Substoichiometric isotope dilution analysis

The quantity of analyte in the sample is obtained by rearranging equation 17.1  

If Ax is much smaller than Aj then 17.2 can be approximated by the following equation  

Having calculated in this way and knowing the mass of sample used, it becomes easy to deduce the concentration of X in the initial sample. 

In such a procedure, the labelled tracer must be uniformly mixed in the sample and the aliquot taken must be of a quantity sufficient to avoid weighing errors. If the concentration of the compound to be measured is too low, then recovery as a pure compound, even partially, could be difficult. The adaptation of this method to very small quantities is discussed in the following paragraphs. 

The preceding method is known as reversed isotope dilution analysis when the compound to be measured is already radioactive. The principle remains the same the activity of the subject compound (measured from a fraction), is carried out before and after dilution with the same compound, non-labelled. The calculations are identical. This analysis is used for the determination of the isotopic carrier in a solution of a radionuclide using one of its stable isotopes. 

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Fractional separation of tin compounds used as stabilisers in PVC was based on substoichiometric isotope dilution analysis [446]. The tin compounds were isolated by extraction and complexed with salicylideneamino-2-thiophenol, followed by controlled addition of y-irradiated tributyl tin oxide and measurement of the y-activity. PVC containing a nominal 0.63% (Bu Sn analysed 0.614 0.016% in nine determinations. 

Substoichiometric isotope dilution analysis was first developed by Ruzicka and Stary (1968) as another variation on the basic IDA technique. The basic idea of... 

Kanda, Y., Suzuki, N. (1980) Substoichiometric isotope dilution analysis of inorganic mercury and methylmercury with thionalide. Anal. Chem., 52, 1672-1675. 

Substoichiometric isotope dilution analysis. It is often inconvenient to determine the mass of the isolated compound in the above direct isotope dilution analysis, as is the case for a large number of samples. Substoichiometric isotope dilution analysis is an excellent solution to this problem (Ruzicka and Stary 1968 Stary and Ruzicka 1976). In this method, the same amount of the compound is separated from each of the standard and mixed solutions using a reagent in an amount stoichiometrically less than that of the compound in the solutions. The activity of the portion separated from the standard solution (Aq) and that from the mixed solution (A) are measured. Since the mass of the portions counted are the same, the ratio of their specific activities SqIS equals the ratio of their activities Aq/A. 

Isotope dilution analysis require the determination of either the chemical yield in the separation process or of the specific activity. This can be avoided by applying the substoichiometric principle, which may also increase the sensitivity of the analytical method. 

When the substoichiometric principle is applied to isotope dilution analysis, the relationship becomes... 

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 ... 

Reverse Isotope Dilution Analysis. 137 Derivative Isotope Dilution Analysis 137 Substoichiometric Isotope Dilution... 

Combination of isotope dilution with the principle of substoichiometric analysis offers the possibility of avoiding determination either of the chemical yield of the separation procedure or of the specific activity in the isolated fraction. Two identical aliquots of the radiotracer solution are taken, both containing the tracer with mass... 

Two radioanalytical methods described in chapter 17 are applied preferentially in the life sciences, activation analysis and isotope dilution, the latter mainly in combination with the substoichiometric principle. 

Isotope dilution in combination with the substoichiometric principle is applied in various ways. The most important examples are radioimmunoassay for protein analysis and DNA analysis. In radioimmunoassay, radionuclides are used as tracers and immunochemical reactions for isolation. Radioimmunoassay was first described in 1959 by Yalow and Berson, and since then has found very broad application in clinical medicine, in particular for the measurement of serum proteins, hormones, enzymes, viruses, bacterial antigens, drugs and other substances in blood, other body fluids and tissues. Only one drop of blood is needed, and the analysis can be per-fonned automatically. Today more than 10 immunoassays are made annually in the United States. The most important advantages of the method are the high sensitivity and the high specificity. In favourable cases quantities down to 10 g can... 

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