This is a process for analyzing an unknown material by incorporating
uniformly into it a small amount of a radioactive test substance and
determining how much the tracer radioactivity is altered by dilution in
the original material.
It works like this: Let’s say that we have an unknown number of atoms,
_x_, of a given element. The normal isotopic composition of this element
is accurately known, as it is for most elements, and the ratio of two of
its isotopes can be expressed as A/B. We now add to _x_ a known (but
usually smaller) amount, _c_, of the same element. This quantity has a
drastically different isotopic ratio, A′/B′. We mix _x_ and _c_
thoroughly together. The ratio A′/B′ can have almost any value, but must
be different from A/B and we must know exactly what it is. (There are
many ways of determining this chemically, or we can use a sample isotope
of known composition obtained from the U. S. Atomic Energy Commission’s
Oak Ridge National Laboratory at Oak Ridge, Tennessee.) The substance
added is known colloquially as the SPIKE.
After the original material and the spike are thoroughly mixed we have:
_x_(A/B) + _c_(A′/B′) = (_x_ + _c_) (A″B″)
in which A″/B″ will be the ratio of the two isotopes in the mixture.
With this information in hand, we can perform any chemical purification
or transfer process with the material (see photo on page 22), without
having to worry about loss. (Even if 90% of the material should be lost
in some operation, the isotopic composition would not be changed, and
that is all we are interested in.) Now we can place the material
containing the isotopic mixture in a MASS SPECTROMETER, which will
determine the ratio A″/B″. When we have that, we may substitute the
value of A″/B″ in the equation and quickly calculate _x_, the unknown
concentration of atoms in the original sample.
[Illustration: _A large (12-inch) mass spectrometer (at left) in use.
Electronic equipment (right) charts results (see page 21)._]
[Illustration: _Essential parts of a mass spectrometer. Atoms to be
analyzed are changed to ions in the source. Then the ions are
accelerated by high voltage, deflected in a magnetic field according to
their mass, and the intensity of the separated beams is measured in the
collector._]