Speciated Monitoring of Gas-Phase Organic Peroxy Radicals by Chemical Ionization Mass Spectrometry: Cross-Reactions...

Organic peroxy radicals ("RO₂", with R organic) are key intermediates in most oxygen-rich systems, where organic compounds are oxidized (natural environment, flames, combustion engines, living organisms, etc). But, until recently, techniques able to monitor simultaneously and distinguish between RO₂ species ("speciated" detection) have been scarce, which has limited the understanding of complex systems containing these radicals. Mass spectrometry using proton transfer ionization has been shown previously to detect individual gas-phase RO₂ separately. In this work, we illustrate its ability to speciate and monitor several RO₂ simultaneously by investigating reactions involving CH₃O₂, CH₃C(O)O₂, c-C₆H₁₁O₂, and (CH₃)₃CO₂. The detection sensitivity of each of these radicals was estimated by titration with NO to between 50 and 1000 Hz/ppb, with a factor from 3 to 5 of uncertainties, mostly due to the uncertainties in knowing the amounts of added NO. With this, the RO₂ concentration in the reactor was estimated between 1 × 10¹⁰ and 1 × 10¹² molecules cm^-3. When adding a second radical species to the reactor, the kinetics of the cross-reaction could be studied directly from the decay of the first radical. The time-evolution of two and sometimes three different RO₂ was followed simultaneously, as the CH₃O₂ produced in further reaction steps was also detected in some systems. The rate coefficients obtained are (in molecule^-1 cm³ s^-1): k_{CH3O2+CH3C(O)O2} = 1.2 × 10^-11, k_{CH3O2+t-butylO2} = 3.0 × 10^-15, k_{c-hexylO2+CH3O2} = 1.2 × 10^-13, k_{t-butylO2+CH3C(O)O2} = 3.7 × 10^-14, and k_{c-hexylO2+t-butylO2} = 1.5 × 10^-15. In spite of their good comparison with the literature and good reproducibility, large uncertainties (×5/5) are recommended on these results because of those in the detection sensitivities. This work is a first illustration of the potential applications of this technique for the investigation of organic radicals in laboratory and in more complex systems.