Determination of the chemical composition of titan4s aerosols analogues using pyrolysis gas chromatography mass spectrometry
Résumé
The in situ chemical characterization of Titan's aerosols with the ACP-GCMS (Aerosol Collector and Pyrolyze-Gas Chromatograph and Mass Spectrometer) experiments onboard the Cassini-Huygens mission showed that the aerosols heated at 600°C were releasing two main gaseous compounds: NH3 and HCN. To better understand the process of production of these species at high temperature, and their relationship to the solid aerosol composition, it is possible to mimic in laboratory the Titan's atmosphere chemistry to produce analogues of Titan's aerosols (tholins), chemical and physical properties of which can be studied with laboratory instrumentation. In the present work, we studied the thermal decomposition of tholins produced with the PAMPRE cold plasma experiment[1]. For this study, pyrolysis at various temperatures, coupled to gas chromatography mass spectrometry (Pyr-GCMS), has been used in order to understand the origin and formation mechanism of the NH3 and HCN compounds. With this aim, two samples were produced using different initial CH4/N2 gaseous mixtures with different concentration ratios: (5:95) and (10:90). Pyr-GCMS was applied to characterize their chemical composition and molecule structure. In order to study the evolution of chemical compounds released as a function of temperature, the final pyrolysis temperature has been set from 100°C to 900°C with a 100°C step increment. The results show that the major compounds released are similar for the two different studied samples. They include hydrocarbon compounds, nitriles, ammonia, hydrogen cyanide, and some pyrrole isomers. Moreover, whatever the final temperature is, acetonitrile is the most abundant compound released by the samples. At 100°C only water contribution can be detected. Then the number of compounds released increases with the temperature up to 600°C. With the temperature increase the nature of the gaseous species detected does not change significantly. Pyrrole was not detected until the temperature exceeds 600°C for (5:95) sample and 400°C for (10:90) sample. Benzene has also been detected at 800°C for the (5:95) sample and 700°C for (10:90) sample. In the same way, toluene has been detected for the (10:90) sample at 700°C but it has not been detected for the (5:95) sample. This could indicate that carbon percentage is directly related to the benzene and toluene production. HCN and NH3 as the major compounds detected in the ACP instrument were also found in our results. It confirms that the aerosol analogues produced in PAMPRE set-up are among the most similar to Titan's aerosols, as found in [2]. [1] C. Szopa, G. Cernogora, L. Boufendi, et al. : PAMPRE: A dusty plasma experiment for Titan's tholins production and study, Planetary and Space Science, vol. 54, no. 4, pp. 394-404, 2006. [2] P. Coll, R. Navarro-González, C. Szopa, et al. : Can laboratory tholins mimic the chemistry producing Titan's aerosols? A review in light of ACP experimental results, Planetary and Space Science, vol. 77, pp. 91-103, 2013.