Structural Study of Analogues of Titan’s Haze by Trapped Ion Mobility Coupled with a Fourier Transform Ion Cyclotron Mass Spectrometer

Abstract

The aerosols present in the atmosphere of the Saturn's moon Titan are of particular planetary science interest and several spacecraft missions already allowed to gather spectroscopic data. Titan haze's analogs, so-called tholins, were produced on earth to push forward the comprehension of their formation and properties. In this study, this highly complex mixture was analyzed here for the first time by trapped ion mobility spectrometry coupled to ultra-high resolution mass spectrometry (FTICR MS). Electrospray ionization revealed the characteristic CHN x -class components, with CHN 5-6 and DBE 6-7 most abundant. Deploying specialized visualization, enabled by accurate mass measurements and elemental composition assignments, the adapted Kendrick mass defect analysis highlights the C 2 H 3 N homolog series, whereas the nitrogen-modified van Krevelen diagram exhibits a clear trend towards H/C 1.5 and N/C 0.5. More interestingly, the representation of m/z versus collision cross section (CCS) allowed hypothesizing a ramified N-PAH structural motif. State-of-the-art IMS is currently not able to resolve the isomeric continuum of ultra-complex mixtures; thus peak parameter other than the CCS-value are explored. As such, analyzing the mobility peak width versus m/z shows a linear increase in isomeric diversity between m/z 170 and 350 and a near plateau in diversity at higher m/z for the N-PAH-like structure. Due to the high complexity of the sample, these structural insights are only to be revealed by TIMS-FTICR MS.

Suggested plausible structures for Titan's haze analogs using tandem mass spectrometry

“…This work confirms structural hypotheses proposed in previous works performed by ion mobility concerning the presence of bulky structures in the soluble molecules (Maillard et al, 2020;Ruger et al, 2019). Such part of these structure were identified in several NMR studies (Derenne et al, 2012;He and Smith, 2014a, b) and liquid-chromatography-mass spectrometry analyses (Gautier et al, 2016).…”

“…The more aromatic one will be called series 1 and the other series 2. To illustrate this observation, putative structural formulas of compounds representative of the two major series were added in the Figure. These putative structures are based on hypotheses made in previous works (Gautier et al, 2017;Maillard et al, 2020;Ruger et al, 2019). Two types of aromatic materials are observed by the series 1 and series 2.…”

“…However, the isomeric and structural diversity remained unknown. In order to obtain an overview of the global structure present in tholins sample, ion mobility coupled with mass spectrometry was performed (Maillard et al, 2020;Ruger et al, 2019). These studies allowed for a narrowing of the possibilities of structures by excluding several families of compounds.…”

Imaging Titan’s Organic Haze at Atomic Scale

“…Our assignment of Titan's haze analogs is based on the AFM measurements of the model compounds and previously published AFM studies (Kawai et al 2018;Kocic et al 2016;van der Heijden et al 2016;Schuler et al 2017b;Zahl & Zhang 2019). In addition, comparison with high-resolution mass spectrometry (HR-MS) data (Ruger et al 2019) is used to qualitatively confirm the AFM-based molecular structures. In the future, the application of machine learning to interpret AFM images might improve the interpretation and structure assignment of non-planar molecules, including non-PAH structures and heteroatoms (Alldritt et al 2020).…”

High‐resolution mass spectrometry for future space missions: Comparative analysis of complex organic matter with LAb‐CosmOrbitrap and laser desorption/ionization Fourier transform ion cyclotron resonance

“…It allows, thanks to the retention of ions in a small electric field and their diffusion according to their collision with the gas flowing through pumping, to separate the isomers according to their size and charge. Its utility has recently been proven for the analysis of laboratory tholins and has allowed to show the isomeric diversity present in this complex environment (Rüger et al, 2019).…”