Anal. Chem. 1993, 65, 2538-2541
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Extraction of Semivolatile Organic Compounds from Aqueous Samples Using High-Density Carbon Dioxide and Hollow Fiber Membrane Module Min J. Yang and Janusz Pawliszyn' The Guelph-Waterloo Center for Graduate Work in Chemistry and the Waterloo Center for Groundwater Research, University of Waterloo, Waterloo, Ontario, Canada, N2L 3Gl The combinedgaslike mass-transfer and liquidlike solvating characteristics of supercritical fluid have led to considerable interest in its use as an extraction solvent or a mobile phase in supercritical fluid chromatography. In many cases, C02 was selected as the solvent for the extractions due to its low critical temperature and pressure, low cost, and nontoxic properties. Although many applications have demonstrated the potential of supercritical fluid extraction (SFE),analytical SFE methods are very limited for extraction of organics from water.' An indirect SFE method has been introduced using a solid sorbent to transfer analytes from water to a supercritical fluid stream.2 This method included three time-consuming processes: adsorption, drying, and extraction. Also, loss of analytes can occur during the drying procedure. Direct SFE methods using a flow-through extraction vessel or a phase separator have been investigated by a number of researchers.1~3-5These methods suffered the common problems of the dissolution of a small amount of water in the supercritical COZand small sample volume, which limited the sensitivity of the technique. The small quantity of water present in the extract can cause restrictor plugging and deactivation of the stationary phase in the separation column during the subsequent chromatographic analysis. In addition, the volume of the sample is limited by the safety consideration, which requires thicker walls for high-pressure vessels. Use of a hollow fiber membrane module (HFM) leads to a simple solution to the water solubility problem in C02. In the HFM, water molecules are prohibited from entering the stripping medium because the silicone rubber membrane is nonporous and hydrophobic. A HFM was previously developed and demonstrated quantitative recovery of volatile organic compounds from water. This resulted in a simple and efficient extraction method for rapid routine analysis and continuous monitoring applications.6~7A good extraction efficiency of the HFM is ensured by the high sample throughput and the large surfaceto-volume ratio of the hollow fiber membrane. The masstransfer process within the HFM can be considered as being similar to liquid-liquid extraction process. In the HFM, however, a third phase is needed to remove and transfer the permeated compounds from the outer wall of the membrane to a collection sample vial or an instrument for analysis. If the compounds are volatile or have high Henry's law constants (above O.l), the removal and transfer can be accomplished (1)Hedrick, J. L.; Mulcahey, L. J.; Taylor, L. T. In Liquid-State Gas Technology;Bright, F. V., McNally, M. E. P., Eds.; American Chemical Society: Washington, DC, 1992. (2)Pawliszyn, J.; Alexandrou, N. Water Pollut. Res. J. Can. 1989, 24,207-214. (3)Ehntholt, D. J.; Eppig, C.; Thrun, K. E. In Organic Pollutants in Water;Suffet, I. H., Malaiyandi, M., Eds.; Advances in Chemistry Series 214;American Chemical Society: Washington, DC, 1987. (4)Ong, C. P.; Lee, H. K.; Li, S. F. Y. Enuiron. Monit. Assess. 1991, 19, 63-66. (5)Thiebaut, D.; Chervet, J. P.; Vannoort, R. W.; Dejong, G. J.; Th. Brinkman, U. A.; Frei, R. W. J. Chromatogr. 1989,477,151-159. (6)Pratt, K.F.; Pawliszyn, J. Anal. Chem. 1992,64,2107-2110. (7)Yang, M. J.; Pawliszyn, J. Multiplex Gas Chromatography with a Hollow Fiber Membrane Interface for Analysis of Trace Volatile Organic Compounds in Aqueous Samples. Anal. Chem. 1993,65, 1758. 0003-2700/93/0365-2538$04.00/0
simply with a stream of low-pressure N2. However, this N2 stream cannot provide sufficient solvation to strip semivolatile compounds such as phenols. The HFM technique can be used for the extraction of semivolatile organic compounds from an aqueous matrix by replacing the low-pressure stripping phase with a high-density gas and modifying the design of the extraction cell. In other words, the use of a high-pressure fluid as the stripping phase widens the volatility range of analytes to which the HFM can be applied, due to the increased solvating power associated with the fluid. Furthermore, the extraction process using the high-pressure HFM module is organic solvent-free. The objective of this work is to construct a HFM which can be operated under high-pressure conditions. The preliminary experiments using this high-pressure HFM are carried out at 3000 psi and room temperature (23 "C). As a result, liquid C02 was used as the stripping phase during the extractions. Isolation of phenols from water was investigated using this high-pressure HFM method. Also, in this paper, the theories and assumptions for a mathematical model describing the extraction of 2,4-dichlorophenol (DCP) from water using the technique are discussed and compared with experimental data.
EXPERIMENTAL SECTION Extraction Cell. The extraction cell or the HFM used in the high-pressure HFM system was constructed as depicted in Figure 1. Stainless steel parts were used for the entire extraction vessel to permit the use of high pressure in experiments. The fittings used at either end of the membrane module were the same and consisted of a Va-in. T-union and two l/~-l/lc-in. bored-through reducers (Swagelok Canada Ltd., Niagara Falls, ON, Part No. SS-200-3 and SS-100-R2BT). The silicone hollow fiber membrane (Dow Corning Canada Inc., Mississauga, ON) had an inner diameter of 0.0305 cm and a wall thickness of 0.0165 cm. The technique for connecting the membrane to a 23-gauge stainless steel needle (Hamilton Co., Reno, NV) was previously described.8 One end of the silicone fiber was first submerged in toluene for -3 min. After it had swollen, -2 cm of the stainless steel needle was inserted into the hollow fiber. When the toluene evaporated, the membrane shrunk, and a tight seal formed between the membrane and the needle. This connection is also illustrated in the inset of Figure 1. The needle at either end of the membrane module was held in place with a 0.8-mm graphite/vespel ferrule (Supelco Inc., Bellefonte, PA, Part NO. 2-2512). Extraction System. The high-pressure HFM system consisted of a high-pressure syringe pump, a water sample reservoir, and the extraction cell (Figure 2). The syringe pump was purchased from Isco Inc. (Lincoln, Nebraska, Model 260D). The water reservoir was made with a l/c-in.-o.d. stainless steel tube and two 1/4-1/16-in.stainless steel reducing unions (Swagelok Canada, Ltd., Part No. SS-400-6-1LV). (8)Melcher, R. G.; Morabito, P. L. Anal. Chem. 1990,62,2183-2188. 0 1993 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 65. NO. 18, SEPTEMBER 15, 1993 Liquid-State Gas Inlet
e
c
b
',
e
e
I ! f a
.~~ . . . . . . .
Fbun 1. High-pressuremembrane module: (a) 'I&. StainleSS Steel tubing, (b) pressure restrictor, (c)'I&. T-union, (d) 'I8-'/,& boredthrough reducer, (e) stainless steel needle. (f) hollow silicon fiber membrane, and (g) 'I8-in. ferrule.
a Z&pm fused-silica capillary (Polymicro Technologies, Phoenix, AZ) was used as a restrictor for the aqueous stream and a 27 pm one was used for the stripping gas stream. A check valve was installed at the outlet of the syringe pump topreventany possible back-flowcontaminationtothepump. A Varian 6500 gas chromatograph (GC) interfaced to the data system of a Varian 6000 GC (Varian Canada Inc., Georgetown, ON) was used in the experiments. The 6500 GC was equipped with a split/splitless injector and a flame ionization detector (FID). The FID was operated with He carrier at a flow of 1.5 mL/min, Nz make-up at 21 mL/min, Hz at 30 mL/min, and air at 280 mL/min. Separations were done on a PTE-5 column, 30.0 m X 0.25 m m with a film thickness of 0.25 pm (J&W Scientific, Folsom, CA). The injector and detector temperatures were set at 250 and 275 "C, respectively. The column temperature was initially set at 37 "C and then ramped to 170 "C at 10 OC/min. Both 2,ldimethylphenol (DMP) and 2,4dichlorophenol (DCP)were used as standards for aqueous sample preparation and had a purity of 99% (Supelco Canada Ltd., Oakville, ON). Deionized water was used in all aqueous sample preparation. Procedure. A 2-mL watersamplecontaining20ppmDMP and DCP was transferred to the water reservoir. The syringe pump was set to deliver 3000 psi COz to both the extraction vesselandthe water reservoir. Theaqueoussamplewasforced through the hollow fiber membrane, which was enclosed in the HFM. The high-density gas was simultaneously introduced into the extraction vessel and flowed around the membrane under the same pressure. The analytes carried out by the high-densitygas were collected in avial containing 1mL of 2-propanol. The 2-propanol sample was analyzed using the Varian 6500 GC. Under the GC conditions stated above, the retention times for the DMP and DCP peaks were 13.55 and 14.01min, respectively. The 2-mLaqueous sample eluted over a period of -40 min via the 20-pm restrictor. The extraction experimentwas repeated to determinethe precision of the system. Because of the nonporous and hydrophobic characteristics of the silicon rubber membrane, water molecules were restricted from passing through the fiber membrane. These properties made the separation of organic compounds from water possible. The actual transfer of the organic compoundsfrom water tothestrippingphase occurred by diffusion. A portion of analytes was either lost during the collection process or eluted with the aqueous phase. A mass balance experiment was performed to determine where the loss of the analytes occurred. In order to measure the escaped portion of the analytes, a second 2-propanol trap was installed. COz gas exiting the HFM expanded in the first trap and was then transferred to the second one through a fused-silica capillary with an inner diameter of 0.5 mm. Both the 2-propanol samples were analyzed with the GC. The area counts of the component peaks were used for quantitative determinations. Tomeasure the retainedportion ofthe analytes, theeluted aqueous sample was treated with an equal volume of methylene chloride and the organic extract was analyzed with the GC. The same liquid-liquid extraction method was used to measure the total amount of analytes in the initial 20 ppm sample. The eluted portion was determined using the ratio of the component peak area counts of the eluted sample and the original samples. The analytes absorbed in the system were measured in a series of contamination testa. A line contamination test was run by removing the extraction cell and plugging the lower end of the water reservoir. A piece of 27-pm restrictor was attached to the high-density gas stream. A 2-mL aqueous sample containing 20 ppm DMP and DCP was loaded onto the reservoir as usual. High-density COzwas allowed to flow through the line at 3000 psi for 0.5 h. The COz exiting from
I Syringe Pump
Extmclion Cell
Flgum 2. High-pressure hollow Rbar membrane extraction system for analysis of semivolatile organic compounds in water.
Within the extraction cell, it was important to maintain the samepressureforboththeinsideandoutaideofthemembrane wall in order to avoid damaging the hollow fiber. As shown in Figure 2, the outlet of the syringe pump, which delivered high-density C0z at 3000 psi, was split into two streams using a 1/16-in.stainless steel T-union. One of the streams pushed the aqueous sample from the reservoir through the center core ofthe hollow fiber whiletheother flowedcountermntly around the fiber as the stripping phase. Because the aqueous sample and the high-density gas were introduced simultaneously using only one pump, the pressure was uniformly maintained throughout the extraction cell. In this constantpressure configuration,theflowrateofastreamwascontrolled by the size of restrictors (15 cm in length). A small restrictor would result in a low volumetric flow rate. For this study,
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ANALYTICAL CHEMISTRY. VOL. 65, NO. 18. SEPTEMBER 15, 1993
Table I. Extraction Result Using the High-Pressure HFM System at a Sample Flow Rate of 0.05 mL/min trial DMP ( % I DCP(%) 1 3
72.69 69.18 81.36
68.57 64.52 73.48
av RSD
74.41 8.43
68.85 6.52
2
0.6 0.5 0.4
the restrictor bubbled in a sample vial containing 1mL of 2-propanol. At the end of the test run, the 2-propanol sample was analyzed by GC. Wall contamination in the water reservoir was also investigated. The water reservoir was detached from the rest of the extraction system. The 2-mL aqueous sample was transferred into the reservoir as usual. One end of the reservoir wasconnectedto alow-pressuresyringe pump (Sage Instruments, Cambridge, MA). The aqueous sample was pushed out of the reservoir by low pressure air at 2 mL/h. The eluent was analyzed with the same liquid-liquid extraction technique mentioned above. Finally, contamination of the extraction cell was measured. The water reservoir and the extraction cell were detached from the rest of the system. The 2-mL aqueous sample was loaded into the reservoir as usual. The upper end of the reservoir was connected to the low-pressure syringe pump and the aqueous sample was pushed out of the reservoir and through the hollow fiber in the extraction cell by low pressure air at 2 mL/h. The eluent was also analyzed by liquid-liquid extraction. Microsoft Excel4.0 with Windows (Microsoft Canada Inc., Mississauga, Ontario) was used for the theoretical modeling.
RESULTS AND DISCUSSION To evaluate the extraction efficiency and precision of the high-pressure HFM system, a standard water sample spiked with 20 ppm of DMP and DCP was extracted several times. By comparing the area counts of the GC component peaks between the sample extract and a 5 ppm standard sample of DMP and DCP in 2-propanol, the recoveries of the phenols during the extractions were calculated and listed in Table I. The extraction efficiency and the relative standard deviation (RSD)of the extraction results were very close for both target analytes. This indicates that the high-pressureHFM system had no selectivity toward either of these two compounds. The RSDvaluesfor bothcompounds are -7 9% whichindicate acceptable precision for the extraction method. In the high-pressure HFM extraction cell, diffusion of analytes occurs in three phases: the aqueous media, the membrane wall, and liquid COz. In general, the mass-transfer process across the three phases within the extraction cell can be described by Fick's second law of diffusion in the form of a differential equation. Two solutions to the differential equation have been founds an analytical solution is based on the concentration distribution across the diameter and along the axis of the hollow fiber membrane; a semiempirical solution is based on the mass transfer in all phases within the membrane extraction cell. Since both solutions lead to very similar predictions, the analytical solution will be emphasized in this paper, and a theoretical model for the high-pressure HFM system will be discussed in terms of the attributes and limitations of the extraction method. Severalassumptionsmust be made to obtain the analytical solution for the theoretical model. Because of the large diffusion coefficients for solute in Con, the low viscosity of the high-density gas, and the thin membrane, mass transfer (9)Pratt, K. F.;Pswliszyn, J. Anal. Chem. 1992,64.2101-2106.
0.1
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Fyure 3. mewetid concentration distribution of 2.4dich1oropheno1 present inskle the sillcone hollow fiber membrane at a sample flow rate of 0.05 mLlmin.
in the stripping media and in the membrane has a negligible effect on the extraction dynamica.9 As a result, the overall mass transfer in the high-pressure HFM system is determined bythediffusionin tbeaqueousphase. In this case,therelative amount of analyte in the aqueous phase inside the fiber membrane can be calculated using the following relationship:
0.385ed3'wJz@ -
... (1)
where Jo, 51, and JZ are functions of r;lo C is the analyte concentration at a given point along the fiber length; Cois the concentration entering the fiber; R is the inner radius of the fiber; and r is the radial distance from the center of the fiber. P is a dimensionless parameter defined as
P =D ~ L I . R ~
(2)
where DL is the diffusion coefficient in the aqueous phase, Y is the linear velocity of the aqueous sample, and L is the active fiber length. In most cases, only the first three terms are used in eq 1, since the later terms are insignificant compared to the first three. In the high-pressure HFM system, the active fiber length is 14 cm and the inner radius of the fiher is 0.0153 em. The diffusion coefficient of DCP in water is 8.07 X 106 cmYs as calculated by Lymanet al.'s estimation method." The linear flow rate of the aqueous stream is 1.14 cmls, whicb corresponds to a volumetric flow rate of -0.05 mL/min. By substituting these parameters into eqs 1and 2, the concentration profile of DCP inside the silicone hollow fiber membrane can be plotted. Figure 3 illustrates the DCP concentration distribution along and across the fiber. From this plot, it can be seen that increasing the length of the fiber will result in a decrease in the analyte concentration in the water exiting the fiber, hence an increase in the extraction efficiency. The extraction efficiency can be predicted by integrating the concentration profile along the cross-sectional area at the (10)Jakob, M.Heat Transfer; John Wiley & Son, Inc.: New York, 1951: Vol. 1. (ll!Lyman, W. J.; +hI. W. F.; RasenblstZ D. H.Handbook of Chemrcal Property Estimation Methodp; McGraw-Hill Boak Ca.: New York, 1981.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 18, SEPTEMBER 15, 1993
I
03 0
0 05
01
02
0 I5
0 25
03
volvmtric nor (mumill)
Flgure4. Theoretlcai effect of aqueoussample flow rate on extraction efficiency of the hlgh-pressure HFM system for the extraction of 2,4dichlorophenol from water uslng highdensity carbon dioxide at 3000 psi and r m m temperature.
exit of the hollow fiber using the following equation: extraction efficiency = (1
c, -ilRm0 q dr r/R=1
Figure 4 shows the effect of the volumetric flow rate of the aqueous phase on the extraction efficiency. This model predicts quantitative recovery of DCP from water at a sample flow rate of 0.004 mL/min using the high-pressure HFM system. Figure 4 also indicates that the extraction efficiency can be significantlyincreased by reducing the volumetric flow rate of the aqueous phase. In the present system, the flow rate was controlled by changingthe size of a fused-silicatubing. Because only a limited number of sizes of the fused-silica tubings were available for use as restrictors, a variable restrictor was needed to facilitate good control over the flowrate conditions. The flow rate of the aqueous sample through the hollow fiber membrane at 3000 psi was reduced from 0.08 to 0.05 mL/min when the 27-pm restrictor was replaced with a 20-pm one. With a smaller restrictor, the possibility of plugging the system would be greater. Based on the theoretical model, for an aqueous volumetric flow rate of 0.05 mL/min, the DCP extraction efficiencyis expected to be 80 % . The experimental recovery shown in Table I is slightly lower than the predicted value due to analyte loss during the extraction. The recoveries in Table I represent only the amounts of analyte trapped in the collection vial. To account for the losses, a mass balance experiment was performed. As shown in Table I, only -69% of the total amount of DCP was recovered from the water sample. Three percent of the DCP escaped to the ambient air during the collectionprocess,which was detected by using a second collection vial. The escaped portion of analyte would be larger if the collection vial was not capped. For more volatile analytes, this loss would constitute a significant error. The amount escaping to the atmosphere can be minimized by directly interfacing the extraction cell to an instrument such as a supercritical fluid chromatograph for quantitative determination. A similar interface between a HFM to a multiplex GC for monitoring trace volatile organic compound levels in water has been applied successfully.7 Nineteen percent of the total amount of DCP was eluted with the aqueous phase. This was in agreement with the theoretical models (Figure 4), which predicted that, at a sample flow rate of 0.05 mL/min, extraction efficiency would be 80%. In other words, the concentration the analyte at the fiber exit would be 20 7%. According to eq 1, the relative concentration of the analyte at the fiber exit decreases as P increases. Based on eq 2, an increase in the diffusion coefficient of the analyte in the aqueous phase (for example, by
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increasing the temperature) or the length of the hollow fiber membrane will result in a decrease in the analyte concentration in the water exiting the fiber, thereby increasing the extraction efficiencyof the system. A decrease in the aqueous sample flow rate or the inner diameter of the hollow fiber membrane will have the same effect. Lengthening the fiber will allow the use of higher flow rates while continuing to obtain good extraction efficiency. This will increase the throughput and improve the sensitivity of the analytical method.9 However, this approach would require a long extraction cell. For the high-pressure HFM experiments, this would not be desirable because it would result in a large dead volume. Alternatively,the fiber can be coiled,thus increasing its length while the compactness of the extraction cell is retained. The improved extraction efficiency in the spiral configuration is also attributed to the increased convection inside the hollow fiber.6 Besides the extracted, escaped, and eluted portions, the other 9% of the total amount of DCP can be accounted for as system contamination. A series of tests were designed to determine the level of analyte contamination within various parts of the extraction system. Because the COz line was also physically connectedto the water sample reservoir, a potential back-flowfrom the reservoir could cause a line contamination as well as plugging of the gas restrictor as water enters the stripping phase. The test result for the high-pressure HFM system showed no line contamination. In other words, there was no back-flow at the water reservoir. To eliminate the possibility of the line contamination, a check valve at the upper end of the water reservoir can be installed. Based on the component peak area counts obtained in the contamination tests, the wall of the reservoir was contaminated with -4% of the total amount of the analyte and the other 5 % of the analyte was absorbed by the membrane wall. The fraction of the total amount of analyte attributed to the system contamination can be reduced by extending the length of the extraction. It is expected that the contamination of the system occurs in the initial stages of the extraction. A two-pump configuration of the high-pressure HFM system could be used for continuous extraction or monitoring applications. In this configuration, an additional highpressure water pump is required. The water reservoir is replaced with the water pump which provides continuous supply of the aqueous sample. The major concern associated with this configuration is maintenance of the same pressure within the two pumps. A differential pressure of greater than 50 psi can damage the hollow fiber membrane. Solid supports such as a porous glass tubing can be fitted around the outside wall of the fiber membrane to reduce the risk of membrane breakage caused by small differential pressure. In the twopump configuration, the sensitivity of the method could be increased by operating the high-pressure HFM system at high water flow rates. In this case, quantitative extraction is no longer obtained as only a very small fraction of the analyte is removed from the aqueous sample. However, significant gain can be obtained due to the increased throughput of the membrane.6
ACKNOWLEDGMENT This project was supported by the Natural Sciences and Engineering Research Council of Canada. RECEIVED for review March 26, 1993. Accepted June 14, 1993.
