Atmospheric pressure chemical ionization (APCI) is a popular complementary soft ionization technique to electrospray. It is commonly used to analyze small polar and nonpolar compounds that are ionized poorly by electrospray but do well in chemical ionization, which generates singly charged ions at higher temperatures.

The Agilent APCI source is sensitive yet extremely robust, thanks to an orthogonal spray geometry and counterflow drying gas. Like the ESI source, it can generate both positive and negative ions, and ion polarity can be switched on a spectrum-to-spectrum basis.