6.3.5 Inlet systems
Gases to be analyzed must usually be reduced from atmospheric pressure to pressures of less than the working pressure of the mass spectrometer (MS). Many vacuum technology processes that are monitored by mass spectrometers occur in pressure ranges $p$ > 10-4 hPa. An essential element of a quadrupole mass spectrometer is therefore a suitable gas inlet system for the particular application. Various pressure reducing procedures are used, depending on the pressure gradient in question.
Gas mixtures should be admitted to the mass spectro-meter without de-mixing if possible:
- At pressures $p$ > 10 hPa, 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.
- At pressures $p$ < 10 hPa, pressure is reduced with an orifice and a mass spectrometer which is differentially pumped with a turbomolecular pump.
- At pressures $p$ < 10-4 hPa the mass spectrometer can be installed directly in the process chamber with an open ion source.
Where an orifice is used for reducing pressure, its gas-type dependent conductivity is canceled out by the equally gas-type dependent conductivity of the flow path towards the pump, which means that the concentrations in the QMS reflect the true gas composition.
| Inlet system | Pressure range | Product example | Characteristics |
|---|---|---|---|
| No pressure reduction | 10 -12 to 10 - 4 hPa | PrismaPlus QMG 220, HiQuad QMG 700 | Open construction ion sources to allow gas from anywhere in the system to be represented in the ionizer volume. |
| SPM ion source | 10 - 9 to 10 hPa | PrismaPlus SPM 220, HiQuad SPM 700 | Special ion source for analyzing sputter processes. This system analyzes gas directly at the sputter pressure of 10-2 hPa with no pressure reduction. |
| Capillary inlet | 100 to 1,100 hPa, depending on the capillary length and the downstream orifice | OmniStar, ThermoStar, Inlet system GES 020 and GES 010 | Differentially pumped inlet system, low mass discrimination, not suitable for varying sample pressures |
| Orifice inlet | 0.01 to 10 hPa, as a function of orifice diameter | PrismaPlus HPA 220 | Orifices with diameters of 0.01 to 0.5 mm, simple, robust design, mass discrimination, changing inlet pressure possible with various orifice diameters, not suitable for fast-changing gas compositions |
| Dosing valve | 0.1 to 1,000 hPa | PrismaPlus HPA 220, Gas dosing valves UDV 040, UDV 146 | Dosing valves are suitable for gas inlets over a very broad measurement range, differentially pumped valves also allow analysis of rapidly changing gas compositions |
| Pressure-regulated gas inlet | 10 - 3 to 1,000 hPa | EVR 016 with RVC 300, OmniStar with pressure-regulated inlet | Differentially pumped inlet system, comprising a control loop with regulating valve and pressure measurement, relatively large dead volume, slow response |
Table 6.3: Various gas inlet systems and their attributes
