Anal. Chem. 1088, 6 0 , 1625
it appears other researchers are encouraged to use this technique for sample preparation. However, heating acids with microwave energy in sealed vessels having no pressure-relief mechanism is very dangerous. Data on temperatures and pressures developed during microwave digestion of biological materials have been published (12,13,19) but were not cited. Based on tests done at CEM Corp., vessel contents can reach pressures of over 1.4 MPa (200 psi) before failure. Two modes of vessel failure were observed. Most often, threads on the cap fail which allows pressure to propel the cap upward. Less frequently the vessel side wall will distend enough to cause a vertical tear which vents the vessel contents. The forces and fumes generated in such failures are cause for great caution (17). Savillex Corp. has provided two types of 60-mL containers made of Teflon PFA in the past. The vessels use different thread designs and differ widely in their ability to retain pressure. It is likely that the vessel described (18)could not hold 1.4 MPa (200 psi). Digestion vessels with built-in pressure relief mechanisms have been commercially available for several years and work quite well for acid digestions (19). However, microwave ovens designed for home use have durability and safety problems when used with vessels designed to release excess pressure. Acid fumes released into a home microwave oven will quickly attack exposed metal parts in the wave guide and in the control electronics. Home microwave ovens have been reported ( 5 , 1 3 )to fail in a matter of months due to damage of the electronics. More seriously, the wave guide, cavity walls, and the magnetron cooling fan bearings can be attacked and corroded. If the cavity or wave guide walls are breached by corrosion, microwave energy can leak into the laboratory. This can expose the user to harmful levels of microwave radiation (20). If the cooling fan bearings fail or develop high resistance to turning, the magnetron will overheat. In addition, digestion of small samples in the microwave cavity will cause much of the applied microwave energy to be reflected back to the magnetron causing it to overheat. Such overheating will cause variations in power output and reduce magnetron working life from years to months. Predicting how long to digest a sample is impossible when power output varies in this manner. We urge readers of the Analytical Chemistry article (18) not to attempt routine acid digestions or indeed microwave
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heating of any liquids in a sealed container which does not have a designed safety relief mechanism. The advantages of rapid closed vessel microwave digestion have been demonstrated and safe techniques are available (20).
LITERATURE CITED (1) Abu-Samra, A.; Morris, J. S.; Koirtyohann, S.R. Anal. Chem. 1975, 4 7 , 1475. (2) Barren, P.; DavMowski, L. J., Jr.; Penaro, K. W.; Copeland, T. R. Anal. Chem. 1978 50, 1021. (3) Nadkarni, R. A. Anal. Chem. 1984, 56, 2233. (4) White, R. T.; Douthlt, G. E. J . Assoc. Off. Anal. Chem. 1985, 68(4), 766. (5) Matthes, S. A.; Farrell, R. F.; Mackie, A. J. Tech. Prog. Rep.-US., Bur. Mines 1983. No. 120. (6) Fernando, L. A,; Heavner, W. D.;Gabrielli, C. C. Anal. C h m . 1986, 58, 511. (7) Fischer, L. E. Anal. Chem. 1988, 58, 261. (6) Lamothe, P. J.; Fries, T. L.; Consul, J. J. Anal. Chem. 1986, 58, 1881. (9) Copeland, T. R. Work Assignment for Office of Solid Waste US EPA, June 1966. (10) Westbrook, W. T.; Jefferson, R. J. J . Microwave Power 1986, 21,25. (11) Jassie, L. E.; Kingston, H. M. 1985 Pittsburgh Conference Abstracts, Paper 108A. (12) Kingston, H. M.; Jassie, L. E. Anal. Chem. 1986, 58, 2534. (13) “Symposium on Microwave Techniques”, Twenty-Fifth Eastern Analytical Symposium, Oct 1966, New York; “Symposium on Microwave Techniques,” Twenty-Sixth Eastern Analytical Symposium, Sept 1987. (14) Gordon, C. L. J . Res. Natl. Bur. Stand. (U.S.) 1943. 30, 107, Research Paper 1521. Gordon, C. L.; Schlecht, W. G.; Wichers, E. J . Res. Natl. Bur. Stand. (U.S.) 1944, 33, 457, Research Paper 1622. Gedye, R.; Westaway, K.; Smith, F. Twenty-Sixth Eastern Analytical Symposium, Sept 1987, Paper 58. Gilman, L. 8.; Grooms, W. G.; Littau, S. E.; Revesz, R., GEM Corp. Dersonal comrnunicatlon and observation. Aysoia, P.; Anderson, P.; Langford, C. H. Anal. Chem. 1987, 59, 1582. (19) Revesz, R.; Hasty, E. 1967 Pittsburgh Conference Abstracts, Paper 252. (20) Kingston, H. M.; Jassie, L. E. “Safety”, in Introduction to Microwave Sample Reparation: Theory and Practice; ACS Reference Book Series; Kingston, H.M.. Jassie, L. B., Eds.; American Chemical Society: Washington, DC, in press.
Lee Gilman Will Grooms* CEM Corporation P.O. Box 200 Matthews, North Carolina 28106 RECEIVED for review December 1, 1987. Accepted February 16, 1988.
Response to Safety Concerns Sir: We are in agreement with the observation made by Grooms and Gilman with respect to the hazards of the use of microwave energy for wet ashing. This issue was not specifically addressed in our note and an ambiguity may have arisen (1). There are two approaches to safe use of microwaves. One is to adapt the microwave for high pressure. The other is to explore low-pressure treatment. Our note (11, in fact, dealt with the second strategy. It is an unfortunate source of confusion, perhaps, that Savillex Corp. supplies two vessels. The one we use (catalog no. 0102) is not pressure-tight. This risks loss of volatile sample components, but the results show this to be unimportant for the metal ions in question. A hazard may also arise from corrosion of the microwave by acid fumes that escape non-pressure-tight vessels. That is the reason for enclosure of sample vessels in large-mouth, largevolume, screw-cap microwavable dishes. Such containment is a critical aspect. We would underline our agreement that both explosion of pressurized vessels and corrosion of control elements of a 0003-2700/88/0360-1625$01.50/0
microwave present serious possible hazards. Analysts should remain cautious. However, we believe that research directed toward reducing cost factors while still providing for safe operation is useful.
LITERATURE CITED (1) Aysola, P.; Anderson, P. W.; Langford, C. H. Anal. Chem. 1987, 59, 1582.
Prasad Aysola Perry W. Anderson Cooper H. Langford* Departments of Chemistry and Biological Sciences Concordia University Montreal, Quebec Canada H3G 1M8 RECEIVED for review March 18, 1988. Accepted March 24, 1988 0 1988 American Chemical Society
