The present invention relates to an apparatus for delivering ions to a vacuum
chamber. The apparatus comprises an ionization chamber, an ionization region within
the ionization chamber, a vacuum interface at a vacuum interface voltage and a
vacuum chamber, wherein the ionization chamber communicates with the vacuum chamber
through the vacuum interface. Sample is introduced into the ionization chamber
from an electrospray assembly at approximately ground potential. Two electrodes
are provided within the chamber such that three electric fields are generated,
a first field extending from the electrospray assembly to the first electrode,
a second field extending from the second electrode to the first electrode, and
a third field extending from the second electrode to the vacuum interface. Ions
are forced to travel through the fields in order before entering the vacuum chamber.
In addition, the invention provides a method of delivering ions to a vacuum chamber.