An apparatus for ionizing an analyte sample with a mass calibrant is provided. The apparatus includes an ionization chamber defining an ionization region, a first passageway coupled to the ionization region for delivering the analyte sample to the ionization region, a second passageway leading to a mass analyzer having an orifice arranged adjacent to the ionization region to receive ions from the ionization region, a third passageway coupled to the ionization chamber at a first end and having a second end with an orifice arranged to receive gaseous neutral mass calibrant molecules, and an ionization device arranged within the ionization chamber. The ionization device generates primary ions from the analyte sample, and the primary ions ionize a portion of the gaseous neutral mass calibrant molecules received into the ionization region via the third passageway.