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.
An alternative method is to introduce the sample material into the
vacuum chamber in the form of a gas (like argon, for example), and then
bombard the gas with electrons streaming from a hot filament. The
electron stream will knock some electrons off the gas molecules and this
also will produce positive ions. Either process of ion production is
satisfactory, depending on the problem to be tackled, but the mass
spectrometers for the two methods are naturally quite different.
Whichever way the ions were produced, they are next exposed to a strong
electric field, accelerated, and electrostatically focused into a beam.
These charged particles are directed into a magnetic field between the
pole faces of an electromagnet. The magnet does the analyzing by the
principle of magnetic deflection that was known to André Ampere and
Michael Faraday more than a century ago. Any moving electric charge has
a magnetic field associated with it. This field interacts with the field
of the analyzing magnet to impress a deflecting force on the charge. The
force acts at right angles to the direction the charge travels and also
at right angles to the direction of the impressed magnetic field. The
pull of this force depends only on the electric charge and the speed of
each particle: A light single-charged particle will be deflected more
than a heavier particle with the same charge. In this way, the ions in
the beam are sorted out into a number of separate beams, each made up of
particles of the same charge/mass ratio. Each beam contains one isotope
of the original material, because isotopes differ on the basis of their
mass. By adjusting the current in the electromagnet we can direct these
separate beams into a “collector” and electrically measure their
intensity one by one. This gives the relative abundance of the separate
isotopes in the sample.