A calibration apparatus for a mass analyzer includes an ion source device and a dual-purpose electron beam generating unit. The ion source device ionizes an analyte of a sample, producing analyte ions. The dual-purpose electron beam generating unit is positioned between the ion source device and the mass analyzer. In a first mode, the dual-purpose electron beam generating unit is used to create fragments of analyte ions of unknown mass-to-charge ratio. In a second mode, the dual-purpose electron beam generating unit is used to create ions of calibration compounds of known mass-to-charge ratio. All ions are subsequently transferred to the mass analyzer.