Systematic study of impact of Perchlorate on the derivatization reagents (TMAH and MTBSTFA) onboard SAM
Résumé
Sample Analysis at Mars (SAM) is an experiment of the Mars Science Laboratory mission and is mainly dedicated to the search for indigenous organic compounds in Gale crater on Mars. To reach its goals, SAM may operate by using pyrolysis-gas chromatography mass spectrometry (GC-MS) and pyrolysis-MS (EGA). In addition, SAM includes wet chemistry capabilities to support the extraction and analysis of polar organic compounds of high interest (e.g. amino acids) from solid samples. Wet chemistry experiments improve the detection of organic compounds both by enhancing the recovery of chemical species from the solid sample matrices, and by changing their chemical structure to make compounds more amenable to GC-MS. Two reactants are contained in SAM for wet chemistry: MTBSTFA/DMF (N-tert-Butyldimethylsilyl-N-methyltrifluoroacetamide/ Dimethylformamide) and TMAH (tetramethylammonium hydroxide) thermochemolysis. The procedures developed for running these wet chemistry experiments were originally optimized for chemical standards and analogues of martian soils. But as oxychlorine salts have since been detected in Gale crater, we have to take into account their presence and all the possible chemical reaction they could induce when in contact with organic material. Indeed, when they are heated, perchlorates and chlorates decompose into O2 and Cl2/HCl and those compounds can react (through oxidation, chlorination, etc.) with the indigenous organic matter. This is also the case for the derivatization reagents as well. For this reason, we systematically studied the possible interaction of derivatization agents with different perchlorates, and its influence on the analysis of organic molecules. With this aim, we have first listed all the organic compounds generated from the reactions of the different perchlorates (CaClO4, Mg(ClO4)2, Fe(ClO4)3) with the derivatization agents TMAH and MTBSTFA/DMF. The study has been carried out at several temperatures ranging from 200°C up to 900°C. Through these experiments, we are now able to identify the contribution of theses reactions to the organic background signal observed in SAM, and to follow it at the different temperatures the samples are exposed. Hence, we can propose optimal temperature parameters for near term SAM analyses with those derivatizing reagents.