A method for acquiring mass spectral data, the method comprising: (a) producing an electron beam in an electron ionization EI source (304, 400) at a first electron energy, the source comprising an ionization chamber between an electron source and a lens assembly, wherein the electron source includes a thermionic cathode positioned between an electron reflector and an ion repeller, wherein the lens assembly includes an extractor and a first lens element for reflecting the electron beam back into the ionization chamber; (b) introducing a sample comprising an analyte of interest into the ionization chamber of the EI source; (c) irradiating the sample with the electron beam at the first electron energy to produce first analyte ions from the analyte of interest; (d) transmitting the first analyte ions into a mass analyzer to generate a first mass spectrum correlated to the first electron energy; (e) adjusting the electron energy to a second electron energy different from the first electron energy; (f) irradiating the sample with the electron beam at the second electron energy to produce second analyte ions from the analyte of interest; and (g) transmitting the second analyte ions into the mass analyzer to generate a second mass spectrum correlated to the second electron energy, wherein the first and second electron energies are set by the voltages applied to the ion repeller and the electron reflector; wherein the method also comprises: setting a potential difference between the thermionic cathode and the ion repeller; maintaining the potential difference at a fixed value as the voltage on the thermionic cathode or ion repeller changes, by adjusting the voltage on the other component; causing the voltage on the first lens element to track the cathode voltage to optimize the electron reflecting function of the first lens element, by applying an additional offset voltage summed in with the applied cathode voltage. (a) producing an electron beam in an electron ionization EI source (304, 400) at a first electron energy, the source comprising an ionization chamber between an electron source and a lens assembly, wherein the electron source includes a thermionic cathode positioned between an electron reflector and an ion repeller, wherein the lens assembly includes an extractor and a first lens element for reflecting the electron beam back into the ionization chamber; (b) introducing a sample comprising an analyte of interest into the ionization chamber of the EI source; (c) irradiating the sample with the electron beam at the first electron energy to produce first analyte ions from the analyte of interest; (d) transmitting the first analyte ions into a mass analyzer to generate a first mass spectrum correlated to the first electron energy; (e) adjusting the electron energy to a second electron energy different from the first electron energy; (f) irradiating the sample with the electron beam at the second electron energy to produce second analyte ions from the analyte of interest; and (g) transmitting the second analyte ions into the mass analyzer to generate a second mass spectrum correlated to the second electron energy, wherein the first and second electron energies are set by the voltages applied to the ion repeller and the electron reflector; wherein the method also comprises: setting a potential difference between the thermionic cathode and the ion repeller; maintaining the potential difference at a fixed value as the voltage on the thermionic cathode or ion repeller changes, by adjusting the voltage on the other component; causing the voltage on the first lens element to track the cathode voltage to optimize the electron reflecting function of the first lens element, by applying an additional offset voltage summed in with the applied cathode voltage. setting a potential difference between the thermionic cathode and the ion repeller; maintaining the potential difference at a fixed value as the voltage on the thermionic cathode or ion repeller changes, by adjusting the voltage on the other component; causing the voltage on the first lens element to track the cathode voltage to optimize the electron reflecting function of the first lens element, by applying an additional offset voltage summed in with the applied cathode voltage.