Experiment All samples were analyzed by using JMS-T100GCV (JEOL Ltd.) with the optionally available EI/FI combination ion source and PI ion source. EI data was acquired by using the standard 70eV ionization energy with the EI/FI combination source. FI data was acquired by using a 5μm carbon emitter (Carbotec Analytik) with the combination EI/FI source. A deuterium lamp with a magnesium fluoride window (Hamamatsu Photonics K.K.) was used as the PI source. This lamp has an irradiation wavelength range from 115 to 400 nm (equivalent to the energy range from 3.1 to 10.7 eV) in which the maximum radiation intensity is at 160 nm (7.7 eV). And finally, the EI, FI and PI measurements were done with the same detector voltage so that the peak intensities could be directly compared for each ionization method.