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.