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Fundamentals of GC-MS Ionisation Techniques

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Electron Impact (EI)

Electrons are emitted from a heated filament (usually made of tungsten or rhenium) and are accelerated across the source by using an appropriate potential (5 - 100 V) to achieve the required electron energy to ionise the molecule (Figure 1).

Usually a potential difference of 70 V is used as :

1 eV ≈ 23 kcalmol -1 ≈ 96 kJmol -1

Ionisation of an organic molecule requires about 7 – 10 eV

The strongest single bonds in organic molecules are approximately 4 eV


 

Figure 1:  Electron impact ionisation process »

 

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The analyte is introduced into the mass spectrometer ion source where it is impacted by the beam of ionising electrons leading to the formation of an analyte radical cation (Figure 2).  The molecular ions produced can fragment by loss of a radical or of a molecule with all its electrons paired (Figure 3).

:  Formation of a cation radical « Figure 2:  Formation of a cation radical and subsequent fragmentation to the molecular ion.

of molecular fragments

« Figure 3: Formation of molecular fragments of butyl acetate.



EI is a relatively harsh form of ionisation and as a result the parent often fragments further producing a pattern of fragment ions which can aid structural elucidation, however, the disadvantage of this is the frequent absence of the parent ion. 


 

 

Chemical Ionisation (CI)

Chemical ionisation involves the ionisation of a reagent gas (e.g. methane, isobutene, ammonia) at relatively high pressure (~1 mbar).  The gas is ionised by using electrons produced from a hot filament (as in EI) with energies up to 300 eV (Figure 4).  Some fragmentation can then occur

 

Figure 4:  Chemical ionisation process »

 

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Once produced the reagent gas ions collide with the analyte molecules producing ions through gas phase reactions (Figure 5).

bimolecular reactions

 

« Figure 5:  Main bimolecular reactions occurring in CI.

Chemical ionisation is a soft process due to the energy of the reagent ions rarely exceeding 5 eV.  As a consequence the spectra produced by this technique usually shows little fragmentation. 

In CI spectra analyte molecules will appear in the mass spectrum with m/z values one greater than the analyte molecular weight.  The spectra obtained with CI are highly dependent on the nature of the reagent gas used, and because of that different structural information can be obtained by choosing different reagent gases.  Ammonia gas works well for the analysis of volatile molecules.

 
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