The invention involves electrospray ionization of dissolved substances at atmospheric pressure in the ion source of a mass spectrometer. A chip with a multitude of spray nozzles is proposed, where each individual spray nozzle is surrounded by several sheath gas nozzles, preferably in a symmetric arrangement, for the jet-like introduction of a sheath gas. A shared attracting-voltage electrode is positioned substantially opposite the spray nozzles. The attracting-voltage electrode may have a tapering (e.g. funnel-shaped) opening above each spray nozzle so that the sheath gas jets are forced to closely envelop the spray jet, which is comprised of ions and very fine droplets. Heavier ions and droplets are thus prevented from discharging on the surfaces of the openings of the attracting-voltage electrode. Special measures can be taken to make all spray nozzles spray uniformly and to supply them with substance peaks from chromatographic or electrophoretic separators as simultaneously as possible.