The Extraction of Organic Compounds from Sulphate Minerals for Astrobiological Exploration

Sulphates on Mars: Sulphates were estimated to constitute 10 % of the Pathfinder and Viking Lander sites [1] and up to 30 % at the MER-B landing site [2]. This is significant in the context of astrobiological exploration as aqueous environments rich in sulphates could have been a habitat suitable for phototrophic life on Early (3.45 Ga) Earth [3] and sulphates are known to provide a habitat for phototrophic life in a cold, desiccating environment in the Canadian Arctic [4]. The former represent conditions similar to those that may have existed on Mars in the past and the latter conditions similar to those existing on the surface of Mars today. Evaporites on Earth: Evaporite deposits on Earth comprise sulphates, carbonates and halides and have been shown to preserve biomarker information from the environment of their deposition for millions of years, although in lesser quantities than faciesequivalent mudstones [5]. More recently fossil lipids present in stalagmites have been used to study past climates [6]. The lipids in the stalagmites are thought to have been derived from primary producers at the surface and transported by groundwater prior to incorporation within a carbonate mineral matrix. We are developing a lab-on-a-chip surfaceenhanced-resonance-raman-scattering system [7] to analyse evaporite minerals for key organic molecules of astrobiological interest. This has necessitated the production of evaporite minerals that contain suitable target analytes for both proof of concept purposes and to provide a reference material for testing extraction and sample handling protocols. Precipitation of model evaporite minerals. In order to precipitate organic bearing evaporites, solutions of dissolved evaporite mineral and solutions containing a model organic compound (anthracene, aspirin and stearic acid) are mixed and evaporated under standard conditions. The solid mineral phase is collected after all of the water has evaporated. Analysis: Samples of the model evaporite mineral had their surface exhaustively cleaned and were then extracted using micro/mini liquid-liquid extraction: samples were dissolved in pre-extracted deionised water and then extracted three times with DCM. Samples of gypsum (CaSO4.2H20) were dissolved in preextracted 20 % HCl (aq) and then extracted three times with DCM. Soil samples from Devon Island were Soxhlet extracted and an aliquot of the product separated using a packed silica column into aliphatic, aromatic and polar products. Products were analysed by GC-MS in SIM mode and quantified by GC-FID. Model crystals: The graph in Fig. 1 is a plot of anthracene yield against crystal size (halite). Two important features are: 1) yields per gram of mineral are much lower than the concentration per gram of mineral in the starting solution and 2) the yields for small volumes of mineral are very chaotic. This generally accords with the observation that the concentration of organics in evaporite minerals is low [5], and with most organics being absorbed onto particulate matter in the parent solution which is included within the mineral phase as it precipitates.

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