Investigation of the ionization mechanism of polycyclic aromatic hydrocarbons using an ethanol/bromobenzene/chlorobenzene/ anisole mixture as a dopant in liquid chromatography/atmospheric pressure photoionization mass spectrometry Ma’an Amad * and Salim Sioud Analytical Chemistry Core Laboratory, King Abdullah University Of Science and Technology (KAUST), Thuwal, 23955-6900, Kingdom of Saudi Arabia RATIONALE: An ethanol-based multicomponent dopant consisting of ethanol/chlorobenzene/bromobenzene/anisole (98.975:0.1:0.9:0.025, v/v/v/v) has been used as a dopant for atmospheric pressure photoionization (APPI) of polycyclic aromatic hydrocarbons (PAHs). In this study the mechanism of ionization of PAHs assisted by the ethanol-based multicomponent dopant is investigated. METHODS: The reactant background cluster ions of the ethanol-based multicomponent dopant observed in the positive ion APPI were studied. These studies were performed to investigate the mechanism behind the generation of a molecular radical cation (M + • ) for PAHs by APPI assisted by the ethanol-based multicomponent dopant. Full scan and MS/MS analyses were conducted using an LTQ Orbitrap mass spectrometer. The effect of acidification of the mobile phase on the dopant cluster ion formation was also investigated. RESULTS: With the ethanol-based multicomponent dopant, a single type of molecular radical cation M + • was observed for the studied PAHs. The characteristic ion signal of the multicomponent dopant mixture consisted of mainly anisole photoions at m/z 108.05697 and its adduct ions at m/z 124.05188 and 164.07061. The anisole ion response at m/z 108.05697 was stable in the presence of acetonitrile, methanol, water and 0.1% formic acid mobile phase composition. CONCLUSIONS: The abundance formation of anisole photoions shows the universality of this multicomponent dopant in ionizing compounds with ionization energy ranging from 7.1–8.2 eV. Since the ionization energy of anisole is 8.2 eV and is lower than those of chlorobenzene (9.07 eV) and bromobenzene (9.0 eV), the mechanism of formation of anisole photoions even with its very minute amounts was not only governed by its photoionization by the krypton lamp photon energy (10.0 eV and 10.6 eV), but also by charge transfer from bromobenzene and chlorobenzene radical cations. PAH molecules were mainly ionized by charge transfer reaction from photoionized anisole and oxidized anisole radical cations as well as by the krypton lamp. Copyright © 2012 John Wiley & Sons, Ltd. Dopant-Assisted Atmospheric Pressure PhotoIonization (DA-APPI) is a novel ionization technique for liquid chroma- tography/mass spectrometric (LC/MS) analysis that was first introduced by Bruins and co-workers in 2000. [1] The APPI ion source is suitable to ionize both polar and non-polar compounds. [2–10] Analyte ion formation in DA-APPI is governed by charge exchange and/or proton transfer reac- tions following the photoionizations of a reagent chemically called a dopant. Typically, a krypton discharge lamp (ionization energy 10.0 and 10.6 eV) is used to form a radical cation of the dopant through electron ejection. The formed dopant cation will then transfer the charge to the analyte molecule through a charge exchange reaction. The formation of the radical dopant cation is governed by its ionization energy. Dopants with ioniza- tion energy (IE) lower than 10.6 eV will be ionized by the Kr lamp. The formation of a radical cation makes it possible to ionize non-polar molecules that cannot be efficiently analyzed by electrospray ionization (ESI) or atmospheric pressure chemi- cal ionization (APCI). [1,11–14] Researchers have used several dopants to assist APPI ionization. Toluene (8.83 eV) was first used as a dopant for charge transfer in APPI. [1] One of the disadvantages of toluene as a dopant was that its radical ion was rapidly consumed and was incompatible under aqueous mobile phase conditions. Benzene, chlorobenzene, bromobenz- ene, tetrahydofuran, acetone, xylene, anisole, 2,4-difluoroanisole and 3-(tri fluoromethyl)anisole have been used as dopants. [12,15–18] However, some of these dopants are not heavily used because of their high toxicity. A mixed dopant of toluene/anisole (99.5:0.5, v/v) was reported by Itoh et al . to enhance the ionization efficiency of 16 polycyclic aromatic hydrocarbons (PAHs). On the other hand, Smith et al. have studied mixed dopants of 2,4-difluoroanisole and 3-(trifluoromethyl)anisole in bromobenz- ene or chlorobenzene for the detection of PAH compounds. [18] * Correspondence to: M. Amad, Analytical Chemistry Core Laboratory, King Abdullah University Of Science and Tech- nology (KAUST), Thuwal, 23955-6900, Kingdom of Saudi Arabia. E-mail: maan.amad@kaust.edu.sa Copyright © 2012 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2012, 26, 2517–2525 Research Article Received: 30 April 2012 Revised: 15 August 2012 Accepted: 22 August 2012 Published online in Wiley Online Library Rapid Commun. Mass Spectrom. 2012, 26, 2517–2525 (wileyonlinelibrary.com) DOI: 10.1002/rcm.6373 2517