In isotope dilution methods, radioactive isotopes are used as tracers. When diluting with a stable isotope, the specific activity decreases and the mixing entropy increases. Thus, the specific activity is measured before and after dilution; the quantity of the diluting substance is determined by the change of the specific activ­ity, as discussed in Section 9.2. Equations (9.26)—(9.30) are the basic formulas of the isotope dilution methods. In this section, the different isotope dilution methods and their applications will be discussed.

As seen in Eq. (9.30), specific activities provide the required analytical informa­tion, so the quantitative isolation of the studied substance is unnecessary. However, the isolated substance has to be pure (selective isolation) and must have a well — defined stoichiometry. In Eq. (9.30), mass or volume can replace the number of moles, so it is possible to determine these quantities as well. The absolute activity is frequently substituted by radioactive intensity (as discussed in Section 4.1.2). Of course, when we use intensities, we must ensure that the conditions of the measurements stay the same.

Isotope dilution methods are frequently applied to the measurement of the con­centration of substances which otherwise are difficult to analyze, such as the con­centration of a particular lantanoid element in the mixtures of rare earth elements or the concentration of a particular hydrocarbon in mixtures of hydrocarbons. The isotope dilution is suitable to measure the volume of substances in large tanks, such as molten metal in furnaces or mercury in electrolytic cells (see Section 11.2.4), in addition to the volume or flow rate of flowing liquids (blood, river, pipelines, etc.). A very important application of isotope dilution methods is the RIA in nuclear medicine (see Sections 12.2.1 and 12.3.1). The main types of isotope dilution methods will be discussed next.