Ions traveling coaxially within an oscillating quadrupole electric
field will travel in a spiral around the axis. If the oscillating
field is offset by a constant voltage (for example, it is oscillating
above and below +10 V rather than 0 V), the radius of the spiral is
mass dependent. As with magnetic sector instruments (Section 5.3.2),
ions other than those selected are removed from the ion beam. The
mass spectrum is collected by scanning the offset voltage.
Quadrupole instruments are relatively simple, rugged and easy to use.
They generally provide nominal mass resolution (unit mass), though
resolutions slightly higher can be achieved with an extreme loss of
signal. Quadrupole mass spectrometers capable of collecting spectra
up to m/z 750 are barely over one foot long, making them useful for
small footprint, benchtop analytical laboratories. The mass spectrometer
in this laboratory is one of these small quadrupole instruments.
A variation on the quadrupole technique that merits mention is the ion trap. These instruments are capable of storing ions for long periods; this allows very detailed experiments to be performed to investigate the nature of the decomposition reactions that produce the 'normal' mass spectrum.