Answer 8.1

a) How is ionization achieved in field ionization (FI)?

In the presence of a strong electric field an electron can leave a molecule by tunneling through a potential barrier. Thus, the molecule is ionized and immediately driven away from the anode by action of the strong electric field.

b) Field ionization describes a process as well as a technique. Can you tell the difference?

Atoms or molecules can be ionized by action of a strong electric field independent of whether they have been adsorbed to the anodic surface or whether they are moving freely through the space between the electrodes. This process is known as field ionization.
If the sample is supplied from a separate inlet system in the gaseous state, this methodology is also termed field ionization to distinguish it from field desorption (below).

c) In field desorption (FD), the analyte is supplied on the surface of an activated wire emitter. What advantage does this offer compared to FI?

FD circumvents the evaporation of the analyte prior to ionization by uniting the processes of ionization and subsequent desorption of the ions on the surface of the field emitter. This allows thermally labile and also ionic analytes to be measured with low or even absent decomposition. Beside of having no need for a separate inlet system, the ion source remains the same for FD and FI operation.