The above two case studies have illustrated the possibility and generality of Chem-I occurring in an EI chamber for the case when the involved ions or molecules are present in a sufficient amount/composition/concentration such that they can easily collide. This inadvertent Chem-I participation can scramble m/z assignments and thus result in a misleading interpretation if not properly analyzed. This is particularly important for EIMS combined with chemical or electrochemical reaction systems, as well as in gas/liquid chromatography-mass spectrometry combined systems (GC-MS or LC-MS) where a mobile phase is usually involved and therefore could easily participate in Chem-I reactions.