J.H., Lewandowski, M., Jaoui, M., Flagan, R.C., and Seinfeld, J.H. (2007). Evidence for organosulfates in secondary organic aerosol, Environmental Science and ...
POLAR ORGANIC COMPOUNDS IN AEROSOLS STUDIED BY ADVANCED MASS SPECTROMETRY (HPLC-QTOF-MS) M. GLASIUS Department of Chemistry, Aarhus University, DK-8000 Aarhus C, Denmark
Keywords: Secondary organic aerosol, carboxylic acids, sulfate esters. INTRODUCTION The organic fraction of atmospheric particles account for 20-70% of the particle mass in both remote and urban areas (e.g. Saxena and Hildeman, 1996) and is composed of hundreds of different compounds. The exact chemical composition depends on sources and dynamic transformation processes during atmospheric transport involving both gas phase and aerosol phase compounds. The properties of aerosol to act as e.g. cloud condensation nuclei may be affected by their chemical composition. There is thus a need to investigate occurrence of organic compounds in relation to sources, transformation and properties of aerosol. Secondary organic aerosol (SOA) may be formed from both anthropogenic and biogenic precursors. Of the biogenic precursors especially isoprene and monoterpenes are important due to their high emission on a global scale. As a result of the high reactivity of these compounds, biogenic SOA contributes significantly to organic particulate matter. Polar organic compounds are often of secondary origin and it is thus important to study sources and concentrations of this group of aerosol phase compounds. Target compounds in the present study are multifunctional polar compounds observed in ambient aerosol, especially multifunctional carboxylic acids. These include monoterpene oxidation products such as pinonic acid and pinic acid. Another group of target compounds are highly polar compounds such as highly hydroxylated compounds (e.g. tetrols) identified in the aerosol phase as oxidation products of isoprene (Claeys et al., 2004). In addition fatty acids (straight-chain carboxylic acids) are analyzed. Recently sulfate esters have been identified in both laboratory studies and ambient particle samples as oxidation products of monoterpenes and isoprene (e.g. Surratt et al., 2007 and 2008), but the exact route of formation is still being investigated (Minerath et al., 2008), as well as their possible role in formation of high molecular weight compounds (oligomerization). Sulfate esters are also target compounds of the present investigation. METHODS We use HPLC coupled with quadrupol time-of-flight mass spectrometry (QTOF-MS) to study polar organic compounds in aerosol samples from both ambient air and laboratory studies. The QTOF-MS instrument (Bruker, Germany) can analyze compounds in the mass range 50-20,000 m/z with an error smaller than 3 ppm (using internal calibration). Using determination of accurate mass and isotopic pattern the chemical composition of a compound can be suggested. Additional fragmentation (using the quadrupol) provides additional information on the chemical structure. Aerosol samples from ambient air are collected on pre-baked quartz fibre filters using a high-volume sampler (Digitel DHA-80) with size-selective inlet (PM2.5 or PM1.0). Aerosol samples from laboratory studies are typically collected on Nucleopore or Teflon filters using low-volume samplers. After sampling, filters are stored at -18ºC until analysis.
Polar organic compounds are extracted in methanol in an ultrasonic bath. The solvent is evaporated, and the sample is redissolved in 1.00 ml methanol:water. Aerosol samples are analyzed together with synthetic standard compounds using a Dionex ultimate 3000 HPLC. The HPLC is coupled to the mass spectrometer through an electrospray ionization interface operating in the negative ionization mode. Two types of advanced C-18 HPLC columns have been investigated, a Synergi Fusion (Phenomenex) and Atlantis T3 (Waters). These columns are suited for analysis of polar compounds at highly aqueous conditions. CONCLUSIONS We have developed an analysis method for polar organic compounds in aerosol using HPLC-QTOF-MS equipped with an electrospray inlet. The method is based on minimal sample preparation procedures in order to minimize artifact formation. Two types of HPLC columns have been applied and both result in adequate separation of most target compounds. Preliminary results from analysis of synthetic standards and aerosol samples from a Danish forest site will be presented. ACKNOWLEDGEMENTS This work was supported by the Carlsberg Foundation and Aarhus University Research Foundation. J.K. Christensen is acknowledged for her skillful technical assistance. REFERENCES Claeys, M., Graham, B., Vas, G., Wang, W., Vermeylen, R., Pashynska, V., Cafmeyer, J., Guyon, P., Andreae, M.O., Artaxo, P., and Maenhaut, W. (2004). Formation of secondary organic aerosols through photooxidation of isoprene. Science, 303, 1173. Minerath, E.C., M.T. Casale, and M.J. Elrod (2008) Kinetics feasibility study of alcohol sulfate esterification reactions in tropospheric aerosols, Environmental Science and Technology, 42, 4410. Saxena, P. and L.M. Hildeman (1996). Water-soluble organics in atmospheric particles: A critical review of the literature and application of thermodynamics to identify candidate compounds, Journal of Atmospheric Chemistry 24, 57. Surratt, J.D., Kroll, J.H., Kleindienst, T.E., Edney, E.O., Claeys, M., Sorooshian, A., Ng, N.L., Offenberg, J.H., Lewandowski, M., Jaoui, M., Flagan, R.C., and Seinfeld, J.H. (2007). Evidence for organosulfates in secondary organic aerosol, Environmental Science and Technology 41, 517. Surratt, J.D., Gomez-Gonzalez, Y., Chan, A.W.H., Vermeylen, R., Shahgholi, M., Kleindienst, T.E., Edney, E.O., Offenberg, J.H., Lewandowski, M., Jaoui, M., Maenhaut, W., Claeys, M., Flagan, R.C. and Seinfeld, J.H. (2008). Organosulfate Formation in Biogenic Secondary Organic Aerosol, J. Phys. Chem. A, 112, 8345.
