Supercritical Fluid Extraction of organic molecules from extraterrestrial samples: A new one-step protocol proposed

The supercritical fluid state is reached for certain compounds at high pressure and temperature, above their critical point. Under these conditions, such fluids combine the properties of gases (high diffusivity, low viscosity) and liquids (high density, strong solvating power), making them particularly suitable for the extraction of organic molecules from complex matrices, such as extraterrestrial samples. For instance, meteorites, and especially carbonaceous chondrites, can contain up to 4% organic matter^1,2. Organic compounds such as amino acids, nucleobases, sugars, and carboxylic acids, considered among the “building blocks of life”, have been reported in meteorite sample^3–5. To better understand the role of meteorites in the emergence of life on Earth nearly 4 billion years ago, their molecular characterisation is therefore essential. However, organic matter, already present in low quantities in carbonaceous chondrites, is often strongly bound to the mineral matrix, making its extraction particularly challenging.

Supercritical fluids therefore offer a promising approach to overcome this limitation. The widespread use of supercritical CO₂ (scCO₂), due to its relatively low critical point (74 bar and 31 °C), as well as its availability, non-toxicity, non-flammability, and easy removal upon depressurisation, makes supercritical fluid extraction (SFE) a green chemistry technique⁶. In addition, scCO₂, commonly used for the extraction of non-polar compounds, can be mixed with polar co-solvents such as methanol, ethanol, dichloromethane, acetonitrile, or water to enable the simultaneous extraction of polar compounds. Moreover, recent instrumental developments allow the mixing of co-solvents with scCO₂ upstream of the sample, making it possible to generate co-solvent mixtures, gradients, and extraction sequences without intermediate sample handling, thereby reducing the risk of contamination. These advances over the past decades make SFE a promising alternative to more conventional extraction methods. It often provides comparable or higher recovery yields^7–10, while overcoming common drawbacks such as high temperatures and pressures, multistep protocols, the use of toxic or polluting solvents, and contamination risks.

In the context of astrobiology, SFE is particularly well suited for complex and valuable samples, as it is a semi-destructive technique that preserves the mineral matrix after extraction of soluble compounds, while providing efficient extraction and a low risk of contamination. Although astrobiology emerged in the late 1950s, this technique has only begun to be explored in this field over the past three decades^11–15, especially for sample return or meteoritic samples. In this study, we focus on meteoritic samples by investigating the capabilities of SFE considering recent developments, particularly the possibility of implementing a one-step protocol with successive co-solvent mixtures or gradients within a 2 mL sample cell, a more realistic volume given the limited availability of meteoritic material. To develop our protocol, we first optimise the large number of parameters using a meteoritic analogue. Using methanol as a co-solvent, an initial screening of the system was performed by focusing on the four most influential parameters (pressure, temperature, static extraction time, and methanol/scCO₂ ratio), allowing us to explore the parameter space with only 19 experiments using a full factorial design. Finally, considering that different molecules are extracted under different conditions, we aim to optimise the protocol using co-solvent gradients to develop a single extraction workflow capable of efficiently targeting multiple molecular classes through successive extraction steps. We will present the first results obtained from this optimisation and compare them with classical extraction methods.

 

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