GENERAL PROCEDURES FOR RADIOASSAY 163 



When seeds or other material low in basic content is ashed, the sample 

 should be mixed with calcium or magnesium acetate or nitrate prior to 

 ashing. These salts give increased oxidation activity, minimize the 

 fusion of ash to crucible, and increase the amount of ash for easier han- 

 dling. Samples like blood and liver often tend to "boil" over, and these 

 must be watched carefully, especially in the preliminary heating from 

 150 to 300°C. It is helpful if small samples are used relative to the 

 crucible size. If the ash does not appear white, it can be treated with a 

 few drops of distilled water, moistened with nitric or perchloric acid, 

 heated to drive off the acid, and re-ashed in the muffle furnace; this also 

 permits the use of a lower maximum temperature. The crucibles are 

 usually allowed to cool in the furnace and then transferred to desiccators 

 to reach room temperature prior to weighing. Care must be taken to 

 avoid blowing of the friable ash, especially in the removal of the desicca- 

 tor cover, if the pressure within has not been equalized. The ash may 

 then be dissolved in HCl solution for further operations. With plant 

 material there is always an acid-insoluble siliceous residue which may 

 retain many of the elements like manganese, copper, or zinc (4). It may 

 be necessary to recover such elements by alkaline fusion or hydrofluoric 

 acid treatment. 



Advantages of wet- over dry-ashing are that more samples can be 

 handled in a shorter time with less e(|uipment required. There are also 

 fewer chances for loss due to volatilization, fusion with crucibles, adsorp- 

 tion onto silica residues, blowing of ash, etc. Dry-ashing requires 

 patience and experience. With some radioisotopes that show losses in 

 transfer, it is convenient to dry-ash small samples directly in porcelain 

 milk-ashing dishes, after which the ash can be spread with a little acid, 

 dried, and counted directly. 



Oxidation for C^^ Assay. Very frequently biological samples cannot be 

 assayed directly for C^^ on account of self-absorption (see page 177) or the 

 physical nature of the sample. It then becomes necessary to oxidize 

 the sample for production of CO2, which maj^ be measured as a gas or 

 absorbed in alkali for conversion to barium carbonate for solid counting. 

 Various oxidation methods have been employed such as Liebig combus- 

 tion, sodium peroxide fusion, persulfate wet oxidation, or Van Slyke- 

 Folch wet oxidation. The text of Calvin (6) and the original papers of 

 Van Slyke and associates (23 to 25) may be consulted for details. Fresh 

 tissues can be oxidized by any of the methods, and dry tissues can be used 

 with any except the persulfate procedure. 



The Van Slyke-Folch method appears to be most generally applicable 

 and convenient. The oxidation reagent suitable for use in the procedure 

 to be described may be prepared as follows (6, 24): In a 1 -liter Erlen- 

 meyer flask equipped with a ground-glass stopper are placed 25 g chro- 



