An Atmospheric Pressure Chemical Ionization (APCI) source interfaced to a mass spectrometer is configured with a corona discharge needle positioned inside an APCI inlet probe assembly. Liquid sample flowing into the APCI inlet probe is nebulized and vaporized prior to passing through the corona discharge region all contained in the APCI inlet probe assembly. The APCI probe is configured to shield the electric field from the corona discharge region while allowing penetration of an external electric field to focus APCI generated ions into an orifice into vacuum for mass to charge analysis. Ions that exit the APCI probe are directed only by external electric fields and gas flow maximizing ion transmission into a mass to charge analyzer. Sample ions and gas phase reagent ions are generated in the APCI probe from liquid or gas inlet species or mixtures of both.