Atmospheric Pressure Chemical Ionisation (APCI) is an ionisation method used in mass spectrometry utilising gas-phase ion-molecule reactions at atmospheric pressure. Ionisation is effected using a corona discharge. This is a method of ‘soft ionisation’, where little fragmentation occurs in the source of the mass spectrometer.
Mass Spec uses APCI for trace analysis - primarily for drugs detection, but can be used for a wide range of other substances.