Anal. Chem. 1995, 67,2424-2430
Independent Comparison of Soxhlet and Supercritical Fluid Extraction for the Determination of PCBs in an Industrial Soil Wren Bewadt*gt and Berit Johansson*
Environment Institute, CEU Joint Research Centre, TP 290, 1-21020 lspra (VA), Italy Samuel Wunderli and Markus Zennegg
Swiss Federal Laboratories for Materials Testing and Research, Uberlandstrasse 129, CH-8600 Dclbendotf, Switzerland Luiz F. de Alencastro and Dominique Grandjean lnstitut de g&nie de I'environnement- Ewtoxiwlogie, Ecole polytechnique f&d&rale,CH-1015 Lausanne, Switzerland
An independent comparison between supercritical fluid extraction (SFE) and Soxhlet extraction methods was conducted as part of a certification of PCB congeners in industrial soil. The study was performed in the framework of the PCB group for the Measurements and Testing Program (former BCR) under the Commission of the European Union. It involved 21 selected and independent laboratories experienced in congener-specificPCB analysis. The comparison was performed using both interlaboratory and intralaboratory data. SFE was performed independently by three laboratories, and the obtained data showed that SFE is a very competitive alternative in terms of both accuracy and precision. The soil was certitied for eight PCB congeners ranging from 7.0 to 137 pg/g with interlaboratory RSDs of 9-13% using all but two (out of 34) individual results from the SFE group. In recent years, supercritical fluid extraction (SFE) has gained increased interest because of the potential for reducing (1) the use of large amounts of hazardous organic solvents and (2) the work connected with conventional methods.'-8 Unfortunately, but also understandably, early SFE methods were developed using spiked or fortified samples on a variety of homemade instrum e n t ~ . ~The - ~ results ~ obtained indicated that SFE had a very + Present address: Energy & Environmental Research Center, University of North Dakota, Grand Forks, ND 58202. Present address: DTI Environmental Technology, Gregersensvej, P.O. Box 141, DK-2630 Taastrup. Denmark. (1) Hawthorne, S. B.Anal. Chem. 1990,62,633A (2) King, J. W. J. Chromatogr. Sci. 1989,17, 355. (3) Hawthorne, S. B.; Miller, D. J.; Burford, M. D.; Langenfeld, J. J.; EckertTilotta, S. E.; Louie, P. K. J. Chromatogr. 1993,642, 301. (4) King, J. W.; France, J. E. In Analysis w'th Supercn'tical Fluids: &traction and Chromatography;Wenclawiak, B., Ed.; Springer: Berlin, 1992; p 32. (5) Langenfeld, J. J.; Hawthome, S. B.; Miller, D. J.; Pawliszyn, J. Anal. Chem. 1993,65,338. (6) Camel, V.; Tambutk, A; Caude, M.J Chromatogr. 1993,642, 263. (7) Janda, V.; Bartle, K. D.; Clifford, A A./. Chromatogr. 1993,642, 283. (8) Chester, T. L.; Pinkston, J. D.; Raynie, D. E. Anal. Chem. 1994,66, 106R. (9) Hawthorne, S. B.; Krieger, M. S.; Miller, D. J. Anal. Chem. 1989,61, 736. (10) Lopez-Avila, V.; Dodhiwala, N. S.; Beckert, W. F. J. Chromatogr. Sci. 1990, 28, 468.
2424 Analytical Chemistty, Vol. 67, No. 14, July 15, 1995
large potential for extraction of a variety of organic pollutants from environmental samples even under very mild conditions. However, later accounts on SFE performed on real samples with incurred contaminants have shown SFE to be more difficult than initially indicated by the spiked and fortitied s a m p l e ~ . ~ < j JThis -'~ finding has prompted a new approach in SFE where methods are developed directly on real samples with trace amounts of incurred contaminants. Using this approach, it was shown to be possible to perform on-line cleanup of real environmental samples in SFE without any manual sample manipulation, thus facilitating automated extraction and a n a l y ~ i s . l ~However, -~~ there is still a lack of robust SFE methods for routine usage, and increased emphasis has to be devoted to this field if SFE shall continue to develop into a real alternative to the conventional extraction methods for environmental analysis. Polychlorinated biphenyls (PCBs) have been among the most studied environmental contaminants for more than two decades. A large effort has been made to determine the toxicity, environmental pathways, chemical and biological stability, and major sources of P C B S . ' ~ - ~The ~ outstanding physical and chemical properties of PCBs are their high thermal stability, resistance to oxidation, acids, bases, and other chemicals, and their excellent dielectric characteristics. These and other desirable properties for industrial applications led to widespread use of PCBs and were (11) Snyder, J. L.; Grob, R L.; McNally. M. E.; Oostdyk. T. S.Anal. Chem. 1992, 64, 1940. (12) Burford, M. D.; Hawthorne, S. B.; Miller, D. J. Anal. Chem. 1993,65,1497. (13) Langenfeld, J. J.; Hawthome, S. B.; Miller, D. J.; Pawliszyn, J. Anal. Chem. 1994,66,909. (14) Yang, Y.; Gharaibeh, A.; Hawthorne, S. B.; Miller, D. J. Anal. Chem. 1995, 67, 641. (15) David, F.;Verschuere. M.; Sandra, P. FreseniusJ. Anal. Chem. 1992,344, 479. (16) Bswadt, S.; Johansson, B.; Pelusio, F.; Larsen, B. R.; Rovida, C. J. Chromatogr., A 1994,662, 424. (17) Bswadt, S.; Johansson, B. Anal. Chem. 1994,66, 667. (18) Bewadt, S.; Johansson, B.; Fruekilde, P.; Hansen, M.; Zilli, D.; Larsen, B.; de Boer, J. J. Chromatogr., A 1994,675, 189. (19) Hutzinger, 0.; Safe, S.; Zitko, V. The Chemistry of PCBs; CRC Press: Cleveland, OH, 1974. (20) Erickson. M. D. Analytical ChemistryofPCBs; Butterworth: Stoneham, MA, 1986. (21) Safe, S. CRC Cn't. Rev. Toricol.1984,13, 319. (22) Lang, V. J. Chromatogr. 1992,595. 1. 0003-270019510367-242459.0010 0 1995 American Chemical Society
also responsible for the persistent contamination of the global envir~nment.~~ The predominant method for detection and determination of PCBs in environmental samples is by gas chromatography (GC) with either electron capture detection (ECD) or mass spectrometric detection (MS) .22 With the development of fused silica capillary columns, GC enabled the separation of a large number of individual PCB congeners (CBs) and therefore also a more precise determination compared to the use of packed columns for the estimation of the total PCB content based on Aroclor m i x t u r e ~ . 2 ~In, ~the ~ last 4-5 years, congenerspeciiic analysis has developed significantly and a great deal of commercial capillary columns are now fully characterized for the elution profile of individual congener^.^^-^^ The standard conventional sample preparation method for the analysis of PCBs in environmental matrices (among which soil is one of the most important) has through the years been Soxhlet extraction with a mixture of solvents, followed by cleanup over acid silica, Florisil, or a combination eventually followed by sizeexclusion chromatography. These methods have remained virtually unchanged during the last 20 years. Soxhlet extraction, however, is not very attractive either when reduction of solvent usage is concerned or from the standpoint of automation. Therefore, the use of SFE for soil samples seems to be a good altemative.36 This is especially the case if solid phase trapping is used, which facilitates the automated extraction and analysis because of the on-line cleanup possibilities. Until now an independent comparison of SFE and Soxhlet extraction for the analysis of PCBs has never been rigorously performed among a large group of laboratories experienced in congener-specific analysis. The aim of this study has been to investigate the independent comparison of SFE and Soxhlet extraction for the congener-speciiic analysis of PCBs from a real industrial soil with incurred contamination in the course of the certitication of this material as a reference material. EXPERIMENTAL SECTION Chemicals. The reference material (CRM 481)37and the PCB standards (neat crystals) used in this study were obtained from the Community Bureau of Reference (BCR), Brussels, Belgium.38-40 (23) de Voogt, P.; Brinkman, U. A Th. Production, proporties and usage of polychlorinated biphenyls. In Halogenated Biphenyls, Terphenyk, Naphthalenes, Dibenrodioxins and Related Products, 2nd ed.; Kimbrough, R D., Jensen, A A, Eds.; Elsevier: Amsterdam, 1989. (24) Ballschmiter IC; Zell, M. Fresenius'Z. Anal. Chem. 1980,302,20. (25) Mullin, M.; Pochini, C.; McCrindle, S.; Romkes, M.; Safe, S.; Safe, L. Enuiron. Sci. Technol. 1984,18, 468. (26) Larsen, B.; Bswadt, S.; Tilio, R Int. J. Enuiron. Anal. Chem. 1992,47, 47. (27) Bmadt, S.; Skejwhdresen, H.; Montanarella, L.; Larsen, B. Int. J. Enuiron. Anal. Chem. 1994,56, 87. (28) Bmadt, S.; Larsen, B. J High Resolut. Chromatogr. 1992,15, 377. (29) Larsen, B.; Bmadt, S.; Tilio, R; Facchetti, S. Chemosphere 1992,25, 1343. (30) de Boer, J.; Dao, Q. T.; van Dortmund, R J High Resolut. Chromatogr. 1992, 15, 249. (31) de Boer, J.; Dao, Q. T.I. High Resolut. Chromatogr. 1991,14, 593. (32) Galceran, M. T.; Santos, F. J.; Barcelo, D.; Sanchez, J. J. Chromatogr. 1993, 655, 275. (33) Fisher, R.; Ballschmiter, IC Fresenius'Z Anal. Chem. 1988,332, 441. (34) Schantz, M. M.; Panis, R M.; Kurz, J.; Ballschmiter, K; Wise, S. A Fresenius J. Anal. Chem. 1993,346, 766. (35) Hillery, B. R.; Girard, J. E.; Schantz, M. M.; Wise, S. A. J. High Resolut. Chromatogr. 1995,18, 89. (36) van der Velde, E. G.; de Haan, W.; Liem, A K D. J, Chromatogr. 1992, 626, 135. (37) Maier, E. A; Chollot, A; Wells, D. E. The cett@xtion ofthe contents (mass fraction) of eight chlorobiphenyls IUPAC no 101, 118, 128, 149, 153, 156, 170 & 180 in industrial soil (CRM 481); CEU report, ER 16215 EN, 1995.
The PCBs used were IUPAC Nos. 28,52,101,105,118,128,138, 149, 153, 156, 170, and 180. The solvents used (acetone, n-hexane, n-heptane, methanol (MeOH) , isooctane, dichloromethane) were all pesticide grade (Merck, Darmstadt, Germany). The MeOH-modified COZmixtures (2 and 5%)used by laboratory 1were obtained as SFE/SFC grade from SIAD, Milan, Italy. Laboratories 2 and 3 both used pure COZ in a purity of 40 and 48, respectively, obtained from Carbagas, Lausanne, Switzerland. Supercritical Fiuid Extraction. All the work presented here was performed with Hewlett-Packard 7680A supercritical fluid extractors. The procedures used at each laboratory are as follows: Laboratory 1 (Lab 1). Industrial soil (CRM 481) in portions of 100 mg were mixed with -10 g of anhydrous Na~S04and packed into 7 mL extraction cells. COZmodified with 2%MeOH (premixed in the cylinder) was used as extraction fluid at the following conditions: 10 min static extraction at a density of 0.75 g/mL (378 bar) at 97 "C followed by 40 min dynamic extraction at the same density and temperature with a flow of 1 mL/min. The completeness of the extractions were examined using sequential extractions with COZmodiiied with 5%MeOH for a 30 min dynamic extraction using the above-mentioned conditions. The nozzle temperature was kept constant at 45 "C, and the trap was kept at 65 "C. The trap was filled with -1 mL of Florisil (0.16-0.25 mm particle size) as trapping material and was eluted with 2 x 1.5 mL n-heptane and then 1 x 1.5 mL of dichloromethane followed by 2 x 1.5 mL of n-heptane after the end of each individual extraction. The extracts were diluted 10 and 50 times in isooctane, and an internal standard (PCB 35 and PCB 169) was added to the diluted fractions resulting in internal standard concentrations of -60 pg/pL for PCB 35 and -12 pg/ pL for PCB 169. Laboratory 2 (Lab 2). Industrial soil (CRM 481) in portions of 100 mg of were mixed with -5 mL of anhydrous NazS04 (premixed with 5.5% (v/v) MeOH) and packed into 7 mL extraction cells. Use of pure COZas the extraction fluid, together with the NaZSO4premixed with MeOH, essentiallyyields the same effect as addition of 4% (v/v) MeOH directly to the extraction cell. The extractions were performed with the following conditions: 16 min static extraction at a density of 0.84 g/mL (371 bar) at 70 "C followed by 30 min dynamic extraction at the same density and temperature with a flow of 3 mL/min. The completeness of the extractions was examined using sequential extractions with the above-mentioned conditions. The nozzle temperature was kept constant at 45 "C and the trap was kept at 25 "C. The trap was filled with -1 mL of octadecylsilica (ODS,Hypersil, 35-45 pm) as trapping material and was eluted with 7 x 1.5 mL of isooctane. The extracts obtained were diluted to -25 mL and two aliquots taken (0.775 and 0.140 g) to which were added internal standard (dichlorobenzyl ether, DCBE 7) in isooctane up to 0.969g. Laboratory 3 (Lab 3). The extraction cells were packed in the following way: A cotton wool plug, a layer of -1 g of (38) Quevauviller, Ph., Maier, E. A, Griepink, B., Eds.; Quality assurance for environmental analysis: Method evaluation within the measurements and testing programme (BCR). In Techniques and Instrumentation in Analytical Chemistry, Elsevier Science B. V.: Amsterdam, 1995; Vol. 17, pp 1-25. (39) Barcelo, D., Ed.; Techniques and Instrumentation in Analytical Chemistry; Elsevier Science B. V.: Amsterdam, 1993; Vol. 13, pp 1-25. (40) Note, the sales of BCR reference materials is now (1995) administrated by the Institute for Reference Materials and Measurements (IRMM), Retieseweg, E2440 Geel, Belgium.
Analytical Chemistry, Vol. 67,No. 14, July 15, 1995
2425
anhydrous NaZS04,600 mg of industrial soil (CRM 481), followed by a layer of -1 g of anhydrous Na~S04and finished by a cotton wool plug. COZmodified with 1.7%MeOH (added by a separate modifier module from Bischoff model 2200) was used as extraction fluid at the following conditions: 20 min static extraction at a density of 0.84 g/mL (371 bar) at 70 "C followed by 20 min dynamic extraction at the same density and temperature with a flow of 3 mL/min. The completeness of the extractions was examined using sequential extractions with the above-mentioned conditions. The nozzle temperature was kept constant at 45 "C and the trap was kept at 70 "C during extractions and 25 "C during rinsing. The trap was filled with -300 mg of Kieselgel KG 60 (Merck) as trapping material and was eluted with 8 x 1.0 mL of n-hexane after the end of each individual extraction. The three first extracts obtained were pooled and diluted -10 times in isooctane, and internal standards (octachloronaphthalene and 1,2,3,4tetrachloronaphthalene) in isooctane were added. Soxhlet Extraction. The different laboratory procedures were as follows: Lab 1. Aliquots (100 mg) of industrial soil were mixed with -10 g of anhydrous Na2SO4 and extracted with 250 mL of a mixture of n-hexane and acetone (2:3) for 48 h. The solventswere evaporated on a rotary evaporator at 30 "C and redissolved in 10 mL of n-hexane. Extracts were loaded on a 15 cm x 6 mm column with activated silica impregnated with 40% (w/w) sulfuric acid and eluted with 50 mL n-hexane. The eluent was evaporated, and the residues were redissolved in 1.5 mL isooctane. Internal standards were added (PCB 35 and 169, as for the supercritical fluid extractions), and the final volume was adjusted to 1.8 mL with isooctane. Lab 2. The residues from SFE were transferred to a Soxhlet thimble and extracted with 250 mL of a mixture of n-hexane and acetone (80:20) for 20 h. After a change of solvents, the residue was extracted a second time with 250 mL of the same solvent mixture for 10 h. The solvents were evaporated and redissolved in isooctane. No additional cleaning was performed before addition of internal standards and quantitation. Lab 3. Aliquots (600 mg) of industrial soil were mixed with -10 g of anhydrous NazS04 and extracted with 250 mL of n-hexane for 24 h. After a change of solvent, the residue was extracted a second time with 250 mL of the same solvent mixture for 24 h. No additional cleaning and evaporation was performed before addition of internal standards and quantitation. BCR-Group Soxhlet Laboratories. Sample sizes ranged from 100 mg to 1 g using solvent quantities from 60 to 500 mL with extraction times from 2 to 24 h. The solvents used were generally mixtures of two solvents with different polarity, of which the most common were n-hexane/acetone and n-pentane/dichlor~methane.~~ Gas Chromatography. Procedures used in each laboratory were as follows: Lab 1. Dualcolumn gas chromatography. The extracts were analyzed using a pressurecontrolled Hewlett-Packard 5890 11gas chromatograph equipped with heatable oncolumn injector (run in oven track mode) and two 63Nielectron capture detectors held at a temperature of 300 "C (purged with 60 mL/min of argon/ methane (10%))and a HP 7673A autosampler. Aliquots (1pL) of the extracts were oncolumn injected on two parallel coupled columns, a 60 m x 0.25 mm, 0.25 pm 50% 2426
Analytical Chemistty, Vol. 67,No. 14, July 15, 1995
diphenyldimethylsiloxaneDB17 column U&W Scientific) and a series combination of a 25 m x 0.25 mm, 0.25 pm 5% diphenyldimethylsiloxaneSIL8 column (Chrompack) and a 25 m x 0.22 mm, 0.10 pm 1,7-dicarba-closododecarborane-dimethylpolysiloxane HT-5 column (Scientific Glass Engineering). The columns were installed in the GC oven together with a deactivated 2 m x 0.53 mm fused silica retention gap using a quick-seal glass 'T'. The GC oven program was as follows: initial temperature 90 "C, retained for 2 min, then increased at a rate of 20 "C/min to 170 "C, retained for 7.5 min, then increased at a rate of 3 "C/min to 275 "C, and retained for 10 min. Hydrogen linear velocity was -43 a d s , held constant by the pressurecontrolled inlet throughout the whole temperature program (starting pressure 1.7 bar at 90 "C). This choice of columns and GC conditions has previously been shown to give optimum separation of CBs and organochlorine pesticide^.^^ Quantitative measurements of PCBs were performed using peak heights after a seven point multilevel calibration curve using the power fit calibration routine provided with the HP Chem 3365 software. PCBs were calibrated in the concentration interval of 1.7-573 pg/pL on each of the two columns, and the lowest results obtained from the quantitation of the extracts were adapted as the value closest to the true value. Standards were injected after every f&h sample to determine deterioration of separation or drift. New calibrations were performed if the results for the standards drifted by more than 10%. Lab 2. The extracts were analyzed using Hewlett-Packard 5890 I1 and Varian 3400 gas chromatographs equipped with split/ splitless injectors (injection temperatures 250 and 260 "C, respectively) and 63Nielectron capture detectors held at a temperature of 300 and 350 "C, purged with 60 and 45 mL/min Nz, respectively. Aliquots (1 and 3 pL) of the extracts were injected in the split mode (split ratios, 1:5 and 1:16) on two different columns, a 60 m x 0.25 mm, 0.25 pm bis(cyanopropy1)phenylsiloxane SP-2331 column (Supelco, on the HP 5890 GC) and a 60 m x 0.25 mm, 0.25 pm 5%diphenyldimethylsiloxaneDB-5 column U&W Scientific, on the Varian 3400 GC). The GC oven programs were the following: initial temperature 130 "C, retained for 4 min (2 min for SP-2331), then increased at a rate of 2.5 "C/min to 285 "C (255 "C for SP-2331), and retained for 10 min (20 min for SP-2331). Helium linear velocity was -26 cm/s. Quantitative measurements of PCBs were performed using peak heights provided by HP 3396 series I1 integrators. The calibrations were performed with bracketing standards calculated to be -&lo% of the concentrations of each PCB as determined by a preliminary analysis. This calibrating routine was performed on each of the two columns, and the lowest results obtained from the quantitation of the extracts were adapted as the value closest to the true value. Bracketing standards were injected after every fourth sample. The dilution of the extracts was performed in order to fall inside the linear range of the detectors. Lab 3. Gas chromatography/mass spectrometry. The extracts were analyzed using a Varian Saturn I GC/MS (ion trap) equipped with cool on-column injector. Full-scan spectra were run in the electron impact (ED mode from m / z 150 to 510 every second. The source temperature was 220 "C, electron energy 70 eV, filament current 30 mA, and interface temperature 280 "C. (41) Rahman, M. S.; Bmadt, S.; Larsen,B. J. High Resolut. Chromatogv. 1993, 16. 731.
Aliquots (2 pL) of the extracts were injected manually on two different columns, a 60 m x 0.25 mm, 0.25 pm 5% diphenyldimethylsiloxane DB-5ms column U&W Scientific) and a 60 m x 0.25 mm, 0.25 pm DEDioxin column U&W Scientific). The columns were connected to the injector using a 80 cm uncoated retention gap. The GC oven program for DBdms (DB-Dioxin) was as follows: initial temperature 60 "C, retained for 1 min, increased at a rate of 50 "C/min (40 "C/min) to 210 "C (190 "C), then increased at a rate 0.5 "C/min (2 'C/min) to 219 "C (220 "C), then increased at a rate of 4 "C/min (3 'C/min) to 280 "C (270 "C), and retained for 7.7 min (7min). Hydrogen linear velocity was 52.6 cm/s at 200 "C (column head pressure 2.07 bar). Quantitative measurements of PCBs were performed using the sum of the heights over their molecular cluster mass traces relative to the internal standards. A multilevel external calibration was applied to calculate the relative response factors. The linear regression line for each PCB was calculated using a in-house program developed from the statistical analysis system (SAS). PCBs were calibrated in the concentration interval of 200 pg/pL to 31.3 ng/pL on each of the two columns, and the lowest results obtained from the quantitation of the extracts were adapted as the value closest to the true value. Standards were injected after every fifth sample to determine deterioration of separation or drift. New calibrations were performed if the results for the standards drifted by more than 10%. RESULTS AND DISCUSSION
Method Development The method development for the SFE was performed independently in each of the three laboratories. Preliminary experiments in the method development phase, performed on the actual soil to be certitied, showed that -90% recovery could be achieved at 60-97 "C with pure COZ, as evaluated by sequential extraction. In order to achieve higher extraction efficiencies, all three laboratories chose to evaluate methanol as a modifer in the COZ. For all laboratories, MeOH at varied concentrations (1.7-4% v/v) was found to give satisfactory results. Each laboratory chose a different way of addition of the modifier to the COZ. Lab 1 used premixed cylinders with 2% MeOH, lab 2 used static addition of 4% MeOH directly to the extraction thimbles, while lab 3 added 1.7%MeOH via a separate modifier pump. The latter procedure is considered a more correct and efficient way of modifier a d d i t i ~ n . ~In~ our - ~ ~case, however, no difference in extraction efficiency among the three addition methods was discovered. For the trapping of extracted PCBs, liquid trapping has been shown to yield good recoveries and has also been the most common trapping method used. However, all three laboratories chose to use solid phase trapping (each using different trapping materials), because of the potential for class fractionation with this technique. It has been demonstrated that the trapping efficiency in SFE using solid phase trapping is very much dependent on the trapping material, modifer identity and concentration, flow rate, and trapping t e m ~ e r a t u r e . ' ~Therefore, ,~~ initial spike recovery studies were conducted to ensure quantita(42) Schweighardt, F. K; Mathias, P. M.J. Chromatogr. Sci. 1993,31, 207. (43) Lee, H.; Peart, T. E.; Hong-You, R L.; Gere, D. R. J. Chromatogr., A 1993, 653, 83. (44) Cross, R F.; Ezzell, J. L.; Porter, N. L.; Richter, B. E. Am. Lab. 1994, (August), 12. (45) Mulcahey, L. J.; Taylor, L T. Anal. Cbem. 1992,64, 2352.
Table 1. lntralaboratory Comparison of SFE and Soxhlet Extraction for Quantitative Determination of PCBs in Soil (CRM481) in Lab la
Soxhlet"
SFEb PCB no.
olg/g)
SD olg/g)
mean olg/g)
SD olg/g)
28 52 101 105 118 128 138 149 153 156 170 180
0.31 3.20 40.3 1.16 9.80 8.29 89.2 102 136 7.75 43.9 139
0.02 0.13 1.6 0.05 0.38 0.30 3.0 4 6 0.37 1.6 4
0.29 3.21 39.6 1.21 10.1 8.77 89.2 103 135 7.82 46.2d 146d
0.01 0.12 2.0 0.03 0.4 0.29 3.4 5 6 0.22 1.1 3
mean
column typeused DB-17
SIL8HT5 DB-17 DB-17
SIL8HT5 SIL8HT5 DB-17 DB-17 DB-17 DB-17
SIL8HT5 SIL8HT5
Quantifiedwithout correction for recovery and water content. Ten minute static and 40 min extraction with 2%MeOH-modified carbon dioxide $97 "C, 0.75 g/mL and 378 a h ) , five replicates. Forty-eight hours ulth 250 mL hexane/acetone (2:3), five replicates. Interference in the Soxhlet extract on this column.
tive trapping. Lab 1used Florisil for trapping since this material previously has been shown to give good results for PCBs with the use of MeOH as modifier.I6 Lab 2 used the ODS traps provided as an integral part of their instrument, while Lab 3 used silica gel 60. The latter choice of trapping material has been shown to be inefficient for the on-line cleanup of PCBs. However, for the studied soil, silica gel did not appear to give any problems either in the trapping or in the on-line cleanup. In order to evaluate the extraction efficiency of the soil with SFE, labs 1and 3 performed independent Soxhlet extractions. Lab 3 found an average recovery of 98%for SFE when compared to a 24 h Soxhlet extraction using n-hexane. On average, a further -2.2% PCBs could be recovered with an additional 24 h Soxhlet extraction. Table 1 shows the results of the intralaboratory comparison between SFE and Soxhlet extraction performed by lab 1. Soxhlet extraction was performed for 48 h using 250 mL of a mixture of n-hexane and acetone (2:3, v/v) while SFE was conducted for 10 min static followed by 40 min dynamic with a flow of 1mL/min at 97 "C and 378 bar using 2 % MeOH-modified COZ. All results are within the standard deviation for the two experiments and clearly show that there is no difference in the overall extraction efficiency of SFE and Soxhlet methods when using the chosen parameters. The relative standard deviation (RSD) ranged from 2.3 to 5.2%with an average of 3.5%for both methods. The RSD values, which contain the reproducibility of both the extraction and the GC analysis, are remarkably small when considering that only 100 mg of sample is used for each extraction. The SFE method appeared to yield cleaner extracts than the Soxhlet method even when the Soxhlet extracts were cleaned up over acid silica, as evidenced by the interferencefound in Soxhlet extracts for PCBs 170 and 180. In contrast, none of the SFE extracts were subjected to a cleanup procedure. Recovery Measurements. Conventional recovery calculation of PCBs from real matrices uses the addition of known quantities of each of the measured PCBs at one or several levels of concentration. In certification exercises performed by the BCR on PCBs, it was previously mandatory to perform these standard additions at three or four different levels in triplicate for each of Analytical Chemistry, Vol. 67, No. 14, July 15, 1995
2427
-boo00
30000
20000
1 0 0 0 0
8 c
1
i
(A)
GO2 + 2 OO/ MeOH, 97 “C
diluted 50 times
s
ii
In
P
0 0
w
20
30
(e)
40
50
C02 + 5 % MeOH, 97 “C undiluted
-
20000
(0 0
I
10000-
0
E : .
E!
2
--
P
0
0 .
Retention Time (minutes) Figure 1. GC-ECD chromatogram (SIL8-HT5)of sequential SF extractions of the BCR soil: (A) 100 mg of soil extracted by SFE using C o g modified with 2% MeOH (10 min static and 40 min dynamic, 0.75 g/mL, 378 bar, 97 “C, 1 mumin); (B) 30 min dynamic extraction with COz modified with 5% MeOH of the soil already extracted in (A) (same parameters as (A)). Sequential Soxhlet extraction performed after the first SFE only gave -0.4% additional extractable PCBs in comparison to the -1% PCBs when SFE was used as shown in (B). For Soxhlet extraction, the peak pattern was identical to the one obtained by SFE.
the congeners concerned. In this exercise, however, it was feasible to use sequential extractions to assess the possible remaining PCB in the soil because of the high level of contamination. In SFE, sequential extractions (using stronger conditions, not the same conditions) have been advocated on several occasions as a means to define the extraction efficiency for quantitative a n a l y s i ~ . ~ J ~The J ~ Jreason ~ for this recommendation arises from the finding that spiked analytes are much more easily extracted than incurred analytes.12 Therefore, assessment of recovery in SFE by help of the standard addition method is a big potential error source and the only viable approach for SFE seems to be sequential extractions at increasingly “stronger” conditions.4‘j Stronger conditions entail increased temperature and pressure, addition of modifiers, or both (if modifier is already used, change to a more potent modifier or increase the amount of modifier used). Another solution is to follow SFE by an additional Soxhlet e x t r a c t i ~ n . All ~ ~ three , ~ ~ SFE laboratories used sequential extractions for their recovery measurements, and all found recovery values were higher than 90%for the first extraction (on average 98%). Labs 1 and 2 used both SFE and Soxhlet extraction for (46) Bswadt, S.; Hawthorne, S. B. /. Chromatop,, A., in press.
2428 Analytical Chemistty, Vol. 67, No. 14, July 15, 1995
sequential extractions. When a second SFE extraction was performed sequentially,
