High-resolution chemical ionization mass spectrometry (ToF-CIMS): application to study SOA composition and processing

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Aljawhary, D. ; Lee, A. K. Y. ; Abbatt, J. P. D. (2013)

This paper demonstrates the capabilities of chemical ionization mass spectrometry (CIMS) to study secondary organic aerosol (SOA) composition with a high-resolution (HR) time-of-flight mass analyzer (aerosol-ToF-CIMS). In particular, by studying aqueous oxidation of water-soluble organic compounds (WSOC) extracted from α-pinene ozonolysis SOA, we assess the capabilities of three common CIMS reagent ions: (a) protonated water clusters (H<sub>2</sub>O)<sub>n</sub>H<sup>+</sup>, (b) acetate CH<sub>3</sub>C(O)O<sup>&minus;</sup> and (c) iodide water clusters I(H<sub>2</sub>O)<sub>n</sub><sup>&minus;</sup> to monitor SOA composition. Furthermore, we report the relative sensitivity of these reagent ions to a wide range of common organic aerosol constituents. We find that (H<sub>2</sub>O)<sub>n</sub>H<sup>+</sup> is more selective to the detection of less oxidized species, so that the range of O / C and OS<sub>C</sub> (carbon oxidation state) in the SOA spectra is considerably lower than those measured using CH<sub>3</sub>C(O)O<sup>&minus;</sup> and I(H<sub>2</sub>O)<sub>n</sub><sup>&minus;</sup>. Specifically, (H<sub>2</sub>O)<sub>n</sub>H<sup>+</sup> ionizes organic compounds with OS<sub>C</sub> &le; 1.3, whereas CH<sub>3</sub>C(O)O<sup>&minus;</sup> and I(H<sub>2</sub>O)<sub>n</sub><sup>&minus;</sup> both ionize highly oxygenated organics with OS<sub>C</sub> up to 4 with I(H<sub>2</sub>O)<sub>n</sub><sup>&minus;</sup> being more selective towards multi-functional organic compounds. In the bulk O / C and H / C space (in a Van Krevelen plot), there is a remarkable agreement in both absolute magnitude and oxidation trajectory between ToF-CIMS data and those from a high-resolution aerosol mass spectrometer (HR-AMS). Despite not using a sensitivity-weighted response for the ToF-CIMS data, the CIMS approach appears to capture much of the chemical change occurring. As demonstrated by the calibration experiments with standards, this is likely because there is not a large variability in sensitivities from one highly oxygenated species to another, particularly for the CH<sub>3</sub>C(O)O<sup>&minus;</sup> and I(H<sub>2</sub>O)<sub>n</sub><sup>&minus;</sup> reagent ions. Finally, the data illustrate the capability of aerosol-ToF-CIMS to monitor specific chemical change, including the fragmentation and functionalization reactions that occur during organic oxidation, and the oxidative conversion of dimeric SOA species into monomers. Overall, aerosol-ToF-CIMS is a valuable, selective complement to some common SOA characterization methods, such as AMS and spectroscopic techniques. Both laboratory and ambient SOA samples can be analyzed using the techniques illustrated in the paper.
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