Preparation and Characterization of Porous Silica-Coated Multifibers

extraction (SPME). The porous multifiber SPME provided larger absorption capacity and higher absorption rate compared to a polymer-coated single fiber...
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Anal. Chem. 2001, 73, 2041-2047

Preparation and Characterization of Porous Silica-Coated Multifibers for Solid-Phase Microextraction Xin-Rui Xia and Ross B. Leidy*

Pesticide Residue Research Laboratory, Department of Toxicology, North Carolina State University, 3709 Hillsborough Street, Raleigh, North Carolina 27607

C18-bonded silica-coated multifibers were prepared and studied as a stationary phase for solid-phase microextraction (SPME). The porous multifiber SPME provided larger absorption capacity and higher absorption rate compared to a polymer-coated single fiber. Its absorption rate was 10 times higher than that of a commercial 100µm poly(dimethylsiloxane) (PDMS)-coated fiber. Its high extraction efficiency enabled the positive identification of unknown compounds at sub-part-per-billion level in fullscan mode with a benchtop quadruple GC/MS. The desorption temperature indicated that the analyte interactions with the C18-bonded silica were stronger than those with the PDMS polymer. The dependence of the equilibration time on the molecular weight was not observed for the porous multifiber SPME. The boundary layer between the fiber coating and the sample matrix could be the absorption control step in SPME under mild agitation. The special experimental conditions in the porous multifiber SPME, such as air interference and polar organic solvent wetting, were investigated. Solid-phase microextraction (SPME) is a newly developed technique for sample preparation in organic analysis. It has been successfully applied to the analyses of various compounds in environmental, industrial, pharmaceutical, and clinical samples.1 SPME has many attractive features compared to the traditional sample preparation methods.2 It combines the extraction, preconcentration, and sample introduction into one simple step. It is solvent-free and readily utilized with gas chromatography (GC) or liquid chromatography (LC). Currently, SPME is predominantly performed on a single fiber coated with a poly(dimethylsiloxane) (PDMS) stationary phase.1-3 SPME with polymer coating depends on the partitioning of the analytes between the sample matrix and the polymer film coating. The coating thickness determines the volume and surface area of the stationary phase and, consequently, the amount and rate * Corresponding author: (tel) 919-515-3391; (fax) 919-515-5462; (e-mail) ross•[email protected]. (1) Pawliszyn, J., Ed. Applications of Solid-Phase Microextraction; The Royal Society of Chemistry: Hertfordshire, U.K., 1999. (2) Pawliszyn, J. Solid-Phase Microextraction: Theory and Practice; Wiley-VCH: New York, 1997. (3) Arthur, C. L.; Potter, D. W.; Buchholz, K. D.; Pawliszyn, J. LC-GC 1992, 10, 656-661. 10.1021/ac001273f CCC: $20.00 Published on Web 03/22/2001

© 2001 American Chemical Society

of absorption. Increasing the coating thickness can improve the sensitivity of the method, while the time to reach equilibrium becomes longer because extraction is a diffusion-controlled process.4 The extraction equilibrium takes hours or days for larger molecular weight analytes (MW >200). Therefore, SPME has to be performed at a nonequilibrium state.5-7 The absorption rate of a thick coating was higher than a thin coating because the thick coating has larger surface area.6 Unfortunately, the surface area of the polymer coating is limited by its outer surface. To improve the SPME technique, researchers studied porous stationary phases for larger surface area and higher absorption capacity. Lee et al.8 prepared a porous-layered SPME by gluing bonded-phase silica particles onto a metal fiber. The porous silica particles provided a large specific surface area. The calculated surface area was 500 times larger than a 100-µm polymer-coated fiber. It was found that the adsorption amount was 8 times larger than that of the 100-µm PDMS. Sol-gel coating technology was also used to prepare coated fibers for SPME in order to enhance its thermal stability and adsorption surface area.9 In our laboratory, porous multifibers were studied for solidphase microextraction. It was believed that a porous stationary phase is the solution for large surface area and large absorption capacity and that the porous multifibers would achieve the largest contact surface and absorption amount without diffusion limit in the stationary phase. In the present work, the porous multifibers were prepared by directly adhering a layer of C18-bonded silica onto glass fibers. The coated porous fibers were examined by scanning electron microscopy (SEM), thermal desorption, and solution extraction techniques. The special experimental conditions in the porous multifiber SPME were investigated. The advantages, extraction equilibrium, absorption process, and desorption temperature of the porous multifibers were compared with the polymer-coated single fiber. EXPERIMENTAL SECTION Chemicals and Materials. Hexane was analytical grade; acetone, acetonitrile and methanol were HPLC grade (J. T. Baker). (4) Louck, D.; Motlagh, S.; Pawliszyn, J. Anal. Chem. 1992, 64, 1187-1199. (5) Ai, J. Anal. Chem. 1997, 69, 1230-1236. (6) Bartelt, R. J.; Zilkowski, B. W. Anal. Chem. 1999, 71, 92-101. (7) Doong, R. A.; Chand, S. M. Anal. Chem. 2000, 72, 3647-3652. (8) Liu, Y.; Shen, Y. F.; Lee, M. L. Anal. Chem. 1997, 69, 190-195. (9) Chong, S. L.; Wang, D. X.; Hayes, J. D.; Wilhite, B. W.; Malik, A. Anal. Chem. 1997, 69, 3889-3898.

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Deionized water was prepared from a Picotech water system (Research Triangle Park, NC). The standard mixture of organochlorine pesticides was purchased from AccuStandard Inc. (New Haven, CT). Glass fibers with an average diameter of 31 µm were a gift from Fiber Glass Industries, Inc. (Amsterdam, NY). C18-bonded silica was obtained from Waters (Milford, MA). The C18-bonded silica was manually ground for 30 min to reduce its particle size. Stainless steel tubing and epoxy glue were obtained from CRS (Louisville, KY). Liqui-Nox cleaning detergent was purchased from Alconox, Inc. (New York, NY). SPME devices with 100-µm PDMS coating were obtained from Supelco (Bellefonte, PA). A stock solution of 4.00 µg/mL of individual components in hexane was prepared from the pesticide mixture. A standard solution of 80.00 ng/mL, abbreviated as PM, was prepared from the stock solution by removing hexane under a stream of dry nitrogen and rediluting with acetone for water fortification experiments. A series of pesticide standard solutions were prepared from the 4.00 µg/mL stock solution by diluting with hexane. A series of pesticide-fortified water samples were prepared from the 80.00 ng/mL standard solution. Preparation of the Multifiber SPME Syringe. The glass fibers were cleaned with a mixture of Liqui-Nox cleaning detergent and acetone (1:1 v/v) 3 times to remove fiber sizing chemicals and rinsed with sufficient water to remove the cleaning chemicals. The fibers were rinsed successively 5 times with acetone and hexane. Finally the fibers were baked at 300 °C in air for 12 h, cooled to room temperature, and cut to a desired length for subsequent preparation. The glass fiber was coated with a porous layer by applying a thin film of epoxy glue over the fiber and pressing onto a C18bonded silica particle bed. The coated fibers were dried at ambient temperature and then heated to 100 °C under a stream of nitrogen for 60 min. After cooling to room temperature, 15 pieces of the coated fibers were attached to a stainless steel tube by gluing the uncoated ends of the fibers into the tubing with the hightemperature epoxy (Figure 1a). The fibers were then cut to a total length of 2.5 cm with a coated length of 2 cm (Figure 1b). The other end of the stainless tubing was sealed and reinforced with a piece of tungsten wire and epoxy glue. A multifiber SPME syringe was modified from a commercial SPME device (Supelco). It was assembled with the fiber attachment tubing, piercing needle, sealing septum, screw nut, plunger, and holder as shown in Figure 1a. The coated fibers were treated in a GC injection port, which was programmed from 100 to 300 °C, at 10 °C/min and held for 30 min under a flow of helium. The treatment was repeated 5 times to stabilize the stationary-phase coating. The prepared multifiber SPME syringes were labeled as C18EP300. Multifiber SPME Technique. Before each extraction, the multifiber SPME syringe was preconditioned for 10 min by exposing the fibers inside the injection port at 280 °C under helium flow (the PDMS fiber was conditioned at 250 °C). The fibers were retracted back to the needle and sealed with a silicon septum during syringe transfer. The extractions were performed under magnetic stirring. The stirring speed was fixed and marked as stir (MH) for all of the extractions except otherwise indicated. A 30-mL high glass bottle 2042 Analytical Chemistry, Vol. 73, No. 9, May 1, 2001

Figure 1. Schematic of the porous multifiber SPME syringe.

containing a 2.5-cm spin bar was used as the extraction container. The pesticide-fortified water sample was measured into the extraction bottle with a pipet. The bottle was capped with a silicon septum, on which two pieces of Teflon tubing for nitrogen inlet and outlet were connected. Nitrogen flow was used to remove the trapped air in the extraction bottle and was kept on during extraction to prevent the extraction fibers from being exposed to air. The fibers were wetted in acetone for 1 min and rapidly placed into the sample solution under continuous stirring and nitrogen flow. The extraction time was defined as the period from the fiber insertion to their removal. Once extraction was completed, the fibers were withdrawn back into the needle and analyzed immediately. The following steps were used to transfer the extracted analytes from the SPME fibers to the chromatographic column: (1) the injection port was pierced with the fibers retracted inside the needle of the multifiber SPME syringe; (2) the plunger was depressed to expose the fibers inside the injection port, which was maintained at 280 °C in a splitless mode (the injection port was maintained at 250 °C for the PDMS fiber); (3) the analytes were thermally desorbed inside the injection port and carried into the column by the carrier gas, where they were focused and separated; (4) the fibers were held in the injection port for 10 min to serve as the preconditioning step for next extraction experiment; and (5) the fibers were retracted back into the needle and removed from the injection port. GC-ECD, GC/MS, and SEM. The quantitative analyses were performed on an HP-5890 series II gas chromatograph equipped with dual electron capture detectors and dual wide-bore fusedsilica capillary columns (DB-5, 30 m × 0.32 mm i.d. × 0.25-µm film thickness and DB-17, 30 m × 0.32 mm i.d. × 0.25-µm film thickness). The signal from the DB-5 column was used for quantitative analysis, and the signal from DB-17 was used as confirmation. Helium was used as carrier gas at a flow rate of 1.00 mL/min, and nitrogen was used as makeup gas at a flow

rate of 52 mL/min. The temperature was programmed as follows: initial temperature at 100 °C, held for 1 min, ramped at 15 °C/min to 150 °C, 2 °C/min to 220 °C, 5 °C/min to 280 and held for 5 min. The injection port was maintained at 280 °C for porous multifibers and 250 °C for the PDMS fiber. The detectors were set at 290 °C. The injection amount of the standard solution was 4 µL by using an HP 7675 automatic sampler. The analyte introduction from the multifiber SPME was carried out manually as detailed in the multifiber SPME Technique section. GC/MS experiments were performed on a HP 5890 II gas chromatograph coupled with a HP 5970B mass selective detector. An HP 7675 automatic sampler was used to inject 2 µL of the standard solutions, while the multifiber SPME syringe was injected manually. The injection port was maintained at 280 °C for sample vaporization and thermal desorption. Separation was performed on a 30 m × 0.25 mm (i.d.) × 0.25 µm (df) Rtx-5MS capillary column (Restek Corp., Bellefonte, PA). The transfer line temperature was set at 250 °C. The column oven was programmed as follows: initial temperature at 100 °C, held for 1 min, ramped at 15 °C/min to 150 °C, 1 °C/min to 220 °C, and 3 °C/min to 280 °C, and held for 5 min. An electronic pressure control was used to maintain a carrier gas flow of 1.00 mL/min helium. The scanning electron microphotograph was acquired with a Hitachi S-520 scanning electron microscope (Hitachi Ltd, Tokyo, Japan). The porous fibers were positioned on a conductive tape and gold vapor coated for 10 min. The microphotograph was undertaken with 2000 magnification at an acceleration voltage of 10 kV. RESULTS AND DISCUSSION Porous Multifibers. In the fiber preparation, the key step was to apply a very uniform thin epoxy film onto the glass fiber, which determined the final coating results. Directly adhering the particles offers several advantages, such as well-characterized stationary phase and the simplicity of fiber preparation. In the case of the multifiber SPME, the epoxy glue has two functions, adhering the particles and reinforcing the glass fibers. After the fiber preparation, 15 pieces of the coated fibers were used to make the porous multifiber SPME syringe as shown in Figure 1. The C18-bonded silica and the epoxy glue were stable to 300 °C. The chromatographic performance was not affected. One multifiber syringe was used more than 100 times under different experimental conditions. However, when it was subjected to a temperature higher than 300 °C, decomposing products or release of monomers in the epoxy glue was observed.8 To prepare the C18-bonded silica-coated fibers, very fine particles were used in order to eliminate the diffusion limit in the stationary phase. From the SEM photo, the average particle size was ∼2 µm and only one layer of the particles was coated on the fiber (Figure 2). The coverage of the fiber by the particles was ∼60%. The C18-bonded silica had a specific surface area of 330 m2/g and a silica particle density of 0.3 g/mL. Thus, the total absorbent volume coated on the 15 fibers is estimated to be 0.037 µL. It is smaller than the 100-µm PDMS, which is 0.612 µL.10 The total surface areas of the porous fibers was ∼36 cm2, while the surface area of the 100-µm PDMS-coated fiber was 0.14 cm2. Air Interferences. It was realized that the absorption rate of the polymer-coated fiber is higher in a gas phase than in water.2 When water samples are analyzed, however, special attention

Figure 2. Scanning electron micrograph of C18-bonded-phase silica-coated fibers. The C18-bonded-phase silica-coated fibers were pretreated at 300 °C under helium. Undertaken with gold coating, 2000-fold magnification, and 10-kV acceleration.

should be paid to interferences picked up from laboratory air (Figure 3a). The porous multifibers have a much stronger absorption capability than the polymer-coated fiber. The interferences picked up from laboratory air were much stronger after the C18-bonded silica-coated fibers were conditioned at high temperature (Figure 3c). Since interferences from laboratory air are unpredictable, precautions should be taken during analyte extraction and syringe transfer. For the rest of the experiment, a silicon septum was used to seal the piercing needle during syringe transfer. In addition, a nitrogen flow was use to prevent the fibers from being exposed to air during sample extraction. These precautions can effectively reduce potential interferences from laboratory air (Figure 3b). It should be mentioned that the strong absorption capability of the porous fibers is not a drawback, as it enables the porous multifiber SPME to have a large absorption capacity and high absorption rate. The strong absorption capability in air would enable the porous multifiber SPME to be used for gas sample collection and headspace analysis. A study of this potential use will be conducted in our laboratory. Polar Organic Solvent Wetting. The multifibers coated with C18-bonded silica showed a strong hydrophobic property. If the multifibers were directly exposed to a water solution, the fibers tended to stick together, which greatly reduced the exposed surfaces to the water phase. The extraction rate was slow (Figure 4). On the basis of knowledge of solid-phase extraction, it is known that the C18-bonded silica cartridge should be wetted by a polar organic solvent prior to water extraction. Therefore, the porous multifibers were wetted in acetone for 1 min and then immediately placed into the water sample for analyte extraction. The fibers were swinging separately and freely in the solution. Thus, all of the exposed surfaces effectively made contact with the water phase. The extraction rate was high. The wetting effect on the porous fibers is similar to the preconditioning stage of the cartridge in conventional solid-phase extraction. Other polar organic solvents, such as methanol and acetonitrile, can also be used as a wetting solvent for water sample analysis. The magnetic stirrer has a significant effect on the absorption rate (Figure 4). The absorption rate was the slowest when the extraction was performed with no wetting and no stirring, while acetone wetting slightly improved the absorption rate. Under Analytical Chemistry, Vol. 73, No. 9, May 1, 2001

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Figure 3. Airborne interferences picked up in laboratory air: (a) 100-µm PDMS exposed to laboratory air for 60 min; (b) C18EP300 exposed to nitrogen flow above DI water; (c) C18EP300 exposed to laboratory air for 60 min; (d) C18EP300 wetted with acetone for 1 min and exposed to laboratory air for 60 min.

Figure 4. Absorption rate of porous multifiber SPME. Extracted from 25 mL of water fortified with 100 ppt pesticide mixtures with or without acetone wetting, and with or without magnetic stir.

magnetic stirring, the absorption rate is higher even without wetting. The absorption rate is highest with acetone wetting and magnetic stirring. These results imply that the use of a polar organic solvent to wet the fibers only guarantees the full effectiveness of the exposed surface; it did not accelerate the absorption rate. The stirring speed, however, can greatly accelerate the absorption rate. In gas-phase absorption, it was also observed that the absorption rate with acetone wetting was about the same as that without wetting (Figure 3). Extraction Efficiency. Preconcentration is one of the advantages of the SPME technique. The response signals of the SPME technique were much stronger than those of a 1.00 ng/mL standard solution (Figure 5). The analytes from the SPME technique were extracted from 25 mL of water sample containing 10 ppt pesticide mixture. In comparison with 4 µL of the 1.00 ng/ mL standard solution, the preconcentrations were equivalent to 4000-fold for a 100-µm PDMS-coated fiber and 40 000-fold for C18bonded silica-coated fibers. 2044

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The absorption amount of the porous multifibers was ∼10 times larger than that of the 100-µm PDMS-coated fiber (Figure 5). Since the extraction equilibrium was not reached, the absorption amount represents the absorption rate. However, the absorption rate is affected by several factors including the loading of the stationary phase, the contact surface area, and the interaction strength between the analytes and the stationary phase. As estimated, the total volume of the C18-bonded silica on the multifibers was less than the 100-µm PDMS-coated fiber, while its surface area was 250 times higher than the 100-µm PDMScoated fiber. Thus, the high absorption rate of the multifiber SPME resulted primarily from its high surface area or the high interaction strength between the analytes and the stationary phase. Trace Organic Analysis and Identification (GC/MS). The high absorption capacity of the porous multifiber SPME can significantly increase the method detection sensitivity and improve its analytical performance in trace organic analysis. Furthermore, this feature can be also utilized with a bentchtop quadruple GC/

Figure 5. Extraction efficiency: (a) 4 µL of a 1.00 ng/mL standard pesticide mixture in hexane; (b) 100-µm PDMS; (c) C18EP300 extracted from 25 mL of water containing 10 ppt pesticide mixture for 30 min. Compared to spectra c, spectra a was enlarged 10 times (×10), and spectra b was enlarged 5 times (×5).

Figure 6. Full-scan spectra with regular GC/MS and porous multifiber SPME. C18EP300 extracted from 25 mL of water fortified with 100 ppt pesticide mixture for 60 min. The spectra were acquired with an HP 5970B mass selective detector, ChemStation software and HP MS database.

MS for trace organic identification. It is known that regular bentchtop quadruple GC/MS systems have a detection limit of 1 ng in scan mode and, typically, 20 ng depending on how well the compound is ionized in the mass spectrometer. This means that analyte concentrations of 1-10 µg/mL (ppm) were required to provide high-quality, full-scan spectra for positive identification of unknown compounds. For sample concentrations in the nanogram per milllilier (ppb) range, a 10-100-fold sample concentration is required. Using the porous multifiber SPME, the analysis and identification in sub-part-per-billion levels become easy and simple. Figure 6 shows the total ion chromatogram obtained by an HP bentchtop quadruple GC/MS and the porous multifiber SPME from 25 mL

of water fortified with 100 ppt pesticide mixture. The signals were strong enough for qualitative identification by matching to the standard MS database. Otherwise, this can be only achieved by ion trap MS or complicated sample concentration processes. Extraction Equilibration. The extraction mechanism of the polymer-coated fiber remains unclear.5,6,10-12 SPME was proposed initially as an equilibrium method for rapid organic analysis.2 Equilibrium can be reached in seconds or minutes for smaller (10) Shumer, B.; Pawliszyn, J. Anal. Chem. 2000, 72, 3660-3664. (11) Yang, Y.; Hawthome, S. B.; Miller, D. J. Anal. Chem. 1998, 70, 18661869. (12) Mayer, P.; Vaes, W. H. J.; Hermenns, J. L. M. Anal. Chem. 2000, 72, 459464.

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Figure 7. Extraction equilibration of the porous multifiber SPME. C18EP300 extracted from 25 mL of water containing 100 ppt pesticides using magnetic stirrer at fixed stirring speed (MH).

Figure 9. Desorption temperatures of the 100-µm PDMS SPME. The 100-µm PDMS extracted from 25 mL of water containing 500 ppt pesticides for 30 min. The injection port was set to different temperatures to determine desorption profiles.

Figure 8. Agitation effect on the absorption rate. C18EP300 extracted from 25 mL of water containing100 ppt pesticides either using magnetic stirring (MH) or remaining static.

molecules. However, it was realized that the equilibrium might take hours or days for larger molecules (MW >200), and the equilibration time is related to the molecular weights of analytes. The data reported from different laboratories were sometimes contradictory or had large discrepancies.7,10,11,13 In this study, the dependence of the equilibration time on the molecular weight was not observed for the C18-bonded silicacoated multifibers (Figure 7). The plotted compounds in Figure 7 were chosen to represent the chromatogram of the 26 organochlorine pesticides with molecular weights ranging from 280 to 540 (Figure 5). From Figure 7 it was also observed that the equilibration times were as long as 4 h. This prolonged equilibration time is not readily explained since the absorption inside the porous silica is a rapid process.8 The absorption rate is significantly affected by the stirring speed. When the extraction was performed statically (i.e., without agitation), the absorption rate was very slow (Figure 3). The absorption amount shows a liner relationship with the extraction time (Figure 8). The absorption rate became much higher when the mixture was stirred. This stirring effect is emphasized because it is directly related to the thickness of the boundary layer, which stays statically on the fibers. All of the analytes absorbed must transport through this boundary layer. Under mild agitation, such as magnetic stirring, this boundary layer would be the extraction control step in the SPME technique. Unfortunately, it is difficult to keep the stirring effect identical from experiment to experiment (13) Langenfeild, J. J.; Hawthome, S. B.; Miller, D. J. Anal. Chem. 1996, 68, 144-155.

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Figure 10. Desorption temperatures of the porous multifiber SPME. C18EP300 extracted from 25 mL of water containing 500 ppt pesticides for 30 min. The injection port was set to different temperatures to determine desorption profiles.

since it is complicatedly related to stirring speed, fiber position in the container, sample viscosity, sample temperature, sample volume, shape of the container, and size and shape of the stirring bar. If the stirring conditions can be properly controlled and carefully defined, the reproducibility of the SPME technique could be improved, and the data contradiction and discrepancy from different laboratories might be solved. Desorption Temperatures. Thermal desorption is a reverse process of absorption. Study of desorption processes can provide useful information on the absorbent and the absorption processes. In the SPME technique, desorption was performed in the GC injection port and the desorbed analytes were directly carried to the capillary column for analytical separation. If an SPME liner were used in the injection port, it proved to be a good heat transfer and gas flow system. For compounds of molecular weights ranging from 280 to 540, the desorption at 280 °C from the porous multifibers can be accomplished in less than 5 s. Thus, the porous multifibers can be used for GC analysis and desorption kinetic studies. Figure 9 shows the dependence of the desorption amount to the desorption temperature for a 100-µm PDMS-coated fiber. The “S”-shaped curves were observed. The middle of the “S”-shaped curve indicates the desorption peak. The R-benzene hexachloride (R-BHC) began to desorb at 120 °C, while cis-chlordane, endosulfan II, and endrin ketone desorption peak occurred at 175 °C. For the porous multifibers R-BHC began to desorb at 150 °C, while

cis-chlordane, endosulfan II, and endrin ketone desorption peak occurred at 210 °C (Figure 10). The desorption temperature was mainly determined by two aspects, the molecular weight and vapor pressure of the analytes, and the interaction strength between the analyte and the stationary phase. These two aspects should be considered in comparison of the desorption temperatures of different compounds in the same stationary phase. However, for a given compound in different stationary phases, the desorption temperature indicates the interaction strength between the analyte and the stationary phase. It was observed that there was a 50 °C difference between the PDMS polymer and the C18-bonded silica (Figure 9 and Figure 10). It is concluded that the analyte interactions with the C18bonded silica are stronger than that with the PDMS polymer.

stronger analyte interactions compared to the polymer-coated single fiber. The absorption rate of the porous multifiber SPME was 10 times higher than that of the 100-µm PDMS-coated fiber. High-quality, full-scan spectra for positive identification of unknown compounds at sub-part-per-billion can be obtained with a benchtop quadruple GC/MS and the porous multifiber SPME. The desorption temperature indicated that the analyte interaction with the C18-bonded silica was stronger than that with the PDMS polymer. The equilibration time of the porous multifiber SPME does not depend on the molecular weights of analytes. The prolonged equilibration time indicated that the analyte diffusion in the porous stationary phase was not the control step in SPME. The boundary layer between the fiber coating and the sample matrix could be the control step under mild agitation.

CONCLUSIONS The C18-bonded silica-coated multifibers were a good absorbent material for the SPME technique. The porous multifibers had larger absorption capacity, higher absorption rate, and

Received for review October 30, 2000. Accepted February 14, 2001. AC001273F

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