Case study I: Electrochemical CO reduction reaction (CORR) CORR was chosen as a case study because it is a model reaction for investigating the electrochemical CO2 reduction reaction. This has been a popular topic in the field of renewable energy, since it enables converting the excessive CO2 in the atmosphere to valuable fuels and chemicals and thus mitigate global warming. Cu was used as the catalyst and working electrode for CORR because Cu is known to be able to produce valuable fuels and chemicals (e.g. methane, ethylene, and ethanol) . Ethylene and ethanol are the two dominant products, but with ethylene being a gas and ethanol being a liquid it is hard to measure both of these simultaneously using the same technique. As ethanol has moderate vapor pressure, EIMS may have the potential to provide this concurrent analysis. In these experiments, the working electrode potential was scanned linearly from 0.2 V vs. RHE to -0.6 V vs. RHE at a scan rate of 2 mV/s. Applying the standard EI energy at 70 eV in the MS causes significant fragmentation, which makes the deconvolution and assignment of m/z difficult. Therefore, CORR was conducted with a reduced EI energy at 28 eV as this reduces fragmentation, allowing for optimal product differentiation and identification. Further details and discussions on the EC-MS setup and optimization of MS setting in CORR have been published elsewhere . Figure depicts the MS ion current (IMX, where "MX" stands for mass-to-charge ratio, m/z, at X) on Cu as a function of working electrode potential (EWE) during the linear sweep voltammetry scans. Targeted product molecules, hydrogen (H2), methane (CH4), ethylene (C2H4), and ethanol (C2H5OH), were monitored and indicated by their corresponding characteristic m/z at M2, M15, M26, and M31, respectively. While hydrogen, methane, and ethylene are all gases that would be expected to be seen in the EIMS, ethanol is a liquid with a vapor pressure of 7.8 kPa at 25 °C, thus its limit of detection (LOD) is much higher than gaseous species. The expected ethanol production from CORR under the given reaction conditions was estimated to be approximately two orders of magnitude of LOD. Thus, the observed IM31 cannot be from the actual ethanol formation. To investigate potential causes of the IM31 increase, a more in-depth analysis of the mass spectrometry procedure is needed to verify whether this is valid. Detailed information on the LOD of all involved species as well as estimation of ethanol production from CORR can be found in references , and in the SI. As the reaction proceeds, gaseous and volatile liquid products are generated and mixed with the carrier gas (i.e., CO in CORR) that enters the mass spectrometer (MS) ionization chamber where ionization occurs. Thus, it is a gas mixture comprised of the pristine carrier gas (major) and product molecules (minor) that enters the MS. Since the total molar flow rate of the gas mixture entering the MS is defined based on its physical properties (e.g., dynamic viscosity, mean molecular dynamic mass and diameter) , its total molar flow rate varies as its composition varies during the reaction. As a result, the background pressure in the ionization chamber may shift because the chamber volume is kept constant. Considering H2 was the major product , its MS ion current IM2 increased the most under the given conditions (Figure ). As H2 is a light gas and known to be easily trapped in the chamber , H2 production and the concomitant pressure build-up may lift the background of other m/z, thus exhibiting a seemingly increased IMS.