MASS SPECTROMETER AND METHOD

MASS SPECTROMETER AND METHOD A mass spectrometer including a vacuum chamber wherein molecular flow conditions are maintained. A sample introduced into the chamber is ionized while a magnetic field through the chamber induces ion cyclotron resonance. Trapping plates are provided for restricting ion movement along the magnetic field while a conductance limit plate divides the chamber into first and second compartments. The conductance limit plate has an orifice configured to allow ion equilibration between the compartments while maintaining a pressure differential between them. The conductance limit plate includes an electrode that is selectively connected to a means for applying trapping potential to selectively trap ions in one of said compartments.