Accelerated Solvent Extraction

Abstract

Extraction of solid and semisolid samples using liquid solvents is a common practice in nearly every analytical laboratory. Years of empirical testing have resulted in rugged and reproducible methodologies for a wide range of analyte classes. However, recent concerns regarding the volumes of organic solvents used (with the associated human exposure), along with increased purchase and disposal costs, have emphasized the need for more efficient sample extraction methods. In response to these concerns, accelerated solvent extraction (ASE ® , Dionex Corporation, Salt Lake City, UT) was introduced. Since its introduction in 1995, ASE has grown rapidly as an accepted alternative to traditional extraction methods. Accelerated solvent extraction takes advantage of the enhanced solubilities that occur as the temperature of a liquid solvent is increased. Increasing the temperature of solvent results in a decrease in viscosity, allowing better penetration of the sample matrix. In addition, analyte diffusion from the sample matrix into the solvent and overall solvent capacity are increased. In traditional Soxhlet extraction, the solvent that comes into contact with the sample has passed through a cooling condenser, and is therefore close to room temperature at the point of contact. The time required to complete Soxhlet extractions ranges from 6 to 48 h. Semi-automated Soxhlet systems that immerse the sample into boiling solvent are available. This increase in the temperature of the contacting solvent shortens the required extraction time to ~2 h. Using these systems, a further increase in temperature beyond the boiling point of the solvent is not possible due to solvent loss because these systems operate at atmospheric pressure. However, a continued increase in the temperature should continue to enhance the extraction process. This can be accomplished by applying pressure, which maintains the solvent in its liquid state beyond its atmospheric boiling point. This is the theoretical basis for ASE technology and represents the next step in liquid solvent extraction of environmental samples. There are, of course, limits to which raising the temperature is feasible, due to thermal degradation concerns. However, as evidenced by data published to date, there is room to continue raising the temperature, thereby improving the extraction efficiency, without risking analyte degradation in environmental samples. As the extraction efficiency is