4.5.5.2 Atmospheric Pressure Chemical Ionization (APCI)

ESI is highly efficient for many systems. However, for systems where the running buffer is not especially volatile and/or the molecules are not strongly charged, alternative ionization techniques may be more efficient. Atmospheric pressure chemical ionization (APCI) and atmospheric pressure photo ionization (APPI) can both be used for systems like these.

APCI relies on the evaporation of an analyte-containing aerosol, where a charged reagent gas collides with analyte molecules and confers the charge. In APPI, an analyte aerosol is again evaporated to a vapor, and then photons ionize the analyte. In situations where the analyte is not easily photoionized, a buffer gas may  be  photoionized and then transfer the charge to the analyte.

APCI and APPI both require substantial heating of the sample and therefore are not appropriate for analytes that easily degrade at higher temperatures. Typically, APCI and APPI systems are heated to between 200-300ºC.  Additionally, low molecular weight (<1kD) analytes are better suited to APPI and APCI as they are more volatile.