Microwave-Enhanced Flow System for High-Temperature Digestion of Resistant Organic Materials

Abstract

Microwave-assisted flow digestion systems open up newpossibilities 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 °C are necessary. To achieve the desired temperatures of ∼250 °C, it is essential that the pressure in the system can be increased up to 35 bar. The teflon tubes used, however, do not withstand the vapor pressure of the digestion mixture at these temperature levels. A high-pressure flow 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 °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 °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 mediumpressure flow 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 °C and 5 min showed good agreement with the certified values.

High pressure microwave-assisted flow digestion system using a large volume reactor-feasibility for further analysis by inductively coupled plasma-based techniques

“…When performed in closed vessels analyte losses and contamination are avoided, a fast and efficient digestion is promoted and reagent consumption and waste generation is reduced. Based on the applied pressure during digestion, they can be broadly classied as ambient pressure, 5-9 medium pressure (<25 bar) [10][11][12] or high pressure (>25 bar) [13][14][15] ow digestion systems. These procedures are time consuming anddespite the fast heating stepadd signicantly to the total time required for the digestion step.…”

“…Moreover, these steps are very difficult to automate, making manual operation mandatory. 17 Pichler et al 13 used a PTFE tube to form an inert reactor for sample decomposition. Moreover, ow digestion systems can be easily interfaced to techniques for analyte quantication (e.g.…”

Microwave-Assisted Extraction

“…16 A subsequent design avoided the need for the nitrogen bomb and enabled direct introduction of solid samples, through which fresh extractant was continuously pumped, and connection to a subsequent step of the analytical process. The highly complex first design required solid samples to be slurried and the assistance of a nitrogen bomb to supply N 2 at a constant pressure up to 5 bar.…”

Sample Preparation for Trace Analysis

“…The sample throughput is given to be 6 samples/h. PICHLER et al [133] have shown that such an on-line microwave procedure can also be adapted to pressurized ashing. In this special case, the sample is injected into a constant stream of nitric acid which passes the microwave heating zone.…”