Aerosol photoionization

In the current LC-MS interfaces, i.e., ESI and atmospheric-pressure chemical ionization (APCl), interfacing and analyte ionization are closely interrelated. The column effluent is nebulized and ionization takes place in the aerosol generated, either with or without a primary external source of ionization. Ionization mechanisms of ESI, APCl, and atmospheric-pressure photoionization (APPl) are discussed in this chapter. 

Oktem, B., Tolocka, M. P., and Johnston, M. V., On-line analysis of organic components in fine and ultrafine particles by photoionization aerosol mass spectrometry. Anal. Chem., 76, 253-261, 2004. 

Northway MJ, Jayne JT, Toohey DW, Canagaratna MR, Trimborn A, Akiyama K, Shimono A, Jimenez JL, DeCarlo PF, Wilson KR, Worsnop DR. Demonstration of a VUV lamp photoionization source for improved organic speciation in an aerosol mass spectrometer. Aerosol Sci Technol. 2007 41 828-39. 

Another ionization technique, which seems closely related to TSI and ESI, is atmospheric-pressure chemical ionization (APCI). In APCI, the solvent stream, e.g., the effluent from an LC column, is pneumatically nebuhzed into a heated vaporizer zone, where (almost) complete evaporation of the aerosol droplets is achieved [71-73]. Analyte ionization is initiated by electrons from a downstream corona discharge needle. The electrons act as primary source of ionization of the solvent or mobile-phase constituents, which in turn by gas-phase ion-molecule reactions in the API source ionize the analyte molecules, mostly by proton-transfer reactions, i.e., formation of [M-hH]+ in positive-ion and [M-H] in negative-ion mode. There are also some results, indicating the Na -cationization can take place under APCI conditions. Atmospheric-pressure photoionization (APPI) is an ionization technique closely related to APCI. In APPI, the analyte ionization is initiated by light from a vacuum-ultraviolet lamp, e.g., a Kr-lamp, instead of by means of a corona discharge. Next to direct photoionization of the analytes, gas-phase ion-molecular reactions greatly contribute to the ionization in APPI [74,75]. 

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