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Analytical Chemistry, Strategic Exercise: Organic Aerosol Analysis

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Analytical Chemistry, Strategic Exercise: Organic Aerosol Analysis WIR SCHAFFEN WISSEN –HEUTE FÜR MORGEN Analytical Chemistry, Strategic exercise: Organic aerosol analysis Urs Baltensperger Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Villigen , Switzerland ETHZ, HS 2020, 8.12.2020 Aerosol size distribution, and sources and sinks Chemical composition of PM1 in Zurich in winter Energiespiegel 19/2008 http://gabe.web.psi.ch/pdfs/Energiespiegel_19_e.pdf Today’s task: Organic aerosol analysis Page 4 The state of the art in 2009 Hallquist et al., ACP 2009 Traditional methods can identify only a fraction of organics Rogge et al., 1993 Most detailed analysis performed with off-line methods: More than 10‘000 different organic compounds isolated with GCxGC-TOF/MS Hamilton et al., 2004 NB: isolated ≠ identified…. Besides extraction, also thermal desorption is applied: TAG (Goldstein et al., 2008) Organic acids formed in the photooxidation of -pinene All acids found in aerosol phase. Some of them are highly volatile (e.g., formic, acetic acid) and not expected to be there. Probably hydrolysis of oligomers Fisseha et al., Anal. Chem. 2004 Developments since 2009 EESI CHARON‐ PTRMS FIGAERO -I-CIMS Adapted from Hallquist et al., 2009 9 Required for mass spectrometry: ionization of individual molecules • For gas phase: relatively easy (avoid fragmentation, clustering) • For aerosol particles: Much more difficult: either dissolution in a solvent or evaporation - Problem with dissolution: only the soluble fraction accessible - Problem with evaporation: fragmentation Page 10 What is Mass Spectrometry ? • Measure of the mass/charge (m/z) ratio of charged particles in vacuum (10‐6 torr). • Molecules are converted to gaseous ions that are separated according to their mass/charge ratio • 3 main components: • These 3 elements drive the capability of an instrument Ion source Analyzer Detector Electron Impact Time-of-Flight Photographic Plate Electrospray Ionization Quadrupole Faraday Cup Photo Ionization Magnetic Sector Electron Multipliers Chemical Ionization Ion-Trap Electro-optical ion Others… Orbit-Trap detector Etc… Etc… Page 11 Overview of ion sources • Two types of ion sources, depending on the energy involved: Highly energetic (hard ionization) –extensive fragmentation Electron Impact – Low Pressure Photoionization Chemical Ionization – High or Low Pressure Less energetic (soft ionization) –ions of the molecular species Electrospray – High Pressure Photoionization Chemical Ionization – High or Low Pressure DART APCI Page 12 Example: current OA measurements AMS CHARON-PTRMS FIGAERO-CIMS 0.1 – 2.5 μm 0.2 – 1.0 μm All sizes Ionization-induced Thermal desorption Semi-continuous sampling fragmentation (collection stage) Loss of chemical information Potential reactions on collection stage Lower time resolution 13 Aerodyne ToF-AMS et Q-ACSM Time of Flight – Aerosol Mass Spectrometer Quad - Aerosol Chemical Speciation Monitor Real‐time or near real‐time chemical composition of the bulk aerosol (Total Organics, Nitrate, Sulfate, Ammonium, Chloride) Advantages • High time resolution • Analysis of the total mass • No sampling artefact Inconvenients • High level of fragmentation • No resolution at the molecular level HR‐ToF‐AMS Page 14 Aerodyne ToF-AMS et Q-ACSM Time of Flight – Aerosol Mass Spectrometer Quad - Aerosol Chemical Speciation Monitor Real‐time or near real‐time chemical composition of the bulk aerosol (Total Organics, Nitrate, Sulfate, Ammonium, Chloride) • Focusing lens • Thermal particle vaporization (600 °C) • Electron impact ionization (70 eV) • Time of Flight Page 15 Extractive Electrospray Ionisation (EESI) • ES flow and aerosol flow intersect before cone • Aerosols are extracted • Solvent evaporation and coulomb explosion inside TOF‐ capillary • TOF‐capillary heated to 270°C Lopez‐Hilfiker et al 2019 Orbitrap MS • Ions are first trapped in an injection trap (C –trap) and then injected at once into the orbitrap. • Ions rotate around the center electrode and the frequency of oscillation is detected via an image charge. • Fourier transform analysis is performed to take the frequency spectrum and turn it into a m/z spectra. Thermo Fischer Page 17 Comparison of LTOF and Orbitrap Lee et al., Anal. Chem.2020 Page 18 Source apportionment Method: PMF (positive matrix factorization) “Factors” = sources or processes measurement factor n x m= n x p x p x m Factor profile (“fingerprint”) measurement Factor time series Our implementation of PMF is via the Multilinear Engine (ME‐2) and SoFi Controlled exploration of solution space (rotational ambiguity) via constraints on factor profiles and/or time series Improves source resolution, facilitates uncertainty analysis 19 Off-line analysis of filter samples with the AMS HR ToF AMS measurement AMSHR measurement ToF AMS measurement Drying with silica gel dryer Aerosol generation with custom nebulizer filtration Allows for analysis of filter samples from multiple sites with a single instrument Reduced time resolution due to filter sampling (typically 1 day) Water extraction Daellenbach et al., AMT 2016 Sources of PM mass and of different markers for oxidative stress (reactive oxygen species) Daellenbach et al., Nature, 2020 Page 21 Thank you for your attention Page 22.
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