Biomedical Engineering Reference
In-Depth Information
Fig. 9 Functional
Principle of the
Synchronous Ion Shield
Mass Analyzer
Both the time and the space domain show field free (white) areas with respect
to the actual potentials. In the time domain these effects emerge during switching.
In the space domain the electric field in the gaps between the finger electrodes can-
not be controlled fully. This is the main reason for some blurr in the filter process.
The relative mass resolution can be approximated as:
(
)
2
2
2
1
2
Ad
m
m
=
,
(
)
Δ
4
dA A d
−−
dd
2
1
1
2
A stands for the overall length of the filter. This formula is optimistic as it assumes
that an ion that crosses a dark region is safely deflected. Experiments show that the
actual selectivity is slightly lower than calculated.
This formula suggests a preferably long filter with small gaps between the elec-
trodes, whereas the width of the fingers hardly affects the selectivity. According to
theory there is no absolute optimum for the resolution, i.e., the more electrodes and
the longer the separator, the higher will be the resolution. However, to stay within
microsystem dimensions, reduce the effort for evacuation (dimensions < ion mean
free path), and limit the loss of “straying” ions, an overall length of the filter of
4 mm was chosen. The minimum width for the gaps between the fingers is limited
to 20 mm due to fabrication restrictions.
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