Common-sense safety precautions include safe handling of solvents and acids. “But users need to pay a certain degree of respect to the corrosive, high-temperature, high-pressure operation of digesters,” says Johan Nortje, product manager at Milestone (Shelton, CT). “Safety, above all other factors, influences microwave digester [MD] design and construction.”
MDs are closed systems, which enables reaching temperatures much higher than the boiling point of the digestion medium (usually concentrated acid). Consequently, construction from high-quality stainless steel is essential to resist mechanical stresses and the corrosive effects of heat and acid. Milestone, for example, uses no polymers either in critical contact surfaces or in components.
Probes that monitor temperature and software that controls heating are essential for safe operation and method development/consistency. Rotors come in many configurations, and are normally selected on the ability to speed digestion. However, rotors play a role in safety as well because their design affects heat generation.
“Temperature is the number-one safety factor,” Nortje explains, “because the pressure generated inside the vessel is directly proportional to temperature.”
Digestion methods consist of two steps: ramp to temperature, and hold at temperature. Heating should be gradual, particularly on samples containing significant carbon matrix, due to the possibility of the sample going exothermic. Rapid digestion of carbon contributes to both temperature and pressure.
Nortje advises users to consider MDs equipped with exhaust hoses for venting corrosive gases after a run to an acid trap or fume hood. Another suggestion is to begin method development on very small samples— no larger than about 100 mg, preferably much lower. This provides a good estimate of what to expect from much larger samples in the event that the sample goes exothermic. If temperature is difficult to control at 100 mg, it will likely lead to unsafe conditions at half a gram or one gram.
Microwave digesters from reputable suppliers will operate well within safe conditions, provided users take these precautions. Moreover, vendor sites and online searches can provide excellent guidance on specific methods in the way of case studies and application notes. While “nothing new under the sun” may not strictly apply, finding a method that is both safe and effective for your samples will not be difficult.
Safety by design
All commercial MDs undergo safety-based design and manufacturing. Systems employ mechanical interlocks to prevent vessel breakage. Many units employ access rights that prevent untrained or otherwise unapproved individuals from operating the equipment. Software controls method parameters such as digestion temperature and duration, temperature ramp-up, and cooling. Firmware or software controllers perform diagnostic checks before a run to ensure that temperature and pressure sensors are working, and that interlocks are performing as specified. After a run, the control panel alerts operators when opening the unit is safe. Opening before reaching safe temperatures and pressures is impossible with Anton Paar digesters due to a hard-lock safety mechanism.
“Thanks to these safety features, despite the high temperatures and pressures they generate, microwave digesters may be located safely anywhere in a lab,” says Reynhardt Klopper, product specialist for microwave synthesis and digestion at Anton Paar USA (Ashland, VA). While fume hoods are not required for operation, the vent hose for releasing gases should be vented to a hood. “The main caution, since microwave digesters contain magnetrons, is that workers with implantable electronic devices should keep their distance.”
Klopper advises laboratory managers to pay special attention to safety-related certifications. European manufacturers apply the CE logo to goods they sell globally, indicating that these products meet strict European Union safety standards. Anton Paar digesters are produced under a somewhat stricter German government standard, GS (Geprüfte Sicherheit or “Tested Safety”). “GS indicates that an independent third-party organization, in this instance Intertek, has confirmed safe design and operation,” Klopper says. “GS certification provides added peace of mind.”
For additional resources on microwave digesters, including useful articles and a list of manufacturers, visit www.labmanager.com/microwave