Intake Levels. Tritium, H', occurs in natural water to the extent of 

 about 10~'** g H^ per gram H' (H-1). It has been of biological interest 

 primarily on account of its use as a label for water and other metabolic 

 compounds. The 30-day LD50 of H2O for mice was determined as 1 mc/g 

 body weight when given as a single injection (H-2). 



Radioassay. On account of the unusually low beta energy, H^ is quite 

 difficult to count. In general, the measurements are accomplished by 

 gas counting in ionization chambers or Geiger tubes or by solid counting 

 with internal counters. Gas counting is highly efficient but requires 

 complicated sample preparation and handling; solid counting is relatively 

 inefficient. About 3 X 10~^ /xc can be detected in an ionization chamber 

 with the vibrating reed. Liquid-scintillation-counting procedures have 

 been described (H-3, H-4) which offer possibilities of easier sample prep- 

 aration than gas techniques and better sensitivity than solid techniques. 

 However, there are many technical difficulties yet to be overcome. 



Chemistry. The chemical behavior of H^ is similar to that of H\ and 

 details of methods are presented below. 



Typical Methods. A procedure has been described for combustion of 

 a 10-mg biological sample for assay of H^ C'^, and C^'^ (H-5). A micro- 

 combustion train is used, and the HoO produced is treated with n-butyl 

 magnesium bromide to yield n-tritiobutane, which can be counted in the 

 gas phase. The actual combustion takes only 5 to 6 min. A one-step 

 method has been described for quantitative conversion of tritium in 

 organic compounds to a mixture of methane and hydrogen by heating 

 with zinc, nickelic oxide, and water in a sealed tube at 640°C (H-6). In 

 a study with mice (H-7), tritium oxide was administered intraperitoneally 

 daily for 13 days in 0.5 ml of physiological saline for a total of L2 mc. 

 The entire carcass was ground with 100 ml benzene in a Waring Blendor, 

 and the body water w^as obtained by distillation of the benzene suspen- 

 sion. To obtain the bound water, the benzene was removed from the 

 carcass by distillation, and the dry residue mixed with copper oxide and 

 burned in a combustion tube at 750°C. The water formed w^as collected 

 in a dry-ice trap. Body water and combustion water were analyzed for 

 tritium by treatment of the water with CaC2 to produce acetylene, which 

 was measured with an ionization chamber and vibrating-reed electrometer. 



In a study of retention of tritium over periods of 4 to 8 months, rats 



