Vessel Microwave Devices

Usually, multi-mode systems use closed type vessels and focused systems use open vessels. /. Closed-Vessel Microwave Devices 

FIGURE 2.25 Schematic view of the microwave oven. (Reprinted from Zlotorzynski, A., Crit. Rev. Anal. Chem., 25, 43-76, 1995. With permission from Taylor and Francis Group.) 

The phenomenon of lower internal pressure at relatively high temperatures is one of the main advantages of the microwave-assisted sample preparation with closed vessels. The pressure inside a vessel may be additionally lowered by cooling the gas phase inside the vessel. There are some designs of closed microwave vessels which make use of heat loss to improve the safety and robustness of digestion procedures.  

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The analytical quality in such cases can be better assessed when CRMs of similar nature are available. For example, samples of arterial blood weighing 5 to 15 mg were withdrawn from different parts of a rabbit and were subjected to mineralization with hot 50% (v/v) HNO3 in a closed vessel microwave device. After adequate dilution end analysis of Ca, Mg and Fe was carried out by ICP/AES . 

Microwave-assisted solid sample treatment Focused closed-vessel microwave devices... 

The experimental conditions for the acid digestion (closed-vessel microwave device) of samples were established. Hydride generation was carried out on the digests before atomizing them. 

Pervaporation of the analytes. An amount of ca. 0.5 g of sample was placed in the pervaporator s donor chamber, which was then closed, connected to the system and placed either in a water bath or in a vessel of a focused microwave device depending on the polar or non-polar nature of the target analytes. These were evaporated into the gas layer above the sample and then diffused through the membrane to the argon acceptor stream. 

Most studies about the microwave-assisted extraction of PAHs from solid samples have been conducted using closed-vessel systems [12,214,226,236,239-246] and only a few with open-vessel focused microwave devices [57,247-252]. Because open-vessel systems operate at atmospheric pressure, the extraction vessel can be used as a reactor in order to perform on-line purification pretreatments of the total extracts (reagents can be readily added to the medium) [53] or directly introduce the extract into the determination instrument, as in the focused microwave-assisted extractor with on-line fluorescent monitoring of Fig. 5.10, which provides a matrix-independent approach to the extraction of PAHs [61]. 

Commercial focused microwave devices have been used for coupling the extraction with other steps such as filtration, preconcentration, and/or detection. The experimental setup showed in Figure 4 uses a piston pump as the interface between the extractor and an FI manifold. Extraction is performed by supplying an appropriate extractant in an automatic manner by means of the piston pump. Then, the sample is irradiated with microwaves for a preset time and the extract aspirated through the other pump channel to a vessel connected to the FI manifold, where the analytes are filtered, preconcentrated, and determined chromatographically in an automatic manner. 

The microwave power could be adjusted in order to allow constant pressure within the vessel. A incorporated pressure release valve permits to use this experimental device routinely and safely. Furthermore, an inert gas as argon could be introduced within the reactor to avoid sparking risk with flammable solvents. This experimental device is able to raise temperature from ambient to 200 °C in less than 20 s (pressure is close to 1.2 Mpa and heating rate is close to 7° s 1). The RAMO system has been designed for nanoparticles growing and elaboration [59-62]. The RAMO system is a batch system. It could be easily transpose to continuous process with industrial scale (several hundred kilograms by seconds). 

This first industrial device has been designed by MES company [65] for drying. It could be used for solid state reactions with powder reactants. Consequently, the reactor cannot be a classical chemical vessel or a classical chemical reactor with stirrer and others associated technical devices but a container able to enclose a reactant powder layer. The geometrical shape of the microwave applicator is parallelepiped box and the reactants are supported by a dielectric conveyor belt with edges as described by the Fig. 1.18. 

This industrial equipment has been designed by MES company [67]. The Thermostar system is constituted by cylindrical vessel associated with parallelepiped applicator. Circular pipes are very classical geometrical shape for industrial reactors. The Thermostar device is constituted of parallelepiped microwave applicators crossing by a dielectric pipe. Two variants of this device have been designed in relation to reactants state (liquid or solid). 

On the other hand, solvents usually show a decrease in dielectric constant with temperature. Efficiency of microwave absorption diminishes with temperature rise and can lead to poor matching of the microwave load, particularly as fluids approach the supercritical state. Solvents and reaction temperatures should be selected with these considerations in mind, as excess input microwave energy can lead to arcing. If allowed to continue unchecked, arcing could result in vessel rupture or perhaps an explosion, if flammable compounds are involved. Therefore it is important in microwave-assisted organic reactions, that the forward and reverse power can be monitored and the energy input be reduced (or the load matching device adjusted) if the reflected power becomes appreciable. 

Although microwave-heated organic reactions can be smoothly conducted in open vessels, it is often of interest to work with closed systems, especially if superheating and high-pressure conditions are desired. When working under pressure it is strongly recommended to use reactors equipped with efficient temperature feedback coupled to the power control and/or to use pressure-relief devices in the reaction vessels to avoid vessel rupture. Another potential hazard is the formation of electric arcs in the cavity [2], Closed vessels can be sealed under an inert gas atmosphere to reduce the risk of explosions. 

As illustrated in Scheme 1, on a small scale under reflux in ace-tone/water, 5% of starting material remained after 12 h reaction time and approximately 20% of the by-product was formed (entry 1). When performing the reaction at the same concentration in a lab-scale micro-wave device (Emrys Optimizer, entry 2) at 120°C, the reaction was complete after 5 min and gave a product of significantly higher purity and in higher yield. In the next step, 400 ml of reaction mixture was reacted in an 8 vessel rotor batch microwave (entry 3) at the same temperature... 

Safety features are essential to a microwave apparatus. An exhaust fan draws the air from the oven to a solvent vapor detector. Should solvent vapors be detected, the magnetron is shut off automatically while the fan keeps running. Each vessel has a rupture membrane that breaks if the pressure in the vessel exceeds the preset limit. In the case of a membrane rupture, solvent vapor escapes into an expansion chamber, which is connected to the vessels through vent tubing. To prevent excessive pressure buildup, some manufacturer use resealable vessels. A spring device allows the vessel to open and close quickly, releasing the excess pressure. 

OF samples were collected with the commercial device Salivette, which consists of a cotton swab that is inserted into the mouth for 2-3 min. Toxitubes A was compared to microwave assisted extraction (MAE). A volume of 1 ml of saliva was poured into each Toxitube and treated similarly as seen previously [109], For MAE different solvents (chloroform, dichloromethane, hexane), temperatures (80, 90, and 100 °C), and time periods (5, 10, and 15 min) have been tested finally 1 ml of saliva was mixed with 10 ml of chloroform and placed in the oven vessel for extraction at 100 °C for 10 min. Recoveries were found to range from 53 to 95 % with Toxitubes and from 83 to 100 % with MAE, so authors demonstrated that microwave-assisted extraction provides recoveries higher than those obtained for opiates with SPE. 

Dynamic systems for high-pressure microwave treatment were developed much later than open-vessel systems. Operating under a high pressure reduces the flexibility afforded by working at atmospheric pressure. However, some recently developed devices allow microwave-assisted high-pressure digestion and extraction in a dynamic manner [33,34]. 

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. 

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