Tritium Tracing 
—A Rediscovery 
Better labeling techniques, more sensitive detection and lower price 
bring new usefulness to tritium. 
Good half-life and versatility make 
this radiohydrogen valuable to biologists, chemists and geologists 
TRITIUM HAS recently been “‘redis- 
covered” as a tracer. Three develop- 
ments have contributed: 
(a) Tritium is cheaper. In recent 
years the price has dropped by a factor 
of 50—from $100 to $2 per curie. 
(b) Detection is more efficient. In 
particular, liquid-scintillator counting 
techniques have made possible full- 
efficiency counting without self-absorp- 
tion. This conquers the greatest tech- 
nical difficulty—detection of the very 
weak (18 kev max) beta particles. 
(c) Labeling is easier and more versa- 
tile. Recoil labeling that makes use of 
energetic tritons from the Li®(n,a)T 
reactions in a nuclear reactor is effective 
for a wide variety of compounds, and 
discharge-tube labeling promises a 
means of using the same technique 
with simpler apparatus (1, 2). More 
recently a still simpler, still more versa- 
tile technique has been developed by 
K. E. Wilzbach at Argonne National 
Laboratory. This is gas-exposure la- 
beling in which tritium serves as both 
a radiation source and the inserted 
label (8). Discharge-tube labeling 
and gas-exposure labeling have the im- 
pressive advantage that they can be 
carried out with simple apparatus. 
There are other contributing causes 
for the tritium revolution. Tritium is 
a very useful tracer for biologists and 
organic chemists because it is the only 
radioactive form of hydrogen, the most 
common element in organics. 
In spite of technical difficulties, tri- 
tium was used as a tracer before World 
War II. With the accelerated de- 
velopment of radioactivity techniques 
carbon-14 displaced it as the most used 
radiotracer for organics because of C14’s 
easier detection and labeling. The 
current revolution is bringing tritium 
back into prominence and forcing scien- 
tists to reread prewar publications. 
Lower costs and wider use are making 
it possible to exploit many of its 
advantages: useful half-life, high spe- 
cific activities, low cost, ete. Even the 
weakness of its betas is sometimes an 
advantage, particularly in producing 
high-resolution radioautographs (see 
box p. 64). 
Ten experts in the use of tritium of- 
fered a detailed picture of the state of 
the science at a recent symposium 
sponsored by the New England Nuclear 
Corp., Atomic Associates, and Packard 
Instrument Co. They spoke on label- 
ing techniques, detection, health-phys- 
ics considerations, and uses in the 
pharmaceutical industry, petroleum- 
reservoir engineering, biochemistry, 
and radioautography. 
TRITIUM LABELING 
Tritium’s usefulness as a tracer has 
been greatly increased by the gas-ex- 
posure labeling technique developed by 
K. E. Wilzbach and described by him 
at the symposium. The compound to 
be labeled is exposed to tritium gas, 
which serves a double function: beta 
particles activate either gaseous tritium 
or the molecules of the compound, and 
this promotes exchange between hydro- 
gen in the molecule and gaseous tritium. 
(It is not clear whether the mechanism 
is activation of tritium or activation of 
the exposed molecule, but the difference 
is not important to people who want to 
use it.) 
Other tritium labeling methods are 
the tritium-recoil method (/) and 
chemical synthesis and exchange tech- 
niques, which can be used in combina- 
tion with gas exposure and recoil. Gas 
exposure has the advantages of smaller 
radiation damage and simpler labora- 
tory techniques. 
In both gas-exposure and recoil label- 
ing the product usually contains small 
amounts of high labeled impurities. 
Removal of these by-products requires 
efficient purification procedures. These 
lead to most of the effort involved 
in using the two methods. 
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