Microwave-assisted processes distillation

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... 

Before applications are dealt with, the main variables governing microwave-assisted processes and the parameters characterizing specific microwave treatments are examined. The applications discussed include not only microwave-assisted digestion and extraction — which are the two most widely implemented and hence those with the highest potential interest to readers — but also others of special significance to solid sample treatment such as microwave-assisted drying, distillation and protein hydrolysis. Finally, some safety recommendations on the use of microwave equipment are made. 

Many investigations have been carried ont of procednres for improving the analytical qnality of GC methods by changing the matrix, increasing the concentration of the pertinent analytes and redncing the interference of other componnds present in the sample. Preconcentration by LLE, before or after derivatization, is most freqnently apphed in GC trace analysis however, other techniqnes, snch as SPE, sample stacking (see Section V.A.l) and some of their modifications, snch as simnltaneons distillation and extraction (SDE) and SPME, are also mentioned. Application of microwave-assisted processes (MAP) dnring sample preparation seems to improve recoveries. 

As noted earlier, not all open-vessel systems (viz. those that operate at atmospheric pressure) are of the focused type. A number of reported applications use a domestic multi-mode oven to process samples for analytical purposes, usually with a view to coupling the microwave treatment to some other step of the analytical process (generally the determination step). Below are described the most common on-line systems used so far, including domestic ovens (multi-mode systems) and open-vessel focused systems, which operate at atmospheric pressure and are thus much more flexible for coupling to subsequent steps of the analytical process. On the other hand, the increased flexibility of open-vessel systems has promoted the design of new microwave-assisted sample treatment units based on focused or multi-mode (domestic) ovens adapted to the particular purpose. Examples of these new units include the microwave-ultrasound combined extractor, the focused microwave-assisted Soxhlet extractor, the microwave-assisted drying system and the microwave-assisted distillation extractor, which are also dealt with in this section. Finally, the usefulness of the microwave-assisted sample treatment modules incorporated in robot stations is also commented on, albeit briefly as such devices are discussed in greater detail in Chapter 10. 

Extraction of essential oils is one of the most time- and effort-consuming processes in the analysis of the constituents of plants. Various extraction methods were traditionally employed, depending on the material or the available devices. The most commonly used methods are steam distillation and distillation-solvent extraction. The introduction of innovative extraction methods, such as microwave-assisted extraction (MAE) and supercritical fluid extraction (SEE), has led to significant improvement, not only in the analytical performance, but also in the accuracy and reproducibility of methods. 

Despite the often large increase in the reaction rate the use of microwave-assisted reactions has still not been implemented on an industrial scale. One of the main barriers for industrial applications is reliable scale-up of microwave reactors [116], but there are also other engineering problems that have to be solved. The use of microwaves to speed-up distillation processes has also been indicated [123]. 

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. 

An essential oil (EO) is internationally defined as the product obtained by hydro-, steam-, or dry-distillation of a plant or of some of its parts, or by a suitable mechanical process without heating, as in the case of Citrus fruits (AFNOR, 1998 Council of Europe, 2010). Vacuum distUladon solvent extraction combined offline with distillation simultaneous distillation extraction supercritical fluid extraction microwave-assisted extraction and hydro-distiUation and static, dynamic, and high concentration capacity headspace sampling are other techniques used for extracting the volatile fraction from aromatic plants, although the products of these processes cannot be termed EOs (Faleiro and Miguel, 2013). 

Some new green extraction techniques, aimed at sparing energy and reducing costs, such as solid state and submerse fermentation, enzymatic, microwave- or ultrasound-assisted extraction, ultrafiltration, flash distillation and controlled pressure drop processing [22, 23] have been studied to improve solvent extraction. 

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