​Characterization of the various petroleum constituents
hydronaphthalenes,thiophenes,alkylsubstitutedbenzenes,pyridines,fluorenes,andpolycyclicaromatichydrocarbons was achieved under ambient conditions without sample preparation by desorption atmospheric pressure chemical ionization DAPCI. Conditions were chosen for the DAPCI experiments to control whether ionization was by proton or electron transfer. The protonated molecule [M+H]+ and the hydride abstracted [MH]+ form were observed when using an inert gas, typically nitrogen, to direct a lightly ionized plasma generated by corona discharge onto the sample surface in air. The abundant water cluster ions generated in this experiment react with condensed-phase functionalized hydrocarbon model compounds and their mixtures at or near the sample surface. On the other hand, when naphthalene was doped into the DAPCI gas stream, its radical cation served as a charge exchange reagent, yielding molecular radical cations M+ of the hydrocarbons. This mode of sample ionization provided mass spectra with better signal/noise ratios and without unwanted side-products. It also extended the applicability of DAPCI to petroleum constituents which could not be analyzed through proton transfer e.g.,highermolecularPAHssuchaschrysene. The thermochemistry governing the individual ionization processes is discussed and a desorption/ionization mechanism is inferred. ⺠Analysis of petroleum constituents is achieved under ambient conditions. ⺠Proton or electron transfer reactions are carried out under ambient conditions. ⺠Charge exchange using naphthalene as the DAPCI reagent gave simple mass spectra consisting of only molecular cation M+ without side products. ⺠Unleaded gasoline and diesel standard mixtures can be characterized.