Microwave-enhanced flow system for high-temperature digestion of resistant organic materials

Microwave-assisted now digestion systems open up new possibilities in fully automated sample preparation for element analysis. For an extensive and fast oxidation of organic materials with nitric acid, temperatures of more than 200 degrees C are necessary. To achieve the desired temperatures of similar to 250 degrees C, it is essential that the pressure in the system can be increased up to 35 bar. The tenon tubes used, however, do not withstand the vapor pressure of the digestion mixture at these temperature levels. A high-pressure now digestion device is described that enables the application of such high temperatures by means of a novel pressure equilibration system. PTFE or PFA tubes can be used up to 250 degrees C if no mechanical stress is applied to the tube wall. The pressure equilibration system keeps the pressure inside and outside the digestion tube equal even for extremely fast oxidation reactions. The digestion of easy, medium, and difficult oxidizable substances (glucose, glycine, and phenylalanine, respectively) shows the importance of digestion temperatures around 250 degrees C, The extremely violent digestion of glucose with concentrated nitric acid can be carried out as easy as the difficult oxidation of phenylalanine by means of this system. The SRM TORT 2 (defatted lobster hepatopancreas tissue) was digested under different conditions to indicate the high oxidation capabilities in comparison with a commercial medium-pressure now digestion device. The analysis of the SRMs milk powder (BCR 063, BCR 151), bovine liver (BCR185), and pig kidney (BCR 186) after digestion at 245 degrees C and 5 min showed good agreement with the certified values.