Technical Note: Detection and identification of radical species formed from α-pinene/ozone reaction using DMPO spin trap

Abstract. The reactions of ozone with monoterpenes proceed via the formation of multiple oxygen- and carbon-centered free radical species. These radical species are highly reactive and thus, have generally not been measureable. A method for their detection and characterization is needed to preserve these radicals for a sufficiently long time to permit analyzes to be performed. Radical-addition reactions, also called spin trapping techniques, allow the detection of short-lived radicals. This approach has been applied to products from the α-pinene/ozone reaction. Secondary organic aerosol (SOA) from a reaction chamber was collected on quartz fiber filters and extracted with a solution of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) (spin trap) followed by analysis with ion-trap tandem mass spectrometry (MSⁿ) using electrospray ionization (ESI) in the positive scan mode. The DMPO adducts with radical species appear as positive ions [DMPO−R+H]⁺, [DMPO−OR+H]⁺ and [DMPO−O−OR+H]⁺ in full MS spectra of the samples. Tandem mass spectrometry (MS²) was performed to identify the radical species. The DMPO adducts with the C-centered radical species [DMPO−R+H]⁺ are characterized by m/z 114 [DMPO+H]⁺ in the MS² spectra and with peaks that represent the loss of [DMPO+H]⁺. The DMPO adducts with O-centered radical species (RO· and ROO·) are identified by m/z 130 [DMPO−OH+H]⁺ and m/z 146 [DMPO−O−OH+H]⁺, respectively, and with peaks that correspond to the loss of those adducts. DMPO was also able to capture OH radicals from the particle phase, and the product ion fragmentation confirmed DMPO/OH structure providing evidence for particle-bound OH radicals.