Atmospheric Pressure Photoionization Proton Transfer for Complex Organic Mixtures Investigated by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

To further clarify the role of dopant solvent in proton transfer in atmospheric pressure photoionization (APPI), we employ ultrahigh-resolution FT-ICR mass analysis to identify M+•, [M + H]+, [M − H]−, and [M + D]+ species in toluene or perdeuterotoluene for an equimolar mixture of five pyrrolic and pyridinic nitrogen heterocyclic model compounds, as well as for a complex organic mixture (Canadian Athabasca bitumen middle distillate). In the petroleum sample, the protons in the [M + H]+ species originate primarily from other components of the mixture itself, rather than from the toluene dopant. In contrast to electrospray ionization, in which basic (e.g., pyridinic) species protonate to form [M + H]+ positive ions and acidic (e.g., pyrrolic) species deprotonate to form [M − H]− negative ions, APPI generates ions from both basic and acidic species in a single positive-ion mass spectrum. Ultrahigh-resolution mass analysis (in this work, m/Δm50% = 500,000, in which Δm50% is the mass spectral peak full width at half-maximum peak height) is needed to distinguish various close mass doublets: 13C versus 12CH (4.5 mDa), 13CH versus 12CD (2.9 mDa), and H2 versus D (1.5 mDa).