Focus
Microwave Dissolution Over the next few years it volatile elements and less appears likely that micro chance of contamination wave-oven-based proce from the environment or Microwave ovens are poised dures will replace some of from the larger reagent vol to replace thermal digestion the conventional hot plate umes that must be used in and other thermal digestion open-vessel work. Accord methods in analytical laboratories techniques that have been ing to Jassie, "People who used for decades in chemi ι., . spend lots of time on hot cal laboratories for acid dis plate techniques will find solution of hard-to-dissolve that microwave dissolution samples prior to elemental is the answer to their analysis. Researchers are prayers." finding that for many sam In their work, the N B S ple types, microwave tech CEM researchers use spe niques are safer and faster cially designed Teflon ves than conventional acid dis sels with screw-on caps, solution techniques and are which are now available less prone to cause contami commercially, to digest nation of samples. samples with nitric, hydro The improvement of mi chloric, or other acids. Ac crowave dissolution proce cording to Jassie, "We're dures is the goal of an ongo trying to eliminate the need ing joint research project of for dangerous and expen the National Bureau of sive perchloric acid proce Standards (NBS) and CEM dures. By raising pressures Corporation (Indian Trail, in the closed containers, we N.C.), a manufacturer of can achieve high internal microwave equipment for container temperatures that research laboratories. Ulti increase the dissolution effi mately the NBS-CEM re ciency of acids many-fold." searchers hope to develop The vessels used at NBS Lois Jassie places a carousel loaded with closed Teflon specific guidelines for the are outfitted with pressure vessels into a research microwave oven. Specimens con rapid dissolution of many relief valves for safety. To tained in the vessels will be dissolved and ready for analy different types of samples— seal the vessels, caps are sis within 10 minutes. Photo courtesy of NBS from biological specimens, ^^^_ torqued to strict specifica such as bone and liver, to tions. The researchers rec botanical, geological, metal ommend particular contain lic, and glassy substances. er pressure limits and equip ment configurations that prevent the The techniques under study at NBS time to reach completion than do de uncontrolled venting of superheated make it possible to speed acid dissolu compositions limited by the boiling acids. They have developed methods tion procedures by combining the rap point of acids. In addition, substances to help chemists predict the internal id heating ability of microwave ovens that ordinarily would not be decom vessel temperatures that are attained with the higher temperatures and posed by these acids at their normal with various microwave power set pressures possible when samples are boiling points react at elevated tem tings, thus eliminating guesswork heated in closed containers. Using the peratures and pressures." about appropriate operating condi new microwave techniques, research The use of closed-vessel techniques tions. And they are presently experi ers Howard M. Kingston (NBS) and is more feasible with microwave menting with a fiber-optic tempera Lois B. Jassie (CEM) have completed sources than with hot plate and other ture-sensing system that may be able digestions in 10 minutes that take four thermal sources, in part because the to monitor vessel temperatures more hours to four days by conventional hot Teflon vessels used to prevent con accurately and efficiently than the plate techniques with open-vented tamination of samples are microwave thermocouples they have been using. beakers. transparent and are thus not directly One scientist who appreciates some According to Kingston and Jassie heated by microwaves. With thermal of the advantages of microwave disso (Anal. Chem. 1986,58(12], 2534-41), sources, on the other hand, Teflon lution over conventional techniques is "The rate of reaction and efficiency of vessels are heated directly, sometimes Edward F. Sturcken of the E.I. du acid decomposition increase dramati to the point of melting. Closed-vessel Pont de Nemours Savannah River cally with temperature. Reactions car techniques afford a number of advan Laboratory, who uses microwave tech ried out at elevated pressures and tages over open-vessel digestion tech nology for sample preparation of hightemperatures require considerably less niques, including better retention of 1424 A · ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
0003-2700/86/A358-1424$01.50/0 © 1986 American Chemical Society
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CIRCLE 51 ON READER SERVICE CARD
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CIRCLE 138 ON READER SERVICE CARD 1426 A · ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
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Focus ly radioactive samples in hot cells. Ac cording to Sturcken, "Determining a weight percent solids used to take 24 hours in the cell, but with the mi crowave system I've got down there now we're doing them in 15-20 min utes. With cell time running $95 an hour, it doesn't take very long to pay for the investment." "For certain sample types, micro wave digestion is definitely at present the best way to achieve dissolution," says Steve A. Matthes of the U.S. De partment of Interior Bureau of Mines, who originated the idea of using closed vessels in microwave ovens to obtain elevated temperatures and pressures. In geology and metallurgy, Matthes explains, microwave dissolution is es pecially advantageous for solders, non-iron-based superalloys, and sili cate materials (with the possible ex ception of high-quartz rocks), includ ing high-calcium silicates such as ce ment and phosphate rock. According to Matthes, "It used to take two or three days using a heated sand bath or hot plate to take superalloy samples into solution, but we can now do that on a microwave in about five minutes. In addition," he says, "the Bureau of Mines used to have a huge backlog of slags [silicate samples] because we didn't have a way to pro cess these fast enough for ICP [induc tively coupled plasma] or atomic ab sorption analysis. But when we devel oped the microwave method, all of a sudden we had same-day turnaround. Using microwave dissolution, we can analyze a set of twelve slags, from weighing the samples to running them, in about an hour." In a session on microwave dissolu tion at the recent Eastern Analytical Symposium, Matthes also discussed methods for doing chemical fusions using a newly developed microwave muffle furnace that will be marketed by CEM. The muffle furnace contains firebrick lined with silicon carbide, which is a strong microwave absorber. According to Matthes, the new fur nace can achieve very high tempera tures in just a few minutes and is much more energy efficient than a conventional muffle furnace. Will the microwave oven eventually replace the hot plate for analytical di gestions? R. Thomas White, who uses microwave dissolution in his laborato ry at R.J. Reynolds Tobacco Compa ny, feels that "most hot plate tech niques will eventually be replaced, just as soon as people see some meth ods out there and start to become aware of what microwave ovens can do. Education is the key." "It's hard to beat a hot plate," adds
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Matthes, "but the microwave has the advantage of taking up very little space, using very little energy, and be ing very versatile. I don't think the microwave oven will supplant the hot plate, partly because the microwave oven has limited space inside and re quires special vessels that are fairly expensive, especially when compared with beakers. So it won't eliminate beakers, but I think it will make a siz able dent in hot plate usage." "The ability to use sealed vessels is very important," says Sturcken, "be cause the environmental people are not going to continue to allow chem ists to boil acids in their hoods for days. Everybody is looking at what
Detection of Hydrazines in Ambient Air Microsensors, passive dosimeters, and photoionization monitors can help protect workers from toxic hydrazine vapors Hydrazine and monomethylhydrazine, two toxic components of rocket fuel, can pose a health hazard to peo ple involved in various rocket-related activities, including space shuttle and missile site operations. An effective method for their detection at trace levels is thus essential to personnel safety, and researchers at the Naval Research Laboratory (NRL) are work ing to develop new methods for deter mination of both hydrazine and monomethylhydrazine in ambient air. The project, sponsored by the National Aeronautics and Space Administra tion (NASA), involves the use of pat tern recognition techniques to detect hydrazine vapors using microsensors, development of passive dosimeters, and evaluation of several new technol ogies for low-level hydrazine detec tion. The National Institute of Occupa tional Safety and Health (NIOSH) re cently recommended lower threshold level values (TLVs) for hydrazine and monomethylhydrazine, explains Susan Rose-Pehrsson, NRL's principal in vestigator for the project. Because current commercially available instru ments, which use electrochemical sen sors or chemically impregnated paper tape, cannot reliably detect hydra zines at the new levels, Rose-Pehrsson and her co-workers are exploring new methodology and working with instru-
CIRCLE 193 ON READER SERVICE CARD 1428 A ·
you're stacking and venting, and I can see where being able to do analyses in sealed vessels in a reasonably rapid time should have a tremendous poten tial. I feel strongly that we've got to keep our chemistry in the bottles. I think the future is in that direction." Industrial, academic, or government scientists interested in learning more about the NBS-CEM research should contact Howard M. Kingston or Lois B. Jassie at NBS, Chemistry A353, Gaithersburg, Md. 20899 (301-9754136). A textbook on the subject of microwave dissolution, to be pub lished by the American Chemical So ciety, is presently in preparation. S.A.B.
ANALYTICAL CHEMISTRY, VOL. 58,
NO. 14, DECEMBER
1986
ment manufacturers to improve the sensitivity of commercially produced detection systems. Chemiresistors, microfabricated sensors that rely on chemical coatings for sensitivity and selectivity, are be ing studied by Rose-Pehrsson for use in the trace detection of hydrazines. A chemiresistor is a set of interdigital
New methodology is being explored to improve the sensitivity of commercial detection systems.
electrodes coated with an organic semiconductor. When a fixed poten tial is applied, it functions as a resistor that changes conductance in the pres ence or absence of a vapor. Jay Grate and co-workers in NRL's microsensor program are working closely with Rose-Pehrsson to develop chemiresis tors specific for hydrazine and mono methylhydrazine. Because the new NIOSH-recommended levels are quite low (3060 ppb), says Rose-Pehrsson, detec tion of hydrazine and monomethylhy drazine in a normal atmospheric back ground, which has relatively high con-
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X-ray fluorescence equipment cannot match the new microanalyzer's detec tion limits or its ability to make mea surements on extremely small (micro meter-size) areas. A commercial de vice based on the X-ray fluorescence portion of the instrument is being de veloped by the Kevex Corporation, Foster City, Calif. Quintel Corporation has agreed to acquire Jerome Instrument Cor poration, which will become a wholly owned subsidiary of Quintel. Quintel is a manufacturer of moisture analysis instrumentation, and Jerome Instru ment is a privately held company that manufactures toxic gas analyzers. At the same time, Jerome announced a $2.1-million contract with the U.S. Navy for gold film mercury vapor ana lyzers. The analyzers will be used on U.S. Navy ships and submarines to protect sailors against contamination from breakage of mercury-containing devices. NUS Corporation is offering a free publication that summarizes meth ods of analysis and that compares regulatory categories (RCRA Ap pendix VII and the proposed Appen dix IX, Clean Water Act priority pol lutants, and Superfund) of environ mentally important compounds. To receive a copy, call Rose Ann Cochran at 412-788-1080 or 800-245-2730, or write to NUS, Park West Two, Cliff Mine Rd., Pittsburgh, Pa. 15275.
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ed in ANALYTICAL
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Porphyrins Excited States and Dynamics fl^W!
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Martin Gouterman, Editor, University of Washington Peter M. Rentzepis, Editor, University of California at Irvine Karl D. Straub, Editor, John L. McClelland Veterans' Hospital Examines theoretical and experimental studies on excited states of porphyrins as presented by classical spectroscopy, photochemistry, very transient spectroscopy, and theoretical calculations of electronic energy levels and dynamics. Offers the newest work in the most active areas of research in porphyrin excited states. An invaluable reference for those interested in porphyrin chemistry and excited states dynamics in general. CONTENTS Photodissociation of Nitrosoheme Complexes • Charge-Transfer States in Porphyrin Heterodimers · Electron Transfer in Chlorophylls, Chlorins, and Porphyrins · Spin-Orbit and Spin-Polarization Effects in Metalloporphyrins • NH-Tautomerism in the Formation of Excited States · Optical Detection of the Lanthanoid Ion Contraction · The S2 Emission of Metalloporphyrins · Triplet-Triplet Annihilation · The Triplet-State Decay Rate Constants of Porphyrin · Dimerization Effects on Porphyrins · Distance Dependent Rates · Picosecond Excited State Relaxation · Picosecond Photodissociation of the CO and 0 2 Forms · Electron Transfer in Bacterial Photosynthetic Reaction Centers · TwoPhoton Absorption and Radiationless Transitions · Nickel Porphyrins and Nickel-Reconstituted Heme Proteins · Porphyrin Radical Cations, Excited States, and Ligation Photodynamics · Transient Photoinduced Ligation Changes · Mechanistic Studies of Thermal and Photoinduced Atropisomerization · Quenching of Low-Lying Excited States in Porphyrins · Photooxidation of Phthalocyanines · Cation Radical Formation · The Principal Tumor-Localizing Porphyrin Photosensitizer Developed from a conference held in Little Rock, Arkansas, November 16-19, 1985 ACS Symposium Series No. 321 384 pages (1986) Clothbound LC 86-14647 ISBN 0-8412-0997-9 US & Canada $64.95 Export $77.95 Order trom: American Chemical Society Distribution Office Oept. 29 1155 Sixteenth St., N.W. Washington, DC 20036 or CALL TOLL FREE 800-424-6747 and use your credit card!
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