Radionuclides analytical chemistry

Radiochemistry involves the application of the basic ideas of inorganic, organic, physical, and analytical chemistry to the manipulation of radioactive material. However, the need to manipulate radioactive materials imposes some special constraints (and features) upon these endeavors. The first of these features is the number of atoms involved and the solution concentrations. The range of activity levels in radiochemical procedures ranges from pCi to MCi. For the sake of discussion, let us assume an activity level, D, typical of radiotracer experiments of 1 p,Ci (= 3.7 x 104 dis/s = 3.7 x 104 Bq), of a nucleus with mass number A 100. If we assume a half-life for this radionuclide of 3 d, the number of nuclei present can be calculated from the equation... 

There are certain unique features to the chemical separations used in radiochemistry compared to those in ordinary analytical chemistry that are worth noting. First of all, high yields are not necessarily needed, provided the yields of the separations can be measured. Emphasis is placed on radioactive purity, expressed as decontamination factors rather than chemical purity. Chemical purity is usually expressed as the ratio of the number of moles (molecules) of interest in the sample after separation to the number of all the moles (molecules) in the sample. Radioactive purity is usually expressed as the ratio of the activity of interest to that of all the activities in the sample. The decontamination factor is defined as the ratio of the radioactive purity after the separation to that prior to the separation. Decontamination factors of 105-107 are routinely achieved with higher values possible. In the event that the radionuclide(s) of interest are short-lived, then the time required for the separation is of paramount importance, as it does no good to have a very pure sample in which most of the desired activity has decayed during the separation. 

NAA is preferred for the analysis of unique samples such as meteorite, the certification of reference materials, and quality assurance in analytical chemistry [21]. Definitive methods are based on the activation of samples, selective separation of analyzed radionuclides by column (ion-exchange) chromatography, and measurement by gamma spectrometry [15, 16, 19]. In definitive methods, radiochemical separation, co-precipitation, extraction, and ionic exchange guarantee the... 

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-... 

The following applications of radionuclides in analytical chemistry can be distinguished ... 

The high sensitivity of detection of radionuclides has already been emphasized in section 17.1 with respect to their application in analytical chemistry. 

Making an Activation-Analysis Measurement. The most prominent technique in nuclear analytical chemistry is Instrumental neutron activation (INAA), in which thermal neutrons from a nuclear reactor are used to irradiate the sample and the induced radionuclides are measured nondestructively with a germanium gamma-ray spectrometer. Sensitivity may be enhanced by chemically separating the elements of interest before radionuclide assay. 

Tolgyessy J, Havranek E and Dejmkova E (1990) Radionuclide X-ray fluorescence analysis, with environmental applications. In Svehla G, ed. Wilson and Wilson s Comprehensive analytical chemistry Vol 26, pp. 1—254. Elsevier, Amsterdam. 

The section Radioactive Methods in volume 9 of the Treatise on Analytical Chemistry (Kolthoff and Elving 1971) discusses radioactive decay, radiation detection, tracer techniques, and activation analysis. It has a brief but informative chapter on radiochemical separations. A more recent text. Nuclear and Radiochemistry Fundamentals and Applications (Lieser 2001), discusses radioelements, decay, counting instruments, nuclear reactions, radioisotope production, and activation analysis in detail. It includes a brief chapter on the chemistry of radionuclides and a few pages on the properties of the actinides and transactinides. 

Chapter 3 presents general information on the purification processes in the wet laboratory that underlie radioanalytical chemistry, as well as the background information in analytical chemistry necessary to apply those processes. Information specifically associated with the behavior of radionuclides in aspects such as their low concentration and the effect of radiation emission is given in Chapter 4. Chapter 5 describes the form of the usual samples submitted to the radioanalytical chemistry laboratory and the treatment of various sample matrices to indicate the context in which analytical results are considered. 

Radionuclide analysis methods are published in analytical chemistry and radiochemistry journals, and in methods manuals issued by nuclear facilities such as government laboratories. For example, the Environmental Measurements Laboratory Procedures manual, HASL-300 (Chieco 1997), is an excellent source. Standard methods for radionuclide analysis (see Section 6.7) are available, and should be used whenever appropriate. If conditions differ from those to which published methods have been applied, radionuclide recovery and decontamination must be tested and additional process steps may have to be inserted. 

The techniques for separating and purifying radionuclides as part of the radio-analytical chemistry process are discussed in the following sections. Although separate sections present the different techniques, the analyst is expected to combine separation techniques that produce optimum analytical efficacy. 

Radionuclides with extremely long half-lives or in very large amounts will not be at such extremely low concentrations, as indicated by Eq. (2.7). Radionuclides with half-lives in excess of 10 years, such as and Th, can be measured by some of the conventional techniques of analytical chemistry as alternatives to radiation measurement. Radionuclides at concentrations much above lO Bq/1 usually are not submitted to a low-level radioanalytical chemistry laboratory because of the threat of contaminating other samples. 

Another distinction pertains to radionuclides that have no stable isotopes. Chemical analysis for these radionuclides has no basis in conventional analytical chemistry except as studies performed with the usual small amounts, based on similarities in chemical behavior to homologous stable elements according to their location in the periodic table. When sufficiently large amounts of these radionuclides are produced and purified to permit observation by microchemical manipulations, any conclusions must consider the impact of the intense radiation on the observed chemical reactions. 

In recent years, the MS has become a useful alternative to the radiation detector for measuring longer-lived radionuclides. It provides a second and completely independent method of measurement to confirm results it also gives the analyst a choice, because some measurements are easier or more reliable by one method than the other. The MS has been used to measure radioactive atoms with half-lives greater than 10 years because the number of these atoms relative to their decay rate is proportional to the half life for half lives greater than 109 y, even the conventional measurements of analytical chemistry are applicable. 

Standardization matches the concept of calibration generally used in analytical chemistry. The number of radionuclides produced per unit time by irradiation of an infinitesimally thin target is determined by the balance of the increase due to nuclear reactions and the decrease due to radioactive decay ... 

Radioactivity was a discovery that was soon brought into the arsenal of analytical chemistry. It is interesting to note that, as in some other fields, radiation from radioactive species was examined for analytical purposes in a variety of different ways at a time when they had little practical importance, because the radiation detectors were primitive and the field of potential applications limited owing to the small number of natural radionuclides. After the production of artificial radionuclides had started the importance of the earlier radiochemical methods increased rapidly and this field became one of the most powerful in trace analysis. 

Some basic knowledge of the structure and rearrangement procedures of unstable atoms, properties of radiation, characteristics of radiation detectors, and production of artificial radionuclides is helpful for the understanding of radiochemical methods and their application in analytical chemistry. Also, it has to be pointed out that for the use of radioactive materials not only do the principles of radiation protection have to be observed but also one has to follow strictly those rules that depend on the legislation of the relevant country. 

Garten Rainer PH and Tolgyessy J (2001) Radionuclides in analytical chemistry. In Ullmann s Encyclopedia of Industrial Chemistry, 6th edn. Weinheim Wiley-VCH. 

Essentially all the separation methods known from classical analytical chemistry can be applied to chemical separations of radionuclides and labeled compounds from samples to be analyzed precipitation, electrolytic deposition, extraction, ion exchange, distillation, chromatography, etc. 

Radiochemistry is a branch of chemistry, but in this context it exhibits various characteristics that make it a somewhat autonomous discipline. To be sure, the radionuclides obey the same chemical laws as their inactive isotopes do there are, however, additional aspects that have to be considered. The mechanisms in the formation of the radionuclides have to be taken into account, as well as the chemical effects of ionizing radiation and of possible chemical reactions of highly excited atoms ( hot atom chemistry ). The properties of ionizing radiation and the techniques to be applied for their measurement are additional aspects specific to radiochemistry. Generally speaking, one can define radiochemistry as an independent branch situated between inorganic chemistry, physical and analytical chemistry and nuclear physics. 

The same considerations apply with even greater force to the use of radioactive tracers in elemental and especially molecular analysis (-+ Radionuclides in Analytical Chemistry). 

Addition of a radioactive tracer may make it possible to monitor losses that occur during the course of an analysis [4] (- Radionuclides in Analytical Chemistry). Isotope dilution analysis is also useful for establishing the correctness of a result [5], [6]. 

The use of radionuclide techniques in analytical chemistry was first reported in 1913 by G. Hevesy and F. Paneth in a study of the solubility of lead sulfide in water, using the natural lead isotope " Pb as indicator [67], Isotope dilution analysis was introduced by O. Hahn in 1923 [68J, using Pa to determine the yield of Pa. The development of radioreagent methods followed, and further development of radioanalytical chemistry has established a range of analytical methods and techniques ll]-[4], [61], [65], [87], [93], [95], [97]. These include the use of artificial radionuclides and labeled compounds, the principles of nuclear activation [4]-[10], [66] (- Activation Analysis), and absorption and scattering of radiation [11], [12]. The most important procedures are shown in Table 1. 

Radionuclides are used in many subdivisions of analytical chemistry (see Table 1). Of major importance are radiotracers in methodological and pathway studies, isotope dilution analysis (IDA), radioimmunoassay, and nuclear activation analysis (AA) (- Activation Analysis) [66]. They are all especially suited to analyze the extremely small amounts of substances encountered in ultra-trace analysis or in trace analysis of microsamples. 

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