Anal. Chem. 1994,66, 1097-1106
Microwave-Assisted Extraction of Organic Compounds from Standard Reference Soils and Sediments Viorica Lopez-Avila' and Richard Young Midwest Research Institute, California Operations, 625-8 Clyde A venue, Mountain View, California 94043 Werner F. Beckerl Environmental Monitoring Systems Laboratory, U.S. Environmental Protection Agency, 944 East Harmon Avenue, Las Vegas, Nevada 89 I19
As part of an ongoing evaluation of new sample preparation techniques by the US. Environmental Protection Agency (EPA), especially those that minimize waste solvents, microwave-assisted extraction (MAE) of organic compounds from solid materials (or "matrices") was evaluated. Six certified reference materials containing polynuclear aromatic hydrocarbons (PAHs) and a few base/neutral/acidic compounds, all of which are common pollutants of interest to the EPA, were subjected to MAE in a closed-vessel microwave system with hexane/acetone (1:l)at different temperatures (80,115, and 145 "C)and for different periods of time (5,10, and 20 min). For comparison, the same samples were subjected to room-temperature extraction by allowing the solvent mixture to stay in contact with the solid matrix the same amount of time as the microwave-extractedsample (including any cooling time). Whereas the average recovery at room temperature was -52%, the MAE recoveries for the 17 PAHs (3 of which were deuterated PAHs that were spiked into these matrices) from the six matrices were 70% a t 80 "C,75% at 115 "C, and 75% at 145 O C . Although the average recoveries increased slightly with extraction time, the increase was not statistically significant. The performance of the technique varied with the matrix and the analyte. Eleven PAHs had average recoveries in the 65-8596 range, and three compounds (acenaphthene, benzo[a]pyrene, and fluorene) had recoveries of 50%. The spiked-compound recoveries were 77% for acenaphthene-dlo, 105% for fluoranthene-dlo,and 85% for benzo[a]anthracene4 2 . Experiments with 14 phenols and 20 organochlorine pesticides indicated that MAE is a viable alternative to the conventionalSoxhlet/Soxtec and sonication techniques. The MAE technique requires smaller amounts of organic solvents, and sample throughput is increased by shorter extraction times (10min) and by simultaneous extraction of up to 12 samples.
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Use of microwave energy to enhance extraction of organic compounds from solid matrices such as soil, seeds, foods, and feeds was reported by Ganzler and Salgo in two publications in 1986 and 1 9 8 7 . ' ~These ~ researchers used a conventional, household microwave oven to irradiate solvent/sample suspensions for 30 s up to seven times each. They reported that the microwave-assisted extraction (MAE) method was more efficient than Soxhlet extraction for polar compounds.' (1) Ganzlcr, K.; Salgo, A.; Valko, K. J. Chromatogr. 1986, 371, 299-306. (2) Ganzler, K.; Salgo, A. 2.Unlers. Forsch. 1987, 184, 274-276.
0003-2700/94/0366-1097$04.50/0
0 1994 American Chemical Sacletv
Recently, Onuska and Terry3used microwave energy to extract organochlorinepesticides from sediment samples; they reported quantitative recoveries and no compound breakdown due to sample exposure to microwaves. Extraction of essential oils and other oils from biological materials such as plant and fish tissue by exposure to microwave energy was recently described in a patent application.4 In a U S . patent? extractionof natural products from mint, sea parsley, cedar foliage, and garlic with hexane, methylene chloride, or ethanol in two or more stages is described. Other researchers have reported use of microwave energy to extract stabilizers from polyolefins.6 As part of an ongoing U S . Environmental Protection Agency (EPA) program addressing sample preparation techniques that prevent or minimize pollution in analytical laboratories, this study addresses the extraction of organic compoundsusing a closed-vesselMAE technique. Six certified reference soil and sediment materials containing PAHs and a few base/neutral/acidic compounds of interest to the EPA were extracted with hexane/acetone (1:l) at different temperatures and for different periods of time to establish whether this technology has merit. Comparative measurements were made using conventional extraction techniques (e.g., Soxhlet/ Soxtec and sonication extraction), room-temperature extraction (as defined later), and MAE. The results indicated that MAE could bea viablealternative to the conventionalSoxhlet/ Soxtec and sonication methods. This technique uses smaller amounts of organic solvents, and sample throughput is increased by reduced extraction time (10 min) and by the capability of extracting up to 12 samples simultaneously (this study was performed with six samples being extracted simultaneously).
EXPERIMENTAL SECTION Standards. Analytical reference standards of 14nonlabeled and three labeled polynuclear aromatic hydrocarbons (PAHs), 14 phenols, and 20 organochlorine pesticides (Table 1) were purchased as composite solutions (concentration 2 mg/mL per compound) from Supelco, Inc. (Bellefonte, PA). The three deuterated PAHs were purchased as neat materials from Cambridge Isotope Laboratory (Woburn, MA). The other (3) Onuska, F. E.;Terry, K. A. Chromarographfa 1993, 36, 191-194. (4) Par€, J. R.J. Eur. Pat. Appl. EP 485668 A l , 1992: Chem. Absrr. 1992,117
(16), 157431~. (5) Par€, J. R. J.; et al. US.Patent 5,002,784, 1991. (6) Freitag, W.;John, 0. Angew. Makromol. Chem.
1990, 175, 181-185.
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Table 1. Compound8 Investlgatd In Thl8 study
PAHs compound name 1 2 3 4
5 6 7 8 9
acenaphthene acenaphthylene anthracene anthracene-dlo benz [alanthracene benz[a] anthracene-& benzo[al pyrene benzo[b+k]fluoranthene benzo[ghil perylene
compound name 10 11 12 13 14 15 16 17
chrysene fluoranthene-dlo fluorene fluoranthene indeno(l,2,3d)pyrene naphthalene phenanthrene pyrene
18 19. 20 21 22 23 24
Base/Neutral Compounds dibenzofuran 25 9H-carbazole 1,2-dichlorobenzene 26 di-n-butyl phthalate 1,3-dichlorobenzene 27 bis(2-ethylhexyl) phthalate N-nitroso-di-n-propylamine28 isophorone nitrobenzene 29 4-chlorophenyl phenyl ether 1,2,4-trichlorobenzene 30 butyl benzyl phthalate 2,4-dinitrotoluene
31 32 33 34 35 36 37
phenol 2-chlorophenol 2-methylphenol 3-methylphenol 2-nitrophenol 2,4-dimethylphenol 2,4-dichlorophenol
45 46 47 48 49
50 51 52 53 54
Phenols 38 39 40 41 42 43 44
4-chloro-3-methylphenol 2,4,6-trichlorophenol 2,4-dinitrophenol 4-nitrophenol
2,3,4,5-tetrachlorophenol 2-methyl-4,6-dinitrophenol pentachlorophenol
Organochlorine Pesticides a-BHC 55 dieldrin b-BHC 56 4,4’-DDE y-BHC 57 endrin I-BHC 58 endoslilfan-I1 heptachlor 59 4,4’-DDD aldrin 60 endrin aldehyde heptachlor epoxide 61 endosulfan sulfate y-chlordane 62 4,4’-DDT endosulfan-I 63 endrin ketone a-chlordane 64 methoxychlor
13compounds (base/neutral) inTable 1 (except dibenzofuran, which was bought from Supelco as neat material) were purchased as individual stock solutions from ChemService (West Chester, PA) and Supelco, Inc., and were combined with the PAH stock solution to make the,working calibration standards for the GC/MS analysis. Dibenzofuran was dissolved separately in methanol at 5 mg/mL and was combined with the PAH stock solution to make the working calibration standards. The purities of all compounds were stated to be higher than 96%. The spiking solution and the working calibration standardswere prepared by serial dilution of the composite stock solution containing either the phenols, organochlorine pesticides, PAHs, or base/neutral compounds; for the analysis of the ERA soil samples (defined below), the calibration standards contained the PAHs, the baselneutral compounds, and selected phenols. Standard Reference Materials. Six certified reference marine sediments and soils were used in this study. HS-3, HS-4, and HS-5 are marine sediments collected from three harbors in Nova Scotia. The materials were purchased from the National Research Council of Canada, Atlantic Research Laboratory (Halifax, NS). According to the certificate of analysis, these materials were freeze-dried, sieved to pass a 125-pm sieve, homogenized in a cement mixer, and then subsampled into 200-g portions. The SRS1941 marine sediment, purchased from NIST (Gaithersburg, MD), is a 1098 AnaEytcal Chemistry, Vol. 88, No. 7, April 1, 1994
sediment collected from the Chesapeake Bay at the mouth of the Baltimore Harbor. According to NIST, this sediment was air-dried, pulverized, sieved (70%) for 10 compounds, 2 compoundshad borderline recoveries (2-nitrophenolat 66.7% and pentachlorophenol at 55%), and 2 compounds (2,4dinitrophenol and 2-methyl-4,6-dinitrophenol)appeared to have degraded since their recoveries were 9.4 and 17.1%, respectively. Catalytic reactions in the presence of the soil may have been the cause of these low recoveries. In the case of the organochlorine pesticides (Table 7), we did not find any degradation when using solvent only
compound name
% avrec
% RSD
% avrec
RSD
%
a-BHC &BHC y-BHC 6-BHC heptachlor aldrin heptachlor epoxide y -chlordane endosulfan4 a-chlordane dieldrin 4,4'-DDE endrin endosulfan41 4,4'-DDD endrin aldehyde endosulfan s k a t e 4.4'-DDT endrin ketone methoxychlor
104 103 104 107 105 107 107 83.0 110 99.3 108 107 109 106 111 117 109 105 107 115
1.6 1.5 1.7 1.5 1.5 1.3 2.3 2.3 4.0 0.5 2.7 0.9 0.6 1.4 2.0 1.0 1.5 1.0 1.2 6.0
82.4 81.9 88.0 95.5 108 92.5 100 74.0 98.2 86.9 125 93.9 123 99.6 118 92.6 101 114 123 169
8.4 9.3 8.5 9.5 12.2 8.4 11.5 11.0 11.4 9.6 14.8 9.6 14.4 8.4 15.4 11.9 11.0 22.7 14.9 17.4
0 The number of determinations was three. The hexane/acetone extrads were not concentrated.
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Table 8. Average Recoverlea and Percent RSDs for PAHs and Selected Base/Neutral Compounds afler MAE Solvent VMW Solvent/Soll Suspendon' solvent and soil solvent only blow% % % % down compound name avrec RSD avrec RSD evap
PAHs acenaphthene acenaphthylene anthracene anthracene-dlo benz[alanthracene benz[alanthracene-dlz benzo[a1pyrene benzo[bl fluoranthene benzo[klfluoranthene benzo[ghilperylene chrysene fluoranthene-dlo fluorene fluoranthene indeno(l,2,3-cd)pyrene naphthalene phenanthrene pyrene base/neutral compounds dibenzofuran 1,2-dichlorobenzene 1,3-dichlorobenzene 1,2,4-trichlorobenzene 2,4-dmitrotoluene isophorone 4-chlorophenylphenyl ether butyl benzyl phthalate. a
72.6 74.8 84.8 76.6 79.5 82.4 101 103 101 100 81.3 82.1 77.5 85.0 109 66.3 81.9 78.1 75.2 62.1 61.6 65.3 96.7 67.4 77.1 84.5
4.0 4.1 2.4 2.5 2.1 3.5 10.3 11.1
11.6 8.9 1.7 4.1 3.9 4.1 9.9 5.0 2.3 2.0 3.7 5.8 7.4 5.4 3.1 4.9 3.8 2.0
103 105 109 95.5 103 106 94.7 93.6 94.5 91.0 105 105 106 109 102 93.3 106 102
2.2 95.8 1.9 97.9 3.0 103 2.6 89.8 2.7 95.9 1.9 98.8 2.2 88.1 2.2 85.2 4.2 90.7 2.3 81.1 2.2 99.0 2.3 99.0 2.8 98.1 3.3 102 2.7 93.3 8.0 90.7 3.5 99.0 2.3 94.9
105 3.2 96.1 85.2 16.7 86.8 87.5 15.3 88.6 93.0 6.8 89.8 135 3.3 126 96.6 1.6 90.9 107 3.2 97.1 112 1.8 105
The number of determinations was three.
(recoveries ranged from 83 to 117%). When soil was present, recoveries were still almost quantitative for all compounds but y-chlordane (recovery 74%). We found some losses for three of the four BHC isomers; however, their recoveries were acceptable (the range was 82-88%). In the case of the PAHs and a few base/neutral compounds (Table 8), we did not find any degradation when using solvent Analytical Chemistry, Vol. 66, No. 7, April 1, 1994
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only (recoveries ranged from 85.2 to 135%). When soil was present, PAHs exhihited 15% loss in recoveries and the other compoundsexhibited 30% loss. Consideringthat losses during the blowdown step could be as high as 15% for some of these compounds and that the measurement error could also be as high as 15%, we concluded that the 30% loss is not unreasonable for these types of compounds. We believe some of the more volatile compounds (e.g., chlorinated benzenes, naphthalene) may be partially lost during the filtration step. In summary, MAE of stable organic compounds from soil samples seems to be a viable alternative to the conventional techniques employing Soxhlet/Soxtec and sonication extraction. The main advantages of sample preparation using microwave energy are reduced extraction time (typical sample preparation time for this technique is 10 min for extraction and 40 min for extract cooling, centrifugation, and extract concentration) and reduced solvent use (30 mL in the MAE versus 300 mL in the Soxhlet/Soxtec extraction). Up to 12 samples can be extracted simultaneously in a few minutes, resulting in increased sample throughput over theconventional extraction techniques that employ Soxhlet and sonication extraction.
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ACKNOWLEDGMENT The authors thank the CEM Corp. for the loan of the microwave unit used in this study and Bob Revesz from CEM Corp. for helpful technical discussions. Nikhil Shah and Robert Kim of the Midwest Research Institute were involved with some early experiments using this technique. This research was funded by the US.Environmental Protection Agency (EPA) through its Office of Research and Development (ORD) and was conducted by the Midwest Research Institute under the management of the Environmental Monitoring Systems Laboratory in Las Vegas (EMSL-LV) to support ORD’s Hazardous Waste Issue. It has been subjected to ORD’s peer and administrative review and has been approved for publication. Readers should note the existence of a patent describing the microwave-assisted extraction of biological materials. Neither the EPA nor ORD endorses or recommends any trade names or commercial products mentioned in this article; they are noted solely for the purpose of description and clarification. Received for review August 20, 1993. Accepted December 6, 1993. Abstract published in Aduunce ACS Absfructs, February 15, 1994.
