Microwave-assisted extraction solvents, organic

Theoretical and applied aspects of microwave heating, as well as the advantages of its application are discussed for the individual analytical processes and also for the sample preparation procedures. Special attention is paid to the various preconcentration techniques, in part, sorption and extraction. Improvement of microwave-assisted solution preconcentration is shown on the example of separation of noble metals from matrix components by complexing sorbents. Advantages of microwave-assisted extraction and principles of choice of appropriate solvent are considered for the extraction of organic contaminants from solutions and solid samples by alcohols and room-temperature ionic liquids (RTILs). 

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. 

In recent years, extraction methods for PhACs have usually been based on liquid partitioning with ultrasonic extraction (USE) [43-47], microwave-assisted extraction (MAE) [48], or the more advanced PLE [49-52]. When compared to the other extraction techniques, PLE provides good recoveries, saves time and organic solvent, which makes it become currently a preferred technique for PhAC analyses. 

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. 

It should be noted that microwave-assisted extraction (MAE) discussed in this chapter is different from microwave-assisted acid digestion. The former uses organic solvents to extract organic compounds from solids, while the latter uses acids to dissolve the sample for elemental analysis with the organic contents being destroyed. Microwave-assisted digestion of metals is covered in Chapter 5. 

Although the traditional Soxhlet and solvent extraction techniques are widely accepted, they have inherent limitations and problems. Thus, Soxhlet extraction requires 12-24 h in most cases and uses high volumes of organic solvents (hundreds of millilitres). In contrast to conventional methods, microwave-assisted extraction can reduce the extraction time to less than 30 min and solvent consumption to under 50 ml [12]. Moreover, the recoveries obtained with microwave-assisted extraction are comparable with those provided by alternative extraction methods [7]. 

The advantages of ASE over other techniques such as supercritical fluid extraction (SEE) and microwave-assisted extraction (MAE) are not so clear [16,60,62,116]. In fact, SEE features similar extraction times and uses little or no organic solvent. Also, SEE... 

In microwave assisted extraction (MAE), microwave energy accelerates the partition, i.e. the mass transfer of an analyte from a sample matrix into a solvent, by directly heating the solution. The extraction is performed at an elevated temperature in a closed vessel. The major benefits are the shorter extraction time, reduced consumption of organic solvents and increased sample throughput. However, there is a need for an additional filtration step and, if the extract is dilute, further concentration, e.g. by evaporation or SEE, may be needed before analysis. 

Microwave-assisted extraction (MAE) utilizes organic solvent and heat to extract organic pollutants from solid matrices. The major difference between this approach and others is the use of a microwave oven as the heat source. For background information on microwave ovens, see Box 5.1. 

The focus in Chapters 7 and 8 is on the specific sample preparation approaches available for the extraction of organic compounds from environmental matrices, principally soil and water. Chapter 7 is concerned with the role of Soxhlet, ultrasonic and shake-flask extraction on the removal of organic compounds from solid (soil) matrices. These techniques are contrasted with newer developments in sample preparation for organic compound extraction, namely supercritical fluid extraction, microwave-assisted extraction and pressurized fluid extraction. Chapter 8 is arranged in a similar manner. Initially, details are provided on the use of solvent extraction for organic compounds removal from aqueous samples. This is followed by descriptions of the newer approaches, namely solid-phase extraction and solid-phase microextraction. 

Microwave assisted extraction (MAE) is also an extraction technique based on heating an organic solvent. The principle is roughly that a sample and an appropriate solvent (or solvent mixture) are put in a vessel, which is then pressurized and heated by microwaves. After typically 5 to 20 min the extraction is complete, and the vessels are allowed to cool down before removing the sample/solvent mixture. The solvent must be filtered to remove sample particles prior to analysis of the extracted components. See figure 5 for a schematic of a MAE equipment. 

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. 

Xiong, G., Tang, B., He, X., Zhao, M., Zhang, Z., and Zhang, Z., Comparison of microwave-assisted extraction of triazines from soils using water and organic solvents as the extractants, Talanta, 48, 333-339, 1999. 

MASE uses microwave energy to heat a liquid organic solvent in contact with a sample. There are two types of microwave extraction - microwave assisted extraction (MAE), which is performed under controlled pressure and temperature in a... 

Soils, sediments, and biosolids are complex matrices and extraction of organic contaminants and their TPs has been more challenging than in aqueous media since co-extracted material present in these samples can severely reduce the efficiency of extraction. Therefore, it is essential to develop effective methods for extraction and purification. Due to the thermolabile properties and polar nature of many TPs, traditional extraction methods such as Soxhlet are not appropriate and other techniques such as pressurized Hquid extraction (PLE), microwave-assisted extraction (MAE), microwave-assisted Soxhlet extraction (MASE), and ultrasonic solvent extraction are more suitable. A summary of extraction methods applicable to the determination of pesticide TPs in soHd matrices (soil) is presented in Table 4. 

The recovery of compour ds trapped inside the membrane is achieved by dialysis using an organic solvent such as hexane [97], cyclohexane [78,138], or by microwave-assisted extraction [149]. Setkova et al. give an overview of SPMD application for monitoring pollutants in various matrices [121]. 

The mode of extraction for PAHs is highly dependent on the matrix. For solid-based matrices such as food samples, sediments, soil, marine organisms, etc. extraction methods such as Soxhlet extraction with nonpolar solvent [35 6], hollow fiber membrane solvent microextraction (HFMSME) [10], pressimzed hquid extraction (PLE) [37,38], sonication extraction [3], microwave-assisted extraction (MAE) [3], supercritical fluid extraction, (SEE) [39], accelerated solvent extraction (ASE) [40], cold extraction [41], soxtec extraction [42], microwave-assisted alkaline saponification (MAAS) [43], dynamic microwave-assisted extraction (DMAE) [44], add-induced cloud point extraction (ACPE) [45], methanolic saponification extraction (MSE) [7], etc. are employed. Of all these, Soxhlet extraction is the most common for solid samples and has achieved excellent extraction with high-level recovery but its setback is the high consmnption of solvent and time associated with it. 

In the case of organic solvents, Soxhlet extraction can be used to enhance the extraction capability of the solvent by pressurizing it. Other methods include using pressurized subcritical solvents, or the application of microwave-assisted extraction [5]. 

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