Maximizing the Output from Filter Sample Analysis: Evolved Gas Analysis from Thermal-Optical Carbon Analysis (TOCA) Using Photoionization Mass Spectrometry (PIMS)

Sven Ehlert, Hendryk Czech, Marco Schmidt, Martin Rigler, Andreas Walte, RALF ZIMMERMANN, Photonion GmbH

     Abstract Number: 299
     Working Group: Instrumentation and Methods

Abstract
The hyphenation of photoionization mass spectrometry (PIMS) to thermal optical carbon analysis (TOCA) provides insights into the composition of single carbon fractions from filter samples. A small part of the evolving carbon is not converted to CO₂, but enters directly the mass spectrometer through a deactivated transfer capillary for evolved gas analysis (EGA) with soft photoionization techniques, such as single-photon ionization (SPI) and resonance-enhanced multi-photon ionization (REMPI). Thus, it enables molecular information during TOCA without pre-treatment or cost of additional sample material, offering its inclusion in existing air quality programs involving routine analysis of organic (OC) and elemental carbon (EC).

We explore how TOCA-PIMS may serve for the identification of different wood types in stove emissions by molecular feature detection. In a comparison of particulate emissions from beech, birch, and spruce logwood combustion, birch as wood fuel can be rapidly identified by thermal decomposition products of betulin and betulinic acid in the m/z range beyond 350, which is representative for such pentacyclic triterpenoids. Spruce contains significant amounts of diterpenoids in its resin. Those resin components thermally degrade in the m/z range from 302 (e.g. abietic acid) down to 234 (common softwood combustion marker retene). In contrast, for the hardwood beech, only polycyclic aromatic hydrocarbons (PAH) appear in the m/z range above 250.

The coupling of high-resolution time-of-flight mass spectrometry with simultaneous electron ionization (EI) and REMPI generates information comparable to offline aerosol mass spectrometry (AMS), such as elemental composition and carbon oxidation state, together with PAH analysis by REMPI. As REMPI-TOFMS may be regarded as a chemical sensor for the detection of aromatic species, interferences from matrix constituents are negligible. Therefore, REMPI-TOFMS coupled to TOCA enables semi-quantification of health-relevant PAH.

Limitations in resolving isomers are overcome by the addition of hyper-fast gas chromatography to PIMS. Larger oligomeric structures in atmospheric organic matter, such as humic-like substances (HULIS), decompose during TOCA, but may be classified from their degradation products. This extent of the accessible chemical space may support the understanding of atmospheric chemistry.