Isotope decay

Calculates radioactive decay during a specified number of days. Select one of the common isotopes, or enter the half-life of another isotope.

Conc. of stock

Enter mCi/ml and Ci/mmole, which should be on the label. If you are using a molecule labeled with 125I, the specific activity equals 2200 Ci/mmole if each molecule is labeled with one iodine.

Also enter the percent of the original isotope remaining (calculated above). The calculations assume that the decay product is not biologically active, so the concentration of stock that is biologically active decreases over time.

Dilution of stock

Enter the concentration in your stock solution, after accounting for decay. Also enter the concentration and volume you want. The result is the volume of stock you need to use.

Specific activity (cpm/fmol)

Enter the specific radioactivity as Ci/mmol which should be on the label. If you are using a molecule labeled with 125I, the specific activity equals 2200 Ci/mmol if each molecule is labeled with one iodine.

Also enter the counter efficiency - the fraction of radioactive disintegrations that are detected. The efficiency depends on the isotope and instrumentation. With low energy isotopes such as tritium, the efficiency also depends on the experimental details such as the choice of scintillation fluid, the amount of water in the sample, and the presence of any colored substances in the sample.

Cpm to fmol/mg

Enter the specific radioactivity as cpm/fmol, the number of cpm counted, and the protein content of the sample in mg. The result is the number of binding sites in fmol/mg protein.

Cpm to sites/cell

Enter the specific radioactivity as cpm/fmol, the number of cpm counted, and the cell count. The result is the number of binding sites per cell.

Cpm to nM

Enter the specific radioactivity as cpm/fmol, the number of cpm counted, and the volume counted. The result is the concentration of radioligand in nM.