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Analytic Centres of the Institute of ChemistryMethods of ionization

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EI - explained in a simple way

Electron impact ionization

For EI measurements the sample is introduced as a pure substance with a push rod into the EI-source of the mass spectrometer directly under high vacuum where your sample – if necessary supported by thermal energy – can be vaporized. The vaporized sample is the bombarded with a 70 eV electron beam. Due to the relatively powerful impact of the electron, you can detect not only the ionization of the molecule, but also various fragmentations of the mother-ion too. This molecular-ion and the fragmentations can be separated in the mass spectrometer by their mass-to-charge ratio and they will be detected.

At high resolution measurements there won’t be a full spectra. The m/z-ratio of the desired substance will be compared to the m/z-ratio of a reference (“peak matching”). The maximum of deviation can be up to 5 ppm. 

The following mass spectrometer are used with an EI-source : MAT 95S, GC8000 Voyager




APCI - explained in a simple way

Atmospheric pressure chemical ionization

APCI is a gas phase ionization technique used mostly for semi-polar samples with low or medium mass. The dissolved sample will be sent through a capillary heated by a ceramic tube so the sample and solvent can vaporize at atmospheric pressure. There is a metal needle between the capillary and entrance where the ions will be generated. This is achieved by vaporizing solvent and air molecules with a corona discharge. These ions react with the analyte ionising them (in positive mode with proton change). Now the analyte ions arrive into the high vacuum of the mass spectrometer via ionoptics.

APCI is a milder ionization method than EI but more powerful than ESI. For very unstable substances can still occur fragmentation. APCI measurements are possible in positive or negative modes.

The following mass spectrometer are used with an APCI-source: Orbitrap XL




ESI - explained in a simple way

Electrospray ionization

ESI is suitable for polar substances or salts within medium or high mass range, especially those which are able to protonate easily  or are already ions in solution. These ions will be transferred from solution to the gas phase. Therefore the solution will be introduced through a capillary with high voltage to separate charged drops from the tip of the capillary. Through the support of a flow of nitrogen, solvent molecules will evaporate from these drops so the charge is concentrated into a smaller area, and via a Coulomb-explosion the drops will fragment. This procedure repeats until mono-cations of the substance without solvent arrives the high vacuum of the mass spectrometer by ionoptics.

ESI can be used in positive and negative mode. It’s a mild method which produces ions where the analyte ions generally should not fragment.

The following mass spectrometer are used with an EI-source: Orbitrap XL, Synapt G2




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