Coupling of a Thermal-optical Carbon Analyser (TOCA) to High Resolution EI/PI Mass Spectrometry: Unravelling the Molecular Organic Signature and Oxidation State of Aerosol Samples

RALF ZIMMERMANN, Hendryk Czech, Kevin Schnepel, Marco Schmidt, Patrick Martens, Thorsten Streibel, Judith Chow, John Watson, Andreas Walte, Sven Ehlert, Helmholtz Zentrum München and University of Rostock

     Abstract Number: 245
     Working Group: Instrumentation and Methods

Abstract
Carbonaceous material in particulate matter (PM) is charcterizable by the organic/elemental carbon content (OC/EC), measured by Thermal-Optical Carbon Analysis (TOCA). The TOCA information-yield can be extended by evolved gas analysis (EGA) by mass spectrometry (Grabowsky et al.-2011; Diab et al.-2015, Miersch et al.-2019a/b). Health-relevant polycyclic aromatic hydrocarbons (PAH) are detectable by resonance-enhanced multiphoton ionization (REMPI)-TOFMS. Here, concept and instrument for an improved TOCA/EGA-TOFMS instrument is presented. Quasi-simultaneous EI and REMPI detection of desorbed molecules by high-resolution TOFMS is realized, enabling detection of both, molecular PAH-information and the oxidation state of organic PM (similar to high-resolution AMS). A quadrupole mass spectrometry approach (qMS) with electron ionization (EI) was privously introduced by Riggio et al. (2018), providing quantitative results on organic matter, nitrate and sulfate, aiming to transfer fragmentation tables from online aerosol mass spectrometry (Canagaratna et al.-2007) to offline filter-analysis. However, no information on the carbon oxidation-state is otainable because qMS cannot provide sufficient mass resolution to distinguish between key isobaric ions, such as C3H7+ and C2H3O+ at m/z 43. In this work EGA was done via on-line sampling from the TOCA-quartz oven. Ions were generated alternating by EI (70 eV) and REMPI (248 nm). Ion were detected by reflectron-TOFMS with a mass resolution of ~2,800. The high mass resolution allows the separation of many hydrocarbon-/oxygenate-isobars, e.g., for atmospherically aged spruce combustion emissions, a decrease in hydrocarbon-like fragments (e.g. C3H7+) can be observed, whereas oxygenate-ions (e.g. C2H3O+ and CO2+) are noticeably increased. Simultaneously, the selective REMPI method addresses PAH and thus indirectly the carcinogenicity of the organic aerosol-component (Miersch et al.-2019b).

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