Atmospheric pressure chemical ionization

Atmospheric pressure chemical ionization (APCI) is an ionization method used in mass spectrometry. It is a form of chemical ionization which takes place at atmospheric pressure.

How it works
APCI allows for the high flow rates typical of standard bore HPLC to be used directly, often without diverting the larger fraction of volume to waste. Typically the mobile phase containing eluting analyte is heated to relatively high temperatures (above 400 degrees Celsius), sprayed with high flow rates of nitrogen and the entire aerosol cloud is subjected to a corona discharge that creates ions. Often APCI can be performed in a modified ESI source. This is basically a gas phase ionisation, unlike ESI which is a liquid phase ionisation process. Also, we can use nonpolar solvent for solution making instead of polar solvent for supporting ions in solution as gaseous state conversion of solvent before reaching to corona discharge pin is carried out here, which well supports the ions formed. Typically, APCI is a less "soft" ionization technique than ESI, i.e. it generates more fragment ions relative to the parent ion.