Gas-Exposure Procedure 
Good gas-exposure labeling requires 
isotopic purity, relatively high tritium 
pressures (~1 atm) and large surface 
area in the exposed sample. Wilzbach 
uses tritium of about 90% isotopic 
purity. The presence of He*, formed 
by tritium decay, has no apparent 
effect on the process, but accumulation 
of hydrogen and methane makes it un- 
wise to reuse the gas. Higher pressures 
increase both the activation and the 
tritium available for exchange; how- 
ever, present techniques limit pressures 
to just below atmospheric (Fig. 1). 
For safety and economy it is advan- 
tageous to use a limited amount Of gas 
and a small sample. Subsequent di- 
lution is better than using a large 
sample to start with. Since the tritium 
beta has a range of only 0.7 mg/cm?, it 
is best to use solids in finely powdered 
form and to shake liquids. Room 
temperature is generally satisfactory, 
but some compounds display better 
labeling at reduced temperatures. 
Apparatus. Figure 1 shows a typi- 
cal reaction vessel and the pumping 
system used to introduce tritium. 
Samples consisting of 0.1-1 gm of 
liquid or powdered organics are intro- 
duced into the 6-ml vessel through the 
constriction, which is subsequently 
sealed off. Volatile samples can be 
introduced into a similar vessel from 
the Toepler pump. 
After the vessel has been filled with 
sample and placed on the pumping 
apparatus, ~2 cm*® (5 curies) of 
tritium are introduced from a storage 
bulb or by distillation from uranium 
tritide. The vessel is then pulled 
off at the capillary. Shaking promotes 
the reaction with liquid samples, and 
with solids it is customary to rotate 
the tube and distribute the sample as 
well as possible over the walls. 
The length of the gas exposure de- 
pends on the extent of radiation 
damage. This is expressed in terms of 
G_m, the number of molecules destroyed 
per 100 ev absorbed. Unfortunately 
this number has been measured for 
only a limited number of organic com- 
pounds, but the available values have 
been tabulated (4). Its value de- 
termines the length of the optimal 
exposure. G_y ranges between 1 and 
10, and optimal exposures are usually 
10-30 curie-days per millimole. 
Incorporation of tritium is expressed 
in terms of Gz, the number of tritium 
186 
Gas-Exposure Labeling—Apparatus and Results 
Reaction 
vessel, 
\ 
Constriction 
pulled off 
Tilting 
McLeod gage 
, 
,Metal-glass seals, 
Gourd 7 Bellows _ 
glass — 
seal~ 
‘ 
Uranium 
furnace 
~- Monometer 
FIG. 1. System above permits introduction of gaseous tritium and powdered or 
liquid samples into vessel at upper left. Tritium comes from storage bulb mounted 
on U-tube or by distillation from uranium tritide 
TABLE 1—Tritium Labeling by Gas Exposure 
Tritium incorporated 
Sample Amount of In pure 
Compound weight Exposure tritium Total Labile product Values of Gr 
exposed (gm) (days) (curies) (mc) (me) (me/gm) Crude “Pure” 
Toluene 0.86 2.9 7.5 42.7 None 22.2 0.22 0.10 
n-Heptane 1.37 9.8 6.9 17.5 None 1.3 0.27 0.003 
Benzoie acid 1.31 5.0 6.4 156 40 14.0 0.57 0.067 
Sucrose 4.0 6.7 14.0 593 480 5.0 0.72 0.024 
Cholesterol 1.88 4.8 7.2 335 90 64.3 1.11 0.40 
Digitoxin 0.50 5.8 es) 438 182 90 1.16 0.13 
eS 
TABLE 2—Distribution of Tritium in Toluene 
The specific activities of these toluene . indicate this tritium distribution 
derivatives ... in gas-exposure-labeled toluene 
Specific 
activity Per cent of tritium 
Compound (uc/mmol) at various positions 
Toluene 18.9 85 CH; 
Pentabromotoluene 1.6 | 
Benzoic acid 17.3 27.2 27.2 
p-Nitrobenzoic acid 14.9 
* * 
2,4-Dinitrotoluene 11.0 12.2 12.2 
2,4,6-Trinitrotoluene 6.2 12.7 
* By difference. 
