Chemical Analysis of an Organic Bubble in the North West Africa 6148 Meteorite Using Raman Spectroscopy Imaging and Chemometrics

Abstract:

The rover Perseverance will collect samples from various locations of the Martian surface, which will be brought to Earth in the future for further analysis. Non-destructive techniques should be used first to obtain as much information from them as possible, as the number of samples will be limited. In this sense, Raman spectroscopy is a well suited technique to analyze the composition of geological samples without altering them. In addition, the use of chemometrics can help to solve complex mixtures extracted from Raman images. In this study, the organic compounds contained inside a bubble of the North West Africa (NWA) 6148 nakhlite was analyzed using an InVia confocal micro-Raman spectrometer (Renishaw). A 532 nm excitation laser was employed to acquire the spectra in high-resolution image mode. The spectral treatment of the map was carried out with the WiRE 4.2 software (Renishaw) and HYPERTools v.2.0 working under Matlab environment (The MathWorks).

The bubbles were located in an olivine matrix. Thus, the Raman spectra collected in the bubble always showed the signals from the organic molecules together with olivine (Figure 1). Principal Component Analysis (PCA), sparse Principal Component Analysis (sPCA) and Multivariate Curve Resolution (MCR) were tested as possible methodologies to solve the mixture. As an example of the results obtained, the loadings for the sparse Principal Components (sPCs) 1 and 3 and their heat map in the bubble are shown in Figure 1. Both in the spectra and in the sPCs 1, the typical saturated carboxylic acid Raman bands can be observed, specifically, the ones from dodecanoic acid. In addition, the sPCA results suggested that there is a hotspot area in the bubble corresponding to carboxylic acid salts (-CO₂^-), where the Raman signals are present around the 1500 – 1700 cm^-1 region of the spectra in sPC3. This could mean that the initial carboxylic acid reacted partially with a base, resulting in the current mixture of the bubble.

This methodology might provide great results for the returning Martian samples, as it can give both geological and chemical composition results. In addition, the distribution in the mapping of the different components can add information to the history and alterations that the sample has suffered.