Isotope Dilution is a method for the quantitative determination of a substance in a mixture by adding a known quantity of an identical but radioactively labeled substance.
The quantity of the test substance can be calculated based on the change in specific activity of the added radioactive substance. In other words the behavior of the radiolabeled compound is assumed to mimic the behavior of the native compound.
If the concentration of 2,3,7,8-T4CDD in a given sample is 1.265 pg. In this case we would use C13-2,3,7,8-T4CDD as an internal standard. All ten carbon atoms are radioactive carbon13 as opposed to carbon12 in the naturally occurring 2,3,7,8-T4CDD. As a result we will see the native 2,3,7,8-T4CDD at mass 292 and the corresponding labeled internal at mass 302. By calculating the concentration of C13-2,3,7,8-T4CDD and comparing it to the known amount spiked, we can determine the % recovery of the native 2,3,7,8-T4CDD.
If the C13-2,3,7,8-T4CDD is spiked at 100 pg and we calculate 78 pg then the recovery is 78/100 x 100% or 78%. To correct the native for the recovery we now multiply 1.265 x 100/78 to get 1.622 pg of native 2,3,7,8-T4CDD in the sample.