An apparatus for ionizing chemical species comprising: a) an Atmospheric Pressure Chemical Ionization (APCI) inlet probe and b) an ion source which comprises a chamber (53, 101, 293, 334), wherein the inlet probe comprises: i) a solution nebulizer, ii) a heater to vaporize said nebulized solution, and iii) an enclosed vapor flow channel comprising a corona discharge needle with a tip, wherein the geometry of the enclosed vapor flow channel is configured to constrain said analyte vapor to pass through a corona discharge region, and wherein the enclosed vapor flow channel further comprises at least one counter electrode (22, 23, 24, 213, 214, 215, 317, 373) surrounding the corona discharge needle and shaped to partially shield a corona discharge electric field from an electric field in the chamber and to allow penetration of external electric fields into an exit end of said enclosed vapor flow channel, said enclosed vapor flow channel comprising walls, and wherein the apparatus further comprises: c) an endplate electrode (37, 218) with voltage, and d) means to apply voltage (50, 30, 51) to said corona discharge needle, said at least one counter electrode (22, 23, 24, 213, 214, 215) and said endplate electrode (37, 218) to form a corona discharge at said tip and provide an electric field extending from the endplate electrode and penetrating into the exit end of said enclosed vapor flow channel to focus and direct APCI generated ions away from the walls of said enclosed vapor flow channel. a) an Atmospheric Pressure Chemical Ionization (APCI) inlet probe and b) an ion source which comprises a chamber (53, 101, 293, 334), wherein the inlet probe comprises: i) a solution nebulizer, ii) a heater to vaporize said nebulized solution, and iii) an enclosed vapor flow channel comprising a corona discharge needle with a tip, wherein the geometry of the enclosed vapor flow channel is configured to constrain said analyte vapor to pass through a corona discharge region, and wherein the enclosed vapor flow channel further comprises at least one counter electrode (22, 23, 24, 213, 214, 215, 317, 373) surrounding the corona discharge needle and shaped to partially shield a corona discharge electric field from an electric field in the chamber and to allow penetration of external electric fields into an exit end of said enclosed vapor flow channel, said enclosed vapor flow channel comprising walls, and wherein the apparatus further comprises: i) a solution nebulizer, ii) a heater to vaporize said nebulized solution, and iii) an enclosed vapor flow channel comprising a corona discharge needle with a tip, wherein the geometry of the enclosed vapor flow channel is configured to constrain said analyte vapor to pass through a corona discharge region, and wherein the enclosed vapor flow channel further comprises at least one counter electrode (22, 23, 24, 213, 214, 215, 317, 373) surrounding the corona discharge needle and shaped to partially shield a corona discharge electric field from an electric field in the chamber and to allow penetration of external electric fields into an exit end of said enclosed vapor flow channel, said enclosed vapor flow channel comprising walls, c) an endplate electrode (37, 218) with voltage, and d) means to apply voltage (50, 30, 51) to said corona discharge needle, said at least one counter electrode (22, 23, 24, 213, 214, 215) and said endplate electrode (37, 218) to form a corona discharge at said tip and provide an electric field extending from the endplate electrode and penetrating into the exit end of said enclosed vapor flow channel to focus and direct APCI generated ions away from the walls of said enclosed vapor flow channel.