10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Original Approach for Determining Surface Binding Energy of Particle-adsorbed PAHs Using L2MS Signal Decay
MARIN VOJKOVIC, Dumitru Duca, Yvain Carpentier, Michael Ziskind, Alessandro Faccinetto, Cristian Focsa, Université de Lille
Abstract Number: 1303 Working Group: Aerosol Physics
Abstract We present an original method for determining surface binding energy of Polycyclic Aromatic Hydrocarbons (PAHs) on black carbon, using signal decay in our Two-Step Laser Mass Spectrometer (L2MS). The reactivity and toxicity of soot particles is driven by the processes on their surface, where the nature and the strength of chemical bonds play an important role. Therefore, the research into the basic chemical properties of PAHs and their interactions with carbonaceous particles can yield valuable insights into soot chemistry, formation and health effects.
The L2MS technique used in our laboratory allows probing the surface chemical composition with great precision (Faccinetto et al. 2015). The two-step desorption-ionization method gives us high sensitivity in sample analysis. In addition to studying real soot, we use laboratory-made samples, synthesized by adsorbing well-determined amounts of selected PAHs onto black carbon particles to reach sub-monolayer coverage. This artificial soot allows us to study the PAH surface chemistry in controlled conditions.
The new approach for surface binding energy determination consists in measuring the shot-by-shot signal decay in L2MS. The method is derived from the measured total number of molecules desorbed after a given number of laser shots (Zhigilei et al. 1999), based upon the assumption of dominantly thermal desorption at low laser fluences (Dreisewerd et al. 1995) Two approaches are used for the binding energy measurement. For the first one, the signal intensity of the peak corresponding to the PAH of interest is recorded for each laser shot. The data is fitted with the pseudo exponential decay of number of molecules in n laser shots. From there, the unknown parameters are then retrieved. For the other approach, the mass spectrum is recorded by averaging the signal over 50-100 laser shots. In this approach, the sample is moved after each laser shot, ensuring that new area of surface is irradiated each time. Therefore, the intensity measured corresponds to the average intensity of the first laser shot. By recording the spectra in this manner, we are able once again to extract the binding energy of the adsorbed species. The two different approaches complement each other helping to achieve a more precise result by reducing the uncertainties introduced by fluctuations in laser fluence or possible non-homogeneous distribution of the molecules on the sample surface.
These results are presented as proof of concept for the new method, which, with refinement and higher precision in measurements, could enable us to identify different types of surface-molecule bonds.
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