Biomedical Engineering Reference
In-Depth Information
Fig. 11
Microchannel plate (MCP) hybrid-integrated in the PIMMS
Fig. 12
Faraday detector
4
Injection System
For an adequate performance of the PIMMS it is necessary to install appropriate
pressures and gas flows in the system, as well as a minimum time delay for the
sample to reach the ionization chamber. The flow time of the sample gas governs the
temporal resolution of the PIMMS measurements. Since the pressure of the plasma
and sample gases are usually much higher at their origin than allowed in the plasma
and the ionization chamber, respectively, there is a need for pressure reduction
which will inevitably introduce a time delay.
Both plasma and sample gases are fed into the PIMMS system via glass capillar-
ies as shown in Fig.
13
. They not only physically connect the gas supplies to the
PIMMS but also reduce the gas pressure via their flow resistance to the adequate
operation range. Typically 10 cm long, 100 mm diameter capillaries are sufficient to
induce the desired pressure reduction in the chambers. These capillaries are glued
to the PIMMS.
As already mentioned the plasma is ignited by a high voltage discharge in the
plasma chamber at a relatively high pressure. In continuous operation, a stable RF
plasma will burn at much lower chamber pressures [
13
] . This requires switching
between two different flow resistances, i.e., capillary lengths, which is accomplished
by a four-way valve operated as shown in Fig.
14
.
Due to the intricate layout of the PIMMS, the pressure distribution throughout the
system is difficult to predict theoretically. Even though an integration of a pressure
