Fundamentals of Microwave Digestion

Conventional wet-sample preparation methods for the decomposition of solid samples are usually carried out in vessels containing the sample and a large volume of decomposition reagent(s), typically 15 to 100 mL. This mixture is heated for long periods of time using a hot plate, heating mantle, or oven. Heating is terminated when the analyst decides that the decomposition of the sample is sufficiently complete. This type of open-vessel digestion has many drawbacks, which include the use of large volumes (and multiple additions) of reagents, a large potential for contamination of the sample by materials and laboratory environment, and the exposure of the analyst and the laboratory to corrosive fumes.

Closed-vessel microwave decomposition uses significantly different technology and fundamentally unique principles to accomplish sample decomposition. Decomposition of most solid samples can be achieved using near stoichiometric quantities of reagents, typically 10 mL, and can usually be completed in 10 to 15 minutes. This decrease in sample preparation time can be attributed to the closed vessels and the rapid heating of the sample mixture. The higher temperatures achieved in the closed system give microwave digestion a kinetic advantage over hot plate digestion, as described by the Arrhenius Equation:

Integration of this equation gives:

In this expression k₁ and k₂ are rate constants for the reaction of interest at T₁ and T₂ respectively, E_a is the activation energy, and R is the ideal gas constant. These equations show that the reaction rate increases exponentially with increasing temperature. This translates into approximately a 100-fold decrease in the time required to carry out a digestion at 175 °C when compared to 95 °C digestion.

In addition, because the mineral acid converts the microwave energy into heat almost instantaneously, rapid heating of the sample is achieved, further decreasing the reaction times. Also, digestions are more complete because many acids (e.g. nitric acid) show improved oxidation potentials at elevated temperatures.

For additional, in-depth information about microwave digestion, read Chapter 6 of Clean Chemistry: Techniques for the Modern Laboratory, by Dr. Robert Richter. The book is available elsewhere on our web site.

For additional information about microwave chemistry in general, read Chapter 2 of Microwave-Enhanced Chemistry: Fundamentals, Sample Preparation, and Applications, edited by H.M. "Skip" Kingston and Stephen J. Haswell. This Chapter is available online, from Duquesne University.

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