164 



RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



miiim trioxide, 167 ml phosphoric acid (density 1.7), and 333 ml fuming 

 sulfuric acid containing 20 per cent free sulfur trioxide. The mixture is 

 heated and swirled occasionally, and heating is discontinued when the 

 temperature reaches 150°C. A beaker is inverted over the mouth of the 

 flask during cooling. Care is taken to keep dust out of the solution during 

 subsecjuent storage and use. A modification of the combustion procedure 

 as described by Claycomb, Hutchens, and Van Bruggen (26) is presented 



below. 



The apparatus is shown and de- 

 scribed in Fig. 5-1. It is essential 

 that all components of the oxidation 

 J //((O)^ apparatus be initially cleaned in a 



' \ '^^ ( r~l ^^^ bath of Van Slyke-Folch oxida- 



tion mixture. After initial assembly 

 and use, routine cleaning of flask A 

 with this reagent is strongly recom- 

 mended. Such treatment destroys 

 all organic contaminants including 

 trace amounts of the silicone lubri- 

 cant used, thus assuring low blank 

 values. After cleaning, the equip- 

 ment is thoroughly rinsed and 

 oven-dried. 



Delivery tube C is solidly clamped 

 to a movable ring stand, the mercury 

 manometer being attached and 

 fastened to the same stand. A 

 minimal amount of silicone lubricant 

 is used on stopcock D and all other 

 glass-to-glass connections. 

 The sample to be combusted, containing 5 to 10 mg carbon, is placed 

 in the bull) of A. Solutions may be pipetted into the flask, or solids may 

 be introduced in a porcelain boat of suitable dimensions. Solutions in A 

 are taken to dryness by attaching the flask to the evaporation sleeve G 

 and immersing flask A in a hot water bath, with the empty oxidant tube 

 B in an upright position. Dry C02-free air is drawn in through tube H 

 by applying a minimal vacuum via the side tube /. To minimize bump- 

 ing and splattering, drying is accomplished largely by the air stream from 

 H rather than by strong vacuum distillation. When the sample appears 

 dry, flask A is removed from G, and with side arm B directed downward, 

 5 ml of the Van Slyke-Folch reagent is carefully added to B with a bent- 

 tip pipette. The flask is attached to tube C, care being taken not to spill 

 any of the oxidant onto the sample to be combusted. The absorption 



Fig. 5-1. Combustion assembly for C^ 

 assay. [From Cecil K. Claycomb, 

 Tyra T. Hutchens, and John T. Van 

 Bruggen, Techniques in the Use of C^* 

 as a Tracer. I. Apparatus and Tech- 

 nique for Wet Combustion of Non-volatile 

 Samples, Nucleonics, 7: 38-41 (1950).] 



