Unsupervised Raman spectroscopy imaging of bio-interfaces in analogue samples in preparation for space missions

In 2020 and 2022, Raman spectrometers will be deployed on Mars to study the composition of the first two meter of Mars’ surface when the NASA’s Mars 2020 and the ESA/ROSCOSMOS’ExoMars rovers are launched [1]. Raman spectrometers have the capability to detect geological substances constituting the rocky surface of Mars (inorganic molecules and inorganic molecular ions), which will provide information on the habitability of the planet, either past and present. In addition, Raman instruments have the ability to detect potential biological-derivative substances, often referred to as biomarkers (organic molecules originating from extent or extinct living organisms). In preparation for space mission, intended to use dedicated instruments developed with constraints such as minimal power budget, mass budget and data budget, documenting the detection capability of miniaturized Raman spectrometers is essential. Terrestrial analogue samples are important to address that capacity [2]. Amongst other analogue samples, those comprising bio-interfaces are of particular interest. Bio-interfaces are almost omnipresent in biological systems. In analogue samples, microorganism colonies are often thriving on or in rocks, using the mineral as both nutrient sources and protection against deathly UV irradiation, mechanical treats and desiccation [3]. Often, the functional properties of the bio-interface are significantly affected by their close micro-chemical environment. Actually, the molecular composition of the bio-interfaces is spatially affected in response to various changes of the micro-environment. To follow the variation of the chemical composition across analogue samples presenting a bio-interface, Raman molecular imaging approaches (implying that spectral data are recorded at different locations on the surface of a sample) have emerged as powerful implementation of molecular analytical techniques [4]. Raman spectroscopy was successfully applied to study the chemical composition of a wide variety of biological samples: from plants to animals, from single cells to tissues (either healthy or diseased). Raman imaging enables to obtain molecular images with a high spatial resolution. Yet, the power of Raman imaging was scarcely implemented with space designed instrumentations for future missions.
Here, we discuss the challenges associated with the integration of Raman Spectroscopy Imaging on analogue samples such as meteorite and biocrusts. From the experimental preparation of the sample to the implementation of simple chemometrics statistical tools to identify underlying molecular trends in association with the microstructure of the samples. In particular, we will compare spectral imaging data recorded with benchtop instruments and miniaturized spectrometers with operating modes similar to the future ExoMars Raman Laser Spectrometer. We will also discuss the possibility and the challenges associated with the combination of Raman imaging with Mass Spectroscopy Imaging, offering a higher molecular specificity, but a higher spatial resolution.

References
[1] Hutchinson I. B. et al., PTRS A, 2014, 372.
[2] Malherbe C. et al., Astrobiology, 2015, 15, 442.
[3] Malherbe C. et al., JRS, 2019, available online.
[4] Bocklitz, T.W. et al., Anal. Chem., 2016, 88, 133.