An ion source can include a magnetic field generator configured to generate a magnetic field in a direction parallel to a direction of the electron beam and coincident with the electron beam. However, this magnetic field can also influence the path of ionized sample constituents as they pass through and exit the ion source. An ion source can include an electric field generator to compensate for this effect. As an example, the electric field generator can be configured to generate an electric field within the ion source chamber, such that an additional force is imparted on the ionized sample constituents, opposite in direction and substantially equal in magnitude to the force imparted on the ionized sample constituents by the magnetic field.