Anal. Chem. 1988, 60. 1624-1625
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not pass through the area of interest. EXPERIMENTAL SECTION The instrument used in this study was an IBM Instruments IR-44 Fourier transform infrared spectrometer, equipped with a Bruker A 590 infrared microscope. Only the beam passing through the microscope was examined. The microscope field of view is about 380 pm in diameter. Both the X and Y axes were examined by use of an adjustable rectangular aperture. In each case, a square, about 10 pm on a side, was created at the extreme end of an axis and 10 scans were taken. The square was then moved 10 pm further along the axis and the measurement repeated. This process was repeated until the entire axis had been scanned. Field position
Figure 1. Relationship for the X axls. 1200
4
,000
4
between instrument counts and field position
i
4001 \ ./
200
i! _.-. .-.-./.
0
I
I
10
\. .-.-.-._._. -. I I I L-
c
I
I
I
20
30
I
40
Field position
Figure 2. Relationship between instrument counts and field position
for the
RESULTS AND DISCUSSION The distributions obtained are plotted in the Figures 1and 2. Along the X axis, counts over background are seen in 6 of the 38 areas counted, or 15.8%; along the Y axis this becomes 7 out of 37, or 18.9%. If these percentages are considered to be the axes of an ellipse, its area will correspond to the area containing nearly all of the beam energy and will comprise only 2.3% of the total field. IR spectroscopists who deal with samples with inhomogeneous sample areas must be sure the area of interest is located at the center of the energy of the IR beam. This will be at the center of the field, if the instrument is properly aligned. It is important to be aware of how s m d a portion of the visible field corresponds to most of the energy of the beam and to its location. Since this kind of experiment is so easily performed, it should precede preparation of spectra from such samples, after every instrument realignment.
Y axis.
tribution is not important. When the sample area is not homogeneous, as may sometimes be the case with an IRmicroscope spectrometer, it is important that this distribution be ascertained, otherwise most of the energy of the beam may
Frank Levy Materials Laboratory IBM Corporation 5600 Cottle Road San Jose, California 95193 RECEIVED for review November 23, 1987. Accepted March 7, 1986.
Safety Concerns Associated with Wet Ashing Samples under Pressure Heated by Microwave Energy Sir: Microwave heating was f i t reported to speed digestion of samples by acids 12 years ago (1). The technique has been used in a variety of sample preparations since then (2-121, and it is rapidly gaining recognition as a useful tool in the analytical laboratory (13). Systems designed specifically for laboratory microwave digestion have been commercially available for at least 5 years. There are now hundreds of such systems in use worldwide for this application. These systems are designed to overcome deficiencies identified by researchers (4, 5 , 9, 1 1 , 12) who performed their initial work with microwave ovens manufactured for use in the home. The deficiencies of home systems include lack of cavity ventilation to remove acid fumes, poor resistance of the system to acid fumes, coarse power control, and short lifetime of the magnetron. But most especially, the lack of suitable digestion vessels had been a serious impediment to the widespread use of acid digestion by microwave heating. Microwave heating of acids occurs much more rapidly than conduction heating used to raise the temperature of acids using 0003-2700/88/0360-1624$01.50/0
a hot plate. The boiling point of an acid mixture is limited to the boiling point of the digesting acid mixture. If heating is done in a sealed pressure vessel, the maximum temperature will rise dramatically (14, 15). However, the pressure in a sealed container will increase quickly during microwave heating. Even with the development of methods that provide real-time monitoring of temperature and pressure, a pressure-relief system is recommended for safe operation. This is most important when sample decomposition products can contribute to internal vessel pressure. In addition to partial pressure of the acids, reaction products (such as COz) will rapidly increase pressure in sealed vessels. For this reason, it is hazardous to attempt sample digestion in sealed containers heated by microwave energy. A number of explosions resulting from overpressurizing sealed digestion containers have been reported (5, 16, 17). A recent article (18) reports that the technique of microwave heating acids in sealed containers is quite versatile. Since the article is printed in the "Aids for Analytical Chemists" section, 0 1988 American Chemical Society
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
