Where X -are the same as defined earlier, and include phosphate groups in the nucleic acid analyte. In the solid sample, some of the nitrogen atoms in the analyte are protonated, and there should be substantial number of nucleobases that carry a positive charge. The charged moiety of the nucleobases and their counter anions may be well separated. Upon laser irradiation, in the gas phase, tight ion pairs form, which can further convert to neutral species via proton transfer (equations 10-11).