Interesting Signals from Electrochemical Mass Spectrometry: Are They Real? Probably Not. Chemical Ionization Can Distort Electron Ionization Mass Spectra

Electron ionization mass spectrometry (EIMS) provides valuable structural and compositional information about unknown compounds. When connected in line with electrochemical reactions, it allows for the fast detection of reaction intermediates and products. Thus, EIMS is optimal for supporting investigations of reaction mechanisms and pathways. However, chemical ionization can inadvertently occur in the electron ionization chamber, scrambling the mass-to-charge ratio (m/z) distribution and thus misleading further analysis. In this work, we start by illustrating the potential influences of this phenomenon in two case studies: (i) the influence of H2 evolution during electrochemical CO reduction and (ii) the influence of Cl2 evolution when analyzing electrochemical C–Cl bond formation. This is followed by demonstrating the generality of this principle in EIMS. We also provide insights into differentiating if an MS signal of interest comes from the expected electrochemical process or is formed inadvertently in the MS chamber due to Chem-I scrambling.