Microwave-assisted processes variables

Before applications are dealt with, the main variables governing microwave-assisted processes and the parameters characterizing specific microwave treatments are examined. The applications discussed include not only microwave-assisted digestion and extraction — which are the two most widely implemented and hence those with the highest potential interest to readers — but also others of special significance to solid sample treatment such as microwave-assisted drying, distillation and protein hydrolysis. Finally, some safety recommendations on the use of microwave equipment are made. 

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. 

Temperature is a key variable in most analytical processes. In microwave-assisted processes, it plays a prominent role and affects the rate of some reactions, the degradation of thermolabile species and the solubilization of some substances, among others. A number of devices have been developed for monitoring or even controlling the temperature, some of which are commented on in Section 5.3. 

The performance in microwave-assisted processes depends on a number of variables including the microwave power output, exposme time, solvent used, and sample size used. 

Microwave-assisted extraction has also been used as a solid sample treatment prior to speciation analysis [264-266], leaving the organometallic compound moiety intact. This is a prerequisite for a successful extraction procedure to be applied prior to speciation analysis and can be met by careful optimization of the conditions of the microwave attack. Open-vessel treatment is preferred to pressurized bomb systems commonly used in the analysis for total metals because it offers milder reaction conditions — the increase in temperature is governed to a great extent by the boiling point of the solvent — and easier control of process variables [266]. 

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