 The negative ion sensitivity of DART gases varies with the efficiency in forming electrons by Penning ionization, which means that the negative ion sensitivity increases with the internal energy of the metastable species, for example nitrogenᐸneonᐸhelium.Analyte ions are formed at ambient pressure during Penning and chemical ionization.The mass spectrometry analysis, however, takes place at high vacuum condition.Therefore ions entering the mass spectrometer, first go through a source - to - analyzer interface (vacuum interface), which was designed in order to bridge the atmospheric pressure region to the mass spectrometer vacuum.