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تسجيل مستخدم جديدMolecular Isomer Identification of Titan Tholins Organic Aerosols by Photoelectron/Photoion Coincidence Spectroscopy Coupled to VUV Synchrotron Radiation
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The chemical composition of Titan organic haze is poorly known. To address this issue, laboratory analogs named tholins are synthesized, and analyzed by methods requiring often an extraction process in a carrier solvent. These methods exclude the analysis of the insoluble tholins fraction and assume a hypothetical chemical equivalence between soluble and insoluble fractions. In this work, we present a powerful complementary analysis method recently developed on the DESIRS VUV synchrotron beamline at SOLEIL. It involves a soft pyrolysis of tholins at ~230 deg C and an electron ion coincidence analysis of the emitted volatiles compounds photoionized by the tunable synchrotron radiation. By comparison with reference photoelectron spectra (PES), the spectral information collected on the detected molecules yields their isomeric structure. The method is more readily applied to light species, while for heavier ones the number of possibilities and the lack of PES reference spectra in the literature limit its analysis. A notable pattern in the analyzed tholins is the presence of species containing adjacent doubly-bonded N atoms, which might be a signature of heterogeneous incorporation of N2 in tholins.
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Titan, the biggest moon of Saturn, has a thick atmosphere which presents similarities with the one thought to be on Earth at its beginning. The study of Titan s photochemical haze is thus a precious tool in gaining knowledge of the primitive atmosphe
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Numerous studies have been carried out to characterize the chemical composition of laboratory analogues of Titan aerosols (tholins), but their molecular composition as well as their structure are still poorly known. If pyrolysis gas chromatography ma
ss spectrometry (pyr-GCMS) has been used for years to give clues about this composition, the highly disparate results obtained can be attributed to the analytical conditions used and/or to differences in the nature of the analogues studied. In order to have a better description of Titan tholins molecular composition, we led a systematic analysis of these materials using pyr-GCMS with two major objectives: (i) exploring the analytical parameters to estimate the biases this technique can induce and to find an optimum for analyses allowing the detection of a wide range of compounds and thus a characterization of the tholins composition as comprehensive as possible, and (ii) highlighting the role of the CH4 ratio in the gaseous reactive medium on the tholins molecular structure. With this aim, we used a radio-frequency plasma discharge to synthetize tholins with different concentrations of CH4 diluted in N2. The samples were systematically pyrolyzed from 200 to 600{deg}C. The extracted gases were then analyzed by GCMS for their molecular identification.
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