Anal. Chem. 1994,66, 4005-4012
Direct Comparison of Soxhlet and Low- and High-Temperature Supercritical COS Extraction Efficiencies of Organics from Environmental Solids Steven B. Hawthorne. and David J. Miller Energy and Environmental Research Center, University of North Dakota, Campus Box 90 18, Grand Forks, North Dakota, 58202
Supercritical fluid extraction (SFE) with pure C 0 2 was used at 50,200, and 350 O C to extract PAHs, N- and S-heterocyclics, chlorinated phenols, and pesticides from a variety of soil and soot samples. The extraction efficiencies were directly compared to those obtained using 18 h of Soxhlet extraction with identical analysis methods. While SFE efficiencies at 50 O C (30 min) typically werelower thanSoxhlet (18 h) recoveries, SFE at 200 O C generally gave good quantitative agreement with Soxhlet extraction. Further increases in SFE temperature to 350 O C did not yield increased recoveries (except for PAHs from the soot sample), but did show evidence of causing thermal degradation of some species (particularly aromatic amines), and the possible production of some PAHs. The increases in SFE extraction efficiencies observed at 200 and 350 O C were not explained by volatility considerationsalone, since thermal desorption at these temperatures did not yield recoveries as high as SFE. Supercritical fluid extraction (SFE) has increasingly been accepted as an alternative to the use of liquid solvents for extracting organic pollutants from environmental Because of practical considerations of low toxicity, high purity, and a good ability to solvate a range of organics, supercritical C02 has received the most use for analytical-scale extractions. However, it is becoming increasingly apparent that pure C02 at "normal" extraction conditions (e.g., 400 atm at 50-80 "C) has insufficient ability to compete with matrix active sites to efficiently extract many organics from heterogeneous environmental solids such as soils, sludges, and air particulate matter.&I4 When recoveries of environmental pollutants have been poor, the most common approach has been to add organic modifiers to increase the polarity of the C02 solvent, either Camel, V.;TambutC, A.; Caude, M. J. Chromafogr.1993, 642, 263. Janda, V.;Bartle, K. D.; Clifford, A. A. J. Chromafogr.1993, 642, 283. Hawthorne, S . B. Anal. Chem. 1990, 62, 633A. Alexandrou, N.; Pawliszyn, J. Anal. Chem. 1989, 61, 2770. ( 5 ) Alexandrou, N.; Pawliszyn, J. Anal. Chem. 1992.64, 301. (6) Langenfeld, J. J.; Hawthorne, S.B.; Miller, D. J.; Pawliszyn, J. Anal. Chem. 1993, 65, 338. (7) Paschke, T.; Hawthorne, S . B.; Miller, D. J.; Wenclawiak, B. J. Chromafogr. 1992, 609, 333. ( 8 ) Hawthorne, S. B.; Langenfeld, J. J.; Miller, D. J.; Burford, M. D. Anal. Chem. 1992.64, 16 14. (9) Onuska, F. I.; Terry, K. A. J. High Resoluf. Chromatogr. 1989, 12, 357. (10) Hawthorne, S. E.; Miller, D. J. Anal. Chem. 1987, 59, 1705. (1 1) Hawthorne, S . E.; Miller, D. J.; Langenfeld, J. J. J. Chromatogr. Sci. 1990, 28, 2. (12) Sauvage, E.; Rocca, J.-L.; Toussaint, G. J. High Resolut. Chromafogr.1993, 16, 234. (13) Langenfeld, J. J.; Hawthorne, S. B.; Miller, D. J.; Pawliszyn, J. Anal. Chem. 1994, 66, 909. (14) Pawliszyn, J. J. Chromafogr.Sci. 1993, 31, 31.
0003-2700/94/0366-4005$04.50/0 0 1994 American Chemical Society
to increase the solubility of the target analyte or to interact with active sites on the sample matrix in order to more efficiently displace the analyte. The addition of organic modifiers, however, does complicate the SFE experiment since additional pumps or premixed fluids are required (unless a single addition of modifier to the sample is sufficient) and the added modifier may also interfere with subsequent analysis (e.g., aniline has been reported to be a good modifier for PCBs and PAH13but is not a particularly good GC injection solvent). Several recent studies have suggested that raising SFE temperature either with pure C02 or with modified C02 can yield substantial increases in SFE r e c o v e r i e ~ . ~ ,Because ~~-~~ of temperature limitations of existing commercial SFE instrumentation, the majority of studies have been limited to temperatures of 276) which might provide a source for pyrolytic production of some PAHs at 350 "C, while the soil sample had no such higher aromatics present.
CONCLUSIONS SFE with 400 atm of pure CO2 at 200 "C generally gave much higher recoveries of pesticides, chlorinated phenols, aromatic amines, and PAHs from real-world environmental solids than SFE at 50 " C despite the drop in C02 density. Further increases to 350 "C generally did not improve extraction efficiencies and were much more difficult to perform than extraction at 200 "C because of the problems sealing high-temperature extraction cells. In addition, extraction at 350 O C may have caused degradation of thermally reactive analytes such as aromaticamines and possibly led to production Analytical Chemistty, Vol. 66,No. 22, November 15, 1994
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Fluorene
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Flgure 3. SFE extractlon rates of representativePAHs from diesel soot using 400 atm of pure COz at 50, 200, and 350 O C . After 60 min, the sample extracted at 50 O C was extracted an additional 30 min at 200 120
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species can be associated with discretely different sites on the sample matrix. With the exception of the recoveries of high molecular weight PAHs ( M 1 252) from the soot sample, 30 min of extraction with pure C02 at 200 OC gave good quantitative agreement with 18 h of Soxhlet extraction and was simple to perform with commercially available SFE cells. These results demonstrate that increasing SFE temperatures with pure C02 to 200 O C is a powerful alternative to adding organic modifiers to increase the recoveries of a wide range of organic pollutants from real-world samples.
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Flgure 4. SFE extractlon rates of total hydrocarbons (based on GCFID analysis) from diesel soot using pure COz at 50, 200, and 350 O C .
of some low molecular weight PAHs from matrix organics in diesel soot. SFE rates of PAHs were shown to differ greatly between soil and soot matrices, and differences in extraction behavior at different temperatures indicate that the same
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Analytical Chemistry, Vol. 66, No. 22, November 15, 1994
ACKNOWLEDGMENT The financial support of the U.S.Environmental Protection Agency (EMSL, Las Vegas) is gratefully acknowledged, as are instrument loans from Isco. The authors also thank the Ontario Ministry of the Environment and Energy and Laszlo Torma (Montana Department of Agriculture) for providing some of the samples used in this study. Received for review April 25, 1994. Accepted July 27, 1994.' Abstract published in Aduance ACS Abstracts, September 15, 1994.
