	The first atmospheric pressure ionization source was developed by Horning, Carroll and their co-works in the 1970s at the Baylor College of Medicine (Houston, TX). Initially, Ni foil was used as a source of electrons to perform ionization. Latterly in 1975, corona discharge electrode was developed, which had a larger dynamic response range. APCI with the corona discharge electrode became the model for modern commercially available APCI interfaces.