Microwave digestion has been used for decades in chemistry, materials, and biology labs to facilitate the preparation of complex and difficult samples for subsequent analysis. Microwaves heat objects by causing vibrations in polar molecules within the material, hence, the not entirely accurate description of microwave cooking as an “inside-out” process.
Microwave safety can be explained intuitively through the ideal gas law, PV = nRT, where PV has units of energy but its component variables of pressure, volume, and temperature become operational limits. Microwave systems, which typically use acid as a digestion aid, generate high temperatures that, without correction or mitigation, cause the pressure to rise quickly.
More than 30 years ago, as a graduate student, I “discovered” that microwave heating could drive off the last remaining water molecules from a hydrated inorganic salt. The aftermath of this earth-shattering breakthrough taught me two important lessons.
First, no matter what you put into a microwave oven, if it’s a “chemical,” nobody will use the oven ever again to heat food or beverages. Second, samples you’d never expect to go “snap, crackle, pop” will indeed go “snap, crackle, pop” under uncontrolled microwave heating. Because a microwave interacts intimately with both the sample and its surroundings, microwave heating occurs much faster than convection heating, so samples not suitably protected and presented will bump, splatter, explode, or decompose.
Dr. Henry Albert, technical director at Parr Instrument Company (Moline, IL), notes the difference between conventional microwave heating and digestion-purposed laboratory models: “Laboratory digesters generate very high temperatures, but only inside the closed reaction vessel, whereas in an open container, the temperature is limited by the boiling point of the digestion aid. Accordingly, the pressure developed in a sealed vessel increases quickly during microwave heating.”
Pressure swings are a function of the digestion aid, its loading density, the sample size, and the sample decomposition products, for example, carbon dioxide. “Thus, sample vessels must not only be closed but incorporate some type of pressure relief mechanism,” Albert adds.
Following these two recommendations—using a closed reaction vessel incorporating pressure relief— eliminates most safety issues one is likely to encounter when using microwave equipment. Another way to state it is: Use only approved microwave digestion systems and always follow manufacturers’ recommendations on operation and sample presentation.
Parr has addressed these issues through the design of microwave digestion vessels incorporating a PTFE liner using a self-sealing, O-ring design. According to Albert, PTFE, a fluorinated polymer better known as Teflon, “eliminates the need to preload the liner closure to make the seal, as well as the effects of differential thermal expansion during the heating and cooling cycle, which can lead to sample loss. This provides for both an effective O-ring seal and the chemical inertness of an all-PTFE system.”
Parr vessels also employ a compressible relief disc that, with an O-ring seal, releases excessive pressure that might otherwise damage the vessel and oven and destroy the sample. When the pressure reaches approximately 1,500 psi, the relief disc compresses to the point that the support on the O-ring is lost and the O-ring blows out, releasing the pressure.
“In most cases, the vessel, except for the O-ring itself, will be reusable if promptly and carefully cleaned and inspected,” Albert says. This release mechanism is designed to protect against the relatively gradual pressure buildup associated with overheating the contents of the vessel. As the pressure in the liner increases and the relief disc is compressed, the retainer screw, which is normally flush with the top of the screw cap, begins to protrude above the top of the screw cap by approximately 1/32 of an inch for every 500 psi of extra pressure inside. “By monitoring the extension of this retainer screw, the user is able to get a visual indication of the pressure inside the vessel.”
Like all modern electronic products, including cars and kitchen appliances, laboratory instruments now incorporate sensors, microprocessors, and digital storage to great advantage. Elaine Hasty, senior chemist at CEM (Matthews, NC), believes that such “smart tools” are the key to the safe operation of microwave digesters. “Look for features like contactless temperature sensors that see through the vessel material and read the temperature of what you are digesting instead of the temperature of the reaction vessel.” This level of control promotes safe operation in the event—really, a when as opposed to an if scenario—the reaction begins approaching dangerous temperatures. “Such a system will have built-in methods and sensors to identify the kind of vessel being used and how many vessels are in the batch.” Smart digesters use this information to determine how to supply enough power to achieve clear digestion, without overshooting and burdening the vessels and system with unnecessary stress.
“Since we are all human and sometimes forget how to do things, having onboard videos will help train and retrain users on everything from vessel assembly to troubleshooting.”
Hasty notes that since reaction vessels are the heart of any microwave digestion system, “proper assembly, inspection, cleaning, and conditioning will ensure long and happy system life.” Microwave digestion is very tough on vessels, and while system developers specify materials that withstand high temperature and pressure stresses, which they experience during every run, vessels will, over time, show signs of age. “By following the manufacturers’ recommendations for conditioning, use, and cleaning, you can extend the life of your vessels and achieve better and more consistent digestion results.”
Following manufacturers’ recommendations and taking advantage of training on the proper use of microwave digesters can also help eliminate errors resulting from ignorance or what amounts to dangerous preconceptions—the notion that “I have a microwave oven at home; this is no big deal.”
CEM holds a class, Microwave Digestion 101, twice a year. The short course covers optimal conditions for organic and inorganic digestions, vessel inspection and assembly, microwave theory, and more. “We take students into the lab and spend a full day assembling vessels, digesting samples, and talking through vessel selection and acid combinations,” Hasty says. “When students graduate from this course, they are amazed at how much they learned, and once they apply that information in their labs, they rarely have difficulties.” For users who miss the CEM course or who need their memories jogged on best operational practices, the company provides more than 100 one-touch preset digestion methods, plus onboard tutorial videos.
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