Interference Suppression in Mass Spectrometers

A method of operating a collision cell in a mass spectrometer, wherein the collision cell comprises an entrance aperture, an exit aperture, and at least one DC exit electrode, the method comprising:

• feeding input ions in a forward axial direction through the entrance aperture into the collision cell;
• producing a gas flow in the collision cell that is directed contrary to the forward axial direction in at least an upstream portion of the collision cell;
• spatially separating ions of interest from interference ions within the collision cell with the gas flow;
• during a first time period, producing a first exit potential with the at least one DC exit electrode to trap a collection of trapped ions comprising the ions of interest within a trap region located nearer to the exit aperture than to the entrance aperture;
• and
• during a second time period:
• producing a second exit potential with the at least one DC exit electrode to release the trapped ions through the exit aperture;
• and
• purging the interference ions out of the collision cell in an upstream direction away from the exit aperture.

A collision cell for use in a mass spectrometer, the collision cell comprising:

• an entrance aperture for receiving input ions in a forward axial direction;
• an exit aperture for emitting ions in the forward axial direction;
• at least one DC exit electrode for producing, during a first time period, a first DC exit potential to trap ions and for producing, during a second time period, a second DC exit potential to release trapped ions in the forward axial direction towards the exit aperture,
• at least one gas inlet port for receiving a gas flow such that the gas flow is contrary to the forward axial direction in at least a portion of the collision cell, so as to separate ions of interest from interference ions in dependence on their collisional cross sections;
• and
• at least one DC axial electrode for producing an axial electric field to exert an axial force on ions within the collision cell;
• wherein the collision cell is configured such that:
• during the first time period, the gas flow and the first DC exit potential confine the ions of interest to a trap region located near the exit aperture;
• and
• the interference ions are restricted from reaching the exit aperture during each of the first time period and the second time period by one or both of the gas flow and the axial electric field.