Anal. Chem. 1995, 67, 1727-1736
Kinetic Study of Supercritical Fluid Extraction of Organic Contaminants from Heterogeneous Environmental Samples with Carbon Dioxide and Elevated Temperatures John J. Langenfeld,t**Steven B. Hawthome,t David J. Miller,t and Januru Pawliszyn*s* Energy and Environmental Research Center, Universrty of North Dakota, P.O. Box 9018, Grand Forks, North Dakota 58202, and Department of Chemistty, Universily of Waterloo, Waterloo, Ontario, Canada, N2L 3G 1
Supercritical fluid extraction (SFE) rates of spiked polychlorinated dibenzo-p-dioxins(PCDDs) from Florisil, spiked [l3C1PCDDs and native PCDDs from fly ash, and spiked [2Hlpolycyclicaromatic hydrocarbons (PAHs) and native PAHs from marine sediment and railroad bed soil were examined at 40, 120, and 200 "C, while constant fluid density (d = 0.67 g/mL) and flow rate were maintained. Over 30 min (150 void volumes of C02) was required to quantitatively remove both spiked 2,3,7,8tetrachlorodibenzo-p-dioxinand 1,2,3,7,8-pentachlorodibenzo-p-dioxinfrom Florisil at 40 "C, while SFE at 200 "C significantly improved the elution rate so complete removal was achieved in 10 min. Elution rates of spiked PCDDs from Florisil were slower with a 5-mLvessel (12 cm long) than a 0.5-mLvessel (6 cm long). Increasing the temperature from 40 to 120 and 200 "C enhanced the SFE rates of spiked [l3C1PCDDs and native PCDDs from fly ash, as well as [2HlPAHsand native PAHs from marine sediment and railroad bed soil. In all cases, native analytes were extracted more slowly than spiked analytes, suggestingthat additional processes affect SFE rates of native analytes. A kinetic model was used that could help distinguish between these processes and included terms for matrix-fluid mass transport, as well as partitioning and bulk mass transport in the supercritical fluid. Using a three-rate constant desorption model to describe mass transport, good correlations (9> 0.9 in most cases) were obtained with experimental data for native analytes, and desorption rate constants suggestthat --matrix interactions are strong. The results of this study show that increasing the extraction temperature is a simple and effective method to increase SFE rates while still exploiting the advantages of supercritical Con, and can be used regardless of whether slow SFE rates are due to poor partitioning into the fluid or limited by slow desorption due to strong analyte-matrix interactions.
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Supercritical fluid extraction (SFE) is becoming an attractive alternative to liquid solvent extraction (i.e., Soxhlet or sonication) for organic compounds from environmental solids, as evident by the number of publications quoted in recent review University of North Dakota. University of Waterloo. (1) Chester, T. L.;Pinkston, J. D.; Raynie, D. E. Anal. Chem. 1992,64, 153R. +
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0003-2700/95/0367-1727$9.00/0 0 1995 American Chemical Society
Unfortunately, researchers have been frequently challenged by poor SFE efficiencies of nonpolar and moderately polar compounds such as polycyclic aromatic hydrocarbons (pAHs), polychlorinated biphenyls (PCBs), and polychlorinated dibenzo-pdioxins OCDDs) from real-world environmental samples, most notably SFE with pure COZfrom samples that are contaminated at trace levels. Since these compounds have high bulk solubilities in supercritical COZbut are difficult to extract at trace levels, it appears that SFE is limited by kinetic factors that can be adversely affected by the extraction conditions, sample matrix, analyte type and concentration,presence of co-contaminants,and contamination pr0cess.~-6 Unfortunately, identifying the ratedetermining step in the SFE process may be very difficult due to the heterogeneous nature of the samples (e.g., presence of co-contaminants on the solid; humin, humic, and fulvic material; etc.). One way to study the SFE process in detail is by the development of mathematical models and their application to experimental data. This approach has been used by chromatographers and engineers studying mass transport through porous pedia using liquids, gases, and supercritical fluids. For example, Erkey et aL7modeled adsorption of organic compounds onto soil using the Freundlich isotherm and desorption breakthrough curves with a model that consisted of a system with plug flow and negligible mass transfer resistances. King and Friedrich*used a SFE model that predicted recoveries based on analyte solubilitymolecular structure correlations. In these and other cases, modeling was performed by spiking target analytes onto the sample matrix or introducing them to the sample as a long plug, such as the frontal elution chromatographic method. Unfortunately, real-world environmental solids often contain pollutants that have undergone several processes (weathering, aging, chemisorption,trapping in interstitial pores, etc.), making them more resistant to extraction. Although studying SFE solely with frontal elution methods or spikes may describe some of the (2) Chester, T. L.; Pinkston, J. D.; Raynie, D. E. Anal. Chem. 1994,66, 106R (3) Hawthome, S. B. Anal. Chem. 1990,62, 633A (4) Langenfeld, J. J.; Hawthome, S. B.; Miller, D. J.; Pawliszyn, J. Anal. Chem. 1994,66,909. (5) Hawthome, S. B.; Langenfeld, J. J.; Miller, D. J.; Burford, M. D. Anal. Chem. 1992,64, 1614. (6) Langenfeld, J. J. Investigation of Kinetics and Mechanisms During Supercritical Fluid Extraction of Organic Contaminants from Environmental Samples. MSc. Thesis, University of Waterloo, Waterloo, ON, Canada, 1994. (7) Erkey, C.; Madras, G.; Orejuela, M.; Akgerman, A Enuiron. Sci. Technol. 1993,27, 1225. (8) King, J. W.; Friedrich, J. P. J. Chromatogr. 1990,517, 449.
Analytical Chemistry, Vol. 67, No. 10,May 15, 1995 1727
particle core organic layer Figure 1. Schematic representationof the steps in an SFE process, where process A is mass transport from the particle surface to the supercritical fluid and process B is chromatographic elution.
processes involved in SFE, Alexandrou and Pawliszyng and Burford et a1.l0 demonstrated that great discrepancies exist between the extraction rates of artificially introduced analytes (e.g., spiked analytes) and analytes that were generated or spilled onto the sample matrix (e.g., native analytes). These results suggest that in order to understand the kinetic proceses involved in analytical-scale SFE of real-world environmental samples, the extraction rates of the native analytes must be studied. The major challenge in developing models is that it is difficult to distinguish between various mechanisms based on experimental SFE results. For example, Bartle et al. developed a diffusionalbased model” to explain the SFE process by adopting the “hotball” model that describes heat transfer within a spherical particle, and more recently extended this model to include terms for partitioning at the sample interface and desorption from the sample surface.12 The model predicted an exponential extraction profile with an initial rapid extraction phase that was associated with analytes located near the surface, followed by a slow diffusionlimited phase from analytes located in the interior of the matrix, and fit the experimental data reasonably well. However, other mathematical relationshipsbased on completely different theories and mechanisms can show the same behavior, making these theories experimentally indistinguishable. Fortunately, the abundance of adjustable parameters of supercritical fluids allows one to easily change extraction conditions and observe their effect on SFE rates. For example, a change in extraction pressure, temperature, or fluid composition can be used to distinguish between mechanisms. The kinetic model used in this study13assumes that the matrix is composed of particles that may be covered by an organic layer, as shown schematically in Figure 1. This is a reasonable assumption since many environmental samples contain humic and fulvic material that is a result of the deposition of decaying vegetation over the years. The native analyte is assumed to be trapped on the core surface (e.g., physisorption or chemisorption), (9) Alexandrou, N.; Pawliszyn, J. Anal. Chem. 1989, 61, 2770. (10) Burford, M. D.; Hawthorne, S. B.; Miller, D. J.Anal.Chem. 1993,65,1497. (11) Bartle, K. D.; Clifford, A. A; Hawthorne, S. B.; Langenfeld, J. J.; Miller, D. J.; Robertson, R J. Supercrit. Fluids 1990, 3,143. (12) Bartle, K. D.; Boddington, T.;Clifford, A. A; Hawthorne, S. B. J. Supercrit. Fluids 1992, 5, 207. (13) J. Pawliszyn J. Chromatogr. Sei. 1993, 31, 31.
1728 Analytical Chemistry, Vol. 67, No. 10,May 15, 1995
and must undergo several processes before being removed from the extraction vessel. Figure 1shows that these include (A) mass transport from the matrix to the matrix-fluid interface (e.g., desorption from the surface and/or diffusion through the organic layer on the matrix) and (B) partitioning at the matrix-fluid interface and bulk mass transport in the supercritical fluid (e.g., diffusion through the static fluid present in the pore, eddy diffusion,longitudinal diffusion, etc.) . One or both of these steps can be rate-determining in SFE. Due to the comprehensive nature of this kinetic model, it may be difiicult to distinguish between the steps described by the model. However, isotopically labeled compounds (e.g., 13C-labeled PCDDs or 2H-labeled PAHs) can be spiked on top of the sample and extracted simultaneouslywith the native analytes. Although native analytes undergo processes A and B to be extracted (see Figure 1), the isotopically labelled analytes only undergo process B, since they are initially present in the fluid phase. Therefore, the steps that are important during the SFE process can be examined by comparing the extraction time profiles for the spiked and native analytes and by employing mathematical deconvolution. The deconvoluted time profiles describe only the mass transfer rate from the matrix to the fluid interface (process A in Figure l), which can be studied independently of process B. This paper describes the application of this kinetic model to explain SFE results obtained for spiked PCDDs from Florisil, spiked [13C]PCDDs and native PCDDs from fly ash, and spiked [2H]PAHsand native PAHs from railroad bed soil and marine sediment. The effects of cell volume and length on SFE rates were examined, as well as experimental parameters such as temperature because of the improved SFE recoveries observed with elevated t e m p e r a t u r e ~ . ~ ~ J ~ EXPERIMENTAL SECTION
Samples and Standards. All of the samples that were used contained native PCDDs or PAHs. PCDD-contaminated fly ash was collected by the Ontario Ministry of Environment from an electrostatic precipitator of a municipal incinerator in Toronto, ON, Canada. The sample contained branched and normal alkanes, PCBs, and polychlorinated dibenzofurans (PCDFs) , as well as nanogram per gram concentrations of PCDDs. The sample was sieved to 150 pm prior to SFE. The PAH-contaminated marine sediment [Standard Reference Material (SRM) 19411 was obtained from the National Institute of Standards and Technology (NIST, Gaithersburg, MD). As determined by thermal gravimetric analysis, the water and organic contants were 4% and 11%, respectively. The PAH-comtaminated railroad bed soil was obtained by the author near a set of railroad ties in Hastings, MN, and was sieved through a 2mm sieve to remove any debris. The sample contained 1%water and 8%organic content. A 10-g sample of Florisil (Supelco Inc.; Oakville, ON) was cleaned by Soxhlet extraction for 48 h with n-hexane and dried under the open air in a fume hood. The clean Florisil was placed in a 20-mL brown glass vial, and n-hexane was added so that it completely covered the Florisil. Individual 50 pg/mL standards of 2,3,7,&tetrachlorodibenzo-p-dioxinCr,CDD) and 1,2,3,7,&pentachlorodibenzo-p-dioxin (OsCDD) (AccuStandard; New Haven, CT) were combined and diluted to 500 ng/mL in n-hexane. A (14) Langenfeld, J. J.; Hawthome, S. B.; Miller, D. J.; Pawliszyn,J. Anal. Chem. 1993, 65, 338. (15) Hawthorne, S. B.; Miller, D. J. Anal. Chem. 1994, 66, 4005.
1-mL aliquot of this standard was spiked into the vial, and the mixture was shaken and sonicated for 1h to ensure homogeneity. The n-hexane was evaporated in a fume hood using a gentle stream of nitrogen, and the dried Florisil contained -50 ng/g of each PCDD (note that PCDDs are extremely toxic, and all work with these compounds was carefully performed in a fume hood with the operator wearing safety glasses, gloves, and a laboratory coat at all times). Prior to SFE, isotopically labelled standards were prepared so that they could be spiked onto the real-world samples at concentrations similar to the native analytes so the chromatographic elution process could be studied. Individual standards of [13C1~1tetrachlorodibenzo-$-dioxin and [13C1~1 octachlorodibenzo-Mioxin (10 pg/mL each in n-hexane) were obtained from Cambridge Isotopes (Woburn, MA), and a single spiking solution of these compounds was prepared so that the final concentrations were 186 ng/mL ([13C1~1tetrach10r~dibenzo-$-dioxin) and 14 pg/mL ([13C~~loctachlorodibenzc-~dioxin). Individual pure [2Hlolphenanthrene, [2Hl~lpyrene,[2Hl~lchrysene, and [2H~~lbenzo[blfluoranthene were also obtained from Cambridge Isotopes, and a stock solution (-0.5-7 mg/mL each component) of these compounds was prepared in methylene chloride. Supercritical Nuid &traction. SFGgrade COZ(Scott Specialty Gases, Plumsteadville, PA) was pressurized to 87-680 atm by an Isco Model l00D syringe pump (Isco; Lincoln, NE). A 0.4g sample of fly ash, a 0.6g sample of railroad bed soil, or a 0.4g sample of marine sediment was placed inside a O.5mL extraction vessel (Keystone Scientific; Bellefonte, PA). Before the cell was sealed, 10pL of the appropriate isotopically labeled standard was spiked onto silanized glass wool that was placed at the inlet end of the extraction vessel. The extraction vessel and a 5 m long coil of l/lsin.-o.d. (0.020-in.-i.d.) stainless steel tubing (used to preheat the fluid before entering the extraction vessel) were placed inside the oven of a Hewlett-Packard 5870 gas chromatgraph (GC; Avondale, PA), which was monitored by a thermometer independent of the GC oven and accurate to fO.10 "C. The extraction vessel was allowed to preheat to 40, 120, or 200 "C before the outlet valve on the syringe pump was opened to pressurize the extraction vessel with COZat 105, 373, or 650 atm, respectively (to maintain constant fluid density at 0.67 g/mL, which is simply the maximum working density that can be used based on the pressure and temperature limitations of our SFE hardware) .I6 The flow rate of COZ was maintained at 0.8 mL/min (liquid C02 measured at the pump) by 8-13-m lengths of 20-41-pm4.d. fused silica tubing (Polymicro Technologies; Phoenix, AZ). Extracted analytes were collected at timed intervals over a period of 60 min by placing the outlet end of the restrictor into a 7.4mL vial containing 5 mL of HPLC-grade n-hexane (PCDDs from fly ash) or methylene chloride (PAHs). Prior to GC analysis, 12 ng of methoxychlor [ l,l,l-trichloro-Z,%bis(4methoxyphenyl)ethane; Aldrich Chemicals; Milwaukee, WII or 12 pg of lchloroanthracene were added as internal standards to PCDD and PAH extracts, respectively. Prior to GC analysis, PCDD and PAH extracts were concentrated to 50 pL and 0.5 mL, respectively, under a gentle stream of nitrogen. No other sample preparation was necessary. PCDDcontaminated Florisil was extracted in a similar manner by placing 0.5-4.5 g of spiked Florisil inside a 0.5 or 5mL extraction vessel. SFE was performed at 650 atm and 40 or 200 (16) Langenfeld, J. J.: Hawthome, S. B.; Miller, D. J.: Tehrani, J. Anal. Chem. 1992,64,2263.
"C. Collection and concentration steps were identical to those discussed earlier. Gas Chromatographic Analysis. PCDD analyses were performed on a Varian Saturn II gas chromatograph/ion trap mass spectrometer (GC/lTMS) equipped with a septum programmable injector (SPI). Autosampler injections (2 pL) were performed into a 30 m x 0.25 mm i.d. (0.25pm film thickness) DE5 column (J&W Scientilic; Folsom, CA). Helium was used as the carrier gas at a linear velocity of 40 cm/s. The SPI was programmed to an initial temperature of 100 "C, ramped to 300 "C at 200 "C/min, and held at 300 "C for 15 min. The column temperature program was initially held at 70 "C for 1min, ramped to 150 "C at 40 "C/min, ramped to 250 "C at 8 "C/min, ramped to 320 "C at 15 "C/min, and held at 320 "C for 9 min. The transfer line and detector temperatures were maintained at 220 and 290 "C, respectively. Prior to analysis, the ion trap was optimized in a similar fashion as describedby Alexandrou et The following conditions were found to give the highest sensitivities, maximum linear dynamic range, and good spectral integrity for the low concentrations of PCDDs that must be detected: Autogain control (AGC), on; ion target, 45 OOO; flament emission current, 30 f i background mass, 150 amu; electron multiplier, 2400 V peak threshold, 5 counts; mass defect, $50 mmu/amu; and scan time, 1 s/scan. Using these optimized parameters, 5-10 pg of each PCDD congener could be detected. PCDD quantitations were based on the intensity of the m / z = M 2 and m/z = M - COCl ions, and by the injection of calibration standards of 2,3,7,&tetrachlorodibenzc~ pdioxin, 1,2,3,7,&pentachlorodibenzc~pclioxin, 1,2,3,4,7,&hexachle rodibenzcPfidioxin, 1,2,3,4,6,7,&heptachlorodibenzo-~ioxin, and octachlorodibempdioxin prepared in n-hexane at concentrations of 500, 100, 50, and 5 ng/mL. PAH analyses were performed using a Hewlett-Packard 5988 GUMS, operating in the selected ion monitoring (SIM) mode for the molecular ion of each PAH. Autosampler injections (1 pL) were performed in the splitless mode for 0.2 min into a 25 m long x 0.32 mm i.d. (0.17-pm film thickness) HP-5 capillary column. The column temperature was initially held at 80 "C for 1min and then ramped to 330 "C at 8 "C/min. The injector and detector temperatures were maintained at 300 and 250 "C, respectively. PAH quantitations were based on the injection of standard mixtures of PAHs in acetonitrile obtained from NIST (SRM 1647b).
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RESULTS AND DISCUSSION Kinetic Model. A detailed mathematical description of the kinetic model has been described previou~ly.~~ The mathematical model was separated into two components, one describing mass transport from the matrix to the matrix-fluid interface (process A in Figure l), and the other describing partitioning at the interface and bulk transport in the supercritical fluid (process B in Figure 1). Process B is similar to the steps that are important in packed column chromatography and will be referred to as "chromatographic elution". Chromatographic elution was described by selecting the mass transfer steps known to be important to chromatographers, with the sum of these steps equal to the height equivalent to a theoretical plate (H): (17) Alexandrou, N.: Miao, 2.; Colquhoun, M.; Pawliszyn, J.; Jennison, C. J. Chromatogr. Sci. 1992,30,351. Analytical Chemistry, Vol. 67, No. 70, May 75,1995
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where ~ R I (is the effect of slow desorption kinetics, h ~ is c the diffusion of the analyte in the organic material on the matrix, ~ D describes the diffusion of the analyte in the static (nodlowing) supercritical fluid in the pores of the matrix, ED is eddy diffusion, and h~ is longitudinal diffusion. The concentration of analyte in a particular region of the extraction vessel as a function of time C(x,t) can be expressed by the dispersion of a plug in a packed column.’*
where L is the length of the extraction vessel, COis the initial concentration of analyte in the extraction vessel, t is time, u = L/to is the chromatographic linear velocity where to is the unretained solute hold up time, k’ is the retention factor at the specified extraction conditions, and 0 is the mean square root dispersion of the band of solute in the extraction vessel, expresed as (5
=
&&
v/n
(5) where Ye is the fast-Fouriertransformed (FFI‘) data corresponding to the extraction rate of native analytes and H e is the corresponding rate for spiked analytes. By inverse FFT, one can transform the quotient X ( n back into the time domain. The retransformed data corresponds to the rate of mass transfer from the sample matrix to the matrix-fluid interface (process A in Figure l),allowing one to study the mass transfer rate independently of the chromatographic elution process. For example, desorption kinetics can control the rate of mass transfer to the matrix-fluid interface (process A). If first-order desorption kinetics is assumed, then the amount of analyte released from the sample matrix can be expressed as md(t):
(3)
By choosing the appropriate boundary conditions for the system, one can generate an elution time profile. For example, when the analytes are evenly distributed on the sample in the extraction vessel at the beginning of the extraction, eq 2 can be integrated
(4)
where m(t) is the mass of analyte extracted from the vessel and mo is the total mass of analyte at the beginning of the extraction. Essentially, the integrationfrom to - (L/2) sums the mass of analyte that has exited an extraction vessel of length L. To model chromatographic elution during SFE, isotopically labeled spikes must be introduced without disrupting the native analytes from their binding sites or location on the matrix. Ideally, spiked analytes would be distributed as uniformly as possible on the sample matrix, similar to the concentration of the native analytes. Unfortunately, recent reports have demonstrated that spiking analytes onto real-world environmental samples can disrupt the binding interactions of native analytes.1° Therefore, in the present study the labeled analytes were spiked onto an inert matrix (silanized glass wool) placed above the sample. Using this procedure, one maintains the integrity of the native sample matrix, but the dispersed “plug” of spiked analytes becomes a narrow “band at the top of the extraction vessel. To compensate, the L/2 terms in eq 2 must be adjusted to L/20 and the upper integration limit in eq 4 must be changed to - (19/20)L. The discussion above corresponds to the elution of spiked isotopically labeled compounds from the sample matrix during SFE (process B). The extraction time profile that is generated during SFE of native analytes is a combination of chromatographic
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(18) Crank, J. The Mathematics ofDifision; Clarendon Press: Oxford, U.K, 1989.
1730 Analytical Chemistry, Vol. 67, No. 10, May 15, 1995
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elution (process B) and mass transfer from the matrix to the fluid interface (process A). Since these processes occur simultaneously, one can deconvolute the experimental data describing the extraction rate of spiked analytes from the extraction rate of native analytes. This is easily accomplished by taking advantage of the fact that deconvolution is the same as division in the Fourier domain:I9
where mo is the initial mass of analyte, X,is the fraction of analyte adsorbed on site type i, and kdi is the desorption rate constant for site i. SFE of PCDDs from Florisil. Prior to kinetic modeling of real-world environmental samples, a homogeneous matrix with some known characteristics was extracted with supercritical C02, and the data were applied to the chromatographicelution portion of the kinetic model to validate its applicability. Florisil, a sorbent that contains Si02 (85%)and MgO (15%),was used as the sample matrix and uniformly spiked 2,3,7,8-tetrachlorodibenzo-~dioxin and 1,2,3,7,8-pentachlorodibenzepdioxin(50 ng/g each compound) as the target analytes. Furthermore, n-nonane was used as an unretained tracer compound to determine the chromatographic linear velocity. The small-diameter Florisil particles (60/ 100 mesh or particle diameter dp = 0.03 cm) were assumed to form a well-packed bed with interparticulate porosity e = 0.3 inside a 0.5mL extraction vessel (note that the extraction vessel was completely filled with Florisil). The flow rate of supercritical fluid was 0.8 mWmin, measured at the syringe pump as liquid COZat 650 atm and 25 “C. The time, to,to elute the unretained tracer compound n-nonane from the extraction vessel was found experimentally to be -0.2 min, and the extraction vessel length, L, was 6 cm, resulting in a chromatographiclinear velocity u = 30 cm/ min. Figure 2 shows a comparison of experimental chromatographic elution profiles of 2,3,7,8-tetrachlorodibenzo-~dioxin and 1,2,3,7,8pentachlorodibenzep-dioxin at 40 and 200 “C (650 atm of COZ) with chromatographic theory. At 40 “C, both 2,3,7&tetrachlorodibenzep-dioxin (Figure 2a) and 1,2,3,7,8pentachlorodibenzop-dioxin (Figure 2b) were eluted very slowly, requiring well over (19) Ramirez, R W. The FFT: Fundamentals and Concepts; Prentice-Hall, Inc.: Englewood Cliffs, NJ, 1985.
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Figure 2. Comparison of SFE elution profiles of 2,3,7,8-tetrachlorodibenzo-pdioxin (a) and 1,2,3,7,8-pentachlorodibenzo-pdioxin (b) from Florisil at 40 and 200 "C with the chromatographicelution model (eq 4).
Figure 3. Effect of extraction vessel size on experimental and theoretical chromatographic elution (eq 4) of 2,3,7,84etrachlorodibenzo-pdioxin (a) and 1,2,3,7,8-pentachlorodibenzo-pdioxin (b) from Florisil at 200 "C using a 0.5(L = 6 cm) and 5-mL vessel ( L = 12 cm).
100 void volumes of C02 to be completely removed from the extraction vessel. For 2,3,7,8tetrachlorodibenzo-pdioxin and 1,2,3,7,8pentachlorodibenzo-f1-dioxin, the best theoretical fit was when k' = 5 (H= 35 cm) and k' = 15 (H= 35 cm), respectively. Essentially, both PCDDs were strongly retained to Florisil at 40 "C as evident by the large values of k'. The large values of H signify that kinetic processes that contribute to band broadening (slow but reversible partitioning from the sample, pore diffusion, eddy diffusion, longitudinal diffusion, etc.) are prevalent, as well as tailing effects that occur during the elution of PCDDs from Florisil (noticeably starting at -10 void volumes). Essentially, the large values of H signify that our system is not necessarily optimized for elution, and steps should be taken to ensure that these effects are m i n i i e d , such as sample grinding to reduce the particle size diameter and m i n i i e the intraparticulate/ interparticulate void volume, as well as careful packing of the sample in the extraction vessel to minimize any dead volume in the system. Increasing the extraction temperature to 200 "C, however, resulted in signiiicantly reduced retention and band broadening/ tailing effects for both compounds (k' = 1, H = 20 cm for 2,3,7,8tetrachlorodibenzo-p dioxin and k' = 5, H = 20 cm for 1,2,3,7,8 pentachlorodibenzo-pdioxin), and complete elution was achieved within -50 void volumes of COz. Assuming a packed bed with interparticulate porosity e = 0.3 and C02 flow rate of 0.8 mL/ min, this corresponds to roughly 10 min. Note that 1,2,3,7,8 pentachlorodibenzo-pdioxinwas retained to the sample much more strongly than 2,3,7,8tetrachlorodibenzo-pdioxinat both 40
(k' = 15 vs k' = 5) and 200 "C (k' = 5 vs k' = 1). This may suggest that the retention mechanism is related to the number of chlorines on the PCDD and their ability to interact with the localized positive charges in the Si-0-Mg lattice of Florisil.2O In any case, the lower values of k' and Hat 200 "C imply that elevated temperatures can be used to reduce retention to the sample matrix and optimize elution conditions for PCDDs from other samples. The effects of extraction vessel volume and length on the elution time profiles of 2,3,7,8tetrachlorodibenzo-pdioxinand 1,2,3,7,8pentachlorodibenzo-pdioxin were investigated at 200 "C. Again, a well-packed bed with interparticulate porosity e = 0.3 was assumed (i.e., the vessel was completely filled with Florisil). The time to to elute the unretained tracer compound n-nonane from the 0.5 and 5mL extraction vessels was found experimentally to be approximately 0.2 and 3 min, respectively. The extraction vessel dimensions for the 0.5 and 5mL vessel were 6 cm long by 3.2 mm i.d. and 12 cm long by 7.2 mm i.d., respectively, resulting in chromatographic linear velocities u = 30 and 3 cm/min, respectively. As discussed earlier, k' = 1 for 2,3,7,&tetrachlorodibenzo-$-dioxin and k' = 5 for 1,2,3,7,8pentachlorodibenzo-~dioxin from Florisil at 200 "C. Figure 3 shows the effect of extraction vessel length and volume on elution of 2,3,7,8tetmchlorodibenzo-~oxinand 1,2,3,7,8pentachlorodibenzo-pdioxin from Florisil at 200 "C. The elution profile for 2,3,7,8tetrachlorodibenzo-pdioxin(Figure 3a) from the (20) Alexandrou, N.; Pawliszyn, J. Anal. Chem. 1992, 64, 301.
Analytical Chemistty, Vol. 67,No. 10, May 15, 1995
1731
could not be completely chromatographicallyresolved, the results for native PCDDs are reported as the sum of the concentrations 10 of the individual congeners. For example, TdCDD, PsCDD, HC CDD, H?CDD, and O&DD represent tetra-, penta-, hexa-, hepta-, u 0.0 and octachlorinated dibenzo-pdioxins, respectively. The yaxis d g! represents the fraction extracted, where 1 is equal to 100% 0.6 recovery based on the total recoveries from a 60-min SFE with 0 zz COZat 650 atm and 200 "C, followed by a 24h sonication of the 0.4 solid residue with methylene chloride. A 48h Soxhlet extraction with toluene was also performed on fly ash, and the PCDD 0.2 concentrations agreed well with the SFE/sonciation results (4 ng/g T4CDD and 350 ng/g 08CDD). Spiked [13C1~12,3,7,80.0 tetrachlorodibenzo-pdioxin was extracted very slowly at 40 "C, 0 10 20 30 40 50 60 since only 8%was eluted during the 60-min SFE. Increasing the Extractionlime (minutes) extraction temperature to 120 "C and finally 200 "C, however, 12 , resulted in significantly faster elution times for the spiked compound. Similarly, native T4CDD was extracted much more rapidly at 200 "C. The SFE time profiles for spiked [13C1~1 octachlorodibenzo-p dioxin and native O&DD are shown in Figure 4b. Again, spiked [13C~~loctachlorodibenzo-$-dioxin and native O&DD were extracted much more rapidly at 200 "C than either 40 or 120 "C. However, the d ~ e r e n c ebetween the extraction rates of spiked [13Cl~loctachlorodibenzo-pdioxin and native O&DD was more pronounced than for [13C~~ltetrach10r~dibenz~-pdioxin and T4CDD; i.e., spiked [13C1~]octachlorodibenzo-$-dioxin eluted much more rapidly than native O&DD at each extraction temperature. 0 10 20 30 40 50 60 SFE was performed under identical conditions (constant Extractiontime (minutes) density) on PAH-contaminated marine sediment and railroad bed Flgure 4. SFE time profiles for spiked [13C12]2,3,7,8-tetrachlorosoil. Similar to the PCDD results, 100%recovery was defined as dibenzo-pdioxin and native T4CDD (a) and spiked [13C~2]octathe sum of the concentrations obtained with a 60-min SFE with chlorodibenzo-pdioxin and native O&DD (b) from fly ash at 40, 120, pure COZat 650 atm and 200 "C, followed by a 24h sonication and 200 "C with constant CO2 density ( d = 0.67 g/mL). with methylene chloride. Note that SFE of marine sediment (SRM 1941) with COn at 200 "C has shown recoveries that were in good 12-cm-longextraction vessel was signiicantly slower than that for agreement with the certified concentrationsbased on two sequenthe km-long vessel, requiring roughly 110 void volumes for tial l&h Soxhlet extractions with methylene chloride. For complete elution, or 206 min using the parameters discussed example, Yang et aLZ5reported recoveries of log%,106%,and 80% above. On the other hand, only 50 void volumes (roughly 10 min) for phenanthrene, pyrene, and benzo[b+k]fluoranthene using a were required to elute 2,3,7,&tetrachlorodibenzo-pdioxinfrom the 30-min SFE with COZat 400 atm and 200 "C. km-long extraction vessel. Similar trends were found (Figure Figure 5 shows the effects of temperature on SFE rates of 3b) for 1,2,3,7,&pentachlorodibenzo-pdioxin,but the elution times spiked [2Hl~lphenanthrene and native phenanthrene (a), [ 2 H ~ ~ l were slightly longer than the 2,3,7,8tetrachlorodibenzo-$-dioxin pyrene andpyrene (b), [2H~~lchrysene and chrysene (c), and [2H~~1congener for both extraction vessel sizes due to stronger retention benzo[blfluoranthene and benzo[b+klfluoranthene (d) from mafor this congener (k'= 5 vs k' = 1 at 200 "C). These results rine sediment (note that native benzo[blfluoranthene and demonstrate that it is best to minimize the interparticulate void benzo [kl fluoranthene could not be mass or chromatographically space e (i.e., reduce the dead volume by carefully packing the resolved, so the results are reported as the sum of their vessel with sample) and the retention factor k when SFE is concentrations and denoted as benzo [b+kl fluoranthene). Spiked performed, especially when large extraction vessels are ~ s e d . 2 ~ - ~ [2Hlo]phenanthrene ~ (Figure 5a) was extracted rapidly at all three In any case, the experimental data correlated well with chromatotemperatures, although there was a short lag at the beginning of graphic theory and gave good predictions on how the spiked the extraction at 40 "C, which would be expected since spiked PCDDs were eluted from the extraction vessel. [2Hlo]phenanthrenemust travel through the entire length of the SFE of Real-World Samples. Figure 4a shows the SFE time extraction vessel before it is eluted. Nevertheless, full recoveries profiles of native T4CDD and spiked [13C1~1-2,3,7,8tetrachloro- of spiked [2Hlo]phenanthrenewere achieved within 10 min at all dibenzo-pdioxin from fly ash using supercritical COZat 40, 120, three temperatures. Native phenanthrene, on the other hand, was and 200 "C (105, 373, and 650 atm, respectively) with constant extracted rather slowly at 40 "C, but the extraction rate increased fluid density (0.67 g/mL) and flow rate (0.8 mL/min liquid COz substantially at 120 "C, and full recovery was obtained within the at 25 "C). Since individual PCDD congeners with the same mass 60-min SFE period at 200 "C. Similar trends were observed for spiked [2Hlo]pyrenefrom marine sediment Figure 4b). However, (21) Rein, J.; Cork, C. M.; Furton, K. G. J. Chromatogr. 1991,545, 149. 12
I
I
-0spiked (40OC) native (40 -€Is oikedH28CI -0-
2
(22) Wheeler, J. R; McNally, M. E. J. Chromatogr. Sci. 1989,27, 534. (23) Furton, K. G.; Jolly, E.; Rein, J. J. Chromatogr. 1993,629, 3. (24) Furton, K. G.; Lin, Q. J. Chromatogr. Sci. 1993,31, 201.
1732 Analytical Chemistry, Vol. 67,No. 10, May 15, 1995
(25) Yang, Y.; Gharaibeh, A; Hawthorne, S. B.; Miller, D. J. Anal. Chem. 1995, 67, 641.
1.2
1.2
1 .o
1 .o
c) chrysene
p 0.8 ;
0.8
C
E
0.6
3
5
e
L
f
0.4
-*-*-
0.0 0
10
20
30
40
50
0.0
1.2
1.2
1 .o
1 .o
p
0.8
-
0
10
20
30
40
50
60
0
10
20
30
40
50
60
0.8
ti
e
-*spiked (40 -0- native (40 OC)
OC)
0.6
-*spiked (120 OC) + native (120 OC)
.-0
c)
f
0.2
60
'ii
EE
0.4
-&- spiked (40 OC) -4- native (40 OC) spiked (120 OC) + native (120 "c) spiked (200 OC) + native (200 "c)
0.2
p
0.6
0
0.4
spiked 200 OC) + native 600 -A-
0.2
0.6
.-0
r
0
0.4
0.2
0.0
0.0 0
10
20
30
40
50
60
Extraction time (minutes) Extraction time (minutes) Figure 5. SFE extraction time profiles for spiked [2Hl~]phenanthrene and native phenanthrene (a),spiked [2H~~]pyrene and native pyrene (b), spiked [*H12]chryseneand native chrysene (c), and spiked [2H12]benzo[b]fluorantheneand native benzo[b+ffluoroanthrene (d) from marine sediment at 40, 120, and 200 "C with constant CO2 density ( d = 0.67 g/mL). the lag at the beginning of the extraction at 40 "C was more pronounced than for spiked [2Hl~lphenanthrene, requiring -20 min for complete elution. Native pyrene was extracted more rapidly than native phenanthrene at all three temperatures. Figure 5c shows the SFE time profiles of spiked [2Hl~lchrysene and native chrysene from marine sediment. Spiked [2H1~1chrysene and native chrysene were extracted extremely slowly at 40 "C, since only 30%of spiked 2Hlzchrysene and 5%of native chrysene was removed after the 60-min SFE. Again, increasing the temperature to 120 or 200 "C greatly enhanced the extraction rates so that >90% recovery of each compound was obtained in 10 minutes at either temperature. Similar results were also obtained for spiked [2Hl~lbenzo[blfluoranthene and native benzo[b+k]fluoroanthene from marine sediment in Figure 5d. The results for SFE of spiked [2Hl~lPAHs and native PAHs from railroad bed soil are shown in Figure 6. Similar to the results obtained for PAHs from marine sediment, increasing the temperature to 200 "C enhanced the elution rate of spiked [2Hl~lphenanthrene and native phenanthrene (Figure 6a), and the spiked [2Hlolphenanthrenegenerally was extracted more easily than native phenanthrene at all three temperatures. This was also observed for spiked [2Hlo]pyreneand native pyrene (Figure 6b), spiked [2Hl~]chrysene and native chrysene V i e 6c),and spiked
[2H12]benzo[B]fluorantheneand native benzo[b+klfluoranthene (Figure 6d) from railroad bed soil. The experimental results in Figures 4-6 for spiked [l3C1PCDDs from fly ash and [2H]PAHsfrom marine sediment and railroad bed soil follow the same trend as for PCDDs from Florisil; i.e., increasing the extraction temperature significantly improved the elution conditions (process B in Figure 1) by reducing the value of k' and H. Band-broadening and tailing effects (i.e., the last fraction that is eluted very slowly) were also observed for SFE of spiked analytes from real-world samples (most notably at 40 "C) and probably arose as significant contributions to H from pore diffusion ( h ~ in p eq 1), organic layer diffusion ( h ~ c )and , slow but reversible desorption ( h d since these samples are likely rather porous, contain signilicant amounts of organic carbon, and possess numerous active sites due to their heterogeneous nature. There were also some other interesting trends. For example, spiked [2H]PAHsfrom marine sediment generally were eluted more easily than spiked [2H]PAHsfrom railroad bed soil, suggesting that spiked PAHs partition more effectively into the railroad bed soil than the marine sediment. High molecular weight PAHs (e.g., [2Hlz]benzo[b]fluoranthene)also tended to elute more slowly than low molecular weight PAHs (e.g., [2Hlo]Analytical Chemistry, Vol. 67, No. 10, May 15, 1995
1733
I.2
1.2
-8-
spiked ( ~ P c )
-e-
spiked (120F)
-B-
-e nalive (40%) -A1.o
1.o
natiwe ( 1 2 0 ~ ~ ) spiked (200%) native (2W"C)
0.8
0.8 tj
tj
e
e
E 0.6
c)
I
C
0
0.6
'c;
E
i
I; 0.4
(a) phenanthrene
0.2
0.4
0.2
0.0
0.0 10
0
20
30
40
50
0
60 1.2
-8-8-
spiked (40%) native (40°C) spiked (12O'C)
-8-A-
nalive (120°C) spiked (200"~) native (200eC)
___----
10
20
30
40
50
60 I
I
1.o
3 E
0.8
E
r
'
0.6
0
LL
02
I
-ll/
0.4
(d) benzo[b+k]fluoranthene
0.2
__----e--------
0.0 0
10
20
30
40
50
60
Extraction time (minutes)
0
10
20
30
40
50
60
Extraction time (minutes)
Figure 6. SFE extraction time profiles for spiked [2H~~]phenanthrene and native phenanthrene (a),spiked [2Hl~]pyrene and native pyrene (b), spiked [2H~2]chrysene and native chrysene (c),and spiked [2H12]benzo[b]fluorantheneand native benzo[btk]fluoroanthrene (d) from railroad bed soil at 40, 120, and 200 "C with constant COz density ( d = 0.67 glmL).
phenanthrene) from both PAH-contaminated samples, especially at 40 "C. In chromatography, the retention factor k' is one of the major factors that determines the solute retention time and can be expressed as the product of the matrix-fluid distribution constant K and the ratio of the stationary phase and mobile phase volumes, and similar arguments should be valid for SFE except that very small values of k' would be desirable to obtain rapid elution. Lowering the retention factor k' so it approaches zero can be performed by reducing the stationary phase volume (or increasing the mobile phase volume). However, altering these parameters can hinder the elution process by creating large void spaces in the extraction v e ~ s e 1 . 2Another ~ ~ ~ ~ approach to lower k' is to decrease K by increasing the solubility of the analyte in the supercritical fluid, since solubility will determine the magnitude of the distribution constant Kbetween the two phases. From the experimental data for SFE or spiked PCDDs from Florisil (Figure 2), spiked [l3C1PCDDsfrom fly ash (Figure 4), and spiked PHIPAHs from marine sediment (Figure 5) and railroad bed soil (26) Iangenfeld, J. J.; Burford, M. D.; Hawthorne, S. B.; Miller, D. J.J. Chromatogr. 1992,594, 297. (27) Hawthorne, S. B.; Miller, D. J.; Burford, M.D.; Langenfeld, J. J.; EckertTilotta, S.; Louie, P. K.J. Chromatogr. 1993,642, 301.
1734 Analytical Chemistry, Vol. 67,No. 10, May 15, 1995
(Figure 6), it appears that elevated extraction temperatures can increase the analyte solubility in the supercritical fluid. The lack of experimentally determined solubility data at the elevated temperatures used in this study makes it difticult to confirm this for PCDDs, but PAH solubilities have been measured in supercritical CO:, and shown to dramatically increase at elevated temperatures (even at constant fluid density)8: even though the solvent strength of a supercritical fluid is presumed to be primarily dependent on its density. Coupled with analyte solubility, the distribution constant K can also be affected by the af6nity of a particular analyte toward sample matrices with different sorption characteristics,clearly evident by the faster elution rates of spiked [:,H]PAHsfrom marine sediment compared to railroad bed soil, especially at 40 and 120 "C.The molecular weights of PCDDs or PAHs on a particular sample can also affect elution conditions, even though they are considered relatively nonpolar and not expected to undergo strong retention to most sample matrices. However, their aromatic nature renders them polarizable and capable of fairly strong interactions with heterogeneous environmental matrices during the elution process. Furthermore, high molecular weight PAHs are less soluble in supercritical COSthan (28) Miller, D. J.; Hawthorne, S. B. Anal. Chem. 1995,67, 273
Table 1. Solutions to a Three-Ratemconstant Curve Fit for Deconvoluted SFE Rate Data of PCDDs from Fly Ash
’ 2 1 A
E
/I 4ooc
0.6
e
U.
04
- theory
:
00 0 0
2
10
20
30
40
50
I
I
8e
k (
(b) OaCDD
irI
0
40aC
A
1209: 200%
- theory
4
I
I
I
I
1
0
10
20
30
40
50
0.0
40 120 200
0.18 0.94 0.99
40 120 200
0.17 0.85 0.81
Xz
X3
0.05
0.82 0.01 0.01
0.17
0.83 0.15 0.02
kdi
kd2
kd3
(min-1)
(min-1)
(min-1)
T4CDD 0.18 0.94 0.96 O&DD 0.17 0.75 0.81
0.05
0.06
72
1.7 x 0.96 3.4 x W4 0.45 6.1 x 0.97 0.78 0.55 0.96
5.1 x 1.7 x 3.2 x
Table 2. Solutions to a Three-Ratemconstant Curve Fit for Deconvoluted SFE Rate Data of PAHs from Railroad Bed Sol1
I
\[
XI
60
Extraction time (minutes)
0.6
temp (“C)
tI 60
Extractiontime (minutes)
Flgure 7. Deconvoluted and theoretical desorption time profiles (eq 6) for T&DD (a) and O&DD (b) from fly ash at 40, 120, and 200 “C with constant COn density ( d = 0.67 g/mL).
low molecular weight PAHs, so they can be more strongly retained. In any case, elevated temperatures were effective at increasing the elution rate of spiked PCDDs and PAHs from all of the samples studied. Matrix-Fluid Mass Transport. The data presented have shown that spiked [l3C1PCDDs and [2HlPAHs generally were eluted much more rapidly than native PCDDs and PAHs from all three real-world samples, indicating that processes other than chromatographic elution affect the extraction rates of native PCDDs and PAHs. Using the deconvolution theorem in eq 5, mass transport rates from the matrix surface to the matrix-supercritical fluid interface (process A in Figure 1) can be calculated and studied independently from the chromatographic elution process. Figure 7 shows an example of deconvoluted experimental data at 40,120, and 200 “C (symbols) for T4CDD and OsCDD from fly ash. Since the rate of analyte release (process A in Figure 1) from environmental samples can dominate the overall SFE rates, the deconvoluted data points were fitted to a desorption model (eq 6) assuming three rate constants (lines), since the deconvoluted data generally showed at least three distinct regions, an initial rapid desorption phase followed by an intermediate phase, and fmally a slow tailing phase. Tables 1-3 show the tabulated desorption rate constants ( k d i ) , mole fractions pi),and nonlinear regression correlation coefficients (r?) from the deconvoluted data fit to eq 6 for all three samples. In general, the threerateconstant curve correlated well with the deconvoluted data points, and the majority of the correlation coefficients (9)were greater than 0.9.
temp (“C)
Xl
40 120 200
0.15 0.61 0.99
40 120 200
0.55 0.83 0.99
0.06
40 120 200
0.09 0.23 0.71
0.40 0.26
40 120 200
0.05 0.11 0.73
0.02 0.04 0.26
kd3
xz
(min-l) Phenanthrene 0.14 0.55 0.87 Pyrene 0.45 0.47 0.11 0.78 0.04 0.01 0.99 Chrysene 0.91 0.11 0.37 0.21 0.09 0.03 0.47 0.03 Benzo[b+klfluoranthene 0.93 0.03 0.02 0.85 0.07 0.02 0.01 0.15 0.02 0.85 0.39 0.01
2.1 3.6 1.7
10-3 10-3 10-4
0.46 0.46 1.00
7.6 10-3 1.5 x 1.2 x 10-2
0.96 0.91 0.99
1.5 10-3 7.3 10-3 6.8 x
0.57 0.99 0.99
1.7 x 2.8 x lo-’ 2.5 10-4
0.85 0.97 0.96
Table 3. Solutlons to a Three-Rateeonstant Curve Fit for Deconvoluted SFE Rate Data of PAHs from Marine Sediment
temp (“C)
X2 0.11
40 120 200
0.31 0.39 0.64
40 120 200
0.82 0.83 0.95
0.13
40 120 200
0.15 0.71 0.91
0.27 0.08
40 120 200
0.36 0.40 0.72
0.03 0.09 0.25
0.20
X3
kdi
(min-1)
kd2
(min-1)
Phenanthrene 0.28 0.01 0.22 0.64 0.01 Pyrene 0.18 0.76 0.04 0.79 0.02 0.05 0.95 Chrysene 0.85 0.14 0.02 0.26 0.02 0.01 0.84 0.03 Benzo [b+klfluoroanthene 0.61 0.18 0.01 0.51 0.23 0.03 0.26 0.04 0.03 0.58 0.61 0.16
kd3
(min-I)
12
7.5 6.7 2.1
10-4 10-3 10-3
0.92 0.98 0.99
1.4 5.3 1.7
10-3 10-4 10-4
0.98 1.00 0.99
6.8 x 5.1 10-4 5.1 10-4
0.74 0.99 0.99
2.6 4.3 6.2
0.45 0.97 0.43
10-4 10-4 10-4
However, poor correlation coefficients were observed at some of the extraction conditions and were a result of the nature of the deconvolution process. For example, the complex division of the Y f l rate data by the H f l data in eq 5 may result in a divide-byzero situation at some experimental points, and steps should be taken to avoid this. Furthermore, the deconvolution procedure is extremely sensitive to “noise”. In our situation, noise refers to Analytical Chemistty, Vol. 67, No. 10,May 15, 1995
1735
scattered extraction rate data, such as SFE of native phenanthrene from railroad bed soil at 40 "C (see Figure 6a). When noise components are present in the original SFE data, they can be amplified in the deconvoluted data causing erratic data points. Table 1shows the solutions to the parametersXi (mole fraction on site z? and kdi (desorption rate constant on site i) from the desorption model curve fit for T4CDD and O&DD from fly ash at 40, 120, and 200 "C. Both T&DD and O&DD had rate constants kdl that were -3 orders of magnitude larger than kd3 regardless of the temperature. Also, kdl increased significantly as the temperature was increased from 40 to 200 "C. The effect of temperature on rate constant kd3 was more difkult to accurately evaluate because of the scatter of the deconvoluted data. For example, at 200 "C there were unmeasurable amounts of T4CDD Yr, = 0.01) remaining, and these fractions are especially prone to measurement errors. Interestingly, the mole fraction XI increased with temperature, while the mole fraction X3 decreased with elevated temperatures in nearly every case. The solutions to the desorption model parameters for PAHs from railraod bed soil and marine sediment at 40, 120, and 200 "C are shown in Tables 2 and 3, respectively. The results appear similar to those in Table 1 for PCDDs from fly ash; i.e., kdl was generally 2-3 orders of magnitude faster than kd3 regardless of the temperature, and the accuracy of kd3 for these samples was subject to the same limitations as for PCDDs from fly ash. In both PAH-contaminated samples, the mole fraction XI increased with elevated temperatures at the expense of a decrease in X3. Although the rate constant kdl was similar for both PAHcontaminated samples, the rate constant kd3 was nearly 1 order of magnitude larger for PAHs from railroad bed soil. This is an interesting result and suggests that PAH desorption (most notably from the resistant fraction X3) from marine sediment is slower due to the higher desorption energies associated with this matrix. Although good experimental fits with the three-rate-constant desorption model were obtained, in reality, heterogeneous environmental samples may have hundreds of different sites with different desorption energies. Furthermore, apparent rate constants that were dependent on SFE conditions were determined, as opposed to theoretical sitespecific rate constants where the mole fraction distribution between sites should be constant regardless of the SFE temperature. For example, SFE at 40 "C only supplied enough thermal energy to desorb the weakly bound analytes, while SFE at 200 "C supplied thermal energy to desorb both weakly and strongly sorbed species. As a result, the apparent (29) Miao, 2.; Zhang, Z.; Pawliszyn, J. J. Microcolumn Sep. 1994, 6, 459.
1736 Analytical Chemistry, Vol. 67,No. 10, May 15, 7995
rate constants at 200 "C deviate from the true site-specific rate constants because the mole fraction distribution shifted so that XI,Xz, and X3 appeared to be extracted simultaneously under the "fast" apparent rate constant kdl. Nevertheless, increasing the extraction temperature has been shown to be effective at increasing the elution rate of species from solid samples (process B), as well as increasing the rate of analyte release from the surface (process A), and the modeling approach was useful for studying analyte-matrix interactions in SFE CONCLUSIONS
The kinetic model proved to be a useful approach for studying the extraction processes in SFE. The chromatographic elution portion of the model accurately fit the data for the elution of PCDDs from Florisil using different extraction vessel lengths and volumes. Increasing the extraction temperature significantly improved elution conditions for spiked PCDDs from Florisil, as well as spiked [WIPCDDs from fly ash and [2HlPAHs from railroad bed soil and marine sediment. Deconvolution of the experimental data describing the extraction rate of spiked analytes from native analytes allowed an investigation of analyte release from the sample matrix to the fluid interface. Increasing the temperature resulted in faster desorption for all of the analytematrix combinations that were studied, and a three-rate-constant desorption model generally fit the experimental data well. From these data, it appears that both chromatographic elution (partitioning and bulk transport in the supercritical fluid) and desorption (matrix-fluid mass transport) have significant impacts on SFE rates of native analytes under the SFE conditions used in this paper, and increasing the temperature may be a useful approach to improve SFE efficiencies regardless of the ratelimiting step. Dioxins could be fractionated from interferences by varying temperature conditions in the extraction vessel.29 ACKNOWLEDGMENT
The financial support of the U.S. Environmental Protection Agency, EMSLLV (Las Vegas) ,the American Petroleum Institute, the US. Department of Energy, the Natural Sciences and Engineering Research Council of Canada, and the NATO grant for international collaboration program, as well as intrument loans from Isco, is gratefully acknowledged. The fly ash sample was donated by Nick Alexandrou and Dave Potter. Received for review August 26, 1994. Accepted February 22, 1995.B AC940850V Abstract published in Advance ACS Abstracts, April 1, 1995.
