448 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 18 



in light of the possibility of serious radioactive contamination. Wherever 

 feasible, therefore, wet ashing should be considered if the radioactive element 

 is likely to be volatilized at muffle-furnace temperatures. 



For radioactive tracer studies, ashing to constant weight is not important. 

 The aim is solely to decompose the tissue to the point where organic com- 

 pounds, which are likely to interfere in the subsequent analysis, are com- 

 pletely decomposed. Ashing beyond this point, to oxidize the last traces 

 of carbon, is unnecessary and time-wasting. The use of perchloric acid after 

 such incomplete ashing should not only be safe but should also facilitate the 

 dissolution of the inorganic salts. The loss of important elements from the 

 tissue ash might very well be eliminated by this less vigorous procedure; com- 

 pounds difficult to dissolve should not be produced so frequently. 



Aids to ashing have been employed with considerable success. Calcium 

 nitrate, magnesium acetate, and mixtures of calcium and aluminum nitrates 

 may be advantageously used. These salts act not only to supply oxygen, but 

 they also increase the bulk of the ash and render it more amenable to dissolu- 

 tion. The use of these aids is indicated with samples producing little or no 

 ash of their own, e.g., fatty tissues and carbohydrates. Magnesium acetate 

 in alcohol is particularly recommended for use with such finely ground mate- 

 rial as flour [4]. Fusion of material with the crucible is practically eliminated 

 when ash aids are employed. Etching of the glaze on porcelain is not com- 

 pletely prevented, however, and chlorine attack on platinum, due to oxida- 

 tion of chloride by nitrate, is likely to occur. 



18.3. Wet Ashing. Less drastic than furnace ignition but more satis- 

 factory in many respects are the procedures for oxidation of organic mate- 

 rial in solution. Wet digestion may require more attention and will fre- 

 quently be slower than dry ashing, but on the whole these disadvantages are 

 more than offset by the superior results obtained when the quantitative 

 recovery of certain elements is desired. The choice between wet and dry 

 ashing is based, therefore, upon consideration of the likelihood of loss by 

 volatilization at higher temperatures versus the inconvenience in increased 

 time and labor of digestion at lower temperatures. Maximum wet digestion 

 temperatures are governed by the boiling point of the solution in which this 

 digesting occurs, provided that the samples are not taken to dryness, and are 

 roughly 400 to 500°C lower than the usual minimum dry-ashing temperatures. 



The Kjeldahl procedure, with modifications, utilizes concentrated sulfuric 

 acid as an oxidizing agent and has been by far the most widely used method 

 for digesting plant and animal tissues. In addition, potassium sulfate or 

 persulfate together with a catalyst such as copper, mercury, or selenium are 

 generally employed. Perchloric acid may be used to great advantage with 

 sulfuric acid as a modification of the original Kjeldahl method. 



Nitric acid and mixtures of nitric acid with hydrochloric acid, hydrogen 



