Radiochemical methods isotope dilution

Three common quantitative applications of radiochemical methods of analysis are considered in this section the direct analysis of radioactive isotopes by measuring their rate of disintegration, neutron activation, and the use of radioactive isotopes as tracers in isotope dilution. 

Isotope Dilution Another important quantitative radiochemical method is isotope dilution. In this method of analysis a sample of analyte, called a tracer, is prepared in a radioactive form with a known activity. Ax, for its radioactive decay. A measured mass of the tracer, Wf, is added to a sample containing an unknown mass, w, of a nonradioactive analyte, and the material is homogenized. The sample is then processed to isolate wa grams of purified analyte, containing both radioactive and nonradioactive materials. The activity of the isolated sample, A, is measured. If all the analyte, both radioactive and nonradioactive, is recovered, then A and Ax will be equal. Normally, some of the analyte is lost during isolation and purification. In this case A is less than Ax, and... 

Radiochemical methods of analysis take advantage of the decay of radioactive isotopes. A direct measurement of the rate at which a radioactive isotope decays may be used to determine its concentration in a sample. For analytes that are not naturally radioactive, neutron activation often can be used to induce radioactivity. Isotope dilution, in which a radioactively labeled form of an analyte is spiked into the sample, can be used as an internal standard for quantitative work. 

The apphed pretreatment techniques were digestion with a combination of acids in the pressurized or atmospheric mode, programmed dry ashing, microwave digestion and irradiation with thermal neutrons. The analytical methods of final determination, at least four different for each element, covered all modern plasma techniques, various AAS modes, voltammetry, instrumental and radiochemical neutron activation analysis and isotope dilution MS. Each participating laboratory was requested to make a minimum of five independent rephcate determinations of each element on at least two different bottles on different days. Moreover, a series of different steps was undertaken in order to ensure that no substantial systematic errors were left undetected. 

Fukai, R. 1965. Analysis of trace amounts of chromium in marine organisms by the isotope dilution of Cr-51. Pages 335-351 in Radiochemical Methods of Analysis. Int. Atom. Ener. Agen. (Vienna). 

The flameless atomic absorption method has a reproducibility of about 2% or better for homogeneous specimens. Checks (3) between AA and NAA (with radiochemical separation after irradiation) and isotope dilution spark source mass spectroscopy on thoroughly homogenized tuna fish and Bureau of Mines round-robin coal specimens indicate good agreement between the methods. (0.425 0.9%, 0.45 3.5%, and 0.45 4.4% for tuna by AA, NAA, and SSMS, respectively, and 1.004 is the average ratio of NAA to AA results for five coal samples.) The similar results indicate that the technique used in sample preparation... 

Materials for use in the determination of stable trace elements in environmental or biomedical research. Radiochemical methods such as neutron activation or isotope dilution analysis, are often used in the determination of such trace elements and constitute an important contribution of nuclear techniques to applied science (Strachnov et al., 1993). 

In clinical chemistry the determination of stable elements by radiochemical methods offers no outstanding advantages over alternative methods, but the use of radioisotopes for determining organic compounds is developing rapidly. In isotope dilution methods (G6), a pure but radioactive form of the compound to be measured is mixed with the sample, a fraction is isolated, and its activity is determined. In radio-metric or derivative analysis (W14), a radioactive reagent is allowed to react with the analyte the labeled compound is separated and its activity is measured. The isotopes commonly used are C,... 

The second general category of radiochemical analysis involves adding a radioactive substance to the sample, manipulating the sample by chemical or physical means, measuring the radioactivity, and ultimately calculating the amount of the component of interest. This category includes direct and inverse isotope dilution analysis, radiochemical titrations, and radiorelease methods of analysis. 

Isotope dilution analysis is probably the most important radiochemical method. A radioactive isotope, whose specific activity is known, is added to a... 

The term specific activity (activity per unit mass) is important for radiochemical methods like isotope dilution. Samples of carrier-free radionuclides have the highest possible specific activity for this radionuclide. By the addition of carrier material the specific activity is lower, the total amount of material (stable and radioactive materials) is increased, and some handling procedures, like precipitation, might be easier. The change of specific activity is the basis of isotopic dilution techniques. 

Isotope dilution analysis is a quantitative analytical technique. The basis of this radiochemical method is that by chemical processing the specific activity (activity per unit mass) of a mixture of stable and radioactive isotopes is not changed. 

Direct isotope dilution analysis is applied if an amount of an analyte cannot be separated quantitatively for analytical determination. A known amount of a radioactive isotope of the element of interest is added to the sample containing the analyte. Then a portion of the analyte is isolated in high purity from the sample. This separation step need not be quantitative. The mass and activity of the isolated portion are measured and used to calculate the amount of analyte in the original sample. There are several varieties known of this radiochemical method, e.g., reverse isotopic dilution. 

See also Archaeometry and Antique Analysis Dating of Artifacts. Bioassays Ovenriew. Drug Metabolism Metabolite Isolation and Identification Isotope Studies. Fertilizers. Food and Nutritional Analysis Meat and Meat Products. Immunoassays, Techniques Radioimmunoassays. Isotope Dilution Analysis. Pesticides. Pharmaceutical Analysis Drug Purity Determination. Process Analysis Overview. Radiochemical Methods Pharmaceutical Applications. Water Analysis Industrial Effluents. 

See also Immunoassays, Techniques Radioimmunoassays. Isotope Dilution Analysis. Radiochemical Methods Radiotracers. Titrimetry Overview. Water Analysis Overview. 

The need for increased reliability and analytical quality control has emphasized the usefulness of radiochemical methods for the certification of standard reference materials (SRM) [9], [22j. 23J. Radioanalytical methods are often suitable for homogeneity testing and di.stribution analysis of traces in SRMs. Activation analysis, radiotracer techniques, and isotope dilution analysis are becoming increasingly important for assessment of analytical quality. 

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. 

The inherent sensitivity of these isotopic methods is especially attractive when a specific reaction is to be studied in detail. Furthermore, the formation of a radioactive product enables isotope dilution analysis to be applied to establishing the specificity of the reaction. A wide range of substrate and enzyme concentrations can be used and it is often possible to measure low enzymatic activity in crude homogenates without the need for extensive purification of the enzyme. This undoubtedly has constituted one of the major practical advantages of the radiochemical techniques. 

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