Pressure Bombs

Technical Notes

Anal. Chem. 1994,66, 751-755

Development of a High Pressure/Temperature Focused Microwave Heated Teflon Bomb for Sample Preparation Henryk Matuslewlcz Department of Analytical Chemistry, Politechnika Poznahska, 60-965 Poznah, Poland

A new high pressurehemperatwe TFM-Teflon bomb for analytical sample preparation is described. The novel prototype system uses focused microwaves, operated at 2.45 CHz, to improve digestion capability. Up to 100-W microwave power can be concentrated into a single polymer vessel containing sample and nitric acid. Methodology was developed using powdered biological reference material. The feasibility of using water for in situ vessel cooling was investigated. The residual carbon content of 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 procedure. The sample preparation time is - 10 min (including subsequent cooling time and preparation of the final solution).

The complete digestion of sample is required to achieve reproducible and accurate elemental results in instrumental analytical methods. Two important developments in sample preparation procedures have been (1) use of sealed high- pressure bombs (vessels) to accelerate sample digestion and minimize contamination and loss of volatile elements and (2) use of microwave radiation to assist in digestion. The merits of high-pressure digestion in Teflon bombs are widely recognized,' and such techniques are attracting considerable attention. Acid digestion bombs are generally Teflon-lined, stainless-steel containers2 which, when sealed, are capable of digesting resistant materials with suitable solvents under elevated temperature and pressure conditions while retaining potentially volatile compounds.

In 1975, Abu-Samra et ala3 described one of the first uses of microwave heating for the rapid wet acid digestion of biological materials. This stimulated a long-term development of microwave technology for the preparation of all types of samples for analysis, as summarized in a recent book4 and general review article^.^-^ These studies have led to com-

(1) Jackwerth, E.; Gomiscek, S . Pure Appl. Chem. 1984, 56, 479. (2) Karpov, Y. A.; Orlova, V. A. Vysokochist. Veshchestua 1990, 2.40. ( 3 ) Abu-Samra, A.; Morris, J. S.; Koirtyohann, S. R. Anal. Chem. 1975, 47,

1475. (4) Introduction to Microwave Sample Preporatiom Theory and Practice;

Kingston, H. M., Jassie, L.B., Eds.; American Chemical Society: Washington, DC, 1988.

( 5 ) de la Guardia, M.; Salvador, A ; Burguera, J. L.; Burguera, M. J . Flowlnjection

( 6 ) Matusiewicz, H.; Sturgeon, R. E. Prog. Anal. Spectrosc. 1989, 12, 21. Anal. 1988, 5, 121.

0003~2700/94/0366-075 1$04.50/0 0 1994 American Chemical Society

mercial microwave digestion systems (CEM Corp., Floyd Inc., Milestone s.r.l., Questron Corp., Anton Paar, and Pro1abo)and microwave digestion bombs (Parr Instrument Co., Berghof GmbH, and GEC Alsthom Int.).

Microwave acid digestion is easily adapted to closed-vessel digestions; hence its application has been limited to decom- positions in closed Teflon-lined vessels made of nonmetallic microwave-transparent materials at a maximum safe upper pressure between 85 (Parr microwave digestion bomb) and 110 bar (Milestone microwave digestion rotor). For some techniques, it is necessary to destroy all electroactive organic matter lo (especially voltammetric determinations of metals); hence there is an obvious need to achieve higher temperatures while containing the reaction products, including copious amounts of gaseous reaction byproducts. Unfortunately, Teflon-lined reaction bombs placed inside stainless-steel jackets are unusable because of their metallic construction; therefore, various jackets (containers) made of high-strength nonmetallic microwave-transparent polymeric materials have been used (with the internal pressure not exceeding the design limits of the vessel shell). Also, the optimum temperature range for Teflon vessels terminates at -200 OC (the recommended maximum temperature for short-term use of Teflon bombs is 250 "C). An additional disadvantage of microwave oven systems is that as much as a 10-20% of temperature error can result from disparity between rated and actual power outputs, and uneven heating (nonhomogeneous distribution of microwave radiation in the oven cavity) may be a problem.11J2 Reproducibility of field strength or density from most manufactures and for different equipment models is essential for reproducible results; therefore, to transfer methods or reproduce them on different equipment, calibration is necessary but is not as common a practice as it should be. The feedback control which is just now becoming available can also help to alleviate this problem. Finally, an additional limitation is the necessary delay in opening digestion bombs, which must first

(7) Sulcek, Z.; Novak, J.; Vyskocil, J . Chem. Listy 1989, 83, 388. (8) Sulcek, 2.; Povondra, P. Methods of Decomposition in Inorganic Analysis;

(9) Kuss, H. M. Fresenius J. Anal. Chem. 1992, 343, 788. CRC Press, Inc.: Boca Raton, FL, 1989; Chapter 6 .

(10) Pratt, K. W.; Kingston, H. M.; MacCrehan, W. A,; Koch, W. F. Anal. Chem.

(1 1) Moralcs-Rubio, A,; Cerezo, J.; Salvador, A.; de la Guardia, M. Microchem.

(1 2) Vereda Alonso, E.; Garcia de Torres, A.; Can0 Pavon, J. M. Mikrochim. Acta

1988,60, 2024. J . 1993, 47, 270.

1993, 110, 41.

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be cooled to room temperature and the internal pressure reduced to a safe level. The capacity of the present generation of microwave ovens for sample digestion is rather limited.

Recognizing these limitations and the fact that rapid heating of only solvents and samples within a polymer vessel can lead to significant advantages over high-pressure steel-jacketed Teflon bombs that are thermally heated, we have sought to develop a focused microwave heated bomb that would exceed the operational capabilities of existing microwave digestion arrangements and permit construction of an integrated microwave source/bomb combination. As a result of this effort, combining advantages of high-pressure Teflon with those of microwave a focused high pressure/ temperature microwave heated digestion structure was pro- duced capable of being water or fluid cooled in situ. Preliminary tests indicate that this arrangement works well with both low- and high-power unpulsed- (continuous-) mode microwave generators, when used with digestion bombs, and permits precise control of the calories absorbed by the sample.

This note describes the design arrangements and preliminary operating conditions established for the focused microwave heated digestion system. The technique developed is an extension of the focused open-vessel microwave heated digestion of materials previously reported;l3-ISthis paper discusses the further application and evaluation of this concept.

I

DESIGN CONCEPT A specific objective was an integrated high pressure/

temperature polymer bomb/focused microwave heated digestion system demonstrably operable under the very high pressure and temperature typically employed with a classical Parr-type acid digestion bomb.

The design criteria for this device included obtaining a configuration that would match the dissolution and decomposition performance of conventional stainless-steel thermally heated bombs but would permit precise control of the calories absorbed by the sample. These were met by the system illustrated in Figure 1 (Patent Pending, Poland). This arrangement utilizes a rectangular steel waveguide to direct and focus the microwaves onto the sample and a polymer bomb to contain the sample and acid(s). The design offers very efficient (ca. 100%) energy transfer from the microwave generator to the sample and acid(s) and can be employed to create low-power as well as high-power microwave energy. Details of a 700 W maximum continuous power output, 2.45 GHz stabilized generator, Model MPC-0 1 a (Enterprise for Implementation of Scientific and Technological Progress, Plazmatronika Ltd., Wrocaw, Poland), have been reported earlier.16 In this system, the energy is directed only at that portion of the vessel that is in the direct path of the focused microwaves. The high pressure/temperature digestion vessel (inner volume of 28 mL) is constructed of a patented polymer, tetrafluorometoxil (Le., TFM-PTFE a registered trade mark of Hoechst Ag, Frankfurt, Germany), that permits use of high boiling point acids (such as sulfuric acid) at temperatures up to 350 C (TFM-PTFE polymer material: the use

(13) Grillo, A. C. Spectroscopy 1989, 4, 16. (14) Didenot, D. Spectra 2000 1990, 146, 44. (15) Feinberg, M. H. Analusis 1991, 19, 47. (16) Matusiewicz. H. Spectrochim. Acta 1992, 47B. 1221.

I

A

10 cm

7

Figure 1. Experimental setup: (A, top) overview photograph. (1) microwave power generator; (2) rectangular waveguide; (3) microwave heated digestion bomb. (B, bottom) schematic of the TFM-Teflon high pressure /temperature focused microwave heated digestion bomb. (1) hexagonal lug; (2) screw cap (stainless steel); (3) compressible rupture disks (safety valve): (4) stainless-steel compression plate; (5) TFM- Teflon lid; (6) vessel rim; (7) TFM-Teflon sample cup vessel; (8) ceramic vessel liner; (9) fluid cooling system; (10) internal coupling antenna system; (1 1) pressure bomb (stainless-steel jacket); (1 2) feeder coaxial: (1 3) external coupling antenna; (14) antirotation post.

temperature is 260-350 C; the melting/decomposition temperature is 350-380 OC17). The vessel is conically shaped with a handle that can be attached to the middle of the top rim, allowing it to be moved up. In addition, the Teflon pouring spout, which can be snapped to the peripheral sides of the vessels curved rim, prevents any contact between the highly corrosive sample solution and the metallic parts and facilitates

(1 7) Lautenschlgger, W.; Schweizer, T. LaborPraxis 1990, 14, 376.

752 Analytical Chemistry, Vol. 66, No. 5, March 7, 1994

quantitative transfer of the digested sample solution. Upon insertion of a new vessel, the distance between the top plate of the vessel and the plate lid is normally -3 mm. During use, this gap will slowly decrease. If it becomes less than 1 mm, a new cover must be used, since otherwise the distance between the cover lid and the vessel plate will be too close to allow blow-off in the case of overpressure. The TFM-Teflon sample digestion vessel is mounted in an autoclave (outer bomb casing) of "acid-proof" stainless steel, which can withstand a pressure of up to -200 bar (20 MPa). Several safety disks will rupture if the internal pressure exceeds 160 bar (1 6 MPa). During operation, a protection jacket is used around the bomb (not shown in the photograph). The hexagonal lug top must always be carefully tightened using a torque wrench, which protects the thread from forces higher than 1.5 Nm. No calculations were made to determine the force on the TFM- Teflon lid, because of thedifficulty in estimating the coefficient of friction in the thread. As the need for a more effective cooling method has been recognized,'* in order to minimize the delay in opening the TFM-Teflon pressure vessel following microwave heated acid digestion, a water or fluid (up to -10 "C) was used for in situ vessel cooling in both a pre- and postdigestion mode. The focused microwave heated digestion concept is versatile, and additional arrangements (i.e., one microwave power generator and two to four separate TFM- Teflon bombs) not described here are being evaluated.

This prototype laboratory version of the system is completely manual, accommodates one sample at a time, and is well-suited for research and development. A special configuration of antennas (internal and external coupling antenna systems) with a fixed penetration depth provides coupling of microwave energy between a magnetron and the sample and acid(s). 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%.

The bomb vessel is presently not equipped with any pressure or temperature indicators.

A significant feature of the new design is the relatively easy machining of the microwave heated bomb assembly in which the only precisely defined dimensions are the length and diameters of antennas as well as their configuration.

EXPERIMENTAL SECTION A high-pressure digestion system (High Pressure Asher,

HPA, Anton Paar, Austria) with 70-mL quartz vessels, autoclave, and microprocessor unit was used for sample decomposition with HN03 alone.

Suprapur grade HNO3 (65% m/V) (Merck, Darmstadt, Germany) was used as reagent for decomposition. Water, doubly distilled in a quartzapparatus (Bil8, 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-

(18) Reid, H. J.; Greenfield, S.; Edmonds, T. E. Anolyst 1993, 118, 443.

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 Laborgerate, Leipzig Germany) and a microcoulometer (Radelkis Model OH-405, Hungary) were used for the determination of total residual carbon in solutions of digested biological material.

Procedure. For the pilot study, both the high pressure/ temperature focused microwave heated TFM-Teflon bomb and the thermal high pressure acid digestion techniques were used to achieve complete decomposition of the biological- material. Theobjective was toobtain conditions which resulted in a nonresidual carbon presence in the final solution. The approaches are outlined below. All sample preparations were conducted under typical laboratory conditions.

The thermal high pressure decomposition method described by Knapp19 was established as the control technique since effectiveness of decomposition is maximized.

For removal of carbon contamination, the TFM-Teflon and quartz vessels were first heated for 3 h at 105 OC, hot leached with concentrated HNO3 for 3 h, and thoroughly rinsed with bidistilled water.

A suitable amount of bovine liver sample (-0.1 g) was transferred into individual TFM-Teflon vessels, and 2 mL of concentrated HNO3 was carefully added (The initial weights of the sample material to be decomposed were limited to the equivalent of -0.05 g of carbon in order to prevent the automaticventing of the digestion system that can occur with greater initial weights. Therefore, Le., 0.25 g of organic material cannot be digested safely). The vessel lid was kept in place and the assembly inserted in a "niche" of the ceramic vessel liner. The rupture disks were placed in position, and the stainless-steel cap was screwed in place and tightened using a torque wrench to 1.4 N m. The complete bomb assembly was then positioned in the rectangular waveguide attached to the microwave power generator. At this time, tap water was used for cooling during digestion in an attempt to reduce the increase in pressure without unduly slowing down the digestion. Additionally, the bomb was water cooled before commencing digestion of the bovine liver sample. This presumably postponed the onset of the pressure increase. The contents of the vessel were heated "unpulsed" at a power of 90 W for 4 min. After completion of the heating cycle, the assembly (stainless-steel bomb and TFM-Teflon vessel) was cooled with circulating water. After ca. 6-7-min cooling, the vessel had returned to room temperature. It was carefully opened using the torque wrench, the lid was washed with a small volume of bidistilled water, and the contents were transferred into a 1 0-mL calibrated flask and diluted to volume with bidistilled water. This cooling time may be decreased by using other fluids (i.e., at -10 "C). Sample preparation for this biological material was about 10-1 2 min, including subsequent cooling time and preparation of the final solution. No residue was observed in the solution, which was completely colorless once all the nitrogen dioxide had escaped. A corresponding blank was also prepared according to the above procedure.

For comparison, the same sample (-0.1 g) was decomposed in the thermally heated high pressure acid digestion system

(19) Knapp, G. Fresenius 2. A n d . Chem. 1984, 317, 213.

AnalyiicalChemistry, Vol. 66, No. 5, March I , 1994 153

Table 1. Rerldual Carbon Content In Dlgerted Sampler of Bovlne Liver (NIST-SRM 1577a) residual carbona

sample final vol, digest. time, in digestate, in dry sample, efficiency of digestion method mass, g mL min pg mL-1 mg g oxidation! %

microwave heated bomb 0.1 10 4 30f3 3 f 0.3 99.4 High Pressure Asher 0.1 10 120 2 0 & 2 2 f 0.2 99.6

a Total carbon content of undigested sample 510 f 10 mg g1 (n = 3), dry mass basis. Mean and standard deviation reported. * Five measurements from a triplicate sample preparation.

(HPA) using the same acid (HNO3), reagent blank, and dilution procedure. A step in this procedure involved the high- pressure decomposition described in detailed by WhiteZo with 2 mL of concentrated H N 0 3 at 320 OC and - 100 bar for 2 h. The resulting solution was clear and colorless.

The total residual carbon content in the resulting solutions was determined according to an earlier procedure* 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 an optical point of view. 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 during pressurized focused microwave heated bomb digestion. Good agreement between the microwave heated bomb decomposition and a traditional thermal acid decomposition procedure in a quartz bomb was obtained. As shown in Table 1, the microwave technique takes only -3% of the time 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 of -0.1 g as this mass is sufficient for the typical analysis of many trace elements (metals) in such samples. 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 the focused microwave heated bomb decomposition system for complete oxidation of such biological material.

The limiting pressure, causing the inner TFM-Teflon vessel to burst, has not been determined.

The cavity and the antennas did not become overheated during this digestion study, indicating efficient transfer of energy to the sample and solution. No microwave radiation leakage above 1 mW cm-2 was detected at a distance of 5 cm from the stainless-steel bomb under all operating conditions.

CONCLUDING REMARKS A recent review of acid microwave heated sample digestion

by Matusiewicz and Sturgeon6 highlighted one interesting problem for future study. A comment in the final section of

(20) White, R. T., Jr. J. Assoc. Ofl. Anal. Chem. 1989, 72, 387. (21) Matusiewicz, H.; Suszka, A,; Ciszewski, A. Acfa Chim. Hung. 1991, 128,

849.

that paper is particularly pertinent to the work reported here: Microwave digestion systems could also be designed specif- ically for use with steel-jacketed Teflon bombs utilizing a waveguide cavity design rather than an oven. This approach (energy from the magnetron focused through a waveguide directly into the sample) should attain much faster heating and higher pressures. Additionally, this should result in higher precision and sample-to-sample reproducibility compared to microwave oven digestions. In this respect, it appears that the high pressure/temperature bomb focused microwave heated digestion system described here can make a significant contribution.

This study has been motivated by the need to fill the gap between the sophisticated, expensive, and effective thermal high pressure digestion system (Le., HPA) and the sophisticated and expensive microwave heated digestion systems (Le., CEM, Milestone) that do not provide complete digestion efficiency.

It was shown, for the first time, that microwave heated digestion can be used to completely oxidize organics in a single- step procedure in a closed system, although general conclusions cannot be drawn from these limited experiments gained in the decomposition of only one matrix. This contrasts with the multistep procedures normally required (i-e., cycles of heating and cooling to limit pressure buildup).

The use of water or other fluid for in situ vessel cooling reduces the delay in opening the vessel and processing the digests. The primary advantage of the proposed design is that the closed TFM-Teflon focused microwave heated bomb enables very high pressure and temperature to be reached. In this apparatus, the pressure increases with rise in the temperature and the generated pressure is not controlled independent of the temperature. In general, the effects of high pressure and temperature during microwave heated digestion remain to be evaluated, especially for the safe application to spontaneous reactions which may occur during sample digestion.

Further development is needed in order to improve the design. The threaded closure of the stainless-steel bomb may be replaced by a bayonet closure (thus avoiding the unnecessary delay in closing and opening the bomb), and one assembly may be augmented by two or even four such arrangements (thereby permitting simultaneous multisample digestions).

This same stabilized generator could be used subsequently to provide microwave power to the bomb assembly and to a microwave cavity for microwave induced plasma atomic emission spectrometry.

Finally, this arrangement brings a new dimension to sample preparation and may combine the qualities of the Parr-type

754 Analytical Chemistty, Vol. 66, No. 5, March 1, 1994

Teflon bomb in retaining volatiles and the High Pressure Asher in completing effective decomposition within a short heating time characteristic of the microwave technique. The final goal of these studies is the development of an on-line apparatus based on the concepts described herein.

ACKNOWLEDGMENT Financial support by the State Committee for Scientific

Research (KBN), Poland, Grant PB618/P3/92/02 Appli- cation of Microwave Techniques to Analytical Chemistry is gratefully acknowledged. The assistance and cooperation of

R. Parosa and E. Reszke, of the Plazmatronika Ltd., in obtaining the microwave digestion assembly, without which this work would not have been possible, is gratefully acknowledged. A preliminary report of this work was presented at the PITTCON93, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlanta, GA, March 1993.

Received for review August 20, 1993. Accepted November 16, 1993.

Abstract published in Advance ACS Abstracts, January 1 , 1994.

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