Ion chemistry of aluminum and boron in methane–oxygen flames

The ion chemistry of aluminum and boron, primarily in the +3 oxidation state, was studied by doping premixed, methane–oxygen flames of both fuel-rich and fuel-lean (oxygen-rich) composition with 1 × 10−6 mol fraction of AlCl3 and 5 × 10−4 mol fraction of B(OC2H5)3. Ions were observed by sampling the flames at atmospheric pressure through a nozzle into a mass spectrometer. At this low concentration level, aluminum exhibits two cation series: (a) AlO+•nH2O (n = 2, 3) formed by proton transfer to AlO(OH) and Al(OH)3 in which hydration reactions are involved; and (b) Al+•nH2O (n = 0, 1) by protonation of AlOH with hydration/dehydration steps. At the higher concentration of the boron additive, four cation series were observed: (a) B(OC2H5)3H+•nH2O (n = 0, 1) based on proton transfer to the B(OC2H5)3 additive; (b) BO+•nH2O (n = 1–4) similar to aluminum; (c) HB2O3+•nH2O (n = 1–3) involving the protonated dimer of metaboric acid, BO(OH); and (d) B3O4+•nH2O (n = 2, 3) involving the protonated trimer of BO(OH) whose structures might be cyclic or linear. Other series members are formed by subsequent hydration or dehydration of the parent cations. The anions BO2− and BO− previously studied by D. E. Jensen were also observed. The formation chemistry and probable structures of these ions are discussed, and compared with similar results obtained previously for flames doped with transition metals; notably Sc, Ti, V, and Cr in the +3 oxidation state. Keywords: aluminum, boron, ions, flame, gas phase.