Theory:
Atmospheric pressure chemical ionisation
(APCI) is an analogous ionisation method to
chemical ionisation (CI).
The significant difference is that APCI occurs at atmospheric pressure
and has its primary applications in the areas of ionisation of low mass
pharmaceutical compounds (APCI is not suitable for the analysis of
thermally labile compounds). The general source set-up (see fig. 1)
shares a strong resemblance to
electrospray
ionisation (ESI) and as such is most commonly used in conjunction
with HPLC or other flow separation techniques. Where APCI differs to
ESI, is in the way ionisation occurs. In ESI, ionisation is bought
about through the potential difference between the spray needle and the
cone along with rapid but gentle desolvation. In APCI, the analyte
solution is introduced into a pneumatic nebulizer and desolvated in a
heated quartz tube before interacting with the corona discharge
creating ions.
Fig. 1: A schematic of the components
of an APCI source.
The corona discharge
replaces the electron filament
in CI - the atmospheric pressure would quickly "burn out" any filaments
- and produces primary N
2°
+ and N
4°
+
by electron ionisation. These primary ions collide with the vaporized
solvent molecules to form secondary reactant gas ions - e.g. H
3O
+
and (H
2O)
nH
+ (see fig. 2). These
reactant
gas ions then undergo repeated collisions with the analyte resulting in
the formation of analyte ions. The high frequency of collisions results
in a high ionisation efficiency and thermalisation of the analyte ions.
This results in spectra of predominantly molecular species and adduct
ions with very little fragmentation. Once the ions are formed, they
enter the pumping and focussing stage in much the same as the other
atmospheric pressure ionisation sources (for example
ESI).
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Fig 2: A more detailed view of
the mechanism of APCI.
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Reactions in the plasma region:
Assuming nitrogen is the sheath and nebulizer gas with atmospheric
water vapour present in the source, then the type of primary and
secondary reactions that occur in the corona discharge (plasma) region
during APCI are as follows:
The most abundant secondary
cluster ion is (H2O)2H+ along with
significant amounts (H2O)3H+ and H3O+.
The reactions listed above are ways to account for the formation of
these ions during the plasma stage.
The protonated analyte ions are then formed by gas-phase ion-molecule
reactions of these charger cluster ions with the analyte molecules.
This results in the abundant formation of [M+H]+ ions.
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©2004 Paul
Gates, University of Bristol
Last updated April 1st 2004
