmitters, or detection of neuropeptides by RIA or ELISA. Biochemical detection, however, lacks the sensitivity and temporal resolution afforded by electrophysiological and electrochemical approaches. As a result, it is not possible to measure individual synaptic events and apply quantal analysis to verify the vesicular nature of neurotransmitter release. 

Distribution in vivo could be studied by positron emission tomography (PET). Positron emission tomography is a sensitive and specific functional non invasive 3-D imaging method that permits rapidly and directly measurement of the total radioactivity from a drug labelled with a positron-emitting radionuclide (Gupta). 

In vivo or direct measurements of uranium in the body are made with radiation detector systems and associated electronics called whole body counters that measure radiation as it leaves the body from internally deposited uranium. In vivo assays are the most direct method of quantifying internally deposited radioactive materials. However, not all radionuclides emit radiations than may be detected... 

High-purity detection systems having a very low background are suitable tools for the direct measurement of low-level radioactivity in environmental samples. The background features of the detection system are of considerable importance because they have to be known for one to obtain an estimate of the detection limit and of the minimum detectable activity (Curie, 1968). The natural radioactivity background originates from the uranium and the thorium series from K and from cosmic rays. Natural radioactivity is found in most materials, and it is necessary to shield the... 

Direct measurements of active centre concentrations have been made by terminating the polymerization with some reagent such as iodine [86], carbon dioxide [115] or an alcohol [116], labelled with an isotope which introduces radioactivity into the polymer e.g. 

A direct measure of the binding of a molecule to a protein target can be made if there is some means to distinguish bound molecule from unbound and a means to quantify the amount bound. Historically, the first widely used technique to do this was radioligand binding. Radioactive... 

NO has a short half-life (2-30 s) and therefore the direct measurement of NO production is impractical. In aqueous solutions, however, it rapidly reacts with oxygen to form the stable water-soluble metabolites nitrite and nitrate (Figure 1). The concentration of these ions, which can be measured via a variety of methods, is used as a measure of the tissue content of NO and/or the synthesis of NO by cultured cells (Hevel and Marietta, 1994 Archer, 1993). An alternative method used to quantify NOS activity is to directly measure the catalytic activity of a cell or tissue extract (i.e., the conversion of radioactive arginine to citrulline Hevel and Marietta, 1994 Archer, 1993). While both methods are sensitive measurements of the overall capacity of a cell or tissue extract to synthesize NO, neither one provides a true kinetic... 

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