Atmospheric Pressure Chemical Ionization (APCI) mode creates ions at atmospheric pressure. In the APCI mode, the liquid flow is forced through a stainless steel capillary. Evaporation of both the analytes and mobile phase solvents occur under a heated nebulizer (note that no voltage is applied to the capillary). After nebulization the analytes and mobile phase solvent are sprayed into a corona discharge needle. As a result, nitrogen is ionized first (most abundant in mobile phase), passes its charge to vaporized solvent and then charged solvent that transmits its charge to the analytes. Finally, ions enter in the mass spectrometer to be detected under a specific scan mode.
APCI is best suited for metabolites with low to medium polarities.