Answer 4.4

a) What forces have to compensate each other in a magnetic sector analyzer in order to keep ions of a certain m/z value on a stable flight path of defined radius?

An ion travelling in a direction perpendicular to a homogeneous magnetic field will follow a circular path of a radius that fulfills the condition of equilibrium of Lorentz force and centripetal force.

b) Explain the term “double-focusing magnetic sector instrument”?

Energy focusing is achieved by combining a magnetic sector and an electric sector in a way that the energy dispersion of the magnetic field is just compensated by the energy dispersion of the electric field. Additional direction focusing is obtained, if the radii and angles of these fields and their mutual alignment does not diminish the focusing properties of each of them. Then, an ion optical system is obtained that is able of focusing ions on a single image point, although these were emerging from the ion source in (slightly) different directions and with (slightly) different kinetic energies. This is called double-focusing.

c) What is the difference between BE and EB instruments? Which of them is necessary to perform MIKES, which is better suited for focal plane detectors?

In EB instruments, the electric sector (E) precedes the magnetic sector (B) – an alignment also known as forward geometry. Vice versa, reverse geometry is realized by the BE type of magnetic sector instruments.
There is virtually no difference between EB and BE in normal operation, however, MIKES requires the BE setup, whereas the EB design is beneficial for the combination with focal plane detectors.