160 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



of laboratory animals, it becomes almost impossible to get accurate fresh 

 weights, and it is usually better to express the results on a dry-weight 

 basis. Depending on the experiment, samples should be dried as soon as 

 possible after collection to minimize chemical and biological changes such 

 as dry-weight losses due to respiration. Material should be dried in a well- 

 ventilated oven at 60 to 70°C. Good ventilation tends to reduce decom- 

 position of organic constituents, and in special cases it may be necessary 

 to dry under vacuum. If dry weights are required, the samples should be 

 finished at 100 to 110°C. If the dried material is hygroscopic, weighing 

 bottles may be needed for the drying and weighing. 



Ashing. Depending upon the characteristics and amount of radio- 

 activity present, fluid samples such as blood, plasma, urine, bile, or milk 

 may be assayed directly by licjuid counting. Also, as discussed later, it 

 is sometimes feasible to count solid samples directly. However, it is 

 often necessary to reduce solid samples to a homogeneous-solution phase 

 for liquid counting, and sometimes large samples of milk, blood, etc., 

 must be ashed to permit concentration. Where simultaneous chemical 

 analyses or chemical separations are required, it is frequently necessary 

 to oxidize completely the organic matter of the sample. 



Pseudo Wet-ashing. This method is convenient when the only require- 

 ment is the production of a homogeneous solution. Small samples of 

 plant or animal tissue may be placed in a graduated test tube, concen- 

 trated nitric acid added to dissolve the tissue, and the solution diluted to 

 volume for direct liciuid counting. For larger samples, a typical proce- 

 dure is as follows (13): A representative sample, up to 50 g, is cut into 

 small pieces and placed in a 400-ml beaker, to which about 40 ml concen- 

 trated nitric acid is added. The tissue is allowed to soak 10 min, after 

 which gentle heat is applied. It there is not too much fat or particulate 

 material at this stage, the acid solution can be made to volume for direct 

 solution measurement. If there is appreciable fatty material present, it 

 is better to make a solvent extraction. In this case the original acid 

 digest is evaporated to about 15 ml on a hot plate and then transferred to 

 a steam bath for continued evaporation to remove the acid. The residue 

 is washed into a separatory funnel with small amounts of warm water. 

 The beaker is rinsed with two 10-ml portions of isoamyl alcohol which are 

 in turn transferred to the separatory funnel. The separatory funnel is 

 shaken gently, and the two solutions are separated and made to volume 

 for measurement with a Geiger counter. The activities as measured in 

 each solution are added to give the total for the sample. Isoamyl alcohol 

 is a suitable general solvent chiefly because of its high boiling point. 



Certain tissues may require slight modifications of this procedure, and 

 the following suggestions are offered : When it is necessary to pseudo-ash 

 more than 35 g of whole blood, it has been found advantageous to slow 



