Abstract: Method for generating ions from thermally unstable, non-volatile, large molecules, particularly for a mass spectrometer such as a time-of-flight mass spectrometer. A specimen substance comprising the molecule is exposed to energy pulses with which molecules are released from the specimen substance, and the released molecules are entrained by a jet of a carrier gas and are cooled upon expansion thereof and are subsequently ionized in an ionization chamber. The molecules are ionized by electron impact, the power per unit area of the electrons employed for the ionization is selected such that a potential trough is generated in the focus of the electron beam, the depth thereof being greater than the translational energy of the molecule ions in the carrier gas stream. The molecule ions generated by the electron impact ionization are respectively collected in the potential trough for a defined time span.