Page 271 - Radiochemistry and nuclear chemistry

Uses of Radioactive Tracers 255

and 4He have all been used. This technique should not be confused with PIXE described
in w In PIXE high energy protons are used to induce X-ray fluorescence in the
sample, and the analysis with the multichannel spectrometers is done simultaneously with
the bombardment. In activation analysis the sample is counted after the end of the
irradiation. Generally, charged particle irradiation techniques are more expensive than
neutron activation analysis.

9.3.4. Substoichiometric analysis

Isotope dilution analysis require the determination of either the chemical yield in the
separation process or of the specific activity. This can be avoided by applying the
substoichiometric principle, which may also increase the sensitivity of the analytical
method.
The method requires three samples: (1) the test sample containing the unknown weight
w u of the element or compound X of interest; (2) a standard of the same element
(preferably in a similar matrix, subscript s); and (3) a nonactive carrier (usually a solution,
subscript c), of the same element or compound X.
If a test sample containing the unknown weight w u of the element or compound X and a
standard containing a known weight Ws ~ of the same are irradiated under identical
conditions,
0
w u = WsA u/A s (9.11)

where A u and A s (or R u and R s, if measured under identical conditions) are the total
radioactivities contained in sample and standard. If the test sample contains other
radioactivities in addition to that in the standard, the preceding relation can be applied only
when the measurement technique is isotope specific, for example, when high-resolution
gamma spectroscopy is used, in which case interference from undesirable radioactivity is
eliminated.
When simple counting equipment must be used or highly complicated test samples are
involved, it may be desirable to isolate chemically the element or compound of interest.
This can be done through selective chemical procedures using a non-radioactive carrier of
element X. The carrier is added to the irradiated test sample (containing Wu), which is
processed through the different purification steps. Finally a sample of almost (or
sufficiently) pure X is isolated, and its activity Ru+ r and weight Wu+ r measured. It is also
necessary to run the irradiated standard with added carrier through the same chemical
separation procedures. With the exception of w u, the final equation (9.12) has terms that
are known or measured:
o {Ru+o (w~ (wO+o+wO)/w +o }
W u -" W s (9.12)
Usually, the amount of career is much greater than the unknown and the standard, that
0 ~, Ws ' and ~, w u. Then if equal amounts (by weight) of the nonactive isotopic carrier
is, w e 0 0
are added to the unknown sample and the standard, the equation reduces to