Reprinted from Eskilsson, C. S. and Bjorklund, E., J. Chromatogr. A, 902, 227— Elsevier.
a Determined at 20°C.
Determined at 25°C. c Determined at 101.4 kPa. d Determined at 1207 kPa. e No microwave heating.
250, 2000. With permission from e. Instrumentation
Recently Luque-Garcia and Luque de Castro have published an excellent review on modern microwave devices.106 In the following paragraphs only a brief overview is given.
Microwave equipment used for sample pretreatment can be classified into two groups, according to how microwave energy is applied to the sample, namely:
• Multi-mode systems, in which the microwave radiation is allowed to disperse randomly in a cavity, so each zone in the cavity and the sample are evenly irradiated (Figure 2.25);
• Single-mode or focused systems, in which microwave radiation is focused on a restricted zone where the sample is subjected to a much stronger electric field than in the previous systems (Figure 2.26).
Usually, multi-mode systems use closed type vessels and focused systems use open vessels.106 i. Closed-Vessel Microwave Devices
Closed vessels have high upper pressure limits and normally are constructed in several layers of microwave transparent polymer. In well-insulated closed vessels, the temperature can be estimated by means of Equation 2.2 for temperatures higher than the boiling point of the reaction mixture. However, in most microwave systems the vessels are not completely insulated therefore a loss of heat occurs, introducing error in the calculation. A significant amount of heat is lost through the walls of the vessels to the cooling system of the microwave oven. Consequently, the pressure of gases inside the closed vessels is significantly lower than that predicted by the temperature of the liquid phase. In the microwave field the assumption that all components of the system
(liquid, gas and vessel) are in equilibrium is no longer valid because the gas phase is heated less effectively than the liquid phase. The ion conduction mechanism is not present in the gas phase because all free ions are left in solution thus leaving only the molecule rotation as a heating mechanism. In addition, the effectiveness of this mechanism is drastically decreased because of the statistically lower number of molecule-molecule collisions in the gas phase. For these reasons there is no thermal equilibrium reached between the liquid and gas phases. The lower temperature of the gas phase creates a vertical temperature gradient from the bottom of the vessel (the hottest part) to the top of it (the coolest part). This temperature gradient causes acid fumes to condense and creates an effective reflux system inside the vessel.92
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.92
ii. Open-Vessel Microwave Devices
For microwave systems operating at atmospheric pressure, the temperature can be calculated by means of Equation 2.2 up to the boiling point of the solution, including any superheating effects. The boiling point limits the oxidation potential (i.e., the ability of reagents to destroy the matrix). This leads to different approaches for increasing the oxidation potential. In addition to using azeotropic mixtures and taking advantage of any superheating effect, acids with higher boiling points, such as H2SO4, are used to produce more rigorous digestion/leaching conditions. Another frequently used approach is the addition of H202, which may be safely used with open vessels. Hydrogen peroxide increases the oxidation potential and also improves the conversion of the microwave energy into heat due to its high dielectric constant value.
One useful aspect of microwave systems with open vessels is that they allow direct adaptation of already existing sample preparation methods. Of particular interest for chromatography is the focused microwave-assisted Soxhlet extractor (Figure 2.17 and Figure 2.27). It is based on the same principles as a conventional Soxhlet extractor but is modified to facilitate accommodation of the sample-cartridge compartment in the irradiation zone of a microwave oven. The modification
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