Electron Impact Ionization
M + e(70eV) -----> M+. + 2e
EI ionization method is suitable for non
thermolabile compounds. The volatility of the sample is required. Sample
molecules in vapor state are bombarded by fast moving electrons, conventionally
70 eV energy. This results in ion formation. One electron from the highest
orbital energy is dislodged, and as a consequence molecular ions are formed.
Some of this molecular ions decompose and fragment ions are formed. The
fragmentation of a given ion is due to the excess of energy that it requires
within the ionization. Fragment ions can be odd electron or even electron.
Molecular ions formed in electron impact ionization are odd electron ions.
Odd electron fragment ions are formed by direct cleavage(e.g. direct cleavage
of a C-C bond). Even electron fragment ions are often formed by rearrangement(e.g.
proton transfer). Sample can be introduced to the EI source via a gas chromatography
device, for example in the case of mixtures, or directly via a solids probe
device. The quantities needed for an experiment is usually less than a
microgram of material.
EI mass spectra, in most of cases,
contain intense fragment ion peaks and much less intense molecular ion
peak. When the molecular ion peak is not observed in the mass spectrum,
chemical ionization
can be used in order to get molecular ion information.
Nitrogen rule:
As indicated above, molecular ions formed
in EI ionization are odd electron ions. If their observed mass to charge
ratio is odd, the molecule under investigation contains odd number of nitrogen
atoms. If that mass to charge ratio is an even number, that molecule contains
no or even Nitrogen atoms.
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