Microwave exposure time

A comparative study [12] of the reactivity of the oxalimide 16 in a variety of solvents (xylene, chlorobenzene, toluene) and of methylphosphinite 17 was performed with the focused microwave Synthewave 402 reactor (Merck Eurolab, div. Prolabo, France), using different conditions of power and exposure time (Scheme 8.8). In all experiments yields were better than those of previous procedures with classical heating (Tab. 8.2), and the authors wrote it is dear that microwave technique is applicable to highly functionalized compounds containing stereogenic centers without appreciable modification of these centers . 

Joachim Walter (Boehringer Ingelheim Pharma KG) presented results on the use of microwave technology for virus inactivation in biopharmaceutical products. By using microwaves, viral inactivation can be carried out with the exposure time of the product solution to high temperatures being limited to 500 ms or less. For antibodies, viral inactivation can be effectively carried out using peak temperatures up to 95°C with hold times as low as 5 ms. 

Figure 15.6 3D plots of Acridan assay emission as a function of time from glass slides without (Top) and with low power microwave exposure / pulses (Middle). Bottom - photographs showing the Acridan emission both before (a) and after a low power microwave pulse (b). Mw - Microwave pulse. The concentration of BSA-Biotin was 1.56 pM. Adopted from Ana/ Chem 78 8020-8027 (2006). 

Microwave treatment, because of its rapid heating of materials, is being explored in a multitude of crops for enzyme inactivation (25-28), for extraction of natural products (29), and oil and fat extraction from seeds and food products (30-32). Microwave treatment of peanut seed prior to press extraction increased oil recovery approximately 10% at an optimum treatment time of 30 seconds (30). However, free fatty acid content initially increased with exposure time as well as peroxide value (30). Research on use of microwave treatment in blanching of peanuts indicated an influence on oil stability depending on treatment conditions (33). 

Performance in microwave-assisted processes depends on a number of variables including the microwave power output, exposure time, solvent and sample size used. However, specific treatments such as digestion are additionally dependent upon factors such as the digesting acid, pressure and its relationship to temperature in closed-vessel systems, the residence time in flow systems, the number of cycles used in an FMAS extractor, etc., all of which should be optimized for each specific situation. 

In theory, one should use a high microwave power to reduce the exposure time as much as possible. This is the rule to be followed in most digestion processes, i.e. to use... 

Other organic compounds such as phenols [40], hydrocarbons [267], polymer additives [268,269] and natural compounds [270] have also been extracted from a wide range of solid matrices with good results. This confirms that, under optimal conditions of solvent, microwave power and exposure time, etc., the MAE technique is suitable for the extraction of organic compounds from solid samples. 

It is clear that the plant leaves exposed to the oil field burns are loaded with contaminants compared to the control (unexposed plant) leaf. This was comfirmed by a simple test using a commercial microwave device (8) exposure time was 2 minutes. The dry palm - tree leaves exposed to environmental pollution by oil field burns contaminant showed severe weight loss compared to the conventional control (dry palm - tree unexposed leaves). Figure 4 shows details. More details are available elsewhere (9). 

A recent report [51] related to process optimization describes a new series of six graft copolymers by microwave-initiated method. For the first four graft copolymers (I-IV) (Table 5.1), the exposure time was varied with the concentration of acrylamide and CMS fixed. For the second set of three graft copolymers (III, V-VI), only the acrylamide concentration was varied keeping the other parameters as optimized in the first set. 

It is obvious from Table 5.1 that with the increase in exposure time (1-4 minutes), the percentage grafting increased up to 3 minutes (which is optimized) after which it decreased. This may be because of the fact that, beyond an exposure time of 3 minutes, the prolonged exposure to microwave irradiation may have degraded the polysaccharide backbone. Hence, the percentage grafting and intrinsic viscosity decreased. 

Diaz-Ortiz described the microwave-induced 1,3-dipolar cycloadditions of mesi-tonitrile oxide to aliphatic and aromatic nitriles under solvent-free conditions. The adducts were obtained in shorter reaction times and better yields than with classical methods [84]. By the same approach, Corsaro reported the successful cycloaddition of arylnitrile oxides to naphthalene or aromatic polycyclic dipolaro-philes under solvent-free microwave activation [85]. In the same report, the yields of bis-cycloadducts were also improved by microwave exposure [85]. 

The principal variables to be considered in developing an MAE method are the applied power and the irradiation (exposme) time. The power and the corresponding time will depend on the type of sample and solvent used. Usually, these two variables have opposing effects for a given process, the use of high microwave power affords a decreased exposure time ... 

---