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Soft Ionisation with Different Reagent Ions

The Kore PTR-TOF-MS is delivered with a water vapour system that produces H3O+ reagent ions from the ion source, but the glow discharge ion source can be operated with other gas species. Alternative gases can be supplied to the glow discharge ion source as a standard instrument feature.

O2+ Reagent Ions

These ions can be generated by passing pure O2 gas into the glow discharge ion source. It is also the case that allowing air to the glow discharge will also produce copious amounts of O2+. Also produced with an air feed are NO+ ions, but the ionisation energy for this species is 9.26eV, as opposed to 12.07eV for O2+, and thus the dominating ionisation method for most analytes is by charge exchange with O2+.

Spectrum of BTX
BTX sample in water (ppm level) with O2+ reagent ions. Note how the molecular ions are M+ species, not (M+H)+, as is observed with H3O+ reagent ions.

NO+ Reagent Ions

Spectrum of NO+ reagent ions

NO gas can also be fed directly into the glow discharge, where it produces an intense beam of NO+ ions at mass 30 (see above). The low intensity peaks at mass 31 and 32 are due to ions 14N17O and 15N16O, and 14N18O and 15N17O respectively

Ar+, Xe+, Kr+ Reagent Ions

We have also operated the Kore glow discharge source on various noble gases such as argon, xenon and krypton.

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: 11:56 05/02/2014

© Kore Technology Limited 2014