Atmospheric-pressure chemical ionization

Chemical ionization at atmospheric pressure ( atmospheric pressure chemical ionization English, APCI ) is an ionization method that is used in the mass spectrometer. It is a form of chemical ionization at atmospheric pressure. A solution of the analyte is sprayed through a capillary in a nitrogen stream, whereby a spray is produced. The spray is passed through a heated ceramic (300-400 ° C), where the solvent is completely evaporated.

The escaping steam is generated by applying a high voltage (about 5 kV ) via a needle-shaped electrode (so-called corona needle, Eng. Corona discharge needle ) into a plasma. In the plasma buffer are first out of the solvent and possibly added (often ammonium acetate ) ions are formed. The ionized solvent molecules in turn ionize the analyte molecules, which are then transferred to the actual measuring apparatus.

Depending on the solvent and analyte, the following reactions are possible:

• Protonation (such as chemical ionization, such as amines)
• Charge exchange
• Deprotonation (e.g. with carboxylic acids, phenols )
• Electron (eg halogen compounds and aromatics )

Multiply-charged molecules of [M nH ] n, as in the case of the electrospray ionization ( ESI), are not observed.

APCI often may be carried out in a modified ESI ion source. The method is less gentle than ESI, that is, there occur more fragment ions. APCI is to use the relatively high flow rates of a standard HPLC directly without the largest part of the volume must be discarded allowed. The use of APCI is useful under the following conditions:

• The analyte is ionized by ESI poor ( eg low functional groups, no nitrogen )
• The analyte contains little reactive functional groups ( for example, hydrocarbons, alcohols, aldehydes, ketones, esters )
• The sample is thermally stable and vaporized