Methods of Ionization

Geräteausstattung__Ionisierungsmethoden_ESI-Quelle_Bild

Methods of ionization

EI (Electron Ionization)

For EI measurements, the pure sample is transferred with a direct inlet to the EI source of the mass spectrometer via a vacuum lock and is evaporated, eventually facilitated by thermal energy. In the source, the gaseous molecules collide with electrons moving with an kinetic energy of typically 70 eV. The high energy collision with the electron results in the loss of an electron from the neutral molecule thus generating radical M^+. ions. Moreover, the molecular ion usually fragments into smaller ions due to the high collision energy. Mass spectrometer of our lab equipped with an EI source are: MAT 95S.

ESI (Electrospray Ionization)

ESI is a proper ionization method mainly for polar compounds which are easy to protonate or medium and high mass salts which are present as ions already in solution. These ions then gets transferred from solution into the gas phase. To this aim, the solution is run through a high voltage capillary, thus producing charged droplets splitting from the tip of the capillary. Enhanced by a flow of nitrogen, the charged droplets are reduced by evaporation of solvent molecules. Because the carrying charge is concentrated in a smaller and smaller space, Coulomb forces cause consecutive droplet fragmentations. Depending on the properties of the analyte, eventually a single ion MH⁺ evaporates from a small highly charged droplet ("Ion evaporation") or multiple charged ions MH_nⁿ⁺ are generated via the evaporation of all solvent molecules ("Charge residue"). ESI is suitable to generate cations or anions and is a quite soft method of ion generation yielding mainly intact molecular ions. Mass spectrometer of our lab equipped with an ESI source are: Orbitrap XL, Synapt G2.

APCI (Atmospheric Pressure Chemical Ionization)

APCI is suitable for medium polar compounds with small to medium molecular mass. The solved analyte is guided through a capillary and analyte and solvent molecules are evaporated by thermal energy under atmospheric pressure. A metal needle is placed in the area of sample evaporation, i.e. between the capillary and the entrance of the mass spectrometer. The high voltage applied to the metal needle yields corona discharge and the ejected electrons ionize - collision-induced as in EI - mainly solvent and air molecules which outnumbers the analyte molecules by far. The thus generated radical M^+. ions react in a series of gas phase reactions in which the protonation of the analyte constitutes the last step (in the positive mode), i.e. as for ESI, closed shell ions MH⁺ are generated which are then transferred to the vacuum of the mass spectrometer. APCI is softer compared to EI but generates usually ions with higher internal energy compared to ESI. Thus, labile compounds generated by APCI do often fragment. APCI measurements are possible in the positive and negative mode. Mass spectrometer of our lab equipped with an APCI source are: Orbitrap XL.

LIFDI (Liquid Injection Field Desorption Ionization)

As with EI, an electron is removed from the neutral analyte by LIFDI. Here, a high electric field is applied on an emitter. The solved sample is transferred to the emitter via a capillary. The electric field is generated by an applied voltage of up to 10 kV between the emitter an an electrode. The LIFDI emitter consists of a tungsten wire carrying a large number of micro graphite dendrites with nano sized diameters. The field strength at the tips of these dendrites are four orders of magnitudes higher compared to a plain electrode using the same voltage. This field strength is sufficient to remove the weakest bound electron from the analyte thus generating radical M^+. ion. Mass spectrometer of our lab equipped with an EI source are: Orbitrap XL in room C63.