Sample digestion is the process of decomposing solid samples into a more workable form, in this case a liquid state. It is a crucial intermediary step for elemental analysis of a wide range of sample types, including ash, cosmetics, minerals, pharmaceutical goods, and more. Typical sample digestion workflows involve dissolving samples in reagents such as strong acids, alkalis, or enzymes which can be expedited by heating the mixtures.
Microwave heating is a method for sample digestion, but chemists are increasingly coming to appreciate the benefits of novel digestion systems based on focused short-wave infrared (FSWIR) excitation. This article will compare and contrast these two heating technologies.
Benefits of Microwave Digestion Systems
Microwave digestion is a well-known technique and is routinely used for dissolving samples such as filters and sediments (50-500mg). Microwave digestion systems are also commonly employed to dissolve organic samples containing heavy metals in order to prep them for analysis. The analysis is then done by inductively coupled plasma mass spectroscopy (ICP-MS), inductively coupled plasma optical emission spectroscopy (ICP-OES), or atomic absorption spectroscopy (AAS).
Usually, microwave digestion systems are used to heat a sample in the presence of a strong acid in a closed vessel by raising the pressure and temperature via microwave irradiation. The increase in pressure and temperature facilitate a faster speed of reaction between the sample and the acid leading to the dissolution of the sample and the release of the analytes from the sample matrix.
The sealed container means that the boiling point of the acid increases substantially as the extra heat energy becomes trapped inside the vessel. This extra heat allows samples to be digested quicker and more effectively. Microwave digestion typically takes between 20 to 40 minutes and digestion temperatures can reach as high as 310°C.
One drawback to the process, however, is that the extra trapped heat also generates higher than atmospheric pressures which can reach levels of several thousand PSI. To ensure the safety of the operator this limits the microwave systems in how much sample weight can be used and adds time to the workflow to allow for the ramp up and cool down periods. Overall, these added precautions mean the time of the overall workflow becomes like that of common Hotblock digesters.
FSWIR Digestion as an Alternative to Microwave Digestion Systems
FSWIR Digestion from ColdBlock® is a revolutionary approach, using FSWIR to target sample particles and accelerate the process of decomposition. FSWIR digestion uses a focused SWIR heat source to energize samples. Coupled with a cooling zone this shortens sample preparation time and accelerates sample throughput to achieve productivity gains in a manner similar to the simplicity of Hotblock but with digestion times similar to Microwave digestion while also being safer than the latter.
FSWIR digestion is extremely fast, especially when compared with a microwave digestion workflow and also leads to accurate and precise results. FSWIR is also more versatile than microwave digestion with capabilities to handle sample sizes from 0.2-30g. To date a number of publications show no need for harmful reagents such as hydrofluoric and perchloric acids when breaking down even the most difficult matrices, resulting in better workplace safety. This yields a versatile system that champions greater automation, higher precision, improved sustainability, and better occupational safety.