In the simple isotope dilution method, the quantity of an inactive substance (m) is determined by the addition of a radioactive indicator. The labeled species (stan­dard) of the substance to be analyzed is added to the unknown samples. The quan­tity (m0 + mx) and the specific activity (a0) of the standard are exactly known. The system is homogenized, and the substance in question is selectively isolated as a well-defined compound; then it is purified to the required high level. As mentioned previously, the yield of isolation is unimportant. The specific activity of the com­pound obtained after the dilution (a) is determined.

If the quantity of the radioactive indicator can be disregarded (mx«m0), the quantity of the unknown substance from Eq. (9.30) is:

(a0 1 m = m0 — — 1 a

or is expressed by radioactive intensities:

m = m^-° — 1j (10.2)

Thus, the specific activity or intensity of the labeled substance has to be measured before and after the isotope dilution. The degree of the dilution is determined by the ratio of m to m0. The precision is usually suitable in the range of m = 0.01 X m0 to m0 = 0.01 X m. The simple dilution method is a fairly good option in all cases when the substance to be analyzed cannot be separated quantitatively.