Calibration of a mass spectrometer for the direct measurement of water concentration and its application to the study of sooting flames

An accurate calibration is essential for the quantitative measurement of species concentrations in fuel-rich hydrocarbon flames when using a mass spectrometer (MS). In sooting flames, the mole balances of the atomic components of the gas phase species cannot be made because there are carbon deposits and soot. A procedure in which binary gas mixtures containing Ar are introduced by a two-stage pressure-reduction system into a vacuum chamber for the MS analysis is described, which allowed the calibration of the H2O response coefficient. This was calibrated using H2O-vapor-saturated air that was saturated at different temperatures to give mixtures with known partial pressures of H2O and N2. The measured response coefficients of H2, CH4, C2H2, CO, N2, O2, CO2 and H2O with respect to Ar were obtained as 7.56, 0.76, 1.18, 0.69, 0.71, 0.60, 0.83 and 0.68, respectively. Species concentrations in sooting flames containing H2O were studied using the calibrated matrix equation of the response coefficients. The MS measurement uncertainty was less than ±6% where this uncertainty does not include errors due to fractionation changes, which may occur for measurements made under conditions that are different from those used in the calibration. The procedure is useful for the quantitative study of combustion processes containing water. (paper)