Ionization at atmospheric pressure

Many implications arise when moving the ionization region from the classical high-vacuum environment to the AP region: As an example, the collision frequency in a supersonic jet expansion downstream of the jet terminus is practically zero as long as the Mach disc is still distant at the place of ionization – at AP, collisions frequencies exceed 10⁹ s^-1 per molecule.

Two facts are becoming quite apparent from this situation:

1. In an API source any induced ion molecule chemistry is preferentially described as a system close to thermal equilibrium.
2. The ion trajectories within the API source are dominated by fluid flow forces and to a lesser extent by electrical field gradients – a situation well known from ion mobility spectrometry.

In other words: Regardless of any selectivity in the ionization process, a careful assessment of potential subsequent reactions of primary ions with matrix constituents is always warranted.