An electrospray ion source assembly for generating ions for analysis by a mass spectrometer , comprising: an ion source housing defining an ionization chamber , the ionization chamber configured to be disposed in fluid communication with a sampling orifice of a mass spectrometer system, the sampling orifice formed in a counter electrode; at least one heater for heating the ionization chamber ; an electrospray electrode defining an axial bore therethrough for transmitting a liquid sample received from a sample source at an inlet end of the electrospray electrode to an outlet end for discharging the liquid sample from the electrospray electrode into the ionization chamber ; a power source configured to provide an electrical voltage to an electrical circuit containing the electrospray electrode and the counter electrode so as to generate an ion emission current between the electrospray electrode and the sampling orifice as at least a portion of the liquid sample is ionized upon being discharged from the electrospray electrode; and an ion emission current controller configured to limit the onset of electrical discharge between the electrospray electrode and the counter electrode without substantially altering the electrical voltage provided to the electrical circuit by the power source ; wherein the ion emission current controller comprises one or more current limiting resistors disposed in series in the electrical circuit between the power source and the electrospray electrode; and wherein the one or more current limiting resistors exhibit a total resistance in a range from about 20% to about 100% of the resistance of the air gap between the electrospray electrode and the counter electrode under normal operating conditions of the ion source in which ion emission current is generated without electrical discharge between the electrospray electrode and the counter electrode. an ion source housing defining an ionization chamber , the ionization chamber configured to be disposed in fluid communication with a sampling orifice of a mass spectrometer system, the sampling orifice formed in a counter electrode; at least one heater for heating the ionization chamber ; an electrospray electrode defining an axial bore therethrough for transmitting a liquid sample received from a sample source at an inlet end of the electrospray electrode to an outlet end for discharging the liquid sample from the electrospray electrode into the ionization chamber ; a power source configured to provide an electrical voltage to an electrical circuit containing the electrospray electrode and the counter electrode so as to generate an ion emission current between the electrospray electrode and the sampling orifice as at least a portion of the liquid sample is ionized upon being discharged from the electrospray electrode; and an ion emission current controller configured to limit the onset of electrical discharge between the electrospray electrode and the counter electrode without substantially altering the electrical voltage provided to the electrical circuit by the power source ; wherein the ion emission current controller comprises one or more current limiting resistors disposed in series in the electrical circuit between the power source and the electrospray electrode; and wherein the one or more current limiting resistors exhibit a total resistance in a range from about 20% to about 100% of the resistance of the air gap between the electrospray electrode and the counter electrode under normal operating conditions of the ion source in which ion emission current is generated without electrical discharge between the electrospray electrode and the counter electrode. <EOS>