MECHANISM OF IONIZATION OF POLYCYCLIC AROMATIC HYDROCARBONS BY A TOLUENE/ANISOLE MIXTURE AS A DOPANT IN LIQUID CHROMATOGRAPHY/DOPANT-ASSISTED ATMOSPHERIC-PRESSURE PHOTOIONIZATION/MASS SPECTROMETRY

Ionization mechanisms of 16 polycyclic aromatic hydrocarbons (PAHs) by dopant-assisted atmospheric-pressure photoionization (DA-APPI) with a toluene/anisole mixture as a dopant were examined in the reversed-phase liquid chromatography mode with a methanol/water to acetonitrile linear gradient mobile phase. When toluene/anisole mixtures were used as dopants, their characteristics were governed by anisole. Since the IE of anisole (8.20 eV) is below that of toluene (8.83 eV), anisole is ionized not only by photons from the krypton lamp but also by charge transfer from toluene radical cations. In the latter half of chromatogram, when acetonitrile was dominant in the eluent, ions originating from both toluene and anisole decreased while acetonitrile-cluster ions began to increase. Because acetonitrile-cluster ions are not formed with anisole alone, but are formed with toluene alone, toluene radical cations would be consumed to form acetonitrile-cluster ions.