Microwave digestion is a well-established preparation technique for converting solid samples into solutions suitable for analysis by ICP-AES, ICP-MS, atomic absorption spectrophotometry or polarography. The solid sample is chemically digested using a liquid reagent (usually a mineral acid) by microwave heating in a closed container. By heating in a closed container, temperatures in the range of 200-260 °C (far in excess of the boiling points of the acids) can be generated, which accelerates the digestion process. A typical microwave digestion takes just 20-40 minutes, making it a powerful preparation tool for the analyst. The heating effect comes from microwave interactions with dipolar molecules and ions in the sample. For dipolar molecules, microwave radiation causes increased molecular rotations and the resulting friction generates heat. For ions, the microwave field increases ionic vibrations which makes them collide with neighbouring molecules, which in turn creates energy and heat. Typical application areas for microwave digestion include food, environmental analysis, medicine, biology, agricultural, pharmaceutical analysis, materials, geology, energy, metals and the plastics industry.
The microwave digestion process
Since the microwave digestion process aims to provide complete solution of the analytes without loss or contamination as well as complete decomposition of the solid matrix, aggressive reagents are used, such as hydrochloric acid, nitric acid, hydrofluoric acid, sulphuric acid and hydrogen peroxide. This means that the digestion sample containers must be chemically inert as well as allowing the transmission of microwave radiation. Rapid heating of the sample in a closed container also leads to rapid increases in pressure, so the microwave digestion vessels must also be able to withstand this. In addition, the rapid heating may generate spontaneous exothermic (heat generating) reactions within the sample which would lead to even higher temperatures and pressures. A particularly good material for digestion vessels is isostatically moulded TFM™ PTFE, which is transparent to microwaves and gives exceptional mechanical stability and resistance to chemical vapours. TFM™ PTFE offers many benefits over conventional PTFE since it provides improved resistance to gas permeation through the material. In addition, its smooth, anti-adhesive surface and hydrophobic properties make cleaning easier and minimise the possibility of cross-contamination.
Safety considerations for microwave digestion process
Since the microwave digestion process generates increased temperature and pressure, both of these parameters must be monitored closely throughout the process to allow the microwave power to be regulated accordingly. For aqueous inorganic samples, it is sufficient to monitor the temperature alone, since the pressure variation with temperature can easily be calculated. For optimal safety, the temperature sensor should be capable of measuring the temperature of the liquid actually inside the digestion vessel, not the external vessel temperature.
Samples containing organic components will generate varying amounts of CO2 during digestion depending on their chemical composition and concentration. The CO2 produced raises pressure in the digestion vessel but also decreases the temperature, so for these samples a dedicated pressure sensor is also required. This combined temperature and pressure monitoring ensures safe operating conditions so that even the most challenging samples can be reliably digested. Again, temperature should be measured inside the digestion vessel. For additional safety all digestion vessels should be equipped with a pressure release mechanism connected to a gas exhaust system. If the overpressure is too high the release mechanism will open and the gases should be vented directly into a manifold for discharge without risk to the heating chamber.
Microwave digestion systems from Lambda
Lambda is a leading supplier of analytical instruments and preparation equipment for a wide range of applications, including a choice of microwave digestion systems. If you would like more information, simply contact a member of the team today.