High vacuum describes pressures from approximately 10
torr, though the lower limit may vary. To achieve
high vacuum, an oil diffusion or turbomolecular pump in series with
a mechanical pump is used. In this role, the mechanical pump is called
a foreline or backing pump.
While interchangable for some applications, there are some advantages for both oil diffusion and turbomolecular pumps. Oil diffusion pumps are much cheaper, but have to operate upright. One of the key disadvantages of diffusion pumps, however, is that backstreaming of oil is possible if the high vacuum side pressure gets too high while the pump is hot; this makes a mess inside the instrument, and pump oil contamination in the mass spectrometer would render it unusable (cleaning this contamination is time consuming and messy). Turbomolecular pumps do not have this backstreaming issue and some can be operated horizontally or upside down. However, turbomolecular pumps are considerably more expensive (5 or more times as expensive as similar capacity diffusion pumps). In addition, turbo pumps can often achieve an operational pressure more quickly than a diffusion pump of similar size since in can be started in parallel with the rough pump and one does not have to wait for the pump oil to heat to operational temperature.