Where Pep stands for peptide. Met + is metal cations such as Na + and K + . R3NH + is nitrogen associated cations such as ammonium, guanidium, pyridinium and imidazolium ions. Pep-CO2 -⁞Met + , Pep-CO2 -⁞R3NH + , and Pep-CO2 -⁞[M+H] + are ion pairs. In solid phase before laser irradiation, due to solvation by residue solvents and shielding by neutral polar matrix and analyte molecules, even in the medium of the usually acidic matrices, there is certain amount of well isolated ions. Upon laser irradiation, once the analyte goes into the gas phase, residue solvents and neutral shielding molecules fall off, and the ions form tightly associated ion pairs. In the case of Met + , the ion pair remains intact (equation 2). In the cases of R3NH + and [M+H] + , the ion pairs may convert to neutral molecules via proton transfer (equations 3-4).