Reactive membrane filter ring in microwave-assisted reaction

Robert A. Weker; Mary A. Sabolefski
Harvard School of Public Health, Boston

Chemical & Engineering News (19 Jun 1995) Vol. 73, No. 25, pp. 4.

SIR: In our laboratory, airborne metals such as lead are collected on a polytetrafluoroethylene (PTFE) membrane filter that has a special plastic ring to hold it taut. During method development, there was an unexpected explosion when the filter was extracted using concentrated nitric acid and microwave-assisted extraction. The experiment was performed with the microwave oven in a closed fume hood. The reaction vessel assembly was further protected by placing it in a rigid plastic container with a lid. As a result, there was no injury and only insignificant damage.

In another medium, R. N. Sah and R. O. Miller [Anal. Chem. 64, 230 (1992)] "identified an exothermic spontaneous nitric acid-hydrogen peroxide dissolution reaction of biological materials in closed vessels (pressure-tight) that may be induced with little microwave energy."

Microwave-assisted extraction is now a primary dissolution technique for many different matrices. However, its use can lead to rapidly increasing pressure with subsequent explosion. Many manufacturers of laboratory microwave digestion apparatus have taken preventative measures by employing a sealed PTFE reaction chamber that is retained in a special screw-cap plastic housing with a pressure-relief valve. The more elaborate microwave oven units also use a pressure-sensing probe that can turn off the applied microwave energy as buildup of pressure is detected.

Subsequent investigation showed that, in our procedure, the special plastic ring (polymethylpentene) continues to react with nitric acid after the microwave energy has been turned off. A PTFE membrane was carefully cut away from its plastic ring. The ring was placed in the reaction vessel, then 2.0 mL of concentrated nitric acid was added. The sealed assembly was placed in the microwave oven for one minute at 720 watts. Approximately 10 to 15 seconds after the one-minute cycle was complete, there was an explosion that was contained by the pressure-relief valve and the plastic container.

The PTFE membrane, when isolated from the plastic ring and carried through the above procedure, did not undergo a violent reaction. In this case, polymethylpentene, unlike its more common analogs polyethylene and polypropylene, is not inert when treated with nitric acid and exposed to microwave irradiation in an oven.

We will continue to use these filters in many of our air sampling methods, but a step will be added to the procedure to isolate the PTFE membrane by cutting it away from the plastic ring. We are examining now an all-PTFE filter that does not have a plastic ring and anticipate using both in our future study.

We hope this information will be valuable not only to groups that may utilize similar techniques, but especially to those laboratories using this method.

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