The reverse isotope dilution method is used for the quantitative analysis of radioac­tive substances, especially in mixtures of radioactive substances. The quantity of the radioactive substance (m0 + mx) is determined by adding inactive substance (m) (standard). The steps of the procedure are the same as in the simple isotope dilution
method. It is usually true that mx«m0 and (mx + m0)^m0. From Eq. (10.1) or (10.2), the quantity is:

m

m0 5 50—~ (1a3)

a

or is expressed by intensities:
m

m0 = i

^ -1 i

Similar to the simple isotope dilution method, the specific activity or intensity has to be measured before and after the isotope dilution. Since the quantity m is known, m0 can be calculated. The sensitivity is determined by the minimum quantity that is needed to measure a0.

Reverse isotope dilution is used in microanalyses (e. g., for the analysis of purity), and the yield of nuclear reactions is used in the activation analysis (see Section 10.2.2.1).