Microwave-enhanced Solvent Extraction of Organics in Environmental Analysis
INTRODUCTION TO ENVIRONMENTAL ORGANIC SAMPLE PREPARATION
Analysis of organic pollutants in the environment is necessary because of past or present industrial activity, accidental spillage, and deliberate (unlicensed) disposal, all of which can have ecotoxicological effects on living organisms. In order to assess the potential damage, remediate known sites, or simply have an early warning of potential problems requires environmental measurement of the levels of organic pollutants. Organic pollutants are located in all compartments of the environment. So, typically, we refer to pollution in terms of natural waters (e.g. lakes, rivers, groundwater, and potable water), solid or semisolid matrices (e.g. soil, sludge, and vegetation), and the atmosphere (e.g. air). In this article, we are concerned principally with solid-type matrices, although some attention is given to aqueous sample preparation. A recently published book discusses the different types of sample preparation approaches for both liquid and solid samples of environmental origin.(1) Preparation of solid samples for organic analysis can traditionally be subdivided into two classes, with heating and without heating. In the latter case, we refer to the use of shake ﬂask, while in the former Soxhlet extraction. Shake ﬂask extraction involves placing a solid sample into a container together with an organic solvent and agitating, either manually or by the use of a laboratory shaker, for a given period of time. Subsequently, the organic solvent containing the extract is removed via ﬁltration, a separating funnel, or decanting. Soxhlet extraction involves reﬂuxing warm organic solvent through the sample repeatedly for several hours. This is achieved by heating organic solvent, contained in a round-bottomed ﬂask, on an isomantle. The vaporized solvent is then condensed, via a water-cooled condenser, which falls into a thimble-containing sample. This, in turn, returns to the round-bottomed ﬂask. The whole process is repeated frequently until the pre-set extraction time is reached. As the extracted organic pollutant normally has a higher boiling point than the solvent, it is preferentially retained in the ﬂask and fresh solvent recirculates. This ensures that only fresh solvent is used to extract the organic pollutant from the sample in the thimble. A disadvantage of this approach is that the organic solvent is below its boiling point when it passes through the sample contained in the thimble. In practice, this is not necessarily a problem as Soxhlet extraction is normally done over long time periods, i.e. 6, 12, 18, or 24 h. More recently, alternative approaches have become available. In addition to the shake ﬂask approach, it is possible to agitate the sample using a sonic bath or sonic probe. Modiﬁcations of Soxhlet extraction have appeared, which offer some degree of solvent reduction Encyclopedia of Analytical Chemistry, Online © 2006–2013 John Wiley & Sons, Ltd. This article is © 2013 John Wiley & Sons, Ltd. This article was published in the Encyclopedia of Analytical Chemistry in 2013 by John Wiley & Sons, Ltd. DOI: 10.1002/9780470027318.a0844.pub22 ENVIRONMENT: WATER AND WASTE and automation, e.g. Soxtec. Perhaps the biggest changes have occurred in terms of the instrumentation available for extraction of pollutants from solids. The 1980s saw
the arrival of commercial apparatus for supercritical ﬂuid extraction (SFE), while the early 1990s the commercial availability of microwave-assisted extraction (MAE) closely followed in 1995 by accelerated solvent extraction (ASE). This article is concerned with the use of microwave technology for the extraction of organic pollutants from environmental (solid and liquid) matrices. The ﬁrst papers on the subject were published in 1986(2) using a domestic microwave oven; it was not until the 1990s that commercial microwave systems became available. This article only considers the research published since 2006 to illustrate the applications of microwave technology.