Fluoropolymers for digestion vessels
Table of Contents
For microwave digestion to be successful and the end result to meet the highest analytical requirements, it is not only necessary to understand the chemistry behind the digestion and control all reaction parameters, but also to carefully select the vessel material. Digestion vessels or pressure digestion vessels are one of the core components of modern microwave digestion systems in sample preparation and must meet a number of requirements.
- Microwave transparency ->In order to ensure a fast in situ infrared temperature measurement without delay, there must be no material between the sensor and the sample that absorbs the IR radiation itself. The material itself must be transparent to microwave radiation.
- Corrosion resistant ->Since aggressive reagents (acids) are used in wet chemical digestions, all materials in contact with the sample must be 100% resistant to chemicals.
- Pressure and temperature resistance -> In closed systems, higher temperatures of up to 260°C can be achieved. The containers must be able to withstand high pressures and temperatures to guarantee safe digestion.
- Ease of handling -> Considering the fact that more and more samples have to be processed in less time, easy and tool-free handling is advantageous.
- Contamination-free -> The risk of contamination and impurities must be minimized when working in trace and ultra-trace analysis. Therefore, optimized surface properties and reduced absorption or adsorption effects are of particular importance.
Vessel materials in general
A number of materials can be used as storage vessels for digestion solutions or as pressure vessels for digestions (e.g. glass, PP, PE, PFA, PTFE). With regard to the above-mentioned requirements, PTFE and quartz have become established. In addition, both Teflon and quartz glass are transparent to infrared radiation in a certain spectral range and allow monitoring of the internal vessel temperature with modern sensors.
Due to their resistance to chemicals, including acids such as hydrogen fluoride, fluoropolymers (PTFE, TFM™ , PFA) are particularly suitable for the manufacturing of digestion vessels. In addition, PTFE has insulating properties. The material is only heated indirectly via the sample, which minimizes material stress and eliminates the need for long cooling times.
Pressure resistance of digestion vessels
In general, commercially available glass materials or polymers are conceivable as materials for pressure vessels. However, the decomposition vessels themselves or the outer mantle must be made of a material that allows work at elevated pressures and temperatures. For this reason, quartz vessels/inserts or vessels made entirely of PTFE can be found. However, the use of quartz vessels carries the risk of breakage. Pre-damage, which can cause the vessel to break when subjected to pressure, is often not detectable or is very difficult to detect. Therefore, quartz inserts are usually only used optionally as inserts within the actual digestion vessels. Fluoropolymers, in contrast to other polymers, exhibit highest service temperatures of up to 270-300°C.
All other plastic materials have a reduced chemical resistance compared to fluoropolymers, which is why their use in decomposition vessels must be assessed as critical. PEEK (polyetheretherketone) in particular is attacked by nitric acid and can also absorb microwave radiation in unfavorable cases (conditional microwave transparency). This can lead to vessel overheating or even vessel melting.