Vessel Venting Technology
Choice of vessel material is a key consideration in minimizing blank contribution for ultratrace analysis and long term performance. Milestone PTFE vessels are manufactured only from TFM, which is the highest density and thermally resistant form of PTFE available. Our vessels are pressed, not molded – which minimizes contamination during manufacturing. Unlike other manufacturers, Milestone does not use PFA, which has a limited temperature range (max. 210°C) and softens at high temperature, shortening its lifetime. Compare a Milestone vessel with any other manufacturer and it is immediately obvious that Milestone’s vessel quality and robustness is superior.
Vessels are placed inside PEEK shields for support. PEEK is the material of choice as it can be cooled in a water bath for faster cool dow after the digestion. Other manufacturers use Kevlar, which absorbs liquids and must be carefully dried when it gets wet: vessels with Kevlar shields cannot be water cooled.
Milestone’s Patented Vent-and-Reseal Mechanism
This patented (US Patent 5,270,010) technology eliminates vessel failure in the case of an out of control exothermic reaction. The illustration shows ‘vent-and-reseal’ in action: the vessel cap is held in place by a dome-shaped spring (1). In the case of over pressure due to a highly exothermic reaction, the spring is flattened, allowing the cap to lift up slightly (2) releasing excess pressure. Immediately the excess pressure is released, the spring reseals the vessel (3), and the digestion continues. There is no loss of sample and no clean up needed. The microwave program continues to completion and no re-run is required. While there are other methods of venting, only Milestone has ‘vent-and-reseal’ technology.
Other Methods of Venting
There are two other approaches to venting, though both have limitations compared to Milestone’s Vent-and-Reseal.
This method, used by some manufacturers, employs a simple burst disk in the cap which is designed to fail in an over pressure situation, instantly releasing all pressure in the vessel. When this happens, instantaneous boiling occurs, the sample contents are lost and the run has to be manually stopped. Significant clean up of the cavity is required, and corrosion of the cavity and internal components will occur. The illustration compares the burst disk approach with ‘vent-and-reseal’: the red line shows the complete loss of pressure when a burst disk fails, and the whole run is stopped. The blue line shows what happens with Milestone’s ‘vent-and-reseal’ technology: excess pressure is gently released at just over 30 bar, and the digestion continues to completion, with no loss of sample.
The final method of venting is “self-regulating” and is used in the MAXI-44 rotor, and high-throughput rotors supplied by other manufacturers. These extreme throughput rotors were developed to address the needs of labs that process larger sample volumes on a routine basis. Self-regulating vessels are very easy to assemble/disassemble and rely on the Teflon sealing plug inside the cap deforming to release pressure. Their compact design results in more moderate temperature and pressure capabilities, but allows for a large number of vessels to fit onto the rotor. These characteristics make self-regulating rotors ideal for easy to moderate applications such as clinical, environmental, food, etc. Most recently, some manufacturers have applied the self-regulating approach to high pressure rotors designed specifically for more challenging sample types. Since self-regulating vessels are designed to vent, incomplete or unsatisfactory digestions may be observed given that the pressure loss does not allow the mandatory temperatures necessary for complete digestion to be achieved.