Anal. Chem. 1994,66,667-673
Analysis of PCBs in Sulfur-Containing Sediments by Off-Line Supercritical Fluid Extraction and HRGC-ECD Serren Berwadt'gt and Berit Johansson* Environment Institute, EC Joint Research Centre, TP 290, I-27020 Ispra (VA), Ita/)
A method for the rapid interference free analysis of PCB congeners in sulfur-containing sediments is presented. The method involves a sulfur cleanup at supercritical conditions and was developed on a sediment from the Rotterdam (NL) harbor that contains 1.5% sulfur and PCB congener concentrations ranging from 2 to 29 ng/g dry wt. Tbe complete analysis time was less than 2.5 h, mainly depending on the GC analysis time, and did not involve any manual cleanup or pretreatment of the samples. HRGC-ECD analysis was done on two parallel coupled columns, a 60-m DB-17 column and a series combination of a 25-m SILS and a 25-m HT5 column. The supercritical fluid extraction was evaluated on three different trapping materials to investigate any differences in the purity of the extracts. The final analytical method was compaired to Soxhlet extraction, applied on a certified sewage sludge sample, and gave quantitative recovery detection limits of 1-2 ng/g dry wt and standard deviations less than 10%.The method developed was used on a survey of 27 sediments from Venice that contained 0.8-2.596 sulfur with a concentration range of 1.5-343 ng/g dry wt for single PCB congeners. Polychlorinated biphenyls (PCBs) are thermally stable chlorinated hydrocarbons with high dielectric constants, which have led to their application in a wide range of industrial products, properties that are also responsible for the widespread and lasting environmental contamination problems associated with PCBs.'s2 The potential environmental threat of the large amount of PCB still in use and the toxicity of certain congeners call for continued future monitoring of these compounds in the envir~nment.~-~ PCBs enter the environment as longrange air-transported pollutants,6 mainly originating from scattered dumps and waste in~ineration.~ One of the major environmental sinks for these pollutants is marine and lacustrine sediments, and these matrices play an important role in environmental monitoring. The analysis of PCB congeners in sediments classically includes three phases: extraction, purification of the extract, and separation and detection of the various congeners. The extraction of sediments can be carried out successfully with sonication or Soxhlet procedures. The primary purpose of the purification of the extract is the elimination of coextracted + Present address: UniversityofNorthDakota,EERC,GrandForks,ND 58202. 8 Present address: DTI Environmental Technology, Gregersensvej, P.O. Box 141, DK-2630 Taastrup, Denmark. (1) Tanabe, S . Ewiron. Pollut. 1988, 50, 5 . (2) Lang, V. J . Chromatogr. 1992, 595, 1. (3) Safe, S. CRC Crit. Reu. Toxicol. 1984, 13, 319. (4) Clarke, J. Chemosphere 1986, 15, 275. ( 5 ) McFarland, V.; Clarke, J. Ewiron. Health Perspect. 1989, 81, 225. (6) Manchester-Neesvig, J.; Andren, A. Enuiron. Sci. Technol. 1989, 23, 1138. (7) Hermanson, M.; Hites, R.; Enuiron. Sci. Technol. 1990, 24, 1138. 0003-2700/94/0388-0867$04.50/0 0 1994 American Chemical Society
major matrix compounds, such as chlorophyll, organic matter, and elementary sulfur. If not removed, these inevitably will lead to significant interference in the third step of the analytical procedure (gas chromatography (GC) with electron capture detection (ECD) or mass spectrometric detection (MS)). Classical extraction and cleanup methods are time and labor consuming and involve the use of large volumes of often harmful and expensive solvents. The useof supercritical fluid extraction (SFE) in analytical chemistry has become increasingly popular in recent years,&' and it has been adequately demonstrated that SFE has the potential of dramatically reducing the time required for sample extraction as well as of eliminating the need for large volumes of organic solvents. Furthermore, SFE has the potential of reducing requirements for the cleanup of the extracts due to someextent of selectivity in the extraction phase.12J3 Several investigations have been published on the use of SFE for the analysis of PCBs in a broad variety of The majority of these studies have been carried out with samples contaminated at rather high level (in the microgram per gram range or higher). The concentration of PCB congeners in environmental samples are typically in the low nanogram per gram range, which requires a high degree of concentration of the sample extract and puts strong demands on its purity. Only a few i n v e ~ t i g a t i o n s ~ ~have J ~ J ~been J ~ published on the use of SFE for analysis of PCBs at these low concentration levels. Three different types of trapping systems have been used for the collection of analytes after SFE: liquid collection, cryogenic trapping and solid-phase trapping.gJO While liquid collection and cryogenic trapping are "simple" techniques and offer fast analysis of the extract, it is only in special cases that samples contaminated at trace levels can be analyzed directly without some kind of cleanup. Thus, the effort gained by the (8) King, J. W. J. Chromatogr. Sci. 1989, 17, 355. (9) Hawthorne, S. B. Anal. Chem. 1990,62, 633A. (IO) King, J. W.; France, J. E. Analysis with Supercritical Fluids: Extraction and Chromatography; Wenclawiak, B., Ed.; Springer: Berlin, 1992; pp 32-60 (1 1) Hawthorne, S. B.; Miller, D. J.; Langenfeld, J. J. J. Chromatogr. Libr.1992, 53, 225 (Hyphenated Tech. Supercrit. Fluid Chromatogr. Extr.). (12) David, F.; Vershuere, M.; Sandra, P. Fresenius J.Anal. Chem. 1992,344,479. (13) Bowadt, S.; Johansson, B.; Rovida, C.; Pelusio, F.; Larsen, B. J. Chromatogr., in press. (14) Hawthorne, S . B.; Krieger, M. S.; Miller, D. J. Anal. Chem. 1989, 61, 736. (15) Nam, K. S.; Kapila, S.; Viswanath, D. S.; Clevenger, T. E.; Johansson, J.; Yanders, A.F. Chemosphere 1989, 19, 33. (16) Nam, K. S.; Kapila, S.;Yanders, A. F.; Puri, R. K. Chemosphere 1990, 20, 873. (17) Onuska, F. I.; Terry, K. A. J. High Resolut. Chromatogr. 1989, 12, 527. (18) Hawthorne, S. B.; Miller, D. J. J . Chromatogr. 1987, 403, 63. (19) Miller Schantz, M.; Cheder, S. N. J . Chromatogr. 1986, 363, 397. (20) Hawth0rne.S. B.; Langenfeld, J. J.; Miller, D. J.; Burford, M. D. Anal. Chem. 1992, 64, 1614. (21) Langenfeld, J. J.; Hawthorne, S. B.; Miller, D. J.; Pawliszyn, J. Anal. Chem. 1993, 65, 338.
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simplertrapping techniquemight be lost. Solid-phasetrapping is more complex than the other two techniques and requires a high degree of optimization of the experimental conditions (i.e., choice of trapping material, modifier, trapping temperature, and elution solvent). However, it potentially allows the simultaneous extraction, cleanup, and concentration of the extract.12J322 We recently demonstrated the use of solidphase trapping after SFE of PCB congeners from sewage sludge contaminated at 100-300 ng/g.13 Clean heptane eluates (ready for GC-ECD analysis) were obtained with Florisil or octadecyl functionalized silica gel (ODS) in the traps after a 30-min dynamic extraction by carbon dioxide at a density of 0.75 g/mL (218 bar and 60 "C) and a liquid flow rate of 1 mL/min. The purpose of the work reported here is to investigate the use of SFE with solid-phase trapping for analysis of sediments environmentally contaminated with PCB congeners at levels down to a few nanograms per gram. A major problem in sediment analysis is the presence of elementary sulfur. The concentration of sulfur depends of the origin of the sediment and can be as high as several percent. Elementary sulfur is easily extracted23and follows the PCB congeners all the way to the final extract if special cleanup procedures are not employed. In the GC-ECD analysisof the extract, the presence of sulfur at high concentrations may deteriorate separations and saturate the detector. Traditionally treatment with metallic Cu and Hg has been the most popular method for the removal of sulfur from environmental matrices, after or during e x t r a ~ t i o n . ~However ~ , ~ ~ Hg, which is the most efficient reagent, creates a waste problem (toxicity). Other methods used for the removal of sulfur are gel permeation26 and conversion of sulfur to thiosulfate by tetrabutylammonium sulfite.27 These approaches are very effective but are also quite labor intensive. In the present work we havegiven special attention to removal of sulfur in supercritical fluid extraction.
EXPERIMENTAL SECTION Chemicals. The certified sewage sludge (CRM 392) and the PCB standards (neat crystals) used in this study were obtained from the Community Bureau of Reference (BCR), Brussels, Belgium. The PCBs used were IUPAC numbers 28,52, 101, 105, 118, 128,138, 149, 153, 156, 170, and 180. The DDE and DDT standards were obtained from Supelco in a solution of known purity and concentration. The solvents used (acetone, n-hexane, n-heptane, isooctane, and dichloromethane) were all pesticide grade (Merck, Darmstadt, Germany). The C02 and the methanol (MeOH) modified extraction fluids were all obtained as SFE/SFC grade from SIAD, Milan, Italy. The copper powder used was electrolytic, pro analysi grade (Cu>99.9%), which was pretreated prior to use: 20 g of Cu powder washed with 3 X 50 mL of deionized water, 3 X 50 mL of acetone, and 3 X 50 mL of n-hexane. The remaining solvent was evaporated on the Rotavap and the powder kept under argon. (22) Bawadt, S.; Pelusio, F.;Montanarella, L.;Larsen, B.; Mapelli, G. Proc. Fifieenth Int. Symp. Capillary Chromatogr., 1993; p 1671. (23) Louie, P.K.;Timpe, R. C.; Hawthorne, S. B.; Miller, D. J. Fuel 1993,73,225. (24) Brooks, J. M.; Kennicutt, M.C.; Wade, T. L.; Hart, A. D.; Denoux, G. J.; McDonald, J. Emiron. Sci. Technol. 1990, 24, 1079. (25) Bossi, R.; Larsen, B.; Premazzi, G. Sci. Total Emiron. 1992, 12Z, 77. (26) Tilio,R.; KapilaS.; B0ssi.R.; FacchettiS., submitted for publication in J . Chromatogr. (27) Jensen, S.; Renberg, L.; Reuterghrd, L. Anal. Chem. 1977, 49, 316.
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Sediment Samples. Samples (27) of surface sediments were collected at different sites in the channels of Venice City and in the Lagoon of Venice. The samples were air-dried at 25 O C and groundin an automatic grinding machine, with contact surfaces in inert ZrO2, to obtain a homogenous powder.3sThe sample from the Rotterdam harbor was a waste sediment and was received wet. The water was decanted, and the remaining sludge was air-dried at 40 OC for 48 h. The sample was ground mechanically twice to obtain a homogenous powder. Use of such mild conditions for drying and grinding should have prevented loss of the low-volatility PCBs. Supercritical Fluid Extraction. A Hewlett-Packard 7680A supercritical fluid extractor was used for all the work presented. Sediment extractions were prepared as follows: 2-g portions of sediments were mixed with 1.5 g of prerinsed Cu and 6.5 g of anhydrous Na2S04 and were packed into 7-mL extraction cells. Sewage sludge extractions were prepared in the same way, except that only 0.5 g of sample was mixed with 9.5 g of Na2S04. The Na2S04 served to ensure a better distribution of the extraction gas (filling the cell) and to enlarge the available surface area of the sample. The supercritical fluid extractions were performed under identical conditions for all samples (sediments and sewage sludge) in the following way: 20-min static extraction with pure C02 at a density of 0.75 g/mL (218 atm) at 60 OC followed by 40-min dynamic extraction at the same density and temperature and with a flow of 1 mL/min of pure C02. The sequential extractions (only for some samples) were performed with the first step being identical to the one described above, followed by 30-min dynamic extraction with C02 + 2% MeOH (step 2, same density, temperature, and flow) followed by 30-min dynamic extraction with C02 5% MeOH (step 3, same density, temperature, and flow). The nozzle temperature was kept constant at 45 OC to prevent plugging. The traps were filled with 1 mL of trapping material, Le., 30-pm octadecyl functionalized silica gel (ODS, Hypersil), silica gel 60 (230-400 US.mesh), or Florisil (60-100 U S . mesh). During extraction the trap was kept at a temperature of 20 OC when pure CO2 was used or 65 OC when methanol was used as modifier. This was done to keep the methanol in the vapor state, which then does not reduce trapping efficienciesof the PCBs.13 The traps were eluted sequentially with 2 X 1.5 mLof n-heptane, 1 X 1.5 mL of dichloromethane, and 2 X 1.5 mL of n-heptane. To the individual fractions was added 50 pL of internal standard (PCB 35 and PCB 169, at 2.16 and 0.43 ng/pL, respectively). The final volume was adjusted to 1.8 mL with n-heptane, giving an internal standard concentration of -60 pg/mL for PCB 35 and -12 pg/mL for PCB 169. Soxhlet Extraction and Sulfur Cleanup. Aliquots (2 g) of sediments were mixed with 8 g of anhydrous Na2S04 and extracted with 250 mL of a mixture of n-hexane and acetone (2:3) for 18 h. Extracts were loaded on a 15 cm X 6 mm column with activated silica impregnated with 40% (w/w) sulfuric acid (concentrated) and eluted with 50 mL of n-hexane. The eluent was evaporated at 30 OC, and the residues were redissolved in 2 mL of isooctane. The extracts were subjected to a sulfur cleanup with a tetrabutylammonium (TBA) sulfite reagent according to the original prescription
+
-
by Jensen et al.27 The final extracts were evaporated to neardryness (at 30 "C) and redissolved in 1.5 mL of isooctane. Internal standard was added (PCBs 35 and 169, as for the supercritical fluid extractions) and the final volume was adjusted to 1.8 mL with isooctane. To the sediments from Venice with medium-level content of sulfur (Table 4) was added 1.5 g of copper powder prior to Soxhlet extraction. Dual-Column Gas chromatography. The extracts were analyzed using a pressure-controlled Hewlett-Packard 5890 I1 gas chromatograph (GC) equipped with two 63Nielectron capture detectors (ECDs) and a HP7673A autosampler. Aliquots (1 pL) of the extracts were on-column injected on two parallel coupled columns, a 60 m X 0.25 mm, 0.25 pm 50% diphenyldimethylsiloxane DB- 17 column (J& W Scientific) and a series combination of a 25 m X 0.25 mm, 0.25 pm 5% diphenyldimethylsiloxaneSIL-8 column (Chrompack) and a 25 m X 0.22 mm, 0.10 pm 1,7-dicarba-closo-dodecarborane dimethylpolysiloxane HT-5 column (Scientific Glass Engineering). The columns were installed in the GC oven together with a 2 m X 0.53 mm fused silica retention gab using a quick-seal glass "T". The GC oven program was the following: initial temperature 90 OC, retained for 2 min, then increased at a rate of 20 OC/min to 170 OC, retained for 7.5 min, then increasing at a rate of 3 OC/min to 280 OC, and finally retained for 10 min. Hydrogen linear velocity was 43.5 cm/s, held constant by the pressure-controlled inlet throughout the whole temperature program. This choice of columns and GC conditions has been shown to give optimum separation of PCB congeners and organochlorine pesticides as well as a relative standard deviation less than 2% for repetitive injections.** Quantitative measurements of PCBs and pesticides were performed using peak heights after a seven-point multilevel calibration curve (five point for the pesticides) employing the power fit calibration routine provided with the H P Chem 3365 software. PCB congeners were calibrated in the concentration interval of 1.72-573.3 pg/pL whereas the intervals for the pesticides were 6.25-200 pg/pL. Standards were used during the analysis of the sediments in place of every fifth sample to determinate deterioration of separation or drift. New calibrations were performed if the results for the standards had drifted by more than 10%.
RESULTS AND DISCUSSION Removal of Sulfurfrom Sedimentsduring SupercriticalFluid Extraction. The performance of analytical methodology can be enhanced if a selective extraction procedure is employed that directly decreases or eliminates the amount of sulfur in the extract. For a more efficient approach to eliminating sulfur in environmental matrices, it would be convenient to combine the sulfur cleanup step with the extraction step. This would save time and minimize loss of analyte (less sample handling). Therefore, Cu was used directly in the extraction cell to test whether this could completely eliminate sulfur from the final extract. Because the literature reports a growing number of serious discrepancies between the SFE of spiked and real samples (28) Rahman, M. S.; Bswadt, S.;Lanen, B. J. High Resoluf. Chromutogr., in press.
40000
7
20000
-
10
x F
30
40
20
30
40
4
P
n
20
10000
4
s
04 0
10
Retention Time (minutes)
Figure 1. QGECD chromatograms (SILBHTS) of different extracts: (A) 1 g of sedlment dynamlcally extracted by SFE with pure CO2 for 30 min (density 0.75 g/mL(218 atm), 60 O C and Row 1 mL/mln), added 1.5 g of prerinsed copper to the extraction cell; 1 pL of the extract eluted from the trap (1.8 mL) was on-coiumn InJected. (B) The same as for (A) except that 2 g of sediment was extracted, a static extraction step of 20 min (60 "C) was added, and the dynamic extraction was extended to 40 min.
contaminated at trace leve1,20,21,33,34 a method for the analysis of PCB in sediments was developed using a real sediment with a high level of elementary sulfur. A sediment from the Rotterdam harbor seemed appropriate as it was known to contain quite high amounts of environmental contaminants (inorganic however) and was measured to have a total sulfur content of 1.5% (dry wt). To test whether this sediment was appropriate for the study, extracts from sonication was injected (1 pL from 3 X 5 min sonication of a 1-g sediment in 100 mL of isooctane) without any sulfur cleanup. The result was saturation of the ECD detectors throughout the whole chromatogram. The same was the case with SFE under conditions that previously have proven successfulon a certified sewage sludge. A comparison between the extraction by SFE in a pure dynamic mode and a miked static and dynamic mode, both with the use of added Cu in the extraction cell, can be seen in Figures 1A,B. Evidently, the extra static step is needed to obtain an efficient reaction between the copper powder and the sulfur. From these two panels it is also apparent that (29) H6fler, F. CLB Chem. Labor Biotech. 1992, 43, 369. (30) Porter, N. L.; Rynaski, A. F.; Campbell, E. R.; Saunders, M.; Richter, B. E.; Swanson, J. T.; Nielsen, R. B.; Murphy, B. J. J . Chromutogr. Scf. 1992.30, 367. (31) HBfler,F. Lecture at the Second European Symposium on Analytical Supercritical Fluid Chromatography and Extraction, May 27-28, 1993. (32) Pyle, S. M.; Setty, M. M. Tulunta 1991, 38, 1125. (33) Hawthorne, S. B.; Miller, D. J.; Burford, M. D.; Langenfeld, J. J.; EckertTilotta, S.;Louie, P. K.; J. Chromutogr. 1993, 642, 301. (34) Burford, M. D.; Hawthorne, S. B.; Miller, D. J. A n d . Chem. 1993,65, 1497.
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30-min pure dynamic extraction is not sufficient to reach a (A) SILICA 100% recovery for the PCBs from the sediment. Recently, an attempt was made to extract organochlorine contaminants selectively from a soil spiked with elementary sulfur (0.15%).26 The authors concluded that a 50-fold increase in selectivity for organochlorine3 over sulfur can be achieved by SFE. Our results on real samples, however, indicate that there is no significant difference in selectivity in SFE between organochlorines and elementary sulfur. This is in accordance with results obtained by Louie et al., who c0 successfully used SFE for extraction of sulfur from coal.23 I3 They achieved quantitative extraction of surfur from spiked 10 20 30 40 20000 samples in less than 30 min by the use of SFE. (8) FLORlSlL The use of Cu directly in the extraction cell has been attempted on at least three occasions. Of these, two were made on the SFE of PCBs in sewage sludge samples, which normally have a relatively low content of elementary s u l f ~ r , * ~ , ~ ~ and one was for the analysis of PAH in soil samples.31 In the first two cases, an additional cleanup step with silver nitrite loaded silica was used before GC-ECD a n a l y ~ i s .Not ~ ~ ,many ~~ details on the last experiment are known because it does not appear in the literature. In our laboratory, SFE has been carried out on sewage sludge, but no interference problems from sulfur or other contaminants was experienced with this 10 20 30 40 matrix. Also, the use of a copper scavenger outside the extraction cell has been applied successfully to the analysis of PAH in sediment samples. This approach, however, necessitates the frequent change of the scavenger column, and the reported recoveries are quite Up to the present day, there are no reports in the literature on the use of Cu directly in the extraction cell during SFE for the complete removal of sulfur interference in GC-ECD analysis. Evaluation of Different TrappingMaterialsfor Supercritical Fluid Extraction of Sediments. An earlier study has shown U that the purity of SFE extracts depends stronglyon the trapping OO 10 20 30 40 20000 7 material used.13 For the development of this method, three different trapping materials were used. Use of a trap filled (D) SOXHLET with stainless steel balls, which is a standard part of the instrument used, was not considered because this trap does not possess any selectivity on rinsing and hence is of no use in this study. As can be seen from Figure 2A the extract made with the trapcontaining silica gel has a higher amount of contaminants when compared with the other two trapping materials used. This renders silica gel unsuitable as a trapping material for SFE of PCBs in this type of matrix without further cleanup. There might, however, be other analytes or matrices where io 20 30 40 Retention Time (minutes) this trapping material would be beneficial. Florisil (Figure Flgure 2. OCECD chromatogram (SIL8Kr5 column) of eluents from 2B) and ODS (Figure 2C) as trapping materials, on the other dlfferent trapplng materials: (A) 2 g of Rotterdam harbor sediment hand, give extracts that are comparable to the extract obtained extracted by SFE with pure Con(20-mln static and 4 0 4 n dynamic, from Soxhlet extraction followed by silica (with 40% H2S04) 0.75 g/mL, 60 'C, 1 mL/mln), added 1.5 g of prerinsed copper powder to the extraction cell, using 1 g of slllca gel as trapplng material; 1 pL cleanup and specific sulfur cleanup (Figure 2D). Panels B-D of the extract eluated from the trap (1.8 mL) was oncolumn Injected. of Figure 2 show that there are still some contaminants left (B) As for (A) but F W l used as trapplng material. (C) As for (A) but in the three extracts, but the use of a dual-column GC system octadecyl functionallzed slllca gel (ODs)used as trapplng materlal. (D) Sediment (2 g) extracted by Soxhlet. The extract was first cleaned should eliminate problems of overlapping contaminant^.*^*^^ ~
( 3 5 ) Marzello, A,; Carrera, F.; Bewadt, S.; Johansson, B.; Bianchi, M.; Muntau, H., in preparation.
(36) Morrison, D. F. Multiuariute Statistical Methods. 2nd ed.; McGraw-Hill: New York, 1976.
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over acld slllca (40% H2S04)and then the sulfur was removed by the TBA procedure.27
(37) B0wadt.S.; Skeja-Andresen,H.; Montanarella, L.; Larsen, B. Int. J . Emiron. Anal. Chem., in press.
Table 1. Comprukon of the Soxhlet Extraction wlth SFE for Dmerent Trapplng Materlak and Extractlon condnlolro
SFE ODsn SFE FlorisiP ng/g % PCB dw+ RSD RZD
?,"
28 52 101 149 118 153 105 138 128 156 180 170 DDE DDT
19.9 4.1 18.6 3.9 21.3 3.3 22.2 2.3 13.5 1.8 27.2 1.7 2.1 8.0 16.7 1.6 2.5 4.0 2.7 1.7 15.4 0.9 9.5 5.2 6.3 11.9 NDf
19.7 18.7 21.4 22.4 14.0 27.0 2.1 16.6 2.5 1.6 15.8 5.0 6.1 ND
5.8 6.3 5.5 6.0 6.0 7.4 3.6 6.9 9.1 8.4 9.5 9.0 3.0
Soxhletb % longext; RSD ng/gdw
?? 18.4 18.8 21.3 24.3 14.0 28.1 2.4 17.0 2.6 1.6 16.5 5.7 6.9 ND
13.7 10.1 12.2 11.9 13.4 13.4 12.5 12.2 12.6 9.9 11.2 9.5 11.6
20.3 18.0 20.4 21.5 12.4 25.3 1.9 15.3 2.5 2.1 14.4 4.8 6.5
nm L 70
MeOH,d ng/gdw 20.1 17.8 20.4 22.2 13.0 26.3 2.0 16.2 2.6 1.6 15.6 5.0 6.1
SFE with carbon dioxide, 20-min static (0.75 g/mL, 218 atm, 60 "C) and 40-min dynamic (1 mL/min), four replicates. Soxhlet extraction for 18 h with 3:2 acetone/hexane, four replicates. SFE with carbon dioxide, 20-min static (0.75 g/mL, 218 atm, 60 "C) and 60-min dynamic (1 mL/min), single experiment. SFE with carbon dioxide + 2% MeOH, 20-min static (0.75 g/mL, 218 atm, 60 "C) and 40-min dynamic (1 mL/min), single experiment. e dw, dry weight. f ND, not detected.
In fact, this is the case, as can be seen from Table 1, which lists values from the trapping with Florisil and ODS as well as the cleaned extracts from the Soxhlet procedure. These values are practically identical even though the contaminants are different for the three extracts. The only significant difference between the results obtained by Soxhlet and SFE is in precision, where Soxhlet extraction has a higher average relative standard deviation of 11.8% while that for SFE ranges from 4.3% (ODS) to 6.7% (Florisil). Probably this is because the mechanical manipulation on the Soxhlet samples is much higher than is the case for SFE. In order not to ruin the GC columns, no attempt was made to quantify the silica extracts. To investigate whether the recoveries of PCBs from the sediment were different for different extraction conditions, two additional extractions were performed using slightly stronger conditions. In the first, the extraction time of the dynamic extraction was extended to 60 min, and in the second, pure C02 as extraction fluid was exchanged with COZ+ 2% MeOH. As can be seen from Table 1, the values obtained are virtually identical to those of the other extraction. The only difference noted was that the extraction with 2% MeOH as modifier gave an extract containing a little more contaminants (data not shown). There is, however, a rather big difference between our data (Table 1, 100% recovery) and data presented by Langenfeld et al.,*l who at even harder SFE conditions (0.91-1.02 g/mL, 350-650 atm, 50 "C) only obtained recoveries in the range of 3347% for the lower boiling PCBs. The reason for this could be the NazS04, which in our case is mixed with the sample to ensure better distribution of the supercritical COZand to enlarge the available surface area of the sample. Another difference is the amount of PCB per sample, which is up to -50 times higher (depending on congener) than for the samples analyzed by us. It is, nevertheless, not likely that the solubility of the PCBs is the problem because at this level we are far from the maximum solubility of PCBs in supercritical C02.14
-
Table 2. Comparlron of the Soxhlet Extractlon wlth SFE of the Rotterdam Harbor Sediment, wlth and wlthout Copper Added to the Extractlon Cell Soxhlet (silica/TBA)a SFE (TBA)b SFE (Cu)f
PCB
1%
28 52 101 149 118 153 105 138 128 156 180 170 DDE
18.4 18.8 21.3 24.3 14.0 28.1 2.4 17.0 2.6 1.6 16.5 5.7 6.9
95% conp
n /g
tw
95% conf
n /g tw
95% conp
4.0 3.0 4.1 4.6 3.0 6.0 0.5 3.3 0.5 0.3 2.9 0.9 1.3
16.7 15.2 19.7 22.0 12.7 25.3 2.0 16.2 3.0 1.8 15.0 4.8 5.4
1.3 1.3 3.3 4.5 1.8 5.3 0.5 3.9 0.5 0.4 3.3 1.0 0.3
19.9 18.6 21.3 22.2 13.5 27.2 2.1 16.7 2.5 1.7 15.4 5.2 6.3
1.3 1.2 1.1 0.8 0.4 0.7 0.3 0.4 0.2 0.1 0.2 0.8 1.2
Soxhlet extraction followed by acid silica (40% sulfuric acid) and TBA cleanup (sulfur). SFE using ODS as trapping material without copper in the extraction cell, followed by TBA cleanup.cSFE using ODS as trapping material with copper in the extraction cell, without further cleanup. dw, dry weight. e 95% confidential intervals based on the standard deviation from four replicates.
Because metallic Cu has the potential of catalysis,z6 we investigated whether the addition of Cu in the extraction cell had any catalytic effect on the breakdown of PCBs during supercritical fluid extraction. Four additional supercritical fluid extractions were performed without the addition of Cu to the extraction cell. For the GC-ECD analysis to be performed, it was necessary to add a sulfur cleanup step after the SFE. This was done with TBA2' in the same way as for the Soxhlet extracts. The results are listed in Table 2. Multiple analysis of variance36 and the 95% confidence intervals show that there is no statistical difference between the three series of experiments. The only apparent difference between the results in Table 2 is that the standard deviation of the values obtained from the SFE experiments without any Cu addition are, on average, of the same magnitude as the standard deviation of the values obtained from the Soxhlet extractions. Whether this perturbation is introduced by the extra sulfur cleanup step or as a consequence of the extra sulfur extracted together with the PCBs is difficult to judge, but the resulting higher standard deviation is something that is worth considering. This means that there is no reason not to use Cu in the extraction cell during SFE for the analysis of PCBs in sediments. In fact, in three out of seven experiments without the use of Cu, the nozzle and the bottom of the trap were plugged by elementary sulfur. Effect of Sequential Extractions. Recently, a new discussion of the quantitative extraction of trace organics from different matrices has emerged. Soxhlet extraction methods have always been considered as the exhaustive extraction method, but more and more results point out that this is not necessarily the case.20,21,33,34 This means that one can talk about 100% recovery only if a sequential extraction has been performed with increasing strength of extraction to test the efficiency of the extraction. Figure 3A-C displays the sequential extraction of the Rotterdam sediment. Figure 3A shows the extraction using pure C02 with the conditions thought to be the optimum, density 0.75 g/mL (218 bar), 60 OC, and flow 1 mL/min. This is followed by 30-min dynamic Analytical Chemism, Vol. 66,No. 5, Mrch 1, 1994
871
20000 3
,
T a m 9. Rmga of PCB, DM, and DDT Concmtratknr Fwnd In the Venke 8.dhnnk range, det limit, range, det limit, PCB ng/gd@ ng/gdw PCB nglgdd ng/gdw 28 52 101 105 118 128 138
w
oi
10000
10
I
~
20
~l1
1
DDE DDT
5.0-149.0 8.7-157.6 1.5-34.3 1.3-13.0 4.0-39.1 5.2-75.6 5.2-232.7
2
1 1 1 1 2 2
s
20
30
40
10
?O
30
40
Retention Time (minutes)
Flgure 9. Sequential extraction of the Rotterdam harbor sediment. GGECD chromatograms on a SIL8HT5 column: (A) 2 g of sediment extracted by SFE wlth pure Con(20-mln static and 40-min dynamic. 0.75 g/mL, 60 O C , 1 mL/mln, added). 1.5 g of prerlnsed copper powder to the extraction cell, using ODs as trapping matlerlal; 1 pL of the eluent(l.8mL)wasow"ninjected.(B)A30-mlnUynamkexiractkn with Con 2 % MeOH of the sediment already extracted In (A) (same SFE conditions as A)). (C) A 30mln dynamlc extractlon wlth COS 5 % MeOHofthesedknentalr~adyextractedln(B)(sameSFE condltbns as (A)).
+
+
+
extraction with COz 2% MeOH (Figure 3B, same conditions) and finally by 30-min extraction with C02 5% MeOH (Figure 3C, same conditions). It is evident that in this case the first extraction is virtually exhaustive, counting for at least 97% of the extracted compounds. In fact, the third extraction only extracts contaminants. Use of the Developed Method for a Survey of Surface Sediments from Venice.35 The developed SFE method was used on 27 sediment samples from Venice. Of these, 21 were taken from different locations in the old city of Venice and 6 were taken from the lagoon in different locations and distances from the city. Table 3 shows the quantitative dynamic range of individual PCB congeners, DDE, and DDT for which the method has been tested together with the individual detection limits. Of the 27 samples obtained, 5 gave values below the detection limits. The detection limits were established as the lowest amounts of individual PCB congeners, DDE, and DDT detected in the real sediment samples giving a signal-to-noise ratio higher than 10. It must
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A ~ l y t i c e Chemlstty, l Vol. 66, No. 5, March 1, 1994
be remembered, however, that no attempt was made to concentrate the extracts further and that a detection limit of 2 ng/g dry wt means an injected PCB amount (1 pL) of approximately 1 pg/congener that is split (1:l) on the two columns. To provide a check of the efficiency of the supercritical fluid extraction, 3 of the 27 samples were selected for Soxhlet extraction (duplicate analysis); the selection was based on previous measurements of the total sulfur content. The three selected samples had total sulfur contents of 2.4%,1.4%,and 1 .O%, respectively (high, medium, and low). The results of thesemeasurementscan beseeninTable4. Twooftheselected samples have nearly the same PCB concentrations (high and medium) but a quitelarge difference in DDE and DDT content. The sample with low total sulfur content has values somewhat lower than the two others but quite similar to the values obtained from the Rotterdam sample. Becausethe extractions of all three samples are done only in duplicate (both Soxhlet and SFE), one should be careful in interpreting the data. Nevertheless, it is clear that SFE on these sediments produces recoveriesthat are very similar to those obtained from Soxhlet extraction and, in fact, shows higher reproducibility, as can be deduced from the average relative standard deviations. For the samples with a higher content of PCB, DDE, and DDT, the relative standard deviations seem to be quite stable whereas the sample with low content has significantly higher relative standard deviations. This could be due to higher inhomogeneity of this sample and the influence of the smaller values obtained. To examine whether it would alter the results, 1.5 g of copper (the same amount used for SFE)was added to the sample with medium total sulfur content prior to Soxhlet extraction, but the results were not significantly different. To further check the entire method (sample preparation and quantitative GC-ECD analysis) a quality control sample (sewage sludge, CRM 392) was analyzed at the same time and in the same manner as the Venice samples. The quality control was only conducted with SFE, and the results are listed in Table 5 . The results of our measurements are in accordance with the certified values even though the values obtained for PCB 28 and PCB 52 appear to be a little low. This, however, does not necessarily have anything to do with the extraction because, as we have previously shown, the use of SFE on this sample gives a recovery as high as for Soxhlet e~tracti0n.l~ Furthermore, in the approximately five years since this sewage sludge was certified, congener-specific analysis of PCBs has developed significantly and today much more is known about the possible coeluting congeners on
-
m
(C) CO, + 5 % MaOH
072
1 1 1 1
149 153 156 170 180
40
30
( 8 ) CO,+Z%MeOH
10
oi
2 2
dw, dry weight.
1
j.
2.8-33.4 2.6-178.9 5.0-286.0 2.6-184.1 9.3-343.1 1.9-62.5 8.2-228.4
Table 4. Evaluation of the Accuracy and Precklon of the Analytical Procdure Used for the Venice Sedlmentr. Comparison of SFE with Soxhlet Edractlon
mean, nglgdfl
% ' RSD
SFE resultea
SFEa
Soxhleta PCB
Table 5. Analytk of a Qualtly Control Sampk (Certtfled Sowag. Sludge, CRM 382)'
mean, nglgdw
% RSD
rec,b %
High Sulfur Content, 2.4%
28 52 101 105 118 128 138 149 153 156 170 180
DDE DDT 28 52 101 105 118 128 138 149 153 156 170 180
DDE DDT
3.1 66.6 66.7 10.6 49.0 7.5 37.0 51.7 55.6 4.2 7.6 22.8 73.0 238.2 2.9 24.5 57.6 16.2 72.0 10.8 46.4 35.5 51.9 6.7 5.1 13.1 11.5
9.3 3.7 6.5 78.7 7.2 66.9 11.3 12.2 5.2 61.6 5.2 8.0 11.3 43.1 4.5 58.3 6.0 61.7 5.5 5.2 7.3 8.0 4.7 24.6 6.2 75.6 22.6 225.6 Medium Sulfur Content, 1.4% 8.7 3.3 5.8 26.1 6.7 64.2 8.0 18.3 6.1 79.5 3.2 11.0 7.3 51.3 2.4 35.5 2.1 52.4 5.3 7.5 12.8 5.1 1.8 12.3 10.7 11.9
NDd
6.0 5.0 6.1 0.3 0.3 0.0 2.9 2.3 0.1 1.6 3.3 2.8 0.8 3.0
120 118 100 115 126 107 116 113 111 123 106 108 104 95
3.0 1.9 4.0 8.0 5.3 5.1 5.2 5.5 5.1 6.1 3.7 3.8 5.0
114 107 111 113 110 102 111 100 101 111 99 94 104
2.3 13.6 15.6 15.2 14.1 11.4 6.2 3.1 12.9 0.4 11.3 8.0
90 93 90 103 83 86 92 82 91 79 80 91
ND Low Sulfur Content, 1.0%
28 52 101 105 118 128 138 149 153 156 170 180
DDE DDT
ND 3.9 12.9 4.5 15.0 4.2 20.3 13.4 21.6 2.7 5.0 12.0 7.9
ND
ND 10.2 20.9 14.6 18.6 35.9 36.1 30.4 34.0 41.4 1.5 31.2 5.6
3.5 12.0 4.1 15.5 3.5 17.5 12.3 17.8 2.5 3.9 9.6 7.2
ND
TWO replicates. bRecovery of SFE compared with Soxhlet extraction (100%). dw, dry weight. ND, not detected.
different GC column^.^^,^^ In fact, these last results have changed the quantitative determination of several PCB congeners.
CONCLUSION This study clearly demonstrates that it is possible to develop a method for supercritical fluid extraction, using pure C 0 2 38) Larsen, B.; Bmwadt, S.;Tilio, R. Int. J. Enuiron. Anal. Chem. 1992, 47, 41. 39) Bmwadt, S.; Larsen, B. J . High Resolut. Chromatogr. 1992, IS, 311. 40) de Boer, J.; Dao, Q.;J. High Resolut. Chromarogr. 1989, 12, 755. 41) Larsen, B.; Riego, J. Int. J . Enuiron. Anal. Chem. 1990,40959. 42) de Boer, J.; Dao, Q. J. High Resolut. Chromatogr. 1991, 14, 593. 43) Griepink, B.; Maier, E. A.; Muntau, H.; Wells, D. EUR Report 12823, 1990.
28 52 101 149 118 153 105 138 128 156 180 170
DDE DDT a
79 63 128 241 82 296 29 222 21 19 364 89 279
2 2 4 2 1 5 2 8 0.4 1 2 1 4
ND
ND
certified value
100 78 134
20 16 21
97 288
21 36
313
49
Two replicates. b dw, dry weight. ND, not detected.
as the extraction fluid on one kind of surface sediment, and to transfer the method to the analysis of other surface sediments that are adequately similar. As spiking procedures often fail to represent the native analytes accurately, this procedure bears a higher likelihood of producing a working method. This study also demonstrates the efficiency of the sulfur cleanup in the supercritical phase within the extraction step as long as a short static extraction step is introduced to allow the copper powder to react thoroughly with the sulfur. With the right choice of extraction conditions,trap material, trap temperature, and eluent, it is possible on a routine basis to perform interference-free, congener-specific analysis of PCBs, DDE, and DDT in real sulfur-containing surface sediments by off-line SFE and GC-ECD without the use of any manual sample workup between extraction and GC analysis and in a time span of less than 2.5 h.
ACKNOWLEDGMENT The authors are grateful to Hewlett-Packard Italiana S.p.A., Cernusco S/N, Milan, Italy, and especially to Giuseppe Candolfi and Costanza Rovida, for making the HP7680A SFE extractor available. The skillful technical assistance of Palle Fruekilde and Michael Hansen (The Engineering Academy of Denmark) is gratefully appreciated, as are the quantitative sulfur measurements by X-ray fluorescence detection done by Franco Bo (Environment Institute, JRC, Ispra). Finally we thank Herbert Muntau (Environment Institute, JRC, Ispra) for making the sediments used in this study available. Received for revlew August 10, 1993. Accepted December 1, 1993." Abstract published in Adoance ACS Abslracrs, January 15, 1994.
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