Anal. Chem. 2000, 72, 1058-1063
Membrane Extraction with a Sorbent Interface for Headspace Monitoring of Aqueous Samples Using a Cap Sampling Device Yu Z. Luo and J. Pawliszyn*
The Guelph-Waterloo Center for Graduate Work in Chemistry and the Waterloo Center for Groundwater Research, Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1
A cap-shaped device was employed for headspace sampling. This sampling device coupled to membrane extraction with a sorbent interface (MESI) is intended to perform on-site and on-line aqueous sample monitoring. A laboratory sampling test was performed both at the water surface and under water, and it showed some advantages in underwater monitoring. A group of volatile organic compounds (VOCs), varying in PDMS/gas and gas/water distribution constants, benzene, toluene, ethylbenzene, o-xylene, and trichloroethylene (TCE), was used for the sampling study. Magnetic stirring of the sample and circulation of the headspace air with a microfan were used for the enhancement of mass transfer between sample matrix and membrane to obtain higher extraction rate and efficiency. The agitation approaches were investigated individually and compared. The results showed that simultaneous agitation in water and air could greatly improve the extraction efficiency. Good linearity and precision and low detection limits were obtained for watersurface monitoring. The study demonstrated that CapMESI is a useful tool for field headspace monitoring of volatile organic compounds. Sampling and sample preparation play important roles in analytical chemistry. Although the development of modern analytical instruments allows great enhancement in many aspects of analysis, sampling and sample preparation are still often a bottleneck for overall throughput because the steps involved are time-consuming. There is also the possibility of contamination and sample loss. Recently, membrane extraction with a sorbent interface (MESI)1-5 has been introduced for sampling and continuous on-line monitoring. The membrane extraction consists of two simultaneous steps: extraction of analytes from the sample matrix by the membrane probe and stripping of analytes from the other side of the membrane by a flowing stripping gas, liquid, * Author for correspondence: (phone) 519-888-4641; (fax) 519-746-0435; (e-mail)
[email protected]. (1) Pawliszyn, J. Solid-Phase Microextraction, Theory and Practice; John Wiley and Sons: New York, 1997. (2) Pratt, K. F.; Pawliszyn, J. Anal. Chem. 1992, 64, 2101-2106. (3) Pratt, K. F.; Pawliszyn, J. Anal. Chem. 1992, 64, 2107-2110. (4) Mitra, S.; Zhang, L.; Zhu, N.; Guo, X. J. Chromatogr. 1996, 736, 165-173. (5) Yang, M. J.; Harms, S.; Luo, Y. Z.; Pawliszyn, J. Anal. Chem. 1994, 66, 1339-1346.
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or vacuum. The membrane is a selective barriersit allows some analytes and prevents others from passing through the membrane wall to the analytical system. Therefore, a purified sample is introduced into the chromatographic or MS system, which contributes to a longer life for the instrument and results in less interference from the sample matrix. Among the membranes available, the nonporous membrane plays an important role in the extraction of volatile organic compounds (VOCs) because of its selectivity. Transport through nonporous membrane occurs by a solution-diffusion mechanism, and selectivity is achieved either by differences in solubility and/or diffusivity,6 which means analytes with a large partition coefficient (membrane/sample matrix) and a large diffusion coefficient (in the membrane) are preferentially extracted. Membrane extraction in air, aqueous phase, and headspace has been investigated, and the corresponding mathematical models can be used for describing the extraction process.7-9 Headspace extraction precludes the membrane contacting the aqueous matrix and keeps the membrane probe clean to ensure good performance. On the other hand, it requires volatile compounds to efficiently distribute into the headspace to be extracted. In highly efficient headspace membrane extraction, the membrane/air partition coefficient is greater than the membrane/ water coefficient. In this paper, a powder funnel was modified as an extraction cap and coupled to MESI (Cap-MESI) to constitute a device capable of performing on-site and on-line headspace monitoring. The cap was placed not only on the water surface but also underwater to carry out headspace extraction. Underwater onsite monitoring is an important environmental concern. It can better depict the background or contamination at different depths of water and help the scientist understand the transport and conversion of contaminates at different water levels and depths. Mass transfer is an important issue in the extraction. Since headspace membrane extraction involves mass-transfer processes from the aqueous phase to the headspace and from the headspace to the membrane surface, agitation in both the water and the headspace is important. For headspace agitation, a microfan was (6) Mulder, M. Basic Principles of Membrane Technology; Kluwer Academic Publishers: Dordrecht, Germany, 1991. (7) Luo, Y. Z.; Adams, M.; Pawliszyn, J. Analyst (Cambridge, U.K. 1997, 122, 1461-1469. (8) Luo, Y. Z.; Adams, M.; Pawliszyn, J. Anal. Chem. 1998, 70, 248-254. (9) Yang, M.; Adams, M.; Pawliszyn, J. Anal. Chem. 1996, 68, 2782-2789. 10.1021/ac990747b CCC: $19.00
© 2000 American Chemical Society Published on Web 02/03/2000
Figure 2. Schematic of Cap design.
Figure 1. Schematic of the MESI system.
used to circulate the air in the headspace. For water agitation, magnetic stirring was used. Both static and agitation extraction, as well as different agitation methods, were experimentally investigated. A group of volatile organic compoundssbenzene, toluene, ethylbenzene, o-xylene and trichloroethyleneswere employed for the sampling study and the test of linearity, precision, and detection limit. Selection of the compounds has been made to consider both aromatic and chlorinated hydrocarbons varying in octanol-water distribution constants (benzene versus substituted benzene) and Henry constants (chlorinated versus aromatic hydrocarbons). These parameters are important in defining the mass-transfer rates in the system. EXPERIMENTAL SECTION An MESI system is composed of four major sections. They are the membrane extraction probe, the sorbent interface, the GC instrument, and the computer. Figure 1 depicts the schematic of the system. In this study, a 4-cm-long hollow fiber silicone membrane (Baxter Healthcare Corporation, McGaw Park, IL) was used as the extraction probe. The inner diameter of the hollow membrane was 305 µm, and the wall thickness was 165 µm. The sorbent interface contains three parts:10 the polymer sorbent, a pulse heating device, and a cooling device. The polymer sorbent was a 1-cm-long poly(dimethylsiloxane) (PDMS) coated fused silica fiber (Supelco Canada, Mississauga, ON). The fiber was located in a 6-cm-long, 0.53-mm i.d. deactivated fused silica capillary tube (Supelco Canada, Mississauga, ON). On the outside of the tubing, a 2-cm heating coil (Ni-Cr wire, 20% Cr, 0.1-mm diameter, 40 cm in length, 47 Ω total resistance, (Johnson Matthey Metals Ltd., Brampton, Ontario, Canada) was tightly wrapped. The coil covered the entire region where the fiber was located. The tubing with the coil was placed on the top plate of a three-stage semiconductive cooler (top plate: 2 × 2 cm in size, Melcor Materials Electronic Products Corporation, Trenton, NJ). The hot surface of the cooler contacted with an aluminum heat sink with thermal grease to facilitate heat release. A 2 × 2 × 0.4 cm aluminum plate with a slot through the center of the plate was used to cover the tubing. The slot was approximately 3-mm wide and 2-mm deep to allow the tubing to fit into the plate. Finally, a (10) Luo, Y. Z.; Yang, M. J.; Pawliszyn, J. J. High Resolut. Chromatogr. 1995, 18, 727-731.
6 × 6 × 3 cm piece of Styrofoam was used to cover the cooling device to provide insulation. A 13- V dc power supply was used to maintain the cooler at -40 °C. The sorbent interface was placed just before the GC injector. The GC injector was modified. The inlet end of the column was pierced through the septum and extended to the sorbent trap. The extraction cap is shown in Figure 2. The cap was modified from a powder funnel (diameter 65 mm, Nalgene). The neck of the funnel was tightly sealed with a Teflon cap and the membrane probe supported by two pieces of silica tubing positioned inside the cap. To agitate the headspace, a CPU microfan (0.5 w, 5 v, Sunon, Taiwan) with dimensions of 1 × 1 × 0.5 cm was positioned in the cap. The microfan was supported by two steel stainless needles attached to the Teflon cap. The microfan was powered by a 5 V dc battery. A Varian model 3500 GC (Varian Canada Inc., Mississauga, ON) equipped with a flame ionization detector (FID) was operated isothermally with a column temperature of 40 °C. A SPB-5 column, 5 m × 0.32 mm i.d., with a stationary phase thickness of 1.0 µm, (Supelco Canada, Mississauga, ON) was used. Nitrogen was the carrier gas, and the flow rate was 3.6 mL/min. The FID was maintained at 250 °C at attenuation 8, range 12. A computer was used for the control of the pulse heating for the sorbent interface and the data acquisition.11 For the pulse heating, the computer sent a series of electric pulses of a preset duration to the solid-state relay, which converted the pulses to more powerful electrical current pulses, which then passed through the heating coil around the trap. The first pulse at time 0 cleared the trap. The following pulses, each after an equal trapping period, were sent to desorb all analytes into the carrier stream for GC analysis. The second pulse also started a computer program for real-time GC detector signal collection and display on the computer monitor. The cycle of trapping and desorption was repeated automatically for continuous monitoring. Benzene, toluene, ethylbenzene, o-xylene, and trichloroethylene (TCE) were purchased from Sigma-Aldrich (Mississauga, ON, Canada). Nitrogen (UHP), compressed air (zero gas), and hydrogen (UHP) gases for flame ionization detection were purchased from Praxair (Waterloo, ON, Canada). Samples were prepared with purified deionized water from a Barnstead water filtration system (Barnstead, Dubuque, Iowa) and with 99% pure standards of benzene, toluene, ethylbenzene, o-xylene, trichloroethylene (TCE). Aqueous standards were prepared at parts per (11) Yang, M. J.; Luo, Y. Z.; Pawliszyn, J. CHEMTECH 1994, 24, 31-37.
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Figure 3. Chromatograms of a group of VOCs by continuous monitoring with water-surface headspace extraction. (1) Benzene, (2) Trichloroethylene, (3) Toluene, (4) Ethylbenzene, (5) o-xylene. Table 1. Comparison of Amount of Toluene Extracted (Peak Height) in Direct Water and Headspace Extraction (Average of Three Measurements) time (min)
4
8
12
16
20
24
28
32
direct water extraction headspace extraction
18 000 1 024 500
29 500 1 029 000
32 500 1 008 000
36 500 949 500
38 500 869 000
38 000 802 000
38 575 754 320
38 675 712 000
million w/w level in 500 mL water: benzene, 5 µL; toluene, 10 µL; ethylbenzene, 9 µL; o-xylene, 9 µL; and TCE, 3 µL. Samples were prepared by diluting the concentrated aqueous standard 100fold, by pipetting 10 mL into a 1000-mL volumetric flask of water. In all aqueous samples, the exact concentrations for benzene, toluene, ethylbenzene, o-xylene, and trichloroethylene were 88, 172, 156, 156, and 88 ppb w/w, respectively. Samples were stirred using a magnetic stir bar (2 cm in length) at 1500 rpm, using a VWR hot plate/stirrer (VWR Scientific Ltd). The extraction temperature for all experiments was 25 ( 0.5 °C. To perform surface-water monitoring, the extraction cap was placed on the aqueous surface in a glass water bath container which contained aqueous sample to a 1-cm depth. The pressure of the headspace was ambient. For underwater monitoring, the extraction cap was positioned in an aqueous sample at a depth of 25 cm. The pressure of the headspace was higher than the room pressure because of the height of the water. To allow a 1-cm depth of water inside the cap, the headspace pressure was adjusted by opening and closing the pressure release switch in Figure 2. During the extraction, the position of the extraction cap was fixed using a clamp to hold the neck of the cap, and the cap was tightly in contact with the bottom of the bath container in each extraction. This contact ensures the proper sealing of the aqueous sample inside the extraction cap and prevents the sample inside from exchanging with the solution outside of the cap. The monitoring of VOCs was performed using 4 min of trapping and 2 s of thermal desorption. Figure 3 shows the chromatograms of continuous headspace monitoring of aqueous sample by Cap-MESI. 1060
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RESULTS AND DISCUSSION Compared with direct water extraction, headspace extraction has many advantages. Membrane headspace extraction allows the membrane to be isolated from sample matrix, which results in a longer membrane life without performance change. VOCs have much larger diffusion coefficients in air than in water, which results in fast mass transfer and a smaller boundary layer (membrane/air) effect in headspace extraction, hence an improved response. In addition, most VOCs have larger partition coefficients for membrane/air than membrane/water, which leads to a higher driving force of the diffusion in the membrane in headspace extraction and, hence, a good sensitivity. Therefore, compared with direct water extraction, headspace extraction is preferred for monitoring of VOCs. A comparison of toluene monitoring in headspace and direct water extraction is shown in Table 1. It can be seen that headspace extraction produced a much higher extracted amount of toluene than direct water extraction in the same time unit. The time to reach steady-state extraction is much shorter for headspace extraction. Both results reflect the much higher diffusion coefficients (4 orders of magnitude) of analytes in gas phase compared with aqueous phase and higher interface area between the water and headspace compared to the surface area of the membrane. The loss in rates at higher extraction times for headspace extraction is coursed by depletion of analyte concentration in the sample. Surface water headspace analysis can supply information on the release of volatile organic compounds to the air from ponds,
Table 2. Comparison of Amount Extracted (Peak Height) During a Continuous Monitoring of o-Xylene (Average of Three Measurements)
a
time (min)
4
8
12
16
20
24
28
32
agitation Aa agitation Bb
49 500 54 500
78 000 92 500
83 500 91 800
82 500 92 500
83 800 89 500
82 800 82 500
79 500 80 500
74 500 76 500
Agitation A: microfan facing the membrane probe only. b Agitation B: microfan facing the membrane probe and aqueous sample.
lakes, and rivers, to help environmentalists monitor the air and water quality. When Cap-MESI is applied, since there is no pressure difference between the capped headspace and the outside, the extraction can be treated as a normal headspace extraction. Analyte needs to diffuse from the bulk solution to the aqueous surface, then partition to the headspace, diffuse from the headspace to the membrane outer surface, partition into the membrane, and then diffuse to the inner membrane wall and partition into the stripping carrier gas which carries it to the sorbent/GC system where it is analyzed. Among these processes, analyte transport from the bulk solution to the solution surface is the limiting step, due to the small diffusion coefficient of analyte in the aqueous phase. Agitation is the most important approach for the improvement of mass transfer, and magnetic stirring is a popular method for aqueous sample agitation. Water stirring can rapidly bring VOCs to the water surface, allowing the analytes to quickly distribute into the headspace; therefore, an equilibrium between the headspace and water can be reached in a short time period. When the concentration equilibrium is reached, the extraction will reach the optimum extraction efficiency. The amounts of benzene extracted by continuous monitoring with different agitation levels are compared in Figure 4. In Figure 4, it can be seen that, for a 48-min monitoring, the static condition produced the lowest steady-state extracted amount and that the steady-state extraction was reached slowly. Both water stirring and headspace microfan agitation increased the extracted amount and shortened the time to reach the steady state. The comparison of magnetic stirring and microfan headspace agitation shows that the former resulted in a shorter time to steady state and the extracted amount was higher at the initial extraction. Peak decline was seen with both agitation modes. This was attributed to analyte concentration depletion in the bulk solution due to the removal of the analytes by the extraction process. From Figure 4, it can be seen that the two different modes of agitation resulted in almost identical extracted amounts for a 48-min extraction. We know magnetic stirring is a very effective method for water agitation; however, the experiment showed that the microfan method produced similar amounts extracted. This experimental result can be explained by the following three reasons. First, with microfan agitation, the fan moved not only the air of the headspace but also the water surface and made many ripples, which could greatly increase the water surface area and, therefore, result in faster mass transfer from the aqueous phase to the headspace. Second, when the microfan was circulating the air, the water was also agitated. As the depth of the solution was only 1 cm, the analytes could easily be brought to the water surface from the bulk solution. Third, the fast-moving air in the headspace could more efficiently strip the analytes from the water surface, move analytes to the membrane faster, and reduce the boundary layer
Figure 4. Extraction-time profiles for benzene using different agitation methods for surface-water headspace extraction.
around the membrane. From Figure 4, it also can be seen that when water stirring and headspace agitation were used simultaneously the highest extraction amount was obtained and the time to reach steady state was too short compared with trapping time to be observed in the experiment. Since the rapid removal of analytes resulted in depletion of concentration, the peaks quickly declined. To understand the effect of headspace agitation on the amount extracted, two experiments were designed. In one experiment, the microfan was adjusted to face both the membrane probe and the water (at a 60° angle). In another experiment, the microfan was adjusted to face the membrane probe only, to reduce the agitation to the water. The results are shown in Table 2. It can be seen that with the microfan blowing on the water surface the amount extracted is increased. This result supports the above explanation that the microfan can significantly agitate water. For underwater headspace monitoring, the extraction cap was completely submerged and, because the top of the funnel was sealed, a headspace formed inside the extraction cap. It was found experimentally that the extraction under water was different from that at the water surface. Compared with the surface-water extraction, the underwater extraction took a longer time to reach steady-state extraction and the total amount extracted (sum of all peaks) was lower after 36 min of extraction. The phenomenon is likely due to the difference of headspace pressure. When the extraction cap was positioned under water, the pressure of the headspace was higher than that at the water surface, because of the depth of the water column. When the headspace is under higher pressure, the speed of mass transfer from the solution to the headspace is reduced and the partial vapor pressure of analyte Analytical Chemistry, Vol. 72, No. 5, March 1, 2000
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Table 3. Precision and Limit of Detectiona benzene toluene ethylbenzene o-xylene TCE %)b
precision (RSD limit of detection (ppb)
4.2 0.1
3.8 0.1
3.9 0.1
4.5 0.1
6.2 1
a A four-minute trapping time was used. b The precision was calculated from 5 replicates at a concentration of 200 ppb.
Figure 5. Calibration curves of benzene, toluene, ethylbenzene, o-xylene, and TCE by Cap-MESI for surface-water headspace analysis. Each point represents the average of 3 measurements, with RSD less than 7%.
in the headspace is decreased.12 When the partial vapor pressure of analyte is decreased in headspace, the extraction amount will obviously be reduced at steady-state extraction, because the extraction amount is proportional to the sample concentration.8 In this study, the extraction was under only 25 cm of water, so the headspace pressure would not be too high. The amount extracted at steady-state was not significantly different from the water-surface extraction, but a difference in the time to reach steady state was observed. Apparently, a better agitation is needed to improve the mass-transfer rate. For field monitoring, agitation can be performed using a motor rotor with two sets of paddles to simultaneously stir water and headspace. Use of higher power motors would better agitate sample matrix and headspace and therefore would result in higher extraction efficiency. The above investigation was performed in only 25 cm of water; however, it can be predicted that in a deeper water extraction, the amount extracted would be significantly reduced and the sensitivity would be correspondingly worse. To deal with the problem of increased pressure, the headspace pressure must be reduced. In practice, water from a depth can be brought to the surface level to be monitored at ambient pressure. To bring the deeper water to the water surface, a long tube or pipe may be connected with the extraction cap and dipped into the water to a target depth. The pressure release switch (Figure 2) can then be opened to gradually guide the water to the surface-water level. In this case, the headspace pressure would be ambient and an increased sensitivity may be obtained. Quantification is an important issue in field monitoring. For external calibration, quantification based on steady-state extraction can provide good reproducibility. To use steady-state extraction, a long steady-state extraction time is needed and the time depends on the sample volume. Previously13 we showed that a large sample volume resulted in a longer steady-state extraction time. In most cases the sample volume for field monitoring is very large, i.e., rivers, lakes, and undergroundwater, so the change from steady state to non-steady state due to the depletion of analyte by the extraction can be ignored. (12) Treybal, R. E. Mass-Transfer Operations; McGraw-Hill Chemical Engineering Series; McGraw-Hill: New York, 1980; pp 346-347. (13) Luo, Y. Z. Membrane Extraction with a Sorbent Interface. Ph.D. Thesis, University of Waterloo, Ontario, 1999.
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In Cap-MESI headspace analysis, the headspace pressure is important for quantification and a constant headspace pressure is needed. Any variation in depth will affect the amount extracted. To simplify the calibration, the headspace pressure can be controlled to ambient, which will also result in the highest extraction efficiency. The temperature in deep water should also be considered. Normally, the temperature in deep water is below the temperature of surface water. This would affect the distribution of analytes in the headspace. In an external calibration, the correction for different temperatures should be used. The calibration values can be stored in the computer controlling the system. Alternatively, an on-line calibration can be used when perfect agitation conditions are ensured. This method for air analysis was discussed previously.14 To simplify the discussion of the quantification, only surfacewater monitoring was investigated in this study. The extraction was performed under microfan agitation because water stirring is not convenient in most applications. To model field measurement, a larger volume, a 1-L aqueous sample, was used. Experimentally, the constant peaks for those VOCs are used for the calibration to ensure the chromatogram obtained was under steady-state extraction conditions. The calibration curves for these compounds are shown in Figure 5. Good linearity was obtained for concentrations from 1 ppb to 5 ppm with r2 values ranging from 0.9831 to 0.9998. The RSD in all cases was below 7%. Good detection limits were obtained for these compounds. These detection limits are listed in Table 3. CONCLUSION The initial laboratory experiments indicate that Cap-MESI is a useful tool for field analysis and monitoring. This approach integrates sampling, sample preparation, and introduction of extracted components into GC and therefore prevents loss of volatile analytes. The extraction cap can be placed at the water surface or at a depth to perform on-site or on-line monitoring. Since headspace extraction is used, the membrane does not contact the sample matrix directly, therefore reducing possibilities of membrane contamination and mechanical damage, which prevents a decrease of membrane performance with time. This ensures that the extraction probe can be placed in the sampling environment for long-time monitoring without the need for replacement. The headspace approach to MESI can be extended to less-volatile analytes by heating the sample, the membrane, and the transfer lines to the sorbent interface. It can also be extended to solid sample analysis by covering, for example, soil, by the “cup”. In deep-water monitoring, headspace pressure and (14) Luo, Y. Z.; Pawliszyn, J. Calibration of Membrane Extraction for Air Analysis, Anal. Chem. 2000, 72, 1064-1071.
low temperatures are major factors that could result in lower extraction efficiency. To attain good extraction efficiency for very deep water monitoring, water should be brought to the surface to allow ambient pressure headspace monitoring. Calibration procedures should take actual extraction temperature and pressure into consideration, particularly for deep-water monitoring. Quantitative data of surface water headspace extraction showed good linearities, precision, and detection limits. This study demonstrates that the Cap-MESI combined with the portable micro gas chromatograph is a potential practical approach for field monitoring because of its simplicity, enhanced sensitivity, con-
venience of deployment and automation, and reliability. Additional information about the system can be obtained in ref 13. ACKNOWLEDGMENT This work has been financially supported by Chrompack and the Natural Sciences and Engineering Research Council of Canada. Ms. Heather Lord assisted in preparing this manuscript. Received for review July 8, 1999. Accepted December 15, 1999. AC990747B
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