Abstract: A method and apparatus for analyzing chemical species includes an ion source at or near ambient pressure and a time-of-flight mass spectrometer which receives the ions, created at the ion source, through an ion supersonic jet forming device. The ion source creates ions from neutral molecules in the sample to be analyzed or serves to introduce already formed ions into the mass spectrometer vacuum chamber. The ion source can use any of the known techniques for ion creation, including a corona discharge or a .sup.63 Ni Beta ion source. The ions are created and are then inroduced into the vacuum region of the mass spectrometer through a small orifice which causes the stream of ions entering the vacuum region to enter as a supersonic jet wherein the kinetic energy of each individual ion falls within a narrow energy band. The ions are then repelled or drawn into the field-free flight tube of the mass spectrometer and separated and identified based on their mass-to-charge ratios.