Thermo Orbitrap Fusion Series - Figure 26. Electrodynamic Squeezing of Ions in the Orbitrap Analyzer (R) Versus the

4
Ion Transmission and Mass Analysis
Mass Analyzers
38 Orbitrap Fusion Series Hardware Manual Thermo Scientific
Figure 26. Electrodynamic squeezing of ions in the Orbitrap analyzer (r) versus the increased field
strength (z)
Figure 26 shows a schematic of the development of an ion packet with the increased electric
field. When the injected ions approach the opposite electrode for the first time, the increased
electric field (from the change of the voltage on the central electrode) contracts the radius of
the ion cloud. A further increase of the electric field continues to move the trajectory closer to
the axis, which allows additional ions (normally with a higher mass-to-charge ratio) to enter
the Orbitrap. After the ions of all mass-to-charge ratios enter the Orbitrap analyzer and move
far enough from the outer electrodes, the voltage on the central electrode is kept constant and
image current detection takes place.
Measuring Principle
In the mass analyzer (Figure 25), stable ion trajectories combine rotation around an axial
central electrode with harmonic oscillations that run along it. The frequency  of these
harmonic oscillations along the z axis depends only on the ion mass-to-charge ratio (m/z) and
the instrumental constant k:
The Orbitrap analyzer’s two split halves of the outer electrode detect the image current that
the oscillating ions produce. By using a fast fragment ion algorithm of the amplified image
current, the instrument measures the frequencies of these axial oscillations and the
mass-to-charge ratios of the ions.
Ion Detection
To avoid mass drift during ion detection, the mass spectrometer maintains very stable voltages
on the central electrode and the additional electrode. Both electrodes deflect ions during
injection and compensate for electric field imperfections during the mass measurement
(Figure 26). The outer electrode is split in half at z = 0, which allows for the detection of the
ion motion in the axial direction through the induced image current. The image current on
each half of the outer electrode is differentially amplified and then undergoes analog-to-digital
conversion before processing by using the fast Fourier transform (FT) algorithm.

z
m
----
k=