Exploring the influence of chemical extraction on the complexity of the Soluble Organic Matter (SOM) in carbonaceous chondrites

The complexity and origin of organic matter in carbonaceous chondrites has been a topic of interest for over 60 years¹. This organic matter has been studied through the bulk, the Insoluble Organic Matter (IOM) and the Soluble Organic Matter (SOM) using different analytical techniques.

The procedures for extracting the soluble fraction significantly influence the quantity and diversity of extracted organic matter, thereby impacting the interpretation of the data in terms of origin and evolution. They vary depending on the analytical approach (targeted or untargeted) and the analytical instruments used for the characterization^2–4.

So far, most of the work done on the analysis of organic matter from carbonaceous chondrites has focused on targeted approaches. For example, numerous studies have worked on the optimization of the extraction of water-soluble organic matter (such as amino acids, sugars, nucleobases…) usually extracted with boiling water and acid hydrolysis and then analysed by gas chromatography coupled to mass spectrometry (GC-MS)^5–7. Other studies have worked on the solvent-soluble organic matter for the characterization of specific hydrocarbons and aromatics, also using GC-MS^8,9. In 2010, Schmitt Kopplin et al. studied the influence of the extracting solvent on the resulting molecular diversity of the SOM using Ultra-High Resolution Mass Spectrometry, and were able to conclude that methanol was the most efficient extracting solvent¹⁰. No other untargeted study has focused on the impact of the extracting protocol since then. Similar conclusions can be drawn regarding the elemental and isotopic analysis of the SOM, that have mostly been studied using targeted approaches^6,11,12. Untargeted elemental and isotopic analysis applied to SOM was used by Becker & Epstein in 1981¹³, and was never systematically applied to other SOM extracts since then.

In the present work, our objective is to implement an untargeted workflow to carry out comprehensive analyses of the soluble organic matter using mass spectrometry techniques.

Firstly, to better understand the influence of chemical extraction on the resulting SOM, we are optimizing different parameters of the extraction procedure (pH, grain size, extraction time, solvent, etc.) and characterizing the resulting soluble organic fractions using a sample of Murchison (ME 2644 #23.13) allocated by the Field Museum. With Orbitrap Ultra-High Resolution Mass Spectrometry, we can assess qualitative information on the molecular diversity and mass distribution of the sample. Thus, the high mass resolution of this instrument (100 000 at m/z = 400) enables to conduct family analysis and to look for polymerization patterns¹⁴. We also aim to develop a new method to characterize the SOM with an Elemental Analyzer – Isotope Ratio Mass Spectrometry (EA-irMS). This technique will provide quantitative information on the elemental (%C, %N…) and isotopic (¹³C/¹²C, ¹⁵N/¹⁴N…) compositions of the sample, enabling comparisons with previous data on IOM and bulk compositions.

With this untargeted workflow of combined analyses, we will provide new insights into the molecular diversity, mass distribution, and elemental composition of the SOM of carbonaceous chondrites. Later on, we will apply these procedures on CM chondrites having experienced various degrees of alteration. This will enable systematic comparison of the effect of post-accretion processes (aqueous alteration and thermal process) on the organic complexity. Moreover, having an optimized and standardized extraction procedure to characterize the SOM as a whole is important for working on pristine samples such as those from Bennu and Ryugu.

 

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