Where Nuc stands for nucleic acid (DNA and RNA). In solid phase with the usually acidic matrix as the media, some of the phosphate groups exist in deprotonated form. The counter cations can be Met + , R3NH + and [M+H] + , which are defined earlier. Upon laser irradiation, in the gas phase the equilibrium represented by equations (7-9) will have a strong tendency toward ion pairs. In the case of equation ( ), the ion pair remains. In the cases of equations (8-9), the ion pairs can further convert to neutral molecules via proton transfer.