Considering the setup configuration which allows continuous H2O evaporation to the MS, as well as the high Cl2 concentration in the carrier gas (see SI for the estimation), there is a chance for gaseous Cl2 and H2O molecules to meet and react/equilibrate (Equation [34]) before entering the MS ionization chamber, or in the chamber but before being ionized. In this way, both produced HCl and HOCl could be ionized and fragmented. Therefore, M36 and M38 were attributed to HCl because HOCl does not contain a H-Cl bond.