446 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS 



Chap. 18 



18.2. Dry Ashing. Ashing at 500 to 800°C is generally carried out in 

 platinum, porcelain, Vycor, or nickel crucibles, although in general, platinum 

 is to be preferred. It cannot be employed, however, if fluxes are used that 

 contain such materials as potassium acid sulfate, sodium peroxide or hydrox- 

 ide, excessive amounts of potassium nitrate, or any compounds likely to 

 liberate free chlorine. Such fluxes are not usually encountered in biological 

 work, but if they are needed, it is advisable to use porcelain or Vycor, pro- 

 vided that etching by the alkali is not objectionable. Admixtures of 

 such materials as magnesium acetate and calcium hydroxide will not result 

 in attack on platinum. If the dissolution of ash requires the use of aqua 

 regia, porcelain or Vycor must be used, whereas platinum is indicated when 

 hydrofluoric acid is employed. 



One of the most difficult problems encountered when porcelain is used is 

 the fusion of the inorganic material with the crucible; this is a particularly 

 undesirable characteristic of some animal tissues. Although the loss of a 

 small portion of the sample due to crucible fusion may not appreciably 

 affect the analytical results, it may preclude the subsequent use of this 

 crucible. The danger of cross contamination due to residual activity in 

 laboratory glassware must be kept in mind at all times; crucibles showing 

 unremovable residual activity should, therefore, be discarded. Further- 

 more, if fusion occurs, recovery of the ashed material is not only difficult but 

 time-consuming. Vycor and platinum are more satisfactory under these 

 conditions, although from an economic standpoint it may be less expensive to 



