Microwave distillation

Armstrong, B.E and Neas, E.D., Development of a microwave distillation system for the analytical laboratory,... 

Grimm, C.C., Lloyd, S.W., Batista, R., Zimba, RV, Using microwave distillation solid-phase microextraction gas chromatography-mass spectrometry for analysing fish tissue, J. Chromatogr. Sci.,2S, 289-296 (2000)... 

The microwave distillation SPE apparatus is shown in Figure 9.12. A filet of catfish is placed in the microwave oven and heated, which drives the geosmin out of the tissue and is swept through the solid-phase extraction sorbent with argon gas. The condenser unit cools the gas, which increases the retention of geosmin, onto the 500 mg of C-18 in Figure 9.12. The sorbent is present in the bottom of the condenser unit. Charcoal, XAD-2, and C-18 were all tried for recovery of geosmin, and C-18 had the best recovery. 

Figure 9.12. Microwave distillation solid-phase extractor for geosmin. [After Conte and co-workers (1996), published with permission.]...

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. 

Ye, H., J. Ji, C. Deng, et al., 2006. Rapid analysis of the essential oil of Acorus tatarinowii Schott by microwave distillation, SPME, andGC-MS. Chromatographia, 63 591-594. 

SW Lloyd, CC Grimm. Analysis of 2-methylisoborneol and geosmin in catfish by microwave distillation-solid-phase microextraction. J Agric Food Chem 47 164-169, 1999. 

S. W. Lloyd and C. C. Grimm, Analysis of 2-methyhsobomeol and geosmin in catfish by microwave distillation—solid phase microexiraction, J. Agric. Food Chem. 47 164 (1999). 

FIGURE 10.5 Elution profile on OH-B12 treated by microwave heating for 6 min during silica gel 60 column chromatography. Fifty milliliters of the treated OH-B12 solution (5 mmol/1) was evaporated to dryness and dissolved in a small amount of w-butanol/2-pro-panol/water (10 7 10, v/v) as a solvent. The concentrated solution was put on a column (1.4 X 15.0 cm) of silica gel 60 equilibrated with the same solvent and eluted with the same solvent in the dark. The eluate was collected at 4.0 ml with a fraction collector. Fractions I to V were pooled, evaporated to dryness, dissolved with a small amount of distilled water, and analyzed with silica gel TLC. Inset represents the mobile pattern of the OH-B12 degradation products of fractions I to V on the TLC plate. Data are typical, taken from one of five experiments. (Reprinted with permission from Watanabe, F. et al., J. Agric. Food Chem., 46, 5177-5180, 1998. Copyright (1998) American Chemical Society.)... 

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. 

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... 

Similarly, Vasudevan and Verzal have found that terminal alkynes can be hydrated under neutral, metal-free conditions using water as solvent (Scheme 4.15) [41], While this reaction typically requires a catalyst such as gold(III) bromide, employing microwave-superheated distilled water allowed this chemistry to proceed without any catalyst. Extension of this methodology led to a one-pot conversion of alkynes to imines (hydroamination). 

The conversion of isothiocyanates to isonitriles under microwave conditions has been studied by Ley and Taylor using a polymer-supported [l,3,2]oxaphospholidine [119]. The use of 3-methyl-2-phenyl-[l,3,2]oxaphospholidine in solution is less favored [120] due to the associated toxicity and instability of the phosphorus-derived reagent, as well as the need to isolate the products from a complex reaction mixture by vacuum distillation. This drawback has been resolved by attaching the active [l,3,2]oxaphospholidine to a polymer matrix. 

With conductive heating, several factors militate against efficient distillation. Transferal of heat to the liquid usually occurs from the inner surfaces of the vessel. Vaporization from the surface and convection preclude a uniform temperature within the liquid. To achieve distillation under those circumstances, the pot temperature must be considerably higher than that of the distillate. With microwaves, energy is absorbed more uniformly, a larger volume of the sample in the pot is heated simultaneously, convection is reduced and distillation is more rapid. 

Method D (using microwave irradiation) TBA-F (0.26 g, 1 mmol) on alumina (1.7 g) is added to the methylene compound (1.5 mmol) and alkylating agent (1.1 mmol) in the minimum amount of THF or Et20 and the mixture is evaporated to dryness, powdered and irradiated at 300 W for 3-7 min. The solid is extracted with Et,0 and the extracts are fractionally distilled to yield the alkylated product. 

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