Mass-spectrometric sampling of 1-ATM flames

Abstract

As part of a program aimed at mass-spectrometric sampling of the burned gas region of 1-atm flames for thermochemical studies, we have investigated the phenomena occurring in forming molecular beams from high-pressure sources. A simple picture of beam formation involving isentropic expansion of gas through the first orifice and into a supersonic molecular beam is justified. Two effects observed in high-pressure beam formation, mass separation, and polymer formation, are consistent with this picture, and their pertinence to the interpretation of high-pressure sampling results is discussed. A semiquantitative description of the time, temperature, and pressure history of a typical sampling situation is given showing that conversion to molecular flow occurs in several microseconds. Our sampling system, which gives molecular beam intensities of 1017 molecules/cm2/sec at 10 cm from the first orifice, was tested by measuring the stable products in CH4−O2−Ar flames and the dissociation of Cl2 in CO−O2−Ar flames. Satisfactory agreement with calculated composition was obtained in both cases. The condensable species HBO2 was successfully quenched from a H2−O2−BCl3 flame.