The University of Bristol, School of Chemistry

MASS SPECTROMETRY RESOURCE

Atmospheric Pressure Chemical Ionisation (APCI)


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.

APCIsource schematic

    The corona discharge replaces the electron filament in CI - the atmospheric pressure would quickly "burn out" any filaments - and produces primary N2°+ and N4°+ by electron ionisation. These primary ions collide with the vaporized solvent molecules to form secondary reactant gas ions - e.g. H3O+ and (H2O)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).

Fig 2: A more detailed view of the mechanism of APCI.

Mechanism of APCI

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:

APCI equations

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.

©2004 Paul Gates, University of Bristol
Last updated April 1st 2004