Reflux pre-digestion in microwave sample preparation

In order to prevent excessive pressure rises in closed microwave acid digestion of fairly large (1 g) samples of high organic content, an open-vessel pre-digestion technique under reflux was designed to allow the escape of oxidation products, such as carbon dioxide, without incurring evaporation losses of acid or analytes. A microwave-transparent coolant liquid was circulated through a specially designed reflux condenser fitted onto an uncapped, commercially available digestion vessel, whilst samples and acid were heated in a microwave oven. Following pre-digestion, the vessels were capped and subjected to microwaves under pressure to complete the digestion. To test the method, 1 g samples of powdered sweet bay were digested with nitric acid using an open pre-digestion with and without reflux cooling, and the mass loss incurred compared with that using a completely closed method (where loss would be restricted to that due to oxidation reactions and release of carbon dioxide). The reflux and closed methods showed similar small mass losses, whereas those where reflux had not been used showed much higher losses, reflecting considerable evaporation of acid. Analysis of the digests for manganese by flame atomic absorption spectrometry showed no significant losses of analyte using either method.