Radiolysis of Labeled Compounds

In many situations, the experimenter will prefer to buy labeled compounds from commercial suppliers rather than attempt to synthesize them. The radiochemical purity of such purchased compounds cannot be assumed. Radiation-induced selfdecomposition (radiolysis) can result in the formation of a variety of labeled degradation products, which must be removed before experimental use of the compounds. The extent of radiolysis depends on the nature of the labeled compound, how long it has been stored, and the manner of storage. Radiolysis is most significant with low-energy (3 emitters (especially tritium) since the decay energy is dissipated almost entirely with the compound itself. Furthermore, impurities involving other radionuclides may be present. 

In many cases in which radiotracers are used, the chemical identity of the tracer is not important. These applications can be referred to as tracing physical processes. For example, consider those experiments that seek to locate an object in some system by labeling it with radioactivity and then measuring the position of the radioactivity in the system. Quite often a tracer that decays by y-ray emission is 

A form of isotope dilution (see below) is frequently used to measure the volume of an inaccessible container. A small volume V, of tracer solution is assayed to give its activity A. The tracer is added to the liquid in the container and mixed, and a sample of size Vi is removed and assayed to show its activity A. The volume of the original container is given as 

Leak testing can also be done using radiotracers. Here the basic idea is simple— namely, to inject radiotracer into a pipe, flask, or whatever is suspected to be leaking and look for activity that appears outside the container. Suitable caution must be exercised, of course, to be sure that the leaked radioactivity is not a hazard. 

For studies involving water, it may only be important to assure that the tracer remains fluid-bound. One can use tritium or almost any metal atom that can be complexed with EDTA (ethylenediaminetetraacetic acid) or DTPA (diethylene-triaminepeutacetic acid) (with a large stability constant). 

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