Radiochemical purity isotopes

In the stage of drag discovery, LC/MS-MS analytics is the method of choice to quantify the unbound drag concentration. The sensitivity can be increased by the use of radiolabeled substance. But, the radiochemical purity, isotope decay, if not 14C-label is used as well a sufficient specific activity must be taken into consideration (Wright et al. 1996). The concentrations of radioactivity in bound and unbound fraction are measured by liquid scintillation counting. The use of radiolabeled material allows easily examination of the potential of adsorption. However, the identity of the drag in unbound fraction should additionally be veri-... 

The chemical and radiochemical purity of the labeled compound must be ascertained prior to use. In practice a value of 95% or greater is usually acceptable. The desired specific activity of the administered radioactive compound depends on the dose to be used as well as the species studied. Doses of 14C on the order of 5 pCi/kg for the dog and 20 pCi/kg for the rat have been found adequate in most studies, while doses of 3H are usually two to three times higher owing to lower counting efficiency of this isotope. 

The product 144 has 98.8% radiochemical purity after chromatography and recrystallization. No kinetic tritium isotope effect and tritium exchange with the solvent in the last two syntheses has been studied. 

The earliest immunoassays made use of radioactively labeled antigens or antibodies. These analytes are referred to as radiolabeled immunoassays. Antibody binding sites are extremely specific and to retain this specifity the best option would be to replace a non-radioactive isotope in the tracer molecule by its radioisotope (e.g., replace hydrogen by H). However when the substitution is made in a part of the molecule away from the antibody binding site, the choice of radioisotope can be governed by other considerations, such as half-life, availability, high activity, and radiochemical purity. 

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... 

To avoid concerns about the radiochemical purity of the labeled dioxin, the solubility of unlabeled, 98% pure (Cambridge Isotope Labs) dioxin was determined by a scaled up version of the method just described. In this experiment, the entire contents of the flask was transferred to a separatory funnel, 13-C dioxin was added as an internal standard, and the combined dioxins were extracted with hexane. A capillary GC/MS method was used for quantitation (5). A summary of results is given in Table II. 

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. 

We have tried to emphasize not only the importance of the problem of the availability, but also that of the precise specifications of C, S, (etc.) labelled compounds, namely the specificity of the labelling position and the radiochemical purity. These two parameters are of vital importance for the future application of the labelled molecule in medicine, biology and the environmental sciences. Furthermore, two factors must be considered for the reliability of metabolic studies of drugs and pesticides correct choice of labelling position, since a position that is too exposed can result in loss of the label and consequently incomplete results the other factor is that of stable isotopes and the so-called isotopic effect. 

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. 

Reverse IDA (indirect IDA, or dilution with inactive isotopes) is particularly important in organic analysis and biochemistry [24], [32] to test for radiochemical purity and stability of labeled compounds. It is often used in radiochemical nuclear AA for the separation of activated element traces from a variety of interfering radionuclides in the analytical sample, for determination of the isolated activity, in comparison with that of a reference sample (i.e., to determine the radiochemical yield according to Eq. 10). 

For most applications in the use of tracers the most important factor in regard to purity of the labeled compounds is radiochemical purity. However, it is almost as important that the chemical purity or identity of the labeled compound be clearly established. Most classical methods for determination of chemical purity (melting point, UV, NMR, etc.) are usually inadequate for determination of radiochemical purity. The two basic techniques widely used to determine radiopurity are radiochromatography and reverse isotope dilution analysis. 

Having identified the isotope and measured the total count a test of radiochemical purity should be carried out, wherever possible, to confirm that the radioactivity of the sample is due to the one species only. In many cases this may be done by diluting the sample until the activity is of the order of about 20,000 counts per minute and then carrying out a suitable paper chromatographic separation. The radioactivity, as identified by scan-... 

The first step requires a determination of the quantity of target needed, identity(ies) of the isotope(s), specific activity, chemical and radiochemical purity, and position(s) of label(s) within the molecule. Each factor is strongly dependent on the type and design of the envisaged study(ies). 

Isotope dilution analysis permits one to determine the purity of a radiochemical. Compound X, molecular weight of 150 (specific activity 1.0 mCi/mmol), was checked for purity by carefully weighing 1.5 mg of the radiochemical and mixing with 1000 mg of unlabeled compound X and recrystallizing until a... 

The CRP work plan was proposed at the project s first Research Coordination Meeting (RCM), held in Bucharest in October 2002. The evaluation of a new therapeutic radiopharmaceutical depends on several analytical techniques to establish the product s stability and chemical, radiochemical and pharmaceutical purity. In addition, specific bioassays must be developed to evaluate its biological efficacy. These bioassays are product specific and thus need to be worked out separately for each radiopharmaceutical. Participants also identified potential lead molecules and isotopes to be used during the CRP for the development of therapeutic radiopharmaceuticals. 

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