A technique for producing ions for mass spectrometry, used especially for obtaining ions from large molecules, which would be likely to produce fragment ions in electron-impact impact ionization sources. The sample is dissolved in a volatile solvent, which may also contain volatile acids or bases so that the sample exists in an ionic form. The solution is forced through a charged metal capillary tube and forms an aerosol. Evaporation of the solvent results in single ions of the sample, which are analyzed by the mass spectrometer. Electrospray ionization is the ionization technique often used in chromatography-mass spectrometry.