Dr. Christine Joblin of the CNRS and her colleague Dr. Hassan Sabbah of the University of Toulouse III – Paul Sabatier highlight the ability of the AROMA molecular analyzer to understand the origin of large carbon molecules in cosmic environments .
The James Webb Space Telescope (JWST, NASA/ESA/CSA) has just revealed its amazing capabilities to study the Universe at infrared wavelengths. Among the data, we expect unprecedented information about large carbonaceous molecules that are key species in star- and planet-forming regions.2 These molecules are dominated by the family of polycyclic aromatic hydrocarbons (PAHs), known terrestrial pollutants that are important by -products of combustion processes. We need to understand why these PAHs are so abundant in astrophysical environments (they typically contain 10% of carbon).
What are the mechanisms that recycle carbon into space and (preferably) form these molecules? So far, research has been guided mainly by our knowledge of flame chemistry and the proposal that astro-PAHs are produced late in the life of carbon-rich stars, in hot, dense envelopes in which produces the formation of dust and molecular species. , like PAHs, is favored. Even so, to date, we do not have a clear observational diagnosis that demonstrates this scenario. In addition, the individual PAHs identified so far (indene C9H8 and cyano-naphthalene C10H7CN)4,5 are present in the TMC-1 dark molecular cloud and are likely products of very low-temperature gas-phase chemistry.
The PAH model
Over the past decade, the unequivocal detection of buckminsterfullerene C60 and its C60+ cation in various astrophysical environments has been a major achievement. It has allowed to support the PAH model. This model, proposed in the 1980s, explains that all the strong aromatic infrared bands observed in the emission come from the radiative cooling of PAHs heated by absorption of ultraviolet photons. We now know that other carriers must be included, such as fullerenes. Compared to PAHs, the formation of fullerenes requires more extreme conditions, in particular, higher temperatures. This issue motivates studies to propose scenarios of C60 formation in astrophysical environments. These include grain processing by heat, shock or high energy ions and UV photoprocessing of HAP grains6,7.
In this paper, we present our approach to address the question of the formation of astro-PAHs and fullerenes from an experimental point of view. This approach is based on two pillars. The first pillar is to use a variety of reactors to produce samples that can be thought of as laboratory analogues of stardust. Although it is impossible to mimic the physical and chemical conditions of dying stars, a new machine, Stardust,8 in Madrid (ICMM/CSIC) has been built to advance this field by offering features not available in other experimental setups, such as the controlled production of atoms (eg carbon, silicon) to drive chemistry.
Cold plasmas are also used as an alternative method to study the impact of complexity on chemistry. In particular, we study the role of metal atoms (e.g. iron) in the formation of dust and large carbon molecules with our colleagues at LAPLACE (CNRS/ Université Toulouse III-Paul Sabatier).9 The second pillar consists to study samples of terrestrial matter (meteorites) rich in carbon. This is the case of carbonaceous chondrites such as Murchison and Allende but also of the polymict ureilite meteorite Almahata Sitta (AhS). These two pillars share the common interest of probing the carbonaceous molecular content of samples that may be in the form of powder deposits or bulk materials, as illustrated in Figure 1. This theme motivated the development of the Research in Organic Astrochemistry with Molecular Analyzer (AROMA) configuration.
Fig. 1: Different types of samples analyzed with the AROMA setting. Left, atomic force microscopic image of Stardust analogues showing the presence of carbon nanoparticles8 and a scanning electron microscope image of dusty plasma analogues showing organosilicon dust decorated with silver nanoparticles.9 In the middle, small fragments of the AhS meteorite observed with a digital. microscope12 and image of one of the Hayabusa sample containers2 (Yada et al., DOI: 10.1038/s41550-021-01550-6). Right, image of the emission of soot particles (Source: COLOA Studio/Shutterstock) and asphaltenes from oil pipelines (Credit: Schlumberger). Prototype structures of PAH, pyrene C16H10, and fullerene C60 are shown at the bottom
AROMA: the molecular analyzer
The AROMA setup10 has been developed in the framework of the Nanocosmos ERC Synergy project.1 The construction has been done by Fasmatech, a young Greek company, following the requirements of our scientific and engineering team. The main objective is to trace the molecular content of various solid samples, especially in large molecules such as polycyclic aromatic hydrocarbons, carbon clusters and fullerenes.
AROMA, as shown in Figure 2a, consists of three main components: the ion source, the ion trap, and the mass analyzer.
At the ion source, millimeter-sized samples are placed on an XY manipulator and subjected to laser desorption/ionization (LDI) techniques. LDI techniques, which use a single laser pass, are powerful tools for directly probing non-volatile organic species in native or artificial matrices without the need for tedious extraction procedures. In the case of high molecular weight compounds, such as biomolecules, a matrix-assisted LDI (MALDI) technique is commonly used. The specificity of AROMA is the use of the L2MS technique, in which the desorption and ionization processes are separated in time and space and are performed with two different lasers. A pulsed infrared laser is used to desorb neutral molecules, followed after a few microseconds by a pulsed ultraviolet laser to selectively ionize the molecules of interest.13 L2MS offers much higher sensitivity than LDI of a single step to target molecules (e.g., in our case, PAHs and fullerenes). In addition, AROMA offers the possibility to isolate and trap ions of a specific mass and to study their fragmentation by collisions with a gas or by absorption of photons from a tunable laser. Both techniques help us elucidate the molecular structures of the dominant species. Finally, the ions are mass-separated using a high-resolution time-of-flight mass spectrometer (resolving power of 104).
Fig. 2: (a) Mass spectrum recorded from a global sample of the Murchison meteorite. Some of the identified peaks are shown with their mass-to-charge ratio (m/z), associated chemical formula and possible molecular structure. (b) Schematic representation of the AROMA configuration highlighting all its components
Figure 2 shows an example of a mass spectrum recorded for a few mg of ground dust from the Murchison meteorite. It represents the intensity of the ion signal as a function of the mass-to-charge ratio. This allows us to unequivocally assign a chemical formula to each detected peak and associate it with a pure carbon (Cx) or hydrocarbon (CxHY) species. Other elements may also be present, especially in atomic form (eg Na, Al, K, Fe, etc.). Our assignment accuracy is close to 0.01 at mass/charge ratio m/z ~ 300. This is our limitation for assigning with high confidence organic compounds with N, O, and S atoms in their chemical formula . Finally, hundreds of peaks are identified in the mass spectrum with notable discrepancies between different samples. The latter can be used to trace the chemical history of each sample and do not bias our analysis.
For example, in the analysis of terrestrial soot samples11, we were able to follow the evolution of the carbonaceous molecular families, as a function of the height above the burner (Z) and demonstrate an efficient thermal processing of the large population of PAHs to produce clusters of hydrogenated carbon. (HC clusters) and then fullerenes (see Figure 3a). To retrieve this chemical information, large mass spectrometry datasets must be reduced to relevant parameters, such as molecular families. For this, our methodology consists of calculating double bond equivalent (DBE) values, which are representative of the level of unsaturation of the molecules. The different ranges of DBE values allow us to disentangle the molecular families: PAHs, HC clusters, aliphatic species (very rare in our samples because the ultraviolet laser is not adapted to their ionization), carbon clusters and fullerenes.
Fig. 3: Compositional diversity in the four molecular families (PAHs, HC clusters, C clusters and fullerenes) obtained after DBE analysis. (a) Soot samples collected at different heights above the burner11 and analogues produced with the Stardust machine using atomic carbon and different gases (H2 de8 and C2H2 from Santoro et al, DOI: 10. 3847/1538-4357/ab9086). (b) Murchison, Allende and two AhS10,12 meteorite fragments
Figure 3 illustrates the method and shows the results of the molecular family analysis for several samples, including terrestrial soot, stardust analogues (Fig. 3a) and several meteorites (Fig. 3b) . PAHs dominate the molecular composition of carbonaceous chondrites (Murchison and Allende). In contrast, the two AhS fragments, AhS#04 and AhS#48, show a greater diversity of molecular families, which have been shown to come from different cosmic repositories.
All mass spectra associated with the published work are publicly available in the AROMA database. The database offers the ability to calculate and represent DBE values, and to perform molecular family analyses. We are also developing a method to assess the similarity between recorded mass spectra and how this can be used to provide faster investigation and additional information about the chemical history of a sample.
Amazing results and perspectives
Initially seen as an analytical tool to support the Stardust machine, AROMA has become a key facility…