XVII. RADIOACTIVE TRACERS G31 



als, particularly if little or no carrier is present, there are rules that 

 must be taken into account and precautions that must be considered 

 if the results arc truly to mean what they are supposed to mean. 

 When dealing with isotopes that are actually carrier-free one is work- 

 ing in a concentration range far below that of ordinary chemical ex- 

 perience. For example, it can easily be calculated that a solution of 

 HsP^^o^^ completely free of stable P^S with an activity of 1 mc. per 

 liter will have a concentration of 10"^" M. At such concentrations 

 the tracer may exhibit a behavior quite different from that at ordinary 

 concentrations. 



In particular, one has to guard against removal of the tracer from 

 solution by adsorption processes. Even the small surface that a 

 glass beaker presents to a solution contained in it may be enough to 

 remove an appreciable fraction of the tracer from the solution. The 

 precipitation of an insoluble substance that is not supposed to carry 

 the tracer is often likely to do so. According to rules set down by 

 men working in the field of natural radioactivity, such coprecipitation 

 is only supposed to occur under certain definite conditions (44), but 

 if the tracer is truly carrier-free it may and probably will b^ brought 

 down when least expected. For these reasons it is suggested that the 

 use of completely carrier-free isotopes be avoided if it is possible to 

 add enough carrier to bring the concentration of the solution to 10~* 

 M or greater. If chemical operations at the lowest concentration 

 range are necessary, each step should be examined carefully to make 

 sure that the expected behavior is actually being obtained. In gen- 

 eral it is wise to be skeptical of precipitation processes or other steps 

 in which a large surface is presented for adsorption. Dry processes 

 such as volatilization are safe from the standpoint of predictability, 

 but every precaution must be taken to avoid the loss of the micro- 

 gram or less of active material and the consequent contamination of 

 the laboratory. A completely closed system is mandatory for such 

 operations. The technique of liquid-liquid extraction is well adapted 

 to the chemistry of carrier-free radioisotopes. In most cases the be- 

 havior of high and low concentration levels is similar and no large sur- 

 face is presented for adsorption. Another technique that has had 

 extensive study as applied to carrier-free substances is that of ion 

 exchange. In this case adsorption is utilized to aid in the desired chem- 

 cal separation or process (4-5) • 



Though it may not seem likely, difficulty is frequently experienced 

 in the simple process of diluting tracer with a carrier isotopic with it. 



