Molecular beam mass spectrometry applied to determining the kinetics of reactions in flames II. A critique of rate coefficient determinations

Abstract

The microstructure of low-pressure methaneoxygendargon flames has been investigated using modulated molecular beam-mass spectrometry. Profiles of radical and stable species concentration, temperature, and area expansion ratio have been used to calculate rate coefficients as a function of temperature for certain elementary reactions occurring in flames, namely, H + O 2 → OH + O, H + CH 4 → CH 3 + H 2, CO + OH → CO 2 + H, CH 3 + O → H 2CO + H, and H + CF 3Br → HBr + CF 3. The profiles have been modified (computationally) to simulate the effect of various perturbations and errors possible in sampling and analyses, and the effect on the rate coefficients is discussed. Detailed consideration is given to data reduction techniques, temperature profile-composition profile alignment, and the possible temperature dependence of mass spectral fragmentation. The rate coefficients are not dramatically sensitive to the imposed perturbations, although the results depend upon the nature of the reaction in question. Rate coefficients determined for high activation energy reactions and for reactions singularly responsible for the chemical behavior of a given stable species are in agreement with values determined by other techniques. Flame structure studies in which all significant radical and stable species are measured by a single technique are judged to be viable sources of high-temperature rate data for elementary reactions, where such reactions have been identified.