PKINCIPLES OF TRACER METHODOLOGY 7 



from a sample is direct!}^ proportional to the radioisotope content, the 

 measurements may readily be made quantitative. Therefore, in many 

 of the applications already described, comparisons can be made of the 

 degree of movement as a function of experimental variables. 



Concept of Specific Activity. If measurements can be made both of 

 the radioisotope content and of the total amount of the element in the 

 sample, then a most powerful approach is available for metabolic inves- 

 tigations. The relationship between the radioisotope and the total 

 element content can be expressed as the specific activity, a term that has 

 been given various meanings. To avoid ambiguity, it will be used here 

 to designate the amount of radioactive element per unit weight of the 

 element present, this weight to include both active and stable isotopes. 

 As will be discussed in a later section, it makes no difference what units 

 are used so long as they are consistent between the values which are to be 

 compared. This usage of the term seems most practical, since the actual 

 value is directly derived from the measurements usually made. For 

 example, if a biological sample is measured to contain A mg phosphorus 

 by chemical analysis and B counts/min of P'^^ under some specific count- 

 ing conditions, the specific activity may be expressed as B/ A counts/min/- 

 mg (for purposes of discussion counts per minute is used as a measure of 

 radioactivity) . 



In any comparison of this value with others it would be necessary that 

 identical counting conditions be used. In practice, this procedure would 

 be feasible since the same counting arrangements are usually employed 

 for all samples. Otherwise it would be necessary to convert all counts 

 to a standard base by the use of a single standard or sample on all the 

 counting instruments used. It may seem unnecessary to caution that 

 the radioassay and chemical assay of the sample must represent the same 

 functional state of the element or compound. For instance, there would 

 be no meaning to a specific activity for P^^ in plasma based on a chemical 

 determination of the plasma inorganic phosphorus and a radioassay of the 

 total P'^- content of the plasma, since the plasma inorganic phosphorus is 

 only a part of the total plasma phosphorus. 



Determination of Ion Movement under Conditions of No Net Mass 

 Transfer. Before the advent of radioisotopes it was virtually impossible 

 to measure the transport of ions between a tissue and a surrounding 

 medium under conditions in which there was little or no net transfer of 

 mass. This led to many theories and concepts of selective accumulation 

 and physiologic transport which were based on the selective permeability 

 of membranes. Radioisotope studies have in many cases provided 

 unequivocal evidence for the transport of ions in both directions through 

 membranes that were previously considered impermeable. 



Let us consider a simple system of a tissue suspended in a medium in 



