Atmospheric Pressure Chemical Ionisation

In APCI the sample is sprayed out of a fine needle that is usually a bit shorter than that used in Electrospray. The solvent is then dried away by a heater. The remaining solutes, hopefully also vaporised, are then blown towards a corona discharge, which I think is just a technical name for a spark.

In the spark the vaporised solvent and analyte are ionised. The solvent can play an important role here, because if it has a lower affinity for protons than the analyte, it can pass a proton onto an analyte molecule, creating an [M+H]⁺ ion.

Since the metabolite of interest has to float its way into the spark in a stream of gas, this method relies on some volatility (unlike electrospray). Ionization events are comparatively rare, so the chances of an ionized metabolite meeting another ionizing species and getting a second charge are even rarer: most ions in APCI are singly charged.