Superheated Water as an Extraction Solvent in Sample Preparation

May 02, 2014

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.

Water at high temperature is often forgotten as a potential extraction solvent for sample preparation, yet it can be used in a wide range of applications, including the extraction of polycyclic aromatic hydrocarbons (PAHs) from soil and nutraceuticals from plant materials, for both non-polar and polar analytes. Unlike many other solvents, water is readily available in high purity at minimal cost, with minimal disposal costs and a negligible environmental impact. Its potential as a solvent is the result of it's unique properties when heated — the polarity changes from a polar solvent at room temperature with a specific permittivity of ε = 80 to ε = 30 at 220 °C in a pressurized system, which is comparable to many organic solvents at room temperature, such as methanol ε = 33. Mechanically it is easy to handle because only low pressures (15 bar at 200 °C) are required to maintain the liquid state. Unlike most organic solvents, it presents no danger from ignition or toxicity, and, although it is more volatile than ionic liquids, its vapours are environmentally benign.

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).

Extraction Methods

Figure 1: Superheated water extraction system.
In common with other solvents, superheated water is most suitable for the extraction of solid samples such as soils, environmental solids, and dried plant material but can be difficult to apply to tissues, impervious food samples, or clinical samples. The equipment (shown in Figure 1) is fairly simple, consisting of a water supply, pump, an oven containing an extraction vessel, and post-oven restrictor to maintain the pressure. Typically, an old gas chromatograph (GC) oven can be used that will have good temperature control up to 300 °C. An empty column or similar configuration can act as the extraction vessel. Extraction can either be performed in a static or dynamic mode. In the latter case, a system as simple as a capillary or narrow-bore tube can provide sufficient restriction and back pressure to maintain the water in the liquid phase in the oven. The solvation power of high temperature water is effectively independent of the pressure, so an accurate control is usually not needed. Some recent extractions have used commercial accelerated solvent extraction systems but these are limited to 200 °C.

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).