A hypothesis for not seeing Chem-I in the 70 eV case with CO is due to the higher ionization energy and more severe fragmentation at 70 eV than those at 28 eV. Assuming the same amount of CO molecules entering the ionization chamber, there will be more [CO]+ molecular ions as well as C+ and O+ fragments produced at 70 eV, and thus fewer CO molecules. Similarly, EI energy at 70 eV produces more [H2]+ as well as H+.