Microwave-assisted decomposition

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. 

As microwave sample preparation has evolved, standard microwave procedures have been developed and approved by numerous standard methods organisations (ASTM, AOAC International, EPA, etc.), see ref. [64]. Examples are standard test methods for carbon black/ash content (ASTM Method D 1506-97), lead analysis in direct paint samples (ASTM Method E 1645-94), etc. Table 8.15 shows some microwave ashing references (detection weight). A French AFNOR method utilises the atmospheric pressure single-mode microwave method as an alternative sample preparation procedure for Kjeldahl nitrogen determination [84], The performance of a microwave-assisted decomposition for rapid determination of glass fibre content in plastics for QC has been described [85]. 

In 2001, Linder and Podlech studied the microwave-assisted decomposition of diazoketones derived from a-amino acids [340]. In the presence of imines, the initially formed ketene intermediates reacted spontaneously by [2+2] cydoaddition to form /3-lactams with a trans substitution pattern at positions C-3 and C-4 (Scheme 6.179) [340], In order to avoid the use of the high-boiling solvent 1,2-dichlorobenzene, most transformations were carried out in 1,2-dimethoxyethane under sealed-vessel conditions. Solvent-free protocols, in which the substrates were adsorbed onto an inorganic alumina support, led only to the corresponding homologated /3-amino acids. Evidently, traces of water present on the support trapped the intermediate ketene. 

The common methods used for dissolving samples for metals analysis are digestion in an open flask, digestion in a pressurized, sealed container, and microwave assisted decomposition. Some common solvents used are listed in Table 5.1. 

The most innovative source of energy for wet decomposition procedures is micro-waves. Because the heating takes place inside the decomposition mixture, microwave-assisted decomposition is more efficient than decomposition using... 

Determination GC—MS MAE and Soxhlet are compared table with found PCB concentrations by both methods is given recoveries ranged from 98 to 123% with MAE and from 86 to 111% with Soxhlet comparison with other extraction techniques confirmed the efficiency of MAE Determination GC-ECD Three microwave-assisted techniques MAE, microwave- assisted saponification (MAS) and microwave-assisted decomposition (MAD) were studied and combined with success tables with recoveries from studied methods are given Determination offline HRGC-HRMS recoveries ... 

Xiong, G., He, X., and Zhang, Z., Microwave-assisted extraction or saponification combined with microwave-assisted decomposition applied in pretreatment of soil or mussel samples for the determination of polychlorinated biphenyls. Anal. Chim. Acta, 413, 49-56, 2000. 

Microwave-assisted techniques (MATs) can also be used in the extraction process. In MAE, microwaves improve efficiency with respect to Soxhlet extraction with a lower extraction time. " Microwave-assisted decomposition (MAD) has been applied in PCB determination in soil " " and sediment." ... 

Begerow j and Dunemann L (1997) Microwave-assisted decomposition. In Stoeppler M, ed. Sampling and Sample Preparation, Practical Guide for Analytical Chemists, pp. 155-166. Springer-Verlag, Berlin. 

With the aid of an internal standard, fluctuations in sample delivery in particular and, to a certain extent, changes in excitation conditions can also be compensated for. To appreciate the full benefit of internal standardization, the intensities of the analytical line and the internal standard line, and in trace determinations also of the spectral background near the analytical line, should be measured simultaneously, which can easily be done in the case of a CCD spectrometer. The full analytical precision of ICP-OES can thus be obtained. This can be demonstrated by the determination of the main components, as is required in stoichiometric determinations of advanced ceramics. After optimizing the sample dissolution procedure in the case of silicon-boron-carbo-nitride, which can be speeded up by microwave-assisted decomposition with acids [455], silicon at the 60% level could be determined with a sequential spectrometer with an absolute error of analysis (standard deviation including replicated analyses as well as decompositions) of 1% or better. 

Leadbeater described the use of Ni(CN)2 for the microwave-assisted cyanation of (hetero)aryl bromides [81]. The use of 0.6equiv of Ni(CN)2 was found to be optimal. Unfortunately, the heteroaryl bromides reported in the study gave relatively low yields due to significant decomposition (Scheme 70). 

Independently, Caddick et al. reported microwave-assisted amination of aryl chlorides using a palladium-N-heterocyclic carbene complex as the catalyst (Scheme 99) [lOlj. Initial experiments in a domestic microwave oven (reflux conditions) revealed that the solvent is crucial for the reaction. The Pd source also proved very important, since Pd(OAc)2 at high power in DMF gave extensive catalyst decomposition and using it at medium and low power gave no reaction at all. Pd(dba)2/imidazohum salt (1 mol% catalyst loading) in DME with the addition of some DMF was found to be suitable. Oil bath experiments indicated that only thermal effects are governing the amination reactions. 

Triazoles have been obtained via microwave-assisted [3-i-2] cycloaddition, under solvent-free conditions [54], starting from organic azides and acetylenic amides at 55 °C for 30 min (Scheme 23). The complete conversion of the reagents into AT-substituted-1,2,3-triazoles 69 was achieved without decomposition and side products. A control reaction carried out at the same temperature in an oil bath did not give the cycHc products, not even after 24 h of reaction time. 

In the context of preparing potential inhibitors of dihydrofolate reductase (DHFR), the group of Organ has developed a rapid microwave-assisted method for the preparation of biguanide libraries (Scheme 6.174) [330]. Initial optimization work was centered around the acid-catalyzed addition of amines to dicyandiamide. It was discovered that 150 °C was the optimum temperature for reaction rate and product recovery, as heating beyond this point led to decomposition. While the use of hydrochloric acid as catalyst led to varying yields of product, evaluation of trimethylsilyl chloride in acetonitrile as solvent led to improved results. As compared to the protic... 

Vessels for microwave-assisted chemistry are usually made from thermal insulators such as PEEK, quartz, borosilicate glass or PTFE. Thus, the benefits of rapid heating can be diminished if the opportunity for workup is delayed by slow cooling. Decomposition of thermally unstable products also can occur. 

The same authors performed a microwave assisted Stille reaction on the Rink amide (RAM) Tentagel polymer-tethered 4-iodobenzoic acid [5 b]. Successful palladium-catalyzed coupling of heteroaryl boronic acid with anchored 4-iodobenzoic acid enabled both >99% conversion of the starting material within 3.8 min (45 W) and a minimal decomposition of the solid support. The coupling reactions were realized in a mixture of polar solvents (H20-EtOH-DME, 2.5 1.5 6). 

Methods have been described that involve microwave-assisted graphite-supported dry media for the cycloaddition of anthracene, 1-azadienes and 1,2,4,5-tetrazines with several C-C dienophiles and carbonyl compounds in hetero-Diels-Alder reactions [35], This technique leads to a shortening of reaction times, a situation that enables work to be undertaken at ambient pressure in an open reactor to avoid the formation of unwanted compounds by thermal decomposition of reagents or products. 

Dominguez, A. and Fidalgo, B. and Fernandez, Y.and Pis, J.J.and Menendez, J.A. (2007) Microwave-assisted catalytic decomposition of methane over activated carbon for C02-lree hydrogen production. International Journal of Hydrogen Energy, 32, 4792-4799. 

A large class of coordination compounds, metal chelates, is represented in relation to microwave treatment by a relatively small number of reported data, mainly p-diketonates. Thus, volatile copper) II) acetylacetonate was used for the preparation of copper thin films in Ar — H2 atmosphere at ambient temperature by microwave plasma-enhanced chemical vapor deposition (CVD) [735a]. The formed pure copper films with a resistance of 2 3 pS2 cm were deposited on Si substrates. It is noted that oxygen atoms were never detected in the deposited material since Cu — O intramolecular bonds are totally broken by microwave plasma-assisted decomposition of the copper complex. Another acetylacetonate, Zr(acac)4, was prepared from its hydrate Zr(acac)4 10H2O by microwave dehydration of the latter [726]. It is shown [704] that microwave treatment is an effective dehydration technique for various compounds and materials. Use of microwave irradiation in the synthesis of some transition metal phthalocyanines is reported in Sec. 5.1.1. Their relatives - porphyrins - were also obtained in this way [735b]. 

Aminoquinolines 62 have been prepared in a two-step, one-pot, three-component reaction of 2-azidobenzophenones, secondary amines and arylac-etaldehydes [110]. The microwave-assisted reaction proceeded via the initial formation of enamines 59. Subsequent addition of 2-azidobenzophenones 60 afforded the triazoline intermediates 61, which underwent thermal rearrangement and cyclocondensation to furnish 2-aminoquinolines 62 (Scheme 41). Direct comparison with conventional thermal conditions demonstrated the superiority of microwave dielectric heating in terms of yields (73% vs. 31% of heterocycle 63 after 10 min at 180 °C). Furthermore, the formation of by-products due to decomposition of azide 60 was diminished in the microwave-assisted synthesis. Purification of the products was achieved using solid-phase extraction techniques. 

Gromboni, C.F., Kamogawa, M.Y., Ferreira, A.G., N6brega, J.A., and Nogueira, A.R.A. 2007. Microwave-assisted photo-Fenton decomposition of chlorfenvinphos and cypermethrin in residual water. Journal of Photochemistry and Photobiology A Chemistry, 185 32-37. 

C.2. Detecting Hydroxyl Radicals in the Microwave-assisted Photo catalytic Decomposition of Hydrocarbons... 

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