New Microwave Digestion Technology Is Aiming For Safety

Sample preparation can be unpleasant, tedious, and time- consuming, but its careful execution is the backbone of a successful laboratory test or experiment. As many researchers are aware, some of the most demanding sample preparation procedures are called digestions. The aim of this process, like biological digestion, is to break a sample down into more basic constituents, but for analysis rather than for food use. Time, heat, and strong acids, oxidants, and bases are the agents of the process. Sample digestion is often involved in hunts for trace metals, to prepare for such analyses as atomic absorption or inductively coupled plasma spectroscopy. The Kjeldahl nitrogen determinations used by food scientists are another form of digestion. Recently, a number of advanced microwave-based sample preparation systems have appeared on the market. These devices promise unprecedented procedural ease, greater control, and improved reproducibility of results with digestions. The new systems also make digestions--which often involve heated acids under pressure- -safer for laboratory workers. A wide range of analytical chemists, from protein analysts to chemists doing environmental tests, can take advantage of these systems. Manufacturers and distributors whose products are discussed in the text: CEM Corp. P.O. Box 200 Matthews, N.C. 28106 (800) 7263331 (704) 8217015 Fax: (704) 8217894 SPEX Industries Inc. 3880 Park Ave. Edison, N.J. 08820 (800) LABSPEX (908) 5497144 Fax: (908) 6039647 Questron P.O. Box 2387 Princeton, N.J. 08543 (609) 5876898 Fax: (609) 5870513 According to H.M. (Skip) Kingston, a professor of analytical chemistry at Duquesne University in Pittsburgh, who also teaches the American Chemical Society course called "Introduction to Microwave Sample Preparation: Theory and Practice," manufacturers of other machines of the same general type as the CEM line, but not discussed here, are: Floyd Inc. 5440 Highway 55E Lake Wylie, S.C. 29710 (803) 8317406 Fax: (803) 8317693 Milestone s.r.l. Via Fatebenefratelli, 1/5 24010 SORISOLE (BG), Italy 035/57.38.57 Fax: 035/57.05.07 Some years ago, digestion involved the distasteful chore of boiling acids in open flasks containing a sample and appropriate catalysts over a flame. The procedure took many hours, and the fumes released were a health problem. Welcome replacements for this procedure involved the use of electric heaters, some of which had exhaust fans to handle the fumes. These hot-plate systems facilitated control of temperature, but the procedure was still time-consuming and unpleasant. Reduction in the time needed for digestion came with closed- vessel heating, which could be done in ovens. As with kitchen pressure cookers, higher temperatures in shorter times were possible with the closed systems. The disadvantage of these systems was a lack of safety: Known to technicians as "bombs," the Teflon-lined, stainless-steel vessels that were used sometimes ruptured. However, since they get the job done and are relatively inexpensive, these systems for digestion are still used in many labs. In the 1980s, microwave heating for digestion of samples emerged to improve the process dramatically. Procedures that took several hours were reduced to less than an hour in many cases. CEM Corp. in Matthews, N.C., developed some of the first systems for microwave digestion sample preparation. "People were doing hot-plate digestions," says Vickie Kiker, marketing services manager at CEM, "and asking, `How can I do this faster and cleaner?' So we used microwaves to heat a closed digestion vessel, but we couldn't monitor or control the process very well. When reactions took place, they could create high pressures very fast." According to Kiker, CEM's latest two systems, the MDS-2000 and MDS-2100 Microwave Sample Preparation Systems, use new pressure and optional temperature feedback controls to fine-tune the digestion of different sample types. Improved safety and more complete digestions under maximized pressures are the result, as well as improved reproducibility, says Kiker. Direct measurement of temperatures inside the sealed vessels in the presence of the microwave field is accomplished with fused silica probes, tipped with a phosphor material, that are thermally and electrically nonconductive and immune to the field. A conventional metallic thermocouple sensor would pose safety problems that are circumvented by this approach. The system is programmable for up to five stages of digestion, controlling at each stage the microwave power, total run time, the point in the run at which pressure is controlled, the run time at that pressure, and the speed of a cavity exhaust fan. Peter Grohse, a senior research chemist at Research Triangle Institute, Research Triangle Park, N.C., has used CEM microwave machines to prepare samples for trace metals analysis since early in the systems' development. His research team has analyzed animal tissues for toxins for the United States Fish and Wildlife Service. It has also developed Environmental Protection Agency (EPA) protocols for solid waste metals analysis. "CEM was the only ball game in town," when his lab started using the company's machines in the mid-1980s, says Grohse. "They [CEM] were the trailblazers." Since then, several other companies have entered the U.S. market, with some advanced products coming from Europe. Grohse praises the durability of his aging CEM machines, which have seen hard duty. "They've really been hammered by acid fumes," Grohse says. "They're rust buckets on the outside, but still working--and still safe." He calls the CEM machines "bulletproof." Kiker says the CEM closed-vessel system may not be appropriate for all samples, in particular organics, metals, or larger samples that release quantities of gases in digestion. For these, an open-vessel system is still the best approach. David Selem, a technical specialist with SPEX Industries Inc., Edison, N.J., says his company's new 7400 Rapid Digestion System is an open-vessel system that addresses the needs of scientists working with these types of sample materials. Much larger samples can be run--up to 10 grams in SPEX's open system as opposed to less than 1 gram in a closed one--and there is no risk of explosion because there is no pressure buildup. Most sample preparation times are 30 to 45 minutes, says Selem. If large quantities of one sample type are being run, there can be a substantial time saving because of the machine's greater capacity, he adds. "The original microwave digesters were closed Teflon jars with the acid and sample inside," says Selem. "Pressure was a problem, and they tended to blow up at times. We use a refluxing column, or condenser, to draw off the vapors. It's a lot safer." The condenser also means that an exhaust hood is not necessary. Joe Strmec is a chemist looking for trace metals in the effluent filters of the Dresden Nuclear Power Plant in Morris, Ill. He prepares his samples, consisting of digested filter sections, for atomic absorption anal- yses. Strmec values the safety of an open digestion system like the SPEX product. "If you can take the `bomb' factor out of the process, why not?" he explains simply. Strmec also uses the CEM microwave system, but says that for his purposes the SPEX unit is easier to use. One feature he finds useful is the fact that the SPEX system can be programmed for up to nine steps in a digestion. "In our case," says Strmec, "we're using three kinds of filters, and they require three kinds of programs to get the job done." The condenser system for dealing with the potentially acrid gases released during digestion also appeals to Strmec. "Besides not having to deal with pressure," he says, "the system doesn't have to be in an exhaust hood. In the past, there were always fumes." David Hunter, manager of technical services at Rineco Inc. in Haskell, Ark., says that his laboratory has been able to replicate a four-hour, open-vessel, hot-plate analysis protocol specified by EPA with a 45-minute procedure using the SPEX machine. His group analyzes paint and resin industry wastes for levels of lead, barium, cadmium, and other metals. The fact that reagents can be added during the digestion process is an attraction of the SPEX product for Hunter. "This is very helpful," he says, "because you don't have to disassemble the system at that point. Paints have a high metal content, and the higher the metal content in the sample, the more acid gas you're going to generate." According to Selem at SPEX, a typical digestion process will have a minimum of three steps: carbonization, oxidation, and solubilization. The SPEX machine can control three parameters at each of nine available steps: the level of microwave power, the duration of the step in minutes, and the action of a peristaltic pump. Acids and heat are used in the first, a carbonization step to "burn" the sample. In the second, the oxidation stage, hydrogen peroxide or another compound might be added through use of the pump, to convert the carbon to CO2 for removal. Power might be reduced at this point to control excessive foaming. The third step, solubilization can be run at a higher power than the carbonization step until a clear solution is obtained. If needed, additional steps of oxidation and solubilization can be programmed to completely digest a sample. The SPEX machine is actually a modified version of a machine manufactured by the Paris-based company Prolabo, a subsidiary of the Rhone-Poulenc Group. SPEX specified a safety cover to be added to the original design, for increased protection against explosive mishaps with caustic materials. The full line of Prolabo products is distributed by Questron Corp., based in Princeton, N.J., according to John Poling, vice president for sales and marketing. In addition to the machines themselves, Questron also offers more than 500 digestion protocols and a range of applications for use with their products. There is little difference between the SPEX version of the Prolabo system and one of the Prolabo systems marketed directly by Questron. (SPEX does offer a well-developed set of educational materials with its product and easily reached technical assistance staff.) Other Prolabo machines sold by Questron include the 301 Series Focused Open Vessel Microwave Digestion Systems. One of these, the A301 system, is equipped with a robotic arm for unattended operation. Questron also offers a closed-vessel system, called the Q WAVE-1000 Computer Automated Microwave Digestion System, that is somewhat comparable to the CEM system. "Our Q WAVE-1000 offers a high degree of computer regulation," Poling says. "It's more of a robotics system that coordinates the dispensing of acids and reagents during the destruction of the sample. Computer control leads to better reproducibility." Questron's Q WAVE-1000 system offers permanent storage of data, graphics, and applications through a 286-chip, 20-MB laptop computer provided with the product. Because laboratory safety is a primary focus of these systems, each is designed so that the magnetron generating the microwaves will not operate if the door to the sample cavity is open. All of the new microwave sample preparation systems are said to be appropriate for a wide range of laboratory analyses besides those described here. CEM Corp. and SPEX materials recommend the compan-ies' systems for polymer, metal, food, oil, and other dissolutions; protein/peptide hydrolysis; organic synthesis reactions; solvent extraction for gas or liquid chromatography; and sample drying or moisture determination, in addition to acid digestions and Kjeldahl nitrogen determinations.

New Microwave Digestion Technology Is Aiming For Safety

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