Pressurized high temperature or superheated water is a green extraction solvent used in food, environmental, and traditional medicine studies for the extraction of non-polar and polar analytes including essential oils and spices, agrochemicals, pharmaceuticals, and petrochemicals. The technique can be used on both the analytical and preparative scale to give a clean, solvent-free product for chromatographic analysis. This article reviews the current status of superheated water extraction, discussing extraction methods, applications, and, briefly, problems encountered with this approach.
Hot water is not often thought of as a typical extraction solvent but it is used every day at 100 °C to extract tea leaves and coffee beans. When we raise the temperature further, the polarity decreases and extraction power increases. From 100 °C up to around 300 °C under low pressures it is referred to as either superheated water, subcritical water, or pressurized hot water and has found application as an extraction solvent, as a chromatographic eluent, and as a solvent for organic synthesis. The recent interest in sample preparation and extraction was aroused in the 1990s by Hawthorne and co-workers who used superheated water as an extraction solvent in environmental studies for the extraction of PAHs from soils (1) and compared it to alternative extraction methods (2). They found that for non-polar analytes the change in extraction power with temperature can be dramatic. For example, the solubility of anthracene, chrysene, and perylene in water increased by 20,000-fold over the range 25–200 °C.In later studies by a number of groups a wide range of analytes and matrices have been examined and these have been presents in reviews by Smith in 2002 (3); by Kronholm, Hartonen, and Riekkola in 2007 (4); and most recently in 2010 by Teo and co-workers (5) and Smith (6).
After the extraction, the eluent water is cooled to room temperature and if sufficiently concentrated can be analyzed directly either by high performance liquid chromatography (HPLC) or gas chromatography (GC). The extraction can also be linked directly to HPLC or superheated water chromatography, for example, to determine herbicides in compost (7). In that study, a single stream of water steam performed a sequential extraction, fractionation, and chromatographic separation by using temperature control.
For less concentrated samples, analytes can be extracted from the polar aqueous phase and concentrated before an assay using either a small volume of organic solvent, or a solid-phase microextraction (SPME) fibre, adsorbent disc, stir-bar, hollow fibre, or a solid-phase extraction (SPE) cartridge before assay. Post extraction derivatization or modification can also be performed in the aqueous solution (8).