Biomedical Engineering Reference
In-Depth Information
approximately 2 GHz are needed. Ion currents typically in the range of femto- to a
few picoamperes are to be measured. Some of these extremes at these small dimen-
sions can only coexist under vacuum conditions, i.e., will damage the chip at atmo-
spheric pressure. Table
2
summarizes the assembly of electronic components and
the PIMMS structures which they are to drive.
6.2
Pumps, Valves, and Pressure Monitoring
The amount of electrons extracted from the microplasma, the ionization rate of the
sample gas, and the mean free path length depend on the pressure at the different
locations of PIMMS-chip. Therefore, exact knowledge and control of pressure and
gas flow rates will be necessary for quantitative analysis. Pressure sensors, valves
for gas inlets, and vacuum pumps are the components, which have to be read out and
controlled, respectively, by electronic and software to install and stabilize appropri-
ate pressure regimes.
Presently a commercially available two stage vacuum system comprising a mem-
brane (Pfeiffer MVP 006-4) and a turbo pump (Pfeiffer HiPace™ 10) in combina-
tion with a pressure sensor (Leybold Vacuum Ionivac ITR 90) establish a pressure
of about 0.1 Pa in the system. Three electric valves are used to control the gas flow
into the capillary system and for the bypasses. The use of macro devices simplifies
the handling of the experimental setup and also the electronic control. Pressure
drops for plasma and sample gases are accomplished by an appropriate combination
of capillaries with different diameters and lengths as described in Sect.
4
.
Presently the electronics controlling the pumps and valves predominantly protect
the turbo pump and the PIMMS from damage. Since the high voltages driving the
system could initiate detrimental arc discharges between electrodes and metal struc-
tures, they are only to be applied at low pressure. Therefore, this control of high
priority is supplied by an independent unit.
6.3
DC-Sources
Extraction and acceleration of electrons and ions, focusing, and energy filtering of
the ion beam rely on stable but tunable electric fields. Different potentials have to be
applied to the electrode structures to generate these fields.
Figure
21
and Table
3
summarize the necessary DC-sources applied to the struc-
tures of the chip. Presently all DC-sources are programmable and variable in the
range of −230 to +230 V. This flexibility is essential to determine the optimal param-
eter setting during system optimization. Once it is known many of these sources can
be replaced by fixed ones. Still, due to production tolerances and long term drift
effects the sources for ion extraction and focus (
U
IFO
), energy fi lter (
U
SI
,
U
SO
), and
MCP (
U
HV
) will have to remain variable for optimum device performance.
Instability or noise of any of these sources will lead to loss of resolution and/or
sensitivity in the mass-spectra. For example, noise on the electron extraction voltage
