THLAIJEH Siveen: Characterization of organic matter by laser desorption-mass spectrometry (LDMS): application to primitive life and the search for life on Mars.

Doctorant : Siveen THLAIJEH, 01/10/2018 – 17/12/2021

Directeur de thèse : C. Focsa (50%)

Co-directeur : K. Lepot (LOG, 25%)

Co-encadrant : Y. Carpentier (25%)

Sujet : Characterization of organic matter by laser-assisted mass spectrometry: application to paleontology and exobiology

Financement: Univ. Lille (50%) + Région HdF (50%)

Summary of the PhD project:

The origin of organic matter on Mars will be investigated with ExoMars’ “Rosalind Franklin” rover using several analytical instruments including the Mars Organic Molecular Analyzer (MOMA). The MOMA instrument combines laser desorption-ionization mass spectrometry (LDI-MS) and gas-chromatography mass spectrometry (GC-MS) to analyze sub-surface samples. The similarity of the LDI-MS of MOMA with our LDI-MS / L2MS (two-step laser mass spectrometry) instrument has encouraged us to characterize organic matter from different types of samples, with the goals of providing new methodology for microscale molecular characterization of early life on Earth and to support future investigations on Mars.

First, we developed a new-side injection ion source for our existing hybrid (radiofrequency/time-of-flight) mass spectrometer enabling transmission and reflection microscopy and chemical imaging in L2MS and LDI-MS with a lateral resolution down to 3 µm.

Second, characterization of reference molecules was carried with L2MS and LDI-MS to help chemical identification of different types organic matter. These included a tryptophan derivative (biotic/abiotic amino acid), hexylthiophene (proxy for sulfurized hydrocarbons), cholestane (eukaryote biomarker), tocopherol (from photosynthesizers) and a porphyrin (chlorophyll derivative). Thus, we provide insight into the sensitivity of the method for these important classes of molecules, and into their fragmentation patterns.

Third, a sedimentary rock from the Jurassic paleolagoon of Orbagnoux was analyzed as a proxy for diagenetically-sulfurized microbial organic matter. This sample was chosen following the recent detection of sulfurized molecules on Mars. Chemical characterizations of different samples of Orbagnoux rock (fresh rock and polished slice) and its corresponding sub-fractions extracted from the sedimentary rock (soluble extracts, maltene and kerogen) were performed using L2MS and LDI. This gives access to organic and inorganic molecular information of various chemical classes of interest such as polycyclic aromatic hydrocarbons, oxygenated, thiophenic, and nitrogenated hydrocarbons, calcium carbonate/iron sulfide clusters, and metals such as Ni⁺ or VO⁺ that point to the preservation of geoporphyrins.

Fourth, a modern Dammar resin and four fossil resins (ambers) were analyzed in LDI-MS to test the technique capability to provide insights into plant-derived fossil biopolymers, including their maturity grade and/or botanic type. Clear differences in hydrocarbon fragment patterns were observed in mass spectra and investigated with principal component analysis.

Keywords: laser desorption/ionization, organic matter, paleontological samples, micro-L2MS, MOMA instrument