Electrospray Ionization Mass Spectrometer (ESI MS)

Electrospray ionization mass spectrometers (ESI MS) use a so-called “soft” ionization technique that has found widespread use in biological mass spectrometry. Soft ionization techniques enable the ionization of large biological molecules (such as proteins) without fragmenting them, meaning they can be analyzed intact. In an electrospray ionization mass spectrometer, the molecules to be analyzed pass into the mass spectrometer in a fine spray, typically following chromatographic separation, whether via traditional liquid chromatography, HPLC, or nano-LC. As the spray emerges from the ESI source, the molecules are ionized by the nozzle’s electrically charged tip. As this mist travels and evaporates, electrostatic repulsion between like-charged ions ultimately forces the molecules apart. They then may be analyzed by a wide variety of mass analyzers, including quadrupoles, ion traps, time-of-flight, and Fourier transform-based instruments.