Unexpected Artifact Formation in Mass Spectrometric Analysis of Aniline under Atmospheric-Pressure Chemical Ionization

Atmospheric-pressure chemical ionization mass spectrometry (APCI-MS) is a widely used technique for the analysis of a diverse range of analytes. Under APCI conditions, a nonthermal plasma, rich in highly oxidative species such as H₂O₂, O₃, atomic O, and radicals such as HO^•, is created. These oxidants trigger unanticipated and often undesirable chemical reactions within the ion source. For example, when aniline was introduced into this environment, it initially underwent oxidative dimerization forming hydrazobenzene (<i>m</i>/<i>z</i> 185). However, with prolonged exposure, there was a marked increase in total ion abundance and the generation of additional artifact ions such as protonated azobenzene (<i>m</i>/<i>z</i> 183) and protonated azoxybenzene (<i>m</i>/<i>z</i> 199). The emergence of these artifacts was found to be highly dependent on the corona-current magnitude. Moreover, the desorption-gas temperature significantly influenced the rate of artifact generation. Recognizing and acknowledging the formation and presence of such artifacts in an ion source is paramount in conducting validated chemical analysis. The existence of artifacts can complicate mass spectral interpretation, potentially leading to erroneous conclusions and misinterpretations of both qualitative and quantitative data. Thus, understanding the intricacies of nonthermal plasma-driven artifact formation is critical for accurate analytical outcomes.