anion attachment. Most work on the APCI-MS analysis has been in positive mode. In the positive mode, when the discharge current of corona discharge is 1-5 μA on the nebulized solvent, N2 gas molecules are excited and ionized, which produce N4+*. The evaporated mobile phase of LC acts as the ionization gas and reactant ions. If water is the only solvent in the evaporated mobile phase, the excited nitrogen molecular ions N4+* would react with H2O molecules to produce water cluster ions H+(H2O)n. Then, analyte molecules M are protonated by the water cluster ions. Finally, the ionization products MH+(H2O)m transfer out from the atmospheric-pressure ion source. Declustering (removal of water molecules from the protonated analyte molecule) of MH+(H2O)m takes place at the high vacuum of the mass analyzer. The analyte molecule ions detected by MS are [M+H]+. The chemical reactions of ionization process are shown below. Primary and secondary reagent ion formation in a nitrogen atmosphere in the presence of water: N2 + e → N2+ + 2e N2+* + 2N2 → N4+* + N2 N4+ + H2O → H2O+ + 2N2 H2O+ + H2O → H3O+ + OH• H3O+ + H2O + N2 → H+(H2O)2 + N2 H+(H2O)n-1 + H2O + N2 → H+(H2O)n + N2 Ionization of product ions: H+(H2O)n + M → MH+(H2O)m + (n-m)H2O Declustering in the high vacuum of the mass analyzer: MH+(H2O)m → MH+ + mH2O If the mobile phase contains solvents with a higher proton affinity than water, proton-transfer reactions take place that lead to protonated the solvent with higher proton affinity. For example, when methanol solvent is present, the cluster solvent ions would be CH3OH2+(H2O)n(CH3OH)m. Fragmentation does not normally occur inside the APCI source. If a fragment ion of a sample is observed, thermal degradation has taken place by the heated nebulizer interface, followed by the ionization of the decomposition products. In a major distinction from chemical ionization, the electrons needed for the primary ionization are not produced by a heated filament, as a heated filament cannot be used under atmospheric pressure conditions. Instead, the ionization must occur using either corona discharges or β- particle emitters, which are both electron sources capable of handling the presence of corrosive or oxidizing gases. History The origins of atmospheric pressure chemical ionization sources combined with mass spectrometry can be found in the 1960s in studies of ions in flames and of ion chemistry in corona