Intentional Chem-I (mostly applied to the organic-/bio-chemistry and pharmaceutics fields) is normally operated at a higher pressure (0.2-2 Torr ) compared to electron ionization (below 10 -6 Torr ). However, the higher pressure guarantees a higher Chem-I efficiency between the reacted ions and molecules with minimal fragmentation whereas Chem-I can still occur at lower pressure in an EI chamber albeit less efficiently.