Environ. Sci. Technol. 1995, 29, 2709-2712
Determination of PCBs in SoildSediments bv Microwave-Assisted Extraction VIORICA LOPEZ-AVILA,* JANET BENEDICTO, CHATAN CHARAN, AND RICHARD YOUNG California Operations, Midwest Research Institute, 625-BClyde Avenue, Mountain View, California 94043
WERNER F. BECKERT National Exposure Research Laboratory, Characterization Research Division, US.Environmental Protection Agency, 944 East Harmon, Las Vegas, Nevada 89119
Introduction Use of microwave energy to extract organic compounds from a contaminated soil was first reported by Ganzler and Salgo in 1986 and 1987 (1,2).The process of microwave extraction has later been patented by EnvironmentCanada (3),which holds rights on a microwave-assisted process (MAP) covering mainly the extraction of substances from biological materials. Recent environmental applications of this technology include extraction of selected pesticides from sediments (4),of polynuclear aromatic hydrocarbons from marine sediments and soils (5),of triazine herbicides from soil (61, of petroleum hydrocarbons from soils (3, and of organochlorine pesticides, organophosphorus pesticides, and over 90 semivolatile compounds listed in EPA Methods 8081,8141A, and 8250, respectively, from various freshly spiked and from spiked, aged soil samples (8). We have evaluated the microwave-assisted extraction (MAE) technique in combination with gas chromatography with electron capture detection (GCIECD) and with enzymelinked immunosorbent assay (ELISA) and present results for the determination of polychlorinated biphenyls (PCBs) in soil and sediment samples. The advantages of combining MAE and ELISA are multifold (a)MAE is fast (Le., extraction time is typically 10 min, and sample throughput can be greatly increased by the use of multivessel systems that allow simultaneous MAE of multiple samples); (b) MAE requires reduced volumes of organic solvents (Le., 30 mL for MAE as compared to 300 mL for Soxhlet extraction); (c) ELISA can provide results in approximately 45 min for a batch of 20 extracts, with considerable cost savings over the gas chromatographic analysis; and (d) both techniques are amenable to field use as screening and monitoring methods (the microwave unit can be put in a mobile van or trailer, and ELISA can be performed using a portable spectrophotometer).
Experimental Section Reagents and Materials. AU immunologic reagents used in this study, including the paramagnetic particles coated ~~
~
* Address correspondence to this author: telephone: (415) 6916844: fax: (415) 691-6845; e-mail address:
[email protected].
0013-936X/95/0929-2709$09.00/0
D 1995 American Chemical Society
with anti-PCB antibody (suspendedin buffer saline solution with preservatives and stabilizers), PCB enzyme conjugate (horseradish peroxidase-labeled PCB analogl, phosphate buffer, hydrogen peroxide solution (0.02% in citric acid buffer), chromogensolution (3,3’,5,5’-tetramethylbenzidine at 0.4 g/Lin an organic base), stopping solution (2M sulfuric acid), and washing solution, were obtained from Ohmicron Corporation (Newtown, PA). The topsoil (pH 7.5; cation exchange capacity 14.6 mequiv/100 g; organic carbon content 0.1%; sand 57.6%, silt 21.8%;clay20.6%)and clay soil (pH7.4; cation exchange capacity 21.3 mequiv/100 g; organic carbon content 1.8%; sand 33.6%, silt 35.4%; clay 31.0%)used in this study were obtained from Sandoz Crop Protection (Gilroy, CAI. Sand was purchased from a local nursery. Spiking of the samples with Aroclors 1016 and 1260 (obtained from Supelco, Inc., Bellefonte, PA) was performed as follows: the soil sample was weighed into an aluminum cup and was transferred to the microwave vessel; a concentrated stock solution containing the Aroclors (concentration 20 pglmL) was added to the sample with a syringe. The spiked samples were carefully mixed with a spatula and allowed to sit for a few minutes in the MAE vessels prior to the addition of 30 mL of hexane-acetone (1:l) to each vessel. One certified reference soil, two certified marine sediments, and 24 soil samples from a PCB-contaminated Superfund site were used in this study. The certified reference soil containing Aroclor 1260 was obtained from Environmental Resource Associates (ERA), Arvada, CO. The marine sediments containing Aroclor 1254, identified as HS-1 and HS-2, were purchased from the National Research Council of Canada, Atlantic Research Laboratory, Halifax, Nova Scotia, Canada. These materials had been freezedried, passed through a 125-pm mesh sieve, homogenized in a cement mixer, and subsampled into 200-g portions. The Superfund site samples contained approximately30% water and were known to contain Aroclor 1248. They had been analyzed by Soxhlet-GCIECD and supercritical fluid extractionlEUSA in a separate study (9). Microwave-Assisted Extraction Procedure. The MAE experiments reported here were carried out with the CEM Model MES 1000 microwave system (CEM Corporation, Matthews, NC). More details on the CEM system can be found in Ref 5. A 5-g portion of the certified reference soil, spiked soil, or soil sample containing “native” PCBs was accurately weighed into an aluminum dish and was transferred quantitatively to the Teflon-lined extraction vessel, and 30 mLhexane-acetone (1:l)was added. The extractionvessel was closed, after ensuring that a new rupture membrane was used for each extraction. Extractions were performed at 115 “C for 10 min at 100% power (1000 W). After extraction, the vessels were allowed to cool to room temperature (approximately10-15 min) before they were opened. The supernatant was first filtered through glass wool prewashed with hexane-acetone and was combined with the 2-3-mL hexane-acetone rinse of the sample residue. The extract was then concentrated to approximately 5 mL using nitrogen blowdown evaporation and was centrifuged twice for 10 min at 2300 rpm to separate
VOL. 29, NO. 10, 1995 /ENVIRONMENTAL SCIENCE & TECHNOLOGY
2709
the fine particulates. The extract was finally concentrated to 1 mL for analysis by GCIECD or ELISA. All extracts subjected to ELISAwere diluted 10-foldwith methanoland subsequently with phosphate buffer to bring the final concentration of the extract within the linear range of ELISA, which is 0.25-5 ngImL. The losses incurred during the nitrogen blowdown evaporation of the extracts were determined by spiking a 30-mL portion of hexane-acetone (1:l) with 2500 ng of Aroclor 1016 and 2500 ng of Aroclor 1260 and evaporating the spiked solution to 1 mL using a gentle stream of highpurity nitrogen. GC/ECD Analysis. Gas chromatographic analyseswere performed on a Hewlett-Packard 5890 Series I1 gas chromatograph (Hewlett-PackardCorporation,Wilmington,DE) equipped with two ECDs, an HP 7673 autoinjector, and an HPDOS Chemstation; the columns used were 30 m length x 0.32 mm i.d. x 0.25 pm film DB-5 fused-silica capillary column and 30 m length x 0.32 mm i.d. x 0.25 pm film DB-1701 fused-silica capillary column. The oven temperature was programmed from 140 to 190 "C (2-min hold) at 12 "C/min and then to 275 "C at 4 "CImin. Helium carrier gas flow was 4 minlmL for each column. The injector was maintained at 250 "C, and the detectors were maintained at 320 "C. The columns were connected to 30 cm length x 0.32 mm i.d. uncoated fused-silica tubing using a Y-shaped fused-silica splitter (Restek, Bellefonte, PA). Injections (1 pL) were made in a splitless mode (60 s) by the Hewlett-Packard autoinjector. Quantification of PCBs was performed using internal standard calibration with decachlorobiphenyl as the internal standard; five major peaks were selected for quantification. ELISA Procedure. ELISA was performed according to instructions supplied by Ohmicron Corporation; 2OOpL of the diluted soil extract, 250 pL of PCB enzyme conjugate, and 400 p L of anti-PCB antibody-coated paramagnetic particle suspensionwere combinedin a test tube. Following 1-2 s of vortexing, the test tube was kept at room temperature for 15 min. The mixture was separated by means of a magnetic separation rack, the liquid phase was decanted and discarded, and the residue was washed twice with reagent water. The rack containing the tubes was removed from the magnet, and 400pL of a freshly prepared hydrogen peroxide-3,3', 5,5'-tetramethylbenzidinechromogenic solution was added to each test tube and allowed to develop color. The reaction was stopped after 20 min with 500 pL of stopping solution (2 M sulfuric acid), and the color intensity in each test tube was determined at 450 nm using an Ohmicron RPA-1 photometric analyzer. The concentrations of the PCBs were determined by comparing the results with a linear regression line using an ln/logit standard curve of Aroclor 1254 or Aroclor 1248 at concentrations of 0, 0.25, 1.0, and 5.0 ngImL. Sample extracts at PCB concentrations greater than 5.0 ng/mL were diluted with phosphate buffer and reanalyzed by ELISA.
Results and Discussion MAE-GCIECD of Spiked Soil Samples. To determine PCB recoveries from freshly spiked soil samples, three different soil types (clay soil, topsoil, and sand) were spiked with Aroclors 1016 and 1260 at two spike levels (100 and 500 ng/g) and extracted. The results presented in Table 1 show that the average recoveries were greater than 70% for Aroclors 1016 and 1260 and the three soil matrices, except for Aroclor 1260 recovery from clay soil, which was only 2710 4 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. I O , 1995
TnBu I
Percent Average Recoveries and RSOs of PCBs Extracted from Three Freshly Spiked Soil Matrices and Determined by GC/ECV clay soil % average
compdname
topsoil %
recovery RSD
%
recovery
RSD 17 5
93 100
3 5
4
87 84
6
AroclorlOl6 Aroclor 1260
78 62
Spike Level 100 ng/g 7 84 3 76
Aroclor Aroclor 1260
80 87
Spike Level 500 ng/g 3 71 2 75
a
sand
YO average
5
YO average
% recovery RSD
4
The number of determinations was 3.
62%. The method precision was better than 7%, with the exception of Aroclor 1016 extracted from topsoil samples where the % RSD was 17%. To evaluate whether extractions were complete under the conditions used, the soil samples were reextracted with fresh solvent under the same conditions. We found that the amounts of the Aroclors recovered in the second fractionwere always less than 5%of the spikes. Losses of Aroclors during extract concentration using nitrogen blowdown evaporation (Table 2) were relatively small (14% for Aroclor 1016 and 8% for Aroclor 1260). Concern has been expressed that heating the soillsolvent suspension using microwave energy may possibly lead to degradation of certain analytes. To evaluate this possibility, we subjected Aroclor 1016 and 1260 to microwave-extraction conditions in the presence of solvent alone, solvent with dry topsoil, and solvent with wet topsoil. These experiments were conducted at 50 and 145 "C for 5 and 20 min at 100%power (1000 W). The results listed in Table 2 show recoveries above 71% for Aroclor 1016 and above 80% for Aroclor 1260 (these data were not corrected for blowdown losses); thus, the possibility of PCB degradation upon heating of solvent/soil suspensions with microwave energy was ruled out. MAE-ELISAof Reference and Real Samples. The results obtained with the MAE-GCIECD technique encouraged us to combine the MAE technique with ELISA, a determination method that is very sensitive and allows analysis of up to 20 soil extracts in about 1 h. We extracted by MAE three reference materials and 24 soils from asuperfund site, most of which contained Aroclors, and analyzed the extracts by both GC/ECD and ELISA. Because ELISA is very sensitive, and its detection range is quite narrow, the hexane-acetone extracts were first diluted with methanol and subsequently with the assay buffer (which contained 50% methanol) to bring Aroclor concentrations to less than 5 ng/mL. Aroclor concentrations of the Superfund site samples had been determined earlier by Soxhlet-GCIECD (10). Table 3 summarizes the Soxhlet-GCIECD,MAE-ELISA, and MAE-GC/ECD data. Excellent agreement was found between the certified SoxhletlGCIECDdata and the MAEELISA data (correlation coefficient 0.9986; slope 1.0168) and the MAE-GC/ECD data and the MAE-ELISA data (correlation coefficient 0.9793; slope 1.0468). In earlier experiments,we have found that Aroclors 1248, 1260,and 1262 gave ELISA responses that agreed well with the ELISA response for Aroclor 1254 (91, but the ELISA responses for Aroclors 1016, 1232,1242,and 1268 were off by more than a factor of 2 (almost 3 in the case of Aroclor
TABLE 2
Percent Recoveries of Aroclor 1016 and 1260 from Solvent Alone, Solvent and D q Topsoil Suspensions, and Solvent and Wet Topsoil Suspensions as a Function of Temperature and Microwave Heating Timea solvent alone
50 "C compd name
solvent
blowdown
Aroclor 1016 Aroclorl260
hexane-acetone(1:l) hexane-acetone(1:l)
86 92
a
5
solvent end dry soil
145 "C
50 "C
145 "C
solvent and wet soil
50 "C
145 "C
min
20 min
5 min
20 min
5 min
20 min
5 min
20 min
5 min
20 min
5 min
20 min
84 88
90 99
89 102
78 97
71 80
73 82
82 95
100 133
84 96
73 91
73 85
86 84
Single determinations. The spike level was equivalent to 500 ng/g. The recoveries were not adjusted for blowdown losses.
TABLE 3
Comparison of Soxhlet-GC/ECD, MAE=ELISA, and MAE=GC/ECD Results concentration I% RSDI certified velue
Soxhlet-GC/ECD
MAE-ELISA
MAE-GC/ECD
matrix
Aroclor type
hancs)
(mflsl*
(mfldb
(mfldb
ERA soil (lot 9801) HS-1 marine sediment HS-2 marine sediment soil 1 soil 2 soil 4 soil 5 soil 8 soil 9 soil 11 soil 12 soil 13 soil 14 soil 15 soil 16 soil 17 soil 18 soil 19 soil 20 soil 22 soil 23 soil 24 soil 25 soil 26 soil 27 soil 28 soil 29
1260 1254 1254 1248 1248 1248 1248 1248 1248 1248 1248 1248 1248 1248 1248 1248 1248 1248 1248 1248 1248 1248 1248 1248 1248 1248 1248
394 000 21.8 112
326 000 (2%) 20 (8%) 81 (13%)
200 260 34 000 < 30 < 30 500 590 110 1400
