Techniques radiometrical determination

A radiometric dating technique uses the decay of U-238 to Pb-206 (the half-life for this process is 4.5 billion years) to determine the age of the oldest rocks on Earth and by implication the age of Earth itself. The oldest uranium-containing rocks on Earth contain approximately equal numbers of uranium atoms and lead atoms. Assuming the rocks were pure uranium when they were formed, how old are the rocks ... 

A second radiometric dating technique, used to measure much longer periods of time, involves U-238. U-238 decays with a half-life of 4.5 X 10 yr through a number of intermediates, and eventually ends as lead. As a result, all rocks on Earth that contain uranium also contain lead. If a rock is assumed to have been only uranium when it was formed (which can be determined by the relative amounts of different lead isotopes in the rock), the ratio of uranium to lead can be used to date the rock. For example, if one half-life has passed, the rock would contain 50% uranium atoms and 50% lead atoms. After two half-lives, the rock would be 25% uranium atoms and 75% lead atoms. 

Highly sensitive determination of "Tc is possible using today s advanced mass spectroscopic methods. However, orthodox determination methods of "Tc involving radiometric techniques or even activation analysis are still used because they are simple and can be done without expensive machines. The detection limits of typical analytical methods are listed in Table 1. 

A force that is as large as the gravitational force can be used to suspend a particle against gravity, provided that it can be controlled and directed upward to balance gravity. One such force is the radiation pressure force or radiometric force. Ashkin and Dziedzic (1977), whose work is discussed in the next section, were the first to use the radiation pressure to levitate a microsphere stably. It was demonstrated by Allen et ai (1991) that the radiometric force can be measured with the electrodynamic balance, and they used the technique to determine the absolute intensity of the laser beam illuminating a suspended particle. This was accomplished in the apparatus displayed in Fig. 13. The laser illuminated the microparticle from below, and... 

There is a wide range of applications for methods of analysis that are based upon the energies and intensities of the radiations emitted by radioactive nuclides. These techniques sometimes are temied radiometric methods of analysis. The methods are not restricted to the determination of substances initially radioactive, since there is wide use of methods involving the irradiation of stable, nuclides to produce radioactive ones, followed by measurement of their radiations, horn which the composition of the original stable substance can be inferred This method is radioactivation analysis. Another method for the use of measurements of radioactivity in the analysis of stable substances is that of tracer techniques, that is, by the addition to them of radioactive nuclides, which can then be used to follow the course of various reactions or processes. There are various ways of introducing the radioactive nuclides, which are discussed later in this entry. 

One of the most popular radiometric techniques is that of radiometric titrations. In a radiometric titration, the unknown is titrated with a radioactive reagent, and the radioactivity of the product or supernate, formed by the chemical reaction of the reagent and the unknown material, is monitored as a function of titrant volume to determine the endpoint. This means that the radioactive component being followed must be isolated during the titration and its activity measured—that is, a discontinuous titration. 

Radiometric techniques, essentially alpha and gamma spectrometry, allow the determination of a number of fissile and fission product isotopes. 

Measurement Techniques. Light isotopes Concentrations of were determined radiometrically after synthesis of propane (13) in a proportional counter after about twenty-fold electrolytical enrichment of (14), The C content of large samples (. 5 g C) were determined by liquid scintillation counting after synthesis of benzene (15) fi-om CO2 which was extracted fi-om the water sample after acidification. The C content of small samples (<0.5 g C) were measured by accelerator mass spectrometry (AMS) W the Isotrace Laboratory of the University of Toronto. The C, and isotopic... 

Samples of all ICE were submitted for analysis. Plutonium and americium were determined by radiometric techniques and other elements were determined by atomic absorption. 

The term radiometric analysis is often used in a broad sense to include all methods of determination of concentrations using radioactive tracers. In a more restricted sense it refers to a specific analytical method which is based on a two-phase titration in the presence of a radioactive isotope. The endpoint of the titration is indicated by the disappearance of the radioisotope from one of the phases. Figure 9.4 illustrates two cases, (a) the determination of Ag in a solution by titration with Nal solution containing ( y t 1.57x10 y),and (b) the determination of Fe in an aqueous solution, to which trace amounts of radioactive Fe (EC 2.73 y) has been added. In case (a) the Agl precipitate is radioactive but the solution has little radioactivity until all the Ag has been precipitated. The activity of the solution is measured by a liquid flow GM-detector (Ch. 8). In the latter case (b) a two-phase liquid-liquid analytical technique is used ( 9.2.6) the titrant contains a substance (oxine) which extracts Fe(II) from the aqueous to the chloroform phase. The radioactivity of the organic phase is followed by liquid scintillation (sampling) to determine the end point of the titration. 

In some cases (where the mentioned metals are not encountered) this may be turned to advantage. Thus, we [71] developed an indirect extraction-radiometric method of sensitive determination of benzylamine and noradrenaline at the level of micromoles through the competition with radio-labeled metal ions, Sr. Indeed, the detection of the isotope is more sensitive than common photometric techniques the analytical response is a decrease in the amount of metal extracted relative to the case in which no amine is present), which is proportional to the content of analyte. However, in general, the affinity of crowns to metals is, undoubtedly, a drawback. 

The introduction of spectral analysis made it possible to reveal the existence in natural objects of elements that could not be seen, felt or weighed. Now the history repeated itself but the role of indicator was played by radioactive radiaton, which could be measured with a radiometric technique. However, the results of the Curies were not faultless. They were wrong in suggesting a chemical similarity between polonium and bismuth. Even a brief look at the periodic system shows that the existence of a heavy analogue of bismuth is hardly possible. But one must not forget that the Curies did not extract pure metal, could not determine its relative atomic mass, and, finally, did not see differences in the spectra of polonium and bismuth. This is why they actually ignored a possible analogy between polonium and tellurium. 

Radiometric methods other than the isotope dilution analysis are not necessarily versatile. But they provide convenient and reliable technique in certain cases (Braun and Tolgyessy 1967). In radiometric titration, a radioactive indicator is used to monitor the end-point of the titration. For example. Cl ions in an aqueous solution sample are titrated with a standard solution of Ag solution labeled with Ag. Until the equivalence point is attained essentially all the Ag ions are precipitated as AgCl, giving little activity in the supernatant solution. At the equivalence point, the activity begins to increase linearly. Plotting the activity of the solution against the amount of Ag solution added, the equivalence point is clearly determined as the intersection of two fines. This technique is reliable and convenient, if both the separation of the supernatant solution from the precipitate and the activity measurement are made automatically. 

Several analytical methods have been deployed for the determination of uranium in ores. Among the older methods that were used are radiometric methods that were already used over 50 years ago for ore sorting (Mal tsev 1960), titrimetric methods in which the nraninm content in the ore was determined with ferrous ion-phosphoric acid reduction (Hitchen and Zechanowitsch 1980), colorimetric methods where complexes of nraninm are formed with standard arsenazo 111 (Onishi and Sekine 1972), exotic siderophores (Renshaw et al. 2003) reagents and instrumental neutron activation analysis (INAA) based on measurement of Np in the ore (Chaudhry et al. 1978) in addition to nnmerons other approaches. Many modem techniques are now employed for destrnctive and nondestructive determination of uranium in ores. 

The methods that can be used for the determination of uranium isotopes in environmental samples were briefly surveyed (Borylo 2013). The methods were divided into radiometric methods that include different techniques of neutron activation analysis, liquid scintillation and alpha spectrometry and nonradiometric methods that include ICPMS and its variations and TIMS, and methods for assaying the amount of uranium (without isotopic composition) like ICP-OES, atomic absorption, and x-ray-based methods. The types of environmental samples listed were plants, mosses, water, soil, phosphates, sediments, and surface water. Sample preparation procedures were mentioned only in passing, stating that digestion (or mineralization) with concentrated acids (HNO3, HCl, and HE) should be followed by anion exchange on suitable resins. For alpha spectrometry, uranium was purified and deposited on steel disks (Borylo 2013). 

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