Radioactive isotopes behave identically to their stable counterparts except for the very small effects of mass dependent isotope fractionation. Thus, by labelling an element in our experiments with a very small amount of radioactive isotope, we can determine relative changes in its concentration from changes in the sample radioactivity. Very, very small differences (ppb to ppt) can be measured, which makes this an excellent method for laboratory experiments. Radiolabelling also makes it possible to probe the rates that an element is exchanged between two pools, e.g., between a solid and the solution.
The activity of the radiotracer is determined using liquid scintillation counting. This method relies the behaviour of a fluorescent cocktail of organic molecules that is added to the sample to be analysed. It absorbs energy that is released by radioactive decay and reemits it as light with a wavelength that is suitable for detection by photomultiplier tubes. The very high sensitivity of the detector means that very little radiotracer is needed (??) to measure concentrations in the range of parts per billion or parts per trillion.