Free Radicals by Mass Spectrometry. I. The Measurement of Methyl Radical Concentrations

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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.

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