The Interdepartmental Equipment Facility
Microwave Digestion System
The Milestone ETHOS 1 is a microwave oven especially designed for sample dissolution and digestion at elevated temperatures and pressures.
Most samples intended for ICP have to be prepared for analysis. The samples are digested and solubilized, usually in strong mineral acids, and converted to inorganic compounds.
Sample digestion for ICP might take a long time if carried out in an open vessel on a hot plate. The process is rate-limited by temperature, i.e. by the boiling point of the acids used for sample digestion.
In the ETHOS-1, Sample Digestion is carried out in sealed Teflon vessels. Microwave radiation heats up the acid/sample mixture and the pressure starts to build up. As pressure increases so does the boiling point of the acids - and the operating temperature. The boiling point of the nitric-acid/water mixture used for sample digestion is typically raised from 120oC to 180-200oC. The digestion processes speed up, and the time required is shortened, by one order of magnitude or more.
A regular pressure-cooker actually works quite similarly. Steam produced during cooking raises the pressure inside by ~1 bar, thus raising the boiling point of water from 100oC to 125oC. This speeds up the cooking about 4-fold.
The Temperature and pressure in the vessels is monitored and controlled. The microprocessor-based digestion program which specifies the duration of digestion and the power applied, can be overridden by the temperature or pressure control. Power will be turned on and off in a duty cycle which will stabilize the temperature (or pressure) at the desired value.