Atmospheric-pressure ionization background

Guo X, Bruins AP, Covey TR (2006) Characterization of typical chemical background interferences in atmospheric pressure ionization liquid chromatography-mass spectrometry. Rapid Commun Mass Spectrom 20 3145-3150... 

A third class is the recently-developed atmospheric pressure ionization (API). Since the background and applications of this technique have been discussed by Professor Horning at this Symposium it will not be considered here. The theory of CIMS has been discussed and reviewed in a number of articles (1-9). Here it is useful to summarize the more common reactions which can occur between the reagent gas (ionization gas) and the molecule of the sample being analysed (Table 1). 

The high potentials required for electrospray show that air at atmospheric pressure is not only a convenient, but also a very suitable, ambient gas for ES, particularly when solvents with high surface tension, such as water, are to be electrosprayed. The oxygen molecules in air have a positive electron affinity and readily capture free electrons. Initiation of gas discharges occurs when free electrons present in the gas (due to cosmic ray or background radiation) are accelerated by the high electric field near the capillary to velocities where they can ionize the gas molecules. At near-atmospheric pressures, the collision frequency of the electrons with the gas molecules is very high and interferes with the electron acceleration process. 

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