Mechanistic aspects of electrospray ionization (a review)

Electrospray ionization (ESI) mass spectrometry can be divided into three steps: Nebulization of a sample solution into electrically charged droplets, liberation of ions from droplets, and transportation of ions from the atmospheric pressure ionization source region into the vacuum and mass analyzer of the mass spectrometer. A sample solution is fed through a capillary tube and a high electric field at the tip of the tube pulls positive charge towards the liquid front. When electrostatic repulsion becomes stronger than the surface tension, a small electrically charged droplet leaves the surface and travels through the surrounding gas to the counter-electrode. Under the majority of experimental liquid chromatography-mass spectrometry and capillary electrophoresis-mass spectrometry conditions, positive charge on droplets is generated by the removal of negative charge via electrochemical discharge of negative ions against the metal wall of the spray capillary. When the ESI source is set up for the detection of negative ions, all power supplies are at reversed polarity. Removal of positive ions inside the tip of the spray capillary provides droplets depleted of positive charge. The supply of negative charge to the solution may also take place; electrons released from the spray capillary can be captured by sample molecules having a high electron affinity. Droplet size decreases and charge density at the droplet surface increases after droplet disintegration and solvent evaporation. When the electric field at the surface of a droplet has become sufficiently high, ions are emitted from the droplet surface into the surrounding gas and are sampled by the mass analyzer. Sample ion intensity is dependent on ion structure and is affected by solvent composition and presence of additives. ESI behaves as a concentration sensitive detector for chromatography. When the sample concentration is increased above 10 mu M, the sample ion signal saturates, which can be explained by the assumption that the surface of ion-emitting droplets is full at 10 mu M. Sample ion abundance over a wide m/z range is further affected by inherently mass-dependent efficiencies of ion transportation, ion separation and ion detection. (C) 1998 Elsevier Science B.V