Biomedical Engineering Reference
In-Depth Information
The layout of the mass analyzer is based on the following geometrical impacts:
1. The selectivity of the mass filter scales with its length and is affected by the
width and number of electrodes.
2. Too narrow gaps between the electrodes may impair the removal of captured ions
out the active channel due to a high flow resistance.
However, to allow for a better evacuation of the ion channel a width of 100 m m
was chosen.
3.6
Energy Filter
The energy filter following the SIS-separator is to deflect ions that do not have the
preset energy, i.e., the energy at which the ions are extracted from the ionization
chamber. Due to their thermal energy distribution ions will start with a non-negligible
kinetic energy as they are accelerated by the extraction field. Furthermore, between
adjacent electrodes field gradients parallel to the ion movement will occur, which
contribute to ion energy variations. These field gradients appear at the edges of the
active finger electrodes and can either increase or decrease the ion energy.
An ion subjected to the electric field inside the semicircular shaped energy filter
will be deflected towards the inner radius
r
i
of the filter. This deflecting force along
with the initial kinetic energy of the ion results in a circular trajectory. For the
cylindrical layout, the applied voltage
U
defines the ion energy that can pass the
fi lter as [
24
] :
qU
EU
()
=
,
kin
⎛⎞
⎜
⎝⎠
r
r
2ln
i
o
r
o
is the outer diameter of the filter channel. Ions that do not have the selected energy
are deflected to the sidewalls and thus neutralized.
Two further important parameters have to be defined for the energy filter layout:
the channel length and the width of the outlet aperture. The channel length is impor-
tant because the ion beam enters the channel with a certain width. Since ions enter-
ing the channel close to the inner or outer radius are deflected differently, ions of the
desired energy are focused at 63 °.
A good configuration therefore is when the focal point is in the center of the
outlet aperture.
Figure
10
shows simulations of different energy filter configurations. In Fig.
10a
the filter ends at the focal point which is exactly in the center of the outlet aperture.
Hence a small outlet aperture is sufficient and a high selectivity is expected. If the
filter is extended to 90 °, as it is usually the case in macroscopic systems (Fig.
10b
)
