Biomedical Engineering Reference
In-Depth Information
Fig. 8
Simulation of the ion optics (
top
) potential in the ion optics (
bottom
)
3.5
Synchronous Ion Shield (SIS) Mass Analyzer
The synchronous ion shield (SIS) mass analyzer is the core component of the
PIMMS [
23
] .
The upper part of Fig.
9
shows a schematic of this analyzer. The working principle
is related to time-of-flight filters, yet the SIS analyzer is operated in a high frequency,
rectangular pulse voltage mode. This voltage is applied perpendicular to the flight
direction between the finger electrodes and the comb structure. The voltage is alternat-
ing both in terms of time and space, which means that neighbored finger electrodes are
actuated at inverted voltages. This results in an electrical field which is traveling in the
same direction as the ion beam. When its velocity matches the ion velocity, a number
of ions can travel through the analyzer inside the momentarily field-free regions.
The lower part of Fig.
9
shows the time-space diagram of the SIS analyzer,
which resembles a chess pattern. The dark gray colored regions depict the channel
area between the comb electrode and the “active” electrodes, where the ions are
deflected sideways by the electric field. The light gray and white regions depict
inactive fingers and gaps in between, respectively.
The mass analyzer works primarily as a velocity filter because the slope in the
time-space diagram corresponds to the ion velocity. An ion moving on a line within
the light gray and white areas will not be deflected and pass the analyzer, ions with
other velocities will be subjected to an orthogonal electrical field and be deflected.
Ions of different mass but equal energy are selected according to:
1
2
E
=
v
,
due to their different velocity, i.e., slope in the diagram, by varying the frequency of
the traveling pulse stream accordingly.
