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only be operated in sequential mode, although the speed with which this can be achieved makes it seem
almost like simultaneous mass analysis. The quadrupole mass analyser has the advantage of being
cheap, reliable and compact, with single mass resolution which is sufficient for most applications, and
is therefore the most commonly used mass analyser. However, if an extremely high degree of resolution
or true simultaneous mass analysis is required, then a magnetic sector must be used.

5.4.2 Magnetic Sector Mass Analysis
Magnetic sector mass analysers rely on the fact that ions are deflected by a magnetic field, with ions of
greater mass or charge being deflected to a greater extent. A typical double-focusing instrument,
combining both an electric and a magnetic sector in normal geometry, is shown in Fig. 5.6. The
electric sector can be placed either before or after the magnetic sector with the former termed normal
and the latter reverse geometry, indicating the chronological order in which the two techniques were
developed. In one typical commercial instrument the ions are acclerated after they are skimmed from
the plasma, then travel through the electric sector, which acts as an energy filter. The ions are then
deflected in a single plane by the magnetic field, with the degree of deflection increasing with
increasing m /z. A mass spectrum can be generated by scanning the magnetic field, allowing ions of
consecutively higher m/z to pass through a slit, and then detected. Alternatively, the magnetic and
electric field strengths can be held constant and several detectors arranged in an array around the plane
in which the

Figure 5.6
Schematic diagram of a high-resoution mass spectrometer using magnetic and electric sectors.