Subcritical-Water Extraction of Organics From Solid Matrices

An apparatus for extracting organic compounds from soils, sands, and other solid matrix materials utilizes water at subcritical temperature and pressure as a solvent. The apparatus, called subcritical water extractor (SCWE), is a prototype of subsystems of future instrumentation systems to be used in searching for organic compounds as signs of past or present life on Mars. An aqueous solution generated by an apparatus like this one can be analyzed by any of a variety of established chromatographic or spectroscopic means to detect the dissolved organic compound(s). The apparatus can be used on Earth: indeed, in proof-of-concept experiments, SCWE was used to extract amino acids from soils of the Atacama Desert (Chile), which was chosen because the dryness and other relevant soil conditions there approximate those on Mars. The design of the apparatus is based partly on the fact that the relative permittivity (also known as the dielectric constant) of liquid water varies with temperature and pressure. At a temperature of 30 °C and a pressure of 0.1 MPa, the relative permittivity of water is 79.6, due to the strong dipole-dipole electrostatic interactions between individual molecular dipoles. As the temperature increases, increasing thermal energy causes increasing disorientation of molecular dipoles, with a consequent decrease in relative permittivity. For example, water at a temperature of 325 °C and pressure of 20 MPa has a relative permittivity of 17.5, which is similar to the relative permittivities of such nonpolar organic solvents as 1-butanol (17.8). In the operation of this apparatus, the temperature and pressure of water are adjusted so that the water can be used in place of commonly used organic solvents to extract compounds that have dissimilar physical and chemical properties. Heretofore, laboratory extractions of organic compounds have involved the use, variously, of toxic organic solvents in Soxhlet extraction, strong acids in amino acid vapor-phase hydrolysis, or carbon dioxide as a solvent at supercritical temperature and pressure. Supercritical CO2 is effective as a solvent for extracting lipids and other very nonpolar organic compounds because its relative permittivity is 1.4 and does not vary significantly with pressure in the range of 7 to 21 MPa or with temperature in the range of 25 to 200 °C. However, supercritical CO2 is often inadequate as a solvent for extraction of other nonpolar and polar organics. Frequently, it is necessary to mix supercritical CO2 with methanol or other more-polar organic solvents to obtain an extraction solvent having a greater relative permittivity. The apparatus (see figure) includes a sample cell, into which a solid sample is