A mass spectrometer ion source structure and its mode of operation are described by which the resolved ion current produced from a given gas concentration in the ionizing chamber can be made one to two orders of magnitude larger than is usually obtained in conventional mass spectrometer ion sources. This structure employs an ionizing chamber whose walls are electrically insulated from each other so they can operate at separate potentials which permit positive ions to be trapped in the ionizing electron beam. The resulting partial neutralization of the electronic space‐charge allows the use of a higher current density ionizing beam and also increases the effective ionizing volume from which a stable ion current can be extracted. Inherently this mode of operation produces more fragmentation species because it increases the probability of several successive electron impacts on each particle.