The M31 increase can be caused by three possible sources: i) adduct formation between 13 C 16 O (at M29) and H2 (at M2), where either of the two species could be the molecule or ion; ii) 12 C 18 O molecule (at M30) and either H+ ion (at M1) following the adduct formation mechanism or [H2O]+ ion (as the reagent gas) following the proton transfer mechanism; and iii) 12 C 16 O molecule (at M28) and [H2-H]+ ion (at M3) following the adduct formation mechanism. In the last case, Chem-I is expected to happen twice. Although statistically, the probability is low for the last source, considering the high IM3 value in Figure , this possibility cannot be fully ruled out.