atmospheric pressure photoionisation (APPI)

A form of API in which the analyte is sprayed into the source, as in ESI and APCI sources, in a  solvent frequently containing a dopant molecule such as toluene or acetone. The spray is irradiated by a powerful UV source that forms excited  species which undergo secondary ion/radical- molecule reactions with the analytes to cause ionisation. It is claimed to be the method of choice for detection of non-polar molecules in HPLC eluates. Protonated or deprotonated molecules or radical molecular ions can be formed, according to  the mechanisms and thermodynamics involved.