Extraction microwave-assisted

Microwave-assisted extraction (MAE) uses microwave energy to heat the solvent/sample mixture in order to partition analytes from the sample matrix into the solvent (see Fig. 4.6). Using microwave energy allows the solvent to be 

The results obtained to date have led to the conclusion that microwave radiation causes no degradation of the extracted compounds, unless an excessively high temperature arises in the vessel. However, a specific effect of microwaves on plant material has been found. Microwaves interact selectively with the free water molecules present in the gland and vascular systems, leading to rapid heating and temperature increase, followed by rupture of the walls and release of the essential oils into the solvent. Similar mechanisms are suspected in soils and sediments, where strong, localized heating should lead to an increase in pressure and subsequent destruction of the matrix macrostmcture. 

Microwave energy may be applied to samples using either of two technologies closed vessels (under controlled pressure and temperature) or open vessels (at atmospheric pressure).These two technologies are commonly termed pressurized MAE (PMAE) or focused MAE (EMAE), respectively. Whereas in open vessels the temperature is limited by the boiling point of the solvent at atmospheric pressure, in closed vessels the temperature may be elevated by simply applying the correct pressure. The latter system seems more suitable in the case of volatile compounds. However, with closed vessels, one needs to wait for the temperature to decrease after extraction before opening 

Akhtar et al. developed a method for MAE extraction of fortified and incurred chloramphenicol residues in freeze-dried egg. Sample extraction time was 10 s 

2-propanol. Akhtar also compared MAE with conventional extraction (homogenization, vortexing) for the determination of incurred salinomycin in chicken eggs and tissues. Raich-Montiu et al. described the extraction of trace concentrations of sulfanomides from soil samples using MAE with acetonitrile, followed by further clean-up with SPE. The extraction efficiency was evaluated using three soil samples with different physicochemical characteristics. Recovery rates ranged from 60% to 98%, and detection limits were between 1 and 6 ftg/kg. 

The success of this technique is related to the development of specialized microwave instruments that provide temperature-controlled and closed-system operation, permitting the processing of many samples simultaneously. 

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]. 

Unlike digestion, microwave-assisted extraction is used mainly with soil and sediments rather than with biological samples. As regards analytes, microwave-assisted extraction is applied primarily to organic pollutants such as PAHs, PCBs and pesticides rather than to metal species. 

The use of MAE is a continuously expanding area of research at present. As a result, it is difficult to provide a completely updated overview. Rather, selected groups of compounds of interest that have been subjected to MAE are discussed here. Unless otherwise stated, the microwave systems used in these applications were of the open-vessel type, which require none of the drastic pressure and temperature conditions employed with closed-vessel systems. 

The increasing risks to human health posed by the widespread use of pesticides in our environment is well established [212,213]. Thus, the determination of pesticides in water, plants, soil, sediments, foodstuff, etc., is of major concern today [214,215]. Microwave-assisted extraction provides an efficient, reproducible alternative to classical methods based on Soxhlet extraction or sonication for the extraction of pesticides from environmental samples. 

A wide range of pesticides including organochlorine [216,217] and organophosphorus compounds [218], triazines [219-222], herbicides [43,62] and imidazolinones [223] have been extracted with the aid of microwaves, with recoveries close to 100% in most cases. 

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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). 

The performance of microwave-assisted decomposition of most difficult samples of organic and inorganic natures in combination with the microwave-assisted solution preconcentration is illustrated by sample preparation of carbon-containing matrices followed by atomic spectroscopy determination of noble metals. Microwave-assisted extraction of most dangerous contaminants, in particular, pesticides and polycyclic aromatic hydrocarbons, from soils have been developed and successfully used in combination with polarization fluoroimmunoassay (FPIA) and fluorescence detection. 

COMPARISON OF MICROWAVE ASSISTED EXTRACTION METHODS FOR THE DETERMINATION OF PLATINUM GROUP ELEMENTS IN SOIL SAMPLES BY ELECTROTHERMAL ATOMIC ABSORPTION SPECTROMETRY AFTER PHASE SEPARATION-EXTRACTION... 

General trends are focused on reduced-solvent extractions or adsorption-based methods — enviromnentaUy friendly solvents for both solid and liquid samples. In recent decades, advanced techniques like supercritical fluid extraction (SFE), ° pressurized liquid extraction (PLE)," microwave-assisted extraction (MAE), ultrasound-assisted extraction, countercurrent continued extraction (www.niroinc.com), solid... 

Weichbrodt et reported on the use of focused open-vessel microwave-assisted extraction (EOV-MAE) for the determination of organochlorine pesticides in high-moisture samples such as fish. The results were comparable to those with closed-vessel microwave-assisted extraction (CV-MAE) and ASE. The main advantage of FOV-MAE is that the use of Hydromatrix is unnecessary as the solvent mixture of ethyl acetate and cyclohexane allows the removal of water from the sample matrix via azeotropic distillation. 

Fast Soxhlet extractions 71 3.4.5 Microwave-assisted extractions. 104... 

In liquid-solid extraction (LSE) the analyte is extracted from the solid by a liquid, which is separated by filtration. Numerous extraction processes, representing various types and levels of energy, have been described steam distillation, simultaneous steam distillation-solvent extraction (SDE), passive hot solvent extraction, forced-flow leaching, (automated) Soxh-let extraction, shake-flask method, mechanically agitated reflux extraction, ultrasound-assisted extraction, y -ray-assisted extraction, microwave-assisted extraction (MAE), microwave-enhanced extraction (Soxwave ), microwave-assisted process (MAP ), gas-phase MAE, enhanced fluidity extraction, hot (subcritical) water extraction, supercritical fluid extraction (SFE), supercritical assisted liquid extraction, pressurised hot water extraction, enhanced solvent extraction (ESE ), solu-tion/precipitation, etc. The most successful systems are described in Sections 3.3.3-3.4.6. Other, less frequently... 

Table 3.10 shows the recovery from PP of Irgafos 168 and its oxidised and hydrolysed by-products by various extraction procedures. As may be observed, One-Step Microwave-Assisted Extraction (OSM) and US lead both to negligible hydrolytic additive degradation. The measured additive decay (by oxidation) is essentially due to the antioxidant activity during the processing (extrusion) step of the polymer and not to the US or microwave heating treatment. 

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. 

Table 3.25 Solvents commonly used in microwave-assisted extraction...

For one-step microwave-assisted extraction (OSM) the following parameters were optimised ... 

S, Soxhlet S , Soxtec R, reflux SF, shake-flask US, ultrasonics SFE, supercritical fluid extraction MAE, microwave-assisted extraction PFE, pressurised fluid extraction (ASE , ESE ) D/P, dissolution/precipitation. 

With the exception of GC-MIP-AES there are no commercial instruments available for speciation analysis of organometallic species. Recently, a prototype automated speciation analyser (ASA) for practical applications was described [544,545], which consists of a P T system (or focused microwave-assisted extraction), multicapillary GC (MC-GC), MIP and plasma emission detection (PED). MCGC-MIP-PED provides short analysis times ([Pg.676]

LD-IMS Laser desorption-ion mobility MAE Microwave-assisted extraction... 

For solid samples PLE, microwave-assisted extraction (MAE) and supercritical fluid extraction (SFE) [39, 40] are the preferred extraction methods. 

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. 

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