Molecular beam sampling from high-pressure sources

The quantitative chemical analysis of species present in high-temperature, high-pressure reacting systems is a continuing task in such research areas as basic chemical kinetics and combustion studies. For example, measurement of the nonequilibrium concentrations of reacting species is an important step in determining the formation chemistry of air pollutants in combustion systems. The present paper treats the development and calibration of a supersonic molecular beam-mass spectrometer sampling system capable of direct sampling and quantitative analysis of gaseous species present in high-pressure reacting systems. Molecular beam sampling systems presently in use have, in general, been designed for continuous sampling from pressures of 1 atm or less. The present system has been designed, with moderate vacuum pumping requirements, to sample continuously from sources at pressures up to 10 atm at room temperature or 100 atm at elevated temperatures. Skimmer interaction effects in sampling such high-pressure sources have been examined with respect to the effects on beam intensity and mass separation. The ability of the system to follow a dynamically varying sample has been experimentally demonstrated. The system has been operated over a wide range of conditions including sample pressure, geometric arrangement of beam forming elements, and various sample compositions.