We present the development of a chemical ionization mass spectrometer which can be operated with either ammonium (NH₄⁺) or hydronium (H₃O⁺) as the reagent ion. We describe a mass spectrometric voltage scanning procedure based on collision-induced dissociation that allows us to determine the stability of detected ammonium–organic ions and hence constrain the sensitivity of the instrument to a wide range of organic compounds that cannot be calibrated directly.