Biomedical Engineering Reference
In-Depth Information
Fig. 5
Geometry of the
ionization chamber
is 100 eV. This energy is about the optimum for a maximum ionization yield. Due
to the contour of the outer walls of both the plasma and the adjacent ionization
chamber, the electrons are efficiently focused into the ionization chamber orifice.
3.3
Ionization Chamber
The ionization chamber is of similar design as the plasma chamber. As there is no
electric field inside the chamber, its geometry matters only with respect to ioniza-
tion yield and gas flow.
Figure
5
shows the outline of the ionization chamber. The opening on top is the
sample gas inlet. The other two apertures are the inlet for the electrons (left) and the
outlet for the ionized sample gas atoms (right), respectively.
The chosen chamber diameter of 320 mm is relevant for the ion yield. For a
larger diameter the electrons entering from the plasma chamber can interact with
the sample gas along a more extended path and, the number of atoms which are
ionized will increase accordingly. However, only ions that are in the vicinity of the
outlet will be extracted from the ionization chamber, i.e., ions generated close to
the electron entrance have to travel to the exit aperture to be exposed to the extract-
ing field. A small chamber diameter therefore is advantageous. A compromise with
respect to the probability of ionization and ion extraction efficiency will generate
the maximum ion current [
20
] .
Besides the chamber diameter, the width of the apertures is an important struc-
tural parameter. It affects the density (pressure) of the sample gas and the overall gas
flow into the system. Ion trajectory simulations identified an optimum ion yield for
entrance and exit orifices of 40 mm and a chamber diameter of 320 m m [
19
] , as
experimentally validated in the real systems.
Figure
6
demonstrates generation of ions due to the incoming electrons (left) and
the extraction of the ions out of the chamber biased at +100 V by the first pair of
electrodes of the ion optics (right).
3.4
Ion Extraction and Ion Focus Electrodes
The ion extraction and focus electrode arrangement comprises two pairs of elec-
trodes located between the ionization chamber and the mass separator. Their function
