Milestone’s unique Vent-and-reseal vessels are the foundation of our microwave digestion technology. 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 overpressure due to an out of control 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. Sample is not lost – 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.

Burst Disk

This method, used by some manufacturers, employs a simple burst disk in the cap which is designed to fail in an overpressure 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.

Self-regulating

The final method of venting is “self-regulating” and is used in the MultiPREP-41 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 their compact design allows for a large number of vessels to fit onto the rotor.  However, where Milestone’s Vent-and-reseal vessels are designed to release pressure in a controlled and predictable fashion, and then reseal, self-regulating vessels simply rely on the Teflon sealing plug inside the cap deforming to release pressure. The pressure at which these vessels vent can vary depending on sample type, reactivity, and even acid chemistry - which can affect digestion quality, reproducibility, and ultimately data quality.  Extreme throughput rotors are typically only recommended for processing easier to digest samples that are non-reactive, or of small sample size. Most recently, some manufacturers have applied the self-regulating approach to high pressure rotors where all of the above considerations still apply, though at higher pressure, the consequences of venting are more severe.

Please note: Sample throughput is not measured by the number of samples digested per run, but how quickly analytical data can be generated.  For laboratories that need to maximize throughput, Milestone recommends running samples in batches of 20 to 24.  This enables the user to prepare a second set of digestion vessels while the first set is being digested.  At the end of the microwave program, the second batch of samples can be digested, while the first set is processed for analysis. In addition, digesting 40 samples at once will significantly increase heating time and therefore overall run time, which will offset the productivity gain.

See Vent-And-Reseal Technology In Action!

To see Vent-and-reseal in action, watch this 30 sec. video taken by a camera inside an ETHOS EZ cavity during a digestion run. For clarity, only a single vessel has been placed inside the cavity. A large sample amount was digested in order to generate an overpressure and to force the vessel to vent. The vessel vents and reseals twice during the video: at 8 sec. and 12 sec. Yellow nitric acid vapors can be clearly seen as the pressure is released. These are removed via the exhaust duct and the run continues to completion – no loss of sample and no clean up required.