The disclosure relates to a mass spectrometer for mass spectrometric examination of gas mixtures, including: an ionization device and an ion trap for storage and mass spectrometric examination of the gas mixture. In one aspect of the disclosure, the ionization device is embodied for supplying ions and/or metastable particles of an ionization gas and/or for supplying electrons to the ion trap for ionizing the gas mixture to be examined and the mass spectrometer is embodied to determine the number of ions and/or metastable particles of the ionization gas present in the ion trap and/or the number of ions of a residual gas present in the ion trap prior to examining the gas mixture. The disclosure also relates to the use of such a mass spectrometer and a method for mass spectrometric examination of a gas mixture.