Radioactive isotopes synthesis

In order to obtain a compound labeled with a radioactive isotope for studies about the pancreas, p-2- and /i-3-sclenienyl alanine were prepared. Initially the synthetic route shown in Scheme 14, which allowed insertion of radioactive selenium as late in the synthesis as possible, was designed for the ji-2 isomer (116).149... 

Although much of the preceding discussion involved the synthesis of new molecules by organic and inorganic chemists, there is another area of chemistry in which such creation is important—the synthesis of new atoms. The periodic table lists elements that have been discovered and isolated from nature, but a few have been created by human activity. Collision of atomic particles with the nuclei of existing atoms is the normal source of radioactive isotopes and of some of the very heavy elements at the bottom of the periodic table. Indeed nuclear chemists and physicists have created some of the most important elements that are used for nuclear energy and nuclear weapons, plutonium in particular. 

Ideally, molecules should be labelled by introducing a radioactive isotope in place of a normal atom, e.g. carbon-14 replacing a carbon-12 in a carbohydrate. This method of labelling involves the synthesis of the molecule either in vivo or in vitro and the use of enzymes often permits the isotope to be introduced in a particular position in the molecule. The position of the labelled atom should be indicated wherever possible as for example in glucose- 1-14C. 

Wolfe has presented an excellent description of the systematic application of stable and radioactive isotope tracers in determining the kinetics of intestinal fat absorption, hepatic triglyceride synthesis, lipid mobilization, triglyceride-fatty acid recycling, and cholesterol turnover. 

Sometimes the nucleus can be changed by bombarding it with another type of particle. This is referred to as induced radioactivity. In 1934, Irene Curie, the daughter of Pierre and Marie Curie, and her husband, Frederic Joliot, announced the first synthesis of an artificial radioactive isotope. They bombarded a thin piece of aluminum foil with ot-particles produced by the decay of polonium and found that the aluminum target became radioactive. Chemical analysis showed that the product of this reaction was an isotope of phosphorus. 

III. SYNTHESIS AND USES OF ENAMINES AND RELATED COMPOUNDS LABELLED WITH RADIOACTIVE ISOTOPES... 

The synthesis of these complexes poses particular problems in the case of the (CpX)Re(CO)3 derivatives because the methods to introduce delicate X functions either in a CpRe(CO)3 complex or binding CpX to low-valent Re(CO)3 precursors like (1) or ReX(CO)s require multistep procedures and/or reaction conditions incompatible with the presence of sensitive X groups. The situation becomes even more difficult when deaUng with radioactive isotopes where fast, high-dUution and high-yield methods, and aqueous saline solutions are required. 

There are nine known radioactive isotopes and six are listed in Table 2. Sulfur-35 has the longest half-life and is produced by cosmogenic synthesis in the upper atmosphere cosmogenic S-35 (Tanaka and Turekian, 1991) is sufficiently long lived to be useful in determining overall removal and transformation rates of SO2 from the atmosphere and an estimated dry deposition flux to total flux ratio is —0.20 in the eastern US (Turekian and Tanaka, 1992). 

The use of labeled molecules is extremely difficult and costly, however, except for the simplest molecules. The synthesis of properly labeled compoimds requires special skills. U.S. Federal regulations make the use of radioactive isotopes most impractical, time-consuming, and expensive. The use of stable isotopes as labels is possible only in cormection with mass spectrometric detection, which is neither an easy nor an inexpensive proposition in liquid chromatography. For these reasons, the pulse methods have been limited to the elution of a pulse on a plateau (or step and pulse method) and are not yet quite popular in liquid chromatography. 

The radioactivity produced by unstable isotopes is described as natural radioactivity. A normally stable, nonra-dioactive nucleus can be made radioactive, and this is termed artificial radioactivity (the process produces synthetic isotopes). Synthetic isotopes are often used in clinical situations. Isotopic synthesis may be carried out in the core of a... 

In the use of radioactive tracers it is assumed that the radioactive isotopes studied are identical in chemical behavior to the nonradioactive isotopes. The first experiments that used radioactive tracers were carried out in 1913 in Germany and were designed to measure the solubility of lead salts via the use of a radioactive isotope of lead. In industry, radionuclides have been used for analytical purposes, for measurements of flow in pipes, and as part of many other apphcations. Another example of an important tracer study has been the investigation of photosynthesis of carbohydrates from atmospheric CO2 in the presence of light and chlorophyll. Scientists used eC, 15P, and iH to identify the intermediate steps involved in the photosynthesis of carbohydrates in plants that had been placed in an atmosphere composed of fyC-labeled CO2 and had been irradiated with hght. The presence of the radioactive carbon in the synthesized carbohydrate was evidence that O2 was involved in the synthesis. 

The labeling of organic compounds with stable or radioactive isotopes now constitutes an extremely important area of preparative chemistry. The number of publications in that field has grown to such an extent that it is almost impossible to find them all. This alone indicates the ever growing importance of labeled compounds in diverse branches of science moreover, the work reactivates the experimental demands on preparative chemists because an extraordinarily large number of compounds of different isotopic composition are open to synthesis. For instance, even such a relatively simple molecule as propane can afford no less than twenty-nine different deuterium-labeled compounds with eight different molecular formulae. 

Klement 43 in the seventh subgroup of the periodic system, technetium, is the lowest atomic number radioelement. Stable, non-radioactive isotopes do not exist according to Mattauch s rule. Technetium isotopes can be produced artificially by nuclear processes. Long-lived isotopes are Tc (2.6 10 a), Tc (4.2 10 a) and Tc (2.1 10- a). The spectroscopic discovery of technetium in several fixed stars provided the first proof of stellar synthesis of heavy nuclides. Traces of Tc occur in the earth s crust where they arise mainly from spontaneous fission of... 

In complex mixtures of compounds (for exanq>le, in organic synthesis or biochemical systems) it may be quite difficult to ascertain the exact amount of a specific component. A widely used technique of considerable value is isotope dilution analysis. This can be iq)plied either with stable isotopes, in which case the detector is a mass spectrometer, or with radioactive isotopes, using measuring techniques presented in Chapter 8. The use of stable isotopes is usually limited to geologic samples, as described in 2.3, but may be applied to biological samples using highly enriched or However, the technique with... 

The synthesis of the radioactive labelled eompounds needed for these studies is a very specialized discipline and it is necessary to observe a number of mles in order to obtain compounds with the desired specifications. The main points to be considered are choice of radioactive isotope, labelling position, specifie activity, radiochemical parity, number of steps used, radiochemical yield and, finally, the cost of the procednre. This latter point must be considered because the eost of obtaining radioactive labelled eompounds is high, mainly because of the great amount of time required rather than because of the cost of the labelled starting materials. Nevertheless, this eost is justified by the quality of the results obtained. 

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