Anal. Chem. 1997, 69, 5001-5005
Solid-Phase Microextraction of PAHs from Aqueous Samples Using Fibers Coated with HPLC Chemically Bonded Silica Stationary Phases Yu Liu and Milton L. Lee*
Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602-5700 Kimberly J. Hageman, Yu Yang, and Steven B. Hawthorne
Energy and Environmental Research Center, University of North Dakota, Grand Forks, North Dakota 58202
In this study, porous layer solid-phase microextraction (SPME) was investigated for the extraction of polycyclic aromatic hydrocarbons (PAHs) from water. The porous layer coatings were prepared using silica particles (5 µm diameter) bonded with phenyl, C8, and monomeric and polymeric C18 stationary phases for use in HPLC. It was found that several factors affected the selectivity for extraction of PAHs, including functional group in the bonded phase, alkyl chain length of the bonded phase, and phase type (monomeric or polymeric). The distribution coefficients of PAHs in the porous layer increased with an increasing number of carbon atoms, and greater selectivity toward solute molecular size and shape were obtained using a polymeric C18 porous layer. The effect of solution ionic strength on extraction was also investigated. A phenyl bonded phase porous layer fiber was used to extract PCB congeners from Arochlor 1254, and headspace SPME was applied to the analysis of contaminated soil samples. The analysis of polycyclic aromatic hydrocarbons (PAHs) in environmental samples has become an important topic in analytical chemistry because of the carcinogenic and mutagenic properties of these compounds.1,2 Current techniques for the extraction and concentration of PAHs from water are liquid-liquid extraction and solid-phase extraction.3,4 Both of these methods require the use of toxic organic solvents. An alternative solvent-free extraction technique is solid-phase microextraction (SPME).5-8 Typically, a fused-silica fiber, which is coated with a thin layer of polymeric stationary phase, is used to extract analytes from water, soil and gaseous samples. The extracted analytes are then thermally desorbed in the injector of a gas chromatograph for analysis. Since SPME is easily automated, it simplifies the extraction process and significantly reduces the analysis time. SPME has already been (1) Sawicki, E. Polynuclear Aromatic Hydrocarbons: Mechanisms, Methods and Metabolism; Battelle Press: Columbus, OH, 1985; pp 1-47. (2) Lee, M. L.; Novotny, M. V.; Bartle, K. D. Analytical Chemistry of Polycyclic Aromatic Compounds; Academic Press: New York, 1981; pp 50-73. (3) Acheson, M. A.; Harrison, R. M.; Perry, R.; Wellings, R. A. Water Res. 1976, 10, 207-211. (4) Symons, R. K.; Crick, I. Anal. Chim. Acta 1983, 151, 237-243. (5) Zhang, Z.; Yang, M.; Pawliszyn, J. Anal. Chem. 1994, 66, 844A-853A. (6) Pawliszyn, J. Trends Anal. Chem. 1995, 14, 113-122. (7) Louck, D.; Motlagh, S.; Pawliszyn, J. Anal. Chem. 1992, 64, 1187-1199. (8) Zhang Z.; Pawliszyn, J. Anal. Chem. 1993, 65, 1843-1852. S0003-2700(97)00686-0 CCC: $14.00
© 1997 American Chemical Society
successfully applied to a variety of compounds, including volatile organic compounds,9-12 phenols,13,14 PAHs/PCBs,15 and herbicides.16 However, SPME with polymer-coated silica fibers still exhibits two significant disadvantages: (1) the fused-silica fiber is very fragile, and therefore, extra care must be taken during use; and (2) SPME with polymer coatings requires thick films to achieve the desired sensitivity. A thick film is necessary to obtain a high sample loading, and there is considerable difficulty in preparing thick, stable coatings. To date, only two polymers, poly(dimethylsiloxane) and polyacrylate, have been shown to be applicable to SPME, and these polymers are general-purpose extractants, demonstrating little selectivity. We recently introduced a new approach for solid-phase microextraction using porous layer-coated metal fibers.17 Porous silica particles were immobilized on stainless steel fibers using a high-temperature epoxy. The capacity of the porous layer was greater than polymer coatings because of the large specific surface area provided by the porous silica particles.17 Furthermore, since many silica bonded HPLC stationary phases are commercially available, selective extraction of analytes of interest can be easily achieved by preparing porous layer coatings with these selective silica stationary phases. Reversed-phase liquid chromatography on chemically bonded alkyl stationary phases is widely used for the separation of PAHs.18-25 Studies show that the retention and selectivity for separation of PAHs on alkyl phases increased with increasing alkyl (9) Arthur, C. L.; Killam, L.; Buchholz, K.; Pawliszyn, J. Anal. Chem. 1992, 64, 1960-1966. (10) Potter, D.; Pawliszyn, J. J. Chromatogr. 1992, 625, 247-255. (11) Arthur, C. L.; Killam, L.; Motlagh, S.; Lim, M.; Potter, D.; Pawliszyn, J. Environ. Sci. Technol. 1992, 26, 979-983. (12) Chai, M.; Arthur, C. L.; Pawliszyn, J.; Belardi, R.; Pratt, K. Analyst 1993, 118, 1501-1505. (13) Buchholz, K.; Pawliszyn, J. Environ. Sci. Technol. 1993, 27, 2844-2848. (14) Buchholz, K.; Pawliszyn, J. Anal. Chem. 1994, 66, 160-167. (15) Potter, D. P.; Pawliszyn, J. Environ. Sci. Technol. 1994, 28, 298-305. (16) Boyd-Boland, A. A.; Pawliszyn, J. J. Chromatogr. 1995, 704, 163-172. (17) Liu, Y.; Shen, Y.; Lee, M. L. Anal. Chem. 1997, 69, 190-195. (18) Sander, L. C.; Wise, S. A. Anal. Chem. 1987, 59, 2309-2313. (19) Jinno, K.; Kawasaki, K. Chromatographia 1983, 17, 445-449. (20) Wise, S. A.; Bonnett, W. J.; Guenther, F. R. J. Chromatogr. Sci. 1981, 19, 457-463. (21) Sander, L. C.; Wise, S. A. Anal. Chem. 1984, 56, 504-510. (22) Wise, S. A.; Sander, L. C. J. High Resolut. Chromatogr. 1985, 8, 248-253. (23) Stalcup, A. M.; Martire, D. E.; Sander, L. C.; Wise, S. A. Chromatographia 1989, 27, 405-411. (24) Wise, S. A.; May, W. E. Anal. Chem. 1983, 55, 1479-1485. (25) Sander, L. C.; Wise, S. A. J. High Resolut. Chromatogr. 1988, 11, 383-387.
Analytical Chemistry, Vol. 69, No. 24, December 15, 1997 5001
chain length of the stationary phase.18,19 Chemically bonded C18 phases have been shown to provide excellent selectivity for the separation of PAHs.20 Furthermore, it was also found that the phase type (monomeric or polymeric) influenced the separation selectivity. Polymeric C18 phases can provide greater selectivity for PAHs than the more common and widely used monomeric C18 phases because of the higher surface coverage of the polymeric phase.21-25 In this paper, porous layer SPME coatings with different reversed phases including octyl (C8), monomeric octadecyl (C18), polymeric octadecyl, and phenyl bonded silica were prepared and the selectivity for extraction of alkylbenzenes and PAHs from water was investigated. Factors affecting the extraction of PAHs were also studied, and Arochlor 1254 and a contaminated soil sample were extracted and analyzed using the porous layer SPME fibers. EXPERIMENTAL SECTION Chemicals and Instrumentation. Monomeric C8 and C18, polymeric C18, and phenyl bonded silica particles (5 µm) with 200 Å pore size and 200 m2 g-1 surface area were provided by the National Institute of Standards and Technology (Gaithersburg, MD). Benzene, toluene, xylene, naphthalene, 2-methylnaphthalene, 2-ethylnaphthalene, acenaphthylene, 1,5-dimethylnaphthalene, 2,3,6-trimethylnaphthalene, fluorene, anthracene, phenanthrene, 9,10-dihydroanthracene, 2-methylanthracene, benz[a]anthracene, chrysene, and triphenylene were obtained from a variety of sources. All other solvents were reagent grade. A mixture of 10 ppm PAHs in methylene chloride or benzene was used as a standard. HPLC-grade water (Fisher Scientific) was used to prepare all of the samples. The water samples were prepared by mixing 1 µL of the standard solution of PAHs with 1 mL of methanol and then diluting with HPLC water by a factor of 104. Analysis of PAHs was performed using both a Carlo Erba HRGC-5300 gas chromatograph (Carlo Erba, Milan, Italy) with flame ionization detection (FID) and HP 5890 gas chromatograph (Hewlett Packard, DE) coupled with a Jeol JMS-SX102A (Jeol, Peabody, MA) double-focusing, reverse-geometry, magnetic sector mass spectrometer. Both the injector and detector were maintained at 320 °C. After extraction, the porous layer-coated metal fiber was inserted into the GC injector and desorbed for 5 min. The carrier gas flow was 1 mL min-1, and the split flow was 0.125 mL min-1. A 30 m × 0.25 mm i.d. fused-silica column coated with 0.25 µm PTE-5 was purchased from Supelco (Bellefonte, PA). A temperature program of 40 °C for 1 min and then 5 °C min-1 to 320 °C was used. An HP 5890 series II gas chromatograph (Hewlett Packard) with electron capture detector (ECD) was used for analysis of PCB congeners. A 25 m × 0.32 mm i.d. fused-silica column coated with 0.17 µm HP-5 (Hewlett Packard) was used for separation. The coated fiber was desorbed at 300 °C in the injection port of the GC for 1 min. The column was held at 60 °C during desorption, ramped at 25 °C min-1 to 130 °C, and then 8 °C min-1 to 320 °C. The ECD was maintained at 300 °C during analysis. Preparation of Porous Layer Metal Fibers. Stainless steel wire (300 µm o.d.) was purchased from Small Parts (Miami Lakes, FL). The wires were cleaned with ethanol in an ultrasonicator (Branson, CT) for 10 min and dried at 60 °C for 15 min. The bonded-phase silica particles were attached onto the metal wire surface using high-temperature epoxy (353ND, Epo-Tek, MA).17 Briefly, the metal wire was first coated with a thin film of hightemperature epoxy and then carefully dipped into the silica 5002 Analytical Chemistry, Vol. 69, No. 24, December 15, 1997
Table 1. Physical Properties of Chemically Bonded Phases phase type
MW of ligand
carbon load (%)
surface coverage
C8 phenyl monomeric C18 polymeric C18
171 163 311 315
6.12 4.46 10.38 18.18
2.21 1.98 2.50 5.72
particles. The coated metal wires were preheated at 70 °C for 30 min and then conditioned at 350 °C under 100 psi He for 18 h. Four different porous layers (octyl, phenyl, and monomeric and polymeric octadecyl) were prepared in this study. The SPME devices were modified from a commercial SPME fiber holder and assembly (Supelco) as described elsewhere.17 Briefly, the porous layer metal SPME fibers were inserted through a septum-piercing needle with a brass ferrule and a sealing septum. The penetration depth of the coated wire in the GC injector could be adjusted directly using the scale near the barrel window. Blank analyses were conducted before daily analyses by inserting the metal fiber in the GC injector for 5 min. A contaminated fiber can be cleaned by inserting it in the GC injector at 320 °C under 100 psi He for 5 h. Extraction Procedure. A 30 mL Erlenmeyer flask with 2.5 cm spin bar inside was used as the sample container. A magnetic stirrer with speed range from 150 to 1200 rpm was used to obtain the different stirring rates. To prevent sample evaporation, the flask was sealed with a septum. During extraction, the septum was pierced with the protecting needle and the porous layer was exposed to the sample solution for extraction. The organic analytes were adsorbed from the water onto the porous layer until equilibrium was reached. Then the adsorbed analytes were thermally desorbed by inserting the porous layer metal fiber into the injector of the GC. The contaminated soil sample was extracted by headspace extraction. A 1 g soil sample was transferred into a sample vial which was sealed with a septum. The vial was then placed in an oven at 60 °C for 2 h to reach thermal equilibrium throughout the headspace and soil. Then the porous layer-coated fiber was exposed to the headspace at 60 °C for 30 min by piercing the septum of the vial. To avoid contamination, the fiber had to be protected from making contact with the soil. After the extraction was finished, the fiber was immediately inserted into the GC injector for analysis. RESULTS AND DISCUSSION Four different silica bonded phases, octyl, phenyl, and monomeric and polymeric octadecyl, were used for the preparation of the porous layer SPME fibers. The physical properties of the bonded phases are listed in Table 1. The silica bonded particles were bonded onto stainless steel fibers by using high-temperature epoxy. The mechanical strength of the fibers was greatly improved when stainless steel wire was used as support. Previous studies showed that the stainless steel wire was uniformly coated with approximately six layers of bonded silica particles.17 The sample capacity of the porous layer coating was significantly increased when compared with polymer-coated silica fibers, mainly because of the high surface area provided by the porous silica particles. For a 250 µm o.d. fiber with film thickness of 100 µm, the total surface area of the porous layer
Figure 1. Blank chromatograms for C8 bonded-phase porous layer desorbed at 320 °C after conditioning the fiber at (A) 350 and (B) 300 °C for 18 h. Conditions: 30 m × 0.25 mm i.d. fused-silica capillary column coated with 0.25 µm PTE-5; 40 °C for 1 min and then 10 °C min-1 to 320 °C, 10 min desorption time, 320 °C detector temperature.
was roughly 500 times higher than that of a polymer-coated silica fiber.17 Previous studies showed that the epoxy glue contributed little to solute partitioning and extraction/desorption characteristics of the porous layer fibers.17 The coating can be operated indefinitely below 250 °C; however, some monomers are released from the epoxy glue when temperatures higher than 300 °C are used. Since the silica bonded phase is very stable, even at relatively high temperatures, high conditioning temperatures (350 °C) could be used to clean the fiber coatings before they were used for semivolatile analytes. Figure 1 shows blank analyses of a porous layer C8 fiber in a GC injector at 320 °C after the fiber was conditioned at 300 (Figure 1A) and 350 °C (Figure 1B). A clean blank chromatogram was obtained when the fiber was conditioned at 350 °C for 18 h. Selectivity for PAHs. Porous layer solid-phase microextraction is a process in which analytes are adsorbed onto a porous layer coating from a sample matrix. The mass adsorbed on the fiber coating can be expressed as follows:
ns ) KVsVaqCaq°/(KVs + Vaq)
(1)
where ns is the number of moles extracted by the solid phase at equilibrium, Vaq and Vs are the volumes of the sample solution and porous layer, respectively, C° is the initial concentration of analyte in the sample, and K is the distribution coefficient of analyte between the porous layer and water. The mass adsorbed on the fiber is dependent on the surface area of the stationary phase and the distribution coefficient K; therefore, selective extraction of analytes should be possible by using selective silica bonded stationary phases. In general, the distribution coefficient (K) of analytes is dependent on the intermolecular interactions between the solute and stationary phase, which includes hydrogen bonding, acidbase, dipole-dipole, dipole-induced-dipole, and dispersion forces. For a hydrophobic alkyl stationary phase, dispersion forces are very important. Figure 2 compares the distribution coefficients of PAHs on C8, phenyl, and monomeric and polymeric C18 porous layer coatings. The logarithm of the distribution coefficients of PAHs on the four stationary phases investigated increased with increasing number of carbon atoms. However, some alkylsubstituted PAHs, e.g., 2,3,6-trimethylnaphthalene, show much higher K values than nonsubstituted PAHs which have the same, or even higher, numbers of carbon atoms. This is because PAHs with side chains are more soluble in the hydrophobic stationary phase and, hence, are more completely extracted. According to the solvophobic theory, the retention of analytes on the stationary phase in HPLC is dependent on the molecular
Figure 2. Logarithm of the distribution coefficient versus carbon number for PAHs on (2) phenyl, ([) C8, (9) monomeric C18, and (b) polymeric C18 bonded phase porous layers. Conditions: 0.1 ppm BTX and PAH solution in water, extracted for 40 min under 1200 rpm stirring rate. Compounds used to determine distribution coefficients were benzene, toluene, o-xylene, naphthalene, 2-methylnaphthalene, 1-ethylnaphthalene, 2,3,6-trimethylnaphthalene, anthracene, 2-methylanthracene, and 9-ethylphenanthrene.
contact area between the solute and stationary phase. For a given solute, retention is expected to increase with the alkyl chain length of the bonded phase until the molecular contact area reaches a maximum. A further increase in the chain length will not significantly affect the retention. A similar behavior was observed for the distribution coefficients of PAHs between the porous layer and water matrix when PAHs were extracted using monomeric C8 and C18 bonded phases. Figure 2 shows that the distribution coefficients for substituted benzene and naphthalene were very similar for both porous layer coatings, which indicates that the maximum contact area is already reached for a C8 bonded-phase porous layer. By increasing the molecular size of the analytes to three rings, the distribution coefficients on the monomeric C18 phase increased greatly, implying that a longer alkyl chain in the porous layer was required for maximum contact. The distribution coefficients for substituted benzene and naphthalene were greater for the phenyl bonded phase than for other bonded-phase porous layers (Figure 2). The polarizable phenyl groups in the porous layer can exhibit π-π interactions with PAHs. Although the phenyl phase had a similar or shorter chain length compared to C8 and C18, the total molecular interaction between the PAHs and the phenyl phase increased. However these π-π interactions were only significant for compounds with one or two aromatic rings. For larger PAHs, the van der Waals dispersion forces became dominant and a slight decrease in distribution coefficients was observed. Comparing all of the bonded phases, the polymeric C18 porous layer showed higher selectivity for large PAHs due to its high carbon content and surface coverage (Table 1). Larger amounts of three-ring PAHs were extracted by the polymeric C18 porous layer fibers. We can define the selectivity factor (R) of two compounds on a porous layer as
Rji ) Kj/Ki
(2)
where Kj and Ki are the distribution coefficients of compounds i and j between water and the porous layer. Table 2 lists the Analytical Chemistry, Vol. 69, No. 24, December 15, 1997
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Table 2. Selectivities of Porous Layer Bonded Phases to PAHsa
Table 4. Effect of Ionic Strength on the Extraction of PAHsa
stationary phase selectivity
C8
phenyl
mono-C18
poly-C18
R2/1 R3/1 R4/1
25.0 25.1 29.4
7.8 12.2 18.0
16.6 18.8 20.0
4.0 54.3 70.6
a Conditions: 0.1 ppm solution, extracted for 50 min under stirring rate of 1200 rpm. Compounds used for the determination of distribution coefficients (K) were (1) toluene, (2) naphthalene, (3) anthracene, and (4) 9-ethylphenanthrene.
total mass extracted (ng) for concn of NaCl compound
0%
3%
6%
7%
benzene toluene o-xylene naphthalene 2-methylnaphthalene 2,4,6-trimethylnaphthalene anthracene
5 40 75 201 245 412 238
9 47 103 206 250 459 230
16 71 145 188 223 380 162
8 20 30 41 104 90 60
a Conditions: 0.1 ppm PAH solution, extracted for 50 min under a stirring rate of 1200 rpm.
Figure 3. Chemical structures of PAH isomers. Table 3. Distribution Coefficients of PAHs and Selectivities of C18 Bonded Phase Porous Layers to PAH Isomersa porous layer
K1
K2
K3
K4
K5
R1/2
R4/5
monomeric C18 polymeric C18
370 540
320 510
1100 2200
640 1200
560 920
1.1 1.1
1.1 1.3
a Conditions: 0.1 ppm solution, extracted for 50 min under stirring rate of 1200 rpm. PAH isomers used for determination of distribution coefficients were (1) anthracene, (2) phenanthrene, (3) chrysene, (4) benz[a]anthracene, and (5) triphenylene.
Figure 4. Gas chromatogram of PCB congeners extracted from 3.8 ppb water solution of Arochlor 1254 using phenyl bonded-phase porous layer fiber. Conditions: 3.8 ppb water solution of Arochlor 1254, extracted for 15 min; 300 °C injection port, desorption for 1 min, 60 °C oven temperature during desorption, then 25 °C min-1 to 130 °C, and 8 °C min-1 to 320 °C; 300 °C ECD.
selectivity factors of several PAHs on the porous layer bonded phases. The polymeric C18 porous layer has the greatest selectivity for high molecular weight PAHs while the C8, phenyl, and monomeric C18 bonded porous layers show similar distribution coefficients for all PAHs. Early studies showed that the retention of PAH isomers on a C18 bonded phase in reversed-phase LC increased with increasing length-to-breadth ratio (L/B), and greater selectivity was achieved using a polymeric C18 bonded phase.22,24 In this study, the extraction of PAH isomers was compared using monomeric and polymeric C18 phases. The structures of the five PAH isomers (molecular weights of 178 and 228) and their length-to-breadth ratios are shown in Figure 3. The distribution coefficients of the PAH isomers increased with increasing L/B, and higher selectivity was obtained using the polymeric C18 phase (Table 3). It was also found that the selectivity of the polymeric C18 porous layer depends on the molecular size of the PAHs. The larger the molecular size of the PAH, the greater the difference between their distribution coefficients. It is predicted that higher selectivity toward PAH isomers of molecular weight 252 and 302 (five- and six-ring PAHs) will be obtained using a polymeric C18 porous layer.
Since these compounds are not easy to thermally desorb in a GC injector, further work must involve SPME coupled with HPLC or SFC. Effect of the Ionic Strength of the Water Solution on Extraction. As shown by eq 1, the amount extracted by the porous layer is dependent on the distribution coefficients of the analytes between the porous layer bonded phase and the water matrix. Decreasing the water solubility of the analytes will increase the distribution coefficient (K) and, therefore, the amount extracted. The addition of salt to the water solution generally causes a decrease in solubility of the organic compounds in the water, and this has been used to enhance the extraction of analytes on polymer-coated silica fibers.16 Therefore, the effect of ionic strength on extraction was investigated in this study (Table 4). It was found that the mass extracted on the porous layer increased with increasing ionic strength at low concentration of sodium chloride. However, when more than 5% sodium chloride was added, a decrease in mass extracted was observed. For extreme conditions in which the solution was saturated with sodium chloride, the mass extracted decreased greatly. This phenomenon becomes more serious when PAHs larger than naphthalene were extracted. The distribution coefficient of anthracene decreased with increasing ionic strength, and only ∼20% of the analytes were
5004 Analytical Chemistry, Vol. 69, No. 24, December 15, 1997
Table 5. Comparison of Extraction Yields of PCBs as a Function of Sorption Timea PCB No. 8 18 128 52 44 66 101 77 118
15 min extraction peak area RSD 23 500 36 200 107 800 73 800 66 400 139 000 103 900 77 700 132 500
7 6 5 13 20 4 5 3 5
120 min extraction peak area RSD 63 569 90 700 362 800 199 900 195 200 404 500 306 900 244 600 410 200
12 7 11 3 8 7 12 13 25
300 min extraction peak area RSD 63 735 91 300 388 300 207 300 207 000 441 500 348 800 279 000 471 900
10 4 7 2 4 3 3 4 4
a Conditions: 3.8 ppb solution of Arochlor 1254, extracted for different times using phenyl bonded porous layer fiber.
extracted from saturated sodium chloride solution when compared to a solution with no salt added. It should be noted that the results obtained here are very different from those reported by Pawliszyn using polymer-coated silica fibers, for which the amount extracted generally increased with salt concentration until maximum extraction was reached at saturated concentration.16 This is probably because sodium chloride can deposit onto the macropores of the porous layer and greatly decrease the porosity. One evidence for this assumption is that the fiber turned white after being used for extraction of high-concentration sodium chloride solutions. Extraction of PCBs from Aqueous Arochlor 1254 Solution. A phenyl bonded-phase porous layer fiber was used to extract PCB congeners from a 3.8 ppb water solution of Arochlor 1254 (Figure 4). Table 5 compares the amount extracted and the RSDs of triplicate sampling using 15, 120, and 300 min sorption times. The amount extracted in 120 min was 2-3 times higher than that extracted in 15 min, but additional sorption time does not significantly influence the extraction. However, lower RSDs were obtained with the longer extraction times. Headspace Extraction of a Contaminated Soil Sample. A contaminated soil sample was analyzed using headspace solidphase microextraction. Headspace SPME is an efficient technique for analyzing volatile organic compounds. A porous layer C8
Figure 5. Reconstructed total ion chromatogram from the GC/MS analysis of a contaminated soil sample extracted by headspace SPME. Conditions: 1 g of soil sample extracted at 60 °C for 30 min. Peak identifications: (1) toluene, (2) xylene, (3) trimethylbenzene, (4) naphthalene, (5) C17 alkane, (6) cycloalkane, and (7) substituted benzene.
bonded fiber was inserted into the headspace above a soil sample in the sealed vial. The volatile organic compounds were extracted onto the porous layer directly from the headspace and then transferred to the GC injector for analysis. Figure 5 shows a total ion GC/MS chromatogram from headspace extraction of the soil sample. As can be seen, this sample contained many alkanes and substituted benzenes. ACKNOWLEDGMENT This work was supported by the U.S. Environmental Protection Agency, Contract No. R 820-495-01-0. We thank Lane Sander of NIST for preparation of the silica bonded particles used in this study.
Received for review July 1, 1997. Accepted October 7, 1997.X AC970686M X
Abstract published in Advance ACS Abstracts, November 15, 1997.
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