Martin Brüggemann1,2 , Thorsten Hoffmann11 Institute of Inorganic and Analytical Chemistry, Johannes Gutenberg Universität, Mainz, Germany2 Max Planck Graduate Center with the Johannes Gutenberg Universität, Mainz, Germany( [email protected])
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Direct analysis of secondary organic aerosol using the flowing atmospheric-pressure afterglow (FAPA) ambient mass spectrometry source
Overview
Introduction
MethodC Source CharacterizationD• the FAPA ion source consists of a ceramic discharge cell in
which a Helium DC glow discharge plasma is maintainedbetween a tip electrode and a capillary electrode
• exited Helium atoms and primary reagent ions can exit thedischarge through the capillary electrode and enter theafterglow region where the desorption/ionization of theaerosol compounds takes place
• the ion source is held in place by a slightly redesignedThermo® ESI flange; aerosol is introduced through a ¼”stainless steel tubing
• during operation a negative potential is applied to the innerelectrode through a 5 k ballast resistor and the discharge ismaintained in current controlled mode; the Helium flow isadjusted to 1 L/min with a rotameter
• laboratory SOA is produced by -pinene ozonolysis in a100 L smog chamber
• SOA gas phase compounds are removed prior to analysisby passing the aerosol stream through an activated charcoaldenuder
background mass spectra
Conclusions and Outlook
• the chemical analysis of atmospheric aerosols is still amajor challenge and bound to big uncertainties inatmospheric research [1]
• online techniques, such as the Aerosol MassSpectrometer (AMS, Aerodyne®), offer highly time- andsize-resolved information but lack in chemical analysisof single organic compounds due to electron impactionization [2]
• offline techniques, such as filter-sampling andsubsequent analysis by LC-MS, offer almost completechemical analysis of organic aerosols but have a verylow time resolution and additional sample preparationis needed [1]
• here we present a new online technique for thechemical analysis of secondary organic aerosols (SOA)which uses the flowing atmospheric-pressure afterglow(FAPA) technique, offering highly time-resolvedchemical information of sub-micrometer aerosolparticles without additional sample preparation [3]
• secondary organic aerosol (SOA) accounts for a majorfraction of atmospheric aerosol and has bigimplications for earth’s climate and human health
• it is formed when low volatile organic compounds(LVOCs), formed by oxidation of volatile organiccompounds (VOCs), undergo gas-to-particleconversion
• to understand the impacts of SOA detailed knowledgeabout formation, properties and chemical evolution isneeded [1]
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Figure 2: A) photograph of the FAPA ion source mounted on a LCQ Deca XPPlus (Thermo®) ion trap MS; B) Cross sectional view of the FAPA assembly.The aerosol inlet is arranged orthogonal to the MS inlet and the FAPA exit.While the original Thermo® ESI flange was used the ESI manifold wasreplaced by a home-built FAPA manifold.
Analysis of laboratory generated SOA
Figure 1: Formation of secondary organic aerosol (SOA) by oxidation ofvolatile organic compounds (VOCs) and subsequent gas-to-particleconversion.
temperature of the afterglow region
Figure 3: A) maximum temperature of the afterglow region versus time fordifferent current modes; B) maximum equilibrium temperature at a chosencurrent.
• temperature of the gas stream in the afterglow region wasmeasured using a thermocouple (type K) which was placeddirectly in front of the exit capillary of the ion source
• correlation between the applied current, the time and themaximum equilibrium temperature in the afterglow region
• temperature shows a logarithmic behavior with time
• linear correlation between applied current and equilibriumtemperature (3.3 °C/mA) covering approximately the rangebetween 100 °C and 200 °C
VOCs LVOCs
natural and anthropogenic
emissions
oxidation byO3, OH, NO3
e.g. carboxylic acids, esters, peroxides
nucleation or condensation
e.g. isoprene, terpenes, SO2
SOA
Acknowledgements
m/z 62
m/z 125
m/z 149
m/z 167m/z 279
m/z 391
Figure 4: background massspectrum in positive ion mode:the four major signalscorrespond to plasticisers fromthe laboratory air which areknown as common MScontaminants (m/z 149 =phtalic anhydride (MH+),m/z 167 = dimethyl phtalate(MH+), m/z 279 = dibutylphtalate (MH+), m/z 391 =dioctyl phtalate (MH+)).
Figure 5: background massspectrum in the negative ionmode: the signals at m/z 62 andm/z 125 correspond to theformation of NO3
- ions in theafterglow from ambient air. Thesignals at m/z ratios above 400are probably caused bypolymeric compounds frombuilt-in plastics in the ion sourceassembly.
This work was supported by the Max Planck Graduate Centerwith the Johannes Gutenberg-Universität Mainz (MPGC).
The background massspectra are similar to
other common atmospheric-pressure ionization techniques, suchas electro spray ionization (ESI) or atmospheric-pressurechemical ionization (APCI). However, much higher signalintensities are achieved due to the higher reagent ion flux.
• SOA was generated in the laboratory from -pineneozonolysis under dark and dry conditions
• SOA was introduced into the afterglow region of the FAPAthrough PTFE tubing with a flow rate of 2 L/min
• signals for typical -pinene oxidation products areobserved in the range between m/z 150 and m/z 200,such as pinic acid (m/z 185, [M-H]-), pinonic acid (m/z183, [M-H]-) and 10-hydroxypinonic acid (m/z 199, [M-H]-)
• many unidentified signals at higher m/z ratios which arepossibly dimeric oxidation products of -pinene
10-hydroxypinonic acid pinonic acid pinic acid
Figure 6: mass spectrum in the negative ion mode (backgroundsubtracted) of laboratory-generated SOA.
a new ion source for the online analysis of secondaryorganic aerosols based on the flowing atmospheric-pressure afterglow technique was developed andcharacterized
first measurements with laboratory-generated aerosolwere conducted
further experiments are needed to improve the analysis ofhigher molecular weight compounds
intercomparison studies with other aerosol instruments toevaluate the obtained data
experiments for calibration of the ion source andmeasurement of detection limits for different substances
application of the instrument in field measurements
References: [1] T. Hoffmann, R.-J. Huang, M. Kalberer, Anal. Chem. 2011, 83, 4649-4664. [2] V. A. Lanz et al., Atmos. Chem. Phys. 2010, 10, 10453-10471. [3] J. T. Shelley, J. S. Wiley, G. M. Hieftje, Anal. Chem. 2011, 83, 5741-5748.
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