Radionuclides chemical purity

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. 

The quality control tests fall in two categories biological tests and physiochemi-cal tests. The biological tests establish the sterility and apyrogenicity, while the physiochemical tests include radionuclidic, chemical, and radiochemical purity tests along with determination of pH, osmotic pressure, and physical state of the sample (for colloids). 

Physicochemical Tests Physicochemical tests include the tests for the physical and chemical parameters of a PET radiopharmaceutical, namely physical appearance, isotonicity, pH, radionuclidic purity, chemical purity, and radiochemical purity. 

Define (a) radionuclide purity, (b) radiochemical purity, (c) chemical purity of a radiopharmaceutical. 

Another situation occurs as a result of an (n,y) reaction, in which an intermediate radionuclide decays to the product of interest. This route is followed to make for example, with the Xe(n, y) Xe process. The neutron capture product Xe beta decays to with a 16.9 h half-life. Because the final product can be chemically separated from the target, specific activity may approach the theoretical value for the pure radionuclide. Obviously, the use of high chemical purity targets and processing reagents is necessary to avoid introducing stable nuclides of the same element as the product. In the example, this means that both the... 

The radionuclidic and chemical purity of all the four organic positron emitters produced is generally >99%. Reference has been made above to radiochemical purity and specific activity, but they are more relevant to the subsequently labeled product rather than to the radionuclide itself. Thus the production technology of the commonly used PET radionuclides is well established. 

Radiochemical purity determinations consist of separating the different chemical substances containing the radionuclide. The radiochemical purity of labeled pharmaceuticals is typically determined by paper chromatography (paper impregnated with silica gel or silicic acid). The most frequently used radioisotope is technetium-99m obtained by daily elution with saline... 

A generator should ideally be simple to build, the parent radionuclide should have a relatively long half-life, and the daughter radionuclide should be obtained by a simple elution process with high yield and chemical and radiochemical purity. The generator must be properly shielded to allow its transport and manipulation. 

The suitability of a radionuclide for a particular medical application will depend upon its availability in a radiochemically pure form, its nuclear properties and its chemical properties. In respect of the first of these considerations it is necessary to eliminate any extraneous radiation sources from a material destined for medical use. This need for very high radiochemical purity has a bearing on the means by which the radionuclide is produced. One potential method is by nuclear fission of a heavy element. This approach has the advant e that carrier free radioisotopes of high specific activity may be produced. However, because the process produces a complex mixture of FPs, painstaking separation and purification of the desired radionuclide will be necessary. The problem is simplified somewhat by using a pure target isotope to produce an FP which has rather unique properties. Thus fission produces which may be separated from the other FPs by virtue of its volatility. Fission in pure may also be used to prepare Mo in carrier free form, although contamination by Ru, I and Te was a problem in early... 

Hammermaier A, Reich E, Bogl W (1986) Chemical, radiochemical, and radionuclidic purity of el-uates from different commercial fission Mo/ Tc generators. Eur J Nucl Med 12 41-46 Lin MS, McGregor RD, Yano Y (1971) Ionic aluminium (III) in generator eluate as an erythrocyte-agglutinating agent. J Nucl Med 12 297-299... 

An inqx)rtant consideration in the use of radionuclides is their radiochemical purity since, should several radionuclides of different elements be present in a tracer sample used in an experiment, the result could be ambiguous and misleading. In a radio-chemically pure sample, all radioactivity comes from a single radioactive element. If the radioactivity comes from a single isotope, the sample may be said to be radio-isotopically pure. 

In liquid-liquid extraction, an immiscible liquid—usually an organic solution—is combined with the sample in aqueous solution in an extraction flask and shaken to achieve good contact between the liquids. A reagent that functions as an extractant may have been added to one phase or the other. Information on the distribution ratio D and the extraction yield E that indicate the extent of purification from specified contaminants is available from many studies (Sekine and Hasegaw 1977). The information should describe the extractant, the organic solvent and the conditions of purity, reagent concentrations, volumes, required time, and temperature. The value of D reflects the ratio of the radioelement solubility in the organic phase to that in the aqueous phase, hence the type of solvent and the chemical form of the radionuclide to be extracted may be inferred from radioelement solubility data. If the initial conditions of the extraction procedure are not identical to those for reported extractions, the extent of extraction must be tested. 

Extractant reagents and solvents are selected for optimum separation of the radionuclide of interest from contaminants, and also with regard to chemical stability, purity, and minimal hazard potential. The reagents and solvents may have to be stored under conditions that avoid degradation, and purified to remove minor contaminants that can interfere with separations. Use of chemicals listed as hazardous material may be feasible with appropriate care, but later disposal may be difficult. 

Activation analysis is the other field of radiochemical analysis that has become of major importance, particularly neutron activation analysis. In this method nuclear transformations are carried out by irradiation with neutrons. The nature and the intensity of the radiation emitted by the radionuclides formed are characteristic, respectively, of the nature and concentrations of the atoms irradiated. Activation analysis is one of the most sensitive methods, an important tool for the analysis of high-purity materials, and lends itself to automation. The technique was devised by Hevesy, who with Levi in 1936 determined dysprosium in yttrium by measuring the radiation of dysprosium after irradiation with neutrons from a Po-Be neutron source. At the time the nature of the radiation was characterized by half-life, and the only available neutron sources were Po-Be and Ra-Be, which were of low efficiency. Hevesy s paper was not followed up for many years. The importance of activation analysis increased dramatically after the emergence of accelerators and reactors in which almost all elements could be activated. Hevesy received the 1943 Nobel prize in chemistry for work on the use of isotopes as tracers in the study of chemical processes . 

Isotopic composition can be expressed as either absolute or relative measurement, depending on the particular application. When isotopically labeled materials are used as tracers absolute values are determined. The isotopic (or radiochemical) purity is the percentage of label present in the specified chemical form that may include the position of the label or the enantiomorphic form of the compound. In contrast, radionuclidic purity is the percentage of the total radioactivity present as the specified radionu-lide and implies nothing about the chemical form of the radionuclides present. 

Gaseous and liquid targets are commonly used for the production of short-lived radionuclides, especially positron emitters. The chemical separation techniques of some important positron emitters are described in the next section. It should be pointed out here that the methods used have to be fast and efficient, in terms of the purity of the product as well as the recovery of the enriched target material. Furthermore, the chemical form of the separated radionuclide has to be well defined to allow a convenient synthesis of the desired radiopharmaceutical. 

Impurities from decay products (i.e., from radioactive decay and from radiation-induced decay of chemical compounds) can interfere with the activity measurements, or with the chemical reactions employed to process the sample. Radionuclide purity is a function of time. 

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