A novel aerosol mass spectrometric approach - Analysis of the organic molecular signature of PM by coupling of thermal EC/OC-carbon analysis to photo-ionization mass spectrometry
Abstract
Carbonaceous material in airborne particulate matter (PM) is of increasing interest e.g. due to its adverse health effects and its potential influence on the climate. Its analytical assessment on a molecular level is still very challenging. Hence, analysis of carbonaceous fractions for many studies is often solely carried out by determining sum parameters such as the overall content of organic carbon (OC) and elemental carbon (EC) as well as the total carbon content, TC (sum of OC and EC). The used thermal procedure, however, allows getting additional interesting information: By defining different thermal OC fractions (i.e. temperature steps) also information on the refractory properties of the carbonaceous material is obtained. In this context it is particularly interesting to investigate the release and formation behaviors of the molecular species responsible for the different OC and EC fractions. Thus after initial promising results of pre-studies [1,2] in the current work an EC/OC carbon analyzer (Model DRI 2000) and a homebuilt photo-ionization time-of-flight mass spectrometer (PI-TOFMS) were hyphenated and applied to investigate individual organic compounds especially from the different OC fractions. The carbon analyzer enables the stepwise heating of PM loaded filter samples and provides the sum values of the "carbon" release ("Improve protocol" [2]: OC1 - 120 °C, OC2 - 250°C, OC3 - 450°C OC4 - 550°C). With the on-line coupled PI-TOFMS evolved organic compounds, as released during the thermal program, are detectable in real time. This is possible by MS with soft photo ionization methods (SPI - single photon ionization and REMPI - resonance-enhanced multi photon ionization). Soft ionization suppresses fragmentation upon the ionization step and generates molecular signatures in the MS. The EC/OC-analyzer-PI-TOFMS instrument was applied to several types of PM samples, such as ambient aerosol, emission samples (gasoline/diesel car, wood combustion) or samples from aerosol aging chambers. Ambient filter samples, e.g., sometimes show a strong impact of organic wood combustion markers and polycyclic aromatics (winter samples) while in summer typical secondary organic aerosol (SOA) signal were obtained. This was revealed e.g. by comparison to the thermal release signatures of pure cellulose, lignin and wood combustion PM with the ones of ambient air PM or samples from smog chambers. Interestingly, at higher temperatures (450 °C) often a shift to smaller molecules occurs. This is due to the thermal decomposition of larger oligomeric or polymeric molecular structures (e.g. lignocelluloses and similar oxygenated humic-like substances). The thermal fractions from gasoline PM exhibit intense signal of larger polycyclic aromatic hydrocarbons, whereas diesel PM showed a much higher total organic content in the REMPI measurements. In conclusion the method is well suited for source apportionment and organic characterization. A particularly interesting aspect furthermore is that the method uses only small quartz-fiber filter punches. Thus relevant samples from archives or governmental monitoring networks can easily be (re-)analyzed. [1] T. Streibel et. al, (2006). Anal. Chem. 78, 5354-5361 [2] J. Grabowsky et al. (2011). Anal Bioanal Chem 401, 3153-3164
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.A41D0030Z
- Keywords:
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- 0305 ATMOSPHERIC COMPOSITION AND STRUCTURE / Aerosols and particles;
- 0345 ATMOSPHERIC COMPOSITION AND STRUCTURE / Pollution: urban and regional;
- 0365 ATMOSPHERIC COMPOSITION AND STRUCTURE / Troposphere: composition and chemistry;
- 0394 ATMOSPHERIC COMPOSITION AND STRUCTURE / Instruments and techniques