Apart from the direct inlet system which permits the introduction of substances with very low volatility and/or thermal stability into the mass spectrometer, the LKB instrument also permits the high speed recording of mass spectra of volatile compounds as single components or components of mixtures eluted from a gas chromatograph. Caution must be exercised in the interpretation of these spectra in view of possible peak or component overlap using the gas chromatographic column. At times it is useful to have another alternative system for the rapid introduction of controlled amounts of gaseous substances and/or highly volatile liquids and solids as well as reference samples such as the perfluoroalkanes directly into the ion source of the mass spectrometer for analysis. A heated molecular leak system for this purpose was developed.