162 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



to a 300-ml flat-bottomed Kjeldahl flask. Four milliliters perchloric acid 

 (sp. gr. 1.54) is added, also about 7 ml nitric acid for each gram of sample, 

 followed by 2 to 5 ml of sulfuric acid. The solution is swirled and heated 

 gently for 3 to 5 min or until the first appearance of dense brown fumes. 

 The flask is removed from the heat for about 5 min, and thereafter the 

 digestion is continued slowly until the appearance of dense white fumes 

 of sulfuric acid. The digestion is continued at low heat for another 5 to 

 10 min and then at full heat for a further 1 to 2 min. If the licjuid is not 

 colorless, 1 to 2 ml nitric acid is added, and the solution again digested to 

 fuming. The individual shoidd be aware of the explosive 'potentialities of 

 perchloric acid before using it. 



Lindner and Harley (22) suggested a simple method for wet digestion 

 of small amounts of plant material which has also been found satisfactory 

 for animal tissues. About 100 mg of dry material is placed in a 50-ml 

 Erlenmeyer flask or beaker, 2 ml concentrated H2SO4 added, and the solu- 

 tion heated gently until the sample is partially dissolved. If nitrates are 

 present, the digestion is continued for about a minute after the appear- 

 ance of dense fumes. After cooling, 0.5 ml of 30 per cent H2O2 is added, 

 and the solution heated gently until dense fumes are given off. After 

 cooling, another few drops of H2O2 is added, and the solution heated as 

 before. If the solution does not become clear and colorless after the con- 

 tinued heating, the addition of H2O2 is repeated. With liver samples, for 

 example, 7 to 10 additions of H2O2 were found necessary. 



Dry-asking . When the conventional method of dry-ashing in the 

 muffle furnace is used, care must be taken to avoid losses due to volatiliza- 

 tion, incorporation of the radioisotope into solid carbon particles, or 

 adsorption onto the walls of the crucible. Platinum, fused-silica, and 

 porcelain crucibles are widely employed, the last being considerably 

 cheaper than the others. The porcelain crucibles can be economically 

 discarded when the glaze has been lost or the residual activity becomes 

 high. Platinum should be used if hydrofluoric acid is to be employed for 

 dissolution of the ash, but porcelain or silica must be employed if aqua 

 regia is required. Muffle furnaces with automatic temperature controls 

 are highly recommended. Also it is preferable if the heating elements are 

 not exposed. 



A general procedure is as follows: The dried tissue is weighed into a 

 tared crucible, which is then placed in a cold muffle furnace. Samples 

 such as bone or ground grain do not need to be dried before ignition. The 

 furnace temperature is raised slowly to about 250°C and held there for 

 several hours, after which the temperature is raised to 500 to 600°C for 

 completion of the ashing, which may take several hours more. If the 

 temperature is raised too rapidly, volatile gases may be produced which 

 will carry material out of the crucible with resultant cross-contamination. 



