Atmospheric pressure photoionization (APPI) source coupled to a krypton discharge lamp permits to specifically generate either radical cation or protonated species for guanine depending on the dopant molecule. In order to scrutinize the consecutive or competitive chemical reactions occurring in the gas phase, a tunable synchrotron VUV radiation source was coupled for the first time to the APPI source of a hybrid quadrupole time-of-flight mass spectrometer. Experiments were achieved at different photon energies close to the ionization energy of the different reagents. This unique set-up allowed the confirmation of theoretical models developed by several groups over the last ten years concerning ionization processes underlying APPI, such as proton or charge transfer reactions. (C) 2012 Elsevier B.V. All rights reserved.