Characterization of Polyacrylate Membrane-Coated Fibers Used in

Xin R. Xia, Ronald E. Baynes, Nancy A. Monteiro-Riviere, and Jim E. Riviere*. Center for Chemical Toxicology Research and Pharmacokinetics (CCTRP), ...
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Anal. Chem. 2004, 76, 4245-4250

Characterization of Polyacrylate Membrane-Coated Fibers Used in Chemical Absorption Studies with Programmed Thermal Treatment and FT-IR Microscopy Xin R. Xia, Ronald E. Baynes, Nancy A. Monteiro-Riviere, and Jim E. Riviere*

Center for Chemical Toxicology Research and Pharmacokinetics (CCTRP), College of Veterinary Medicine, North Carolina State University, Raleigh, North Carolina 27606

A polyacrylate (PA) film was coated onto a fused-silica fiber as a permeation membrane in a membrane-coated fiber (MCF) technique and a solid-phase microextraction technique. The molecular changes of the PA membrane after different temperature treatments were studied with FT-IR microscopy. The absorption bands of the PA aliphatic backbone at 2902, 2795, and 2740 cm-1 remained unchanged over the elevated thermal treatments, indicating that the polymer backbone was stable over these conditions. The spectra of the PA membrane remained unchanged when the thermal treatment temperature was under 150 °C. When the temperature was 250 °C, the O-H stretching band in the -COOH groups of the poly(acrylic acid) at 3315 cm-1 was significantly reduced. When the temperature was higher than 280 °C, this O-H band disappeared. These evidences suggested that the PA membrane underwent dehydroxyl reaction to form an anhydride when the thermal treatments were higher than 250 °C. Thermal treatments of a deuterated PA MCF confirmed the anhydride formation mechanism. The anhydride formation explained the absorption property of PA MCFs in GC applications where they must be preconditioned at 300 °C. The absorption data suggest that a PA fiber does not preferably absorb polar compounds (with permanent dipole moment); instead, it absorbs preferably aromatic compounds. A new emerged membrane-coated fiber (MCF) technique uses a polymer membrane coated onto a section of inert fiber as a permeation membrane to study membrane permeability, partition equilibrium, and intermolecular forces between the membrane and solution.1 The MCF technique was developed from a solidphase microextraction (SPME) technique in analytical chemistry, where it is used as a stationary phase to extract analytes from sample matrixes for quantitative analysis.2 For SPME applications, analyte extraction can be based on any mechanism including absorption and adsorption since efficient extraction for sampling * Corresponding author. Tel.: (919)513-6398. Fax: (919)513-6358. E-mail: [email protected]. (1) Xia, X. R.; Baynes, R. E.; Monteiro-Riviere, N. A.; Leidy, R. B.; Shea, D.; Riviere, J. E. Pharmacol. Res. 2003, 20, 275-282. (2) Zhang, Z.; Yang, M. J.; Pawliszyn, J. Anal. Chem. 1994, 66, 844A-852A. 10.1021/ac0355146 CCC: $27.50 Published on Web 05/25/2004

© 2004 American Chemical Society

is the only requirement. In fact, most of the newly developed SPME fibers are based on the adsorption mechanism or mixed absorption and adsorption.3 In the MCF technique, porous adsorptive materials cannot be used; only absorptive membrane can be utilized to probe a wide range of intermolecular forces that determine the distribution and permeability of a compound between the membrane and solution. Polyacrylate (PA) is one of the most widely used MCFs in both SPME and MCF techniques. It is made of a poly(acrylic acid) polymer coated onto a fused-silica fiber. Poly(acrylic acid) polymer is a hydrophilic material due to its polar carboxyl groups.4 In fact, polyacrylate is one of the most widely used superabsorbents and thickening agents in pharmaceuticals, cosmetics, food processing, coatings, and agricultural chemicals.5 It is designed to extract polar analytes when used as a SPME fiber.6-8 In practical analytical application, however, it behaves differently. It absorbs more hydrophobic compounds than polar compounds.7,8 The mechanism of this effect has not been understood. For gas chromatography (GC) applications, PA MCFs were conditioned at 300 °C for 2 h to obtain a stable working membrane (manufacture recommended). The molecular structure and composition of the membrane could change under high-temperature treatment. Understanding the molecular structure and its hydrophobicity is critical for the MCF technique for probing intermolecular forces. It is also important for the SPME technique to optimize the extraction conditions and find new applications. FT-IR spectroscopy is used traditionally to study the molecular changes of various materials. For the miniature structure of the membrane-coated fiber, it is difficult to use this conventional technique. For example, the diameter of a typical PA MCF is about 200 µm with a membrane thickness of 85 µm. Different size membranes (e.g., 3.2 cm × 3.2 cm × 61.2 µm poly(dimethylsiloxane)) have to be used for FT-IR study.9 A newly developed (3) Supelco Bulletin 925B. SPME applications guide; Sigma-Aldrich Corp.: St. Louis, MO, 2001. (4) Tamura, T.; Kawauchi, S.; Satoh, M.; Komiyama, J. Polymer 1997, 38, 2093-2098. (5) Zhu, S.; Pelton, R. H.; Hamielec, A. E. Eur. Polym. J. 1998, 34, 487-492. (6) Supelco Application Note 17; Sigma-Aldrich Corp.: St. Louis, MO, 1998. (7) Doong, R. A.; Chang, S. M. Anal. Chem. 2000, 72, 3647-3652. (8) Hall, B. J.; Satterfield-Doerr, M.; Parikn, A. R.; Brodbelt, J. S. Anal. Chem. 1998, 70, 1788-1798. (9) Stahl, D. C.; Tilotta, D. C. Environ. Sci. Technol. 2001, 35, 3507-3512.

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FT-IR microscopy is a powerful tool for acquiring chemical information of small samples or microsamples.10 The spatial resolution of a contemporary FT-IR microscope is about 10 µm. Therefore, it can be used to study the chemical changes of the membrane-coated fibers. The absorption property of PA MCFs is largely determined by the -COOH functional groups.11 Deuterium oxide exchange of the functional groups to -COOD functional groups is an effective method for studying its property in traditional FT-IR spectroscopy.12-14 In the present paper, FT-IR microscopy, incorporated with programmed thermal treatments and deuterium-hydrogen exchange method, is used to study the molecular structure changes of PA MCFs. Their membrane structure and performances were characterized at different temperatures. EXPERIMENTAL SECTION Chemicals and Materials. Acetone was capillary GC grade (99.9+%) and 17 aromatic compounds (Figure 7) were reagent grade purchased from Sigma-Aldrich (St. Louis, MO). Deuterium oxide (100.0 atom % D) was purchased from Aldrich (Wilwaukee, WI). Solid-phase microextraction (SPME) devices, 100-µm poly(dimethylsiloxane) (PDMS), and 85-µm ployacrylate (PA) coated fiber assemblies were purchased from Supelco (Bellfonte, PA). A stock solution of 10.00 mg/mL of each individual component in acetone was prepared from the neat chemical. A standard mixture containing the 17 aromatic compounds with a concentration of 1000 µg/mL (individual component) was prepared in acetone from the individual stock solutions. A series of standard solutions in acetone were prepared from the standard mixture to be used as external calibration standards for quantitative analysis. A water solution with a concentration of 0.800 µg/mL (individual component) was prepared from the standard mixture. Programmed Thermal Treatment. The programmed temperature treatment was performed in an injection port of a HP 5890 gas chromatograph (GC). The temperature was controlled precisely and programmed with HP ChemStation software. A stream of helium (0.75 mL/min) flowed through the injector to protect the fiber from being exposed to air. A PA fiber was injected into the GC injector with its piecing needle through the GC septum. The PA membrane-coated fiber protected inside the piecing needle was pushed out of the needle and exposed to the helium flow at a given temperature for 30 min. After the temperature treatment, the membrane-coated fiber was withdrawn into the piecing needle and removed from the injector for FT-IR analysis. The PA fibers were treated stepwise at 100, 150, 200, 250, 280, and 300 °C and cooled to room temperature after each thermal treatment for FT-IR analysis. Seven new PA fibers were (10) Sommer, A. J. In Handbook of Vibrational Spectroscopy, Vol.2, Sampling Techniques; Chalmers, J. M., Griffiths, P. R., Eds.; John Wiley & Sons Ltd.: Chichester, UK, 2002; pp 1369-1385. (11) Maurer, J. J.; Eustace, D. J.; Ratcliffe, C. T. Macromolecules 1987, 20, 196202. (12) Chalmer, J. M.; Hannah, R. W.; Mayo, D. W. In Handbook of Vibrational Spectroscopy, Vol.3, Sample Characterization and Spectral Data Processing; Chalmers, M. J., Griffiths, P. R., Eds.; John Wiley & Sons Ltd.: Chichester, UK, 2002; pp 1893-1918. (13) Dong, J.; Ozaki, Y.; Nakashima, K. J. Polym. Sci. B: Polym. Phys. 1997, 35, 507. (14) Shurvell, H. F. In Handbook of Vibrational Spectroscopy, Vol.3, Sample Characterization and Spectral Data Processing; Chalmers, J. M., Griffiths, P. R., Eds.; John Wiley & Sons Ltd.: Chichester, UK, 2002; pp 1783-1816.

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used to ensure the experimental reproducibility (three PA fibers for the Polyacrylate MCF, two for the polyacrylate film and two for the deuterium-hydrogen exchange experiments as detailed in the following sections). FT-IR Microscopy. FT-IR spectra were acquired with a PerkinElmer Spectrum 1000 FT-IR with AutoImage FT-IR Microscope system. The FT-IR microscope has a liquid nitrogen cooled, 64 × 64 mercury cadmium telluride (MCT) detector. The infrared beam was switched from the Spectrum 1000 FT-IR position to external AutoImage Microscope position. After the detector was cooled and filled up with liquid nitrogen, infrared energy was maximized by optical focusing and sample stage adjustment. After a sample was loaded into the sample holder on the automatic stage, an optical beam was used for focusing or taking photo images of the sample. Then the microscope was switched to infrared transmission mode to acquire FT-IR spectra with the following parameters: The sampling size was set 100 × 100 µm. The scan range was 400-4000 cm-1 with a resolution of 1 cm-1. Each spectrum was obtained with a sum of 100 scans. Polyacrylate MCF. A new PA fiber was placed on the sample holder of the FT-IR microscope without windows. Its FT-IR spectrum was acquired without temperature treatment, with air as a background. The PA fiber was removed from the sample holder and injected into the GC injector at 100 °C and held for 30 min. After the temperature treatment, the PA fiber was removed from the injector, cooled to room temperature, and placed on the sample holder for acquiring its FT-IR spectrum. This same procedure was repeated for different temperature treatments and FT-IR spectrum recording. Polyacrylate Film. A section of PA film (∼100 × 500 µm) was harvested with a razor blade from a new PA fiber. The PA fiber was injected into the GC injector at a given temperature for 30 min. After each temperature treatment, a section of PA film was harvested with a razor blade. Each of the PA films treated at different temperatures were placed on a KBr window (2 × 13 mm) on the sample holder. Their FT-IR spectra were acquired with the KBr window as a background. Deuterium-Hydrogen Exchange. A new PA fiber was placed into 300 µL of deuterium oxide in a sealed vial insert. The entire membrane was immersed in deuterium oxide for 43 h at room temperature. The MCF was removed slowly from deuterium oxide to allow the surface tension of deuterium oxide to keep the deuterium oxide off the fiber film. The deuterium-exchanged PA fiber (PAD) was placed on the sample holder of the FT-IR microscope to acquire its initial room-temperature spectra. The PAD was treated at 100 °C for 30 min. After the temperature treatment, the FT-IR spectrum was acquired with air as background. This same procedure was repeated for different temperature treatments and FT-IR spectrum recording. PA Absorption of Aromatic Compounds. Five PA and three PDMS fibers were conditioned as manufacture recommended at 300 °C for 2 h and 250 °C for 30 min, respectively. A stir bar and 5.00 mL of the 0.800 ug/mL water solution were transferred into each 7-mL tall glass vial (n ) 8). The glass vials with sample solution were placed in a water bath on a 15-position magnetic stirrer. The solution was stirred at 800 rpm for at least 30 min to equilibrate the solution temperature to 25 °C. The absorption experiments were performed manually by inserting the precon-

was used to inject 2 µL of the calibration standard solution, while the membrane-coated fibers were injected manually. The injection port was maintained at 280 °C for sample vaporization and thermal desorption. The analytical conditions were improved to reduce analytical time and increase analytical sensitivity. Separation was performed on a 30 m × 0.25 mm (i.d.) × 0.25 µm (df) HP-5MS capillary column (Agilent, Palo Alto, CA). The column oven was programmed as follows: the initial temperature was 40 °C and held for 3 min, ramped at 10 °C/min to 165 °C and 30 °C/min to 280 °C, and held at 280 °C for 5 min. An electronic pressure control was used to maintain a carrier gas flow of 1.00 mL/min helium. The selected ion monitoring (SIM) mode was used for quantitative analysis in which the studied compounds were grouped according to their retention times and one to three character ions were monitored for each compound depending on the ion intensity produced by the compound.

Figure 1. Photo images of a whole PA MCF and a PA film. (A) A whole PA MCF; (B) a PA film pilled off a PA MCF. The white center strap was the smooth surface from the fused-silica rod. The designated squire marks indicate the locations where the FT-IR spectra were acquired.

ditioned PA or PDMS fibers into the sample solutions under constant stirring at 800 rpm. The absorption time was 20 min for each PA or PDMS fiber. Finally, the fibers were removed from the vials and transferred directly into an injector of a gas chromatograph for quantitative analysis. GC/MS Analyses. Quantitative and qualitative analyses were performed on a HP 5890 II gas chromatograph coupled with a HP 5970B mass selective detector. A HP 7675 automatic sampler

RESULTS AND DISCUSION Photo Images of PA MCF and PA Film. Figure 1 shows the photo images of a whole PA MCF (A) and a PA film (B) under the FT-IR microscope. The designated squire marks indicate the locations where the FT-IR spectra were acquired. The white center strap was the smooth surface peeled off from the fused-silica rod. It is evident that the infrared beam passed through two layers of the PA membrane and the fused-silica rod of the whole PA MCF, while the infrared beam only passed through one layer of the PA film. Temperature Effects on PA MCF. Figure 2 shows the FT-IR spectra of a whole PA MCF after different temperature treatments. Numbers on lines are characteristic FT-IR absorption bands. Labels on curves indicate the treatment temperatures after which the spectra were acquired. A new PA fiber before any temperature treatment had eight infrared absorption bands between 2000 and 3700 cm-1, two sharp bands at 2740 and 2795 cm-1, one broad band at 2902 cm-1, three more sharp bands at 3061, 3131, and 3446 cm-1, and two more broad bands at 3315 and 3502 cm-1 (PA25). PA MCFs were made of a polymer formula

Figure 2. FT-IR spectra of a whole PA MCF after thermal treatments. Label on curve indicates the treatment temperature after which the spectrum was acquired.

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Figure 3. FT-IR spectra of PA films after thermal treatments. Label on curve indicates the treatment temperature after which the spectrum was acquired.

of poly(acrylic acid) for extraction of polar compounds from various sample matrixes. Infrared spectra of polymers are usually broader, particularly for self-associated amorphous polymers as PA. The broad band at 2902 cm-1 is a characteristic absorption band of aliphatic polymer chain (-CH2-). Two shoulder bands at 2795 and 2740 cm-1 are CH2 or CH stretching. The broad band at 3502 could be from alcohols or phenols. The broad band at 3315 cm-1 is assigned to the O-H stretching in the -COOH groups of the poly(acrylic acid). The bands at 3446, 3131, and 3061 cm-1 could be from polymer additives containing NH2 groups or benzene groups. The spectra of the PA fiber after 100 and 150 °C treatments (PA100 and PA150) are similar to that of PA25. A new band at 2270 cm-1 emerged after 200 °C treatment, which could be from new double bonds species (PA200). When the treatment temperature increased to 250 °C, the broad band at 3315 cm-1 was significantly reduced by comparing its relative peak heights with that of band 3502 cm-1 (PA250). When the PA fiber was treated at 280 and 300 °C (PA280 and PA300), the broad band at 3315 cm-1 disappeared from the spectra. The new emerged band at 2270 cm-1 and the bands at 3061, 3131, and 3446 cm-1 also disappeared. The two sharp bands at 2740 and 2795 cm-1 and the two broad bands at 2902 and 3502 cm-1 remained unchanged over the elevated thermal treatments. Temperature Effects on PA Films. Figure 3 shows the FTIR spectra of the PA films after different temperature treatments. The PA film before any temperature treatment also had eight infrared absorption bands between 2000 and 3800 cm-1 (PAF25). The spectrum of the PA fiber after 200 °C treatment (PAF200) is similar to that of PAF25 except for the new band at 2270 cm-1. When the treatment temperature increased to 250 °C, the broad band at 3315 cm-1 was significantly reduced by comparing its relative peak heights with the band at 3502 cm-1 (PAF250). When the PA fiber was treated at 280 and 300 °C (PAF280 and PAF300), the broad band at 3315 cm-1 disappeared from the spectra. The new emerged band at 2270 cm-1 and the bands at 3061, 3131, and 3446 cm-1 also disappeared. The two sharp bands at 2740 and 2795 cm-1 and the two broad bands at 2902 and 3502 cm-1 remained unchanged over the temperature treatments. 4248

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The spectra from the whole PA MCF (Figure 2) are similar to those of the PA films (Figure 3). This indicates that the infrared absorption from the whole PA MCFs are from the PA membranes in the range of 2000-3800 cm-1. In this range fused-silica optical fiber does not absorb infrared. In the range of 1200-1900 cm-1, the fused silica has strong infrared absorption. Therefore, the membrane absorption can only be studied with PA films (Figure 4). The broad band at 1715 cm-1 is from the CdO stretching in the -COOH groups. After 280 and 300 °C treatments, a band at 1685 cm-1 emerged and made the CdO band broader, indicating some chemical changes occurred. The absorption bands at 1619 and 1600 cm-1 could come from the aromatic polymer additives, which were released from the membrane at high-temperature treatments. PA Molecular Structure after Thermal Treatments. The absorption bands at 2902, 2795, and 2740 cm-1 have not changed after different temperature treatments, indicating that the polymer aliphatic backbone was stable over the elevated thermal treatments. The FT-IR spectra of the whole PA MCF and the PA films remain unchanged over a temperature range from 25 to 150 °C. This reveals that the PA molecular structure remains the same as designed. When the temperature reached 250 °C, dehydroxy reaction occurred. When the temperature reached 280 or 300 °C, the hydroxyl groups disappeared. These results are consistent with the thermal degradation of poly(acrylic acid) studied by NMR15 and thermal characterization.11,16 It suggests an anhydride formation after thermal treatments higher than 250 °C:

Temperature Effects on Deuterated PA MCFs. Deuteriumhydrogen exchange is an effective IR method to study chemical structure. It is a classic infrared method used to determine the (15) Fyfe, C. A.; McKinnon, M. S. Macromolecules 1986, 19, 1909-1912. (16) Buzanowski, W. C.; Cutie, S. S.; Howell, R.; Papenfuss, R.; Smith, C. G. J. Chromatogr. A 1994, 677, 355-364.

Figure 4. FT-IR spectra of PA films after thermal treatments. Label on curve indicates the treatment temperature after which the spectrum was acquired.

Figure 5. FT-IR spectra of PA MCFs before and after deuteration. PA150 is the spectrum before deuteration; PAD150 is the spectrum after deuteration. Absorption frequencies on wavelength axis are reference frequencies from a FT-IR handbook.12

concentration of alcohol and carboxylic end groups in polymers.12,17 The heavier atom reduces the vibrational frequency of the infrared absorption. The key to successful analysis is that the interrogated polymer is dry. In the present paper, the band positions of the -OH in alcohols (3502 cm-1) and -COOH (3315 cm-1) were established by isotopic exchange with D2O to produce -OD and -COOD groups (Figure 5). Then the deuterated PA MCF was treated at different temperatures to confirm the anhydride formation mechanism. Figure 5 shows the spectra of PA MCFs before and after deuteration. PA150 is a spectrum of a PA MCF acquired after 150 °C treatment, while PAD150 is a spectrum of a deuterated PA MCF acquired after the same thermal treatment. Numbers above the wavelength axis are vibrational frequencies from an FT-IR handbook.12 A strong -COOD band at 2479 cm-1 emerged after deuteration (PAD150). An O-D band from -CH2CH2OD was not observed at 2604 cm-1, even though an O-H band existed at 3502 cm-1. The -COOH band at 3335 cm-1 reduced considerably after (17) Chalmers, J. M.; Everall, N. J. In Handbook of Vibrational Spectroscopy, Vol.4, Applications in Industry, Materials and the Physical Sciences; Chalmers, J. M., Griffiths, P. R., Eds.; John Wiley & Sons Ltd.: Chichester, UK, 2002; pp 2389-2418.

deuteration since part of the -COOH groups was converted to -COOD. The total amounts of the -COOH and -COOD groups after deuteration (PAD150) should be equal to the amount of the -COOH groups before deuteration (PA150). The deuteration of -COOH groups are easier than that of alcohols or phenols. Since there was still a significant amount of -COOH groups in the deuterated PA membrane, deuteration of alcohols and phenols had not started yet in the competition reactions. This explains why the -CH2CH2OD group was not observed in the spectrum (PAD150). Figure 6 shows the FT-IR spectra of the deuterium-exchanged PA MCF after different temperature treatments. The spectrum of the PA fiber after 100 °C treatment (PAD100) is similar to that of PA25 except that the broad band at 3502 cm-1 is reduced considerably. This could have resulted from the H2O produced during D-H exchange, which was trapped in the membrane and released at 100 °C. The spectrum of the PAD fiber after 150 °C (PAD150) is similar to that of PAD100. The broad band at 2477 cm-1 after 200 °C treatment (PAD200) decreased considerably compared to that of PAD150, while the broad band at 3345 cm-1 become broader but not decreased. A new band at 2271 cm-1 also emerged after 200 °C treatment. When the treatment temperature increased to 250 °C, the broad bands at 2477 and 3345 cm-1 both were reduced considerably. It is interesting to note that the decrease of -COOD is more than -COOH at a given temperature (PAD200 and PAD250). It reveals that the -COOH groups easier to be deuterated were easier anhydrided. When the PA fiber was treated at 280 and 300 °C (PAD280, PAD300), the broad bands at 2477 and 3345 cm-1 both disappeared from the spectra. The new emerged band at 2271 cm-1 and the bands at 3126 and 3448 cm-1 also disappeared. The two sharp bands at 2739 and 2797 cm-1 and the two broad bands at 2897 and 3502 cm-1 remained unchanged over the temperature treatments. These results are consistent with those from the whole PA MCF (Figure 2) and the PA films (Figure 3 and Figure 4) and confirms the anhydride formation at a temperature higher than 250 °C. PA Absorption Properties. PA MCFs are designed to extract polar compounds from sample matrixes by utilizing its polar -COOH groups in poly(acrylic acid). It is noted in Figure 7 that Analytical Chemistry, Vol. 76, No. 14, July 15, 2004

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Figure 6. FT-IR spectra of a deuterated PA MCF after thermal treatments. Label on curve indicates the treatment temperature after which the spectrum was acquired.

PA fibers, where the extraction amounts of the less polar acetate derivatives of phenols were essentially the same as those for the free phenols.18 The PA fibers also had very high absorption for nonpolar aromatic compounds, such as p-xylene, propylbenzene, naphthalene, and biphenyl. These results suggest that a PA fiber does not preferably absorb polar compounds (with permanent dipole moment); instead, it absorbs preferably aromatic compounds. This is consistent with the present FT-IR findings that the designated polar groups (-COOH) were converted to the anhydride forms during the thermal conditioning of the PA fibers at 300 °C.

Figure 7. Comparison of PA and PDMS absorption of aromatic compounds. The absorption amounts were average amounts determined in 5 mL of 0.800 µg/mL water solution under constant stirring at 800 rpm for 20 min with PA fibers (n ) 5) and PDMS fibers (n ) 3).

PA had higher absorption amounts than PDMS for the polar aromatic compounds, such as phenol, 4-florophenol, m-cresol, 4-ethyl phenol, 4-chlorophenol, p-nitrotoluene, and 3-bromophenol. This phenomenon has been observed in many reports and led to the conclusion that a PA fiber is optimal for the extraction of polar compounds. However, it was observed that addition of a polar group into a nonpolar aromatic molecule will dramatically reduce its absorption amount by PA fibers. For example, the difference between propylbenzene and phenethyl alcohol is the -CH3 group of propylbenzene being substituted by a polar -OH of phenethyl alcohol. The absorption amount of propylbenzene by PA fibers was 27 times higher than that of phenethyl alcohol. This fact was also evident when comparing p-xylene with m-cresol. Buchholz and Pawliszn also observed this special absorption behavior of (18) Buchholz, K. D.; Pawllszyn, J. Anal. Chem. 1994, 66, 160-167.

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CONCLUSION FT-IR microscopy is a useful tool to study the molecular changes of the membrane-coated fibers. Incorporated with the thermal treatment and deuteration methods, it was demonstrated that the molecular structure of the PA membrane changed from -COOH functionality to anhydride formation at a temperature higher than 250 °C. This explains the absorption property of PA MCFs in GC applications that cannot be explained by the molecular structure of the originally designed material. The hydrophobic property of PA MCFs for GC applications comes from the anhydride formation where they must be preconditioned at 300 °C. If PA MCFs do not undergo thermal treatments higher than 150 °C, their molecular structure remains unchanged. The partitioning properties of PA MCFs for GC application could be different from HPLC applications where the absorbed analytes are desorbed by solvent rather than by thermal desorption. ACKNOWLEDGMENT This work was supported by the U.S. Air Force Office of Scientific Research, Grant F49620-01-1-0080.

Received for review December 19, 2003. Accepted April 22, 2004. AC0355146