Detection of RO_2 radicals and other products from cyclohexene ozonolysis with NH_4~+ and acetate chemical ionization mass spectrometry

摘要

The performance of the novel ammonium chemical ionization time of flight mass spectrometer (NH4+-CI3-TOF) utilizing NH4+ adduct ion chemistry to measure first generation oxidized product molecules (OMs) as well as highly oxidized organic molecules (HOMs) was investigated for the first time. The gas-phase ozonolysis of cyclohexene served as a first test system. Experiments have been carried out in the TROPOS free-jet flow system at close to atmospheric conditions. Product ion signals were simultaneously observed by the NH4+-CI3-TOF and the acetate chemical ionization atmospheric pressure interface time of flight mass spectrometer (acetate-CI-APITOF). Both instruments are in remarkable good agreement within a factor of two for HOMs. For OMs not containing an OOH group the acetate technique can considerably underestimate OM concentrations by 2-3 orders of magnitude. First steps of cyclohexene ozonolysis generate ten different main products, detected with the ammonium-CI3-TOF, comprising 93% of observed OMs. The remaining 7% are distributed over several minor products that can be attributed to HOMs, predominately to highly oxidized RO2 radicals. Summing up, observed ammonium-CI3-TOF products yield 5.6 x le molecules cm" in excellent agreement with the amount of reacted cyclohexene of 4.5 x 10(9) molecules cm(-3) for reactant concentrations of [O-3] = 2.25 x 10(12) molecules cm(-3) and [cyclohexene] = 2.0 x 10(12) molecules cm(-3) and a reaction time of 7.9 s. NH4+ adduct ion chemistry is a promising CIMS technology for achieving carbon-closure due to the unique opportunity for complete detection of the whole product distribution including also peroxy radicals, and consequently, for a much better understanding of oxidation processes.