APCI (Atmospheric Pressure Chemical Ionisation)

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Pechar

As in ESI, small droplets of the solvent are created with the mobile phase and the analyte, which enter a vaporiser. The vaporiser is an insulated ceramic tube, which, when heated, evaporates the aerosol droplets from the mobile phase by thermal radiation. The temperature of the vaporiser is about 350 °C, but the temperature can be adjusted. The corona discharge electrode (high voltage needle) generates electrons in a strong electric field that ionise the nebuliser gas (nitrogen), and these ions react with the solvent molecules to form reactive ions which, in turn, react with the analyte molecules by proton transfer. This series of cascading chemical reactions results in the mild chemical ionisation of the analyte molecules at atmospheric pressure and usually at high temperature. This needle is located in a fixed position at the gas outlet of the vaporiser, close to the analyte gas stream.

The mechanism of Chemical Ionisation, Atmospheric Pressure, in positive mode, starts with the formation of primary and secondary reactive ions in a nitrogen atmosphere in the presence of mobile phase (water).

This is followed by ionisation of the product ions, which then go to the mass analyser.

APCI has the advantages of ESI regarding compatibility with previous separation techniques (HPLC, UHPLC...) and that it is also a soft technique, with little fragmentation. This technique is suitable for low polarity compounds containing heteroatoms such as ureas or benzodiazepines; and/or which are thermally stable.