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

Abstract

A new high pressure (40 bar) continuous flow system with a large volume reactor (13.5 mL heated volume inside the microwave cavity) has been developed. The microwave-assisted digestion of the samples occurred in a coiled perfluoroalkoxy (PFA) tube reactor. As the mechanical stability of the PFA-tube is insufficient at the used digestion conditions (40 bar, >200 °C), it was placed inside an autoclave constructed from a thick-walled borosilicate tube and pressurized by nitrogen. Nitric acid and mixtures of HNO₃ with HCl and/or HF were used for sample digestion and no elevated blank levels caused by contamination with corrosion products from the flow digestion system were encountered. For glucose, glycine and phenylalanine a residual carbon content (RCC) of 2.3 ± 0.5, 37 ± 3 and 77.9 ± 0.7% (mean ± standard deviation, n = 5), respectively, was obtained under optimized digestion conditions (500 W microwave power and 5.0 mL min⁻¹ carrier flow rate). The accuracy of the method was evaluated using certified reference materials (NIST SRM 1577b, SRM 1515). The determined values were in good agreement with the certified ones using inductively coupled plasma optical emission spectrometry (ICP-OES) for analyte quantification. Moreover, a comparison between closed vessel microwave-assisted digestion and high pressure flow digestion was performed using several plant- and animal tissue samples. These materials were less finely ground than CRM's making slurry generation more difficult. Nevertheless, the element concentrations obtained by ICP-OES after flow digestion were in good agreement with those from closed vessel batch digestion.