Like Time-Of-Flight, magnetic fields can be used to analyze ions based
on velocity. A magnetic field causes an ion trajectory to curve, and
the radius of curvature is related to the velocity of the ion. Magnetic
sector instruments have a 'bent' flight tube with an electromagnet
with an adjustable field at the bend. For a given magnetic field strength,
only one velocity (for singly charged ions) has a trajectory radius
equal to the instrument radius; this implies that ions of only one
(nominal) mass can traverse the instrument. Ions of all other masses
either don't curve enough or curve too much and crash into the flight
tube walls. To collect the mass spectrum, the magnetic field strength
is scanned.
Magnetic sector instruments are typically combined with an electrostatic sector analyzer (ESA) that provides kinetic energy focusing. This combination, double focusing sector instruments, provides sufficiently high resolution mass analysis that ion molecular formulae can typically be measured. Further, by controlling the ratios of scanning parameters and instrument geometry for the ESA and magnetic sectors, specialized experiments such as kinetic energy release (during decomposition reaction), constant neutral loss spectra, common parent ion spectra, etc, can be conducted. While there are analytical applications that require high resolution, these instruments have more of a research role than as work-horse bench instruments.