Anal. Chem. 1987, 59, 1705-1708
120-12-7; fluoranthene, 206-44-0; pyrene, 129-00-0; benz[a]anthracene, 56-55-3;chrysene, 218-01-9; benzo[a]pyrene, 50-32-8; dibenz[a,h]anthracene, 50-70-3; benzo[ghi]perylene, 191-24-2.
LITERATURE CITED Otson, R.; Armstrong, V.; Leach, J. M. Am. Ind. Hyg. Assoc. J . , in press. Manual of Analytical Methods, 2nd ed.; Publication No. 77-157A: DHEW (NIOSH): Cincinnati, OH, 1977; Vol. 1. Stenberg, U.; Alsberg, T.; Westerholm. R. EHP, Environ. Health Perspect. 1983, 4 7 , 43-51. Stenberg, U.; Alsberg, T.; Westerholm, R. EHP, Environ. Health Perspect. 1983, 4 7 , 53-63. Scheutzle, D. EHP, Environ. Health Perspect. 1983, 4 7 , 65-60. Cautreels. W.; Van Cauwenberghe, K. Atoms. Environ. 1978, 12, 1133-1 141. Yamasaki, H.; Kuwata. K.; Miyamoto, H. Environ. Sci. Technol. 1982, 18, 189-194. Thrane. K. E.; Mikalsen, A. Atoms. Environ. 1981, 15, 909-916. Andersson, K.; Levin, J.-0.; Nilsson, C . 4 . Chemosphere 1983, 12. 197-207. Otson, R.; Leach, J. M.; Chung, L. T. K. Anal. Chem. 1987, 5 9 , 58-62.
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(11) TLVs Threshold Limit Values for Chemical Substances and Physical Agents in the Work Environment and Biological Exposure Indeces with Intended Changes for 1984-85; American Conference of Governmental Industrial Hygienists: Cincinnati, OH, 1984. Lee, F. S.-C.; Prater, T. J.; Ferris, F. I n Polynuclear Aromatic Hydrocarbons; Jones, P. W., Leber, P., Eds.; Ann Arbor Science Publishers: Ann Arbor, MI, 1979; pp 83-110. Jones, P. W. VDI-Ber. 1980, No. 358, 23-38. Otson, R.; Hung, I.-F. Polynuclear Aromatic Hydrocarbons: Mecha nism, Methods and Metabolism; Cooke, M., Dennis, A. J., Eds.; Battelle Press: Columbus, OH, 1984; pp 999-1012. You, F.; Bidleman, T. F. Environ. Scl. Technol. 1984, 18, 330-333. Bidleman, T. F.; Billings, W. N.; Foreman, W. T. Environ. S d . Technol. 1986, 20, 1038-1043. Pupp, C.; Lao, R. C.; Murray, J. J.; Pottie, R. F. Amos. Environ. 1974, 8 , 915-925. Sonnefeld, W. J.; Zoller, W. H.; May, W. E. Anal. Chem. 1983, 5 5 , 275-280. Spitzer, T.; Dannecker, W. Anal. Chem. 1983, 5 5 , 2226-2228.
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RECEIVED for review November 5, 1986. Accepted March 4, 1987* This work was supported by the Canadian Intel&partmental Panel on Energy Research and Development.
Extraction and Recovery of Polycyclic Aromatic Hydrocarbons from Environmental Solids Using Supercritical Fluids Steven B. Hawthorne* and David J. Miller University of North Dakota Energy Research Center, Grand Forks, North Dakota 58202
The use of supercritical fluids for the extractlon and recovery of polycycllc aromatic hydrocarbons (PAH) from environmental sollds has been developed and tested by uslng urban dust, fly ash, and river sedlment. Supercrltlcal N,O with 5 % methanol modlfler gave the best recoveries of PAH from all three samples when compared to supercrklcal CO, wkh 5 % methanol, N,O, CO,, and ethane. Quantltailve recovery of PAH from National Bureau of Standards SRM 1649 (urban dust) and of deuterlated PAH spikes (phenanthrene-d,,, pyrene-d,,, and perylene-d,,) from the river sedlment was obtalned wlth supercritical fluid extractlons In as little as 30 mln. In most cases, 30-60 mln extractions of the river sedlment and fly ash with supercritical N,0/5 % methanol gave better recovery of the deuteriated PAH spikes than the recoveries obtalned by uslng 4 h of sonication or 8 h of Soxhlet extractlon wlth either benzene or methylene chloride. Supercritical fluid extractions yield good PAH recoverles, requlre only small amounts of sample, minimize analyte concentration steps, and are simple and rapid to perform.
The extraction and recovery of organic species from environmental solids are critical and often the limiting steps in analysis schemes used to identify and quantitate organic pollutants. Extractions of polycyclic aromatic hydrocarbons (PAH) from solid samples such as airborne particulates, soil, and fly ash are usually performed with liquid solvents either in a Soxhlet apparatus or with sonication, although variations such as extraction with liquid COz have been described ( I ) . These traditional liquid solvent extraction methods generally take several hours to perform, require relatively large volumes of ultrapure solvent (often making subsequent concentration steps necessary), and may yield incomplete recovery and/or degradation of sample species (2-5).
Besides their strong solvating ability, supercritical fluids have several characteristics that may make them useful for the rapid and quantitative extraction and recovery of organic pollutants from environmental solids. The solvent strength of a supercritical fluid is directly related to its density as can be described by the empirical correlation 6 = 1.25Pc1/2(p,/pl) where 6 is the Hildebrand solubility parameter, P, is the critical pressure of the fluid, pg is the density of supercritical fluid, and p1 is the density of the fluid in its liquid state (6). Thus, the solvating ability of a particular supercritical fluid toward a particular species can easily be modified by changing the extraction pressure (and, to a lesser extent, the temperature). Supercritical fluids with widely varying polarities are available (i.e., pentane, COz, and ammonia have Hildebrand solubility parameters a t liquid densities of 7.0, 10.6, and 13.2 ( c a l / ~ m ~ ) 'respectively), /~, and the polarity of a supercritical fluid can also be controlled by the addition of solvent modifiers. The low viscosities and high solute diffusivities of supercritical fluids should yield good mass transfer during extraction. Supercritical fluids that have low critical temperatures (e.g., COP,nitrous oxide, and ethane have critical temperatures of 31,36.5, and 32 "C, respectively) can be used to avoid decomposition of thermally labile compounds. Finally, many supercritical solvents are gases at room temperature which should simplify analyte concentration steps and help reduce the loss of volatile sample species. Recent studies have demonstrated analytical uses for supercritical fluid extractions including the extraction of diesel fuel components from silica gel and marine sediments (7),the coupled supercritical fluid extraction/supercritical fluid analysis of caffeine from coffee beans (8),the silica column fractionation of oil residuals (9) and of polycyclic aromatic hydrocarbons (PAH) (IO), and the direct mass spectrometric analysis of supercritical fluid extraction products (7, 2 1 ) . The ability of supercritical COz extractions to yield quantitative extraction and recovery of PAH from diesel exhaust partic-
0003-2700/87/0359-1705$01.50/0 0 1987 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 13, JULY 1, 1987 A
B
c
D
E
F
Figure 1. Supercritical fluid extraction cell. The components of the cell are (A) 10 cm X 20-30 pm i.d. fused silica capillary outlet rein. nut, (C) 15/85 graphtte/polyimide (Supeltex M-2A) strictor, (B) ferrule, (D) in. NPT X ’Ile in. tubing union, (E) I/, in. X 10 pm stainless steel frit, and (F) ’116 in. low dead volume union. All fittings were stainless steel “Parker” brand.
ulates and from urban dust has also recently been reported (12-14). The purpose of the present study is to investigate and develop the use of supercritical fluid extractions to extract and recover PAH from three different environmental solids including fly ash, river sediment, and urban dust. Five different supercritical solvent systems including ethane, C02, nitrous oxide (N,O), COBwith methanol modifier, and nitrous oxide with methanol modifier were tested for their ability to extract PAH from each of the three samples. A comparison between supercritical fluid extraction and the more traditional liquid solvent extraction methods, sonication and extraction using a Soxhlet apparatus, is also reported.
EXPERIMENTAL SECTION Sample Description. The urban dust sample (SRM 1649) was purchased from the National Bureau of Standards (Washington, DC). The fly ash sample was an ESP hopper ash (no. 3401.9) supplied by the DOE Fossil Fuels Research Materials Facility (Oak Ridge National Laboratory, Oak Ridge, TN). River sediment was collected near the banks of the Red River in Grand Forks, ND. The sediment was allowed to air-dry and sieved through a screen having 0.6-mm holes to remove debris before use. Both the fly ash and river sediment were spiked with 2 wg/g each of phenanthrene-dlo, pyrene-dlo, and perylene-d12by suspending 2-g samples in 10 mL of methylene chloride and adding the appropriate amount of the deuteriated spikes. The samples were stirred occasionally and the solvent was allowed to evaporate overnight. Methods. All of the supercritical fluid extractions were performed with an SFT Model 250-TMP supercritical fluid pumping system (Lee Scientific, Inc.). Five percent by volume methanol modified COBand N20 were prepared by pipetting 12.5 mL of methanol (Baker “Resi-Analyzed”) directly into the 250-mL syringe pump (held a t 15 “C) and then filling the entire pump volume with liquid COz (or NzO). The extraction cell was constructed with “Parker” brand 316 stainless steel fittings as shown in Figure 1 and had an internal volume of approximately 0.2 mL. (Caution: Care must be taken to ensure that fittings used to construct extraction cells are capable of withstanding the required extraction pressures. Since N20 is a strong oxidant, extractions involving large amounts of easily oxidized material, particularly at elevated temperatures, may represent an explosion hazard and should be avoided. As an added precaution for this study, a pressure relief valve set to vent a t 400 atm was installed a t the top of the pump so that both the pump and extraction cells could be rapidly vented.) Supercritical pressures were maintained inside the extraction cell by using 10-cm lengths of capillary fused silica tubing (Polymicro Technologies, Phoenix, AZ) for outlet restrictors. Flow rate through the extraction cell was controlled by using tubing with internal diameters ranging from 20 to 30 wm. No cross-contamination between samples has been observed when the same restrictor is used for muItiple extractions. However, since restrictors grow fragile after several uses, a new restrictor was used for each extraction in this study. (This poses no limitation since the capillary tubing is very inexpensive and a new restrictor is made by simply cutting off a new piece of the tubing.) Temperature was maintained by inserting the extraction cell into a thermostated tube heater. Extractions were performed on 20-mg samples of the urban dust that had been weighed into the extraction cell. Sample size was approximately 50 mg for the fly ash and the river sediment. The extracted species were collected by inserting the end of the outlet restrictor into a 19 X 65 mm glass vial containing approximately 2 mL of methylene chloride (Baker “Resi-Analyzed”)
and 0.5 pg of 4,4’-dichlorobiphenyl as an internal standard. Because of the cooling effect caused by the rapid expansion of the supercritical fluid as it exited the restrictor capillary, no significant evaporation of the methylene chloride occurred during the extraction. After sample collection the solvent was evaporated under nitrogen to approximately 100 pL. Analysis of the extracts was performed with a Hewlett-Packard Model 5985B GC/MS using selected ion monitoring (SIM). Separations were achieved with on-column injections into a 60 m X 0.25 mm i.d. (0.25 Fm film thickness) DB-5 chromatographic column (J&W Scientific, Inc.). Standards for the quantitation of the PAH extracted from the urban dust were prepared from National Bureau of Standards SRM 1647, which contains certified concentrations of various PAH species in acetonitrile. Quantitation of the deuteriated PAH extracted from the fly ash and river sediment was based on gravimetrically prepared solutions of the pure deuteriated compounds. The concentration of each PAH and deuteriated PAH fell within a three-point calibration curve. Each standard and sample vial contained 0.5 pg of 4,4‘-dichlorobiphenyl as an internal standard. During SIM analysis the molecular ion of each PAH, the deuteriated PAH species, and the 4,4’-dichlorobiphenylwas monitored in groups of two ions with a dwell time of 50 ms for each ion. Soxhlet extractions of 1-g samples of fly ash and river sediment were performed with a micro-Soxhlet apparatus with 50 mL of either benzene or methylene chloride. Solvent cycle time was approximately 15 min. One-half-gram samples were also extracted with 10 mL of both solvents using sonication at 100 W. After extraction, the 4,4’-dichlorobiphenyl internal standard was added to each extract, the solvents were evaporated to near dryness, and the extracts were analyzed as described above.
RESULTS AND DISCUSSION A comparison of the extraction efficiencies obtained by using the five different supercritical fluids (ethane, CO,, N20, CO, with 5% methanol by volume, and N,O with 5% methanol) is shown in Figure 2. The extraction conditions used were purposely chosen to yield less than quantitative recovery of the PAH so that comparisons between the different solvent systems could be made. Each sample was extracted in triplicate for 30 min with each supercritical solvent at 300 atm. A 20 gm i.d. restrictor outlet was used to maintain supercritical pressures in the extraction cell resulting in approximately 10 mL of solvent (measured in the liquid state a t the pump) being used for each extraction. Extraction temperatures were 45 “ C for the pure solvents and 65 “C for the solvents modified with methanol. Recoveries of the PAH from the urban dust were based on the certified values for each individual PAH. Recoveries of the deuteriated PAH from the fly ash and river sediment were based on 2 Hg/g spikes as described above. The resultant extraction efficiencies showed good reproducibility as indicated on Figure 2 by the error bars representing one standard deviation unit (lr)for triplicate extractions of each sample with each solvent. Supercritical ethane and supercritical CO, gave the poorest recovery of PAH from each of the three samples, while the recovery with N 2 0 was moderately higher (Figure 2), as might be expected based on reports that N 2 0 is a stronger eluant than CO, (15, 16). The similarity of extraction efficiencies obtained with ethane and COz might be explained based on the similar solubilities reported for PAH in supercritical ethane and CO, (17). The addition of the methanol modifier yielded much better extraction efficiencies with both CO, and N 2 0 . Supercritical Nz0/5% methanol extraction gave the best extraction efficiency for each of the three sample matrices. Figure 2 also demonstrates that higher molecular weight PAH are extracted at lower efficiency than lower molecular weight PAH from the same sample matrix. This result is consistent with the decrease in solubility of PAH with increasing molecular weight reported for supercritical CO, and supercritical ethane (2 7). The extraction efficiency of PAH from the fly ash sample was much lower than from the urban
ANALYTICAL CHEMISTRY, VOL. 59, NO. 13, JULY 1, 1987
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Table I. Recovery of PAH from Urban Dust (SRM 1649) Using Supercritical Nitrous Oxide/5% Methanol Extraction concentration.. .ug/e 3O-min extractionb Y ,
PAH (mol wt) fluoranthene (202) benz[a]anthracene (228) benzo[a]pyrene (252) benzo[ghi]perylene(276) indeno[l,2,3-cd]pyrene(276)
certified value" 7.1
* 0.5
7.4
2.6 f 0.3 2.9 f 0.5 4.5 f 1.1 3.3 f 0.5
Y
60-min extraction*
* 0.4 *
8.0 f 0.6 2.9 f 0.5 3.2 f 0.3 4.4 f 0.3 3.1 f 0.2
2.5 f 0.3 2.9 0.2 3.2 f 0.3 2.3 f 0.2
"PAH concentrations on the urban dust sample (Standard Reference Material 1649) are given as certified by the National Bureau of Standards. PAH concentrations and standard deviations ( l u ) are based on tridicate extractions of 20-ma samdes.
dust and the river sediment as might be expected since quantitative revovery of PAH from fly ash has been reported to be extremely difficult to achieve using extractions with liquid solvents (4, 5 ) . On the basis of the results just discussed, additional extractions of each sample were performed in order to determine the ability of supercritical N20/5% methanol extractions to yield quantitative recovery of PAH. The extraction pressure was raised to 350 atm, and higher solvent flow rates were achieved by using an outlet restrictor capillary with a larger internal diameter (25 wm). Under these conditions, approximately 20 mL of solvent was used for a 30-min extraction. Each sample was extracted in triplicate and the PAH concentrations were determined as described above. Table I shows the quantities of PAH extracted from the urban dust sample (SRM 1649) using supercritical N 2 0 / 5 % methanol compared with the values certified by the National Bureau of Standards. PAH recoveries obtained from triplicate supercritical fluid extractions showed good reproducibility with relative standard deviations generally
