Anal. Chem. 1997, 69, 190-195
Porous Layer Solid Phase Microextraction Using Silica Bonded Phases Yu Liu, Yufeng Shen, and Milton L. Lee*
Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602-5700
In this paper, solid phase microextraction (SPME) using porous layer-coated fibers is reported. Bonded silica particles (5 µm diameter), which are typically used for reversed-phase high-performance liquid chromatography, were fixed onto the surface of metal wires using hightemperature adhesives. The adsorption and desorption properties of the new porous layer SPME fibers were investigated using aromatic hydrocarbon test compounds, and the results were compared with those of commercially available polymer-coated fused silica fibers. It was found that the desorption time using the porous layer fibers was less than 10 s, which provides high chromatographic efficiency without cryogenic trapping. The total mass of toluene adsorbed from a 0.1 ppm aqueous solution on a fiber with a 30 µm C-8 porous layer was 8 times larger than that obtained using a fiber with a 100 µm thick polydimethylsiloxane coating. It was also found that the selectivity of the porous layer in SPME greatly depends on the properties of the bonded particles. Both C-8 and C-18 bonded phases were evaluated. The stability of the resultant porous layer coatings for SPME was investigated by introduction into a GC injector at different temperatures. It was found that the new porous layers were very stable for thermal desorption below 250 °C; however, some monomers were released from the epoxy glue at 300 °C. Solid phase microextraction (SPME) is a rapidly developing new technology to extract and concentrate contaminants from aqueous samples.1,2 It has advantages of simplicity, rapid extraction, and easy quantitation.3 SPME can be easily utilized with gas chromatography (GC)4-14 and liquid chromatography (LC).15 Currently, polymer-coated fused silica fibers for SPME are (1) Zhang, Z.; Yang, M.; Pawliszyn, J. Anal. Chem. 1994, 66, 844A-853A. (2) Pawliszyn, J. Trends Anal. Chem. 1995, 14, 113-122. (3) Arthur, C. L.; Pawliszyn, J. Anal. Chem. 1990, 62, 2145-2148. (4) Arthur, C. L.; Pratt, K.; Motlagh, S.; Pawliszyn, J. J. High Resolut. Chromatogr. 1992, 15, 741-744. (5) Belardi, R. P.; Pawliszyn, J. Water Pollut. Res. J. Can. 1989, 24, 179-191. (6) Arthur, C. L.; Killam, L.; Buchholz, K.; Pawliszyn, J. Anal. Chem. 1992, 64, 1960-1966. (7) Potter, D.; Pawliszyn, J. J. Chromatogr. 1992, 625, 247-255. (8) Arthur, C. L.; Killam, L.; Motlagh, S.; Lim, M.; Potter, D.; Pawliszyn, J. Environ. Sci. Technol. 1992, 26, 979-983. (9) Chai, M.; Arthur, C. L.; Pawliszyn, J.; Belardi, R.; Pratt, K. Analyst 1993, 118, 1501-1505. (10) Buchholz, K.; Pawliszyn, J. Environ. Sci. Technol. 1993, 27, 2844-2848. (11) Buchholz, K.; Pawliszyn, J. Anal. Chem. 1994, 66, 160-167. (12) Potter, D.; Pawliszyn, J. Environ. Sci. Technol. 1994, 28, 298-305. (13) Hawthorne, S.; Miller, D.; Pawliszyn, J.; Arthur, C. J. Chromatogr. 1992, 603, 185- 191. (14) Boyd-Boland, A. A.; Pawliszyn, J. J. Chromatogr. 1995, 704, 163-172. (15) Chen, J.; Pawliszyn, J. Anal. Chem. 1995, 67, 2530-2533.
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commercially available. By using different polymer coatings, such as polyimide,3 polydimethylsiloxane,4 and liquid crystal polyacrylate,5 SPME has been successfully applied to the analysis of many compounds types, including substituted benzenes,6-8 chlorinated hydrocarbons,9 phenols,10,11 polychlorinated biphenyls,12 caffeine,13 and herbicides.14 To date, all of the research reported on SPME has involved the use of silica fibers coated with polymeric coatings. SPME with polymer coatings depends on the partitioning of the analytes of interest between the sample water matrix and the polymer film coated on the fiber.16 The coating thickness determines the volume of the coatings and, consequently, the amount of analyte absorbed. Generally, the amount doubles when the film thickness is doubled. Thus, the sensitivity of the method can be improved by using thick film coatings. However, when the film thickness is doubled, the equilibration time increases 4 times because extraction is a dynamic diffusion-controlled process.16 Both extraction and desorption become much slower when using thick coatings, which results in long analysis time and low efficiency. Furthermore, it is difficult to prepare stable, thick films, especially with polar or selective polymers. Currently, the polymer coatings on silica fibers are less than 100 µm. Therefore, there is a sensitivity limitation for SPME that is dependent on the thickness of the polymer coating. In this study, we introduce a new approach to prepare SPME fibers. Porous layers were prepared by fixing porous silica particles onto the fibers. Porous silica particles provide a large specific surface area; therefore, the capacity of the coating can be greatly increased. As a result, the sensitivity of the method is increased. Furthermore, since the bonded phase on the particle surface is a very thin layer (less than 100 Å), we can predict that diffusion processes will be very fast, and the separation efficiency can be greatly improved. Although fused silica fibers are widely and successfully used as the sampling matrix in SPME, they have a disadvantage of fragility. Using metal wires which have good mechanical strength should make SPME an even easier method for routine analysis. In this work, porous layer fibers were prepared by coating stainless steel fibers with bonded silica particles. These fibers were compared with commercial polymer-coated SPME fibers. The adsorption and desorption kinetics were studied, and the effects of different alkyl bonded phases on extraction were also investigated. EXPERIMENTAL SECTION Chemicals and Instrumentation. Totally porous silica and C-8 and C-18 bonded silica particles (5 µm) with 100 Å pore size (16) Louck, D.; Motlagh, S.; Pawliszyn, J. Anal. Chem. 1992, 64, 1187-1199. S0003-2700(96)00791-3 CCC: $14.00
© 1997 American Chemical Society
and 500 m2 g-1 surface area were obtained from Phenomenex (Torrance, CA). Benzene, toluene, and xylene (BTX) were purchased from Fisher Scientific (Fairlawn, NJ). All other solvents were reagent grade. A mixture of 10 ppm BTX in methylene chloride was used as a standard. HPLC grade water (Fisher Scientific) was used to prepare all of the solutions for analysis. The water samples were prepared by mixing 1 µL of BTX solution with 1 mL of methanol and then diluting with HPLC water by a factor of 104. An HRGC-5300 (Carlo Erba, Milan, Italy) with FID detector was used in this study. The injection port was equipped with a 0.25 mL glass liner. The injector and detector were maintained at 250 and 300 °C, respectively. After extraction, the porous layer metal wire was inserted into the GC injector and desorbed for 1 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). The column was maintained at 40 °C for 1 min and then heated at 3 °C min-1 to 150 °C and held for 10 min during each analysis. The SPME devices used in this study were modified from a commercial SPME fiber holder and assembly (Supelco). The holder consists of a stainless steel barrel with a scale on the window side, a black polymeric plunger, and a stainless steel retaining nut. The porous layer metal SPME fibers were protected by 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 by using the scale near the barrel window. Preparation of Porous Layer Metal Fibers. Stainless steel wire (450 µm o.d.) was purchased from Small Parts (Miami Lakes, FL). The wires were first 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 immobilized on the metal wire surface using high-temperature epoxy (353ND Epo-Tek, MA). The coated metal wires were preheated at 70 °C for 30 min and then conditioned at 300 °C under 100 psi He protection for 18 h. Three different porous layers (untreated silica, C-8 bonded silica, and C-18 bonded silica) were prepared in this study. The film thicknesses of the porous layers were measured using a Toolmaker microscope (Mitutoyo, Japan). Extraction Procedure. A 30 mL Erlenmeyer flask with a 1 in. spin bar inside was used as the sample container. A magnetic stirrer with speed range from 150 to 1200 rpm was used to obtain different stirring rates. To prevent sample evaporation, the flask was sealed with a septum. During extraction, the septum of the flask 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. The adsorbed analytes were then thermally desorbed by inserting the porous layer metal fiber into the injector of the GC. Scanning Electron Microscopy. The fibers were first coated with 200 Å of gold and then analyzed using a JEOL JSN 840A scanning electron microscope (10 kV accelerating potential). RESULTS AND DISCUSSION Polymer-Coated and Porous Layer SPME. Figure 1 shows electron micrographs of a porous layer SPME fiber (450 µm stainless steel wire coated with a 30 µm porous layer) and a polymer-coated SPME fiber (250 µm silica fiber coated with 100
Figure 1. Scanning electron micrographs of (A) porous layer and (B) polymer-coated film SPME fibers.
µm polydimethylsiloxane). The porous particles were immobilized on the metal fiber using an epoxy. Clearly, a detailed knowledge of the structure of the final fibers seems necessary for understanding their detailed behavior during extraction. However, this information is not readily available. The probable structure of the coatings is a multilayer of particles bonded to the surface of the fiber and bonded to each other with epoxy. Since the epoxy is very hard when it is cross-linked, it should contribute little to solute partitioning and the extraction/desorption characteristics of the porous layer fibers, as demonstrated in Figure 2. The metal wire was uniformly coated with approximately six layers of the bonded phase silica particles. For the polymer coated SPME fiber, the total surface area for the coating can be expressed as
SPC ) π(D + 2a)L
(1)
where SPC is the exposed surface area of the polymer coating for a specific length (L) of silica fiber, D is the outer diameter of the silica fiber, and a is the film thickness. For the porous layer SPME fiber, if we assume that the porous layer is an idealized bed of uniform spherical microparticles, the total surface area can be Analytical Chemistry, Vol. 69, No. 2, January 15, 1997
191
Figure 2. Gas chromatogram of BTX extracted using a 30 µm epoxy-coated metal fiber. Conditions: 0.1 ppm solution of BTX in water, 20 min extraction time, 1200 rpm stirring rate.
expressed as
[(D2 + a) - (D2 ) ]Fs 2
SPL ) πL
2
(2)
where SPL is the total surface area of the porous layer, F is the density of the silica bonded particles, is the porosity of the porous layer, and s is the specific surface area. Therefore,
SPC ) SPL
(D2 + a)L ) 1 1 + 1 D D Fs(a D + a) πL[( + a) - ( ) ]Fs 2 2 2π
2
2
(3)
The ratio of the surface areas of the porous layer and polymer coatings increases with an increase in the film thickness and outer diameter of the fiber. For a 250 µm fiber with a film thickness of 100 µm, a silica particle density (F) of 0.3 g mL-1, a specific surface area (s) of 200 m2 g-1 , and a porous layer porosity () of 0.5, the total surface area for a porous layer C-8 phase is 0.97 × 10-2 m2, and the surface area for a polymer-coated fiber is 1.41 × 10-5 m2. The surface area is 500 times greater when using a porous layer compared to a polymer coating in SPME. As a result, the sensitivity of the method is greatly improved. Mass Transfer Considerations. When the porous layercoated metal wire is exposed to a water sample, the analytes will be adsorbed onto the surface of the porous layer. This process involves four basic steps: (1) Transfer of the analytes from the bulk solution to the outer surface of the porous layer coating by molecular and convective diffusion. When the fiber is immersed into the bulk solution, the analytes must diffuse through the water to a thin layer adjacent to the fiber before they can be absorbed. Under perfect agitation, the extraction kinetics are not affected by diffusion in the bulk solution. However, when magnetic stirring is used in practice, the extraction equilibrium time has some minor contribution from diffusion. (2) Transfer of the analytes from the outer surface of the coating into the interior of the coating by diffusion through the macropores between and inside the particles. Since the pore size of the silica that we used was quite large (approximately 100 Å), the diffusion process is very fast and has little effect on the adsorption kinetics. (3) Low-energy physical adsorption of the analytes at the surface of the organic bonded phase on the particles. We assume that there is no activation energy involved in the transfer of analytes between the solution and the coating; therefore, this step has a high rate constant and does not influence the kinetics. 192
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Figure 3. Effect of stirring rate on extraction equilibrium in SPME. Conditions: 0.1 ppm solution of toluene in water, 30 µm porous layer C-8 bonded phase. (9) 1200 rpm stirring rate, ([) 600 rpm stirring rate, and (2) unstirred.
(4) Diffusion of the analytes into the layer of the stationary phase. According to Tchapla et al.,17 the analytes are immersed in the stationary phase, which is similar to a partition process. However, the thickness of the bonded layer on the silica surface is below 20 Å, and thus, diffusion within this ultrathin layer is very fast and does not significantly influence the kinetics. Therefore, in contrast to SPME with polymer-coated fibers, which involves a diffusion-limited process,16 the kinetics of SPME with a porous layer are mainly controlled by mass transfer in the bulk solution. The extraction rate depends greatly on the degree of agitation rather than on film thickness. It can be predicted that, under perfect conditions of agitation, there is only a little mass transfer resistance, and the adsorption equilibrium can be achieved in a few seconds. Thermal desorption of the analytes in the GC injector is the reverse process of adsorption. Analytes which were adsorbed by the porous layer diffuse from the porous layer into the carrier gas. Since the mass transfer of analytes into the carrier gas is very fast, it should not influence the desorption kinetics. Furthermore, as discussed above, diffusion in the porous layer is a low-energy process and does not influence desorption. Therefore, there is no mass transfer resistance during the desorption process, and the desorption rate is very fast. Figure 3 shows the influence of degree of agitation on extraction equilibrium. A 0.1 ppm solution of toluene in water was used as a test sample. It was found that the equilibrium time depended greatly on the stirring rate. For an unstirred solution, extraction equilibrium could not be reached within 2 h. By increasing the stirring rate, it was found that the equilibrium time (17) Tchapla, A.; Heron, S.; Lessellier, E.; Colin, H. J. Chromatogr. 1993, 656, 81-112.
Figure 4. SPME desorption time profile using a C-8 bonded phase porous layer of different film thicknesses and pore sizes. Conditions: 0.1 ppm solution of toluene in water, 20 min extraction time, 1200 rpm sirring rate. (2) 30 µm porous layer with pore size of 300 Å, (b) 30 µm porous layer with pore size of 100 Å, and ([) 70 µm porous layer with pore size of 100 Å.
decreased accordingly. Under the highest stirring rate (1200 rpm), extraction equilibrium could be achieved in 8 min, compared to about 20 min to reach equilibrium at a stirring rate of 600 rpm. However, this is still a much slower process than expected from theoretical prediction (a few seconds). This is probably because the stirring rate is still not fast enough to achieve perfect mixing of the solution, and there is probably a stagnant layer close to the porous layer which decreases the mass transfer of analytes from the bulk solution to the porous layer surface. The desorption time profile is illustrated in Figure 4. As was predicted, the desorption process is very fast and can be completed within 10 s. There is no discernable difference between SPME thermal desorption and syringe injection. Therefore, when using porous layer SPME, a precolumn cryo-cooling system is not necessary to preserve the chromatographic efficiency.4 Mass transfer in the porous layer is a diffusion process through the micropores. Therefore, the porosity of the porous particles and the layer thickness should be factors that influence the extraction dynamics. If the extraction dynamics are micropore diffusion limited, the equilibrium time for adsorption should increase with decreasing pore size. Figures 3 and 4 compare extraction and desorption time profiles using porous layers with different pore sizes (100 and 300 Å). Clearly, both extraction equilibrium and desorption can be achieved in nearly the same time for both porous layers. Therefore, we can conclude that the pore sizes of typical silica bonded phases do not influence the sorption kinetics, and diffusion within the micropores of the porous layer is not a dynamically controlled process. However, the total mass extracted using a porous layer with 300 Å pore diameter is much lower than that possible with a 100 Å pore adsorbent. This is because the porous particles with 100 Å pore diameter have higher specific surface area (300 m2 g-1) than those with 300 Å pore diameter (150 m2 g-1). The higher the total surface area of the fiber, the more mass extracted. To investigate the influence of the porous layer thickness, 30 and 70 µm C-8 porous layers were compared for the extraction of toluene (0.1 ppm) in water. The desorption from both porous layers was very fast, and there is no discernable difference between their desorption rates (Figure 4). This result shows that there is no transfer resistance during diffusion in the porous layer. Furthermore, the extraction process is quite slow compared with desorption, because instantaneous mixing of the solution cannot
Figure 5. SPME adsorption time profile using a C-8 bonded phase porous layer with different film thicknesses and pore sizes. Conditions: 0.1 ppm solution of toluene in water, 1200 rpm stirring rate. (2) 70 µm porous layer with pore size of 100 Å, ([) 30 µm porous layer with pore size of 100 Å, and (9) 30 µm porous layer with pore size of 300 Å.
Figure 6. Extraction rate profiles for polymer-coated and porous layer SPME fibers. Conditions: 0.1 ppm solution of toluene in water, 20 min extraction time, 1200 rpm stirring rate. (2) 30 µm C-8 bonded phase porous layer and ([) 100 µm polysiloxane coating.
be achieved (Figure 5). This is supportive of the fact that the extraction process is dynamically controlled by diffusion in the bulk solution. However, there was a slight increase in equilibrium time when the thick porous layer was used. The reason for this is presently unknown, and further work is being done to investigate this phenomenon. The sorption kinetics of porous layer SPME were compared with those of conventional SPME with a polymer coated film. A commercial fiber containing a 100 µm polydimethylsiloxane film (Supelco) and 30 µm C-8 porous layer were used for extraction of BTX from a 0.1 ppm solution in water. To minimize the effect of diffusion in the bulk solution, the highest stirring rate (1200 rpm) was used. Figures 6 and 7 give a comparison of their adsorption and desorption time profiles. Desorption equilibrium can be reached in 10 s for the C-8 porous layer, while it takes approximately 2 min to reach equilibrium using the polydimethylsiloxane coating. This is because desorption from a polydimethylsiloxane coating is a diffusion-limited process. It takes time for analytes to migrate from the interior site to the surface of the coating. The rapid desorption from the porous layer makes it possible to achieve high efficiency and better quantitative results compared to those possible with the use of polymer-coated fibers. Adsorption equilibrium is easy to reach when using porous layer C-8 bonded silica coatings. It took 8 min with the porous layer C-8 coating to reach equilibrium, while it took more than 10 min with the polydimethylsiloxane-coated fiber. It should be Analytical Chemistry, Vol. 69, No. 2, January 15, 1997
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Figure 7. Desorption rate profiles for polymer-coated and porous layer SPME fibers. Conditions: 0.1 ppm solution of toluene in water, 1200 rpm stirring rate. (2) 30 µm C-8 bonded phase porous layer and ([) 100 µm polysiloxane coating.
noted that the equilibration time for polydimethylsiloxane coatings obtained in this study (10 min) is much longer than that reported by Louck et al. (100 s).16 This may be because the container used in this study (30 mL) was much larger than that used by Louck et al., which resulted in greater fluid convective transfer resistance and a decrease in adsorption rate. Extraction Efficiency. The extraction efficiency of SPME is dependent on the mass adsorbed by the coating, which can be expressed as
n)
KVsVaqC° KVs + Vaq
(4)
Figure 8. Gas chromatograms of BTX extracted using (A) 100 µm polydimethylsiloxane coated fiber and (B) 30 µm C-8 bonded phase porous layer fiber. Conditions: 0.1 ppm solution of BTX in water, 20 min extraction time, 1200 rpm stirring rate. Peak identification: (1) methanol, (2) benzene, (3) toluene, (4) m-xylene, and (5) o-xylene. Table 1. Peak Asymmetry Factors for Porous Layer and Polymer-Coated Fiber SPMEa tailing factor (Tf)
where n 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 analytes in the sample, and K is the distribution coefficient of the solute between the porous layer and the aqueous solution. Increasing the film thickness of the porous layer increases the volume of stationary phase and thus the mass adsorbed. Figure 4 shows that the mass adsorbed by the porous layer increased when the layer thickness was increased. Since the adsorption and desorption dynamics are not affected by film thickness, porous layer SPME has the advantage of improving the extraction efficiency by increasing the layer thickness without sacrificing performance. For example, approximately 40 ng of toluene can be extracted from a 0.1 ppm toluene solution in HPLC water using a 30 µm porous layer C-8 bonded phase, while only 5 ng of toluene can be extracted using a 100 µm polydimethylsiloxane-coated fiber under the same conditions. Even though the volume of the polydimethylsiloxane coating was more than 2 times higher than that of the C-8 porous layer, the amount adsorbed by the porous layer was 8 times higher than that of the polydimethylsiloxane film. This is because the surface area of the porous layer is very large due to the porosity of the silica bonded particles. The sample capacity of the porous layer is greater than that of the polydimethylsiloxane film, even though the volume of the coating is smaller. Therefore, the extraction efficiency of SPME is greatly improved by using porous layer coatings. Figure 8 shows chromatograms of BTX extracted using a C-8 bonded phase porous layer and a polydimethylsiloxane-coated fiber from a 0.1 ppm solution in water. The amount extracted using the polymer194 Analytical Chemistry, Vol. 69, No. 2, January 15, 1997
compound benzene toluene m-xylene o-xylene
porous layer SPME
polymer-coated SPME
1.31 1.17 1.14 1.21
1.67 1.78 1.81
a Conditions: 0.1 ppm solution of BTX in water was extracted using 30 µm porous layer C-8 bonded phase and 100 µm polydimethylsiloxane coatings for 20 min at a stirring rate of 1200 rpm.
coated fiber is much less than that extracted with the porous layer. Table 1 lists the peak asymmetry factors (Tf) measured at 10% of peak height18 for the compounds extracted using the two different coatings. The Tf values are higher for polymer-coated fiber SPME than for porous layer SPME, which indicates that peak tailing becomes more serious with the former. Selectivity. Adsorption of organic compounds from aqueous solution depends on the interaction between the analytes and the stationary phase in the porous layer. To perform a preliminary investigation of the influence of the stationary phase on adsorption, pure silica, C-8, and C-18 bonded silica phases were used for extraction of BTX from a 0.1 ppm solution in water. When a pure silica porous layer was used, it could only extract a small amount of methanol from the sample (Figure 9A). This is because the silanol groups on the silica particle surface impart a degree of polarity to the surface, so that polar compounds, such as water, alcohols, amines, etc., which can form hydrogen bonds with the (18) Poole, C. F.; Schuette, S. A. Contemporary Practice of Chromatography; Elsevier: Amsterdam, 1984; p 244.
Figure 10. Blank chromatograms for C-8 bonded phase porous layer SPME desorbed at (A) 300 and (B) 250 °C. Conditions: 30 m × 0.25 mm i.d. fused silica column coated with 0.25 µm PTE-5, 40 °C for 1 min, then heated at 3 °C min-1 to 150 °C and held for 10 min, 10 min desorption time in injector, 300 °C detector temperature.
Figure 9. Gas chromatograms of BTX extracted using porous layer fibers with different bonded phases. Conditions: 0.1 ppm solution of BTX in water, 20 min extraction time, 1200 rpm stirring rate. (A) 70 µm pure silica, (B) 70 µm C-8 bonded phase, and (C) 70 µm C-18 bonded phase. Peak identification: (1) methanol, (2) benzene, (3) toluene, (4) m-xylene, and (5) o-xylene. Table 2. Distribution Coefficients of Analytes on C-8 and C-18 Porous Layersa distribution coefficient (K) compound
C-8 bonded phase
C-18 bonded phase
benzene toluene m-xylene o-xylene
31.8 96.3 227.9 105.6
10.4 42.3 201.7 137.2
a
Distribution coefficients between the porous layer and water.
silanol groups are adsorbed in preference to nonpolar compounds. Therefore, it is difficult to use pure silica to extract nonpolar compounds from water. When the silica surface is chemically bonded with a C-18 or C-8 stationary phase, the surface becomes hydrophobic and has a strong tendency to adsorb nonpolar compounds from water (Figure 9B,C). Table 2 compares the distribution coefficients (K) of BTX between the porous layer and water for different stationary phases. The C-18 bonded phase has high selectivity for the substituted benzenes. The K value for xylene on the C-18 bonded phase is higher than the K value for benzene. This indicates that selectivity can be obtained in SPME
by using porous layers of various properties. Further work is underway to develop more selective porous layer SPME fibers. Stability. The mechanic strength of the fiber was greatly improved when using metal wire as the support. For polymercoated fiber SPME, the polymeric stationary phase is coated on silica fibers. Since silica fibers are fragile, caution must be taken to prevent them from breaking during stirring and injection. The porous layer-coated metal wires are very stable toward fast stirring and ultrasonication in water. We have used one C-8 bonded phase porous layer more than 400 times, and it is still stable and usable. The thermal stability of the porous layer also was evaluated. Figure 10 shows blank chromatograms at 250 and 300 °C. The porous layer can be operated indefinitely at 250 °C. However, when it was subjected to 300 °C, some extraneous peaks were observed. This is probably due to the release of some monomers in the epoxy glue used to bond the porous layer. Further work is needed to find a more thermally stable adhesive so that the porous layer coatings are stable above 300 °C. ACKNOWLEDGMENT This work was supported by the U.S. Environmental Protection Agency, Contract No. R 820-495-01-0, to the University of North Dakota, through a subcontract to Brigham Young University. The authors thank Steven B. Hawthorne and Janusz Pawliszyn for helpful discussions and Supelco for supplying the polydimethylsiloxane-coated fibers for comparison to the porous layer fibers in this study. Received for review August 6, 1996. Accepted October 17, 1996.X AC960791G X
Abstract published in Advance ACS Abstracts, December 1, 1996.
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