Liquid chromatography/dopant-assisted atmospheric pressure chemical ionization mass spectrometry for the analysis of non-polar compounds

Liquid chromatography dopant-assisted atmospheric pressure chemical ionization (LC/DA-APCI-MS) has been introduced and demonstrated as a promising method for the analysis of non-polar compounds. Polycyclic aromatic hydrocarbons (PAHs) were studied as non-polar representatives. Chlorobenzene, toluene and anisole were explored as dopants with anisole achieving the best results. Using anisole as the dopant, signal to noise ratio (S/N) for PAHs were up to two orders of magnitude better than those achieved with LC/APCI-MS under identical conditions. While predominant M+ ions were observed for fourteen of the sixteen PAHs by LC/DA-APCI-MS, predominant [M−H]+ ions were observed for acenaphthene and fluorene whose structure is different from other PAHs by containing cyclopenta-fused rings formed by methylene (–CH2–) insertion. PAH and solvent adduct ions, e.g., [M+CH2CN]+, were also observed for a few PAHs. Reactant ion mass spectra of LC/DA-APCI-MS were examined in detail. All three dopants generated chemical background ions at m/z≤250, which possibly limited the use of high dopant flow rates to improve the S/N of low-molecular-mass PAHs. They also generated predominant D+ ions. In comparison with toluene, almost two and seven times more D+ ions were observed by chlorobenzene and anisole, respectively. In addition to predominant D+ ions, anisole also generated noticeable amount of [D+H]+ ions. Toluene generated not only [D+H]+ but also [D−H]+ ions. Therefore, while a charge exchange reaction could be mainly responsible for the ionization of PAHs, a proton transfer reaction followed by in-source fragmentation could also be responsible.