66 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



that no Sr-'" had been deposited there. In other words, if an equihbrium 

 mixture of this type is placed in the system and the samples taken there- 

 from are allowed to stand for about 6 to 10 half-lives of the daughter, then 

 the measurement of the activity of the daughter will give a direct measure 

 of the amount of parent in the sample. 



Interference by radiocontaminants may often be ehminated by appro- 

 priate radioassay procedures. If the radioisotope is separated as a spe- 

 cific chemical compound, then any substance that did not follow it 

 through the chemical reactions would not be counted. For example, 

 Ca*^ is often precipitated as calcium oxalate for measurement — a proce- 

 dure which would automatically eliminate radiocontaminants that did 

 not precipitate or adsorb on the precipitate under the conditions used. 

 Depending upon the comparative radiation characteristics, it is often 

 possible to screen the contaminant activity from the counter while meas- 

 uring that of the principal activity by gamma or hard-beta counting (see 

 Chap. 5). 



Chemical methods may be used for purification but are usually more 

 tedious than physical procedures. The classical separation techniques 

 may be used such as coprecipitation, ion exchange, solvent extraction, 

 and electroplating. If the contaminants are not isotopic with the princi- 

 pal activity and are not daughters, then purification can be performed at 

 any stage prior to measurement. If the contaminants are nonisotopic 

 daughters, then the removal must be effected after the samples have been 

 taken and treated so that translocation cannot occur. 



RADIATION EFFECTS 



It is essential that the experimental results not be affected by any 

 response of the system to radiation from the isotope used. The amount 

 of radioactivity employed should be the minimum necessary to permit 

 reasonable counting rates in the samples to be measured and will be a 

 function of accumulation in the sample and of the sensitivity of the radio- 

 assay. The possibilities of interference due to physiologic response to 

 radiation are minimized by the extreme sensitivity of the activity meas- 

 urements, plus the fact that most studies are of short term and are com- 

 pleted before latent effects come into play. 



It is possible to calculate the radiation dosage delivered to a tissue of 

 known isotope concentration for comparison with known effects from 

 external radiation. However, such comparisons should be accepted with 

 reserve because the radioisotope may become deposited within the cells 

 of a tissue or even incorporated into the metabolic constituents. This 

 may result in a greater biological effectiveness than otherwise anticipated. 

 For example, evidence is now accumulating that P'^- internally located in 

 tissues is mutagenically effective (15, 16). Some of the factors respon- 



