Page 272 - Radiochemistry and nuclear chemistry

256 Radiochemistry and Nuclear Chemistry

w u = w ~ (Ru+ o w +o )/(&+o Wu+ ) (9.13)
This equation can be simplified even further. In the chemical separation procedure a
substoichiometric amount of reagent is added, provided the conditions are such that this
reagent quantitatively reacts with X. For example, zinc is extracted quantitatively from
aqueous solutions buffered at pH 8 by dithizone in chloroform. If the amount of dithizone
is less than that of zinc after adding a carrier of nonradioactive ZnSO 4, only part of the zinc
(e.g., 25 %) is extracted, but the dithizone is quantitatively bound to zinc in the organic
phase. Thus varying amounts of zinc in the aqueous phase do not change the amount in the
organic phase, which is constant, though the specific activity varies. The two chloroform
solutions, from test plus career, and from standard plus carrier, contain equal amounts of
Zn. Liquid-liquid extraction is commonly used for this technique.
The foregoing is an example of the basic principle of substoichiometric analysis. Under
these conditions Ws+c=Wu+c and

w u = w 0 Ru+ c/Rs+ c (9.14)

Thus by carefully choosing proper experimental conditions, the analytical procedure is
reduced to two radioactivity measurements. For precise results the value of w u must be
comparable to w 0, which can be ascertained by a few initial survey experiments.
This substoichiometric analysis technique can be applied to most metals with a high
degree of accuracy and a sensitivity of 10 -6 to 10 -10 g of X.
When the substoichiometric principle is applied to isotope dilution analysis, the
relationship becomes

o (R s/R u +s
Wu ~ Ws - 1) (9.15)
where w 0 is the weight of the standard added, and R s and R u +s the radioactivities measured
from the substoichiometrically separated samples of the standard and of the mixture of
standard and unknown. The specific activities neeA not be determined as in (9.8) because
equal weights of standard (ws) and sample plus standard (Wu+s) are isolated by using the
substoichiometric principle.
The usefulness of this technique for routine determination of numerous chemical dements
in various matrixes has been demonstrated by Ruzicka, Stary, and others. It is also
applicable to organic compounds and known in medicine as radioimmunoassay.

9.4. Applications to general chemistry

In no other area have radioactive tracers played such an important role as in the studies
of chemical and biological reaction paths. This is, of course, due to that, in principle, in
each radioactive decay the atom announces its position. Thus the detection sensitivity can
approach the ultimate limit. A radioactive nuclide, as for example 14C (fl- t,,~ 5730 y), 32p
(fl- tth14.282 d), or 198Au (fl-7, tth 2.6935 d), can be followed through a number of
different chemical reaction steps, revealing details of metabolic or process reactions
impossible to discover by other techniques.