Impurities radionuclides

Decay products of the principal radionuclides used in tracer technology (see Table 1) are not themselves radioactive. Therefore, the primary decomposition events of isotopes in molecules labeled with only one radionuclide / molecule result in unlabeled impurities at a rate proportional to the half-life of the isotope. Eor and H, impurities arising from the decay process are in relatively small amounts. Eor the shorter half-life isotopes the relative amounts of these impurities caused by primary decomposition are larger, but usually not problematic because they are not radioactive and do not interfere with the application of the tracer compounds. Eor multilabeled tritiated compounds the rate of accumulation of labeled impurities owing to tritium decay can be significant. This increases with the number of radioactive atoms per molecule. 

Bischoff JL, Fitzpatrick JA (1991) U-series dating of impure carbonates An isochron technique using total-sample dissolution. Geochim Cosmochim Acta 55 543-554 Bonotto DM (1998) Implication of groundwater weathered profile interactions to the mobilization of radionuclides. J South Am Earth Sci 11 389-405... 

The applications for ICP-MS are broadly similar to those for ICP-AES, although the better sensitivity of the former has resulted in applications such as the determination of ultra-low levels of impurities in semiconductors and long-lived radionuclides in the environment. Also, ICP-MS is better suited to the determination of the lanthanide series of elements in many geological applications because the mass spectrum is much simpler than the equivalent optical spectrum. 

The distribution of the product radionuclides was studied as a function of irradiation periods of from 2-14 days (Table I). These data show that irradiation periods of greater than three or four days do not result in large increases in Os-191 yield but do lead to rapid increases in the Os-193, Ir-192, and Ir-194 impurities. As a result, a three day irradiation period has been chosen as a compromise between Os-191 yield and increasing levels of isotopic impurities. The rapid approach of Os-191 yield to a maximum value during a 14-day irradiation period suggests a maximum attainable specific activity of approximately 600 mCi/mg of enriched 0s-190 target. 

The only radionuclidic impurity detected in the 1-122 is less than 0.1% radioxenons and other radioiodines, which neither interfere with scintigraphic imaging nor result in a high radiation exposure to the patient. Further improvement of the radioiodine contamination could be attained with an iodine trap between the storage reservoir and the growth chamber. The milking efficiency is about 40%. We consider this generator assembly to be a preliminary version that can be refined considerably. Further details may be obtained from Richards and Ku (8). 

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. 

Carriers frequently are stable isotopes of the radionuclide of interest, but they need not be. Nonisotopic carriers are used in a variety of situations. Scavengers are nonisotopic carriers used in precipitations that carry/incorporate other radionuclides into their precipitates indiscriminately. For example, the precipitation of Fe (OH)3 frequently carries, quantitatively, many other cations that are absorbed on the surface of the gelatinous precipitate. Such scavengers are frequently used in chemical separations by precipitation in which a radionuclide is put in a soluble oxidation state, a scavenging precipitation is used to remove radioactive impurities, and then the nuclide is oxidized/reduced to an oxidation state where it can be precipitated. In such scavenging precipitations, holdback carriers are introduced to dilute the radionuclide atoms by inactive atoms and thus prevent them from being scavenged. 

The first step after exchange often concentrates the radionuclide and carrier to perform subsequent steps more easily in a smaller volume. Further steps by precipitation, solvent extraction cycles, ion exchange, or distillation improve sample purification. In these processes, the analyte is separated from various known impurities. One type of separation step is adjusting the oxidation states of analyte or impurity. A special step may be inserted to enhance separation of an impurity that is difficult to remove. 

An example of removing multiple interfering elements is strontium purification in the presence of fission products. Ferric ion is added as a holdback carrier for the rare earths (and other radionuclides) and then precipitated as Fe(OH)3, the scavenger that carries these radioactive impurities. This or any other step can be repeated for enhanced removal of impurities. 

The sample is purified by distillation to separate the tritium-containing water from both non-radioactive and radioactive impurities. Various substances can cause scintillations by means other than radionuclide emission - by chemical fluorescence or luminescence - or interfere with ( quench ) detection of scintillations due to radionuclides. Even after purification, both processes are inevitable, but to a limited extent. Luminescence due to visible light will decay when the sample is stored in a darkened region of the LS system before the sample is counted. The degree of quenching, notably due to water in the sample, is determined instrumentally by reference to comparison sources and recorded, so that any deviation from the quenching observed for the tritium standard can be taken into account. 

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 radioactive decay follows first order kinetics with a half-life of the radionuclide. Theoretically, after one half-life, 50% of the labeled protein should remain intact in the stored sample. In practice, this is not the case. Some radioactive molecule of the protein change their identity and in this manner they are converted to impurities during the preparation of the radioactively labeled protein. Radiochemical conversions f H He, - N, - C1, 1 - Te, - Xe) cause destruc-... 

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. 

Br (t]/2 = 1.6h) is applied as a positron emitter in nuclear medicine. The most suitable reactions for the production of this radionuclide are Se(p, 2n) Br and As( He, 3n) - Br. Irradiation of highly enriched Sc with 30 MeV protons leads to yields of about 4 GBq per pAh. The Br impurity is about 0.9%. Elemental Sc or selcnidcs such as Ag2Sc or Cu2Se may be used as targets. After irradiation, Br can be separated from elemental Se by thermochromatography at 300 °C and taken up in... 

The activity of naturally radioactive elements is a measure of their mass. Prerequisites of application of the correlation between mass and activity according to eq. (17.1) are that the isotopic composition of the element to be determined is constant and that interfering radioactive impurities are absent. If the daughter nuelides are also radioactive, radioactive equilibrium must be established or the daughter nuclides must be separated off quantitatively. Interference of radioactive impurities may be avoided by measuring the a or y spectrum of the radionuclide considered. 

Magnox Reactor waste streams include a wide range of materials such as ion exchange (IX) resins, sludge, Magnox fuel element debris (FED), reactor graphite and carbon and stainless steels. Some wastes will exhibit heterogeneity and for many construction materials such as steels there will be radionuclides present which are neutron activation products of trace impurities and were un-quantified at the time of manufacture. 

Technetium-99m containing drugs are prohibited from sale if these contain a radionuclide impurity mentioned in the monograph for Sodium Pertechnetate Tc-99m injection. 

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