Anal. Chem. 2003, 75, 1002-1010
Thin-Film Microextraction Inge Bruheim, Xiaochuan Liu, and Janusz Pawliszyn*
Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1
The properties of a thin sheet of poly(dimethylsiloxane) (PDMS) membrane as an extraction phase were examined and compared to solid-phase microextraction (SPME) PDMS-coated fiber for application to semivolatile analytes in direct and headspace modes. This new PDMS extraction approach showed much higher extraction rates because of the larger surface area to extraction-phase volume ratio of the thin film. Unlike the coated rod formats of SPME using thick coatings, the high extraction rate of the membrane SPME technique allows larger amounts of analytes to be extracted within a short period of time. Therefore, higher extraction efficiency and sensitivity can be achieved without sacrificing analysis time. In direct membrane SPME extraction, a linear relationship was found between the initial rate of extraction and the surface area of the extraction phase. However, for headspace extraction, the rates were somewhat lower because of the resistance to analyte transport at the sample matrix/headspace barrier. It was found that the effect of this barrier could be reduced by increasing either agitation, temperature, or surface area of the sample matrix/headspace interface. A method for the determination of PAHs in spiked lake water samples was developed based on the membrane PDMS extraction coupled with GC/MS. A linearity of 0.9960 and detection limits in the low-ppt level were found. The reproducibility was found to vary from 2.8% to 10.7%. Solid-phase microextraction (SPME) is a sample preparation technique with many advantages over traditional analytical methods such as the convenient integration of extraction, preconcentration, and sample introduction.1 The technique is rapid, solventfree, and relatively inexpensive. In addition, it can be easily coupled on-line to gas chromatography (GC) and high-performance liquid chromatography (HPLC). Since its introduction, SPME has gained increasing acceptance in many areas, including applications in environmental, food, and drug analysis.1-5 * Corresponding author. Tel: +1-519-8851211. Fax: +1-519-7460435. E-mail:
[email protected]. (1) Pawliszyn, J., Ed. Solid-Phase Microextraction: Theory and Practice; WileyVCH: New York, 1997. (2) Pawliszyn, J., Ed. Applications of Solid-Phase Microextraction; The Royal Society of Chemistry: Hertfordshire, U.K., 1999. (3) Pawliszyn, J., Ed. Sampling and Sample Preparation for Field and Laboratory. Fundamental and New Directions in Sample Preparation; Elsevier: Amsterdam, 2002. (4) Lord, H.; Pawliszyn, J. J. Chromatogr., A 2000, 885, 153-193. (5) Smyth, W. F. TrAC, Trends Anal. Chem. 1999, 18, 335-346.
1002 Analytical Chemistry, Vol. 75, No. 4, February 15, 2003
To date, SPME is typically performed using a fused-silica fiber coated with a polymeric phase.1-5 The principle of SPME is based on the interactions of analytes between the sample matrix and the extraction phase (coating) via absorption or adsorption (depending on the nature of the coating). The extraction selectivity and efficiency of SPME mainly depend on the coating’s properties and size as well as its interactions with the analytes. For large sample volumes, the amount of analyte extracted, n, can be expressed by the following equation.1
n ) KesVeCs
(1)
where Kes is the analyte’s distribution constant between the extraction phase and sample matrix, Ve is the volume of the extraction phase, and Cs is the initial concentration of analytes in the matrix. Recently, efforts have been made to obtain high extraction efficiency for SPME, including the use of porous coatings for SPME characterized by high distribution constants6,7 or simply using a larger volume of extraction phase.8,9 New extraction materials, such as sol-gel10 and poly(pyrrole),11-13 have also been utilized, due to their porous extraction phases and multifunctional properties. Higher extraction selectivity than commercial SPME fibers has been achieved for some compounds. Since the amount of analyte extracted in SPME is proportional to the volume of the extraction phase (eq 1),1 the sensitivity of a method can be improved by increasing the volume of the extraction phase. An increase in the coating thickness can increase the volume of the extraction phase and therefore improve the sensitivity of the method as demonstrated by a coated stir bar8 and a coated multifibers method9 or by other SPME methods using thick coatings.1,2 However, much longer equilibration time (te) is required because the extraction rate is controlled by the diffusion from the sample matrix through the boundary layer to the extraction phase as illustrated by1
te ) t95% ) 3δKes(b - a)/Ds
(2)
where (b - a) is the coating’s thickness, Ds is the diffusion (6) Liu, Y.; Shen, Y.; Lee, M. L. Anal. Chem. 1997, 69, 190-195. (7) Liu, Y.; Lee, M. L.; Hageman, K. J.; Yang, Y.; Hawthorne, S. B. Anal. Chem. 1997, 69, 5001-5005. (8) Vercauteren, J.; Peres, C.; Devos, C.; Sandra, P.; Vanhaecke, F.; Moens, L. Anal. Chem. 2001, 73, 1509-1514. (9) Xia, X. R.; Leidy, R. B. Anal. Chem. 2001, 73, 2041-2047. (10) Chong, S. L.; Wang, D. X.; Hayes, J. D.; Wilhite, B. W.; Malik, A. Anal. Chem. 1997, 69, 3889-3898. (11) Wu, J.; Pawliszyn, J. Anal. Chem. 2001, 73, 55-63. (12) Wu, J.; Pawliszyn, J. J. Chromatogr., A 2001, 909, 37-52. (13) Wu, J.; Yu, X.; Lord, H.; Pawliszyn, J. Analyst 2000, 125, 391-394. 10.1021/ac026162q CCC: $25.00
© 2003 American Chemical Society Published on Web 01/23/2003
Figure 1. Drawing of the headspace membrane SPME system. 1. Deactivated stainless steel rod. 2. Flat sheet membrane. 3. Sample solution. 4. Teflon-coated stirring bar. 5. Rolled membrane. 6. Injector nut. 7. Rolled membrane. 8. Glass liner. 9. Capillary column.
coefficient of the analyte in the sample matrix, and δ is the thickness of the boundary layer surrounding the extraction phase. In addition, it has been emphasized recently that the initial rate of SPME extraction is proportional to the surface area of the extraction phase:14
dn/dt ) (DsA/δ)Cs
(3)
where n is the mass of analyte extracted over the sampling time t and A is the surface area of the extraction phase. Therefore, the ideal way to increase the volume of the extraction phase and thus the sensitivity of the method is to use a thin extraction phase with a large surface area. In other words, the extraction phase should have a large surface area-to-volume ratio. This results in enhanced sensitivity without sacrificing analysis time. In this study, a thin sheet of poly(dimethylsiloxane) (PDMS) membrane was employed as an extraction phase and compared to PDMS-coated fiber geometry. The operating principles of the membrane in both direct and headspace extraction mode have been investigated. EXPERIMENTAL SECTION Chemicals and Materials. Naphthalene, acenaphthylene (ACEY), acenaphthene (ACE), fluorene, anthracene, fluoranthene, and pyrene were all purchased from Supelco (Bellefonte, PA). Acetone (HPLC grade) and methanol (HPLC grade) were both acquired from Fisher Scientific (Springfield, NJ). Helium (99.999%) was obtained from Praxair (Waterloo, ON, Canada). Water was obtained from a Barnstead/Thermodyne NANO-pure ultrapure water system (Dubuque, IA). The 100-µm PDMS SPME fiber and the fiber holder were obtained from Supelco (Oakville, ON, Canada). The cross-linked PDMS membrane (SSP-M100, 0.001 in. thick) was obtained from Specialty Silicone Products Inc. (Ballston Spa, NY). Prior to use, the membrane was conditioned for ∼2 h at 250 °C in a GC injection port. Deactivated stainless steel rod with silicosteel treatment (55 mm in length, 0.56 mm in diameter) was obtained from Restek (Bellefonte, PA). Cleaning Procedure. All glass vials and glass jars were silanized and prior to each use were cleaned with pure water, methanol, and acetone in sequence followed by drying at 105 °C. (14) Koziel, J.; Jia, M.; Pawliszyn, J. Anal. Chem. 2000, 72, 5178-5186.
Instrumentation. Chromatographic analysis was carried out on a GC (3800) coupled to ion trap MS (Saturn 2000) obtained from Varian (Mississauga, ON, Canada). The GC was equipped with a glass insert (54 mm × 5.0 mm × 3.4 mm i.d.) obtained from Varian and a fused-silica capillary column (Rtx-5, 30 m × 0.25 mm, 0.25-µm film thickness) obtained from Restek Corp.. The column temperature program used was as follows: 35 °C (hold 2 min) and then 10 °C/min to 250 °C (hold 6.5 min). The injector was operated in the splitless mode. During membrane desorption, an injector temperature program was used as follows: initial temperature 35 °C (hold 0.1 min) and then 200 °C/ min to 250 °C. The temperatures for the transfer line, the manifold, and the ion trap were set at 260, 50, and 150 °C, respectively. The analytes were inonized by electron ionization (70 eV), and results were recorded in scanning mode in the range m/z ) 120170 for the first 18 min and m/z ) 170-210 for 18-30 min. The MS was tuned and calibrated using perfluorotributylamine. Membrane Extraction Procedures. Figure 1 shows the experimental setup of the membrane SPME experiments. A thin sheet of PDMS membrane was attached to a deactivated stainless steel rod and positioned inside the sample container as shown in Figure 1a. In both direct and headspace extraction, care was taken to ensure that the membrane was shaped like a flag (Figure 1a). The samples were stirred at either 1250 revolutions/min (rpm) (length of stir bar, 24 mm) or at 600 rpm (length of stir bar, 50 mm). After the extraction, the membrane was rolled around the rod (Figure 1b) and finally placed in the GC injector (positioned in the center of the liner) for immediate thermal desorption (Figure 1c). Membrane blanks were checked periodically between the analyses to confirm the absence of any contaminant or carryover on the membrane. The detection limits (S/N ) 3) were determined by extracting from a sample solution containing 25 pg/mL. RESULTS AND DISCUSSION PAHs were used as model compounds in this study since they are semivolatile chemicals of environmental significance and easily separated by GC. The following compounds were chosen in this work: naphthalene, ACEY, ACE, fluorene, anthracene, fluoranthene, and pyrene to cover a range of volatilities and KOW. The KOW value is the compound’s partition coefficient between water and octanol and is the most accepted indicator of a compound’s hydrophobicity. The KOW values along with other physiochemical Analytical Chemistry, Vol. 75, No. 4, February 15, 2003
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Table 1. Physicochemical Properties of the Selected PAHs Studied
a
property
naphthalene
ACEY
ACE
fluorene
anthracene
fluoranthene
pyrene
Tboil (°C) WSa (g/m3) Log Kow
218 30.2 3.45
270 3.93 4.08
278 3.93 4.22
295 1.90 4.38
340 0.076 4.54
383 0.260 5.20
393 0.135 5.30
WS, water solubility.
Table 2. Amount Extracted by Direct Membrane (1 cm × 1 cm) SPME (nm) Relative to Direct Fiber (100-µm PDMS) SPME (nf) Using a Large Sample Volume (1 L)a amount ratio (nm/nf) time (min) 1 3 45 a
naphthalene 8 7 5
ACEY 14 10 6
ACE 16 11 6
fluorene
anthracene
fluoranthene
pyrene
19 11 5
n/db
n/d 18 8
n/d 20 8
16 8
The stirring was performed at 600 rpm. bn/d, not detected.
properties15 of the PAHs are listed in Table 1. However, working with semivolatile compounds there is always a risk of their loss to the surrounding environment. This is most likely to happen when the thin-film membrane is transferred from the extraction vial to the GC injector port. To estimate potential error, investigations were performed to measure amount of analyte loss during the transport from the sample vial to the GC injector port. The results indicated that there were no measurable losses from the membrane during rapid transfer as the amount detected did not change, even for naphthalene, when the transfer time was increased from 30 to 120 s. The average transfer time varied from 30 to 45 s; therefore, loss of analytes from the membrane during membrane transfer to the injector port can be neglected. Direct Extraction. According to eq 3, the extraction rate (dn/dt) is expected to be directly proportional to the surface area (A) of the extraction phase. The surface area of a 1 cm × 1 cm membrane sheet is 200 mm2, which is ∼20 times that of a 100µm PDMS fiber (A ) 10 mm2). Direct extraction of seven PAHs (4 ng/mL) from a 1-L sample solution (stirred at 600 rpm) were performed by both fiber SPME and membrane SPME. After 3 min, the amounts extracted by the membrane were 18 and 20 (fluoranthene and pyrene, respectively) times higher than the 100µm PDMS fiber. For the other PAHs, the values varied from 7 to 16 and increased with molecular weight (Table 2). By reducing the extraction time to only 1 min, the ratios increased to 8, 14, 16, and 19 for naphthalene, ACE, ACEY, and fluorene, respectively. Indeed, these results show clearly that at initial stages of extraction the extraction rate is proportional to the surface area as predicted by the theory. The results illustrate that the further away a given compound is from its equilibrium conditions the higher its extraction rate. The rate slows as the extraction progresses since equilibrium is reached at almost the same time for both membrane and fiber SPME (compare Figure 2A and B). According to eq 1, the amount extracted at equilibrium is proportional to the extraction-phase volume. The extraction phase volume of a 1 cm × 1 cm membrane sheet (Ve = 2.55 mm3) is ∼4.2 times larger (15) Huckins, J. N.; Petty, J. D.; Orazio, C. E.; Lebo, J. A.; Clark, R. C.; Gibson, V. L.; Gala, W. R.; Echols, K. R. Environ. Sci. Technol. 1999, 33, 39183923.
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than that of a 100-µm PDMS fiber (Ve = 0.61 mm3). After 45 min of extraction, the amount extracted by the membrane compared to the fiber was 5-6 for all the compounds that had reached equilibrium (naphthalene, ACE, ACEY, fluorene), and 8 for the compounds not at equilibrium (anthracene, fluoranthene, pyrene) (Table 2). Indeed, the benefit of direct membrane SPME is not only the enhancement in surface area but also in extraction-phase volume. Combined, these advantages lead to enhanced sensitivity without the sacrifice of analysis time. Equation 2 indicates that the initial rate of extraction is controlled by the rate of diffusion through the boundary layer; therefore, parameters that effect diffusion coefficient will impact the extraction kinetics. This was confirmed by membrane SPME at two different temperatures (28 and 50 °C) and two different stirring speed (1200 and 600 rpm) (Figure 3). The seven PAHs (7 ng/mL) were extracted for 10 min from a sample volume of 40 mL using a 1 cm × 1 cm PDMS membrane. For all compounds except naphthalene and ACEY, a higher extraction rate was found at 50 °C than at 28 °C as expected. The reason for this change is that the rate of diffusion increases with temperature. Furthermore, the effect becomes stronger for high molecular weight PAHs with low diffusion coefficients and high partition coefficients. However, for naphthalene and ACEY, a decrease in amount extracted after 10 min was found. The extraction time profile for naphthalene and ACEY shows that the compounds are at or close to equilibrium after 10 min (see Figure 2A) and the amount extracted is now given by eq 1. Since the partition coefficient decreases with increasing temperature, a decrease in amount extracted for naphthalene and ACEY was expected. In addition, enhanced stirring improves the amount extracted for all compounds except naphthalene and ACEY. Again, the reason is that naphthalene and ACEY have already reached equilibrium after 10 min at low agitation conditions. The improved stirring reduces the boundary layer thickness (δ) and therefore according to eq 2 reduces the time needed to reach equilibrium. The results in Figure 3 show that, for direct membrane SPME, the initial rate of extraction is controlled by the rate of diffusion in the boundary layer (in agreement with eq 2). As expected, the largest effects of temperature and stirring enhancements on extraction rates were found
Figure 2. Extraction time profiles for PAHs. (A) Direct membrane extraction (1 cm × 1 cm). (B) Direct fiber extraction (100-µm PDMS). Extractions were performed from a 1-L aqueous sample solution spiked with each PAH in the range 2-4 ng/mL. Stirring was performed at a rate of 600 rpm.
for the compounds most distant from equilibrium such as anthracene, fluranthene, and pyrene. Headspace Extraction: Investigation of Rates. The presence of the gaseous phase makes the process of headspace extraction more convoluted compared to direct extraction. During headspace extraction, the first stage corresponds to extraction of analytes from the gaseous phase only. As soon as the headspace concentration of the analytes falls below the equilibrium level with respect to the aqueous-phase concentration, the analytes are transported from the liquid sample to the headspace. On the other hand, if the amount extracted is negligible compared to what is present in the headspace, only a very small fraction of the analyte has to be transported from the sample to the headspace phase and the process can be very fast. However, in the case of heavier PAH compounds, the amount in the headspace is expected to be very small compared to the extraction equilibrium amount. This is particularly true for high-volume membrane extraction. To investigate the extraction process, the extraction time profile for the seven PAHs were determined using both a 1 cm × 2 cm membrane (Figure 4A) and a 100-µm PDMS fiber (Figure 4B). Clearly, the extraction time profiles for the membrane and the fiber look similar in that most analytes reached the equilibrium
at roughly the same time scale. The difference between the two profiles lies in the amount of analyte extracted at a certain time before reaching the equilibrium, i.e., the difference in the extraction rates. The membrane SPME technique obtained, as it did in direct extraction, much higher extraction rates compared to the fiber SPME approach. After 60 min of extraction, the membrane obtained a rate of extraction around 5-6 times higher than the fiber for fluorene, anthracene, fluoranthene, and pyrene. Although the rate slows down closer to equilibrium, it was expected to be higher if it was proportional to the surface area (predicted by eq 3 and shown for direct extraction). The surface area of a 1 cm × 2 cm membrane sheet is 400 mm2, which is ∼40 times that of a 100-µm PDMS fiber (∼10 mm2). In direct extraction, there exists only one barrier to mass transport, namely, the sample/extraction-phase interface. In headspace extraction, both the headspace/extraction phase and the sample/headspace interfaces are barriers to mass transport. For the membrane, both barriers impact the extraction rate; however, due to the small surface area of the fiber, the headspace/extraction-phase interface is a more significant barrier for mass transfer in the fiber technique. Thus, the result is an increase in rate of extraction for the membrane compared to the fiber (Figure 4A and B). Analytical Chemistry, Vol. 75, No. 4, February 15, 2003
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Figure 3. Effects of temperature and stirring speed on extraction rates in direct membrane extraction (1 cm × 1 cm). Extractions were performed for 10 min from a 40-mL aqueous sample solution spiked with each PAH in the range 4-8 ng/mL. Stirring was performed at a rate of either 600 or 1250 rpm.
Figure 4. Extraction time profiles for PAHs. (A) Headspace membrane extraction (1 cm × 2 cm). (B) Headspace fiber extraction (100-µm PDMS). Extractions were performed from a 40-mL vial containing 20 mL of aqueous sample solution spiked with each PAH at ∼4 ng/mL. Stirring was performed at a rate of 1250 rpm.
Furthermore, if agitation is reduced, the sample/headspace interface becomes the dominating barrier for both techniques. This was confirmed by comparing the rates of extraction obtained by the membrane and the fiber for PAHs in static headspace 1006
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extraction. The results showed that the membrane (1 cm × 1 cm)/ 100-µm PDMS fiber extraction rate ratio dropped to 2 and below for heavier hydrocarbons under static conditions. The sample/ headspace barrier to mass transport in headspace membrane
Figure 5. Effect of temperature on extraction rates in headspace membrane (1 cm × 1 cm) SPME. Extractions were performed for 60 min from a 40-mL vial containing 20 mL of aqueous sample solution spiked with each PAH at a level of 2-4 ng/mL. Stirring was performed at a rate of 1250 rpm.
extraction (with high agitation) was investigated even further. By increasing the surface area of the barrier by a factor of 6, the extraction rate increased by a factor of 1.3. It should be emphasized that the stirring efficiency decreased for larger vessel diameter; therefore, rate increase is expected to be more significant if the agitation conditions are kept constant. In addition, the effect of temperature on extraction rate was investigated (Figure 5). For the compounds that had not reached equilibrium (fluorene, anthracene, fluoranthene, pyrene), the rate of extraction increased at higher temperature. It was also found that the relative increase in extraction rate increased with molecular weight of the PAH. Clearly, as the rate of diffusion increases with increasing temperature, the effect of the sample/headspace barrier becomes less. For the other PAHs, which are at equilibrium, the amount extracted has decreased because the partition coefficients were decreased. Headspace Extraction: Determination of Extraction Efficiency. The extraction efficiency can be evaluated from the amount extracted under the same conditions, which is determined by several parameters including the volume of the extraction phase and the extraction phase/sample matrix partition coefficient. For headspace SPME in a three-phase system (liquid sample/ headspace/extraction phase), the amount of analyte extracted by a coating based on absorption can be expressed by the following equation:16
n)
KesCsVsVe KesVe + KhsVhs + Vs
(4)
where n is the amount of analytes extracted, Ve is the volume of the extraction phase, Vs is the sample volume, Khs is the headspace/liquid sample partition coefficient, and Vhs is the volume of headspace. Semivolatile compounds have much lower values of Khs; therefore, the KhsVhs term can be negligibly small and eq 4 can be simplified to
n)
KesCsVsVe KesVe + Vs
(5)
However, for volatile compounds such as naphthalene, this (16) Gorecki, T.; Pawliszyn, J. Analyst 1997, 122, 1079-1086.
simplification is not always true. The extraction efficiency in headspace was compared between a 100-µm PDMS fiber and two pieces of PDMS membranes of different size (1 cm × 1 and 1 cm × 2 cm) (Figure 6). As illustrated in Figure 6, the amount of analytes extracted by membrane SPME was as expected much higher than that obtained by fiber SPME due to the larger extraction-phase volume (Ve). The volume of a 1 cm × 2 cm membrane sheet (Ve = 5.1 mm3) is more than 8 times larger than that of a 100-µm PDMS fiber (Ve = 0.61 mm3). However, the amount of analytes extracted by the 1 cm × 2 cm membrane was only 5-7 times larger than that obtained by the 100-µm PDMS fiber and did not double the amount obtained by the 1 × 1 cm membrane. This difference can be easily understood when the small sample volume and relatively large membrane volume used for extraction is considered. According to eq 5, only when the Ve is negligibly smaller than Vs, can the amount of analyte extracted (n) be directly proportional to the Ve. Due to the larger extraction phase to sample volume ratio in this work, KesVe in the denominator of eq 5 could not be neglected; thus, the amount of analyte extracted (n) was not linearly proportional to the volume of the extraction phase. For the same reasons, the concentration of the analyte left in the vial decreased with extraction time (was depleted). At equilibrium, the membrane (1 cm × 2 cm) had extracted 30% of the ACEY in the sample, far more than the 100µm PDMS fiber which had only extracted 5%. To minimize this depletion effect and increase the amount extracted, larger sample volumes should be used. The minimum volume of the sample required for achieving less than 1% of the initial concentration depleted can be estimated16
Vs g 100KesVe
(6)
For a 1 cm × 1 cm membrane sheet, Ve = 2.5 mm3 ) 2.5 µL; thus, Vs g 0.25Kes (mL). For a compound with a Kes value as low as 2000, the minimum sample volume required to obtain a linear relationship between n and Cs or between n and Ve was 500 mL. The extraction efficiency for the membrane (1 cm × 1 cm) and the 100-µm PDMS fiber was compared using sample and headspace volumes of 500 mL (total 1000 mL). The results show that, for the compounds reaching equilibrium (naphthalene, ACE, ACEY), the improvement in extraction efficiency was equal to the increase in extraction phase volume (which is 4.2 with a 1 cm × Analytical Chemistry, Vol. 75, No. 4, February 15, 2003
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Figure 6. Compison of the headspace extraction efficiency for 100-µm PDMS fiber, 1 cm × 1 and 1 cm × 2 cm PDMS membranes. Experimental conditions and concentrations were the same as in Figure 4, except the extraction time was 60 min.
Figure 7. Effect of agitation rates on extraction efficiency of membrane SPME. Other experimental conditions were as for Figure 4.
1 cm membrane). The depletion using a 500-mL sample volume instead of 20 mL, was reduced from 12%, 30%, and 46% to 0.3%, 0.9%, and 1.1% for naphthalene, ACE, and ACEY, respectively (as predicted by eq 6). Clearly, a reduction in depletion resulted in increased amount extracted in headspace SPME. Since partition coefficients of semivolatile compounds are usually high (Kes > 1000 for the compounds studied in this work17), sample volumes have a significant effect on the amount of analyte extracted when small sample volumes are used. It is also intuitive from eq 6 that this effect is larger for compounds with larger Kes values, which is in good agreement with the experimental data obtained in this work. However, it was impossible to obtain the same level of agitation for the large sample vial (500 mL) as the small vial (20 mL). With a reduced agitation, mass transfer through the sample matrix/headspace interface increases and the rates of extraction decrease, leading to longer equilibration times. (17) Shurmer, B.; Pawliszyn, J. Anal. Chem. 2000, 72, 3660-3664.
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The effect of agitation in headspace membrane SPME is important and was investigated further. Figure 7 illustrates the dependence of the amount extracted over 60 min at different stirring rates. For naphthalene, the extracted mass increased initially with the increase in stirring rate and then leveled off at ∼500 rpm since the compound reached equilibrium at these conditions. Similar behavior was observed for ACEY and ACE at higher agitation. For heavier PAH compounds, the extraction amount increased throughout the agitation range since the equilibrium was not reached for them at any of the conditions used. The highest stirring rate of 1250 rpm was selected in the application study described below to achieve the best extraction efficiency. Application of Headspace Membrane SPME. Lake water samples were collected from a pond in a nearby park and spiked with PAHs (1 ng/mL). Both headspace fiber SPME and headspace membrane (1 cm × 2 cm) SPME extractions were performed for
Figure 8. Reconstructed ion chromatograms obtained by SPME-GC/MS for the lake water samples: (a) membrane blank; (b) nonspiked water sample extracted by a piece of 1 cm × 2 cm membrane; (c) spiked water sample extracted by a 100-µm PDMS fiber; (d) spiked water sample extracted by a piece of 1 cm × 2 cm membrane. Peak identification: (1) naphthalene, (2) acenaphthylene, (3) acenaphthene, (4) fluorene, (5) anthracene, (6) fluoranthene, and (7) pyrene. The concentration of each analyte in the spiked water samples was 1 ng/mL; the extraction time was 60 min, and other experimental conditions were as for Figure 4.
60 min in order to obtain high sensitivity. A sample size of 10 mL was used and placed in a 40-mL vial, which was then stirred at 1250 rpm in order to obtain maximum agitation. This sample size was chosen because it was found more practical than 500 mL in 1-L jars. The chromatograms of fiber SPME (c) and membrane SPME (d) are compared In Figure 8. Results clearly show that membrane SPME is more sensitive technique. In Figure 8, the chromatograms of membrane blank (a) and membrane SPME of nonspiked lake water (b) were added for comparison. The blank generated by the membrane is low, indicating that this material is acceptable for trace analysis. Using membrane SPME, the analyte recoveries from the spiked lake water samples were found between 82% and 115% relative to the amount extracted from the pure water samples. The linearity was investigated over a range from 0.1 to 10 ng/mL and found to be good (Table 3). Furthermore, the reproducibility was investigated and found to be at an acceptable level (Table 3). The relative standard deviation (RSD) (n ) 7) varied from 3.3% to 10.7%. The variations among different pieces of membranes of the same size were from 3.9% to 13.6% (Table 3). However, compared to commercially available SPME fibers, the precision obtained with the membrane was worse.2 The detection limits were determined as well and found to be in the low-ppt area (Table 3).
CONCLUSIONS The capability and potential application of a thin sheet of PDMS membrane for SPME have been demonstrated in this study. The main advantage of the thin-membrane SPME approach over the thick-phase-coated stir bar or fiber SPME methods is that high extraction efficiency and thus high sensitivity can be achieved
Table 3. Linearity, Precision, and Detection Limits for PAHs Studied by the Headspace Membrane SPME-GC/ MS Methoda
compound
correlation coefficient (r)
detection limit (pg/mL)
naphthalene ACEY ACE fluorene anthracene flroranthene pyrene
0.9998 0.9998 0.9999 0.9999 0.9989 0.9985 0.9972
11 4.5 2.5 3.4 4.8 14 19
% RSD (n)7) one seven membrane membranes 9.9 3.3 2.8 3.5 8.7 9.7 10.7
9.0 10.3 11.9 9.0 3.9 11.1 13.6
a Results are obtained from the extraction of 10-mL samples containing 10 ng/mL each analyte in 40-mL vials with 1 cm × 2 cm membrane. The extraction time was 60 min, and stirring was performed at a rate of 1250 rpm.
without sacrificing the overall analysis time. This is due to the larger surface area to extraction-phase volume ratio. This work represents the first example of using a thin PDMS film for microextraction, which has resulted in promising application for analysis of semivolatile compounds in water samples. The advantages of this SPME approach can be explored further when new SPME devices are designed with optimized surface area-to-volume ratio of the extraction phase. For example, an automated SPME device can be designed by using folded membrane, folded stirrer, or a thin-film-coated multifiber with the configuration of a brush. In addition, with the development of material science and membrane engineering technology, new membrane materials can be used to achieve higher selectivity and sensitivity. Using large Analytical Chemistry, Vol. 75, No. 4, February 15, 2003
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surface area, thin-film sorbents, and small enough sample volume, rapid quantitative extraction is possible. Results in Figure 4 show linear uptake over a long period time for heavier PAHs, indicating that headspace extraction might be a suitable tool for timeweighted average measurement of flowing streams. Compared with the standard fiber SPME technique, the major drawback of the membrane approaches discussed above, as with any other larger volume extraction-phase technique, is the increased complexity of introduction and desorption in the analytical instrument. This can be addressed to some extent by developing a dedicated large-volume injection system.
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ACKNOWLEDGMENT The authors thank the Natural Sciences and Engineering Research Council of Canada (NSERC), the Norwegian Research Council, the Norwegian Institute for Water Research, Varian, Restek, and Leap Technologies for financial support. The work was presented at 2002 Pittsburgh Conference, New Orleans, LA, paper 1264. Received for review September 23, 2002. Accepted December 15, 2002. AC026162Q
