Radioisotope dilution analysis

A mixture is being assayed by radioisotope dilution analysis. 10 mg of the labelled analyte (0.51 pCi mg-1) was added. 1.5 mg of the pure analyte was separated and its specific activity measured and found to be 0.042 pCi mg1. What was the amount of analyte in the original sample ... 

Ba solutions using radioisotope dilution analysis. A discussion of the trend when quartz or TiO are used as adsorbents appears in Chem.. Ind. (1975)704. 

It is not necessary that there be two isotopes in both the sample and the spike. One isotope in the sample needs to be measured, but the spike can have one isotope of the same element that has been produced artificially. The latter is often a long-lived radioisotope. For example, and are radioactive and all occur naturally. The radioactive isotope does not occur naturally but is made artificially by irradiation of Th with neutrons. Since it is commercially available, this last isotope is often used as a spike for isotope-dilution analysis of natural uranium materials by comparison with the most abundant isotope ( U). 

I Andersson, R Lundqvist, R Oste. Analysis of vitamin B12 in milk by a radioisotope dilution assay. Milchwissenschaft 45 507-509, 1990. 

Assays radioisotopes are used in several quantitative detection methods of value to chemists. Radioimmunoassay is a quantitative method for measurement of a substance (the analyte) using antibodies which bind specifically to that analyte. Isotope dilution analysis works on the... 

The principle of the substoichiometric analysis is as follows To the element of interest (M ), one adds a known amount of its labeled radioisotope with the specific activity S = A/M, where A is the radioactivity and M the amount of carrier hence the specific activity of the mixture becomes S = A/(M -1- M). Knowing the change in specific activity from S to S, the element of interest can be sinqily determined. This is the same as the principle of isotope dilution, but in practice the accurate determination of S and S is very tedious. This is one important reason why isotope dilution analysis is not very popular in trace analysis. However, in substoichiometric analysis, equal amounts of the element (m) are isolated substoichiometrically from the radioisotope solution and the mixed solution, and subsequently the radioactivities of the separated portions (a and a ) are measured, then the amount of the element of interest can be calculated according to the equation ... 

The principle of isotope dilution analysis (IDA) [31], [47], [90]. [97], [98] involves measurement of the change in isotopic ratio when portions of a radiolabeled and nonlabeled form of the same chemical species are mixed. To perform a radioisotope IDA. an aliquot of a radioactive spike substance of known specific activity o,=A,/ni/ is... 

The desired compound in labelled form of known specific activity is added to a measured quantity of the mixture to be analysed. After equilibration, the substance is isolated in pure form and the specific activity determined. The degree of dilution obtained enables the amount of compound originally present in the mixture to be calculated. There are many situations where direct dilution analysis is unsuitable. The method requires an accurate determination of the specific activity and when the amount recovered falls to a fraction of a microgramme the accuracy falls off sharply because of the difficulty in measuring these amounts. However, the determination of radioactivity is both easy and accurate and such determinations serve, therefore, as a measure of the recovery of the purified compound. In any separation procedures, technical losses occur and the use of labelled compounds as internal standards represents one of the most widespread and important uses of radioisotopes. 

In non-nuclear teehniques, isotopes of the same element generally eaimot be distinguished, while in nuclear techniques, specific isotopes are measured instead of elements. Therefore, direct quantitative information on the associated elements ean be obtained since poly-isotopic elements have eonstant isotope ratios. Further, isotopes of a given element may be diseriminated therefore, analytieal information may be obtained by using elements enrlehed In respeet to a partieular stable isotope or labelled with a radioisotope, e.g. in isotope dilution analysis. In addition to analytical information, isotope studies may also yield kinetie and mechanistic information. 

IEC continues to have numerous applications to the detection and quantification of various inorganic ions.1 1 This is particularly true in water analysis.5-14 Inorganic ions in a variety of other sample types, such as food and beverages,1518 rocks,19-23 biological fluids, (blood, urine, etc.),24-31 pharmaceutical substances,32 33 concentrated acids,34 alcohols,35 and cleanroom air36 have also been analyzed by IEC. IEC has also been employed in isotopic separation of ions,37 including the production of radioisotopes for therapeutic purposes.3839 Typical IEC sample matrices are complex, and may contain substances that interfere with measurement of the ion(s) of interest. The low detection limits required for many IEC separations demand simple extraction procedures and small volumes to avoid over-dilution. Careful choice and manipulation of the eluent(s) may be needed to achieve the desired specificity, especially when multiple ions are to be determined in a single sample. 

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 operation, since 1945, of nuclear reactors has made available radioisotopes of most elements. The isotopes are useful in a variety of chemical investigations, including those concerned with solubility, diffusion, reaction mechanism and structure. They have given rise to new analytical techniques, such as isotopic dilution and radioactivation analysis. In industry also, they have a wide and rapidly expanding application. All this is made possible by the ease with which small quantities of the nuclides can be detected, often remotely, and quantitatively determined by commercially available and easily operated equipment. 

Isotope dilution mass spectrometry (ID-MS) is widely accepted as a quantification procedure of proven accuracy in elemental analysis and isotope ratio measurements [4]. Several areas of research in nuclear science, geochronology, medicinal chemistry, environmental science, and agricultural science have benefited from this technique. ID-MS is applicable to all elements that have at least two stable isotopes. Monoisotopic elements can be analyzed only if they have a long-lived natural or artificial radioisotope. For example, iodine and thorium have been determined with spikes of the long-lived isotopes 29i and 25 Th, respectively [44]. TI-MS and ICP-MS are the methods of choice for accurate ID-MS analysis. ICP-MS has the advantage that several elements can be analyzed simnltaneously under the same experimental conditions. Other ionization techniqnes discussed in this chapter have also been coupled with ID-MS. 

Other applications of photochemistry include the development of sensitive fluorescent chemosensors for analysis of dilute solutions of inorganic cations and anions and the study of the diffusion of individual molecules in solution at room temperature. Fluorescent compoimds have been used as replacements for radioisotopes in the analysis of biological compoimds and the study of biologically active compounds and living systems. Photochemical reactions also offer alternative probes for the characterization of the microenvironments in diverse solid and liquid media, including crystals, zeolites, alumina, silica and clay surfaces, semiconductor surfaces, liquid crystals and host-guest inclusion complexes, polymer films, monolayers and supported multilayers of surfactant molecules, mi-celles, and dendrimers. ... 

There will of course continue to be a place for radioisotope methodology, especially for dilution and activation analysis, and some of the radionuclides will continue to be measured to greatest advantage by liquid scintillation counting. 

With radioisotopes now available for many elements, the tracer technique became generally applicable. New variants were developed, such as neutron activation analysis, which was introduced in 1936 for the determination of dysprosium in rare-earth samples (Hevesy and Levi 1936) and subsequently became a widely used technique for sensitive trace analyses, particularly when much larger neutron fluxes became available with the advent of nuclear reactors. Another frequently applied method for trace determination is isotope dilution the species to be determined in the sample is diluted by addition of a known amount of the same species labeled with known specific activity. From the specific activity then resulting and measured, the original quantity of the species is derived, even if only a fi action of the species is finally recovered. The impact on biosciences was revolutionary, when suitable isotopes of key elements in the biosphere were soon discovered (Ti/2 = 10 min) was one of I. Curie and... 

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