Purity radionuclides

Radionuclidic Purity Radionuclidic purity is defined as the fraction of the total radioactivity in the form of the desired radionuclide present in a radiopharmaceutical. Radionuclide impurities may arise from impurities in the target material or from fission of heavy elements in the reactor [2], In radionuclide generator systems, the appearance of the parent nuclide in the daughter nuclide product is a radionuclidic impurity. In a "Mo/"mTc generator, "Mo may be found in the "mTc eluate due to breakthrough of "Mo on the aluminum column. The presence of these extraneous radionuclides increases the radiation dose to the patient and may also obscure the scintigraphic image. 

The SI unit of activity is the becquerel (Bq) 1 Bq = 1 transformation/second. Since activity is proportional to the number of atoms of the radioactive material, the quantity of any radioactive material is usually expressed in curies, regardless of its purity or concentration. The transformation of radioactive nuclei is a random process, and the rate of transformation is directly proportional to the number of radioactive atoms present. For any pure radioactive substance, the rate of decay is usually described by its radiological half-life, T r i.e., the time it takes for a specified source material to decay to half its initial activity. The activity of a radionuclide at time t may be calculated by A = A° e ° rad where A is the activity in dps, A ° is the activity at time zero, t is the time at which measured, and T" is the radiological half-life of the radionuclide. It is apparent that activity exponentially decays with time. The time when the activity of a sample of radioactivity becomes one-half its original value is the radioactive half-life and is expressed in any suitable unit of time. 

The Landmark diatomite material is treated and processed to a purity and grade that will collect angstrom-sized particles. According to the vendor, because of the diversity and shapes of the diatoms, different blends may be formulated to filter out almost any kind of material from a liquid including heavy metals, organics, and radionuclides from low-level waste streams of nuclear installations. 

Figure 6. Elution pattern of 70 mCi generator. Key A, mCi Au-195in per eluate and RN, radionuclidic purity of eluates ( /1 Ci Hg-195m/mCi Au-195m).

Production of Sr-82. An important consideration in the development of radioisotope generators is the availability, cost, and radionuclidic purity of the long-lived parent. In the case of Sr-82, the 25 day radionuclide is needed in 100-200 mCi amounts in order to provide adequate elution yields of Rb-82 from one loading of Sr-82 every three months. Initially the Sr-82 for the generator was produced at the Lawrence Berkeley Laboratory (LBL) 88-inch cyclotron by the Rb-85 (p,4n) Sr-82 nuclear reaction (12). However, because of the long irradiation time required to produce... 

Breakthrough. Eluate radionuclidic purity is determined by Nal scintillation spectrometry on 50 ml of eluate. Samples must be held at least one hour before measurement to allow full decay of Rb-82. To improve sensitivity of measurement, the most prominent 511+514 keV peak is counted. Computations are based upon comparison with an aliquot of Sr-82+Sr-85 solution used to prepare the generator.. Data are expressed in units of yCi Sr-82/ml of eluate/ mCi Rb-82 at end of elution. 

Studies of short-lived radionuclide generators (4-6) do not adequately treat the quantitative problems of the daughter nuclide elution or those specific to their optimal clinical use. Two essential physical characteristics of a generator are the yield of the daughter nuclide and its radiochemical and radionuclidic purity. To realize the full potential of a short-lived radionuclide generator for medical studies requires that these two characteristics are optimized and are compatible with parameters important to clinical use such as total perfused volume and duration of the scintigraphic examination. 

The collection and preparation of water samples requires individual approaches for different analytical tasks. If heavy metals or long-lived radionuclides at the trace and ultratrace concentration range are to be determined in water samples by ICP-MS, especially careful sampling is necessary to avoid possible contamination (using clean bottles and containers washed and cleaned before use, for example, with 2 % nitric acid and high purity water to stabilize traces in the samples), and the loss of analyte by adsorption effects or precipitation should be also considered. 

In many situations, the experimenter will prefer to buy labeled compounds from commercial suppliers rather than attempt to synthesize them. The radiochemical purity of such purchased compounds cannot be assumed. Radiation-induced selfdecomposition (radiolysis) can result in the formation of a variety of labeled degradation products, which must be removed before experimental use of the compounds. The extent of radiolysis depends on the nature of the labeled compound, how long it has been stored, and the manner of storage. Radiolysis is most significant with low-energy (3 emitters (especially tritium) since the decay energy is dissipated almost entirely with the compound itself. Furthermore, impurities involving other radionuclides may be present. 

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. 

An alternative means of radionuclide production employs neutron capture reactions in nonfissile nuclides. Again a high-purity target is used and a mixture of the unconverted target material, the... 

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... 

Radiochemical Purity. A sample is radiochemically pure at the time of counting if no other radionuclide is detected in it. As a general rule, the radiochemical procedure is chosen to separate from the radioanalyte all other radionuclides that are in the sample. Purification steps must be added to the usual procedure if the level of contaminant radionuclides is very high relative to the concentration of the radioanalyte. 

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 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). 

Radionuclidic purity is determined by measuring the characteristic radiations emitted by individual radionuclides. Gamma emitters are distinguished from another by identification of their y energies on the spectra obtained from a Nal crystal or a Ge (germanium) detector. This method is called y spectroscopy. 

If the number of radionuclides present in the sample is low, the decay curve can be separated by subtraction into the individual decay curves of the radionuclides, either graphically or arithmetically, as shown in Fig. 7.2. The analysis of decay curves is of practical importance for the investigation of radionuclide purity. As examples, contamination of a sample by a short-lived impurity is shown in Fig. 7.3, and contamination by a long-lived impurity in Fig. 7.4. 

The task of quantitative and effective separation of small amounts of radionuclides has appreciably enhanced the development of modem separation techniques. High radionuclide purity is of great importance for application in nuclear medicine as well as for sensitive measurements. In this context, impurities of long-lived radionuclides arc of particular importance, because their relative activity increases with time. For example, if the activity of Sr is only 0.1% of that of Ba after fre.sh separation, it will increase to 11.5% in three months. 

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