Significance of appearance energies

Kinetic and competitive shifts are instrumental effects which, in principle, can always be avoided given sufficient sensitivity. There are other considerations germane to the determination of critical energies from appearance energies which are essential characteristics of the experimental approach. 

A basic consideration is whether the ions detected at the appearance energy correspond to the process of interest. Ion-pair formation has long been recognised as a low-energy pathway to positive ions. The process 

It has, for example, been suggested that the threshold for formation of (C3H7)+ from propane represents ion-pair formation [172], Ion-pair contributions to ionization efficiency curves are eliminated in PIPECO. Autoionization may also provide alternative pathways to fragment ions however, these are difficult to identify and have not been widely discussed. All that is necessary to obtain a false critical energy for the second step in 

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