An apparatus for generating ions includes an Electrospray ionization source configured to provide a spray of charged droplets from a sample solution during operation of the apparatus; an atmospheric pressure chemical ionization (APCI) source including a corona discharge needle configured to produce a corona discharge that further ionizes the spray during operation of the apparatus; and a gas delivery system configured to deliver a gas flow to the corona discharge needle during operation of the apparatus, wherein the gas flow comprises a reagent ion gas which facilitates ionization of the spray by the corona discharge.