RAPID STOPPED-FLOW MICROWAVE DIGESTION SYSTEM

A prototype system for stopped-flow microwave assisted wet digestions has been developed. A coiled Teflon PFA tube serves both as a sample container and as a digestion vessel. A sample plug, consisting of a water slurry mixed with an acid mixture, is pumped into the coil. Sample flow is stopped, the coiled tube is sealed (by closing an input and an output valve) and microwave power is applied for 2 min for digestion of the sample. Methodology was developed using powdered botanical reference samples and was tested with powdered botanical and biological reference materials. The digests were analysed by inductively coupled plasma atomic emission spectrometry. In addition to comparisons with certified values, the results were compared with those obtained by conventional open-vessel hot-plate digestions, by open-vessel microwave digestions and by digestions taking 32 min by using the coiled tube system. Precise, and in many instances, quantitative digestions were obtained using a net digestion time of 2 min. Elemental recoveries were sample type and digestion time dependent and were found to be comparable with, and sometimes, superior to those obtained when using a 3 h long hot-plate digestion. In this preliminary study, characteristics, limitations and future directions are discussed.