Liquid-liquid Extraction of Included Organic Compounds from Dissolved Sulphate Minerals Performed on a Microfluidic Format

Introduction: The presence of sulfates on the surface of Mars was indicated by a strong Mg-S correlation in the fluorescence data obtained by the Viking landers [1]. Sulfates were subsequently identified as being present in the soil at the Pathfinder landing site at a concentration of ~ 10 % MgSO4 [2]. Data from the Opportunity rover indicates that sulfates of probable evaporite origin are present in Eagle crater, Meridiani Planum where the sulfates constitute ~ 40 % by weight of the bedrock and could have a formed by direct precipitation from water or during early burial [3]. Thermal emission spectrometer data suggest that the major sulfate phases are magnesium and calcium sulfates [4]. Laboratory spectra of hydrated magnesium sulfates and sodium carbonates are a close match to the spectra obtained by Calisto’s Near Infrared Mapping Spectrometer for the dark regions that are present surface of Europa [5]. Therefore hydrated sulfates may be present on the surface of Europa. Both Mars and Europa have astrobiological significance because liquid water may have been or may be present on both bodies; indeed magnesium sulfates and other mineral phases that precipitate from water are indicators of potential past habitability. Water can become incorporated within a sulfate crystal as it precipitates from solution and form a separate aqueous phase in the form of discrete inclusions that are entirely surrounded by a host mineral matrix. Particulate matter and liquid organic phases can also be incorporated as inclusions. Because included organic matter is completely protected from destruction by its external environment, any organic matter included within minerals is resistant to destruction by an oxidating environment. However, in order to analyze organic inclusions within mineral phases it is necessary to break apart the host mineral matrix and extract the contents of inclusions. One method applicable to inclusions in water-soluble minerals is to dissolve the mineral matrix and perform a liquid extraction. This also serves as a signal concentration step for analytical purposes. Liquid-liquid extraction can be performed on microfluidic devices called H-Cells [6]. These have a small footprint, low mass and low power requirements. By establishing a stable interface between the two liquid phases within a low Reynolds number microfluidic format, liquid-liquid extraction by diffusion across a laminar interface can be performed using two miscible liquid phases (e.g. methanol and water). Methodology: Sample preparation. Samples of magnesium sulfate possessing inclusions of stearic acid (C18H37OOH) and anthracene (C14H10) were prepared by precipitating a mineral phase from an aqueous solution or mixture that contained the organic compound. Images of inclusions within the sulfate minerals are shown in Fig.1. Properties of the sample materials are shown in Table 1.