Solvent selection microwave-assisted extraction

Microwave-assisted extraction (MAE) of analytes from various matrices using organic solvents has been operative since 1986 [128], In this process microwave energy is used to heat solvents in contact with a solid sample uniformly and to partition compounds of analytical interest from the sample matrix into the solvent. The way in which microwaves enhance extraction is not fully understood. The main factors to consider include improved transport properties of molecules, molecular agitation, the heating of solvents above their boiling points and, in some cases, product selectivity. 

This chapter covers techniques for the extraction of semivolatile organics from solid matrices. The focus is on commonly used and commercially available techniques, which include Soxhlet extraction, automated Soxhlet extraction, ultrasonic extraction, supercritical fluid extraction (SFE), accelerated solvent extraction (ASE), and microwave-assisted extraction (MAE). The underlying principles, instrumentation, operational procedures, and selected applications of these techniques are described. In a given application, probably all the methods mentioned above will work, so it often boils down to identifying the most suitable one. Consequently, an effort is made to compare these methodologies. 

Microwave-assisted extraction (MAE) and supercritical fluid extraction (SEE) are well established techniques for the determination of different pollutants from solid samples, providing faster extractions and less usage of organic solvents than conventional solvent extraction. SFE was proved to be useful in the selective removal of analytes in different types of samples. 

Supercritical fluid extraction (SEE) is a selective technique of sample preparation that enables the preparation of matrices by varying several physical parameters. Nowadays, it is considered to be the best replacement for many extraction technologies, such as accelerated solvent, Soxhlet solvent, microwave assisted extraction and so on. It was originally marketed as a universal extraction tool in 1988 by Isco Inc. (Lincoln, Nebraska, USA), Lee Scientific (Salt Lake City, Utah, USA) and Suprex Corp. (Pittsburgh, Pennsylvania, USA). The basic components of the SFE instmment are a carbon dioxide reservoir, a pump, an extraction vessel, an oven, a restrictor... 

The choice of solvent in a microwave-assisted sample treatment is a direct function of the type of treatment used. Thus, digestion and hydrolysis are best done with aqueous solutions of pure or mixed acids, whereas selective extraction of organic compounds from a sample matrix usually requires an organic solvent. 

In the sample preparation of semi- and nonvolatile compounds, solvent extraction is typically used for extracting the analytes of interest from a sample matrix. For volatile analytes, head-space or thermal extraction are good alternatives to solvent-based techniques. Several novel extraction systems that utilize elevated temperatures of pressures in the extraction have been developed particularly for solvent-based extraction methods. These new methods typically are much faster and often more selective than older methods and consume smaller amounts of organic solvents and reagents. Commercially available systems with the ability to heat and pressurize liquids include pressurized liquid extraction (PLE), microwave-assisted extinction (MAE) and supercritical fluid extraction (SFE). Also sonication-assisted extraction (SAE) has given promising results. 

The analysis of liquid and solid samples very often requires some form of solvent extraction to isolate organic constituents. Conventional solvent extraction can be used, and the pH may be adjusted to achieve some selectivity, for example, extracting from acid solution to prevent basic compounds from extracting. More efficient means of extraction are commonly employed today, such as microwave-assisted or accelerated solvent extraction for solid samples (Chapter 19). See... 

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