Environ. Sci. Technol. 1994, 28, 298-305
Rapid Determination of Polyaromatic Hydrocarbons and Polychlorinated Biphenyls in Water Using Solid-Phase Microextraction and GC/MS Davld W. Potted and Janusz Pawliszyn'
Guelph-Waterloo Center for Graduate Work in Chemistry and the Waterloo Center for Groundwater Research, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 ~~~~~
-
Solid-phase microextraction (SPME) was investigated as a solvent-free alternative method for the extraction and analysis of nonpolar semivolatile analytes. Analytes were extracted into a polymeric phase immobilizedonto a fusedsilica fiber. The fiber was then inserted directly into the injector of a gas chromatograph, and the analytes were thermally desorbed. This new technique allows sampling directly from the source (lake, drinkingfountain, etc.) and, therefore, eliminates the loss of analytes through adsorption onto container walls and saves transport costs. Using a 15-pmpoly(dimethylsi1oxane)or a Carbopack B coating and an ion trap mass spectrometer, detection limits ranging from 1 to 20 pg/mL were obtained for naphthalene, anthracene, benz[alanthracene, benz[alpyrene, 2,2',5trichlorbiphenyl, and 2,2',3,4,5'-pentachlorbiphenyl after only a 10-min sampling time. The detection limits obtained exceed the regulatory requirements of US. Environmental Protection Agency (US.EPA) method 525. Linearity extended from low picogram per milliliter to nanogram per milliliter levels for all of the compounds studied. The relative standard deviation was comparable with US. EPA method 525, ranging from 10% for the polyaromatic hydrocarbons (PAH) to 20% for the polychlorinated biphenyl (PCB). Initial experimental data demonstrate that SPME can be used to quantitatively determine nonpolar semivolatiles in clean aqueous matrices and as a screening tool for wastewaters.
water are liquid-liquid extraction and solid-phase extraction. Neither of these methods is ideal, and both have several significant disadvantages (IO). The major disadvantage of liquid-liquid extraction is the use of large volumes of expensive, high-purity organic solvents, which eventually must be disposed of. This technique is extremely time-consuming and not very amendable to automation. Other drawbacks include emulsionformation and a lack of sensitivity for more volatile analytes. While solid-phase extraction (SPE) reduces solvent requirements considerably compared to liquid-liquid extraction, they are not eliminated. SPE cartridges and disks are susceptible to plugging and require sophisticated, expensive technology to be fully automated (11-15). Recently, several papers have appeared in the literature describing a solvent-free, fully automatable technique, solid-phase microextraction (SPME) (16-22). Previous research has focused on the analysis of volatile analytes by SPME. The aim of this manuscript is to demonstrate that solid-phase microextraction is an analytical technique that can meet the required detection limits of the US. Environmental Protection Agency (U.S. EPA) method 525 for the analysis of semivolatile analytes in an integrated process and can also serve as a suitable screening method for heavily contaminated waters.
t Presentaddress: WellingtonLaboratories,398 LairdRd.,Guelph, Ontario, Canada N1G 3x7.
Experimental Section Naphthalene (199% ), anthracene (299% ), benz[alanthracene (99%), and benzo[alpyrene (98%) were obtained from Aldrich Chemical Co. (Milwaukee,WI). 2,2',5Trichlorobiphenyl and 2,2',3,4,5'-pentachlorobiphenyl (299%) were obtained from Ultra Scientific (North Kingston, RI). Deuterated benzo[alpyrene (>99%) was obtained from Accu Standard (New Haven, CI), while the 525 internal standard mixture was purchased from Supelco Canada (Oakville, Ontario). ORGANICpure (SybronBarnstead, Boston, MA) water was used to prepare all the samples. All glassware and magnetic stir bars were washed for 30 min in a Branson 2200 ultrasonic bath with Sparkleen detergent followed by rinsing with copious amounts of deionized water. This was followed by rinsing with pesticide-grade acetone and drying for 30 min at 150 "C. Glassware was then capped with aluminum foil until required for use. Volumetric flasks were never placed in the oven, instead they were allowed to air dry inverted. All glassware used for PAH and PCB analysis was silanized prior to use. The glassware was exposed overnight to a 10% solution of dichlorodimethylsilanein toluene, Supelco Canada (Oakville, Ontario). This was followed by rinsing with toluene and then methanol and oven drying at 150 "C for several hours. Silanized glassware was kept in the dark or wrapped in aluminum foil. Glass inserts for the gas chromatographic injectors were also silanized by this method. SPME devices were prepared with either a 15-pMpoly(dimethylsiloxane) stationary phase or a Carbopack B
298 Environ. Sci. Technoi.. Voi. 28, No. 2, 1994
0013-836X/84/0828-0288$04.50/0
Introduction Contamination of water supplies is a global problem. The complexity and diversity of environmental contaminants has resulted in the development of many different analytical techniques and protocols for their extraction and analysis. Ideally an analytical technique should be sensitive while providing accurate and precise results. It should also be inexpensive,rapid, simple, easily automated, and adaptable to on-site analysis. A rapid, simple technique translates to faster analysis times and greater throughput. Accuracy and precision are improved as analyte loss from adsorption to glassware and commonly used laboratory polymeric materials (latex, Tygon, polypropylene, etc.), microbial degradation ( I , 2), and other systematic errors will be reduced with fewer sample preparation steps. Polycyclic aromatic hydrocarbons (PAH) and polychlorinated biphenyls (PCB) are ubiquitous in the environment (3-9). These compounds originate from both natural and anthropogenic sources and include suspected and known carcinogens (5-9). Currently, the two methods of choice for the extraction of these semivolatile compounds from ~~~
* Author to whom correspondence should be addressed.
0 1884 American Chemical Society
coating. In the first case, fused-silica capillary fibers (catalogue no. FL100110140) with an inner silica rod diameter of 111f 5 pm and an outer diameter of 142 f 6 pm including the poly(dimethylsi1oxane) film were obtained from Polymicro Technologies (Tuscon, AZ). Approximately 1cm of a short length of fiber (4cm) was dipped into epoxy resin (EPO-TEK 353ND,Billerica, MA) and inserted into one end of a 1-ft length of 30-gauge stainless steel tubing (Hamilton, Reno, NV). The glue was cured for 1min at approximately 150 "C to secure the fiber, and then the exposed fiber was trimmed to 1 cm. The plunger of a 7105 Hamilton syringe was withdrawn completely form the syringe barrel. The plunger cap was unscrewed, and the plunger wire was removed by gripping the silver solder with pliers. The stainless steel was inserted up through the needle and plunger of the Hamilton 7105 syringe and then cut to a length of 21.5 cm. (Note: It is important to insert the stainless steel through the plunger and then cut the steel since cutting usually crushes the end of the steel and it may prevent it from passing through the small opening in the plunger.) A dab of standard 5-min epoxy glue was placed on the end of the stainless steel. Once the glue had cured, the plunger button was replaced and the syringe assembly was lead checked in agas chromatograph injector using a GOW-MAChelium leak detector. Leaks may arise where the needle joins the syringe body and also at the top of the syringe barrel. These can be corrected by tightening the knurled nut which holds the needle in place. Fused-silica fibers coated with Carbopack B were obtained from Supelco Inc. (Bellefonte, PA) and prepared as above. AVarian Saturn I benchtop gas chromatograph-ion trap mass spectrometer was used for the separation and analysis of all compounds. The gas chromatograph was a Varian 3400 equipped with a SPI (septum-programmable injector) and oven cryogenics. Separations were conducted using a SPB-5, 0.25 pm film thickness, 30 m X 0.25 mm i.d., Supelco Canada (Oakville, Ontario). For a fiber injection, the injector was held isothermal at 300 "C. The column was held at 60 "C for 1min, ramped at 15 "C/min to 280 "C, and held for 5 min. The temperature was then increased 20 "C/min to 300 "C and held for 4 min (Bakeout only). The transfer line and ion trap manifold were held constant at 280 and 250 "C, respectively. For syringe injections, the injector temperature was held at 50 "C for 0.1 min, ramped to 300 "C at 250 "C/min, and held at 300 "C for 2 min. The linear velocity at 280 "C was 43 cm/s. A similar program was used for the Carbopack B fibers, except the column was held at the initial temperature for 3 min and the fibers were desorbed for 10 min. The mass spectrometer was operated in the E1 mode and tuned to decafluorotriphenylphosphine (DFTPP) in accordance with U.S.EPA method 525. This is a very conservative tune and requires a target of =7000-10 000 ions. The filament emission current was set at 10-15 PA. The segment breaks required to tune successfully to DFTPP were 10-99,100-190,191-399, and 400-650. The corresponding segment tune factors were 50/140/100/45. The mass range scanned was from 45 to 450 as specified in U.S.EPA method 525. The ions used for quantitation were naphthalene = 128; anthracene = 178; benz[alanthracene = 228; benzo[alpyrene = 252; 2,2',5-PCB3 = 256; and 2,2',3,4,5'-PCB5 = 326.
A mixed PAH/PCB standard containing 1000pg/mL of naphthalene, anthracene, bsnz [alanthracene, and benzo[alpyrene was prepared by dissolving 100 mg of each analyte in 100 mL of toluene. The two polychlorinated biphenyls, 2,2',5-trichlorobiphenyl (PCB3) and 2,2',3,4,5'pentachlorobiphenyl (PCBB) were 2500 and 1000 pg/mL, respectively, in toluene. A spiking standard was prepared by spiking 250 pL of the PAH stock, 250 pL of the PCB5 stock, and 100 pL of the PCB3 stock into 25.0 mL of acetone to give a final concentration of 10 pg/mL of each analyte. A 10mpg/mL isotopically labeled spiking solution containing 1,4-dichlorobenzene-d4, naphthalene-da, acenaphthene-dlo, phenanthrene-dlo, chrysene-dla, and perylene-dl2 was prepared by spiking 125 pL of the 2000 pg/mL stock solution (Supelco, Canada; Oakville, Ontario) into 25.0 mL of acetone. The spiking standard and the isotopicallylabeled standard mixtures were spiked into reagent water inside 40-mL amber EPA vials (Supelco, Canada; Oakville, Ontario). The EPA vials contained a stir bar and 0.5 mL of headspace to prevent the sample from wicking up the syringe needle. Exposure profiles were obtained using reagent water spiked with 2.5 ng/mL of the target analytes. When required, the reagent water was always spiked with 2.5 ng/mL of the recovery surrogates. Deuterated benzo[alpyrene-dlz, 10 pg/mL was prepared by spiking 250 pL of the stock solution (1000pg/mL) into 25.0mL of acetone. This was used periodically to check the integrity of the standard. Desorption was performed with the fiber just above the restriction in the SPI injector. A signal-to-noise ratio of 3:l was used as the criteria for the limit of detection (LOD). Precision was determined by analyzingeight water samples spiked at the same concentration. The relative standard deviation (RSD) was calculated by dividing the standard deviation by the mean and multiplying by 100. Carryover was determined by analyzing a known concentration and then running consecutive fiber blanks to determine the fraction of the original mass desorbed remaining on the fiber.
Results and Discussion In solid-phase microextraction, analytes partition between the stationary phase on the fiber and the aqueous phase until an equilibrium state is reached. There is a linear relationship between the amount of analyte absorbed by the fiber (n,) and the concentration in the sample (Caq) (17). The linear range and sensitivity will depend upon two factors, the volume of the stationary phase (V,) and the distribution constant ( K ) . If K for a particular analyte is very large, as is often the case with nonpolar semivolatile analytes, or the sample volume is small, then at equilibrium the initial analyte concentration may be significantly depleted (19). In this case, the equilibrium is described by
where V,, is the volume of the aqueous sample and C: is the initial concentration in solution. The term I&, decreases the amount of analyte absorbed by the coating; however, this decrease is only significant if the magnitude of KV, is comparable to Vaq (19). If the distribution constant for an analyte exceeds the phase ratio V,/Va, by Environ. Scl. Technol.. Vol. 28. No. 2, 1994 288
25 p d m L
3
4
5
6
LOG,,(DISTRIBUTION CONSTANT)
Flguae 1. Predicted detection limits vs distribution constant for a I-cm length of fused silica coated with a 15-pm poly(dimethylsi1oxane)film.
an order of magnitude then, KV, >> Vaq and a virtual total extraction occurs (19). The two main factors which affect the LOD and linear range are the stationary phase and the GC detector (20). Sensitivity can be improved by increasing the volume of stationary phase or by changing the selectivity of the stationary phase. Anthracene, benz[al anthracene, benzo[al pyrene, 2,2’,5trichlorobiphenyl (PCB3), and 2,2’,3,4,5’-pentachlorobiphenyl (PCB5) were chosen as representative polyaromatic hydrocarbons and polychlorinated biphenyls for this study. The Ko,’s of these compounds range from 35000 to 7 100 000. Naphthalene has a KO, of only 1000 and is considered a volatile analyte by the U.S. EPA and included in method 524.2, but it is a borderline semivolatile analyte according to the definition of Keith (6). Since KO,% have been shown to be a good approximation of poly(dimethylsi1oxane)-water distribution constants for nonpolar analytes and the LOD for the ion trap mass spectrometer has been established as approximately 1.5 pg for substituted benzenes (22), the detection limits were estimated for the target analytes by rearranging eq 1to
At equilibrium, the predicted LOD, Caq,for a particular analyte can be determined for a stationary phase of volume, V, in a solution of volue, Vas, where n, is the LOD of the mass spectrometer. Assuming n, = 1.5 pg and using a 1-cm length of a 15-pm poly(dimethylsi1oxane) coating immersed in a 1000-mLsample, the LODs for each of the semivolatile target analytes were predicted as shown in Figure 1. For an analyte such as benzo[a]pyrene which has an extremely large KO,of 106, an incredibly low LOD of 27 fg/mL (parts per quadrillion) is predicted. Naphthalene which has the lowest KO,(1000) is still predicted to have a LOD of approximately 25 pg/mL (parts per trillion). The LOD should decrease by an order of magnitude as the distribution constant increases by an order of magnitude. However, even for a 1000-mLsample size, a small effect of KV, is evident as the estimated LODs range from 25 pg/mL to 27 fg/mL. Although the effect of KV, is insignificant for a 1000-mL sample size, the impact increases as the sample volume decreases. Since many of the analytes in US.EPA method 525 have verylargeKow’s, 300
Environ. Sci. Technoi., Vol. 28, No. 2, 1994
a sample may be significantlydepleted after one extraction. For a distribution constant of lo6, almost 100% of the analyte will be extracted from a 1-mL sample vial when using a 15-pmpoly(dimethylsi1oxane)-coatedfiber. Even a sample volume of 1000 mL will have 6% of the total mass available extracted. While these theoretical predictions show the sensitivity of the technique, they also demonstrate the importance of sampling each sample vial only once if K is large. Using a 15-pm poly(dimethylsi1oxane)-coated fiber, naphthalene reached equilibrium in approximately 6 inin while anthracene required a 20-min equilibration time. Benzblanthracene, PCB3, and PCB5 required 60 min to reach equilibrium while benzo[alpyrene required 120min. Since K describes the distribution of analyte between the fiber coating and the aqueous phase, as K increases so will the mass of analyte absorbed by the coating. The ratedetermining step of the equilibrium process is the diffusion of analyte across a thin stationary, aqueous layer at the coating-aqueous-phase boundary (19). Therefore as K increases, so will the equilibration time since a greater mass must diffuse across the static layer (19). Since the required detection limit is 40 pg/mL, there is no need to sample for such a long time. If isotopically labeled standards are added to the sample before the extraction, the nonlabeled analytes can be determined directly using response factors without having to achieve 100% extraction into the fiber coating. Ideally an analyte should be quantitated using its isotopically labeled analog. However, to limit the cost of the project, the analytes were matched according to retention time with a few isotopically labeled compounds specified in U.S. EPA method 525. By choosing an exposure time of 20-30 min, those compounds with K values less than lo4should be at equilibrium, the higher K compounds will not yet be at equilibrium, but should still have ample sensitivity. For semivolatile analytes, a GC run is usually in excess of 30 min; and therefore, for convenience and optimum sensitivity, a sample can be extracted for the same amount of time required for the GC to complete a chromatographic run and return to the ready state for the next injection. Using a 15-pmpoly(dimethylsi1oxane)-coatedfiber and matching the target analytes to the deuterated surrogates as in US. EPA method 525, all native analytes were linear from 6.25 pg/mL to 3.75 ng/mL after only a 10-min agitation time. Correlation coefficients greater than 0.990 were obtained for all calibration curves. The worst sensitivity was obtained for naphthalene, which is to be expected since it haa the lowest K value. For 6.25 pg/mL naphthalene, a signal-to-noiseratio of 2 was obtained while a signal-to-noise ratio of 22 was obtained for 125 pg/mL naphthalene. The actual limit of detection is probably about 20 pg/mL for this analyte. This is in good agreement with the LOD of 25 pg/mL predicted earlier for a distribution constant of 1000. A signal-to-noise ratio of =6 was obtained for 6.25 pg/mL of anthracene, indicating a LOD of approximately 3 pg/mL. This also agrees favorably with the predicted LOD of 2.5 pg/mL for a distribution constant of lo4. Benzlalanthracene and benzo[alpyrene were linear from 1.25 pg/mL to 3.75 ng/ mL. At 1.25 pg/mL, signal-to-noise ratios of 3 and 4, respectively, were obtained. The ion chromatogram for 6.25 pg/mL of benzo[alpyrene ( m / z = 252) is shown in Figure 2A while the full-scan background subtracted spectrum which allows for easy library searching is shown
a .in&
Table 1. Comparison of RSDs Achieved Using SPME with U.S.EPA Method 525
7
U.S.EPA method 525 ( n = 7) SPME (n = 8) C18-cartridge C18-dish (250 pg/mL) (200 pg/mL) (200 pg/mL) naphthalene anthracene benz[alanthracene benzo[al pyrene trichlorobiphenyl pentachlorobiphenyl
8 8 10 11 19 16
7.7
13.7 33.2 21.7 21.4 15.1
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Flgure 3. Carryover for the 15-pm poly(d1methylsiloxane) flber after sampling a 4 ng/mL solution of the target analytes.
MIZ.
Flgure 2. (A) Sampling 6.25 pg/mL of benzo[a]pyrene in water for 10 min generates a signal-to-noise ratio of 1O:l. (B) The full scan background subtracted spectrum allows for easy library searching.
in Figure 2B. With the exception of naphthalene, the signal-to-noise ratios obtained suggest LODs of 1.25 pg/ mL for benz[alanthracene and benzo[alpyrene while the remaining compounds have LODs of approximately 2-3 pg/mL. The required detection limits for U.S. EPA method 525 are 40 pg/mL for the PAHs studied, 60 pg/mL for trichlorobiphenyl, and 100 pg/mL for pentachlorobiphenyl. These detection limits were easily exceeded. None of the analytes were detected in fiber or water blanks analyzed prior to sampling the standards. The standards were run in order of increasing concentration, and a fiber blank was run between each standard to minimize carryover, which will be discussed below. These detection limits were achieved using only a 10-min extraction time. In order to reduce the equilibrium time and realize low picogram per liter detection limits, efficient agitation methods such as sonication can be used. Using sonication, the equilibration time for benzo[a]pyrene was reduced from 120 to 20 min while that of anthracene was reduced to 6 rnin from 20 min. After 20 min, the amount of anthracene absorbed by the fiber began to decrease, and it is believed this was due to destruction of the analyte molecule. Using approximately 50 W of power, previous authors have used sonication to destroy PAH molecules, specifically benzo[alpyrene (23). In this study, a sonication power of 12 W was used, and there was no effect on any of the compounds with the exception of anthracene. To evaluate the precision of the SPME, eight spiked water samples each containing 250 pg/mL of the target analytes and 2.5 pg/mL of the deuterated surrogates were analyzed for 20 min. A fiber blank was run between each
sample to reduce memory effects. Relative standard deviations (RSD) of 8 were obtained for naphthalene and anthracene. Benz[a]anthracene and benzo[alpyrene had RSDs of 10 and 11,respectively, while PCB3 and PCB5 were 19 and 16, respectively. These results are tabulated in Table 1 along with the RSDs obtained in US. EPA method 525 for solid-phase extraction using an ion trap mass spectrometer. As can be seen in Table 1,the results obtained using SPME are comparable to U.S. EPA method 525 and less than the upper limit of 30%. Naphthalene and anthracene have the two lowest KO, values in the mixture and, therefore, reach equilibrium more quickly than the other compounds. They are also better matched to the surrogates used to quantitate them. Since naphthalene-& was in the deuterated spiking mixture, native naphthalene was quantitated using isotopic dilution. Anthracene was quantitated using deuterium labeled phenanthrene. The poor precision for the PCBs indicates that chrysene-cllzis a poor choice for a recovery surrogate for this type of method. The native compounds should be quantitated with a compound that has a similar distribution constant. The loglo KO,for PCB5 is reported as 6.85 while the loglo KO,for chrysene-dlz is 5.61-5.84. A better choice of an internal standard for the PCBs would likely be 37Cl-or 13C-labeledisomers since the K values of the labeled analytes should be essentially identical to the native analytes. Poor precision of the high K compounds may also be partly due to carryover, which can be corrected by using higher desorption temperature and more thermally stable coatings. Carryover of semivolatile compounds was significant when using a 15-pm poly(dimethylsi1oxane)-coated fiber. As shown in Figure 3, carryover ranged from 9 % for trichlorobiphenyl to 23 % for pentachlorobiphenyl immediately after sampling a 4 ng/mL solution and desorbing at 300 OC for 1min. After the 4th fiber, blank carryover has dropped to below 1% only for the lower K compounds Envlron. Scl. Technol., Vol. 28, No. 2. 1994 301
Table 2. Comparison of Ootanol-Water (Kow) Partition Coefficients and Experimentally Determined Distribution Constants for PAH/PCB Target Analytes
distribution constant analyte naphthalene anthracene benz[alanthracene benzo[alpyrene PCB3 PCB5
loglo K,,
1% (experimentally determined)
3.01-3.59 4.54 5.61 6.44 5.64 6.85
naphthalene, anthracene, and trichlorobiphenyl. Carryover is significant for benz[alanthracene, benzo[al pyrene, and pentachlorobiphenyl even after four fiber blanks. In order to prevent erroneous results due to carryover, all fiberswere first checked for background levels of the target analytes. Normally memory effects could be reduced to insignificant levels after several consecutive desorptions. When analyzing unknown samples, a new fiber proven to be free of background levels of the analytes was used. If a fiber became severely contaminated, it was disposed of. A new fiber could be prepared in 10-15 min at a cost of only a few dollars. Some of the carryover of semivolatile components may be due in part to chemisorption of the analyte molecules to the silica surface. Silica with only 0.28 pmoI/m2 of strongly adsorptive sites is capable of dissociative adsorption of hydrocarbons (24). Trace metal impurities are thought to be responsible for the strong adsorption of some analyte molecules to silica. Even chromatographic grade silica contains 0.1-0.3 '?6 metal oxide impurities including Na, Ca, Al, Mg, Ti, and Fe (25). The use of high-purity chromatographic-grade silica should help minimize carryover due to irreversible adsorption. Carryover can also be reduced by increasing the desorption temperature and the length of time the fiber is held in the injector; however, high background levels are observed. High phthalate background levels are likely from the poly(dimethylsiloxane)since it was not chromatographic grade and may have contained a significant number of hydroxyl groups. The absence of hydroxy-terminated end groups is essential for thermal stability since these groups can accelerate depolymerization reactions up to 30-fold as compared to end-blocked siloxanes (26). Impurities in the silica substrate can also contribute to the instability of a stationary phase. Poly(dimethylsi1oxane) stationary phases have been shown to be stable up to 400 "C when carefully prepared on substrates free of impurities which promote thermal degradation (26). At high temperatures, many bis-substituted compounds appeared in the chromatographs. It is believed that these compounds arise from the epoxy resin used to glue the fiber inside the stainless steel. Epoxy resins capable of withstanding temperatures well in excess of 400 OC are now available and should be investigated as an alternative glue. The distribution constants for the semivolatile analytes were determined by calculating the mass absorbed after a 2-h equilibration time. Syringe injections of the analytes in acetone were made in order to determine the actual mass desorbed from the fiber onto the column. The results are shown in Table 2. The experimentally determined distribution constant for naphthalene is in excellent agreement with the Kow. Anthracene is in fairly good 302 Envlron. Sci. Technol., Vol. 28, No. 2, 1994
I
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3.01 4.10 4.96 4.86 4.94 4.89
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Figure 4. Organics extracted from a groundwater sample include naphthalene, methylnaphthalene, biphenyl, acenaphthene, dibenzofuran, and SKfluorene.
agreement as well. All of the other compounds show very poor agreement between the experimentally determined K value and the Kow In fact, for benzo[alpyrene and PCB5 the differenceis 2 orders of magnitude. One possible explanation for this disagreement is the large amounts of carryover for these high K compounds. For example, after four fiber blanks, the sum of the carryover is equal to 40% of the initial mass desorbed for benzo[a]pyrene. Recalculating the distribution constant with this additional mass considered increases the loglo K value to 5.02 from 4.86. Another contributingfactor to the discrepanciesbetween the experimentally determined K values and the KO,% may be exhaustive extraction of the sample. For aKvalue of lo6, 60-70% of the sample will be extracted from a 40-mL sample vial at equilibrium. This is assuming that the only equilibrium occurring in the sample vial is between the aqueous phase and the coating on the fiber. However, analytes will also be partitioning between the aqueous phase and the glass walls and between the aqueous phase and the septum. If exposure profiles are collected using unsilanized glassware, a significant decrease in the mass absorbed is noticed especially for benzo[alpyrene. After 180min,the mass of benzo[alpyrene absorbed by the fiber was only 40% of that absorbed at 60 min. Significant losses of PAHs to the walls of glass storage bottles are well known. A 53 % loss of benzo[alpyrene has been observed after only 1 h (27). Distribution constants could be obtained most accurately by using a continuous flowthrough system since in this case the volume is essentially infinite. The 15-pmpoly(dimethylsi1oxane)coating used in these experiments was a commercial grade used to manufacture optical fiber for communication purposes and was obviously not an ideal coating because of its thermal instability and high memory levels. However, this coating may be well suited for screening samples where only qualitative information is required. Quantitation capability of the SPME/GC/ITS system was demonstrated using a groundwater sample obtained from an experimental site at Canadian Forces Base Borden. This sample was found to contain 19 pg/mL naphthalene. A duplicate sample analyzed by conventional liquid-liquid extraction gave a result of 15 Fg/mL naphthalene. A total ion chromatogram is shown in Figure 4. In addition to naphthalene, methylnaphthalene, biphenyl, acenaph-
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Table 3. Comparison of Surrogate Standards Recovered from Spiked Reagent Water Relative to Sample Containing Particulate Matter
surrogate standard
sample 1
sample 2 (duplicate)
Percent Recoveries Relative to Spiked Reagent Water 78 92 1,4-dichlorobenzene-d4 naphthalene-ds 100 120 acenaphthene-dlo 96 112 phenanthrene-dlo 34 33 chrysene-dlz 8 8
thene, dibenzofuran, and 9H-fluorene were also detected. These were known to be present in the sample at levels ranging from 80 ng/mL for dibenzofuran to 1pg/mL for 2-methylnaphthalene. The SPME/GC/ITS system is ideal for screening purposes, as can be seen on the ion chromatograms corresponding to the water sample from the spill site (see Figure 5). Ten minutes exposure of a 15-pm poly(dimethylsiloxane)-coated fiber resulted in positive identification of naphthalene, anthracene, benz[al anthracene, and benzo[alpyrene as contaminants. The SPME device has also the potential to be used as a more quantitative tool for analysis of dirty aqueous matrices as demonstrated by analysis of duplicate water samples obtained after primary treatment from a sewage treatment plant. These were spiked with 2.5 ng/mL of the deuterated spiking mixture and analyzed using a 15pm poly(dimethylsi1oxane) coating. Duplicate results showed 3.6 and 3.4 ng/mL native naphthalene, respectively, indicating good reproducibility of the technique. However the matrix effects can play havoc with surrogate recoveries as was evident when sampling the sewage sample. The recoveries obtained for the deuterated analytes from the wastewater sample are compared to those obtained from a clean-water sample in Table 3. The low K compounds 1,4-dichlorobenzene-d4, naphthalene-da, and acenaphthene-dlo showed virtually identical recoveries from both wastewater and clean water. However, there was a significant decrease in recoveries of the high K value
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internal standards. Recoveries for phenanthrene and chrysene were 34 and 8%, respectively, indicating strong competition between suspended matter and the fiber coating. While SPME works well for clean water samples such as those addressed by U S . EPA method 525, the effects of suspended and dissolved organic matter on precision and accuracy need to be fully investigated before this technique can be applied quantitatively to wastewaters. In order to improve the carryover and blanks, new thermally stable coatings need to be investigated. These include thinner poly(dimethylsi1oxane) coatings as well as other coatings. As an alternative coating, fibers prepared with a thin layer of Carbopack B were investigated. Carbopack B is a nonspecific graphitized carbon adsorbent which has a surface area of approximately 100 m2/g. As well as being a nonspecific adsorbent Carbopack B has been shown to posses anion-exchange properties due to contamination by positively charged chemical impurities (28). For neutral molecules,surface adsorption depends primarily upon van der Waals dispersion forces and is a function of the adsorbents surface area. The Carbopack B fibers produced virtually zero bleed when desorbed at 300 "C for 10 min. Several nitrogencontaining compounds were present in the fiber blanks, but these were thought to arise from the glue. A time exposure profile shown in Figure 6 shows that all compounds have reached equilibrium in 60 min. As can be seen from the mass adsorbed, excellent sensitivity was obtained for all compounds. Linearity was investigated using a 20-min sampling time with impressive results. All compounds were linear from 10 pg/mL to 2.5 ng/mL and had correlation coefficients greater than 0.99 with the exception of PCB5, which had a correlation coefficient of 0.97. Because of the low bleed of the Carbopack B, at concentrations near the detection limit virtually zero noise was detected after the analyte peaks, and the signal-tonoise ratios could not be accurately determined. Past experience with the poly(dimethylsi1oxane) coating and ion trap mass spectrometer has shown that a signal-tonoise ratio of 3:l corresponds to an integrated peak area Environ. Sci. Technol., Voi. 28, No. 2, 1994 303
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EXPOSURE TIME (min) Flgure 6. Exposure profile showing mass adsorbed (concentration = 250 pg/mL) using a fiber coated with Carbopack B. All analytes have equilibrated In 60 mln.
of ~ 2 0 counts. 0 Using this criteria, the detection limits for the Carbopack B coating range from 7 pg/mL for naphthalene to 1 pg/mL for benzo[a]pyrene. Precision was determined at 20 and 250 pg/mL (n = 8) and ranged from 7 to 20%. As expected, precision was worst for the low-level standard; however, the results obtained are still much less than the criteria of 30% established for U S . EPA method 525 (29). One of the objectives in switching to the Carbopack B fiber was to reduce the amount of carryover for the target analytes. After sampling a 250 pg/mL sample spiked with the target analytes for 2 h and then desorbing for 10 min at 300 "C, some promising results were obtained. Carryover was not detected for naphthalene while anthracene and benz[alanthracene had about 2 % carryover. Benzo[alpyrene showed carryover while PCB3 and PCB5 had 10 and 17 % , respectively. Samples from a sewage treatment plant were spiked in duplicate with the deuterated spiking mixture and analyzed using the Carbopack Bcoated fiber. Native naphthalene and anthracene were found in both these samples, The first sample was found to contain 10 ng/mL naphthalene and 404 pg/mL anthracene while the duplicate sample contained 13 ng/mL and 407 pg/mL of these analytes, respectively. These samples should not be compared with the sewage samples analyzed using the poly(dimethylsi1oxane) coating discussed above since they are different samples. Conclusions SPME is a promising analytical technique for the quantitative analysis of semivolatile pollutants in present in clean aqueous samples. Using a thin poly(dimethy1siloxane) coating, the detection limits and precision required by U S . EPA method 525 can be easily met and exceeded for naphthalene, anthracene, benz[alanthracene, benzo[al pyrene, trichlorbiphenyl, and pentachlorobiphenyl. This coating can also be used for analysis of other groups of nonpolar contaminants such as polychlorinated dioxins (30). This technique is inexpensive, rapid, and simple. The large distribution constants of semivolatile analytes translate into very low picogram per liter (part per quadrillion) detection limits. Sonication can be used 304
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to reduce the equilibration time if these low detection limits are required, otherwise 30 min (less than a chromatographic run) is ample for low part per trillion detection limits. In many cases, the objective of an environmental study is long-term monitoring, and equlibration time would not be an issue. SPME would be ideal for the long-term monitoring of lakes and rivers. After several days of sampling,the fiber would simply be desorbed in the injector of a portable GC or GC/MS on-site or sent to the main laboratory for analysis. There would not be any need to transport solvents and samples. For routine laboratory analysis, the use of deuterated surrogate standards permits low (pg/mL) detection limits to be obtained for large Kcompounds using sampling times as short as 10 min while maintaining relative standard deviations (RSDs) of less than 20%. Acknowledgments This work was supported in part by the National Sciencesand Engineering Council of Canada, Supelco Inc., Supelco Canada, Varian Associates, and Dow Chemical. We are especially grateful to Supelco for providing the Carbopack B-coated fibers. Thanks to Kim Hamilton of the Waterloo Centre for Groundwater Research, Brian MacGillivray of the Waste Water Technology Centre, Burlington, Ontario, and David Andrews from the Regional Municipality of Waterloo Environmental Laboratory for providing the samples and analytical data. The help of Zhouyao Zhang with the analysis of the samples was greatly appreciated. Literature Cited (1) Futoma, D. J.; Smith, S. R.; Smith, T. E.; Tanaka, J.
Polycyclic Aromatic Hydrocarbons in Water Systems;CRC Press, Inc.: Boca Raton, FL, 1981. (2) Cseh, T.; Sanschagrin, S.; Hawari, J.; Samson, R. Appl. Enuiron. Microbiol. 1989, 55, (12), 3150-3154. (3) Hoff, R. M.; Muir, D. C. G.; Grift, N. P. Enuiron. Sci. Technol. 1992, 26, 266-275. (4) Hoff, R. M.; Muir, D. C. G.; Grift, N. P. Enuiron. Sci. Technol. 1992,26, 276-283. (5) Vo-Dinh, T. Significance of Chemical Analysis of Polycyclic Aromatic Compounds and Related Biological Systems. In
Chemical Analysis ofPolycyclic Aromatic Compounds;VoDinh, T., Ed.; John Wiley & Sons Inc.: New York, 1989;pp 1-30. Keith, L. H. Environmental Sampling and Analysis: A Practical Guide;Lewis Publishers Inc.: Chelsea, MI, 1992; p 44. Lane, D. A. The Fate of Polycyclic Aromatic Compounds in the Atmosphere and During Sampling. In Chemical Analysis of Polycyclic Aromatic Compounds; Vo-Dingh, T., Ed.; John Wiley & Sons Inc.: New York, 1989;pp 31-58. Erickson, M. D. Analytical Chemistry of PCBs, Butterworth Publishers: London, 1986,pp 1-5. Harlem, R. L.; Oswald, E. 0.; Wilkinson, M. K.; Dupuy, A. E., Jr.; McDaniel, D. D.; Tai, Han. Anal. Chem. 1980,52, 1239-1245. Liska, I.; Krupcik, J.; Leclercq, P. A. J. High Resolut. Chromatogr. 1989,12,577. Junk, G. A,; Richard, J. J. Anal. Chem. 1988,60,451. Garrigues, Ph.; Bellocq, J. J. High Resolut. Chromatogr. 1989,-12,400. Pankow, J. F.; Ligocki, M. P.; Rosen, M. E.; Isabelle, L. M.; Hart, K. M. Anal. Chem. 1988,60,40. (14) Durhan, E.J.; Lukasewycz, M. T.; Amato, J. R. Environ. Toxicol. Chem. 1990,9,463. (15) Schuette, S.A.; Smith, R. G.; Holden, L. R.; Graham, J. A. Anal. Chim. Acta 1990,236,141. (16) Belardi, R. G.; Pawliszyn, J. Water Pollut. Res. J . Can. 1989,24, 179. Arthur, C. L.; Pawliszyn, J. Anal. Chem. 1990,62,2145. Arthur, C. L.; Killam, L. M.; Buchholz, K. D.; Pawliszyn, J. Anal. Chem. 1992,64,1960-1966. Louch, D. S.;Motlagh, S.; Pawliszyn, J. Anal. Chem. 1992, 64,1187-1199. Arthur, C. L.; Potter, D. W.; Buchholz, K. D.; Motlagh, S.; Pawliszyn, J. LC-GC 1992,10,656-661.
(21) Arthur, C. Lo; Killam, L.; Motlagh, S.; Potter, D. W.; Pawliszyn, J. Environ. Sci. Technol. 1992,26,979. (22) Potter, D. W.; Pawliszyn, J. J. Chromatogr. 1992,625,247255. (23) D'Silva, A. P.; Laughlin, S. K.; Weeks, S. J.; Buttermore, W. H. Polycyclic Aromat. Compd. 1990,1 (3))125-135. (24) Morterra, C.; Low, M. J. D. J. Catal. 1973,28, 265. (25) Nawrocki, J.; Buszewski, B. J.Chromatogr. 1988,449,l-24. (26) Rotzsche, H. Stationary Phases in Gas Chromatography; Journal of Chromatography Library Vol. 48;Elsevier Science Publishing Co. Inc.: New York, 1991;pp 186-286. (27) May, W. E.; Brown, J. M.; Chester, S. N.; Guenther, F.; Hilpert, L. R.; Hertz, H. S.; Wise, S. A. In Polynuclear Aromatic Hydrocarbons;Jones, P. W., Leber, P., Eds.; Ann Arbor Science: Ann Arbor, MI, 1979;pp 411-418. (28) Andreolini, F.; Borra,C.; Caccamo, F.;Di Corcia, A.; Samperi, R. Anal. Chem. 1987,59,1720. (29) Eichelberger, J. W.; Behymer, T. D.; Budde, W. L. Determination of organic compounds in drinking water by liquidsolid extraction and capillary columngas chromatography/ mass spectrometry, Revision 2.1;Environmental Monitoring Systems Laboratory Office of Research and Development, U.S. Environmental Protection Agency: Cincinnati, OH, 1988. (30) Potter, D. W. Rapid Determination of Organics in Aqueous Samples Using Solid Phase Microextraction and Capillary Gas Chromatography-Ion Trap Mass Spectrometry. M.S. Thesis, University of Waterloo, 1993. Received for review April 29, 1993.Reuised manuscript receiued September 16, 1993.Accepted October 29,1993." Abstract published in Advance ACS Abstracts, December 1, 1993.
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