Chemistry for Everyone
Products of Chemistry
Microwave Ovens Out of the Kitchen
Sarah L. Cresswell*
School of Applied and Molecular Sciences, University of Northumbria at Newcastle, Newcastle Upon Tyne NE1 8ST, UK;
[e-mail]
Stephen J. Haswell
Department of Chemistry, Hull University, Cottingham Road, Hull HU6 7RX, UK
We are all aware of the advantages of cooking our meals To understand how reactions can occur faster under micro-
in a microwave oven, but what of the advantages of using wave irradiation, we must first consider the heating process
microwave ovens to do laboratory experiments? Since domestic itself. When a sample is heated on a hot-plate, a convection
ovens first appeared in the early 1980s homes around the process takes place. The heat is transferred from the hot-plate
world have been preparing meals in far less time than was to the vessel and in turn from the vessel to the liquid inside
required by a standard convection oven and this technology it an inefficient method of heating. Microwave dielectric
was only a small step away from similar usage in laboratories. heating uses the ability of some liquids to transform electro-
In 1986, papers by Richard Gedye and co-workers at magnetic energy into heat and propagate chemical reactions
Laurentian University (1) and George Majetich and R. J. (26 ), removing the need to heat the container.
Giguere at the University of Georgia (2) demonstrated that If a liquid is exposed to microwave radiation, the micro-
the rate of a number of organic reactions could be increased waves induce rotation of the dipoles within the liquid, causing
using a commercially available microwave oven. These papers polar molecules to align and relax in the field of oscillating
formed the basis of an ever-increasing range of research electromagnetic radiation. Energy is dissipated from these
applications over the next 15 years. dipole rotations, which causes the liquid to become hot. In
In the early years, the majority of published work described such a way, the heat is produced within the liquid and not
digestion reactions in direct comparison to hot-plate digestion transferred from the vessel as in the hot-plate system (Fig. 1). In
methods (3 7). The samples that are now being studied range a microwave oven, the liquid is therefore often at a higher tem-
from environmental specimens (8), coal and ash materials (9),
and foodstuffs and oils (10) to metals in wine, beer, and other
alcoholic beverages (11). Today microwave radiation is used in
Hot-Plate Heating Microwave Heating
nearly all areas of chemistry. Synthetic reactions are carried
out to prepare organic (1), organometallic (12, 13), and in-
ab
organic materials (14) such as catalysts. Ashing (15 17), ex-
traction (18 21), and digestion (22 24) procedures can also
be routinely carried out.
´+ ´+
H H
O
Heating
´-
Microwave radiation lies between infrared and radio fre-
HOT PLATE
quencies of the electromagnetic spectrum at wavelengths from
1 cm to 1 m (corresponding to 30 GHz to 300 MHz). Because
Figure 1. Comparison of methods of heating. (a) Heat from the hot-
much of this range is dedicated to radar and telecommuni-
plate heats the glass base of the beaker and is transferred to the
cations, microwave ovens are restricted to 12.2 cm or 33.3 cm
solution by convection. The solution heats slowly. (b) Microwaves pass
(2.45 GHz or 900 MHz) in order to prevent interferences (25).
through the vessel walls directly into the polar liquid. Microwaves
Domestic microwave ovens are usually at a frequency of
interact with dipolar molecules, causing rotation and vibration of
2.45 GHz. the molecular bonds, which results in heating of the mixture.
900 Journal of Chemical Education " Vol. 78 No. 7 July 2001 " JChemEd.chem.wisc.edu
Chemistry for Everyone
perature than the vessel in which it is held. This efficient heat-
ing has been reported to lead to the increases in reaction rates,
increases in yields, and improved extraction efficiencies (27).
Microwave Ovens
Most early research work was carried out using com-
mercially available domestic microwave ovens, although
considerations for safety and the need to have controllable and
reproducible heating has led to the development of specially
designed equipment. The new microwave ovens were fitted
with temperature and/or pressure measurement devices,
which made it possible to monitor a reaction while it was
taking place. This, coupled with improved safety, allowed
advances in the field to continue.
In parallel with the development of improved microwave
ovens, the vessels used in microwave experiments have evolved
too. In many cases the vessels are sealed (closed vessels) and
pressure therefore plays a part in the reaction. Vessels need
to be transparent to microwaves for efficient heating to take
place and are usually made of polytetrafluoroethylene (PTFE)
or a similar material. They have been designed with a pres-
sure outlet valve or rupture membrane, which will vent if
safe pressure is exceeded (see Fig. 2). In line with the wide
range of reactions for which microwaves can be used, there
is a wide range of vessel types. Recent advances in the manu-
facture of microwave ovens and reaction vessels has led to a
Figure 2. Closed vessel for microwave extraction and digestion.
system in which each vessel can withstand up to 1500 psi
Courtesy of CEM (Microwave Technology) Ltd, UK.
and can be independently monitored for temperature
throughout the heating process.
Components of a Microwave Oven
A schematic diagram of a microwave oven is shown in
wave guide
Figure 3. Some components and features of a microwave are
mode
described below.
stirrer
magnetron
Magnetron  where microwaves are generated.
microwave
Waveguide  rectangular channel of metal with reflec-
cavity with
tive walls to allow transmission of microwaves from the
reflective walls
magnetron to the microwave cavity.
turntable
microwave
Microwave Cavity  internal space of the oven where
transparent
vessels
samples can be placed for heating. Usually contains a
turntable to ensure that each sample experiences the same
Figure 3. Schematic diagram of a microwave oven.
average heating. The cavity has reflective walls to prevent
leakage of microwaves and to increase the efficiency of
the oven.
Mode Stirrer  a reflective fan-shaped paddle, which
ensures that incoming energy is distributed evenly
throughout the cavity.
refluxing and
Door Interlocks  safety devices to prevent the door from
reagent addition
adapter
being opened while the microwave energy is on.
Safety
sample
As for all laboratory equipment, safety is an important
tube
issue. Since many reactions are performed in closed vessels
magnetron
and involve the heating of solvents beyond their boiling point,
there is a risk of explosion. Decomposition reactions, which
involve the use of acids, often produce gaseous by-products
wave guide
that can result in an increase in the pressure within a closed
vessel. For example, if one heats a sample having a large Figure 4. An open-vessel microwave system.
JChemEd.chem.wisc.edu " Vol. 78 No. 7 July 2001 " Journal of Chemical Education 901
Chemistry for Everyone
number of similar bonds (a polymer), when the bonds are the lengthy sample-preparation step rather than the speed of
degraded there is a rapid release of carbon dioxide. the chromatography limited the total analysis time (32). They
This type of problem led researchers to look at the pos- used low-power focused microwaves to reduce the time
sibility of developing microwave-assisted systems that were needed for quantitative isolation of analytes from 24 hours
open to the atmosphere and therefore unable to reach elevated to between 3 and 5 minutes. They were also able to sharply
pressures. These types of microwaves are often referred to as reduce the volumes of solvents they used.
 open-vessel systems . Since the formation of gaseous by- Over the past few years a number of papers have looked at
products is no longer an issue with this type of system, larger the use of microwave energy in extraction procedures. In all cases,
sample sizes are possible up to 1 2 g. An example of such the researchers concluded that faster reactions and higher
a open-vessel-type microwave system can be seen in Figure extraction efficiencies are possible with microwave-assisted
4. With this apparatus it is possible to perform a number of techniques than with conventional methodology.
reagent addition steps, with controllable microwave heating, It is not only in the field of extraction that microwaves
in six individual vessels. can be used for sample preparation; microwave digestion of
materials for inorganic elemental analysis is also an important
process. Sample digestion is usually performed by wet or dry
Microwaves in Synthetic Chemistry
decomposition, which releases and stabilizes the analytes of
Even with all these advances in technology the question interest as a liquid sample.
still remains, can we use the microwave heating effect to our In microwave digestion, closed vessels are again employed
advantage? In the field of combinatorial chemistry microwave in a batch method, although online  continuous-flow systems
ovens have found a niche. Combinatorial chemistry relies on have been developed for routine analysis of multiple samples
the fast preparation of a large number of similar compounds (33). Like microwave extraction procedures, microwave diges-
for screening. Ian Cotterill and colleagues at EnzyMed in Iowa tion decreases time requirements and increases controllability
City have developed a highly efficient microwave-assisted and reaction yields. However, the use of microwaves as a di-
combinatorial synthesis (MICROCOS) technology (28). Using gestion energy source depends upon the correct selection of
microwave energy as the power source, the system has enabled the acid or acid mixture and method employed.
a rapid increase in their high-throughput, automated, one- Metal speciation studies using microwave energy sources
step parallel synthesis of diverse substituted pyridines based have also been published. Paul Worsfold and colleagues of
on the Hantzsch reaction. They have shown that there are Plymouth University described an online microwave system
many advantages to using this technique, including improved for determining the oxidation states of selenium in biological
product recovery, shorter reaction times, and increased yields. and environmental samples (34). They first analyzed for
Some excellent reviews are available on this area of microwave- selenium(IV) and then with the assistance of rapid micro-
enhanced chemistry (14, 29). wave heating chemically reduced the selenium(VI) to its IV
Although a great deal of research is carried out on organic form. This allowed them to measure the total selenium
reactions, this is not exclusive. Inorganic-chemistry-based present and, by difference, to determine the selenium(VI)
research into the use of microwaves in the synthesis and present in samples.
modification of zeolite catalysts has been carried out by Colin Chun-mao Tseng and co-workers in Pau studied the
Cundy at UMIST (14). He found that the use of this technique leaching of mercury species from sediments and biomaterials
results in rapid reactions, which give pure products with good (35). Using a two-step procedure in open microwave vessels
crystal structure. He also found that in some cases microwave they were able to reduce the sample preparation time from
energy can enhance the selectivity of a reaction. 1 2 hours to just 2 4 minutes.
Microwaves in Sample Preparation Continuous-Flow Systems
The trend toward processing larger numbers of samples Most of the applications described here are batch processes.
for analysis has promoted the desire to speed up this process Each microwave vessel holds a discrete sample. In some cases
considerably (30). When looking at the extraction of organic the efficiency of microwave processes is limited by the re-
molecules from a sample matrix it is usual to attempt to extract stricted number of samples that can be digested or extracted
the analyte molecules into an organic solvent, usually under in any one run. Then these vessels must be left to cool
reflux conditions, as with Soxhlet extraction methodology. before analysis. If this process could be made continuous, so
Soxhlet extraction is both time consuming and solvent that samples could be extracted or digested in an online system
hungry. In addition it usually requires extraction times in and directly analyzed, the throughout of samples would be
excess of 6 hours and is not an easy process to automate. greatly increased.
Therefore there is an obvious advantage to introducing micro- Since the first step toward this goal was made by Strauss, at
wave extraction prior to chromatographic or spectroscopic CISRO in Australia (36 ), continuous-flow microwave systems
analysis in order to greatly reduce the total analysis time. In have been manufactured by CEM Corporation in North
1994, Jocelyn Pare and co-workers described the fundamental Carolina and more recently by Milestone. Using this instru-
physical phenomena of this process and showed how micro- mentation, we developed a method for the online extraction
waves could be used in extractions from plant and animal of polycyclic aromatic hydrocarbons (PAHs) from sediment
tissues, water, soil, consumer products, and cosmetics (31). (37). Our results show that our extraction is as effective as
Looking at the preparation of solid samples for compo- and in some cases more effective than the U.S. Environmental
sitional analysis, Joanna Szpunar and colleagues found that Protection Agency (EPA) batch microwave method (38).
902 Journal of Chemical Education " Vol. 78 No. 7 July 2001 " JChemEd.chem.wisc.edu
Chemistry for Everyone
Ultimately the aim is to produce a system that can be run in Israel (43). When irradiated with microwaves the bilayer
without constant supervision. Complete automation of the became depleted, and this effect could not be repeated using
sample extraction and analysis steps would lead to a substan- a conventional thermal heat source.
tial increase in sample throughput and accuracy and would It is also possible to influence the retention time of
significantly reduce the chance of sample contamination. components on a silica diol column held within a microwave
The disadvantage of making a continuous-flow system field (44), an effect that was attributed to nonthermal micro-
at the moment is that most magnetrons work on a duty cycle. wave properties. Although these findings are not conclusive,
This means that the duration of microwave production is there does appear to be evidence to support the existence of
varied rather than the power of the microwaves. For example, nonthermal microwave effects. More research is obviously
to obtain 50% power, the magnetron would be on full power needed.
for 10 seconds and then off for 10 seconds on average, 50%
power. This is a problem for continuous-flow samples because
Microwave Ovens in Undergraduate Laboratories
if a sample happens to be passing through the microwave cavity
during the power-off section of the duty cycle, it will not be Laboratory classes are an important part of any under-
irradiated. graduate degree course and should provide students with the
Another advantage of continuous-flow systems is a reduc- chance to undertake experiments that mirror industrial reac-
tion in the volumes of solvent required. During the extraction tions. In microwave chemistry there are a number of reactions
of PAHs we used 40% less solvent than is required for the that lend themselves to undergraduate experiments (45 48).
EPA batch method. This may be significant if a large number For example, Ng et al. (49) used a domestic microwave oven to
of samples are analyzed or if regulations governing the disposal demonstrate the advantages of microwave curing of polymers.
of organic solvents are an issue. They used Fourier-transform infrared spectroscopy (FTIR)
to compare the percentage curing of methyl methacrylate by
thermal and microwave oven methods. They concluded micro-
Process Applications
wave curing offers faster curing times, improved efficiency,
Microwave instruments are starting to find uses in other, and enhanced properties of the polymer.
less obvious areas for example, process control. Theisen (39) The use of microwaves to enhance the rate of organic
has produced a low-power microwave sensor capable of making reactions is well documented by Bose (50) and co-workers, who
density and concentration measurements, which is currently have devised a number of undergraduate laboratories that
in use as a process-control mechanism in the sugar industry. demonstrate the advantages of microwave heating. Students are
Another example is the use of high-power microwave able to carry out synthetic organic reactions in open vessels
energy to cure resins. Boey s group at Nanyang Technological in domestic microwave ovens. Promising results have been
University in Singapore (40) compared microwave curing achieved with a number of reactions, including the Bischler
with conventional curing of a thermosetting resin and found Napieralski reaction, the Wolff Kishner reduction, and free-
that the microwave procedure was faster and easier to control. radical dehalogenation reactions. These reactions come un-
Like Theisen s sensor, its main advantage, however, was that der their heading of MORE (Microwave-Induced Organic-
it could be operated within a process environment. Reaction Enhancement) chemistry techniques. Using MORE,
Since microwaves are able to selectively heat materials Bose et al. developed and tested a number of meaningful,
on the basis of their structure, Robert Osiander and co-workers safe, and inexpensive synthetic experiments for undergraduate
are attempting to develop nondestructive thermographic students.
evaluation techniques for the analysis of materials (41). This will
work because the defects in a material and the rest of the
Conclusions
sample matrix will heat at different rates, allowing identifi-
cation of the presence, quantity, and distribution of defects. Microwave ovens have successfully made the transition
from our kitchens to laboratories and have been used to en-
hance many reactions. But the story does not end here; re-
Nonthermal Effects
search in the many fields of microwave chemistry will con-
No review on microwave chemistry would be complete tinue. Already this year some 45 or so papers have been pub-
without mention of nonthermal effects. As anyone who owns lished in this field. These include  Automated Microwave
a mobile phone knows, there are numerous articles in the Digestion of Certifiable Color Additives for Determination of
press about the possibility of microwaves causing damage to Mercury by Cold Vapor Atomic Absorption Spectrometry (51)
brain tissue. Evidence for the existence of nonthermal effects and  Microwave Assisted Solid Support Synthesis of Novel
has been mounting over the past 5 years or so. 1,2,4-Triazolo[3,4-b]-1,3,4-thiadiazepines As Potent Anti-
Work at Ohio State University by Sheryl Barringer and microbial Agents (52). The relationship between microwave
co-workers (42) showed that the rate of heating of oil water power applications and spectroscopic measurements in the
mixtures and emulsions depends upon the dispersion of the microwave region seems also to be an exciting area for future
two liquids. This was attributed to the increased power ab- development.
sorption at the large number of interfaces present, and while
interesting, it may or may not be a real nonthermal effect.
Acknowledgment
A search for specific local microwave interaction in a
single amphiphilic bilayer assembled on silicon was carried out Figure 2 was produced with kind permission of CEM
by Rirka Moaz and co-workers at the Weizmann Institute (Microwave Technology) Ltd, UK.
JChemEd.chem.wisc.edu " Vol. 78 No. 7 July 2001 " Journal of Chemical Education 903
Chemistry for Everyone
Literature Cited 26. Mingos, D. M. P.; Baghurst, D. R. Chem. Soc. Rev. 1991, 20,
1 47.
1. Gedye, R.; Smith, F.; Westaway, K.; Ali, H.; Baldisera, L.; 27. Whittaker, G. Microwave Chemistry Pages; http://www. ed.ac.uk/
Laberge, L. Tetrahedron Lett. 1986, 27, 279 282. ~ah05/microwave.html (accessed Apr 2001).
2. Giguere, R. J.; Bray, T. L.; Duncan, S. N.; Majetich, G. Tetra- 28. Cotterill, I. C.; Usyatinsky, A. Y.; Arnold, J. M.; Clark, D. S.;
hedron Lett. 1986, 28, 4945 4948. Dordick, J. S.; Michels, P. C.; Khmelnitsky, Y. L. Tetrahedron
3. Abu-Samra, A.; Morris, J. S.; Koirtyohann, S. R. Anal. Chem. Lett. 1998, 39, 1117 1120.
1995, 47, 1475. 29. Caddick, S. Tetrahedron 1995, 51, 10403 10432.
4. Abu-Samra, A.; Morris, J. S.; Koirtyohann, S. R. Trace Subst. 30. Kingston, H. M. SamplePrep Web; http://www.sampleprep.
Environ. Health 1975, 9, 297. duq.edu/sampleprep/ (accessed Mar 2001).
5. Abu-Samra, A.; Koirtyohann, S. R.; Morris, J. S. Anal. Chem. 31. Pare, J. R. J.; Belanger, J. M. R.; Stafford, S. S. Trends Anal.
1975, 47, 1475 1477. Chem. 1994, 13 (4), 176 184.
6. Kingston, H. M.; Jassie, L. B. Anal. Chem. 1986, 58, 2534 32. Szpunar, J.; Schmitt, V. O.; Donard, O. F. X.; Lobinski, R.
2541. Trends Anal. Chem. 1996, 15 (4), 181 187.
7. Kingston, H. M.; Jassie, L. B. In Introduction to Microwave 33. Burguera, M.; Burguera, J. L. Anal. Chim. Acta 1998, 366
Sample Preparation: Theory and Practice; Jassie, L.B.; Kingston, (1 3), 63 80.
H. M., Eds.; American Chemical Society: Washington DC, 34. Pitts, L.; Worsfold, P. J.; Hill, S. J. Analyst 1994, 119, 2785
1988; p 93. 2788.
8. Martinez-Garcia, M. L.; Zubieta, A. C.; Lorenzo, S. M.; 35. Tseng, C. M.; Garraud, H.; Amouroux, D.; Donard, O. F.
Lopez-Mahia, P.; Rodriguez, D. P. Anal. Lett. 1999, 32, 139 X.; de Diego, A. J. Automat. Chem. 1998, 20(4), 99 108.
157. 36. Cablewski, T.; Faux, A. F.; Strauss, C. R. J. Org. Chem. 1994,
9. Lachas, H. Analyst 1999, 124, 177 184. 59, 3408 3412.
10. Pare, J. R. J.; Matini, G.; Belanger, J. M. R.; Li, K.; Rule, C.; 37. Cresswell, S. L.; Haswell, S. J. Analyst 1999, 124, 1361 1366.
Thibert, B.; Yaylayan, V.; Liu, Z.; Mathe, D.; Jacquault, P. J. 38. Microwave-Assisted Solvent Extraction; EPA Method 3546; U.S.
AOAC Int. 1997, 80, 928 933. Environmental Protection Agency: Washington, DC, 1996.
11. Lopez, F. F.; Cabrera, C.; Lorenzo, M. L.; Lopez, M. C. Sci. 39. Theisen, K. H. Zuckerindustrie 1999, 124, 157 158.
Tot. Environ. 1998, 220, 1 9. 40. Chia, L. H. L.; Jacob, J.; Boey, F. Y. C. J. Mater. Process.
12. Ali, M.; Bond, S. P.; Mbogo, S. A.; McWhinnie, W. R.; Technol. 1995, 48 (1 4), 445 449.
Watts, P. M. J. Organomet. Chem. 1989, 371, 11 13. 41. Osiander, R.; Spicer, J. M. W.; Murphy, J. C. Mater. Eval.
13. Dabirmanesh, Q.; Roberts, R. M. G. J. Organomet. Chem. 1995, 53, 942 948.
1993, 460, C28 C29. 42. Barringer, S. A.; Ayappa, K. G.; Davis, E. A.; Davis, H. T.;
14. Cundy, C. S. Collect. Czech. Chem. Commun. 1998, 63, 1699 Gordon, J. J. Food Sci. 1995, 60, 1132 1136.
1723. 43. Moaz, R.; Cohen, H.; Sagiv, J. Langmuir 1998, 14, 5988
15. Zhang, H.; Dotson, P. Commun. Soil Sci. Plant Anal. 1994, 5993.
25, 1321 1327. 44. Haswell, S. J.; Howarth, N. Anal. Chim. Acta 1999, 387 (2),
16. Gillain, G. Talanta 1982, 29, 651 654. 113 120.
17. Dunemann, L.; Meinerling, M. Fresenius J. Anal. Chem. 1992, 45. Bari, S. S; Bose, A. K.; Chaudhary, A. G.; Manhas, M. S.;
342, 714 718. Raju, V. S.; Robb, E. W. J. Chem. Educ. 1992, 69, 938 939.
18. Sarawati, R.; Vetter, T. W.; Watters, R. L. Analyst 1995, 120, 46. Nelson, D. A.; Devin, C.; Hoffmann, S.; Lau, A. Abstracts of
95 99. Papers, Part 1, 213th National Meeting of the American
19. Llompart, M. P.; Lorenzo, R. A.; Cela, R. F.; Li, K.; Belanger, Chemical Society, San Francisco, Apr 13 17, 1997; Ameri-
J. M. R.; Pare, J. M. J. J. Chromatogr. A 1997, 774, 243 251. can Chemical Society: Washington, DC, 1997; CHED 101.
20. Tomaniova, M.; Hajslova, J.; Pavelka, J.; Kocourek, V.; 47. Freeman, R. G.; McCurdy, D. L. J. Chem. Educ. 1998, 75,
Holadova, K.; Klimova, I. J. Chromatogr. A 1998, 827, 21 29. 1033 1034.
21. Xiong, G. H.; Tang, B. Y.; He, Z. Q.; Zhao, M. Q.; Zhang, 48. Banik, B. K.; Barakat, K. J.; Wagle, D. R.; Manhas, M. S.;
Z. P.; Zhang, Z. X. Talanta 1999, 48, 333 339. Bose, A. K. J. Org. Chem. 1999, 64, 5746 5753.
22. Zischka, M.; Kettisch, P.; Kairnrath, P. Atom. Spectrosc. 1998, 49. Ng, L. T.; Chia, L. H. L. Polym. Int. 1999, 48, 952 955.
19, 223 227. 50. Bose, A. K.; Manhas, M. S.; Banik, B. K.; Robb, E. W. Res.
23. Perez-Cid, B.; Lavilla, I.; Bendicho, C. Anal. Chim. Acta 1999, Chem. Intermed. 1994, 20, 1 11.
378, 201 210. 51. Hepp, N. M.; Cargill, A. M.; Shields, W. B. J. AOAC Int.
24. Sahuquillo, A.; Rubio, R.; Rauret, G. Analyst 1999, 124, 1 4. 2001, 84, 117 122.
25. Kappe, C. O. Microwave Enhanced Chemistry; http://www-ang. 52. Kidwai, M.; Sapra, P.; Misra, P.; Saxena, R. K.; Singh, M.
kfunigraz.ac.at/~kappeco/microwave.htm (accessed Mar 2001). Bioorg. Med. Chem. 2001, 9, 217 220.
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