Chemical Ionisation by Ion-Molecule Reaction

(Ionisation of Analyte by ion - molecule reactions)

In this mode, the ion source is run on an inert gas such as argon, xenon or krypton, instead of N₂/H₂O mixture.

The primary ion produced from the glow discharge source has a specific energy in its ionised state. If it impacts a molecule in the collision cell and that molecule requires less energy to be ionised, then that molecule can become ionised and the primary ion will convert to a neutral particle. The sequence is as follows:

Kr⁺ (13.99 eV) + CO → CO⁺ (13.98 eV) + Kr
(Chemical Ionisation by Ion-Molecule Reaction)

However, if the molecule requires more energy than is available in the rare gas ion, the molecule remains neutral, e.g.

Kr⁺ (13.99 eV) + N₂ → Kr⁺ + N₂
(No Chemical Ionisation)

This is because 15.5 eV is required to ionise N2.

Notice that N₂ and CO both have masses of 28. Their exact masses are 28.0061 and 27.9949 respectively. Ordinarily, a mass spectrometer would need to have a certain 'resolving power' to separate these two masses. However, by using a chemical ionisation technique, it is possible to achieve selective ionisation of such 'isobaric interferences'.

Ionisation energies of inert gases that can be run in the glow discharge source:

Ar⁺	(15.76 eV)
N₂⁺	(15.50 eV)
Kr⁺	(13.99 eV)
Xe⁺	(12.13 eV)

The glow discharge has been run with both Ar and Xe, producing Ar⁺ and Xe⁺ ion beams Krypton may also be used.

The table below lists the ionisation energies of various components, and the primary ion beams that could be used.

Ionisation Energies for Common Components
Species of Interest	Ionisation Energy (eV)	Primary Species
CO	13.98	Ar⁺, Kr⁺
CO₂	13.78	Ar⁺, Kr⁺
SO₂	12.32	Ar⁺, Kr⁺
CH₄ (methane)	12.61	Ar⁺, Kr⁺
O₂	12.07	Ar⁺, Kr⁺
C₂H₆ (ethane)	11.52	Ar⁺, Kr⁺, Xe⁺
COS	11.18	Ar⁺, Kr⁺, Xe⁺
C₃H₈ (butane)	10.94	Ar⁺, Kr⁺, Xe⁺
C₂H₄ (ethylene)	10.51	Ar⁺, Kr⁺, Xe⁺
H₂S	10.45	Ar⁺, Kr⁺, Xe⁺
NO₂	9.75	Ar⁺, Kr⁺, Xe⁺,
C₄H₈ (isobutene)	9.55	Ar⁺, Kr⁺, Xe⁺
NO	9.26	Ar⁺, Kr⁺, Xe⁺
Benzene	9.25	Ar⁺, Kr⁺, Xe⁺
Toluene	8.82	Ar⁺, Kr⁺, Xe⁺
Xylene	8.56	Ar⁺, Kr⁺, Xe⁺

Organic molecules have lower ionisation energies. In a collision, if the difference in ionisation energy is large, then the excess energy can cause fragmentation of the organic molecule. Thus the ionising gas species should be matched where possible to the analyte molecules of interest. For this reason, some researchers also run a primary ion source on Hg vapour to produce Hg⁺ ions (10.44eV) to minimise fragmentation.

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Last updated: 29 September 2005 23:23