An electron-ion reaction module for use in a mass spectrometer, comprising: a chamber, and an electron source for generating electrons and introducing said electrons into the chamber, characterized in that the electron-ion reaction module further comprises: a gate electrode positioned relative to the electron source and the chamber, a DC voltage source operatively coupled to said gate electrode for applying control voltages to said gate electrode, and a controller operably coupled to said DC voltage source and configured for adjusting the DC voltage applied to the gate electrode to adjust flow of electrons into the chamber; wherein said controller is configured to adjust the DC voltage applied to the gate electrode by switching the DC voltage between a plurality of discrete voltage levels; wherein one of said discrete voltage levels corresponds to an "on" state of the gate during which the gate allows introduction of the electrons into said chamber and another one of said discrete voltage levels corresponds to an "off" state of said gate during which the gate inhibits introduction of the electrons into said chamber; wherein said controller is configured to: adjust the periodicity and the duty cycle of said "on" and "off" voltages so as to adjust the electron current introduced into said chamber; and switch the DC voltage applied to the gate electrode between said discrete levels at a switching frequency in a range of about 100 Hz to about 100 kHz. a chamber, and an electron source for generating electrons and introducing said electrons into the chamber, characterized in that the electron-ion reaction module further comprises: a gate electrode positioned relative to the electron source and the chamber, a DC voltage source operatively coupled to said gate electrode for applying control voltages to said gate electrode, and a controller operably coupled to said DC voltage source and configured for adjusting the DC voltage applied to the gate electrode to adjust flow of electrons into the chamber; wherein said controller is configured to adjust the DC voltage applied to the gate electrode by switching the DC voltage between a plurality of discrete voltage levels; wherein one of said discrete voltage levels corresponds to an "on" state of the gate during which the gate allows introduction of the electrons into said chamber and another one of said discrete voltage levels corresponds to an "off" state of said gate during which the gate inhibits introduction of the electrons into said chamber; wherein said controller is configured to: adjust the periodicity and the duty cycle of said "on" and "off" voltages so as to adjust the electron current introduced into said chamber; and switch the DC voltage applied to the gate electrode between said discrete levels at a switching frequency in a range of about 100 Hz to about 100 kHz. a gate electrode positioned relative to the electron source and the chamber, a DC voltage source operatively coupled to said gate electrode for applying control voltages to said gate electrode, and a controller operably coupled to said DC voltage source and configured for adjusting the DC voltage applied to the gate electrode to adjust flow of electrons into the chamber; wherein said controller is configured to adjust the DC voltage applied to the gate electrode by switching the DC voltage between a plurality of discrete voltage levels; wherein one of said discrete voltage levels corresponds to an "on" state of the gate during which the gate allows introduction of the electrons into said chamber and another one of said discrete voltage levels corresponds to an "off" state of said gate during which the gate inhibits introduction of the electrons into said chamber; wherein said controller is configured to: adjust the periodicity and the duty cycle of said "on" and "off" voltages so as to adjust the electron current introduced into said chamber; and switch the DC voltage applied to the gate electrode between said discrete levels at a switching frequency in a range of about 100 Hz to about 100 kHz. adjust the periodicity and the duty cycle of said "on" and "off" voltages so as to adjust the electron current introduced into said chamber; and switch the DC voltage applied to the gate electrode between said discrete levels at a switching frequency in a range of about 100 Hz to about 100 kHz.