Sec. 18.2] TREATMENT OF TISSU E FOR RECOV ERY OF RADIOISOTOPES 447 



use porcelain and to discard the crucibles if appreciable activity remains 

 in them. 



For dry ashing the muffle furnace which has heating elements at the sides 

 as well as top and bottom is much to be preferred to gas burners. Not only 

 is the ignition more convenient, but it is also more uniform. The muffle 

 furnace has the disadvantage, however, that when completely filled the 

 available oxygen inside is rapidly exhausted. When this condition is reached, 

 the combustible gases produced will leave the furnace as a tarry and soot- 

 laden smoke; the latter must usually be removed by forced ventilation. To 

 avoid this and to aid in combustion, oxygen may be introduced into the 

 furnace through the thermocouple housing, using a quartz tube. When this 

 procedure is followed, the heating units must be reversed if possible so that 

 the coils do not face toward the inside of the furnace, and furthermore the 

 stream of oxygen must not be directed onto the thermocouple itself. 



The general ashing procedure consists of air-drying the weighed sample 

 at a temperature of 100°C or slightly less followed by ignition at 500 to 

 800°C for a minimum of 2 hr. Samples that'have been ground or macerated 

 may be dried at reduced pressures; 100 mm or less is recommended for grain 

 and stock feeds, meat meal, plant tissues, etc. [3]. Caution must be exercised 

 in drying whole tissues, such as liver, since they tend to swell at elevated 

 temperatures or reduced pressures or both. Temperatures above 100°C are 

 permissible for removing the last portions of water and volatile oils. Drying 

 by means of overhead radiant heating has been found to be particularly 

 useful with nearly all types of biological tissues as well as with solutions of 

 both organic and inorganic compounds. 



In many cases the material to be ashed is already dry enough to place in 

 the muffle furnace. Ground grains, bone, and the woody portions of plants, 

 for instance, seldom need to be dried before ignition. 



When possible cross contamination or contamination of the muffle furnace 

 is likely to influence the results, it is best to place the samples in a cold fur- 

 nace and to allow the temperature to rise gradually to the value desired. 

 Rapid ashing produces volatile gases at rates that are sufficient to carry small 

 amounts of the material out of the crucible. Thus radioactive deposits will 

 collect on the walls and floor of the furnace as well as in the adjacent crucibles. 



The duration of ashing and the temperature must be considered in relation 

 to the volatility of certain compounds. For example, some iron generally 

 will be lost as ferric chloride, phosphorus as phosphoric acid, sulfur as sulfur 

 di- and trioxide, fluorine as hydrogen fluoride or silicon tetrafluoride, and 

 chlorine as hydrogen chloride. Because of the difficulty of completely 

 mixing samples with additives that are designed to prevent these losses, 100 

 per cent recovery of some elements is impossible. From a quantitative 

 standpoint these losses may be inconsequential, but they must be considered 



