Chemical ionization

Chemical ionization (CI ) is an ionization method that is used in the mass spectrometer. It is particularly used to determine the molar mass slightly fragmentierender substances. Since the sample must be transferred prior to ionization in the gaseous state, the method can therefore only in high vacuum without decomposition vaporizable substances.

The chemical ionization is similar to the electron impact (EI ) are very strong. In general, however, the CI spectra are much less fragmented as appropriate EI spectra. Since the force transmitted to the sample molecule, excess energy is small, the fragmentation is suppressed. One therefore mainly generated quasi-molecular ions.

Reactant gases

In this case, in the ion source to the thermally vaporized sample molecules, an excess of reactant gas (such as ammonia, isobutane, methane) is introduced, the by electron bombardment (150 eV) is ionized. The primary ions formed by the electron bombardment of the reactant gas to react through a series of collisions with other Reaktantgasmolekülen to the ionizing actually acting stable ions, the CI plasma ions.




Formation of the molecular ions

If the molecules to be analyzed is introduced (M), they react with the formation of charged molecular ions.

In addition to the protonation by addition of analyte ions formed Reaktantgasionen observed ( eg even with NH4 ).


In the chemical ionization and negative ions which can be detected by reversing the polarity of the voltages in the ion source occur (negative chemical ionization, NCI). A method in which one to dispense with the high vacuum, the chemical ionization at atmospheric pressure.