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._] Mass Spectrometry The mass spectrometer measures isotopic abundances using a magnetic field to sort electrically charged particles into groups according to their masses. It works this way: A small drop of material to be analyzed is placed on a metal filament and dried. The filament, in its holder, is placed inside the mass spectrometer, and heated electrically in a vacuum, like the filament in a light bulb. As the wire begins to glow, some of the sample begins to radiate, or “boil off”, losing an electron or two in the process. In other words, some of the atoms will be changed into positive IONS.