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Concept
Ion source
Summary
An ion source is a device that creates atomic and molecular ions. Ion sources are used to form ions for mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters and ion engines.
Electron ionization
Electron ionization is widely used in mass spectrometry, particularly for organic molecules. The gas phase reaction producing electron ionization is
M{} + e^- -> M^{+\bullet}{} + 2e^-
where M is the atom or molecule being ionized, e^- is the electron, and M^{+\bullet} is the resulting ion.
The electrons may be created by an arc discharge between a cathode and an anode.
An electron beam ion source (EBIS) is used in atomic physics to produce highly charged ions by bombarding atoms with a powerful electron beam. Its principle of operation is shared by the electron beam ion trap.
Electron capture ionization (ECI) is the ionization of a gas phase atom or molecule by attachment of an electron to create an ion of the form A−•. The reaction is
A + e^- ->[M] A^-
where the M over the arrow denotes that to conserve energy and momentum a third body is required (the molecularity of the reaction is three).
Electron capture can be used in conjunction with chemical ionization.
An electron capture detector is used in some gas chromatography systems.
Chemical ionization
Chemical ionization (CI) is a lower energy process than electron ionization because it involves ion/molecule reactions rather than electron removal. The lower energy yields less fragmentation, and usually a simpler spectrum. A typical CI spectrum has an easily identifiable molecular ion.
In a CI experiment, ions are produced through the collision of the analyte with ions of a reagent gas in the ion source. Some common reagent gases include: methane, ammonia, and isobutane. Inside the ion source, the reagent gas is present in large excess compared to the analyte. Electrons entering the source will preferentially ionize the reagent gas. The resultant collisions with other reagent gas molecules will create an ionization plasma.
Official source
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