Anal. Chem. 1QQ2,64, 1614-1622
1814
Comparison of Supercritical CHCIF2, N20, and C02 for the Extraction of Polychlorinated Biphenyls and Polycyclic Aromatic Hydrocarbons Steven B. Hawthorne,' John J. Langenfeld, David J. Miller, and Mark D. Burford Energy and Environmental Research Center, University of North Dakota, Grand Forks, North Dakota 58202
Supercrltlcal fluld extractlon (SFE) recoverles for natlve pollutants including PCBs from a standard reference materlal sediment (SRM 1939), PAHs from a petroleumwaste sludge, and PAHs from rallroad bed soli were compared udng (UIpercrltlcai CHCIFz(Freon-22), N20,and COz. While SFE wlth pure CO, ylelded the lowest recoveries for each sample, CHCIFzconrlstently yleldedthe highest extractlonemclendes, most ilkely because of lts hlgh dipole moment. Extractlon efflclenclesobtained in 30-45 mln wing supercrltlcal CHCiF2 generally matched or exceeded those obtalned uslng 183 2 4 llquld solvent extractlons. Methanoknodlfled C02 a b yielded acceptable recoveriesof the PCBs from the sedtment (average of 90% versus the certified values). Extractlon rates of lndivklual PAHs from the petroleum waste sludge were slmllar wlth CHCIF2but decreased wlth Increasingm e iecuiar welght udng COz and NzO. Based on comparative extractlonsof samples ranging from ecwentlai oils from spruce neediestolonksurfactankifromeewagedu~,CHClFzyidds condstently higher recoverles(ca. 2-10 times) than C02.The results of thls study lndlcate that the abllity of a supercrltlcal extraction fluid to Interact wlth sorptlve sltes on sample (and poesbty to modny the matrlx,e.g., by the removal maof water) may ultlmately control SFE rates and recoverks from envhonmentai solids.
INTRODUCTION The use of supercritical fluids for the extraction of organic pollutants from environmental samples has received increasing attention because of the potential to dramatically reduce the time required for sample extraction as well as to eliminate the need for large volumes of liquid ~olventa.l-~ The majority of SFE investigations have used supercritical C02 because of ita low toxicity and cost, ita reasonable critical parameters, and ita ability to solvate a broad range of low- and moderatepolarity organics4 (e.g., organics which are sufficiently nonpolar to allow for their separation using conventional GC techniques). However, quantitative extractions from many environmental matrices appear to require SFE fluids that are not only capable of solvating the target analytes but are also capable of somehow interacting with, or altering, the matrix (or analyte) to facilitate partitioning of the analytes into the bulk supercritical fluid.3,&20 Unfortunately, the Corresponding author. (1) Veuthey, J. L.; Caude, M.; Rosset, R. Analusis 1990,18,103. (2)Vannoort,R. W.;Chervet, J.-P.;Lingeman,H.;DeJong,G. J.;Brinkman,U. A. Th. J. Chromatogr. 1990,505,45. (3)Hawthome, S. B. Anal. Chem. l990,62,633A. (4)Bartle, K. D.; Clifford, A. A.; Jafar,S. A.; Shilstone, G. F. J. phys. Chem. Ref. Data 1991,4,713. (5)Wheeler, J. R.; McNally, M. E. J. Chromatogr. Sci. 1989,27,534. (6)Ramsev, E.D.;Perkins, J. R.; Games, D. E.; Startin, J. R. J.Chromatogr. 1989; 464,353. (7) Hawthorne, S. B.; Miller, D. J.; Walker, D. D.; Whittington, D. E.; Moore, B. L. J. Chromatogr. 1991,541,185. 0003-2700/92/0364-1614$03.00/0
preparation of organically-modified fluids can be experimentally difficult, and the criteria for selecting an organic modifier and ita concentration are also unclear, poseibly because the nature of the analyte/matrix/supercriticalfluid interactions are poorly understood. The developmentof SFE conditionsthat yield quantitative recovery of moderate and high polarity analytes using a pure fluid would be useful, both for experimental simplicity and to aid in an understanding of the mechanisms which control SFE recoveries from different environmental matrices. Unfortunately, most pure supercritical fluids that are significantly more polar than C02 either have unreasonably high critical temperatures and pressures or are too reactive (e.g., ammonia) to be useful for analytical SFE. However, SFE with slightly more polar N2O (permanent dipole moment of 0.2 D versus 0.0 for COz) can increasethe extractionefficiencies of higher molecular weight PAHs and chlorinated dioxins from sediment and fly ash*J6.2°,21indicating that fluids with higher permanent dipole momenta may be useful for increasing SFE extraction efficiencies. (Note that NzO is an oxidizer, and therefore extraction of large quantities of organic material at high temperatures should be avoided.) While the selection of fluids for analytical SFE is severely limited by the need for a reasonable critical temperature and pressure, chemical inertness,low toxicity,and a gaseousstate at ambient conditions (to facilitate the collection of extracted analytes), some freons do exhibit these qualities as well as have significantdipole momenta. Although the widespread use of CFC (chlorofluorocarbon) freons for industrial processee is being reduced (and would therefore make future analytical uses less attractive), the likely replacements for CFCs, hydrogen-containingfreons (HCFCs),have much lower omne depletion and global-warming potentials and therefore are (8)Alexandrou, N.; Pawliszyn, J. Anal. Chem. 1989,61,2770. (9) Hills, J. W.; Hill, H. H., Jr.; Maeda, T. Anal. Chem.1991,63,2152. (10)Hawthorne, S.B.; Miller, D. J.; Nivens, D. E.; White,D. C. Anal. Chem. 1992,64,405. (11)Bartle, K. D.; Clifford, A. A.; Hawthorne, 5.B.; Langenfeld, J. J.; Miller. D. J.: Robinson. R. J. Suoercrit. Fluids 1990.3. 143. (12)Hawthorne, S.B.;Miller,b. J.; Langenfeld, J.'J.'Proceedings of the International Symposium on Supercritical Fluid Chromatography and Extraction; Park City, UT, January 1991; p 91. (13)Mulcahey, L. J.; Hedrick, J. L.; Taylor, L. T. Anal. Chem. 1991, 63, 2225. Kao, C.-P. C.; Dooley, K. M.; Knopf, F. C.; Gam(14)Brady, B. 0.; brell, R. P. Ind. Eng. Chem. Res. 1987,26,261. (15)Dooley, K. M.; Ghonasgi, D.; Knopf, F. C. Enuiron. Progr. 1990, 9,197. (16)Onuska,F. L;Terry, K. A. J.HighReso1ut. Chromutogr. 1989,12, 357. (17)Onuska,F.L;Terry,K.A. J.HighResolut. Chromutogr. 1989,12, 527..
(18)Wright, B. W.; Wright, C. W.; Gale, R. W.; Smith, R. D. Anal. Chem. 1987,59,38. (19)Yu, X.; Wang, X.;Bartha, R.; Rosen, J. D. Enuiron. Sci. Techml.
0 1992 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 64, NO. 14, JULY 15, lQ92 1818
Table I. Temperature, Pressure, and Fluid Densities for PAH and PCB Extractions fluid Tc ("'2) P, ( a h ) DMa P (atm) T ("0 CHCLFz
96
49
1.4
c02
32
72
0.0
37
72 74b
0.2
NzO
COdMeOH
41b
400 110 62 400 400 400 400
100 100 100 50
100 50
70
density (g/mL)b
8b.c
1.13 0.93 0.75 0.93 0.75
7.3 5.9 4.8 7.9 6.5 8.1
0.90 0.84
a Dipole moment (debye). Values were calculated with the ISCO "SF-Solver". The methanol modifier was 5% (v/v)in C02. Hildebrand solubility parameter.u.2s
much more suitable as analytical extraction fluids.22 Notably, CHClFz (Freon-22, the replacement refrigerant of choice for home air conditioning, ref 22) has a dipole moment of 1.4 D and has been shown to dramatically increase extraction efficiencies of steroids compared to those achieved using pure C02.23 In addition, CHClF2 is relatively inexpensive, nontoxic, and nonflammable. This report describes comparisons of the SFE rates and recoveries obtained for the extraction of PCBs and PAHs from environmental matrices using supercritical COZ,NzO, and CHClF2. Recoveries are compared to those obtained using conventional liquid solvent extractions. The effects of SFE pressure, temperature, and fluid density are described. Comparisons of extraction efficiencies obtained using C02 and CHClFz for several nonpolar to polar analytes from a variety of sample matrices are ale0 reported. EXPERIMENTAL SECTION Samples. All samples used in this study contained native (not spiked) analytes and were used without any pretreatment. The PCB-contaminated river sediment standard reference material (SRM 1939) was obtained from the National Institute of Standards and Technology (NIST, Gaithersburg, MD) and used as received. SRM 1939 is a sample of Hudson River sediment which contains low microgram per gram concentrations of individual PCB congeners. The sample was air-dried by NIST and contains ca. 3 w t % water and ca. 10% organic content (determined by thermal analysis). Concentrations of the individual congenersreported by NIST were based on two sequential 16-h Soxhlet extractions using 200 mL of 1:lhexane/methanol followed by 200 mL of 1:l hexane/acetone. The petroleum waste sludge was obtained from a commercial refinery and contained ca.45% water and 35% organic content (versus the wet weight as determined by thermal analysis). Portions of the sample were air-dried overnight to ca. 2 % water. All extractions were performed on the sample as received (45% water),except for the extractions specifyingthe air-dried material. Liquid methylene chloride extractions of 310-mg samples were performed using 50 mL of solvent and 18 h of sonication. After the extraction was completed, the internal standard (described below) was added and the solvent was evaporated to ca. 2 mL under a gentle stream of nitrogen. The railroad bed soil,collected adjacent to a railroad track set on wooden ties, contained ca. 1% water and 8% organic content. Before extraction, the sample was sieved through a 2-mm screen to remove rocks and sticks. Additional samples used to compare SFE efficiencies with C02 and CHClFz were PAHs from diesel exhaust particulate matter (NISTSRM1650),elementalsulfur from bituminouscoal(Illinois Coal Bank, sample 101), bulk abietic acid (Aldrich Chemical Co.), linear alkylbenzenesulfonates(LAS) from municipal wastewater treatment sludge, and essential oils from fresh spruce tree needles. Supercritical Fluid Extractions. All SFE extractions with pure fluids were performed in the dynamic mode (i.e., continual flow) using an ISCO Model 260D syringe pump (Lincoln, NE). SFC-grade COZand NzO and >99.9% purity CHClFz were all (22) Zurer, P. S. Chem. Eng. News 1989, July 24, p 7. (23) Li, S. F. Y.; Ong, C. P.; Lee, M. L.; Lee, H. K. J. Chromatogr. 1990,
515, 515.
purchased from Scott Specialty Gases (Plumsteadville, PA). Methanol-modified COZ(5vol % ) was prepared using two Model 260D pumps coupled with check valves and a mixing tee aa per the manufacturer's instructions. The SFEpumps were connected to the extraction cells with '/le-in.-o.d. stainless steel tubing and 'Slip-free" finger-tight connectors (Keystone Scientific, Bellefonte, PA). A 0.5-m-long coil of the tubing (placed at the inlet of the extraction cell to prewarm the fluid) and the extraction cells were placed in a thermostated tube heater to maintain the extraction temperature within k2 OC. Extraction cells having internal volumes of 1.0 (JASCO Scientific, Easton, MD), 0.5 (Keystone Scientific), and 2.2 mL (Keystone Scientific) were used for 310-mg samples of the petroleum waste sludge, 350-mg samples of the river sediment, and 3-g samples of the railroad bed soil, respectively. Unless otherwise noted, SFE flow rates were controlled at 0.5-0.8 mL/min (measured as liquid fluid at the pump) using 28-30-pm4.d. fused silica tubing for outlet restrictors. All extracts were collected by inserting the outlet end of the restrictor into a vial containing 2.5-5 mL of methylene chloride (for the PAHs) or acetone (for the PCBs). No attempt waa made to control the temperature of the collection solvent during the SFE, although slight warming of the restrictor and collection solvent with a heat gun was occasionally needed to maintain constant extraction flow rates during the extraction of the petroleum waste sludge. To determine whether these collectionConditionscouldquantitatively trap the e h a d PCBs and PAHs, a range of PAHs and PCBs were spiked onto sand and then extracted and collected in a manner identical to that used for the real-world samples. Recoveries of the spiked analytes were found to be quantitative (>95%) for all of the individual PCBs and PAHs, demonstrating that the collection conditions used in this study were quantitatively efficient for the individual PAHs and PCBs extracted from the real-world samples. The characteristics of the individual supercritical fluids at the various extraction conditions used in this study are summarized in Table I. The majority of extractions were performed at 400 atm and 50 OC for C02 and N2O or 100 OC for CHClF2 (because of its higher critical temperature). To compare CHClF2and C02 extractions on a constant density and temperature basis, CHClFz extractions were ala0 performed at 110and 62 atm to provide the same density as the 400-atm COz extractions performed at 50 and 100 OC, respectively. As shown in Table I, CHClFz has the highest dipole moment but the lowest solvent strength based on the calculated Hildebrand solubility parameters.%@ Extract Analysis. All SFE extracts were analyzed without any additional sample preparation, except for the addition of an appropriate internal standard. Quantitations of individualPCBs were performed using a Hewlett-Packard Model 5890 capillary GC equipped with a 60-m J&W DB-5 column (250-pm i.d., 0.25pm film thickness, Folsom, CA) and an electron capture detector (ECD). Autosamplerinjectionswere performed in the split mode at an oven temperature of 120 OC followed by a temperature ramp of 8 "C/min to 330 OC. An internal standard, 125 ng of 1,2,3,5-tetrachlorobenzene,was added to each extract prior to GC analysis (but after SFE). Quantitations of individual PCB S. R. Znd. Eng. Chem. Process Des. Dev. 1984,23, 344. (25) Giddinga, J. C.; Myers, M. N.; McLaren, L.; Keller, R. A. Science 1968,162 (lo), 67. (26) Hawthorne,S . B.; Krieger, M. S.; Miller, D. J. Anal. Chem. 1989, 61, 736. (24) &da,
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 14, JULY 15, 1992 C15
Q 2
!2 Fluoranthene
z a anthracene
Retention Time (minutes) Flguro 1. GC/FID chromatogram of the supercrltlcal CHCIF2 extract of the petroleum waste sludge. Chromatographic condltions are ghren in the text. Peak Identities were based on GC/MS analysisperformed
under Identical Chromatographic conditions.
congeners were based on standard solutions that were prepared gravimetrically from the individual pure (>99 %) congeners. Quantitative values are reported only for congeners which have concentrations reported by NIST and for which authentic standards were available during this study. PAH analyses for the petroleum waste sludge extracts were performed with a Hewlett-Packard 5988 GUMS equipped with a 25-m HP-5 column (320-pm i.d., 0.17-pm film thickness). Injectionswere performed by an autosampler at a split ratio of ca. 1:20. GC oven temperature at injection was 50 O C followed by a temperature ramp at 8 OC/min to 330 O C . Quantitations were based on GC/MS analyses performed in the selected ion monitoring mode (SIM)by monitoring the molecular ion of each PAH. Quantitative PAH standards were prepared from NIST SRM 1647which containscertified concentrationsof 16individual PAHs in acetonitrile. Chrysene-dlz (50 pg) was added as an internal standard to each PAH standard and extract. The PAH extracts from the railroad bed soil contained only PAHs as significant species, thus allowing the individual PAHs to be quantitated using FID detection with heptadecane (8fig) added to each extract as an internal standard. GC separations were performed using the same column as for the GUMS analyses with a oven temperature of 80 "C followed by a temperature ramp at 6 OC/min to 320 "C.
RESULTS AND DISCUSSION Chromatograms of the SFE extracts from the petroleum waste sludge and the railroad bed soil are shown in Figures 1 and 2. GC/ECD chromatograms of the SFE extracts from the PCB-contaminated sediment are virtually identical to those previously reported.27 "Blank" extracts (generated by performing SFE of an empty extraction cell) showed no significant impurities in any of the SFE fluids tested when analyzed in a manner identical to that used for sample extracts. GC/ECD chromatograms of the PCB-contaminated river sediment extracta showed PCBs as the primary speciespresent in all of the SFE extracts (confirmed by GC/MS analysis). GC/FID analysis of the petroleum waste sludge (Figure 1) showed an extremely complex mixture of hydrocarbons which were identified by GC/MS to be primarily branched and normal alkanes, alkylbenzenes, alkylnaphthalenes, and additional PAHs. Because of the complexity of the petroleum waste sludge and diesel exhaust particulate extracts, quantitation of the individual PAHs could not be performed using FID detection. However, GC/MS analysis using selected ion monitoring (as described above) allowed direct quantitation (27)Rebbert, R. E.;Cheder, S. N.; Guenther, F. R.;Koster, B. J.; Parris, R.M.; Schantz, M. M.; Wise, S. A. Fresenius'J. Anal. Chem. 1992, 342,30.
of the individual PAHs in the unfractionated extracts. In contrast to the petroleum waste sludge and diesel exhaust particulate extracts, the railroad bed soil contained a relatively simple mixture of PAHs and heteroatom-containing PAHs which allowed for their quantitation using FID detection (Figure 2). Comparison of Fluids for SFE of PCBs from River Sediment. Preliminary extractions of the river sediment with CHClFz and COZdemonstrated that CHClFz yielded significantly faster extraction of the PCBs than COZand that the CHClFz extractions were essentially completed in 40 min as shown in Figure 3 for the 2,2',3,5' and 2,2',3,3',4,4',5 congeners. Therefore, subsequent extractions of the river sediment were performed for 40 min with each fluid. A comparison of the PCB recoveries obtained using pure C02, NzO, and CHClFz with those obtained by NIST using two sequential 16-h Soxhlet extractions are shown in Table 11. The extraction efficienciesobtained using pure COZand N2O were essentially identical for each PCB congener and ranged from 36 to 91% with an average recovery of 62% for each fluid. Since PCBs ranging from dichloro to hexachloro congeners have been quantitatively extracted from polyurethane foam sorbents using 10-min extractions with pure COZ under similar SFE the relatively low recoveries for pure COSextractions indicate that interactions between the PCBs and the river sediment matrix may be responsible for the relatively poor recoveries. Interestingly, the recoveries showed a general increase in extraction efficiency as the molecular weight of the PCB congener increased, which was surprising since the solubility of PCBs in supercritical C02 might be expected to decrease with increasing molecular weight.' It is also interesting to note that N2O showed no advantage over COZ,in contrast to earlier reports of increased extraction efficiencies with NzO obtained for PAHs from marine sediment and chlorinated dioxins from fly ash and sediment.8J6921 CHClFz had substantially higher recoveries for each of the PCB congeners than either COz or NzO, and in general, the recoveries obtained using CHClFz gave good agreement with the Soxhlet extraction (NIST) values with an average recovery of 97% for the nine PCB congeners. The only significant variation from 100% recovery (versus the NIST values) was for the most volatile PCB congener, 2,4,4'-trichlorobiphenyl, which showed only a 63 5% recovery. Since the COZand N2O extractions also showed the lowest recoveries for the more volatile PCB Congeners, loss of the more volatile species from the collection solvent during the 40-min extractions was indicated. However, when spiked PCBs were extracted from glass beads and collected under identical SFE conditions, the recoveries of the individual congeners were all essentially 100%, indicating that inefficient collection of the lower molecular weight PCBs using the SFE step was not responsible for the lower recoveries. A possible explanation for the low recovery of the 2,4,4'-trichlorobiphenylis found by inspecting the chromatogram provided by NIST with the SRM certificahZ7Their chromatogram and those that we obtained are virtually indistinguishable and both show significant overlap of the 2,4,4'-trichlorobiphenyl peak with a different PCB congener which likely leads to error in peak area integration. Since our value for 2,4,4'-trichlorobiphenyl is based only on GC/ECD analysis, while the NIST value is also based on dual column GC/MS, it is likely that our low value is a result of poor chromatographic resolution rather than poor SFE efficiency when using CHClFz. Quantitative reproducibilities obtained from the triplicate extractions using CHClFz were excellent, with a range in relative standard deviations (RSDs) of only 1-5 5%. RSDs obtained from the replicate COz and NzO extractions were
ANALYTICAL CHEMISTRY, VOL. 64, NO. 14, JULY 15, 1992 Heptadecane IS
\I
Fluorene
Phenanthrene
/
1617
Pyrene
/
Fluoranlhene
\ Benz[a]anthracene
4
Anthracene
\
Acenaphthylene
Benw[b]fluoranthene Benzo[k]fluoranthene I
30 20 Retention Time (minutes) Flgwe 2. GC/FID analysis of the supercritical CHCIF2extract of railroad bed soil. Identificationswere based on the retention times of authentlc standards and verified by GClMS analysis performed under Identical chromatographic conditions as descrlbed In the Experimental Sectbn.
10
2,2',3,5'
-t
400 atm CO1 (50 'C)
I
0
10
20
30
Extraction Time (minutes)
40
2,2',3,3',4,4',5
Ob 0
10
1
1
20
30
I 40
Extraction Time (minutes)
extraction kinetics of representative PCB congeners from river sediment using supercritical CHCIF2and Con. The curves show the cumulative amounts of each PCB extracted at each tlmed fraction collected during the SFE step. Percent recoverles are based on those reported for CHCIFp extractions in Table 11. Flgure 9. SFE
also acceptable with a range of ca. 2-10%. Since the reproducibilitiesshown in Table I1 are based on the extraction of triplicate sediment samples with each fluid, quantitative variations includeany variations introduced by a combination
of the SFE step, sample inhomogeneity, and/or the GC/ECD analysis. The relatively low RSDs obtained (particularly for CHClFz extractions) demonstrate that the SFE and GC steps are reproducible and that the relatively small samples used (350mg) were representative of the bulk sample matrix. (Note that small samples were used only because of limited sample availability and that the extraction of samples weighing a few grams causes no experimental difficulty.) While the recoveries obtained using pure COZand NzO were unacceptable (Table II), methanol-modified COZhas previously been reported to yield quantitative recovery of PCBs from a soil and a harbor sediment.'5,17 Therefore, the use of COZmodified with 5 % (v/v) methanol (400 atm, 70 "C) was investigated in the hopes of obtaining PCB recoveries similar to those obtained using CHClFz. As shown in Table 11,the recoveries obtained using the methanol-modified COZ were nearly as high as those obtained using CHClFz and yielded reasonable quantitative agreement with the NIST values except for the most volatile PCB congeners.
Comparison of Fluids for SFE of PAHs from Petroleum Waste Sludge. Preliminary extractions of the petroleum waste sludge (Figure l) demonstrated that the sludge sample contained concentrations of PAHs that were high enough for severaltimed fractions to be collected during SFE so that extraction kinetic curves (i.e., the quantity of analytes extracted versus extraction time) could be generated. Typical extraction curves for individual PAHs, plotted as cumulative amount extracted versus time, are shown in Figure 4 for phenanthrene (MW = 178))pyrene (MW = 202), and benzo[alpyrene (MW = 252). As shown in Figure 4,CHClFz yielded faster and higher recoveries of the PAHs from the native (45 % water) sample than either COz or NzO, especially for the higher molecular weight PAHs. NzO yields substantially better extraction efficiencies of the higher molecular weight PAHs from the wet sample than COz during the first few minutes of an extraction (Figure 4)) which is similar to the results of 10-min extractions of PAHs from marine sediment using COZ and N20 as reported previously.21 However,the advantage of NzO over COz diminishes as longer extraction times are used, and the extraction efficiencies obtained from the wet sample with either NzO or COz do not approach those obtained using CHClFz.
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 14, JULY 15, 1992
Table 11. Comparison of Soxhlet Extraction (NIST) with SFE Using CHClF2, N20, COz, and Methanol-Modified COZfor the Recovery of PCBs from River Sediment (SRM 1939) % recovery f S D PCB congene9 PCB concn (fig/g),NIS’P CHClF’z Nz0 coz COdMeOH 2,4,4’ 2,2‘,5,5’ 2,2’,3,5’ 2,3‘,4,4’,5 2,2‘,3,4,4’,5’ 2,2’,3,4’,5,5’,6 2,2’,3,3’,4,4’ 2,2’,3,4,4‘,5,5’ 2,2’,3,3’,4,4’,5 av recovery ( % )
2.21. f 0.10 4.48 f 0.06 1.07 f 0.12 0.51 f 0.01 0.57 f 0.01 0.18 f 0.01 0.10 f 0.01 0.16 f 0.01 0.11 f 0.01
63 f 3 83 f 4 108 f 4 124 f 2 85 f 0.5 91 f 4 88f1 104 f 3 128 f 7 97 f 21
37 f 2 41 f 2 46 f 1 63 f 2 61 f 2 76f7 72 f 7 91 f 7 71 f 5 62 f 18
36 f 3 38f 1 44 f 1 75 f 10 57 f 3 89f6 65 f 7 90f9 66f 7 62 f 20
65 f 6 72 f 5 80f4 97 f 1 96 f 5 94 f 7 101 f 7 105 f 9 99f 11 90 f 14
a Percent recoveries (fone standard deviation unit) versus the NIST Soxhlet concentrations are based on triplicate extractions using each fluid. * Each individual PCB congener is identified by ita chlorine substitution pattern, e.g., 2,2’,5,5’ denotes 2,2’,5,5’-tetrachlorobiphenyl. Concentrations reported by NIST. Values for 2,4,4’-trichlorobiphenyland 2,2’,3,5’-tetrachlorobiphenylare based on GC/ECD and dual column GC with MS detection. The concentrationsof the remaining congeners are based on GC/ECD analyses of two sequential 16-h Soxhlet
extractions.
The quantities of individual PAHs extracted from triplicate samples of the petroleum waste sludge using supercritical COZ,supercritical CHClFz, and liquid methylene chloride with 18h of sonication are shown in Table 111. Although the extraction kinetics shown in Figure 4 demonstrate that the CHClFz extractions were essentially completed in ca. 35 min, each SFE extraction was performed for the longest time considered to be practical (85 min) because of the relatively slow extraction kinetics achieved using COz. As shown in Table 111, the extraction with supercritical CHClFz yielded substantially higher extraction efficiencies than either the supercritical COZor the 18-h methylene chloride sonication. Compared to supercritical CHClFz, liquid methylene chloride extracted nearly as much of the lower molecular weight PAHs (e.g., ca. 85% of the phenanthrene and anthracene), but comparativelyless of the higher molecular weight PAHs (e.g., 65% as much benzo[alpyrene). Supercritical COZ also performed reasonably well with the lower molecular weight PAHs (e.g., recovering 70% of the phenanthrene versus CHClFz) but yielded substantially poorer recoveries of the higher molecular weight PAHs (e.g., 24% of the benzolalpyrene). In addition to yielding the highest recoveries of each PAH, CHClFz extractions of triplicate samples also displayed good reproducibility (RSDsranging from 2 to 9 %), demonstrating that the SFE technique itself is reproducible, and that the 310-mg samples used were representative of the bulk sludge matrix. Even though the PAH concentrations shown in Table I11 were based on 85-min SFE extractions for the CHClFz and COz, the extraction kinetic plots shown in Figure 4 clearly demonstrate that a 40-min SFE with CHClFz would yield essentially identical extraction efficiencies. Additionally, almost exactly 80% of all of the PAHs were extracted after only 10 min of SFE with CHClFz (compared to 85 min of CHClFz extraction). Interestingly, these results correspond closely to the extraction efficiencies obtained using liquid methylene chloride extraction. For example, 10 min of SFE with CHClFz yielded ca. 94% as much phenanthrene, 114% as much pyrene, 110% as much chrysene, and 120% as much benzo[a]pyrene as the 18-h sonication with methylene chloride even though the polarity of methylene chloride (dipole moment = 1.5 D) is similar to that of CHClFz and methylene chloride is generally considered to be the ideal extraction solvent for PAHs. While the extraction efficiencies of PCBs obtained using COZand NzO generally increased with increasing molecular weight (Table 11),extraction of PAHs from the waste sludge generally declined with increasing molecular weight as has
been previously reported for PAHs from soil, marine and river sediment,fly ash, and urban air particulate matter.l+Z1*28 This phenomenon is easily seen in the extraction kinetic curves for several representative PAHs extracted with supercritical COz (Figure 4). The initial extraction rate of the lower molecular weight PAHs is much faster than the higher molecular weight PAHs, a trend that fits with the generally lower solubility of higher molecular weight PAHs in supercritical CO2.4,a In contrast, the extraction kinetics of individualPAHs using supercritical CHClFz are virtually identical (Figure 41, regardless of their molecular weight. Unfortunately, solubilities of PAHs in supercritical CHClFz are not available, but, since it is not expected that all PAHs have the same solubility in supercritical CHClFz, the relatively high efficiency of CHClFz appears to be based on its ability to interact with the matrix and compete efficiently with the PAHs for matrix active sites, or to somehow modify the matrix to expose the PAHe for more efficient extraction (by the removal of water, for example, as discussed below). The extraction kinetic curves for both the PCBs and the PAHs (Figures 3-5) all show the same general shape that has been previously describedby a diffusion-based modelrecently proposed to explain SFE extraction rates.” This model assumes that diffusion of an analyte from the interior portions of a sample matrix to the surface of the matrix particles (not diffusion in the supercritical fluid) is the step that limits SFE rates. While the extraction rates obtained for PAHs and PCBs from the samples used in this study definitely display a kinetic limitation that fits this model, diffusion limitations do not seem to be the most likely rate-controlling step for the results shown in Figures 3-5. For example, if diffusion in the sample matrix was the rate-limiting step, changing the SFE fluid would not be expected to have a major effect on the recovery rates of the extracted analytes since the fluid identity would have no large effect on the diffusion of the analytes in the sample matrix (unless the fluid caused a dramatic physical change in the matrix itself). Since CHClFz always yielded substantially faster extraction rates than either COZor NzO for all of the samples in this study, and since no dramatic effect on the sample matrix could be observed (except for the drying of the wet samples when using CHClFz discussed below), it seems likely that the superiority of CHClFz is based on its ability to interact with the matrix and compete efficiently with the analytes for matrix active sites, leading (28) Wright, B. W.; Wright, C. W.; Fruchter, J. S. Energy Fuels 1989, 3, 414. (29) Johnston, K. P.; Ziger, D. H.; Eckert, C. A. Znd.Eng. Chem.Fundam. 1982,21, 191.
ANALYTICAL CHEMISTRY, VOL. 04, NO.
14, JULY 15, 1902 1610
Table 111. Comparison of Methylene Chloride Sonication to SFE Using COz and CHClFz for the Recovery of PAHs from Petroleum Waste Sludge PAH concentration (pg/g) ( % RSD)"
100
80
6
CHzClz PAH
d 3
5
2 $:
40 Wet sludge 400 a m N,O (50 "c)
-A-
20
I
0
10
20
I
I
30
40
50
60
I
I
I
70
80
90
Extraction Time (minutes) 100
phenanthrene anthracene fluoranthene Pyrene benz[a]anthracene chrysene benzo[bl-, - [kl fluoranthene benzo [ a ]pyrene
CHClFf
412 (13.0) 43 (17.0) 45 (7.3) 190 (2.8) 97 (17.0) 233 (11.0) 67 (8.2) 79 (4.9)
486 (9.2) 51 (5.1) 62 (4.9) 272 (8.6) 136 (6.2) 319 (7.9) 93 (2.4) 121 (1.9)
C0zC 341 (1.4) 32 (4.5) 40 (23.0) 138 (5.4) 52 (12.0) 122 (17.0) 42 (11.0) 29 (7.6)
a Relative standard deviations (%) for triplicate extractions are given in parentheses. Baaed on 18-h sonication with methylene chloride. Baaed on 85-min SFE.
i
80
A
/
I
I
20
__ 0
10
20
30
40
50
70
60
80
90
Extraction Time (minutes)
6
8"
d 3
40
k + Air-dnsd sludge 400 alm C 0 2 (50 'C)
20
400 atm
!fl 1
1
1
1
I
20
30
40
50
4 Wet
1
N,O
(50 "C)
sludge
400atmC0, (5O'Cc)
I
. _ -
0
IO
60
70
SO
90
Extraction Time (minutes) Figure 4. SFE extraction kinetics of representative PAHs from petroleum waste sludge using supercritical Con, N20, and CHClF2 of the native (45% water)and C02extraction of the airdrled (2 % water) samples. The curves show the cumulative amounts of each PAH extracted at each timed fraction collected during the SFE step. Extractionefficiencies were normailzed to 100% based on the 85-min CHCIF2 results shown in Table 111.
to the conclusion that chemisorption of analytes to matrix active sites is the most important fador to overcome to achieve quantitative SFE recoveries. Effect of Extraction Temperature, Fluid Density, and Sample Matrix on SFE Recoveries. All of the comparative
recovery data discussed above and in Tables I1 and I11 were generated using the same pressure (400 atm) for all three fluids, and extraction temperatures of 50 "C for COz and NzO and 100 "C for CHClFz (the higher temperature was necessary because of ita 96 "C critical temperature). As shown in Table I, COz and NzO have similar densities (0.93 and 0.90 g/mL, respectively) at those conditions; however, the density of CHClFz is higher (1.13 g/mL). Since the solvent strength of
a supercritical fluid increases with increasing density, it is possible that the higher density of the CHClFz contributed to the higher PAH and PCB recoveries. Additionally, the higher temperature used for the CHClFz extractions could affect the extraction efficiencies by increasing the vapor pressure of the analytes. To inveetigatethe possibilitiesthat either the higher density or the higher temperature used for the CHClFz extractions could be responsible for ita higher recoveries, two additional conditions were used to extract the river sediment and the petroleum waste sludge samples. First, the samples were extracted with CHClFz at 110 atm (100 "C), the pressure where ita density would be equal to the COZdensity at 400 atm (50 "C), i.e., 0.93 g/mL. Second, the samples were extracted with C02 at 100 "C, i.e., the CHClFz extraction temperature (all other extraction conditions were identical to those described previously). Table IV shows the effect of density and temperature on the extraction efficienciesof the PCBs from river sediment. The quantity of each PCB extracted was compared on a percentage basis to that extracted using CHClFz at 400 atm (i.e., the values reported in Table 11). Reducing the pressure of the CHClFz extraction to 110 atm (0.93 g/mL) only caused a small reduction in extraction efficiencies, with the average recovery reduced to 857%. However, the recoveries were still considerably higher than the 400-atm COZ extraction performed at the same density (Table IV). Increasing the temperature of the COZ extraction to 100 "C resulted in a small increase in recoveries for the lower molecular weight PCBs, but no significantchange in the recoveries of the higher molecular weight PCBs. In any case, the 110-atm CHClFz extraction yielded substantially higher extraction efficiencies than 400 atm of COz at either 50 or 100 OC, suggesting that the polarity of CHClFz, rather than the fluid density or temperature, was responsible for the higher extraction efficiencies. In contrast to the PCB results, the extraction efficiencies obtained from the petroleum waste sludge using 110 atm of CHClFz (0.93 g/mL) were substantially lower than those obtained using 400 atm of CHClF2, with an average recovery of only 58% (Table V). However, the recoveries obtained with 110atm of CHClFz were still generally higher than those obtained using 400 atm of COZat either 50 "C (average 49% recovery) or 100 OC (average 43% recovery). It is interesting to also note that the higher temperature appeared to reduce the COZ extraction efficiencies of the lower molecular weight PAHs while appearing to increase the extraction efficiency of the lower molecular weight PCBs (Table IV), although the changes were not large. Also, similar to the 400-atm CHClFz results, the 110-atm CHClFz extractions showed no trend in extraction efficiencieswith the molecular weight of the PAH,
ANALYTICAL CHEMISTRY, VOL. 64, NO. 14, JULY 15, 1992
1620
I
I
Phenanthrene
Table IV., Effects of Density and Temperature on the Recovery of PCBs from River Sediment Using CHClFt and
coz
% extracted versua 400 atm of CHCWZO
CHClF2
PCB congener
* 400 atm C02 (100% + 400 atm '2% C d C )
I 20
0
40
SO
60
100
2,4,4' 2,2',5,5' 2,2',3,5' 2,3',4,4',5 2,2',3,4,4',5' 2,2',3,4',5,5',6 2,2',3,3',4,4' 2,2',3,4,4',5,5' 2,2',3,3',4,4',5 av recovery (% )
Extraction time (minuted
co2
COa
(110 atm, 100 OC, 0.93 g/mL)
(400 atm, 50 OC, 0.93 g/mL)
(400 atm, 100 OC, 0.75 g/mL)
102 i 6 81 i 4 74 i 3 72 f 3 95 i 7 93 f 10 100f4 90*9 62 f 2
57 f 5 46 i 1 41 i 1 60 i 8 67 f 4 98i5 74 f 8 87 f 9 52 i 6
76 i 4 60 i 4 53 i 7 53 4 79 i 8 91 i 7 M i 8 76 i 6 51 f 4
65 i 19
70 i 16
85
* 14
*
a Each recovery was reported versua the quantity of each PCB extracted wing 400 atm of CHClF'2 as shown in Table 11. Standard deviations are for triplicate extractions at each condition.
I
1
Carbazole
100
80
2
b
-
60
I
c
f
(lO0OC)
40
8 110 atm C H C I F ~(roo%
a 0
j+
20
82 atm CHCIF, ( 1 0 8 ~ 1
* 400 atm C O- (IOO~C) ~ -c 400 atm CO,CSO~C)
20
0
40
80
60
100
Extraction Time (minuter)
1
Benzlalanthracene
w
loot 80
-
".
#
u
+400 atm CHCIF,
zor 0 4
20
(100%)
e 110 atm CHCIF, (iooOc~ ++ 82 atm CHCIF, (100Oc)
40
60
80
100
I
Extracllon Tlme (minuter)
Flgure 5. Effect of fluid density and SFE temperature on the extraction of PAHs from raliroad bed soil. Samples were extracted with 400 atm of Con at 50 OC (0.93 g/mL) and 100 O C (0.75 g/mL). Extractions with CHCIFp were performed at 100 O C and 62 atm (0.75 g/mL), 110 atm (0.93 g/mL), and 400 atm (1.13 g/mL). while the 50 "C CO2 extraction showed a definite decrease in recoveries for the higher molecular weight PAHs (Table V). Since the solubilities of individual PAHs in CHClFz might be expected to drop with molecular weight as they do in COz, the fact that all of the individual PAHs were extracted with the same efficiency using 110 atm of CHClF2 demonstrates that the PAH recoveries were not limited by a decrease in solubility at the lower pressure; rather, they appear to be limited by a decreased ability of the CHClF2 to interact with the sample matrix and displace the PAHs from the matrix. It should also be noted that much less water was removed
from the sludge sample using CHClF2 at 110 atm than at 400 atm, and since all of the PAHs were extracted at the same efficiency (ca. 58%, Table V), these results indicate that the presence of unextracted water may have reduced the amount of analytes that were exposed to the supercritical fluid for extraction. The large drop in PAH extraction efficiencies obtained from the waste sludge with 110 atm of CHClFz, compared to the minimal drop in extraction efficiencies obtained for the PCBs from the river sediment, could be a result of either the differencesin extraction characteristics of PAHs versus PCBs or a result of the large differences in the matrices. While the river sediment had very low extractable organic content (based on GC/FID analysis) and was relatively dry (ca. 3 % water), the waste sludge had extremely high concentrations of extractable organics (Figure 1)as well as a high water content (ca. 45%). To determine whether the greater reduction in extraction efficiencies of the PAHs was a result of the high water content in the sludge matrix, a sample of the sludge was air-dried overnight to ca. 2 % water, and the extraction rates were determined using 400 atm of C02 (50 OC). The extraction rate of the lower molecular weight PAHs such as phenanthrene, fluoranthene, and pyrene increased dramatically from the dried sample (values are normalized for the weight loss which occured upon drying), and the recoveries were similar to those obtained with 400 atm of CHClFz as shown in Figure 4. In contrast, the C02 extraction rates of the higher molecular weight PAHs such as benzo[alpyrene showed only slight increases with the dried sample (Figure 4). While solubility data for water in supercritical CHClF2 is not available, it appears that supercritical CHClF2 is a better solvent for water on the basis of the observations that the wet sludgesamples were quite dry after CHClFz extraction and that droplets of water were found in the methylene chloride collection solvent. In contrast, the C02 extraction of the wet sludge did not show significant drying (and no water droplets were observed in the collection solvent). Based on these results, it appears that CHClF2 has two mechanistic advantages over COZfor the PAH extraction from the sludge. First, CHClFz is better able to remove water to expose the PAHs for extraction, although once the sample is dry, C02 can efficiently extract the lower molecular weight analytes (which have high solubilities in C02). Second, whether the matrix is wet or dry, CHClFz is a better extraction fluid than C02 for the higher molecular weight PAHs. To further investigate the effect of the sample matrix on the recoveries of PAHs, extractions at constant density and temperature with CHClFz and COZwere performed using a PAH-contaminated matrix that was much lower in extractable
ANALYTICAL CHEMISTRY, VOL. 64, NO. 14, JULY 15, 1892
1621
Table V. Effects of Density and Temperature on the Recovery of PAHs from Petroleum Waste Sludge Using CHClFa and CO, % extracted versus 400 atm of CHCIFza
PAH
phenanthrene anthracene fluoranthene pyrene benz [a]anthracene chrysene benzot bl-, -[klfluoranthene
benzo[alpyrene av recovery ( % )
CHClFz (110atm, 100 OC, 0.93 g/mL) 59 5 55 4 61 i I 59 f 3 56f3 56 f 3 58f5 59 5 58 f 2
* * *
coz
(400atm, 50 OC, 0.93 g/mL) 70 f 1 63 3 64 15 51 3 38 f 5 38 i 7 45i5 24 2 49 f 16
* *
*
coz
(400atm, 100 OC, 0.75 g / d ) 37 f 5 41 5 47 4 40 f 4 48 & 5 49 i 4 43 f 5 38 f 3 43 f 5
a Each recovery was reported versus the quantity of each PAH extractedusing 400 atm of CHClFz as shown in Table 111. Standard deviations are for triplicate extractions at each condition.
organic content and water content, i.e., railroad bed soil. This soil had ca. 1%water content, and essentially all of the extractable organics were PAHs as shown in Figure 2. Additionally, the railroad bed soil contained concentrations of PAHs (e.g., low pg/g) that were similarto the concentrations of the PCB congeners present in the river sediment and were also high enough to allow the extraction kinetic curves to be generated. One additional extraction condition was added, 62 atm of CHClFz, to obtain a CHClFz density at 100 "C that was the same as the COZdensity at 400 atm and 100 "C (0.75 g/mL). Extraction kinetic curves that are representative of the PAHs extracted from the railroad bed soil, phenanthrene and benz[a]anthracene, along with the nitrogen-containing PAH, carbazole, are shown in Figure 5. As would be expected from the result of the previous PCB and PAH extractions, extraction with COz at 50 "C yielded the slowest recoveries of all of the PAHs and the heteroatom-containing PAHs including carbazole, dibenzothiophene, and dibenzofuran. Increasing the temperature of the COz extraction to 100 "C did yield some increase in extraction efficiency, particularly for the most volatile PAH, phenanthrene (Figure 5, top). However, the recoveries of the less-volatile PAHs obtained using CHClFz at either 400 atm, 110 atm, or 62 atm were superior to 400-atm COz extraction at either 50 or 100 OC. These results are very similar to those found for the PCB extractions from river sediment (Table IV), i.e., low-pressure CHClFz extractions yielded extraction efficiencies nearly as high as those obtained using higher pressure extractions. This indicates that the matrix of the petroleum waste sludge was responsible for the large reduction in PAH extraction efficiencies (Table V) when the CHClFz extraction pressure was reduced from 400 to 110 atm, most likely because of the decreased ability of the 110-atm extraction to remove the matrix water (45% ) and expose the PAHs to the supercritical fluid. While the results discussed above indicate that CHClFz is superior to COz (and NzO) as an SFE extraction fluid because of ita ability to remove water from wet matrices ae well as ita ability to better displace the analytes from sorptive matrix sites, it is also possible that reactions with water during CHClFz extractions could result in the generation of HF (and possibly HC1) with a resultant increase in SFE recoveries. This possibility is supported by the fact that the fused silica restrictors become brittle during SFE with CHClFz but not with COz. (Because of this degradation in the restrictor strength, a new restrictor was used for each extraction in this study. This is not a serious drawback since the restrictors used in this study can be replaced in a few seconds and cost only a few cents.) However, when CHClFz extracts from wet
samples were collected into a carbonate/bicarbonate buffer and analyzed by ion chromatography, no significant fluoride or chloride ion concentrations were detected (
