Environ. Sci. Technol. 1992, 26, 979-983
Analysis of Substituted Benzene Compounds in Groundwater Using Solid-Phase Microextraction Catherine L. Arthur, Lisa M. Klllam, Safa Motlagh, Megan Llm, David W. Potter, and Janusr Pawlisryn" The Guelph-Waterloo Centre for Graduate Work in Chemistry and the Waterloo Centre for Groundwater Research, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
rn Solid-phase microextraction (SPME) is applied to the analysis of benzene, toluene, ethyl benzene, and xylenes in groundwater. The inexpensive SPME method reduced the sample preparation time by 3-7-fold when compared to purge and trap methods. The relative standard deviation ranged from 3 to 5% for the single-operator relative standard deviation using a methyl silicone fiber. Limits of detection of 1-3 ppb (w/v) were obtained when using a fiber coated with 56-pm methyl silicone film and FID detection. The linear range extended from 15 to 3000 ppb (w/v). Solvents have been completely removed from the sample preparation step. Introduction The analysis of groundwater for organic contaminants can require any one of several well-established methods of sample preparation. These include liquid-liquid extraction ( I , Z),solid-phase extraction (3),purge and trap ( 4 4 3 , headspace analysis, and a variety of less widely used techniques such as concentration onto uncoated, capillary columns (7). The appropriate technique can be chosen based upon sample concentration, analyte volatility, the number of samples to be analyzed, and available equipment. The primary problems with liquid-liquid extraction are the difficulties in automating the method and the high volume of organic solvent required. Solid-phase extraction can be automated and uses very little solvent; however, solvent blanks are high and the cartridges are not consistent between manufacturers. Thermal desorption of solid-phase cartridges generally requires extensive modification of a gas chromatograph or addition of a desorption module (8). Purge and trap is used for volatile components but can suffer from contaminated traps, is highly susceptible to leaks, and can use large amounts of liquid nitrogen. In combination with full-scan mass spectrometry, purge and trap is the method used by the United States Environmental Protection Agency (EPA). Headspace analysis is confined to those samples that are both volatile and relatively highly concentrated, although it can be an excellent method for sampling "dirty" or viscous samples. Solid-phase microextraction (SPME) is a simple alternative to the above techniques. SPME eliminates the use of solvents, is fast, and can be easily automated. It can be used with liquid, gaseous or "dirty" samples. This technique is based on chemically modified fused-silica fibers (9). These fibers are widely used in optical communication and are often referred to as optical fibers. Their diameters vary between 0.05 and 1.0 mm. The SPME process consists of a few simple steps. The fiber, with immobilized organic film, is inserted into the aqueous sample (Figure 1). An equilibrium is established between the fiber and the water when the organic components are extracted from the water into the stationary phase. The fiber is then inserted directly into a conventional gas chromatographic injection port, such as a split-splitless, septum programmable (SPI), or on-column. The organic analytes are thermally desorbed and analyzed. The fiber is supported in a syringe to allow it to be handled 0013-936X/92/0926-0979$03.00/0
easily and to protect the fiber during penetration of septa.
A linear relationship is expected between the amount absorbed by the fiber and the concentration in solution (9). Experimental Section SPME devices were assembled as in ref 9 except the top 15 cm was replaced by either 30-gauge stainless steel (Hamilton Syringes) or fused-silica capillary with an inner diameter of 200 pm and an outer diameter of 300 pm (Polymicro Technologies, Tuscon, AZ). The fiber was attached to the casing by stripping off 1-2 cm of coating and inserting the stripped portion inside the tubing. The fiber was immobilized in the sheathing using polyimide sealing resin (Restek Corp., Bellefonte, PA) or epoxy glue. The steel prevents coating from being abraded from the fiber at the ferrule seal at the base of the syringe barrel. The ferrule ensures that the sample cannot "back-flash" up the syringe and that oxygen cannot leak into the gas chromatograph. When a new syringe was assembled, the syringe was tested for leaks for desorbing the fiber in the injector of a cold GC. The knurled knob of the syringe was tightened until leaks could not be detected either at the knurled knob or at the top of the syringe barrel. All fibers were obtained from Polymicro Technologies. Fibers used as "uncoated" were received from the supplier with a polyimide coating. The bottom 4 cm of the fiber was stripped by soaking the fiber in hot, concentrated sulfuric acid for approximately 5 min. Fibers coated with methyl silicone were used as is from the manufacturer. All method development was completed with a 56-pm film thickness. The effect of film thickness was investigated using E - , 56-, and 97-pm film thicknesses. All separations were done using a Varian 3500 gas chromatograph equipped with a SPI injector, oven cryogenics, and a flame ionization detector (FID). The separation was done on a 30 m X 0.32 mm fused-silica column coated with a 1.0-pm SPB-1 film. The FID was operated with He carrier at a flow rate of 39 cm/s, nitrogen make-up at 25 mL/min, hydrogen at 30 mL/min, and air at 300 mL/min. Gas chromatographic conditions were as follows: injector, 15 "C for 0 min, 250 "C/min to 250 "C, hold 5 min; oven, 15 "C for 2 min, 15 "C/min to 90 "C, hold 1 min; detector, FID range 11or 12,275 "C. The oven was held at 15 "C for 2 min to cryofocus the analytes onto the column. Without this focusing, peaks are broad and tail. The location of the fiber in the injector is important as it affects the efficiency of the desorption and the relative standard deviation. For the SPI injector, the optimum location for the fiber is just above the restriction in the glass insert. To find this location, the syringe needle-with the fiber retracted-is placed in the injector until it "bottoms" on the restriction in the glass insert. The needle is withdrawn 1.1cm for a l-cm fiber length, and the needle is marked so that the injections are made to this same depth each time. Spiking standards were prepared in methanol and spiked into water inside sealed, 50-mL hypovials. A stir bar must also be inside the vial, and 0.5 mL of headspace
0 1992 American Chemical Society
Envlron. Sci. Technol., Vol. 26, No. 5, 1992 979
--t -t
. Syringe Barrel
--t
. ? 0
0
BENZENE TOLUENE ETHYLBENZENE MIPXYLENE OXlLENE
~
200
400
I
6OC
i l m e Is)
Needle
Figure 2. Absorptlon-time profile for BTEX compounds absorbed by a 56-pm methyl silicone film from a solution containing 740 pg/L of each compound.
Table I. Comparison between Experimental Distribution Constants and Literature Values for the Octanol-Water Partition Coefficient
An aly te In Water
Magnetic Stirring Bar
Flgure 1. Method of sampling using an optical fiber. The flber is dipped into the vial for 2-5 min with the solution stirred. Normally, 1 cm of fiber is exposed.
was left. The headspace prevented the sample from wicking up the syringe needle. Relative standard deviation was determined by analyzing the same solution eight times on a single day with a single operator. The relative standard deviation is calculated by dividing the standard deviation, s, by the average and multiplying by 100. Linear range and limit of quantitation were determined by spiking water with standards prepared in methanol. The limit of quantitation was defined as the concentration that gave an analyte peak height that was 10 times the standard deviation of the baseline noise. The limit of detection was defined as three times the standard deviation of the baseline noise. Sample preparation time was taken to be the time required for the stationary phase and water to attain equilibrium.
Results and Discussion A method was developed for benzene, toluene, ethyl benzene, and the three xylene isomers (BTEX) because these compounds represent common groundwater contaminants (IO);the technique has considerable potential for environmental analysis, both in the laboratory and for field sampling. The method was developed using a coated and an uncoated fiber; this provides the analyst with some choice in linear range and illustrates the effect of the coating. The initial step in the development of a method using SPME was determining the time required for an equilibrium to form between the stationary phase and the sample. The relative standard deviation, linear range, limit of quantitation, and limit of detection were then established. The absorption-time profile using a methyl silicone coating is shown in Figure 2 for a water sample containing equal amounts of each compound in the BTEX mixture. The rn- and p-xylene isomers were not resolved on the chromatographic column. The equilibration time ranges from 2 min for benzene to 6 min for the xylenes, and greater amounts of the xylenes are also absorbed relative to the benzene. The longer equilibration time for the xylenes is a result of the larger distribution constant, which 980
Envlron. Sci. Technol., Vol. 26, No. 5, 1992
chemical
KO,
K
benzene toluene ethylbenzene o-xvlene
135 489 1412 589
126 340 528 654
increases the amount of analyte which needs to diffuse through the unstirred layer next to the fiber. The amount absorbed by the stationary phase is primarily affected by three factors: the distribution constant, the volume of the stationary phase, and whether the solution has been stirred (9). This relationship is described mathematically by eq 1,where the number of moles of analyte absorbed into the n, = KV,C,,
(1)
stationary phase, n,, is proportional to the distribution constant, K, the volume of the stationary phase, V,, and the concentration in the water, Caq. For the compounds in the BTEX mixture, the volume of the Stationary phase and the amount of stirring are constant and the difference in the amount absorbed is caused by the differences in the distribution constant. By calculation of the volume of the stationary phase (56-pm film thickness and 1cm exposed), the distribution constant was calculated. These values are shown in Table I, where they are compared to the octanol-water partition coefficient (K0J (11). The experimental and literature values are of the same order of magnitude and trend in the same direction, with the exception of ethylbenzene. Thus, the distribution coefficient for a methyl silicone film can be estimated from the KO, value. This can be valuable when new methods are being developed as the linear range and limit of quantitation can be estimated before experimental work is started. The effect of the volume of the stationary phase on the absorption-time profile is shown in Figure 3. Three fibers with different thicknesses and a 1-cm exposed length were equilibrated with a 1 ppm solution of benzene in water. As expected from eq 1, the amount absorbed increased with the film volume. This can be used in method development to optimize the limit of quantitation and linear range that is desired-where high sensitivity and f or greater linear range is required a thicker film can be used. The effect of stirring on the absorption-time profile for benzene is shown in Figure 4. If the sample is insufficiently stirred, equilibrium will not be reached in a short period of time as maximum amount absorbed is limited by the rate of diffusion through the water (12). Equilib-
loo
1
Table 11. Comparison of BTEX Method Parameters for an Uncoated Fiber and a Fiber Coated with a 56-cm Methyl Silicone Film
Prep time, min
analyte
linear range, pg/L
LOD, pg/L
re1 SD, %
200 100 50 50 50
50 11 4.3 5 4.2
5 2 1 1
5.5 3.3 6.5 6.9 6.1
Uncoated Fiber benzene toluene ethyl benzene m- + p-xylene o-xylene 0
"
0
"
"
"
20
"
'
40
~
"
"
EO YOlUrne
'
~
E0
120
100
x 10'ym'
Figure 3. Amount of benzene absorbed by the methyl silicone film as a function of film volume. The benzene concentration was 1 mg/L. The film thickness Is shown beside each data point.
benzene toluene ethylbenzene m- p-xylene o-xylene
+
2 2 2
200-1500 200-1500 200-1500 200-1500 200-1500
2
2
56-pm Methyl Silicone 2 15-3000 3 15-3000 5 15-3000 5 15-3000 5 15-3000
1
BENZENE TOLUENE ETHYLBENZENE MPXYLENE 0-XYLENE
e
. 6
0
0
100
200
I
400
300
%stirring
Figure 4. Effect of stirring on the amount of benzene absorbed from a 1 ppm solution by a 56-pm film. The 50 and 100% refer to the settings of a stir bar/hot plate.
TIME ( S ) 500
I
I BENZENE TOLUENE ETHYL BENZENE
e
I
MPXYLENE 0-XYLENE
I
I iNZENE
i
100
5
1
.*. O D D
.
. 1
.
.
ooo 100
200
300
400
TIME IS)
Flgure 5. Adsorption-time profile for BTEX compounds adsorbed by an uncoated fiber from a solution contalnlng 740 Mg/L of each compound.
rium will eventually be reached if the sample is unstirred; however, it will require 10 min for benzene to equilibrate. With effective stirring maximum sensitivity is attained in the shortest possible time; for benzene this is 2 min. The absorption-time profile for the BTEX method using an uncoated fiber is shown in Figure 5. All the compounds had equilibrated in 1.5 min. The faster and consistent times occur because the silica gel surface has a thickness of approximately 0.1 pm so the effect of the volume is minimal. After the minimum equilibration time had been established, the relative standard deviation was determined for both the coated and uncoated fibers. These data are summarized in Table 11. Precision was measured at 1500 bg/L for both types of fibers. Three things are evident. First, the overall relative standard deviation is somewhat better than previously reported (9). Second, the coated
& I
To1 -
i
1:40
3:a
Flgure 7. Total Ion chromatogram obtained by desorbing a fiber in the Injector of an ion trap GC-MS. The fiber had been exposed for 2 min to a water sample that had been saturated with gasollne.
same phenomenon. This was determined by doing the following experiment. The fibers were exposed to an aqueous sample for 6 min, and the fiber was drawn into the syringe needle and held in the room air for various lengths of time before it was desorbed in the gas chromatograph. The results are shown in Figure 6. The analytes evaporated from the uncoated fiber very quickly-in less than 30 s for benzene. In contrast, the benzene is stable in the methyl silicone coating for 2 min while the xylene compounds are stable for 5 min. As the fiber must be exposed to the air for the short time it takes to transfer the fiber from the vial to the injector, this is a major source of poor relative standard deviation for benzene and explains why the coated fiber has better relative standard deviation for the more volatile components than the uncoated fiber. The relative standard deviation of the fiber method when used with a coated fiber is now good. The linearity of the two coatings was tested over the concentration ranges listed in Table 11. The limits of detection for the two coatings are also shown. The two coatings that were used offer two distinct linear ranges and limits of quantitation, reflecting the different stationaryphase volumes and distribution constants. The fiber coated with 56 pm of methyl silicone has a larger stationary-phase volume than an uncoated fiber, which has only -0.1 pm of silica gel. The methyl silicone film is also nonpolar, which will absorb the BTEX compounds better than the polar silica surface. Consequently, the fiber coated with the methyl silicone film is more sensitive than the uncoated fiber. This allows the analyst to choose the linear range and limit of quantitation best suited to the sampling site. 982
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For a sample preparation method to be useful in most environmental laboratories, it must be possible to readily obtain mass spectra of the prepared sample either for confirmation of peak identity or as a routine sample preparation for mass spectral analysis. The fiber has been used to inject samples into a GC-ion trap mass spectrometer. A methyl silicone coated fiber was exposed to a solution of gasoline in water and desorbed into the injector of a GC-MS. The total ion chromatogram is shown in Figure 7. As nanograms of material are desorbed from the fiber, most benchtop GC-MS systems should be capable of detecting the analytes. The elimination of solvents substantially decreased sample preparation time, and good relative standard deviation indicates that this technique warrants further development for environmental applications. The fiber method combined with the ECD detector offers limits of quantitation comparable to EPA method 624 (3,5). While the fiber method combined with an FID detector is not sufficiently sensitive to meet COD specificationsfor either EPA method 524.2 (11)or 624 (31, both of which use purge and trap/mass spectrometry, by desorbing the fiber into a GC-ion trap mass spectrometer it is possible to improve the limit of quantitation to the required levels as the sensitivity of the full-scan ion trap mass spectrometer is -2 orders of magnitude more sensitive than the FID (13). The amount of analyte the fiber extracts depends upon the distribution constant; for o-xylene it can be calculated from eq 1that the fiber would extract 5 ng from 5 mL of a 10 ppb solution, whereas the purge and trap would remove 50 ng if there was 100% extraction efficiency. The fiber method becomes particularly advantageous as sample volumes decrease because the amount the fiber extracts
Environ. Sci. Technol. 1992,26,983-990
depends upon the aqueous concentration, not the volume, provided the volume is large enough to prevent significant depletion of the sample. A methyl silicone film decreases the limit of detection to 1ppb (w/v) for compounds with moderate distribution constants, while reducing the realtive standard deviation for volatile compounds from 20% to 6%. The further development of unique coatings will enhance the application of the method to complex matrices such as heavily contaminated water from landfill sites where the distribution constant may be affected by the high organic content of the water; the effect of other contaminants on K can be minimized by choosing a stationary phase specific to certain classes of compounds so that the distribution constant is maximized. The range of possible stationary phases can also be extended to physically fragile coatings by incorporating stainless steel tubing into the fiber design. Increasing the stationary-phase volume by either increasing the film thickness or increasing the exposed length of fiber will reduce the limits of detection even further (13). The method is readily portable and has enormous potential for field monitoring, by desorbing the fiber either into a conventional GC in a mobile laboratory or for use with portable instruments. The method is inexpensive, as the fiber/steel tubing combination costs approximately $2.00/ft.
Acknowledgments Many thanks to Professor Jim Barker of the groundwater center for his encouragement and advice. Registry No. Water, 7732-18-5; benzene, 71-43-2; toluene, 108-88-3; ethylbenzene, 100-41-4; xylene, 1330-20-7.
Literature Cited (1) Liska, I.; Kurpick, J.; LeClerc, P. A. J. High Resolut. Chromatog. 1989, 12, 517. (2) Glaze, W. H.; Lin, C. C.; Burleson, J. L.; Henderson, J. E.; Mapel, D.; Rawley, R.; Scott, D. R. Optimization of L i p uid-Liquid Extraction Methods for the Analysis of Organics in Water; U.S. Dept. of Commerce, National Technical Information Service, Springfield, VA, 1983. (3) Junk, G. A.; Richard, J. J. Anal. Chem. 1988, 60, 451. (4) USEPA Method 624. Fed. Regist. 1984, 49, 141. (5) Ho, J. S.; Hodaklevic, P.; Bellar, T. A. Am. Lab. 1989, July, 41. (6) Doherty, L. Am. Environ. Lab. 1991, 6 , 11. (7) Zlatkis, A.; Ranatunga, R. P. J.; Middleditch, B. S. Anal. Chem. 1990,62, 2471. (8) Pankow, J. F.; Ligocki, M. P.; Rosen, M. E.; Isabelle, L. M.; Hart, K. M. Anal. Chem. 1986,58,429. (9) Arthur, C. L.; Pawliszyn, J. Anal. Chem. 1990, 62, 2145. (10) Mackay, D. M.; Roberts, P. V.; Cherry, J. A. Environ. Sei. Technol. 1985, 19, 384. (11) Chiou, C. T. Environ. Sei. Technol. 1985, 19, 57. (12) Louch, D.; Motlagh, S.; Pawliszyn, J. Extraction Dynamics of Organic Compounds from Water Using Liquid-Coated Fused Silica Fibers. Anal. Chem., in press. (13) Potter, D. W.; Pawliszyn, P. Parts per Trillion Detection of Substituted Benzenes in Water Using Solid Phase Microextraction and Gas-Chromatography-Ion Trap Mass Spectrometry. Submitted to Anal. Chem.
Received for review August 30,1991. Revised manuscript received December 16, 1991. Accepted December 28, 1991. Financial support from the Natural Sciences and Engineering Research Council of Canada, Varian Canada, Varian Associates, Imperial Oil Canada, and Supelco Canada is gratefully acknowledged.
Estimating the Equilibrium Aqueous Concentrations of Polynuclear Aromatic Hydrocarbons in Complex Mixtures Wllllam F. Lane” Remediation Technologies, Inc., 127 Kingston Drive, Suite 105, Chapel Hill, North Carolina 27514
Raymond C. Loehr Department of Civil Engineering, The University of Texas at Austin, Austin, Texas 78712
By use of a two-phase liquid-liquid equilibrium model, the distribution of nonpolar solutes between water (polar phase) and soil organic matter (nonpolar phase) was related to principles of equilibrium chemistry. Batch equilibrium experiments were conducted with field-contaminated soils. Aqueous concentrations were measured directly, predicted through the use of organic cosolvents, and calculated from Raoult’s law, thereby providing a three-way comparison of solute behavior in water. Results showed that composition of the nonpolar phase strongly influences the solute concentrations in the polar phase, suggesting that Raoult’s law is applicable to complex mixtures. Tar-water partitioning experiments demonstrated that the distribution of solutes in complex mixtures is analogous to partitioning among multiple solvents.
Introduction The deposition of polynuclear aromatic hydrocarbons (PAHs) in soils has occurred at many locations, including manufactured gas plant (MGP) sites and wood-treating facilities. A t such sites, these constituents have the potential to impact human health and the environment. 0013-936X/92/0926-0983$03.00/0
Many research efforts have been undertaken to better understand the risks associated with the presence of PAH compounds in soils. Selection of technical options and implementation of management practices must include an understanding of the fundamental relationships between the components of the complex mixtures in the environment (soil, water, natural organic matter, and contaminant phase) which contain the PAH compounds. The behavior of the compounds in the aqueous phase is of critical importance because solute transport and transformation processes are known to occur predominantly in water. The presence of 16 PAH compounds in the aqueous phase was the focus of this research. The equilibrium aqueous concentrations were estimated in three ways: (a) by use of batch equilibrium experiments, they were measured directly; (b) they were predicted by using mixtures containing varying fractions of water and miscible organic cosolvents; and (c) they were calculated from Raoult’s law, which describes the equilibrium behavior of a solute between two phases. The use of cosolvents to enhance solubilization of sparingly soluble compounds has been demonstrated and described in the pharmaceutical literature (1). The ap-
0 1992 American Chemical Society
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