High-resolution chemical ionization mass spectrometry (ToF-CIMS): application to study SOA composition and processing
Abstract. 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 (H2O)nH+, (b) acetate CH3C(O)O− and (c) iodide water clusters I(H2O)n− 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 (H2O)nH+ is more selective to the detection of less oxidized species, so that the range of O / C and OSC (carbon oxidation state) in the SOA spectra is considerably lower than those measured using CH3C(O)O− and I(H2O)n−. Specifically, (H2O)nH+ ionizes organic compounds with OSC ≤ 1.3, whereas CH3C(O)O− and I(H2O)n− both ionize highly oxygenated organics with OSC up to 4 with I(H2O)n− 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 CH3C(O)O− and I(H2O)n− 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.