Biomedical Engineering Reference
In-Depth Information
Fig. 21
Con fi guration of DC-sources applied to electrodes of the PIMMS
Table 3
Voltages applied on electrodes of PIMMS [
19
]
DC-Source
Signal
Possible range (V)
Typical values (V)
U
WFP
Traveling fi eld potential
−230 … 230
0
U
EEX
Electron extraction
−230 … 230
100
U
IEX
Ion extraction
−230 … 230
−25
U
IFO
U
IFO
ion focus
−230 … 230
30 …75
U
SI
Inner sector
−230 … 230
0 … −25
U
SO
Outer sector
−230 … 230
0 … 25
U
DCO
DC-offset
−230 … 230
2.5
U
HV
High Voltage
−2,000 … 0
−1,300 … − 400
U
EEX
will cause varying ionization rates in the ionization chamber as well as a broad-
ening of the ion energy distribution focused in the separator:
⎛
⎞
(
)
⎜
⎟
E
=
·U
+
U
−
U
ion
EEXstable
noise
WFP
⎜
⎟
⎝
⎠
U
EEX
with
q
the unit charge. Noise of
U
IFO
,
U
SI
, and
U
SO
is even more critical, since these
voltages strongly affect the shape of the ion beam.
As already mentioned
U
IFO
,
U
SI
, and
U
SO
are variable DC-sources. Their settings
are optimized for each chip individually with respect to maximum ion throughput at
deactivated separator. This follows the algorithm:
1. Energy fi lter optimization by varying
U
SI
and
U
SO
: The voltages are swept in the
ranges [
U
WFP
,
U
WFP
− 25 V] for
U
SI
and [
U
WFP
,
U
WFP
+ 25 V] for
U
SO
simultane-
ously, while the ion current is measured at the detector and plotted versus the
