An apparatus for connecting an ionization probe assembly to a mass and/or ion mobility spectrometer is disclosed. The apparatus comprises: an attachment member for releasably attaching a probe assembly to the apparatus; a cap for enclosing the attachment member; wherein the apparatus is operable to deliver a voltage to a probe assembly only when the cap is arranged to enclose the attachment member; and wherein the cap is configurable to enclose the attachment member when a probe assembly is attached to the apparatus.