Microwave-assisted sample digestion using mineral acids is a well-established and widely used sample preparation technique for atomic and mass spectrometry techniques, such as atomic absorption spectrometry (AAS), inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS). Though widely used, the theoretical understanding of the interaction of microwave radiation with concentrated mineral acids is surprisingly limited. In fact, it is commonly assumed, that mineral acids absorb microwave radiation of 2.45 GHz much like water. Based upon this hypothesis, the penetration depth of microwave radiation into the digestion acid mixture is believed to be about 20 mm – much as in the case of water. Therefore, volumetric heating has been postulated. Unfortunately, these assumptions are wrong.
Due to the very high electrical conductivity of mineral acids used for sample digestion, it will be shown, that microwave radiation is absorbed in a thin (< 1 mm) liquid layer at the digestion vessels inner surface. Heat conduction and convection have to ensure the energy transfer from this boundary region to the center of the digestion vessel and thereby limit the heat transfer. Volumetric heating, that is the heating of the entire sample volume directly by microwave radiation cannot occur. In fact, the heating mechanism is closer to a beaker glass on a hotplate than commonly anticipated. In the latter case a thin liquid layer is heated directly by the hot glass surface and the bulk volume temperature increase of the liquid phase is dominated by convection and conduction.
Furthermore it will be demonstrated, that by drastically reducing the frequency of the electromagnetic radiation volumetric heating of highly conductive liquids can actually be achieved. This paves the way for a new approach for sample digestion: radio-frequency (RF) heated sample digestion.