Electrospray and atmospheric pressure chemical ionization mass spectrometer and ion source

An ion source for generating ions for analysis, comprising an extraction chamber formed in a body, said extraction chamber being in communication with an evacuation port, evacuation means connected to said evacuation port for maintaining the pressure in said extraction chamber less than 100 mm Hg, an entrance orifice leading into said extraction chamber and disposed opposite to said evacuation port so that at least some molecules entering said extraction chamber through said entrance orifice may pass through said extraction chamber on linear first trajectories and enter said evacuation port, exit orifice means leading through said body from said extraction chamber, means for generating a potential gradient in said extraction chamber for deflecting said ions for analysis through said exit orifice on second trajectories which are inclined at between 30.degree. and 150.degree. to said linear first trajectories, particle generating means for receiving a solution in which a sample may be dissolved and generating therefrom a stream of particles which intersects outside said body a notional backwards projection of at least one of said linear first trajectories through said entrance orifice, and means for electrically charging at least some of the particles comprised in said stream before they reach said notional backwards projection, said particle generating means being disposed with respect to said entrance orifice so that immediately on leaving said particle generating means at least the majority of particles comprised in said stream have a velocity whose resolved component towards said entrance orifice in a direction parallel to any one of said linear first trajectories is smaller than the resolved component in a perpendicular direction.