An ion source comprising a chamber and an electron collector is described. In one configuration, the chamber comprises a sample inlet and an ion outlet. The chamber may also include an electron inlet configured to receive electrons from an electron source. The electron collector can be arranged in opposition to the electron inlet. The chamber can be configured to direct an electron beam from the electron source along a path with the chamber transverse to a path between the gas inlet and the ion outlet. The chamber may comprise an ion guide that includes a guide axis offset from an axis of the ion outlet.