Microwave Digestion – the Way Forward for Metals Analysis

July 11, 2012 - PRLog -- Microwave technology is making a significant contribution to many areas of science, including organic synthesis, fat analysis and microwave-assisted extraction. One specialised but important niche it is also beginning to occupy is as a preferred preparation procedure for elemental analysis, more particularly metals analysis.

Elemental analysis, as the name suggests, is simply the process whereby a sample e.g. human waste, soil, bodily fluids, mineral compound is probed for its elemental (or isotopic) composition. For organic chemists this means looking primarily for carbon and hydrogen and associated heteroatoms (usually those like nitrogen, oxygen or sulphur that have replaced carbon in a molecule’s backbone) and this can be accomplished by combustion analysis. If the analyst is interested in inert metals and alloys, refractory inorganics and suchlike materials, then more specialist preparation must be undertaken.

Concentrated acids, including nitric and hydrofluoric acids, have been frequently used to digest samples, but this is time consuming work and inadequate for some materials that are particularly resistant to dissolution. However, when combined with microwave heating in a pressure- resistant vessel (closed vessel digestion) a particularly effective and speedy digestion method is produced.

This is carried out in a variety of specially designed instruments, far removed from the domestic microwave we are all familiar with; only to be expected when pressures in excess of 1500 psi, and temperatures over 300°C, are being produced. As would be expected, such state-of-the-art equipment also features very reassuring safety features! As well as the time saving inherent in the microwave technology, whereby the sample is effectively heated all the way through without heat transmission via the vessel wall, the ability to run 12 samples simultaneously (in top systems) is also a major benefit to any busy analysis lab.

The smaller volumes of concentrated acid used is another saving, both in cost of reagents and environmental impact. The latest microwave digestion equipment also offers other benefits such as ease of operation (one-touch control) and retention of volatiles. The lowered possibility for cross contamination is also important at a time when the levels of elements being detected is being pushed lower and lower.

Many modern metals analysis techniques such as Inductively Coupled Plasma Atomic Emission Spectroscopy, also called Inductively Coupled Plasma Optical Emission Spectrometry (ICP – AES & ICP- OES), Graphite Furnace Atomic Absorption (GFAA) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) can all benefit from prior microwave-assisted sample digestion. With reduced costs and reduced run times there seems very little reason to proceed otherwise.

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