4.1.2.5 Inlet system
Many vacuum technology processes that are monitored by mass spectrometers occur in pressure ranges of more than 10-4 mbar. Gases to be analyzed must also be relieved from atmospheric pressure to pressures of less than 10-4 mbar. Differing pressure reducing procedures are used, depending on the pressure gradient in question.
Gas mixtures should be admitted to the mass spectrometer without de-mixing if possible:
- At pressures p of more than 10 mbar, pressure is reduced by means of a (heatable) capillary, in which laminar flow prevails, with a downstream gas inlet valve. Under some circumstances, pressure reduction by means of an additional pump will be necessary upstream of the valve
- At pressures p of less than 10 mbar, pressure is reduced by means of an orifice and with a mass spectrometer that is differentially pumped by means of a turbomolecular pump
- At pressures p of less than 10-4 mbar, the mass spectrometer can be installed directly in the process chamber with an open ion source
Table 4.3:
Various gas inlet systems and their attributes
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