Microwave-assisted Solvent Extraction MASE

In the last decade there has been an increasing demand for new extraction techniques, amenable to automation, with shortened extraction times and reduced organic solvent consumption, to prevent pollution and reduce the cost of sample preparation. Driven by these goals, advances in microwave extraction have resulted several techniques such as microwave-assisted solvent extraction (MASE) [32, 36-39], vacuum microwave hydrodistillation (VMHD) [40, 41], microwave hydrodistillation (MWHD) [42, 43], compressed air microwave distillation (CAMD) [44], microwave headspace (MHS) [5], and solvent-free microwave hydrodistillation (SEME) [45, 46]. Table 22.3 summarizes the most common microwave extraction techniques for plant matrices and lists their advantages and drawbacks. Over the years procedures based on microwave extraction have replaced some of the conventional processes and other thermal extraction techniques that have been used for decades in chemical laboratories. 

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 

Name Microwave-assisted Vacuum microwave Microwave Compressed air Solvent-free microwave 

Brief description Sample is immersed in Sample is submitted Sample is immersed in Sample is irradiated Sample without any 

Solvent usage 10-40 mL No solvent 1 L Water Air pump No solvent 

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In microwave-assisted solvent extraction (MASE), the sample and solvent are heated directly rather than in more conventional schemes where the vessel is heated to extract the sample. The sample solvents are placed into a closed vessel that does absorb microwaves. This facilitates the extraction of the samples... 

Molins, C., Hogendoom, E. A., Heusinkveld, H. A. G., Van Harten, D. C., Van Zoonen, P., and Baumann, R. A., Microwave assisted solvent extraction (MASE) for the efficient determination of triazines in soil samples with aged residues, Chromatographia, 43, 527-532, 1996. 

A microwave-assisted solvent extraction (MASE) method was optimized by Chee et al. to extract phthalate esters from marine sediments, soils, and results were compared with those obtained by the same authors using conventional Soxhlet and sonication techniques. The analysis was performed by GC-ECD or GC-MS. The overall optimal conditions for the extraction of phthalate esters by MASE included the use of 1 1 acetone or hexane at 115°C for 10 min. Recoveries for six individual phthalate esters (DMP, DEP, DAP, DBP, BBP, DEHP) ranged from 71% to 91%, and were better than those obtained with Soxhlet (66% to 90%) or sonication (65% to 89%). The authors stated that advantages of MASE extraction over sonication or Soxhlet are larger sample throughput, lower usage of hazardous solvents, and less laborious cleanup steps. 

Current trends have been to accelerate the extraction of pesticides from the sample matrix by accelerated solvent extraction (ASE) or pressurized solvent extraction (PSE), microwave-assisted solvent extraction (MASE), and super critical fluid extraction (SEE). 

There are many sample preparation procedures published in the scientific literature, and within the scope of this chapter, only the most current and popular methods will be discussed. By far, the commonest and most popular method used for pretreatment of liquid samples is solid phase extraction (SPE) [40,41]. For solid samples, several techniques are available including supercritical fluid extraction (SFE) [42,43], microwave-assisted solvent extraction (MASE) [44,45] and accelerated solvent extraction (ASE) [46,47]. Solvent extraction methods have long been established as the standard approach to sample preparation, but the increasingly demanding needs of industries like the pharmaceutical, agrochemical and petrochemical for greater productivity, faster assays, and increased automation have led to the development of newer ways of sample preparation summarised in Fig. 2.3. 

An increase of the extraction yield can be obtained by the simultaneous application of a microwave field to the extraction chamber. The resulting procedure is known as microwave-assisted solvent extraction (MASE). Only static procedures are necessary. Atrazine, simazine, terbumeton, terbuthylazine, terbutryn, molinate, and bromacil were extracted from soils by using MASE with methanol, resulting in recoveries >80% for the 5-200 ng per g... 

For many years, the traditional sample preparation methods, such as the Soxhlet extraction, were applied. Most of these methods have been used for more than 100 years, and they mostly require large amounts of organic solvents. These methods were tested during those times, and the analysts were familiar with the processes and protocols required. However, the trends in recent years are automation, short extraction times, and reduced organic solvent consumption. Modern approaches in solid sample preparation include microwave-assisted solvent extraction (MASE), pressurized liquid extraction, accelerated solvent extraction (ASE), matrix solid-phase dispersion (MSPD), automated Soxhlet extraction, supercritical fluid extraction (SEE), gas-phase extraction, etc. 

MAE simply involves placing the sample with the solvent in specialized containers and heating the solvent using microwave energy. MAE is also sometimes called MASE, which can stand for microwave-assisted solvent extraction or microwave-accelerated solvent extraction. In any event, the extraction process is more rapid than Soxhlet extractions, can be run in batches, and reduces solvent consumption. As in the case of sonication, MAE may overcome retention of the analyte by the matrix, but analyte degradation can be a problem at higher temperatures in certain applications. 

LSE, liquid solid extraction p.b., phosphate buffer MASE, microwave assisted solvent extraction HWE, hot water extraction SPE, solid phase extraction. 

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. 

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