Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

302 DETECTION AND ESTIMATION OF INTERMEDIATESthe flow velocity of reactants, or, more conveniently, by varying the length of theilluminated zone.Foner and Hudson, working in the Applied Physics Laboratory of the JohnsHopkins University, Silver Spring, made an important advance in the study ofunstable intermediates in flames'14. Although Eltentong' had successfully detectedsome free radicals in flames, he had experienced difficulty in the detection ofoxygenated radicals. Foner and Hudson emplayed a molecular beam samplingdevice to ensure that the species entering the spectrometer would have been as freeas possible from collisions during sampling. In addition, they employed a beamchoppingtechnique together with phase sensitive detection to discriminate betweentrue reaction intermediates and species formed on the ionizing filament or on thewalls of the ionizing chamber. The molecular beam was chopped at 200 cps with amagnetically driven chopper, and entered the ionization chamber co-axially withthe ionizing electron beam. Fig. 7 shows the gas inlet system employed, and Fig. 8is a block diagram of the mass spectrometer-amplifier arrangement. Using thisapparat~s"~, atomic oxygen and hydrogen, and hydroxyl radicals, were detectedin the hydrogen-oxygen flame; and in the methane-oxygen flame, methyl radicalswere found together with the "stable" intermediates C,H2, CO, CH,O, CH20or C,H, and C4H2. Although in this work there was no positive evidence forHO, formation in the H,-02 flame, the HO, radical was subsequently found in thereaction between atomic hydrogen and molecular oxygen, both at pressures ofaround 30 torr'" and around 1 torr'16. Hydroxyl radicals were also detected inboth cases. Foner and Hudson were able to extend the use of their apparatus in an