Anal. Chem. 1999, 71, 3145-3149
Development of a High-Pressure Asher Focused Microwave System for Sample Preparation Henryk Matusiewicz
Department of Analytical Chemistry, Politechnika Poznan´ ska, 60-965 Poznan´ , Poland
The development of high-pressure Asher focused microwaves (HPA-FMs), a novel approach to microwave assisted digestion, is described. The system uses focused microwaves, at 2.45 GHz, to improve digestion capability with up to 650 W microwave power concentrated into six quartz pressure vessels containing samples and nitric acid. The device combines microwave heating with highpressure vessel technology (reactions can be conducted at pressures and temperatures up to 130 bar and 320 °C, respectively). Methodology was developed using powdered biological reference material. The residual carbon content of digested bovine liver sample was determined by Coulometry after combustion in an oxygen stream to evaluate the effectiveness of the decomposition procedure. With this new decomposition device, organic material is totally oxidized with nitric acid in a single-step digestion. Although sample preparation is an area of analytical chemistry that is often overlooked, it is generally this step in particular which affects the accuracy and reproducibility of the entire analytical procedure. Decomposition of solid samples or digestion of aqueous solutions containing an organic matrix that can interfere with the analytical determination of an element can be achieved in many different ways, e.g., open-vessel hot plate digestion or closed-vessel technique using a variety of commercially available digestion systems. In principle, the highest temperatures are required to achieve a decomposition as complete as possible.1,2 Convectively heated pressurized vessels have proved to be the most reliable systems which guarantee complete or almost complete decomposition of samples, because they provide elevated digestion temperatures in the range 200-230 °C.3 One of the most perfected systems is the high-pressure Asher (HPA) by Knapp,4 which allows for acid decomposition at extremely high pressures (up to 130 bar) and high temperatures (up to 320 °C) in a closed quartz vessel,5 thereby being the best solution for complete destruction of organic material. This system suffers from only one disadvantage: the digestion procedures are, though complete in most cases, very time-consuming. In addition, the prohibitive cost of HPA technology somewhat limits the utility of this technique. * Corresponding author: (phone) +48 (61) 878 23 12; (fax) +48 (61) 878 25 71; (e-mail)
[email protected]. (1) Wu ¨rfels, M.; Jackwerth, E.; Stoeppler, M. Anal. Chim. Acta 1989, 226, 1-16. (2) Wu ¨ rfels, M.; Jackwerth, E.; Stoeppler, M. Anal. Chim. Acta 1989, 226, 1730. (3) Jackwerth, E.; Gomisˇcˇek, S. Pure Appl. Chem. 1984, 56, 479-489. (4) Knapp, G. Fresenius Z. Anal. Chem. 1984, 317, 213-219. (5) AP PAAR, High-Pressure Asher HPA, 1987. 10.1021/ac9901743 CCC: $18.00 Published on Web 06/29/1999
© 1999 American Chemical Society
Remedial measures concerning time consumption and laborintensive manipulation were taken when microwave assisted digestion systems appeared,6 since they facilitate direct heating of the sample and therefore generally offer much shorter times for sample preparation.7 This stimulated a long-term development of microwave technology for the preparation of all types of samples for analysis and has led to commercial microwave digestion systems.8 Because power is supplied by microwave energy, the vessel materials have to be microwave transparent and, as a consequence, are mainly based on polymer (Teflon) compounds (polytetrafluorethylene, PTFE; perfluoralkoxy, PFA; tetrafluorperthylene-hexafluorpropylene, FEP; tetrafluormetoxil, TFM). Teflon has the advantage of being chemically inert and therefore is a suitable containment vessel for acids and most commonly used organic solvents. It does, however, suffer several disadvantages: it has a tendency to flow and creep, particularly at temperatures above 150 °C; it is slightly porous, therefore it is able to absorb and desorb, particularly mercury, and due to limited mechanical strength, working temperatures and pressures are restricted to values not always sufficient for the complete decomposition of certain sample materials. To realize the advantage of rapid reaction rates at high temperatures and pressures while recognizing these limitations, a focused high-pressure/temperature microwave heated digestion approach was presented by Matusiewicz.9 The main advantage of such a focused microwave heated digestion system is the possibility for complete oxidation of organics in a single-step procedure9,10 (the closed TFM-Teflon focused microwave heated bomb enables very high pressure and temperature to be reached). This concept led to the release of commercial focused microwave closed digestion systems (UniClever, Plazmatronika, Wroclaw, Poland, and MDA-2000, Berghof, Germany). However, such “microwave bombs” are not applicable to multiple determinations (sample throughput limited to one vessel per run), quite apart from a consideration of cost. It should be mentioned here that, only on the basis of the product information brochure,11 Milestone markets the “ultraCLAVE” microwave heated autoclave for high(6) Abu-Samra, A.; Morris, J. S.; Koirtyohann, S. R. Anal. Chem. 1975, 47, 14751480. (7) Microwave-Enhanced Chemistry. Fundamentals, Sample Preparation, and Applications; Kingston, H. M. (Skip), Haswell, S. J., Eds.; American Chemical Society: Washington, DC, 1997. (8) Erickson, B. Anal. Chem. 1998, 70, 467A-471A. (9) Matusiewicz, H. Anal. Chem. 1994, 66, 751-755. (10) Levine, K. E.; Batchelor, J. D.; Rhoades, C. B., Jr.; Jones, B. T. J. Anal. At. Spectrom. 1999, 14, 49-59. (11) Milestone S.r.l. ultraCLAVE Microwave Autoclave for High-Pressure Reactions, 1997 (e-mail:
[email protected]).
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Figure 1. Experimental setup. (A) Overview photograph of the high-pressure Asher focused microwaves system: (1) microwave power generator; (2) rectangular waveguide; (3) microwave heated stainless steel high-pressure chamber; (4) remote control panel; (5) electronic pressure sensor for the digestion vessel; (6) pressure detector for the digestion vessel; (7) ring retainer; (8) gas pressure inlet; (9) pressure gauge for pressure indication in the gas cylinder; (10) safety valve (130 bar); (11) expansion chamber; (12) safety disk tube; (13) gas pressure outlet; (14) pressure gauge for pressure indication in the chamber. (B) Schematic of the digestion vessels and high-pressure/-temperature focused microwave heated chamber: (1) quarter lambda antenna system; (2) quartz digestion (reaction) vessel with special cover; (3) rack; (4) high-pressure chamber; (5) gas pressure inlet; (6) gas presure outlet; (7) ring retainer; (8) safety lid; (9) metal rupture disk; (10) expansion chamber; (11) safety disk tube. (C) Schematic of the digestion vessel: (1) quartz decomposition vessel; (2) PTFE seal strip; (3) quartz lid; (4) tungsten spiral spring; (5) PTFE cap.
pressure chemical reactions (200 bar, 350 °C, 0-1000 W). Samples are not processed in closed vessels; rather the unit applies pressure to the samples in open vessels. In the author’s opinion, this concept was earlier suggested by Matusiewicz9 and the system appears to be an extension of the prototype of the pressurized sample autoclave (4 bar) previously reported by Linn High Therm,12 the model Lifumat Mic 1,2/2450. We have sought to develop a focused microwave heated closedvessel system that would exceed the operational capabilities of existing microwave digestion arrangements and permit construction of an integrated microwave source/closed-vessel/high-pressure chamber combination. As a result of this effort, combining advantages of the HPA4,5 with those of microwave heating,7 a highpressure Asher focused microwaves (HPA-FM) digestion structure was produced. Preliminary tests indicate that this arrangement works well with both low- and high-power unpulsed-mode (continuous-mode) microwave sources, when used with quartz digestion closed vessels. The present report describes, for the first time, the design arrangements and preliminary operating conditions established for the focused microwave heated multiple digestion system. Several digestion vessels can be heated simultaneously in a gas pressurized metal chamber, while measuring and controlling the (12) Linn High Therm GmbH. Lifumat MIC 1, 2/2450; 1995.
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pressure in one vessel. The technique developed is an extension of the focused single-closed-vessel microwave heated digestion approach previously reported;9 this paper discusses the further application and evaluation of this concept.
DESIGN CONCEPT A specific objective was an integrated pressure/temperature closed-quartz-vessels/focused microwave heated digestion system (chamber) demonstrably operable under the very high pressure and temperature typically employed with a conventionally heated high-pressure Asher HPA system. It is actually a high-pressure microwave chamber (autoclave); the closed quartz vessels themselves comprise the inside of the chamber. The design criteria demanded a configuration that would match the digestion performance of a hermetically closed stainless steel high-pressure chamber and permit precise control of the microwave energy absorbed by the samples. These were met by the system illustrated in Figure 1. This arrangement utilizes a rectangular steel waveguide to direct and focus microwave energy onto the samples and quartz vessels to contain the samples and acid(s). The apparatus incorporates a high-pressure hermetically closed chamber which is part of the original HPA system reported
earlier.4,13 Briefly, a high-strength acid resistant stainless steel chamber (volume, 1.5 L; Plimit ) 200 bar; Tlimit ) 320 °C) hermetically closed with a quick-lock lid contains a special rack system for up to six digestion vessels. A special pressure cap can be fitted onto one vessel to monitor the actual pressure inside the vessel during digestion at this time; there is no provision for simultaneous measurement of temperature during the digestion process. In the event of an unexpected temperature increase or an unexpected spontaneous violent reaction explosion that would increase the pressure of the quartz digestion vessel above 200 bar, the expansion would be contained in the expansion chamber (due to the design of the HPA, explosions inside the device will not cause damage to the pressure chamber; if the inside pressure is too high, it will be let out through a safety disk placed in the lid of the chamber; the safety disk is designed to burst at 200 bar). The design offers very efficient (ca. 93-98%) energy transfer from the microwave generator to up to six samples and acid(s) (through the quarter lambda antenna) and can be employed to use low-power as well as high-power microwave energy. A 650 W maximum continuous (unpulsed operation of the magnetron, providing a feedback controlled power regulation ranging from 10 to 650 W) power output, 2.45 GHz stabilized generator, model MG-650 (Plazmatronika Ltd., Wroclaw, Poland), was used. The energy is directed only at that portion of the vessels in the direct path of the focused microwaves. The HPA digestion vessels (inner volume of 30 or 70 mL) are made of high-purity quartz to reduce adsorption and contamination errors during acid digestions and are sealed with quartz lids, which are held in a rack using PTFE tape. The rack, with quartz sample digestion vessels, is housed in the high-pressure chamber (HPA autoclave). A safety valve prevents overpressure caused by heating not to exceed 130 bar (13 MPa). With completion of the time microwave heating program, the chamber cools, the nitrogen slowly releasing from the chamber, allowing the pressure in the chamber to drop below that inside the sample vessel. The quartz lid lifts up and allows CO2 and NO decomposition products of the organic material to escape from the digestion vessel. (It is recommended to put a cup of coarse-grained calcium carbonate into the pressure chamber when starting the decomposition procedure. This way eventually escaping acid fumes will be neutralized. This recommendation is valid for the use of acids, such as HNO3 and/or HCl.) To minimize the delay in opening the high-pressure chamber and at this same time the quartz pressure vessels following microwave heated acid digestion, an air cooling system, externally applied, is used for cooling during both a pre- and postdigestion mode. Unfortunately, this cooling system was not very efficient (cooling time is ∼20-30 min). It should be noted that any reference to power refers to that measured at the generator. Qualitative estimates of microwave radiation hazard were made with a microwave leakage detector, model MPD 10 (Plazmatronika Ltd.), with a full-scale calibration of 2 mW cm-2 and accuracy of 20%. A significant feature of the new design is the increased throughput compared to the one-closed-vessel design9 in which the only precisely defined dimensions are the lengths and diameters of quarter lambda antennas as well as their configuration. Further, because in the quartz digestion vessel (HPA system) (13) Anton Paar, K. G. High-Pressure Asher HPA; 1987.
the seal is achieved by pressing a quartz lid and the rim of the quartz vessel against a PTFE tape (the tightness of the system is guaranteed by an outside pressure of 130 bar, which presses a quartz lid onto the vessel), there is no possibility of sample contact with the stainless steel chamber (as in the closed, conventional Parr “Teflon bomb” with steel casing). The advantage of this system is less risk of contamination. EXPERIMENTAL SECTION A high-pressure digestion system (high-pressure Asher, HPA, Anton Paar, Graz, Austria) with 70 mL quartz vessels, autoclave, and microprocessor unit was used in comparative experiments for sample decomposition with HNO3 alone. Suprapur grade HNO3 (65% m/V) (Merck, Darmstadt, Germany) was used as reagent for decomposition. Water, doubly distilled in a quartz apparatus (Bi18, Heraeus, Hanau, Germany), was used as diluent. The National Institute of Standards and Technology (NIST) standard reference material (SRM) 1577a bovine liver (available in powder form) was analyzed for carbon before and after digestion to evaluate the efficiency of the microwave heated decomposition method. For this purpose an elemental analyzer (Perkin-Elmer model 240) was used for the determination of the total carbon in the original dried sample before decomposition, and a Knobloch apparatus for elementary microanalysis (VEB Laborgera¨te, Leipzig, Germany) and a microcoulometer (Radelkis model OH-405, Budapest, Hungary) were used for the determination of residual carbon in solutions of digested biological material. Procedure. For the pilot study, both the experimental HPAFM system and the thermal high-pressure acid digestion system were used for sample decomposition. The objective was to obtain conditions which resulted in nondetectable carbon in the final solutions. The approaches are outlined below. All sample preparations were conducted under typical laboratory conditions. The thermal high-pressure decomposition method described by Knapp4 was established as the control technique since effectiveness of decomposition is maximized. For removal of any carbon contamination, the quartz vessels were first heated for 3 h at 105 °C, hot leached with concentrated HNO3 for 3 h, and thoroughly rinsed with bidistilled water. A suitable amount of bovine liver sample (∼0.25 g) was transferred into each of six vessels, and 3 mL of concentrated HNO3 was carefully added (the initial weights of the sample materials to be decomposed were equivalent to ∼0.1 g of carbon, and 1 mL of HNO3 is used per 0.1 g sample, with a minimum of 3 mL). The vessels were sealed with quartz lids, held in place using PTFE tape, and inserted in a rack. The complete assembly was then carefully positioned in the autoclave chamber in which the coupling antenna system was connected through the rectangular waveguide attached to the microwave power generator. The chamber was then hermetically closed with a quick-lock lid and pressurized (130 bar) with argon or nitrogen. At this time, forced air-cooling was attempted in order to reduce the increase in pressure without unduly slowing down the digestion. The contents of the vessels were heated “unpulsed” at a power of 350 W for 5 min. After completion of the heating cycle, the chamber and the assembly (rack and quartz vessels) were completely cooled with the air fan. After ca. 20-30 min, the chamber and vessels had Analytical Chemistry, Vol. 71, No. 15, August 1, 1999
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Table 1. Residual Carbon Content in Digested Samples of Bovine Liver (NIST-SRM 1577a) residual carbona
digestion method
in dry efficiency sample final digest in of mass, vol, time, digestate, sample, g mL min µg mL-1 mg g-1 oxidation,b%
high-pressure Asher 0.25 focused microwaves high-pressure Asher 0.25
10
5
70 ( 3
7 ( 0.3
99.5 ( 4
10
120
50 ( 2
5 ( 0.2
99.6 ( 4
a Total carbon content of undigested sample of 510 ( 10 mg g-1 (n ) 3), dry mass basis. Mean and standard deviation reported. b Five measurements from a triplicate sample preparation.
reached room temperature. This cooling time may be decreased by using water or original thermal HPA air-cooling system. The gas was slowly released from the chamber, which was carefully opened, the reaction vessels were then safely opened, and the contents of quartz vessels were diluted to 10 mL volume with bidistilled water. Sample preparation for these biological material was about 30-40 min, including subsequent cooling time and preparation of the final solutions. No residue was observed in these solutions, which was completely colorless once all the nitrogen dioxide had escaped. A corresponding blank was also prepared according to the above procedure. For comparison, samples (∼0.25 g) were also decomposed in the Anton Paar thermally heated high-pressure acid digestion system (HPA) using the same acid (HNO3) and dilution procedure. A step in this procedure involved the high-pressure decomposition described in detail by White14 with 3 mL of concentrated HNO3 at 320 °C and ∼100 bar for 2 h. The resulting solutions were clear and colorless. The total residual carbon content in all resulting solutions was determined according to an earlier procedure15 and used as a measure of the efficiency of decomposition. RESULTS The primary purpose of this work was to evaluate the performance of the newly designed system for the decomposition of organic materials as measured by the completeness of sample destruction. The usefulness of the applied digestion technique was judged from the residual carbon content, not from a visual point of view. Reference material decomposed in this system resulted in a clear, colorless solution, undistinguishable from water. NIST SRM 1577a bovine liver contains 51% C (dry weight). The results, summarized in Table 1, clearly show that the organic matrix appears to have been completely oxidized following sample digestion with the pressurized focused microwave heated ashing system. Good agreement between this microwave heated ashing system and a traditional thermal acid decomposition procedure in a quartz bomb was obtained. As shown in Table 1, the microwave technique takes ∼4% of the time (not including subsequent cooling time and preparation of the final solution) required by the thermal high-pressure technique to decompose biological powdered material. In this study, the completeness of decomposition was investigated for the case of sample amounts (14) White, R. T., Jr. J. Assoc. Off. Anal. Chem. 1989, 72, 387-393. (15) Matusiewicz, H.; Suszka, A.; Ciszewski, A. Acta Chim. Hung. 1991, 128, 849-859.
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of ∼0.25 g, as this mass is sufficient for the typical analysis of many trace elements (metals) in such samples and is recommended from the point of view of homogeneity. The amount of acid should be minimized in order to avoid contamination from this reagent as well as for safety and economy. This supports the validity of the present decomposition procedure and operating conditions and confirms the suitability of this focused microwave heated ashing system for complete oxidation of such biological material. The chamber and the antennas did not become overheated during this digestion study (force cooled with a fan during operation), indicating efficient transfer of energy to the samples and solutions. No microwave radiation leakage above 1 mW cm-2 was detected at a distance of 5 cm from the stainless steel chamber under all operating conditions. CONCLUDING REMARKS It was shown, for the first time, microwave heating can be combined with high-pressure vessel technology. This concept fills the gap between expensive effective thermal high-pressure digestion systems such as the high-pressure Asher and the expensive microwave heated digestion systems that do not provide complete digestion efficiency; however, it is very difficult to predict if this technology would be any less expensive when commercialized. The HPA-FM system permits simultaneous multisample digestions (up to six samples) and can be used to completely oxidize organics in a single-step procedure in a closed high-pressure chamber. This contrast with the multistep procedures normally required (i.e., cycles of heating and cooling to limit pressure buildup). The longer cooling time can be accepted in favor of the higher production rates (the method can process six samples at a time). The primary advantage of the proposed design is that very high pressure (up to 130 bar) and temperature (up to 320 °C) can be reached. In addition, when decomposition is complete, the samples can be diluted to volume in the quartz digestion vessel without additional transfer of the sample. Further development is needed in order to improve the design. The air fan cooling may be replaced by a more efficient water system or much more effective original air-cooling HPA system for reducing the delay in opening the chamber and the reaction vessels. This device may be augmented by a more technically advanced HPA-S system16 (thereby permitting simultaneous 21 sample digestions and further increase the sample throughput). The material PTFE-TFM17 might be a good replacement for the quartz; therefore HF could be used as dissolution reagent. This material has a melting point above 350-380 °C. Last, because the system has no means for monitoring temperature during a digestion, future research on the high-pressure/temperature focused microwave heated ashing system should be directed at combining a recently introduced system,18 with simultaneous control of pressure and temperature in all vessels. (16) Perkin-Elmer, HPA-S High-Pressure Asher for Unmatched Digestion Quality, 1996. (17) Lautenschla¨ger, W.; Schweizer, T. LaborPraxis 1990, 14, 376-382. (18) Zischka, M.; Kettisch, P.; Schalk, A.; Knapp, G. Fresenius J. Anal. Chem. 1998, 361, 90-95.
ACKNOWLEDGMENT Financial support by the State Committee for Scientific Research (KBN), Poland, Grant 3 T09A 061 10 “Application of Microwave Technique to Trace Analysis in Biology, Medicine, and Environments”, is gratefully acknowledged. The assistance and cooperation of Plazmatronika Ltd., Wroclaw, Poland, and Anton Paar KG, Graz, Austria, in obtaining the microwave assembly and
high-pressure chamber, respectively, are gratefully acknowledged. A preliminary report of this work was presented at the 1998 Winter Conference on Plasma Spectrochemistry, Scottsdale, AZ, January 1998. Received for review February 15, 1999. Accepted April 29, 1999. AC9901743
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