Interesting signals from (electro)chemical mass spectrometry: are they real? – Probably not. Chemical ionization can mislead electron ionization

Electron ionization mass spectrometry (EIMS) provides valuable structural and compositional information on unknown products. When connected in-line with (electro)chemical reactions, it allows for a fast detection of reaction intermediates and products. Thus, EIMS is optimal for supporting investigations on 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 potentially misleading further analysis. In this work, we start by illustrating the potential for this phenomenon in two case studies: i) influence of H2 evolution during electrochemical CO reduction and ii) influence of Cl2 evolution when analyzing electrochemical C-Cl bond formation. We then demonstrate the generality of this principle in EIMS.