Free radicals by mass spectrometry. I. The measurement of methyl radical concentrations

Abstract

A method has been developed for measuring the partial pressure of methyl radicals in thermally decomposing gases, using a modified mass spectrometer. The sensitivity of the instrument to methyl radicals has been determined by decomposing known quantities of mercury dimethyl and di-t-butyl peroxide in a stream of carrier gas and measuring the amounts of the stable products and the net height of the mass 15 peak after correction for contributions from these stable products. Assuming a 100 percent carbon balance, the sensitivity of the instrument to methyl radicals was found to be 0.47±0.07 of the sensitivity to methane under the conditions used. With the present instrument this means that an ion current of 3×10-14 ampere at mass 15 (a peak 1 cm in height) is given by a partial pressure of methyl of 0.02 micron Hg above the leak in the reactor. The rate constants for the decomposition of di-t-butyl peroxide have been measured with this instrument. Combining the values obtained with values from the literature yields a value of E=38 kcal/mole and a frequency factor of 7×1015. The rate of decomposition of mercury dimethyl was found to be dependent on the pressure of carrier gas in the reactor in the range from 5 to 20 mm. An estimate of the collision efficiency for the recombination of methyl radicals has been made using the partial pressures of methyl radicals measured by the method presented. At 850°C this value of the collision efficiency is 2-3×10-2.