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Effects of Temperature and Pressure on Supercritical Fluid Extraction Efficiencies of Polycyclic Aromatic Hydrocarbons and Polychlorinated Biphenyls John J. Langenfeld2J Steven B. Hawthorne,'.?David J. Miller2 and Janusz Pawliszynl University of North Dakota, Energy and Environmental Research Center, Grand Forks, North Dakota 58202, and Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
Three certified reference materials, polychlorinatedMphenyb (PCBs) from river sedknent, pdycyclk aromatk hydrocarbons (PAHs) from urban air particulate matter, and PAHs from hlghly contaminated soil, were extracted wlth pure COz at conventional(50 "C) and high (200 "C) temperatures. At 50 OC, raislng the extraction preswre (350-650 atm) had no effect on extraction efflciencies from any of the samples. High recoverles were obtained In 40 mln from the hlghly contamhated soil regardless of temperature. However, PCBs from sediment and the PAHs from air particulates were effkiently extracted only if the temperature was raked to 200 OC. At 200 OC, PCBs were effectively extracted at any pressure (150-650 atm), whlie both higher temperature and pressure Increased the recovery of PAHs from alr partlcuiates. These results demonstrate that temperature Is more Important than pressure for achlevlng high extractlon effkiencies when the Interactlonobetweenpollutantmoleculesand sample matrkes are strong and Indicate that increasingSFE temperatures may be a wefui siternatheto addlng organic modlfh?for achlevlng high extractlon eff lciencles from environmental samples.
INTRODUCTION Supercritical fluid extraction (SFE) has gained increased attention as a potential replacement for conventional liquid solvent extractions (e.g., sonication or Soxhlet extraction) because of SFE's ability to quantitatively extract organic pollutants from environmental solids in a fraction of the time while nearly eliminating the need for liquid solvents.lpZ The most popular fluid for SFE has been COZbecause of its low critical properties (T,= 32 OC, P,= 72 atm), low toxicity and coat, and ita ability to solvatea wide range of organics including high molecular weight and moderately polar organics.3 Unfortunately, COZ cannot quantitatively extract many organicpollutants from environmental solids, even when they exhibit sufficient solubility in supercritical COz, which demonstrates that a suitable supercritical fluid not only must be capable of solvating target analytes but also must be able to efficiently interact with the analyte-matrix complex to promote rapid partitioning of the analyte into the bulk supercritical fluid.'@ The addition of organic modifiers (e.g., methanol) to supercritical COz has been shown to dramatically increase
* To whom correspondence should be addressed.
University of North Dakota. University of Waterloo. (1)Hawthorne, S. B. Anal. Chem. 1990,62,633A. (2)Vannoort, R. W.; Chervet, J.-P.; Lingeman, H.; DeJong, G. J.; Brinkman, U. A. Th. J. Chromatogr. 1990,505,45. (3)Bartle, K.D.; Clifford, A. A.; Jafar, S. A.; Shilstone. G. F. JPhys. Chem. Ref. Data 1991,20,713. (4) Alexandrou, N.; Pawliszyn, J. Anal. Chem. 1989,61, 2770. (5)Alexandrou, N.; Pawliszyn, J.; Lawrence, M. Anal. Chem. 1992,64, 301. +
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extraction efficiencies7-9of some organicpollutants. However, since the mechanisms that control SFE of environmental samples are poorly understood, choosing a modifier for a particular application can be difficult. For example, it has been reported that COdlO% v/v methanol quantitatively extracts PCBs from river sediment1*12 but fails to quantitatively extract nitroaromatics from diesel exhaust particulates13 or chlorinated dibenzo-p-dioxinsfrom fly ash.4 The preparation of modified fluids for dynamic (continuous flow) SFE also requires an additional high-pressure pump or the purchase of premixed fluids. The use of modifiers can also complicate the analysis of the extracts since the extracts will contain high concentrations of the modifier which may (for example) degrade chromatographic performance or directly interfere with the detection of target analytss (e.g., total petroleum hydrocarbon determinations using infrared detection14J5). An alternate approach to improve SFE efficiencies is to choose pure fluids that are more polar and selective than COz.5 Unfortunately, the selection of other fluids is limited by the desire for reasonable critical parameters, chemical inertness, low toxicity and coat, and low environmentalimpact. Recent reports have demonstrated that NzO can increase the extraction efficiency of high molecular weight PAHs and chlorinated dibenzo-p-dioxins from fly ash and sediment.4.5.9J6 However, it has been shown that Nz0 does not always improve extraction efficiencies10and can be explosive in the presence of reactive organics.17 Other polar fluids such as CHClFz have been shown to increase the extraction efficiencies of environmental pollutants1OJ8 but are undesirable because of their environmental impact. Recently, it was suggested that the desorption of analytes from environmental matrices requires overcoming the energy barrier of desorption? which could be accomplished by using selective fluids or elevated pressures. Increasing the extraction temperature could also (6)Hawthorne, S.B.; Miller, D. J.; Langenfeld, J. J. Proceedings of the International Symposium on Supercritcal Fluid Chromatography and Extraction; Park City, UT, January 1991;1991;p 91. (7) Wright, B. W.; Wright, C. W.; Gale, R. W.; Smith, R. D. Anal. Chem. 1987,59,38. (8) Wheeler, J. R.; McNally, M. E. J. Chromatogr. Sci. 1989,27,534. (9)Onuska, F.I.; Terry, K. A. J.High Resolut. Chromatogr. 1989,12, 357. (10)Hawthorne, S.B.; Langenfeld, J. J.; Miller, D. J.; Burford, M. D. Anal. Chem. 1992,64,1614. (11)Dooley, K. M.; Ghonasgi, D.; Knopf, F. C. Enuiron. Progr. 1990, 9, 197. (12)Onuska, F.I.; Terry, K. A. J.HighResolut. Chromatogr. 1989,12, 527. (13)Paschke, T.; Hawthorne, S. B.; Miller, D. J.; Wenclawiak, B. J. Chromatogr. 1992,609, 333. (14)Lopez-Avila, V.;Benedicto, J.; Dodhiwala, N. S.; Young, R. J. Chromatogr. Sci. 1992,30,335. (15)Eckert-Tilotta, S.E.; Hawthorne, S. B.; Miller,D. J. Fuel, in review. (16)Hawthorne, S. B.; Miller, D. J.; Langenfeld, J. J. J.Chromatogr. Sci. 1990,28,2. (17)Sievers, R. E.; Hansen, B. Chem. Eng. News 1991,69,No. 29,2. (18)Li,S. F.Y.;Ong,C.P.;Lee,M.L.;Lee,H.K.J.Chromatogr. 1990, 515, 515.
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be effective, but this has not yet been demonstrated experimentally for analytical SFE, although changing the temperature of an extraction has been used to enhance extraction efficiencies and class-fractionation capabilities for processing applications, as well as to enhance chromatographic separation using supercritical fluid chromatography.1*22 In most reports to date, SFE has been performed using mild temperatures (e.g., 50 "C) and high pressures (e.g., 400 atm) in order to obtain the maximum fluid density, while the effect of temperature and pressure on SFE efficiencies from environmental samples has received little attention. In the present study the effects of temperature and pressure on SFE efficiencies were determined for three contaminated environmental samples including PAHs from urban air particulate matter, PCBs from river sediment, and PAHs from highly contaminated soil. Extractions were performed at two different temperatures (50 and 200 "C) and three different pressures (650,350, and 150atm),and the results are compared to the certified concentrations obtained using conventional liquid solvent extractions. Possible mechanisms involved in SFE will also be discussed.
EXPERIMENTAL SECTION Samples. The three samples used in this study were chosen from available certified reference materials to represent major environmental matrices and included PCBs from river sediment (NIST SRM 1939, National Institute of Standards and Technology, Gaithersburg, MD), PAHs from urban air particulate matter (NISTSRM 1649),and PAHs from a highly contaminated soil ("US.EPA Certified" PAH Contaminated Soil, Lot No. AQ103, Fisher Scientific,Fair Lawn, NJ). All samples contained native (notspiked) pollutants ranging from low pg/g of individual PCB congeners and PAHs (SRMs 1939 and 1649, respectively) to high pg/g concentrations (Fisher Scientificsoil). On the basis of thermal gravimetric analysis, the water contents of the three samples (for the river sediment, urban air particulates, and soil, respectively)were ca. 3% , 4 % ,and 5 % and the organic contents were ca. lo%, 38%, and 8%. Supercritical Fluid Extractions. All supercritical fluid extractions were performed by placing 0.5 g of the contaminated soil,0.4 g of the river sediment, or 0.3 g of the urban air particulate matter into a 0.5-mL extraction cell (Keystone Scientific; Bellefonte,PA) and extracting for 40 min using SFC grade carbon dioxide (Scott Specialty Gases; Plumsteadville, PA) which was pressurized (150,350,or 650 atm) by an ISCO Model lOOD syringe pump (ISCO, Lincoln, NE). The extraction temperature was controlled at 50 or 200 "C (*l "C) by placing both the extraction cell and a 5-m-longpreheating coil made from l/l6-in.-o.d. (0.020in.4.d.) stainlesssteeltubing insidea Hewlett-Packard 5890 Series I1 gas chromatograph. Since changing the extraction pressure and temperature altered the supercritical fluid flow rate through the extraction cell,the flow (measured as liquid COz at the pump) was maintained at 0.7-0.9 mL/min by using fused-silica tubing with inner diameters ranging from 26 to 32 pm (15 cm long) as outlet restrictors (Polymicro Technologies; Phoenix, AZ). Extracted analytes were collected outside of the oven at room temperature by placing the outlet end of the restrictor into a 7.4-mL vial (48mm long X 14mm i.d.) containing 5 mL of Fisher Optima Grade acetone (for the river sediment) or methylene chloride (for the urban air particulates and highly contaminated soil). Prior to gas chromatographicanalysis(but after extraction), an appropriate internal standard (125 ng of 1,2,3,5-tetrachlorobenzene for PCBs from river sediment and 101.2 or 1.0 pg of chrysene-dll for the PAHs from soil and urban air particulates, respectively)was added to each extract. To determine whether (19) Stahl, E.; Gerard, D. Perfum. Flauor. 1985, 10, 29. (20) Hutz, A.; Schmitz, F. P.; Leyendecker,D.; Klesper, E. J . Supercrit. Fluids 1990,3, 1. (21) Berger, T. A. J. Chromatogr. 1989,478, 311. (22) Frye, S. L.; Yonker, C. R.;Kalkwarf,D. R.; Smith, Chemistry and Processing in Supercritical fluids, ACS Symposium Series; American Chemical Society: Washington, DC; Vol. 30, p 7.
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these collection conditionscould quantitatively trap the extracted PCBs and PAHs, each of the PAHs and PCB congenersmeasured in this study were spiked onto sand, 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. Gas Chromatographic Analysis. Quantitation of individual PCB congeners extracted from river sediment was performed without any additional sample preparation steps except the addition of the internal standard. PCB quantitations were performed on a Hewlett-Packard 5890 gas chromatograph equipped with an electron capture detector using hydrogen as the carrier gas and nitrogen as the detector makeup gas. Autosampler injections were performed in the split mode (ca. 1:15 split ratio) into a 60-m X 0.25-mm4.d. (0.25-pm film thickness)DB-5column (J&W Scientific;Folsom,CA). The oven temperature was held at 150 "C for 40 mins, ramped at 1"C/min to 220 "C, then ramped at 3 "C/min to 330 0C.23 Calibration standards of PCB congeners in isooctane (ca. 2.08 pg/mL each congener) were obtained from Supelco (Bellefonte, PA) and diluted as appropriate. Analysesof urban air particulate extracts were performed using a Hewlett-Packard 5988 GC/MS operating in the selected ion monitoring mode by monitoring the molecular ion of each PAH. The PAH-contaminated soil extracts were analyzed using a Hewlett-Packard 5985 GC/MS in the full scan mode (50-350 amu). Autosampler injections were performed at a split ratio of 1:20 into a 25-m X 0.320-mm4.d. (0.17-pmfilm thickness) HP-5 column (Hewlett-Packard;Avondale,PA). The oven temperature was 80 "C followed by an 8 "C/min temperature ramp to 330 "C. Quantitationof individual PAH Componentspresent in the urban air particulate matter extracts was based on calibration curves generated by a PAH standard mixture prepared by NIST (SRM 1647b). A separate standard solution of PAHs, heteroatomcontaining PAHs, and pentachlorophenol was prepared gravimetrically from the individual pure components for quantitation of the analytes in the highly contaminated soil extracts.
RESULTS AND DISCUSSION Preliminary analyses of the SFE extracts from all three samples showed that no additional sample preparation (except the addition of an internal standard)was required after SFE. A GC/ECD chromatogram of an SFE extract of the PCB contaminated sediment (not included here but available in ref 23) demonstrated that PCBs were the primary constituents in the river sediment extracts (confirmed by G U M S analysis and by the injection of standard mixtures of PCBs). GC/MS analysis of an SFE extract of urban air particulates showed mainly branched and normal alkanes which are typical of diesel exhaust particulates.24 Since the PAHs of interest were only a minor portion of the extractable organics present on the urban air particulate matrix, GC with FID detection could not be used and quantitation of PAHs was based on GC/MS operating in the selected ion monitoring mode. In contrast to the urban air particulate extracts, the soil extracts contained a relatively simple mixture of PAHs, heteroatom-containing PAHs, and pentachlorophenol (Figure 1). The recoveries (versus certified concentrations) of PCBs from river sediment, PAHs from urban air particulates, and PAHs from the soil at different temperatures and pressures are shown in Tables 1-111. Triplicate 40-min extractions using COZ were performed at each condition, and the reproducibilities obtained were generally good, with relative standard deviations (RSDs)typically
