Product Review
Standardizing the world w i t h microwaves
Tremendous advances have been made in automated analytical instrumentation, offering rapid and accurate detection. However, sample preparation has lagged behind and continues to be the slowest and most errorprone step in many chemical analyses. Despite the importance of sample preparation, relatively few new techniques have emerged over the past few decades. For centuries, chemists have digested samples in acid, a process that traditionally required several hours of heating on a hot plate. Around 1975, researchers began heating samples in domestic microwave ovens to reduce the amount of time required for sample dissolution. Although domestic microwave ovens greatly improved the rate at which samples could be digested, they were not designed to handle corrosive acid fumes.
resistant walls and temperature Microwave-enhanced chemically and pressure monitors to overcome the concerns. sample preparation safetyAccording to H. M. "Skip" Kingston, a professor of analytical chemistry at Duleads tofaster,better quesne University who specializes in developing standard microwave-digestion chemistry. methods, developments in microwave instruBeginning in the 1980s, researchers turned to closed vessels to reach temperatures above the atmospheric boiling point of the acids, leading to even faster digestion rates. These closed vessels were not specifically designed for microwave use, and the increases in pressure that accompanied their use were a potential safety hazard. Today, microwave digestion systems are designed specifically for laboratory purposes, with
mentation stem largely from the need for standardized methods in environmental and analytical chemistry. Basically three different types of microwave-digestion systems have evolved: elevated pressure (closed vessel), atmospheric pressure and flow-through. According to the manufacturers more people use elevated-pressure systems than atmospheric pressure or flow-through terns However atmospheric-pressure units are still relatively new and manufacturers
Analytical Chemistry News & Features, July 1, 1998 4 6 7 A
Product
Review
T a b l e 1 . S u m m a r y of s e l e c t e d e l e v a t e d - p r e s s u r e m i c r o w a v e d i g e s t i o n s y s t e m s ' Model
Multiwave
MWS-1
MARS 5
Manufacturer
Anton Paar GmbH Karntner StraBe 322 A-8054 Graz Austria 43 316 257-0
Berghof Laborprodukte GmbH HarretstraBe 1 72800 Eningen Germany 49 7121 894-0
CEM Corporation P.O. Box 200 Matthews, NC 28106 704-821-7015
URL or e-mail
www.anton - paar.com/ap
www.berghof.com
www.cemx.com
Dimensions W x D x H (cm)
64 x 67 x 49
55 x 42 x 37
51 x 64 x 58
Weight (kg)
45
29
54
Microwave power (Watts)
1000
850
1500
75 300 6(12 option) 25- or 50-mL quartz; 50- or 100-mL TFM'
75 240 6 30- or 80-mL TFM'; 10- or 20-mL quartz
100 300 12 (14 at 33 bar) 100-mL TFM'or quartz
Dedicated atmospheric pressure apparatus
No
No
No
Digestion vessels Max. pressure (bar) Max. temperature ( C) Max. no. of vessels Vessel size/liner material
Pressure control
Yes
No
Option
Temperature control
Yes
Yes
Option, internal and external
Options
12-position rotor; high-pressure 50mL TFM'vessels; 20-mL quartz vessels for micro-samples; software for external PC-documentation of reaction data
Exhaust unit; quartz inserts
Solvent detector for safe extractions; evaporation/concentration accessories; software for PC interface and data collection; 52-place turntable for industrial hygiene samples
Special features
Vessels operate at 75 bar; temperature and pressure monitored in each vessel; unpulsed microwave power; air-cooled vessels; built-in methods library; full system control with built-in PC; maintenance- and installation-free sensors
In situ contactless and contamination-free IR temperature control
Programming in six languages; vent and reseal vessels; high-impact safety flex door; built-in applications library
Reader service no.
401
402
403
a
Several manufacturers offer microwave digestion systems and vessels other than those shown above. Contact manufacturers for their full product line. Distributed by Perkin-Elmer, 761 Main Ave., Norwalk, CT 06859-0012; 203-762-4000; www.perkin-elmer.com; reader service no. 408. = Distributed by Berghof/America, P.O. Box 6029, Concord, CA 94524; 510-827-1868; www.berghofusa.com; reader service no. 409. d Distributed by Leeman Labs, 6 Wentworth Dr., Hudson, NH 03051; 603-886-8400; www.leemanlabs.com; reader service no. 410. " W/C = water cooled; 'TFM = tetrafluoroethylene, modified.
told Analytical Chemistry that they are selling quite well. There is not a large price difference between elevated- and atmosphericpressure units; estimates range from $10,000 to $25,000 for a complete syssem. The eeneral trend is that microwave systems are becoming "flexible microwave chemistry platforms," says Kingston. 'You no longer have to buy one unit to do organic extractions and another to do digestions now you can buy one unit that can do both." In addition manufacturers are just beginning to sell systems that can operate at both elevated and atmospheric pressure Analytical Chemistry hah surveyey several manufacturers of microwave digestion systems to provide potential buyers with 468 A
Analytical
Chemistry
information on what is commercially available. The specifications and features for selected elevated-pressure and atmosphericpressure units are provided in Tables 1 and 2, respectively. This summary is not intended to be a comprehensive review of all microwave digestion systems currently on the market; rather it aims at providing general information on the types of systems that are available. Enclosed and under pressure
Closed-vessel units were thefirstlaboratory microwave systems to become commercially available. Compared with traditional hot-plate methods, the use of closed vessels allows for higher reaction tempera-
News & Features, July 1, 1998
tures and therefore faster reaction rates. In addition, such closed systems are designed to minimize contamination and to prevent loss of volatile elements. According to Kingston, the first generation of closed vessels were made of Teflon and could only withstand about 7 bar of pressure. These early vessels were prone to venting when their pressure limits were exceeded. "The vessel was the weakest point in the first decade, and it was still troubling up until a few years ago " he says. Today closed vessels are much safer and reliable. Most closed vessels are jacketed for strength; have a liner made of a fluoroDolymer such as Teflon or perfluoroalkoxv (PFA) ; and can withstand pressures of 10-150 bar
ETHOS 9 0 0 / 1 6 0 0
Model 7 1 9 5 d
UniClever
QLAB 6 0 0 0
Milestone 160B Shelton Rd. Monroe, CT 06468 203-261-6175
Ol Analytical P.O. Box 9010 151 Graham Rd. College Station, TX 77842-9010 409-690-1711
Plazmatronika ul.Osobowicka 70 51-008 Wroclaw Poland 48 71 72 66 66
Questran Canada Suite 317 1761 W. Hillsboro Blvd. Deerfield Beach, FL 33442 954-429-1577
www.milestonesci.com
www.oico.com
[email protected] www.QuestCan.com
55 x 55 x 70
57 x 46 x 34
20 x 40 x 60
61 x 46 x 56
90
27
20
64
1600
950
300
1200
100 300 12 120-mLTFM'
41 200 12 90 mL
110 270 12 110-mLTFM'
42 (closed) 230 (closed); 500 (W/C atmospheric) 12 (closed); 10 (W/Ce atmospheric) 100-mLTFM', PFA, glass, or quartz
Yes
No
Yes
Yes
Yes
Yes
Yes
Yes
Yes, internal and external
Option
Yes
Yes
"Open-vessel" rotor; segmented rotors for increasing the number of vessels; quartz inserts for trace and ultratrace analysis; dual exhaust; evaporation rotor; acid scrubber
Ventable vessels (7 bar); moderatepressure vessels (14 bar); high-pressure vessels (41 bar)
Open digestion adapter
Interchangeable high-pressure and W / C atmospheric modules
Touch-screen LabPC; rotor-relief valve mechanism (vent and reseal); dual magnetron with pre-mixing chamber and rotating diffuser
Winwave software with applications library; dual stage pressure relief for 41 bar vessels for maximum safety
Triple safety pressure system; watercooled vessels; stainless-steel jacket; self-matching antenna system
Interlock for all vessels; automated reagent dispensing; water-cooled vessels; Pentium notebook included
404
405
406
407
Any vessel that is chemically inert and transparent to microwave radiation can be used for microwave digestions, says Henryk Matusiewicz, a professor of analytical chemistry at the Technical University of Poznari (Poland). He recommends that vessels be made from a material with a melting point above 150° C. Most manufacturers offer more than one type of vessel. Medium-pressure vessels can typically handle up to 35 bar and high-pressure vessels up to 100 bar with temperatures up to 200 °C. According to Kingston only one vessel Ccin be used above 300 °C (over 200 bar) and that is a unique design "It's actually a microwave autoclave the vessel itself is the inside of the autoclave " he explains "Some systems claim they can handle UD °C but they can handle 7 bar inside the vessel at that temperature which means none of the common mineral
acids, such as nitric acid, can be safely used," he cautions. High pressure and the risk of explosions caused by the generation of hydrogen gas with alloys and metals are the biggest concerns with closed-vessel systems, says Jean-Michel Mermet, a spectroscopist and professor of analytical chemistry at Universite Claude Bernard-Lyon I (France). In addition, materials such as PTFE are not capable of reaching the temperatures necessary to completely decompose organic compounds, and there is no possibility of adding reagents after the reaction has started, he says. Other drawbacks to high-pressure vessels are the limited amount of sample that can be used (tvcically less than 1 g for inorganic Dies and even less for organic samples) and the need for cooling before opening the vessel
Atmospheric pressure The limitations of closed-vessel systems spurred the development of atmosphericpressure microwave digestion (commonly referred to as open-vessel digestion). "The vessels used in atmospheric-pressure microwave digestions are not truly open," says Kingston. "They may be purged with gas, and they may have tubes in them to remove gases and to put liquids in. The term 'open vessel' makes people think these systems are like hot plates, but they are not really." Because such systems are at atmospheric pressure, gas-forming species no longer pose a safety concern. Other advantages include the possibility of sequentially adding reagents at any time during the digestion and obtaining higher temperatures with the use of quartz vessels, leading to more complete decomposition of organic
Analytical Chemistry News & Features, July 1, 1998 4 6 9 A
Product
Review
T a b l e 2. S u m m a r y of s e l e c t e d a t m o s p h e r i c - p r e s s u r e m i c r o w a v e d i g e s t i o n s y s t e m s Model
STAR System 6
Microdigest 3.6
Manufacturer
CEM Corporation P.O. Box 200 Matthews, NC 28106 704-821-7015
Prolabo 54 rue Roger Salengro F-94126 Fontenay-sous-Bois France 33 1 45 14 86 80
URL
www.cemx.com
www.prolabo.fr
Dimensions W x D x H (cm)
75 x 33 x 36
45 x 45 x 45
Weight (kg)
30
55
Microwave power (Watts)
1500
250 for each cavity
500 6 250-, 100-, or 50-mL Pyrex, quartz, or Teflon
450 3 or 6 100- or 250-mL, borosilicate glass, quartz, or PTFE
Temperature control
Yes
Yes
Options
Automatic reagent addition; acid scrubber with vapor containment; sample stirring; software for PC interface and data collection
Automatic reagent addition; acid scrubber
Special features
LCD temperature display for each vessel; individual temperature control of each vessel; automatic reagent addition during digestion; automatic concentration and evaporation accessories
Digestion vessels Max. temperature ( C) Max. no. of vessels Vessel size/liner material
Reader service no.
403
material. Moreover, digestions can be carried out until complete dryness, thus eliminating excess acid, and there is no need for a cooling down, depressurizing period. The first commercially available atmospheric-pressure microwave digestion system was based on "focused" technology, which makes use of a waveguide to focus the energy into a single-mode cavity so that the energy is directly coupled with the sample, rather than dispersing the energy throughout the entire microwave cavity. "With a conventional microwave oven, it is difficult to obtain homogeneity within the cavity, although several vessels can be used at the same time," says Mermet. Single-mode cavities provide greater homogeneity of the enhowever they are typically limited in that only a single vessel be heated A few manufacturers, however, do provide atmospheric-pressure microwave systems that can handle more than one sample at a time. The samples are housed in individual cavities that run independently of each other. Each cavity can therefore
According to Kingston, flow-through systems are a problem area because all samples must be homogeneous and small enough to get into the tube (sample size is limited to only about 0.1 g). "We've been trying for twenty years to get analytical results from slurries of solid materials. You can get meaningful numbers, but they are subject to matrix interferences." Nevertheless, there are unique advantages to using flow-through systems, says Kingston, but only if you are using a particular sample that lends itself to the technique. For example, blood, milk, and baby food samples are homogeneous and well suited for flow methods. However, the majority of samples require sample processing before they can be put into the tube, which defeats the purpose of using microwaveenhanced digestion in thefirstplace, he says. Kingston would like to see multiple channels and a better way of homogenizing samples at the front end. Staying in control
411
have its own method and temperature feedback control. In one system, a single magnetron is used to generate the energy for the entire system, and the microwaves are directed into the cavities through shutters that open and close. As many as six samples can be run simultaneously. On-line (flow-through) microwave dissolution
Discrete vessel systems, whether at elevated or atmospheric pressure, require a large amount of handling. Processes such as assembling, closing, opening, and positioning the vessel in the microwave are laborious and time-consuming. Flow-through microwave systems were designed to overcome some of the limitations by replacing the vessels with a flow-through linear tube. These systems can handle reactions that produce sudden increases in temperature and pressure, or unstable samples. The earliest work in this area involved the on-line coupling of microwave sample dissolution to flame atomic absorption
470 A Analytical Chemistry News & Features, July 1, 1998
The real breakthrough in microwave instrumentation in the past decade has been the advent of commercially available pressure and temperature feedback control, says Kingston. By monitoring the pressure and temperature conditions during a reaction, researchers gained the ability to control acid digestions and study their mechanisms. Up until that time, standard microwave sample-preparation methods were based on power settings and were limited because of fluctuations in microwave power from one unit to another According to the experts improperly controlled methods simply not reproducible even when performed on two systems that are manufactured by the same companv In order for standard methods to be "documented, transferred, and reproduced," reactions must be precisely controlled, says Kingston. In a microwave system, reactions can be controlled by relying on power settings, pressure feedback control, calibration, or temperature feedback control. Kingston believes that temperature is the primary control parameter. "The pressure actually follows the temperature and is an artifact but is not in direct control " he says. With precise temperature control "we are down to a point of plus or
minus about 2 to 2.5 °C." In contrast, methods that rely on power settings can vary as much as 20 °C, he says. Kingston prefers to use temperature control rather than calibration. "In calibration of a microwave, the only thing you are trying to do is determine the strength of the microwave field as a function of percent power setting. The problem is that you are using calibration to get a precise temperature." The direct measurement of the microwave field requires expensive, specialized equipment, and therefore indirect methods which determine the amount of energy absorbed by a strongly absorbing material such as water are commonly used instead "Calibration does work" says Kingston "but you have to use the exact same vessels because vessels that have different heatings rtiaracteristics will crive vnu a totally
different temperature profile " Kingston continues to include calibration as an option in the standard methods that he writes for EPA because of its costeffectiveness. "If you analyze samples for the same thing over and over again, then calibration is sufficient; you can have a much less expensive microwave." Calibration would be appropriate for a routine testing laboratory; however, warns Kingston, it is not a research avenue. It is difficult to assess the accuracy of microwave-leaching procedures because there are no real numbers (only relative results based on the procedure). What's important is that the reaction goes to completion, says Kingston. "What's happening is consistency or reproducibility in another laboratory becomes a sort of de facto accuracy, but that is really precision being used as accuracy," he explains. "What gives microwave methods their consistency and their reproducibility is that you can control the reaction precisely" How accurate are the methods? In Kingston's mind "Ifyouare measuring the temperature in the liquid phase then they are all very accurate" So accurate that he believes microwave digestion is becoming the most standard way to f\o ditrp^tion rVipmicttry arnnnd the world "You can rrpt ttip
same results worldwide because you can control the temnpratnrp of the reaction very preciselv No other set of methods has such reprorincibilitv "
Safety first
Because microwave digestions are performed with strong acids, safety is a big issue. In addition, many digestion products are poisonous gases; therefore, all closed vessels should be carefully opened under a fume hood, says Kingston. Why not store the entire microwave system under a hood? "That would be the worst thing you could do." Most laboratory microwave units exhaust gases from the sample cavity, which must be kept separate from the air that cools the circuitry. "If you put it in a hood the air that comes out of the cavity will be sucked right back into the electronics " he savs "If the air cominsr out has acid fumes in it what you normallv do is keep
“What gives microwave methods their consistency and reproducibility is that you can control the reaction precisely”
the microwave in the room and you take the exhaust from the cavity to a hood." Kingston recounts an accident that occurred when a home microwave unit was used in a laboratory fume hood. "The circuits that control the switches that turn off the microwave when you open the door were corroded and severed. An individual opened the door while the microwave was on full power. Luckily the man was rotund, which meant all of his organs were shielded. His blood was heated and he fell to the floor unconscious from thermal shock." Endless possibilities
Over the past decade, laboratory microwave systems have found many uses, particularly for environmental applications such as sediment and soil digestion. Although the experts claim that environmental applications are what drove the development of laboratory microwave systems to where they are today, that is by no means
the only area that has benefited from their use. Medicine is just starting to make use of microwaves, says Kingston. "It takes 24 hours to prepare a slide for a pathologist to determine whetiier a lump is cancer. The entire process consists of dehydration and fixing oo f atain—it's sll lampll prep. You can prepare that same needle biopsy in 30 min in a microwave," he explains. Another area that is likely to benefit is the drug testing industry. "We are analyzing blood samples for drugs of abuse. Normal methods take six hours. We get better results in one minute " says Kingston With the help of microwaves, researchers can significantly decrease the amount of time (from hours to seconds) it takes to complete inorganic and organic syntheses, says Kingston. Changes in the way undergraduate chemistry labs are taught are also likely to take place because of microwaves. "Students can dofivesyntheses per threehour class period, rather than one." Sample preparation is an area of analytical chemistry that is often overlooked; however, Kingston believes that is changing. He envisions that microwave-enhanced sample preparation will become a standard laboratory tool, right up there with spectroscopy and chromatography. "Microwave methods can be used as a standard because they're really better chemistry, not just faster chemistry. They are more consistent and more appropriate for many applications." Britt Erickson For further information:
Microwave-Enhanced Chemistry: Fundamentals, Sample Preparation, ond Applications; Kingsson, H. M., Haswelll S. .., Eds.; American Chemical Society: Washington, DC, 1997. http://www.sampleprep.duq.edu/ sampleprep Upcoming product reviews for 1998 November 1: MALDI-TOF MS December 1: GC/IR accessories
If your company manufactures either of these instruments, please let us know. Email (
[email protected]) or call us (202872-4570) at least three months prior to the listed date of publication.
Analytical Chemistry News & Features, July 1, 1998 471 A
