Atmospheric pressure chemical ionization (APCI). In this method of ionization, an aerosol of sample solution is sprayed at atmospheric pressure into a heated region in which a sharp metal pin held at high potential sustains a corona discharge. The action of the discharge on the solvent creates reagent ions that react with the neutral sample molecules to create protonated ions of the molecule. These ions then pass through a sampling aperture into the mass analyzer of the mass spectrometer.