Table of Contents
The physical and chemical characteristics of microwave heating present some challenges that must be taken into account during construction. Microwave radiation, unlike other types of electromagnetic radiation, is very low in energy and is not capable of breaking real chemical bonds. Thus, microwave radiation is relatively harmless. In addition, microwave radiation is used to stimulate dipoles and generate heat. Thus, a microwave serves as an efficient heat source. The actual digestion of the samples itself is caused by the interaction of pressure and temperature, as well as the action of the digestion reagents. The basis for the operation of a microwave is the dielectric heating of different materials by dipole rotation and ionic conductivity. In order to heat substances, they must contain a dipole. Dipolar molecules align themselves uniformly, according to their charge, in the oscillating microwave field. This results in rotational movements and occurring frictional forces. The energy generated in this process is converted into heat. Ions oscillate under the influence of a microwave field. During this movement, collisions occur with neighboring molecules, which generates energy or heat. When working with a microwave, it becomes clear that different samples and also digestion reagents react in different ways to an applied microwave field. This is due to the individual properties of the reagents and their ability to convert the electromagnetic energy into heat. This is described by the dielectric loss factor tan δ.
dielectric loss (efficiency with which energy is converted into heat)
dielectric constant (ability of a dielectric material to store energy)
Reagents with low tan δ values have higher transparency, absorb less microwave radiation and are more difficult to heat (e.g. HF in contrast to HCl). Heating in the microwave field depends on many factors such as sample type or sample amount. Two samples almost never react in the exact same way. To ensure optimum reproducibility and guarantee safety, unequal heating of samples in the microwave field must always be taken into account. The rapid heating of the sample solution can induce exothermic reactions during the digestion process, which is why a wide variety of sensor systems have been developed to monitor the reaction parameters of pressure and temperature and thus control the microwave power.
The different heating properties of a microwave become particularly clear if one considers the different aggregate states of water. In the liquid state, the molecules are mobile enough to move in the electric field according to dipole rotation and generate heat. In the gaseous state, as water vapor, on the other hand, no heat generation is possible because the molecules move chaotically and no collisions can be generated. In the frozen state, on the other hand, the particles are rigidly bound, so that movement and thus heat generation is practically impossible.