To obtain an overview of the influence of H2 on the MS background during the reaction, Figure compares the mass spectra in the 0-50 m/z range with or without an electrochemical (EC) reaction in the CO-saturated KOH electrolyte on Pt. Figure enlarges the m/z regions that exhibit noticeable differences. Before CORR started, the gas entering the MS was pure CO. As CO is being consumed during the reaction (red), the composition of gas entering the MS contained product molecules and thus the CO composition was below 100%, which results in a slightly decreased M28 intensity.