A direct electron impact ion source is disclosed that includes a vaporizer for producing a process gas; an electron source for generating an electron beam; and an ionization chamber. The electron source is located outside the ionization chamber. Aligned apertures are provided in opposing walls of the ionization chamber to allow an electron beam to pass through the ionization chamber. The process gas is directed into the ionization chamber and ionized and extracted from the ionization chamber by way of an extraction aperture. In one embodiment, the direct electron impact ion source is configured with a form factor to enable it to be retrofit into the volume of an existing ion source , for example, an arc discharge type ion source. Alternatively, the direct electron impact ion source may be used together with an arc discharge ion source to create a dual mode or universal ion source.