Ionization mechanisms related to negative Ion APPI, APCI, and DART

A recent report found that negative ion atmospheric pressure photoionization (Ni-APPI) and direct analysis in real time (Ni-DART) ionize compounds by electron capture, dissociative electron capture, proton abstraction, and anion adduction. The authors of this report suggested that the common ionization of Ni-APPI and Ni-DART demonstrated that these techniques ionize a wider array of compounds than negative ion atmospheric pressure chemical ionization (Ni-APCI). Here we show that Ni-APCI, using the atmospheric sample analysis probe (ASAP) technique, in the absence of solvent vapors, ionizes the same and similar compounds by the same reported mechanisms. These results are supported by previous publications, which show that each mechanism is active for Ni-APCI. This work demonstrates that irrespective of the initial method of ionization, at atmospheric pressure, similar ion/electron-molecule chemistries prevail.