Closed vessel, acid-assisted microwave digestion is a popular and effective way of converting solid samples into solutions for the direct quantitative determination of trace elements by spectroscopic techniques. Heating of the sample in a closed vessel is not limited by the boiling point of the acid solution, enabling significantly higher working temperatures (typically in the range of 200-260°C) to be achieved. The vapour generated during heating increases the pressure in the closed vessel and the increases in pressure and temperature are both key parameters that speed up the reaction rate and lead to the reduction in digestion times.
The need for temperature and pressure monitoring
Monitoring the temperature and pressure inside the reaction vessels during digestion is extremely important in order to regulate the microwave power to maintain the temperature and pressure at the required levels. For inorganic samples, increases in pressure will be due to vaporisation from the acid and any gaseous products of the digestion, such as H2. Since the pressure/temperature curve for these can be predicted, sample volumes can be chosen so that the resulting pressure in the reaction vessel will never exceed the pressure capacity of the vessel. This means that for these types of application, temperature measurement in the vessel is sufficient. However for samples containing carbon or organic material, acid reaction will result in the formation of carbon dioxide. The volume of CO2 (and hence resulting pressure) produced will be dependent on the amount of organic matter in the individual sample which may vary even for samples of the same weight. For these applications, monitoring of pressure in each vessel is also vital in order to ensure that pressure capacity is not exceeded.
Any particular digestion routine will require the sample solution to be heated through as specific temperature and time cycle. The temperature of the solution inside the vessel should be measured and not just the temperature of the outer wall of the vessel. Detecting the thermal radiation emitted from the vessel, using the middle infrared wavelength range that cannot be absorbed by the vessel material allows direct non-contact measurements of temperature of the sample solution. By also filtering out the IR radiation given off by the pressure vessel surface itself, sample temperature measurements can be made in real-time. This approach avoids the need for complex temperature sensors coated with fluoroplastics immersed in the solution inside the pressure vessel which can be both expensive and slow to react to rapid changes in temperature. This is a particularly important consideration if spontaneous induced exothermic reactions take place in the vessel leading to sharp temperature rises. Separate measurements of the infrared radiation emitted from the wall of the vessel provide the outer temperature of the vessel itself, to ensure that the temperature characteristics of the vessel itself are not exceeded.
Non-contact pressure measurement
It is essential that the pressure is measured inside each individual vessel to allow for variations in sample composition, but many systems allow monitoring in only one reference vessel. This can be addressed using non-contact optical pressure measurement in all vessels and this method is deployed in the Berghof Speedwave Xpert microwave digestion system. Feedback from the pressure sensor is used to regulate the microwave power in the event of overpressure in any of the vessels and even enables automatic shut down of the system if necessary. The pressure sensor works by measuring a change in the polarisation properties of a glass ring under mechanical stress resulting from the pressure in the vessel. The glass ring is mounted in the PTFE cap of the pressure vessel. This provides completely contact-free pressure measurement and does not require the measurement in a reference vessel. A good example of the importance of pressure measurement can be found in a case study which looks at the digestion processes needed for heavy-metal analysis of samples from the environment sector, such as soil, potatoes, different types of fruit, earthworms, etc.
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.