GENERAL PROCEDURES FOR RADIOASSAY 159 



PRETREATMENT OF SAMPLES 



Collection. The proper collection of samples is just as important as 

 the reliability of suhsecjuent analysis. The sample should be truly typical 

 of the material that it represents. There may be many biological factors 

 that can be taken into account only by the investigator. For instance, 

 special consideration may be given to the sampling of pasture to simulate 

 the grazing animal, or if an animal is being fed hay, it may selectively 

 reject the stemmy portions, which must then be analyzed separately to 

 give a true measure of the nutrient intake. Care must be taken to mix 

 feces well before sampling, because successive increments may differ 

 widely in activity, especially at short times after administration of the 

 radioisotope. Sampling of bone presents a particularly difficult problem 

 which will be discussed in Chap. 7. Conventionally, large samples of 

 dried material are often reduced by grinding to allow selection of repre- 

 sentativ^e subsamples. This poses considerable difficulties with radio- 

 active material on account of the dust produced, and grinding is not 

 usually recommended unless precautions are taken against this hazard. 

 Analysis of numerous small samples may overcome this problem, espe- 

 cially where the analytical procedure is simple. With plants, excessive 

 separation of leaf, stem, and seed should be prevented; otherwise it 

 becomes difficult to subsample the material. 



In radioisotope studies, cross-contamination must be particularly 

 guarded against on account of the extremely high dilutions that are often 

 measured. For example, care must be taken to avoid adhering soil 

 particles on plant material taken from radioactive soil. In animal 

 studies, precautions must be observed to ensure that the tissue samples 

 do not become contaminated by the original dose or active excreta. Any 

 radioactivity found at the site of administration may well have arrived 

 there mechanically rather than metabolically; after intraperitoneal injec- 

 tion, activity found in the liver or other abdominal organs may also have 

 been deposited there mechanically. If the radioisotope has been given 

 orally and the animal sacrificed shortly thereafter, it is essential that the 

 tissue samples not be allowed to come into contact with intestinal con- 

 tents. As a further illustration, in tissue distribution studies with ele- 

 ments like calcium or strontium which selectively deposit in bone, soft- 

 tissue samples that become contaminated with bone particles may give 

 erroneous measurements. 



Weighing and Drying. When fresh weights are necessary for concen- 

 tration calculations on this basis, the samples should be weighed before 

 any appreciable moisture loss has occurred. If they cannot be weighed 

 immediately, they should be kept in closed containers to minimize weight 

 loss. With some small samples such as adrenals, thyroids, or pituitaries 



