Utilizing gas flows in mass spectrometers

The invention relates to ions guided by gas flows in mass spectrometers, particularly in RF multipole systems, and to RF quadrupole mass filters and their operation with gas flows in tandem mass spectrometers. The invention provides a tandem mass spectrometer in which the RF quadrupole mass filter is operated at vacuum pressures in the medium vacuum pressure regime, utilizing a gas flow to drive the ions are through the mass filter. Vacuum pressures between 0.5 to 10 pascal are maintained in the mass filter. The mass filter may be enclosed by a narrow enclosure to guide the gas flow. The quadrupole mass filter may be followed by an RF multipole system, operated at the same vacuum pressure, serving as fragmentation cell to fragment the selected parent ions. The fragmentation cell may be enclosed by the same enclosure which already encloses the mass filter, so the ions may be driven by the same gas flow at the same vacuum pressure, greatly simplifying the required vacuum pumping system in tandem mass spectrometers. There are many other applications utilizing gas flows including supersonic gas jets in mass spectrometry.