Anal. Chem. 1996, 68, 4316-4320
Comparison of Microwave-Assisted and Conventional Leaching Using EPA Method 3050B Elke M. L. Lorentzen and H. M. “Skip” Kingston*
Department of Chemistry & Biochemistry, Duquesne University, Mellon Hall, Pittsburgh, Pennsylvania 15282-1503
A microwave-heated EPA method 3050B for the leaching of key elements (cadmium, chromium, copper, lead, nickel, zinc) of environmental importance was tested and compared to conventional hot plate-heated EPA method 3050B. All commercially available temperature and power-controlled atmospheric pressure microwave systems were used for the adaptation of EPA method 3050B. Three temperature feedback control systems were evaluated for regulating the temperature of the leachate including outside (IR sensors) and inside the sample flask (gas bulb thermometer). Results, which show the efficiency and effectiveness of the microwave sample preparation method, are discussed for the leaching of three NIST Standard Reference Materials: 2704, 2710, and 2711. The elements were determined either using ICPMS, ETAAS, or F-AAS. This study demonstrates that microwave heating with enhanced reaction control leads to improved precision compared to conventional heating sources. Leaching is a term that has been applied to the extraction of metals from environmental samples and has become common terminology of the EPA and in the environmental analytical field. Leaching is not a total decomposition, and leachable recoveries of analytes are generally lower than total concentrations. Recoveries can only achieve total values if an element is completely soluble in the leaching solvent. Leaching studies are an assessment of worst case environmental scenarios where components of the sample become soluble and mobile. Temperature is a key parameter for all leaching sample preparation methods as well as for extractions and digestions. The control of temperature is paramount in achieving reproducible leaching of elements. Temperature is a primary parameter used to increase the rate of leaching and to bring these tests into appropriate duration for laboratory evaluation. Previously, most leach methods have been accomplished in beakers on a hot plate. These methods are traditional, time consuming, fairly inefficient, and in general imprecise. During the early 1980s, fundamental research established temperature control as the most significant contributor to leach test error.1,2 These experiments focused on the analysis of simulated nuclear waste glass materials. Temperature was found to be the dominant parameter in leaching uncertainty and imprecision. The control of temperature to within (0.04%, instead of (1% over a 28-day leach period, changed the interelement leaching uncertainty from 50% to 3%. By substituting microwave (1) Kingston, H. M.; Cronin, D. J.; Epstein, M. S. Nucl. Chem. Waste Manage. 1984, 5, 3-15. (2) Liggett, W. S.; Inn, K. G. W. Pilot Studies for Improving Sampling Protocols. In Principles of Environmental Sampling; Keith, L. H., Ed.; Professional Reference Book; American Chemical Society: Washington, DC, 1996; Chapter 10.
4316 Analytical Chemistry, Vol. 68, No. 24, December 15, 1996
heating for conventional heating devices, advantages are gained in efficiency, precision, accuracy, and reduced waste.3-8 This is due to direct microwave energy inductance into leachate solutions. The application of microwave energy as the heating source significantly improved many sample preparation methods. Microwave instrument technology coupled with closed vessels has become a standard for EPA methods 3015, 3051, and 3052.8-10 To date, the range of applications for microwave sample preparation extends from “closed-vessel” to “open-vessel” to “flowthrough” systems.9,11-13 The control of temperature in leaching is responsible for much of the precision of these procedures. However, in dedicated atmospheric pressure microwave systems, only power control has been available until recently. Two different commercial systems of focused microwave power with temperature feedback control14,15 are now available and are evaluated for EPA method 3050B. Method 3050B, included in Test Methods for Evaluating Solid Waste SW-846, Update III, has traditionally been a leach test performed on a hot plate.10 In recent years, the EPA has proposed to revise and update certain testing methods used to comply with the requirements of subtitle C of the Resource Conservation and Recovery Act (RCRA) of 1976. Method 3050B is one of several methods included in the list of draft revised methods reviewed in SW-846, Update III, in the U.S. Federal Register.16 This method has become more broadly applicable by adapting the prescriptionbased method to create a performance-based method and to permit newer technological implementations, such as microwave heating.17 By adding the words “or equivalent” to the hot plate designation, and specifying that the heating device be “adjustable and capable of maintaining a temperature of 90-95 °C”, the use (3) Matusiewicz, H.; Suszka, A.; Ciszewski, A. Acta Chim. Hung. 1991, 128, 849-859. (4) Feinberg, M. H.; Suard, C.; Ireland Ripert, J. Chemom. Intell. Lab. Syst. 1994, 22, 37-47. (5) Barnes, R. M. Anal. Chem. 1990, 62, 1023A-1033A. (6) Burguera, J. L.; Burguera, M. J. Anal. At. Spectrom. 1993, 8, 235-241. (7) Alvarado, J. S.; Neal, T. J.; Smith, L. L.; Erickson, M. D. Anal. Chim. Acta 1996, 322, 11-20. (8) Kingston, H. M.; Walter, P. J.; Chalk, S. J.; Lorentzen, E.; Link, D. D. Environmental Microwave Sample Preparation: Fundamentals, Methods, and Applications. In Microwave Enhanced Chemistry; Kingston, H. M., Haswell, S., Eds.; American Chemical Society: Washington, DC, in press. (9) Kingston, H. M., Jassie, L. B., Eds., Introduction to Microwave Sample Preparation: Theory and Practice; American Chemical Society: Washington, DC, 1988. (10) Test Methods for Evaluating Solid Waste-SW846, Update III, 3rd ed.; U.S. EPA: Washington, DC, 1995. (11) Matusiewicz, H.; Sturgeon, R. E. Prog. Anal. Spectrosc. 1989, 12, 21-39. (12) Feinberg, M. H. Analusis 1991, 19, 47-55. (13) Haswell, S. J.; Barclay, D. A. Analyst 1992, 117, 117-120. (14) Lorentzen, E. M. L.; Kingston, H. M. Pittsburgh Conference, New Orleans, LA, 1995; Paper 1305. (15) King, E. E.; Pittsburgh Conference, Chicago, IL, 1996, Paper 1247. (16) Fed. Regist. 1995, 60, (142), 37974-37978. (17) Friedman, D. Environ. Lab. 1993, (April/May) 37-39. S0003-2700(96)00553-7 CCC: $12.00
© 1996 American Chemical Society
Table 1. Instrumentation for Conventional and Microwave-Assisted Method 3050B atmospheric pressure microwave-assisted methods temp control 1 heating device temp meas vessel a
microwave system MX 350a with temperature control IR sensor (M 402a) quartz glass, 250 mL
temp control 2 401a
microwave system with temperature control gas bulb thermometer (Megal 500a) borosilicate glass, 250 mL
temp control 3
calibrated power control A301a
hot plate
microwave system Star System 2b with temperature control IR sensor
microwave system with power control
hot plate with power settingsc
fiber-optic sensord
thermocouple thermometere
borosilicate glass, 250 mL
borosilicate glass, 250 mL
Prolabo, Paris, France. b CEM Corp., Matthews, NC. c Fisher Scientific, Pittsburgh, PA. d Luxtron 750, Santa Clara, CA. e Fluke 52, Paramus,
NJ.
of heating blocks and microwave energy is permitted as acceptable heating alternatives within the structure of the method. In addition, with the incorporation of feedback control of the leachate temperature, it is easier to increase the reproducibility of the measurement while automating the process. In comparison to indirect heating by convection and conduction in hot plate digestions, acids and polar solvents directly absorb microwave radiation by the mechanisms of ionic conductance and dipole rotation.18,19 Since the sample is heated directly, equilibrium conditions are obtained much more rapidly. In 3050B, the sample preparation time can be reduced from over 2.5 to 1 h by using microwave technology rather than conventional means. As well as allowing greater throughput of samples, this may also improve the reproducibility and reduce contamination by decreasing the quantity of reagent and exposing the sample to a controlled atmosphere for a shorter period of time.8,19 These procedures may be automated or semiautomated using microwave instrumentation so the attendance by the chemist may be minimized. Newer instrumentation with two, four, six, or more samples and multiple instruments reduce the limitation in throughput. Furthermore, there are several advantages to atmospheric pressure microwave digestion, such as effective handling of gas-forming digestion products, sequential and automated incremental addition of reagents during digestion, handling of larger and dynamic sample sizes, and the use of quartz, glass, or Teflon vessels.20 The elements Ag, Al, As, Ba, Be, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, Pb, Sb, Se, Tl, V, and Zn can be determined by EPA method 3050B in sediments, sludges, and soils. An optional procedure for improved recoveries of the analytes Sb, Ba, Pb, and Ag has been included in the revision. Two digestions of 1.00 (dry weight) or 1.00-2.00 g (wet weight) of sample are required to analyze all 23 elements.10 Elements determined by F-AAS were leached with nitric acid, hydrogen peroxide, and hydrochloric acid. For analysis with ET-AAS21,22 and ICPMS,22-24 (18) Copson, D. A. Microwave Heating; Avi Publishing Co. Inc.: Westport, CT, 1975. (19) Mingos, M. Microwave Theory for Chemistry. In Microwave Enhanced Chemistry; Kingston, H. M., Haswell, S. J., Eds.; American Chemical Society: Washington, DC, in press. (20) Grillo, A. C. Spectroscopy 1989, 4, 16-21. (21) Kingston, H. M.; Walter, P. J.; Lorentzen, E. M. L.; Lusnak, G. P. Duquesne University, Pittsburgh, PA, 1994. (22) Walter, P. J.; Chalk, S. J.; Kingston, H. M.; Lorentzen, E. M. L. A Review of Microwave Assisted Sample Preparation. In Microwave Enhanced Chemistry; Kingston, H. M., Haswell, S., Eds.; American Chemical Society: Washington, DC, in press. (23) Jarvis, K. E.; Gray, A. L.; Houk, R. S. Handbook of Inductively Coupled Plasma Mass Spectrometry, 1st ed.; Chapman and Hall: New York, 1992.
the samples were leached without hydrochloric acid. The reaction conditions and reagents have an influence on the results of the leach analysis, with a large number of different species being involved.25,26 In this paper, we report the comparison of samples leached with controlled microwave heating to conventional hot plate heating and their ability to accurately reproduce the method’s specified 95 °C temperature. EXPERIMENTAL SECTION Reagents. Concentrated nitric acid, concentrated hydrochloric acid, and hydrogen peroxide (30%) were obtained from Fisher Chemical (ACS Reagent Grade, Fisher, Pittsburgh, PA). The acids were subboiled distilled before use, using either a quartz still (Milestone s.r.l., Sorisole, Italy) or an all-PFA Teflon still, built in-house from Teflon components (Savillex Corp., Minnetenka, MN). The Standard Reference Materials, SRM 2704 (Buffalo river sediment), SRM 2710 (Montana soil, highly elevated trace element concentrations), and SRM 2711 (Montana soil, moderately elevated trace element concentrations) were obtained from NIST (the National Institute for Standards and Technology, Gaithersburg, MD). Equipment. The atmospheric pressure microwave procedures were performed using equipment from Prolabo Corp. (Paris, France) and CEM (Matthews, NC). The sample leaching and reaction control equipment used in this study are summarized in Table 1. A Perkin-Elmer atomic absorption spectrometer PE1100A (Norwalk, CT) with both flame (F-AAS) and electrothermal modes (ET-AAS, HGA 300) and an inductively coupled plasma mass spectrometer (ICPMS, VG Plasma Quad 2STE, Fisons Instruments, Beverly, MA) were used under standard conditions in a class 1000, 100, and 10 combination clean room facility for the analysis of elemental concentrations. Leach Procedures. The comparative procedural outlines of method 3050B and modified microwave assisted 3050B are shown in Table 2 and discussed below. Procedure for Conventional Hot Plate EPA Method 3050B. A 1.0-g sample, known to 0.01 g, was weighed in an Erlenmeyer flask and 10 mL of HNO3 1:1 (v/v) was added. The solution was heated on a hot plate to ∼95 °C without boiling and this temperature was maintained for 15 min. After cooling to less than 70 °C, 5 mL of concentrated HNO3 was added and the sample was refluxed for 30 min at ∼95 °C without boiling. This step was (24) Montaser, A.; Golightly, D. W. Inductively Coupled Plasmas in Analytical Atomic Spectrometry, 2nd ed.; VCH Publishers: New York, 1992. (25) Binstock, D. A.; Grohse, P. M.; Gaskill, A.; Sellers, C.; Kingston, H. M.; Jassie, L. B. J. Assoc. Off. Anal. Chem. 1991, 74, 360-366. (26) Kingston, H. M.; Walter, P. J. Spectroscopy 1992, 7, 20-27.
Analytical Chemistry, Vol. 68, No. 24, December 15, 1996
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Table 2. Comparative Procedural Outlines of EPA Method 3050B and Modified Microwave Assisted 3050B method 3050B
vessel sample size reagents
time procedure
microwave assisted
hot plate
open microwave vessel at atmospheric pressure 1-2 g 15 mL of conc HNO3 17 mL of H2O 5 mL of conc HCl 10 mL of 30% H2O2 60 min weigh sample into vessel and add 10 mL of 1:1 HNO3 reflux at 95 °C for 5 min cool, then add 5 mL of conc HNO3 reflux at 95 °C for 5 min cool, then add 5 mL of conc HNO3 reflux at 95 °C for 5-10 min cool, add max 10 mL of 30% H2O2 heat until effervescence is minimal reflux at 95 °C for 5-10 min cool, add 15 mL of 1:2 HCl, reflux at 95 °C for 5 min
usually a covered beaker or flask at atmospheric pressure 1-2 g 15 mL of conc HNO3 10 mL of H2O 10 mL of conc HCl 10 mL of 30% H2O2 2-6 h weigh sample into vessel and add 10 mL of 1:1 HNO3 reflux at 95 °C for 10-15 min cool, then add 5 mL of conc HNO3 reflux at 95 °C for 30 min cool, then add 5 mL of conc HNO3 reflux at 95 °C for 30 min, evaporate to 5 mL, or heat for 2 h cool, add max 10 mL of 30% H2O2 heat until effervescence is minimal evaporate to 5 mL or heat for 2 h cool, add 10 mL of conc HCl, reflux at 95 °C for 15 min
repeated a second time. The sample was evaporated to ∼5 mL without boiling. After cooling to less than 70 °C, 2 mL of 18-MΩ water was added followed by the slow addition of 10 mL of H2O2 (30%). Care must be taken to ensure that losses do not occur due to excessively vigorous effervescence caused by rapidly adding the strong oxidizer, hydrogen peroxide. The solution was then heated until effervescence subsided. After cooling to less than 70 °C, 5 mL of concentrated HCl and 10 mL of 18-MΩ water were added and the sample was refluxed for 15 min without boiling. After cooling to room temperature, the sample was filtered and diluted to 100.0 mL using 18-MΩ water. Procedure for Power Control Microwave Implementation of Method 3050B. A 1.0-g sample, known to (0.01 g, was weighed in a borosilicate glass vessel and 10 mL of HNO3 1:1 (v/v) was added. A microwave digestion program consisting of 80 W for 2 min and 30 W for 5 min was applied by a Microdigester A301. After cooling to 70 (10 °C, 5 mL of concentrated HNO3 was automatically added and a second power program of 80 W for 2 min and 30 W for 5 min was applied. This step was repeated a second time. After cooling to 70 (10 °C, 3 mL of H2O2 (30%) was slowly added (2 mL/min) and a third power program of 40 W for 5 min was applied. This step was repeated twice. After cooling to 70 (10 °C, 5 mL of concentrated HCl in 10 mL of 18MΩ water was added and a fourth-power program of 80 W for 2 min and 30 W for 5 min was applied. After cooling to room temperature, the sample was filtered and diluted to 100.0 mL using 18-MΩ water. Procedure for Temperature Feedback Controlled Microwave Implementation of Method 3050B. A 1.0-g sample, known to (0.01 g, was weighed in a borosilicate vessel or quartz glass vessel (see Table 1) and 10 mL of HNO3 1:1 (v/v) was added. A microwave temperature program consisting of heating the solution to 95 (2 °C in 2 min and maintaining the temperature for 5 min was applied. After cooling to 70 (5 °C, 5 mL of concentrated HNO3 was automatically added and a second temperature program of heating the solution to 95 (2 °C in 2 min and maintaining the temperature for 5 min was applied. This step was repeated a second time. After cooling to 70 (5 °C, 10 mL of H2O2 (30%) was slowly added (2 mL/min addition by temperature controls 1 and 2, and in 0.5-mL aliquots by temperature control 3). The solution was then heated to 95 (2 °C in 6 4318 Analytical Chemistry, Vol. 68, No. 24, December 15, 1996
min, and the temperature was maintained for 5 min. After cooling to 70 (5 °C, 5 mL of concentrated HCl in 10 mL of 18-MΩ water was added. The solution was then heated to 95 (2 °C in 2 min and the temperature was maintained for 5 min. After cooling to room temperature, the sample was filtered and diluted to 100.0 mL using 18-MΩ water. For analysis by ET-AAS and ICPMS, digestion procedures as described above were performed without hydrochloric acid addition, as possible in the EPA procedure. RESULTS AND DISCUSSION The results of metal analysis, using a modified microwave assisted 3050B (temperature feedback control and power control) and conventional method 3050B, are listed in Tables 3-5 for SRMs 2704, 2710, and 2711, respectively. Results are compared to NIST leachable concentrations using method 3050,27 and certified values for total digestion are included for convenience. Leach concentrations for SRM 2704 are not provided by NIST. All data are in the range of the NIST or other published leachable concentrations25 with minor exceptions. The majority of recoveries for six analytes, as a percentage of certified values for total digestions, are in the same range or higher than the leach reference data published by NIST.27 These six specific elements were chosen as being of universal interest in environmental standards in consultation with NIST.28 The NIST leach values are not certified but are a compilation of a 17 laboratory collaborative leach study. In this study, all laboratories used conventional hot plate equipment. The recoveries obtained with temperature control (IR sensor and gas bulb thermometer) are slightly lower than the corresponding values of power control and hot plate. The lower values obtained by using microwave assisted modified temperature-controlled 3050B can be explained by more accurately controlled, but relatively lower, temperatures during the leaching procedures since the temperature with power control and on the hot plate was higher than the required 95 °C, as will be described. For EPA method 3050B, the procedure requires heating the sample solution to 95 °C and holding this approximate temperature (27) Addendum to the Certificate of Analysis for SRMs 2709, 2710, 2711, NIST, Gaithersburg, MD, August 23, 1993. (28) Personal communication, Jean Kane, NIST, 1992.
Table 3. Results of the Analysis of NIST Standard Reference Material 2704 Using Method 3050B element Cu Pb Zn Cd Cr Ni a
atmospheric pressure microwave-assisted methodsa (µg g-1 ( SD) power control temp control 2 temp control 1 temp control 3 101 ( 7 (102) 160 ( 2 (99) 427 ( 2 (97) nac 82 ( 3 (61) 42 ( 1 (95)
89 ( 1 (90) 145 ( 6 (90) 411 ( 3 (94) 3.5 ( 0.66 (101) 79 ( 2 (58) 36 ( 1 (82)
98 ( 1.4 (99) 145 ( 7 (90) 405 ( 14 (92) 3.7 ( 0.9 (107) 85 ( 4 (63) 38 ( 4 (86)
101.6 ( 4.8 (103) 134 + 5 (83) 407 ( 7.4 (93) 3.05 ( 0.65 (88) 82 ( 8 (61) 35.4 ( 3 (80)
hot platea (µg g-1 ( SD)
NISTb certified values for total digestion (µg g-1 ( 95% CI)
100 ( 2 (101) 146 ( 1 (91) 427 ( 5 (97) nac
98.6 ( 5.0 161 ( 17 438 ( 12 3.45 ( 0.22
89 ( 1 (66) 44 ( 2 (100)
135 ( 5 44.1 ( 3.0
Numbers in parentheses are percent recoveries. b 3050 and 3051 leach data are presented in ref. 25. c na, not available.
Table 4. Results of the Analysis of NIST Standard Reference Material 2710 Using Method 3050B
element Cu Pb Zn Cd Cr Ni
atmospheric pressure microwave-assisted methodsa (µg g-1 ( SD) power control temp control 2 temp control 1 temp control 3 2640 ( 60 (89) 5640 ( 117 (102) 6410 ( 74 (92) nad 20 ( 1.6 (51) 7.8 ( 0.29 (55)
2790 ( 41 (95) 5430 ( 72 (98) 5810 ( 34 (84) 20.3 ( 1.4 (93) 19 ( 2 (49) 10 ( 1 (70)
2480 ( 33 (82) 5170 ( 34 (93) 6130 ( 27 (88) 20.2 ( 0.4 (93) 18 ( 2.4 (46) 9.1 ( 1.1 (64)
3080 ( 22 (104) 5065 ( 89 (92) 6212 ( 84 (89) 17.8 ( 0.8 (82) 20.9 ( 0.5 (54) 10.2 ( 0.55 (71)
hot platea (µg g-1 ( SD) 2910 ( 59 (99) 5720 ( 280 (103) 6230 ( 115 (90) nad 23 ( 0.5 (59) 7 ( 0.44 (49)
NIST leachable concns using method 3050b (µg g-1)c (Range)
NIST certified values for total digestion (µg g-1 ( 95% CI)
2700 (2400-3400) 5100 (4300-7000) 5900 (5200-6900) 20 (13-26) 19 (15-23) 10.1 (8.8-15)
2950 ( 130 5532 ( 80 6952 ( 91 21.8 (0.2 39e 14.3 ( 1.0
a Numbers in parentheses are percent recoveries. b Reference 27. c Numbers in parentheses are ranges. d na, not available. e Non-certified values - for information only. NA - Not available
Table 5. Results of the Analysis of NIST Standard Reference Material 2711 Using Method 3050B
element Cu Pb Zn Cd Cr Ni
atmospheric pressure microwave-assisted methodsa (µg g-1 ( SD) power control temp control 2 temp control 1 temp control 3 107 ( 4.6 (94) 1240 ( 68 (107) 330 ( 17 (94) nad 22 ( 0.35 (47) 15 ( 0.2 (73)
98 ( 5 (86) 1130 ( 20 (97) 312 ( 2 (89) 39.6 ( 3.9 (95) 21 ( 1 (45) 17 ( 2 (83)
98 ( 3.8 (86) 1120 ( 29 (96) 307 ( 12 (88) 40.9 ( 1.9 (98) 15 ( 1.1 (32) 15 ( 1.6 (73)
113 ( 8.1 (99) 1119 ( 60 (96) 326 ( 3.7 (93) 39.4 ( 1.2 (94) 17.3 ( 1.3 (37) 15.5 ( 0.75 (75)
hot platea (µg g-1 ( SD) 111 ( 6.4 (97) 1240 ( 38 (107) 340 ( 13 (97) nad 23 ( 0.9 (49) 16 ( 0.4 (78)
NIST leachable concns using method 3050b (µg g-1)c 100 (91-110) 1100 (930-1500) 310 (290-340) 40 (32-46) 20 (15-25) 16 (14-20)
NIST certified values for total digestion (µg g-1 ( 95% CI) 114 (2 1162 ( 31 350.4 ( 4.8 41.7 ( 0.25 47e 20.6 ( 1.1
a Numbers in parentheses are percent recoveries. b Reference 27. c Numbers in parentheses are ranges. d na, not available. e Noncertified values, for information only.
without boiling. At a temperature of 95 °C, concentrated nitric acid should not boil. Since temperature control is difficult to maintain on a hot plate, this is easier to reproduce and maintain by using microwave temperature feedback control instruments. However, the temperature profile for the microwave assisted modified method 3050B using power control, obtained by simultaneous temperature measurement with a fiber-optic temperature sensor, was more like that of a hot plate. The resultant temperature was typically found to be 10-15 °C above the required 95 °C. Programming the power to maintain a certain temperature is difficult, since microwave digestors with power control are restricted in the control to the power increments and time settings
available similar to hot plates. In addition, heat loss and evaporation of reagents must be considered. The alternative to this procedure is to control the power by temperature feedback during microwave leaching. Until 1994, power control was the only form of control in dedicated atmospheric pressure microwave systems. This was the single most limiting feature of these earlier microwave instruments. Temperature feedback control makes atmospheric pressure microwave systems much more capable. However, temperature measurements in a microwave field are limited to devices that are transparent to the field. The use of shielded thermocouples is inappropriate in comparison with its use in a closed-vessel Analytical Chemistry, Vol. 68, No. 24, December 15, 1996
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system, where its shielding is grounded to a cavity wall, as the microwave field will be conducted from the vessel on the surface of the shielding. Fiber-optic thermometry is commonly used in microwave cavity systems29 and is compatible with open vessel systems. Gas bulb thermometers and IR sensors are two alternatives currently developed for the use in atmospheric pressure microwave digestors. The gas bulb thermometer is based on gas law principles, measuring the pressure difference of heated air in a glass bulb inside the sample flask. The gas bulb has to be fully covered by solution to assure an accurate temperature measurement. IR sensors were used for noninvasive temperature control. The intensity of IR radiation emitted from the vessel base is measured through a hole in the bottom of the cavity under the flask. In the Prolabo instrumentation, the IR radiation emitted is reflected by a mirror in an angle of 90° toward the IR detector. IR sensor calibration was accomplished by simultaneous temperature acquisition with a fiber-optic sensor or the gas bulb thermometer. This allows the emissivity of the quartz glass vessel used during the experiments to be corrected by an emissivity factor (range 0.10-1.00 (0.01) within the controlling software. In the CEM microwave unit, the IR sensors are placed several centimeters below each vessel. The IR sensors were individually calibrated by a low (concentrated nitric acid 121 °C) and high solvent boiling point (concentrated sulfuric acid 330 °C) and verified by an independent fiber-optic measurement system. A major advantage of the IR sensor is that no cross contamination can occur between samples from this source since the probe has no contact with the sample. The IR sensors measure energy reflected or emitted by any surface directly behind the vessel base and therefore have a response that changes slightly with solution volume. The IR sensors require frequent checking of the calibration and have been found to be volume-dependent. The temperature accuracy of all systems was checked with simultaneous temperature acquisition using an independent fiberoptic thermometry system. Temperature spikes, seen as a result of overshooting the goal temperature, were mainly observed with temperature controls 2 and 3, primarily due to deficiencies in the control algorithms. Adjustments of the feedback control algorithm could improve their temperature control. Less major thermal and power spikes were observed with temperature control 1. The maximum temperature is maintained to a much higher accuracy, as shown in Figure 1. The regulation of a certain temperature on a hot plate has been found to be much more difficult than with the use of microwave digestors. Calibration of a hot plate to produce 95 °C in a single flask resulted in the other flasks’ temperatures ranging from 85 and 118 °C. It was commonly observed that, depending on the flask measured, the distribution pattern, number of flasks, surface temperature, and air movement, the temperature may vary by 35 °C. This is in agreement with results found by other researchers.30 The advantage of preparing several samples at the same time (hot plate) is counteracted by the problem of regulating and controlling a certain temperature for all sample flasks. In comparison, the microwave IR sensor is able to regulate and control 95 °C with a mean temperature of 95.4 (1.1 °C (four replicates, 300 data points in 5 min). Furthermore, reagents were programmed to be added automatically on all microwave systems. Temperatures and speed of (29) Wickersheim, K. A.; Sun, M. H. J. Microwave Power 1987, 22, 85-93. (30) Kane, J. S. Fresenius’ J. Anal. Chem. 1995, 352, 209.
4320 Analytical Chemistry, Vol. 68, No. 24, December 15, 1996
Figure 1. Temperature vs time profile of microwave-assisted method 3050B using temperature feedback control 1 (IR sensor).
reagent addition were coordinated through sensor and software control. This is in contrast to the manual hot plate implementation. Both designs of atmospheric pressure, temperature-controlled, microwave equipment produced similar results, demonstrating the appropriateness of microwave-induced heating and its ability to be implemented in very different manners while producing consistent results. CONCLUSIONS This study demonstrates that microwave control is an efficient and effective alternative to conventional heating sources for EPA method 3050B. Control of temperature rather than microwave power is better at maintaining specific sample preparation temperatures. Temperature feedback control microwave systems are capable of controlling the temperature at the required 95 °C with an accuracy of (2 °C that is not achievable by either hot plate or microwave power control. Moreover, precision is improved, leach time is reduced by 60%, and reagent addition is automated. This application demonstrates that atmospheric pressure microwave sample preparation, applying temperature feedback control, is an appropriate alternative to traditionally implemented convection and conduction heating on hot plates. Additional environmental methods, such as total decomposition of oils, and polymers in EPA method 3031, or species extractions, such as chromium(VI) in method 3060, are also being implemented using this newer technology with similar improvements. Other traditional sample preparation methods requiring control of reaction temperature will find advantages in microwave temperature feedback control implementation. ACKNOWLEDGMENT The authors thank Dr. Peter Walter for providing the ICPMS data. In addition, we acknowledge Dr. Stuart Chalk for his valuable suggestions. The research was supported by a grant from Prolabo Corp. (France) and CEM (Matthews, NC). Received for review June 5, 1996. Accepted September 25, 1996. AC960553L X
Abstract published in Advance ACS Abstracts, November 1, 1996.
