Ion Formation in APPI

Statistically, the light emitted by the UV lamp will be absorbed by solvent molecules or eventually by nitrogen used for spraying rather than by the analyte molecules. Therefore, the energy of the UV photons and the ionization energies of the compounds are decisive for the initiation of the APPI process (Fig. 12.44) [186]. 

Provided the analyte (AB) has a UV chromophore, direct photoionization of AB may occur (Chap. 2.10.4), but as explained above, is rarely observed. Often, photon absorption just causes photoexcitation instead of photoionization [186]  

Electronically excited molecules AB can either undergo radiative decay, photodissociation, or collisional quenching with neutrals (N), or can form ions by ejection of an electron  

Even if ions AB have formed they can be neutralized by recombination with free electrons present in the plasma, eventually along with moderating ion-neutral collisions  

Instead of relying on one of the above ionization processes to occur, a chemical ionization process where the reactant ions are formed by photoionization of a reagent gas is normally preferred. If the solvent (S) cannot be photoionized to play this role, a UV-absorbing dopant (D) may be added [185,187-189]. The dopant ions may then react with the analyte by charge transfer 

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