Ions moving in a magnetic field travel with a curved trajectory. If
the ions are initially traveling perpendicular to the magnetic field,
the trajectory is actually a circle. If the field is strong enough,
the ions can be trapped in this circular trajectory within the field.
Such an experimental apparatus is called an ion cyclotron. Plates
are placed near the radius of trajectory; as packets of ions pass
the plates, they induce a charge on those plates. This creates a time
dependent signal, with the signal large when the packets are close
to the plate and small when they are far. The signal actually varies
as a sine wave with the same period of oscillation as the ion packets
in their circular trajector.
To collect the mass spectrum, a radio frequency pulse is focused on
the ions. This causes the trajectory radius to expand; after the pulse,
decay to the original trajectory radius is mass dependent. These time
dependent decays cause more complicated waveforms to be induced on
the plates. A Fourier Transform (as is used in FT-IR) is applied to
the time dependent signal and produces the mass spectrum. Such Ion
Cyclotron Resonance (ICR) instruments have produced the highest mass
resolution of any mass selection technique. The mass of two electrons
(much, much less than the mass of the lightest atom, H) can be resolved.
Advances in this technique are leading to isomer resolution where
the 'mass difference' is the relativistic (using 
) energy
difference due to the different heats of formation.
ICR instruments are very expensive and used for research. The are not generally found in production analytical laboratories.