Organic Compounds Analysis by TOF-SIMS in the Frame of Rosetta/COSIMA Space Mission

Abstract

Comets are remainders of the Solar System formation. They also have been discussed as possible sources of water (Deloule 1998) and molecules relevant in the chemical evolution leading to the emergence of life on Earth (Oro 1961). Thus, cometary grains are interesting objects for planetological and astrobiological studies: their analysis can give us insights into physical and chemical conditions in the early Solar Nebula (Irvine and Lunine 2004) and on the prebiotic chemistry which occurred in the early Earth (Pascal et al. 2006). The Rosetta mission launched by ESAonMarch2004willreach the comet 67/P-Churyumov-Gerasimenko in 2014 to perform the most exhaustive study ever achieved on comets (Glassmeier et al. 2007). A time-of-flight secondary ion mass spectrometer (TOF-SIMS), named COSIMA (COmetary Secondary Ion Mass Analyser), is onboard the Rosetta spacecraft. It will focus on chemical analysis of solid cometary grains collected in situ (Kissel et al. 2007). Spectra interpretation of complex unknown samples with COSIMA instrument will be difficult due to the limited mass resolution which excludes unambiguous compounds identification and requires preliminary work with reference samples. Calibration work on organic compounds is in progress using the prototype of COSIMA in Orleans (France) and the reference model of COSIMA in Lindau (Germany). In this work, we have used the COSIMA prototype based in Orleans to analyze organics deposits. We are interested in N-heterocyclic compounds which are believed to be present in comets or which belong to the chemical family of such molecules. Our first main study is focused on two purines: adenine and guanine. These molecules are made-up of two N-heterocyclic compounds fused together: respectively an amino-pyrimidine and a nucleobase substructure plus an imidazole substructure for each. Therefore experiments have been performed on five compounds: adenine, guanine, imidazole, 2-aminopyrimidine and cytosine. The first step was to perform high statistics of positive and negative spectra of these five pure organic molecules and characterize all the studied compounds. Afterwards, deep examination of the purines spectra has been performed to look for peculiarity: we tried to differenciate the fragmentation peaks of a complexe molecule from the quasi-molecular peak of pure compounds at the same mass. In a second step, we have analyzed a blend of two pure organics in order to spot if we can separate the contribution of each species on the spectra. We observe a good detection of the studied compounds. For almost all of them, the major peak of the positive spectra is the peak of the quasi-molecular analyzed molecule enriched with a hydrogen atom ([M+H]+). We pointed out that no ambiguity was possible between the peaks from the actual molecule and the same one in a substructure form: they have two different spectral features. We can also distinguish the two molecules from the mixture of organic compounds.