Question

8. Describe an electrospray ionization source and the mechanisms involved for the

generation of gas-phase ions. How does this compare to a DESI source for mass

spectrometry?

Answer 1

An electrospray ionization high pressure source. In this, the ESI source is set under the pressurized condition up to 6 bar. The ions are exit the high pressure chamber from a nozzle to an surrounding environment and again re-enter to the vacuum system of the mass spectrometer through the ion sampling skimmer.  The stable spray that is water eliminate the corona discharge by the high voltage.

mechanisms involved for the generation of gas-phase ions- all solvent molecules are depart from the droplets, and only analytes and ionic charges are remain. This has leads to the formation of gas-phase analyte ions, which obtain charges from the surface of the vanished droplets. This mechanism is also known as charge residue model.

In DESI, the solvent mixture is sprayedthrough the conventional ESI method assisted with a sheath gas flow onto a surface at an angle ranging from 30˚ to 70˚. The gas pressure is in the range from 8 to 12 bar. The ESI spray usually generates charged droplets with diameter smaller than 10 μm and a velocity of roughly 120 m/s. The first droplets that arrive wet the surface and dissolve/collect the analytes on the surface in a thin liquid layer. Subsequent droplets impact this layer and break it up due to the high momentum of the charged droplets, generating numerous offspring droplets containing the analytes and charges. Subsequently, these offspring droplets experience solvent evaporation and Coulomb fission, and finally form gas-phase ions analogous to the conventional ESI process. This is referred to as the “droplet pickup” mechanism.