Open vessel digestion is the classic technique that requires complex mixtures of acids and takes a few hours to a few days to digest a sample. For ICP-MSICP-QQQ applications, this technique is not recommended and should be avoided. There is a contamination problem during the digestion and acid fumes cause pollution of the environment. Unfortunately, the EPA method requires this technique for the digestion of environmental samples.
The closed vessel digestion is a better technique than the open vessel. PTFE is commonly used as a container with a stainless steel jacket. The advantages of this technique over the open vessel are as follows:
All these features reduce the time of digestion. However, extreme care should be taken, or the pressure inside the vessel may becomes too high and the vessel could explode.
The most convenient technique is the microwave digestion. Microwave energy is absorbed by dipole rotation and ionic conductance, which causes rapid heating of not only the aqueous phase, but also the sample itself. PTFE is transparent to microwave radiation and favored for the vessel. The microwave digester is different from a conventional microwave oven used for cooking foods and is specially designed for digestion of samples, taking into account safety issues. The temperature of the inside vessel is measured and controlled; the pressure of the inside vessel is also measured and controlled. Pressure that is generated suddenly by reactions between chemicals and samples is vented to prevent the vessel from exploding.
There are several types of vessels for microwave digestion. PTFE is the most popular material but its purity is not high enough for ICP-MSICP-QQQ. PFA has recently been applied to high purity samples, its purity is better than PTFE. Quartz is also used for high purity samples.