Choice of Radiotracers:
It is not probable to use all the radionuclides especially along with too long (>10 y) or too short (< few min) half lives as radiotracers. Same, type of emitted radiation is also a significant criterion. In sequence to make use of a radionuclide as tracer certain assumptions and criteria have to be made.
The most important assumption is that the radiotracer should mix perfectly well with the system under investigation. It implies that the radiotracer should preferably be in the similar chemical form (ionic or covalent) as the analyte along with that it is being mixed. It means in which the tracer will behave as an indistinguishable from the nonradioactive material except for the emitted radiation. Additionally, it should not adversely affect any component by way of radiation damage which means that the energy of radiation should not be too large to cause bond damage. Of course tracer is present in small amounts so that its radioactivity does not significantly affect the system in physical appearance or otherwise. Various isotopes of a radiotracer differ in mass among the active and non active species. Though isotopes of different masses show different reaction rates kinetically because of isotope effect and suitable corrections should be made but these are not really significant for heavy elements.
NOTE: It may be noted that by substituting 'H with 2D, isotopic mass effect is quite significant whereas for other elements of higher mass, it will be insignificant. Thus, main criteria for the choice of a radiotracer are type of radiation emitted, and half-life of radioisotope including its availability.