a) Briefly describe the following soft ionization methods:
In atmospheric pressure ionization (API) ion-molecule reactions at atmospheric pressure are employed to generate analyte ions in the gas phase. The gas phase neutrals are obtained by evaporating the solvent in a stream of hot nitrogen.
Thermospray (TSP) also makes (made) use of heat to evaporate an analyte solution, but ions have to be formed prior to the evaporation process, e.g., by adduct ion formation in solution.
Electrohydrodynamic ionization (EHI) uses high voltage to spray an electrolytic solution. EHI is limited to solvents of extremely low volatility like glycerol, because the spray is created in vacuum.
Electrospray ionization (ESI) also uses high voltage to spray an electrolytic solution that already contains preformed analyte ions. Different from EHI, the dissipation of the liquid is carried out under atmospheric pressure.
b) What properties of ESI effected its breakthrough to become the most widely employed among the beforementioned methods?
By spraying under atmospheric pressure and by supplying heat to the droplets, in ESI, desolvation can be carried out with a wide range of solvents and over a comparatively wide range of liquid flow rates. This allows ESI to be adapted to most liquid chromatographic systems.
c) What is a nozzle-skimmer system? How does contribute to the efficiency of an ESI interface?
Basically, ESI interfaces are derived from a nozzle-skimmer system that delivers an intense cool molecular jet into the high vacuum environment. The adiabatic expansion of the gas on the first pumping stage reduces random motion of the particles due to extensive cooling. Furthermore, a portion of the thermal motion is converted into directed causing the heavier, analyte ion-containing solvent clusters to travel close to the center of the flight path through the interface, whereas light solvent molecules escape from the jet.