Answer 4.10

a) Fourier transformation (FT) and image current detection are also employed in the operation of another type of mass analyzer. Which one?

Orbitraps also employ Fourier transformation and image current detection.

b) Briefly describe the superimposed motion of ions in an Orbitrap analyzer.

In an Orbitrap, radial electrostatic ion trapping forces the ions to rotate around a spindle-shaped central electrode. The shape of the electrode causes a gradient of the axial electric field symmetrically from the center to the ends of this central electrode. As a result, while rotating around the spindle, the ion cloud at the same time moves back and forth in axial direction. Overall, ions of the same m/z move in a ring-shaped ion cloud oscillating along the spindle electrode.

c) Which ion motion is exploited for mass analysis in an Orbitrap analyzer?

The axial oscillation is detected via image currents induced in two outer ring electrodes each of them enclosing one half of the central spindle. The transient is recorded and axial oscillation frequencies are obtained by FT of the signal.