Question

Describe the ionization mechanisms for (a) electron impact (EI) ionization, (b) chemical ionization (CI), (c) electrospray ionization (ESI), and (d) atmospheric-pressure chemical ionization (APCI).

Answer 1

Electron impact ionization (EI): In this process, an electron beam is used to ionize the gas phase sample (analyte molecules). Electron beam are generated from a tungsten filament. An electron from the beam knocks an electron off of analyte atoms or molecules to create ions.

Chemical ionization (CI): In this process a reagent ion is used which react with sample molecules to form ions either by proton or hydride transfer. The reagent ion is produced by placing an excess amount of methane (relative to the sample amount) into an electron impact (EI) ion source.

Electrospray ionization (ESI): In this process, a very fine needle and a series of skimmers are used in a chamber. A sample solution is sprayed into this source chamber to form droplets.The droplets carry charge when the exit the capillary and, as the solvent evaporates, the droplets disappear leaving highly charged analyte molecules. ESI is particularly useful for large biological molecules that are difficult to vaporize or ionize.

Atmospheric-pressure chemical ionization: In this process, the sample is typically dissolved in a solvent and pumped through a capillary inside an uncharged quartz tube. At the end of the capillary, but still within the tube, the sample is converted into an aerosol and then vaporized with the help of nitrogen gas and by heating to very high temperature (~350-550 °C). The gaseous solvent (S) and sample (M) are then ionized by a corona discharge, in which a highly charged electrode creates an electric field strong enough to ionize nearby molecules.