A ion source for a mass spectrometer comprises: a capillary having a nozzle for emitting a nebulized fluid sample; an electrode of the capillary; a high voltage power supply; a second electrode disposed within or configurable to be disposed within a path of the nebulized fluid sample; and at least one switch for selecting application of an electrical potential provided by the high voltage power supply to either or both of the capillary electrode or the second electrode, wherein the capillary and capillary electrode are configurable so as to ionize the nebulized fluid sample by electrospray ionization and the second electrode is configurable so as to ionize the nebulized sample by atmospheric pressure chemical ionization.