Multiple isotope dilution is used when a compoundCis present in a mixture in so minute a quantity that ordinary chemical extraction is difficult. If the compound in the mixture can be obtained in a radioactive state, the mixture is diluted with known amounts of inactive compoundC. The relevant equations are Si(m+xi) =ki, (i= 1, 2, …,p), wheremis the original (unknown) amount of compoundCpresent in each aliquot,xiis the amount of inactive compoundCadded to thei-th aliquot, and Siis the specific activity of the diluted compoundCextracted from thei-th aliquot;kis an unknown constant.