Anal. Chem. 1995,67, 600-605
Determination of BTEX Compounds in Water by Solid-Phase Microextraction and Raman Spectroscopy Brian L. Wittkamp and David C. Tilotta* The Department of Chemistv, Box 9024, University of North Dakota, Grand Forks, North Dakota 58202
A new method is described for determining benzene, toluene, ethylbenzene, and xylenes (BTM)in water that combines solid-phase microextraction and spontaneous Raman spectroscopy. In this method, the BTEX analytes are extracted from the water solution into a solid phase before direct detection by Raman spectroscopy. The solid phase consists of a small volume (ca. 55 pL) of poly(dimethylsiloxane) which has optid windows in the 7501300- and 1410-2800-cm-' Raman shift regions. The time required for each BTM compound to reach equilibrium between the solid phase and the aqueous phase is in the range of 16-30 min and affords preconcentration enhancements of 2-3 orders of magnitude. Limits of detection using the most intense Raman bands are in the 1-4-ppm range and produce relative standard deviations of 3-9%. Preliminary application of this new method to the detection of BTM compounds in real-world water samples shows no sigdcant interferencesfrom river and well water matrices. Benzene, toluene, ethylbenzene, and the xylene isomers (generally referred to as the BTEX compounds) are common contaminants in ground and surface water. In fact, these chemicals, some of which are suspected carcinogens, have been detected at levels exceeding their water solubility limits in wells in Iowa.' Typically, a standard analytical procedure for determining BTEX chemicals in water relies on collecting and transporting water samples to a laboratory for analysis using purge and trap gas chromatography (GC).2 Such an analytical procedure is costly, time-consuming, and labor intensive and introduces errors from contamination and losses. Field determination of BTEX contamination by optical remote sensing is attractive because it eliminates many of the problems connected with collecting and transporting samples (e.g., representative sampling, contamination,loss of volatiles, storage, etc.) . In general, optical remote sensing methods have been based on laser-induced fluorescence (LIF) spectroscopy and spontaneous, surface enhanced, and resonance Raman spectroscopies. Although LIF spectroscopic methods tend to be sensitive, they are generally nonselective. For example, Chudyk and co-workers have shown that far-ultraviolet LIF is a viable means of detecting aromatic species in groundwaters at the parts per billion concentration levels? However, because the fluorescence spectra were (1) Rajagopal, R; Li, P. C. Am. Environ. Lab 1994, 6,16-19. (2) USEPA Method 624. Fed. Regist. 1984, 141. (3) Chudyk, W. A; Carrabba, M. M.; Kenny, J. E.Anal. Chem. 1985,57,12371242.
600 Analytical Chemistry, Vol. 67, No. 3, February 1, 1995
broad and overlapped one another, limited analyte selectivitywas noted. Spontaneous Raman spectroscopy, on the other hand, offers a high degree of analyte selectivity but can be insensitive. For example, Marley et al. showed that phenols could be uniquely detected in water.4~~ However, they found that unless the scatterer was strong (e.g., a nitrophenol), detection limits were in the midparts per million range. Novel fiber-optic probe designs can enhance the sensitivity of spontaneous Raman spectroscopy but often at the price of complexity.6 Resonance Raman and surface enhanced Raman spectroscopic methods (RRS and SERS, respectively) have been shown to dramatically increase the sensitivity of Raman spectroscopy. Van Haverbeke and Herman applied laser excited RRS to the detection of phenolic compounds in water and obtained detection limits in the mid-parts per billion range.7 However, their method relied on a derivatization step and would not be generally applicable (e.g., to the BTEX compounds). Similarly, SERS has been shown to be both selective and sensitive for detecting organic compounds in water matrices.6 However, the application of SERS methodology is tedious with respect to the preparation of the metal substrate (the sensor). Recently, Carron and co-workers have expanded the SERS methodology to an elevated level which incorporates analyteselective extraction with an octadecylthiol monolayer! They have shown that SERS combined with a sample preconcentration step provides both selectivity and sensitivity (e.g., a detection limit of 7.5 ppm for benzene). However, in addition to the complexity in the preparation of the silver/octadecylthiol sensor, the octadecylthiol monolayer itself presents many Raman bands which obscure the Raman bands of the analytes. Solid-phasemicroextraction (SPME) is a relatively new analytical technique which relies on the establishment of equilibrium between the analytes in an aqueous phase and a stationary phase (the solid phase).'OJ SPME is a solventless method which has the capability of selectively preconcentrating organic compounds from water. In its current form, the SPME procedure involves extracting organic compounds directly from a water matrix with (4) Marley, N. A; Mann, C. IC; Vickers, T.J. Appl. Spectrosc. 1984, 38, 540. (5) Marley, N. A; Mann, C. K; Vickers, T. J. Appl. Spectvosc. 1 9 8 5 , 3 9 , 628633. (6) Schwab, S. D.;McCreery, R L. Appl. Spectrosc. 1987, 41, 126-130. (7) Van Haverbeke, L.; Herman. M. A Anal. Chem. 1 9 7 9 , 5 1 , 932-936. (8) Carrabba, M. M.; Edmonds, R B.; h u h , R D. Anal. Chem. 1987,59,25592563. (9) Carron, IC; Pelterson, L.; Lewis, M. Environ. Sci. Technol. 1992,26,19501954. (10) Arthur, C. L.; Potter, D. W.; Buchholz, K D.; Motlagh, S.; Pawliszyn, J. LCGC 1994, 10, 656-661. (11) Arthur, C. L.; Killiam, L. M.; Motlagh, S.;Lim,M.; Potter, D. W.; Pawliszyn, J. Environ. Sci. Technol. 1992,26, 979-983. 0003-2700/95/0367-0600$9.00/0 0 1995 American Chemical Society
the use of a thin layer of a solid phase (typically poly(dimethy1siloxane)) coated on the exterior of a short length of glass fiber. At equilibrium, the concentration of analyte in the solid phase is governed by classical extraction theory by
C, = KC,
(1)
where C, is the concentration of analyte in the stationary phase, K is the partition constant for the analyte in the stationary phase/ water system, and C, is the concentration of the analyte in the aqueous phase. Buchholz and Pawliszyn have shown that existing octanol-water partition constants (Kowvalues) can be used as approximations for the partition constants (Kvalues) in eq 1when poly(dmethylsi1oxane) is used as the stationary phase.12 S i c e the Kowvalues for many environmentally important contaminants are quite large (e.g., in the range of 135-1585 for the BTEX compounds) ,I3 the sensitivity of SPME methodology arises from the preconcentration of the analytes. The selectivity of SPME is inherent in the properties of the solid phase (e.g., polarity). Pawlisyzn and co-workers have shown that SPME coupled with GC methods provide detection limits in the parts per billion to parts per trillion levels for the determination of aromatics in groundwater.l’J2 This article is a report on the feasibility of using spontaneous Raman spectroscopyas a detector for SPME. In this preliminary work, conventional Raman spectroscopy (Le., Raman spectroscopy performed in the visible spectral region with a cooled photomultiplier tube) is used as a means of detecting BTEX compounds, extracted from water, directly in the solid phase. The SPME step affords a preconcentration of the analytes which effectively increases the sensitivity of the Raman measurements without adding complexity to the experimental setup. Raman spectroscopy is an excellent optical detector for SPME because it provides molecular recognition of the extracted compounds directly in the extraction medium. In addition, Raman spectroscopy is nondestructive so that the samples can be further analyzed by GC/MS, if desired. As will be shown, the preconcentration of the BTEX compounds via SPME enhances the sensitivity of the Raman measurements by 2-3 orders of magnitude and allows the BTEX compounds to be determined in water down to the 1-4ppm concentration range. The eventual incorporation of fiber optics to this system should provide a means of performing in situ field measurements while maintaining a simple and rugged instrumental design. EXPERIMENTAL SECTION
Instrumentation. The Raman spectrometerwas constructed in-house and employs a conventional 90” sampling geometry. The excitation source is a Coherent Innova Series 70 AI-+laser (Palo Alto, CA) operating at 488 nm with an output power of 0.9 W. A Pellin-Broca prism (Continental Optical Corp., Hauppauge, NY) was installed at the head of the laser to provide a means of filtering plasma lines. The laser radiation was focused to a 1.5”diameter spot on the sample by a 150.mm-focal length biconvex lens (Edmund Scientific, Barrington, NJ>. The laser power at the sample was determined to be 0.72 W. The scattered radiation from the sample was collected with a Nikon camera lens (collec(12) Buchholz, K D.; Pawliszyn, J. Enuimn. Sci. Technol. 1993,27,2844-2848. (13) Chiou, C. T.; Schmeddmg, D. W.; Manes, M. Enuimn. Sci. Technol. 1982, 16, 4-10.
tion throughput of f/3.9) and focused onto the entrance slit of a scanning ‘/em Digikrom DK 242 double monochromator (CVI Corp., Albuquerque, NM). The double monochromator is fitted with two 1800 g/mm holographic diffraction gratings and employs unilaterally adjustable slits. Unless otherwise stated, the entrance, middle, and exit slits of the monochromator were fixed at widths of 100, 300 and 100 pm, respectively, and corresponded to an effective spectral resolution of 8 cm-l at a Raman shift of lo00 cm-1. Spectra were scanned at a speed of either 1 or 2 nm/min (ca. 38 or 76 cm-’/min at lo00 cm-I, respectively). All spectra were scanned linearly with respect to wavelength and then converted to Raman shiftsin wavenumbers (cm-l). The spectra shown in this work have not been corrected for the instrument response function. The detection system is comprised of a red-sensitive Hamamatsu W28 photomultiplier tube (Hamamatsu City, Japan) operating at -900 V and held at a temperature of 40 “C below ambient (-17 “C) by a Thom/EMI Model WCTS02 thermoelectric cooler (Fairlleld, NJ). Photon counting measurements were performed with a Thorn/EMI Model C10 photon counter. However, analog data from the experiments were displayed on an Omni-Scribe Model 5211-12 stripchart recorder (Bellaire, TX) possessing a 1-s time constant. Reagents. Apiezon-L and SP-2300 were used as purchased from Supelco (Bellefonte, PA). The extraction media OV-1 and SE30, both poly(dimethylsiloxanes),were purchased from Alltech (Deerfield, IL) . The voids in the OV-1 were removed by placing approximately 3 g of OV-1 into a 7 5 x 15@” test tube with 3 mL of toluene. The toluene was allowed to remain in contact with the OV-1 for 5 min to soften the material and was then decanted from the test tube. The “soft”OV-1 was then compressed withii the test tube via a plunger. The remaining toluene in the OV-1 was evaporated by drying the OV-1 in an oven at 130 “C for 10 min with a stream of helium passing over it. For actual use, the extraction medium was molded into cylindrical “beads”measuring ca. 3 mm in length x 5 mm in diameter by forcing it into the cap of a standard 5mm NMR tube. The volumes of the resultant beads were 55 f 1pL. For convenience in performing parallel extractions, seven beads were utilized in this study. The BTEX chemicals were spectroscopic grade and were used as purchased from Fisher Scientific Co. (Itasca, IL). Standard solutions of each of the BTEX compounds were prepared by spiking the appropriate amount of the chemical into 1 mL of methanol and then diluting to 250 mL with distilled, deionized water. In order to avoid evaporative loss of the BTEX compounds, all solutions were used within 10 h of preparation. The helium used to purge the extraction beads was commercial grade and was used as purchased from local sources. Procedure. The extraction media were shaken in 45mL aliquots of the water solutions using a Burrell Corp. wrist action shaker (Model 75, Pittsburgh, PA). All water solutions were held in 5@mLround-bottom flasks (with ground glass stoppers) for the duration of the extraction. Analyte losses from the use of these flasks were determined to be negligible. After the shaking, the beads were removed from the water solution and placed in 1 mL of distilled water in a standard 175 x 5@mmtest tube. Raman spectra of the extraction bead were collected in distilled water to avoid analyte evaporation. The bead in the test tube was positioned in the path of the laser beam with the use of an X-Y-2 translation apparatus constructed in-house. Analytical Chemistry, Vol. 67, No. 3, February 1, 1995
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Following the collection of the Raman spectrum, the extracted analyte was removed from the extraction medium by heating it over helium (50 mL/min flow rate) in a Blue M Electric Co. oven (Blue Island, IL) operating at a temperature of 130 "C (the highest boiling point of the BTEX compounds is 127 "C). The duration of heating was found to be 10 min before the Raman signal of the analyte(s) present in the extraction medium had diminished to within the noise of the system. Raman spectra of liquids were obtained in a standard 5mm NMR tube. Calibration curves for each of the BTEX compounds were obtained by plotting the average Raman signal (two measurements each from two extractions) versus concentration. The equilibration time measurements were performed in duplicate. Limits of detection were obtained by immersing clean extraction media in successively dilute solutions of the individual BTEX compounds in water until the corresponding signal of the BTEX compounds measured 2 x noise,k.tepk. RESULTS AND DISCUSSION
In order for Raman spectroscopy to be useful as a detection means in SPME, the stationary phase must possess several characteristics. Similarly to the application of SPME in GC analyses, the stationary phase must possess a high affinity for the analytes of interest and thermal stability (for desorption of the extracted compounds following Raman spectroscopic analysis in order to clean it), and it must be stable in the matrix of interest. For this application, the stationary phase must also be structurally rigid in order for it to hold its shape (i.e., a gum or rubber) and have optical transparency in the region of the Raman shifts (freedom from spectral interference). In this preliminary study, the poly(dimethylsi1oxane) OV-1was chosen because it possessed all of the characteristics delineated above. Other stationary phases examined, such as SP-2300, Apiezon, and SE-30 (a poly(dmethylsiloxane) manufactured by the General Electric Co.), were rejected because they were viscous fluids and all exhibited some degree of fluorescence under both 488nm and 514.5nm laser excitation. The ideal shape of the extraction medium for this application would correspond to a thin cylinder which would match the dimensions of the entrance slit of the double monochromator. Thus, the optical fibers developed for SPME syringe devices (134320 pm diameters) would be ideal. However, due to the difficulty and length of time required to align and image such a small size, the poly(dimethylsi1oxane) was molded into thick cylindrical shapes (almost spherical). These "beads" were easily and quickly (ca. 3-4 min) aligned in the Raman spectrometer before significant analyte evaporation could occur. With care, it was determined that the extraction medium could be used between 50 and 100 times before it needed to be replaced because of poor partitioning from thermal decomposition (during the cleaning step) and the accumulation of dust particles on its surface. However, the elimination of the air voids from the poly(dimethylsiloxane) prior to laser illumination was critical if the thermal decomposition of the medium was to be avoided. Air voids present in the medium, observable as small bubbles, allowed the laser radiation to be internally reflected and resulted in localized heating and eventual decomposition (typically within 3 min). In addition, photodecomposition of the extraction medium was also accelerated if the laser beam did not strike it perpendicularly. Properly oriented void-free media did not photodecom602 Analytical Chemistry, Vol. 67,No. 3, February 7, 7995
c
84
1
I
1000
2000
1
1
s
3000
4
3643
Raman Shift ( c m - 1 ) Figure 1. Raman spectrum of OV-1 (poly(dimethylsi1oxane)).
pose and were used at laser powers up to the maximum output of our laser (0.72 W at the sample). Figure 1 shows a Raman spectrum of OV-1 in the spectral region of 84-3643 cm-l obtained at a scan speed of 2 nm/min (ca. 76 cm-l/min). This spectrum shows the characteristic Raman bands of poly(dimethy1siloxane) and is similar to the spectra obtained by Maxfield and Shepherd14and Frank et The major optical windows of OV-1 (Le., those spectral regions where Raman scattering is minimal) are in the regions of 750-1300 and 14102800 cm-l. The spectral regions greater than 2800 cm-l and less than ca. 750 cm-1 in the OV-1 are obscured by the strong C-H stretching (centered at ca. 2900 cm-l) and Si-0 and Si-C stretching, respectively. It should be noted that the Raman spectrum of OV-1 shown in Figure 1 exhibits a slight amount of fluorescence,as evidenced by the elevated baseline in the region of 1000-2800 cm-'. This fluorescence, while not seriously impairing the limits of detection (LODs) of the BTEX compounds, does increase the noise by a factor of ca. 5. Although little difference was observed in the fluorescence of the OV-1 under either 488.0 or 514.5 nm excitation, it could be eliminated with the use of a red/near-infrared excitation source. A CCD (chargecoupled device) camera could then be used as a detector to recover the loss in the signal-to-noise (S/N) when using the lower energy laser. For the analysis of substituted benzene compounds, the appropriate spectral window in poly(dimethylsi1oxane) is 7501300 cm-'. This window is nearly ideal because it encompasses a large fraction of the spectral region where the unique skeletal vibrations of organic compounds, including the BTEX compounds, occur. Thus, this window can be used for speciation. The spectra shown in Figure 2 illustrate the Raman bands available for BTEX determination in poly(dimethylsi1oxane) in this spectral region. A resolution of 8 cm-1 (as demonstrated in Figure 2B-D) was found to be satisfactory for resolving most of the BTEX compounds from one another (as discussed below) while still providing an adequate S/N. For comparison purposes, a Raman spectrum of the OV-1 in the 750-1300-~m-~spectral region is shown in Figure 2D. All spectra shown in Figure 2 were acquired at a scan speed of 1 nm/min (ca. 38 cm-l/min). Figure 2B shows a Raman spectrum in the region of 7501300 cm-l of the OV-1 bead following a 30-min extraction of a ~
~~
(14) M d e l d , J.; Shepherd, I. W. Chem. Phys. 1973,2,433. (15) Frank,C. J.; McCreery, R L.; Redd, D. C. B.; Gander, T. S. AppE. Spectrosc. 1993,47,387-390.
A
Table 1. Comparison of the Limit8 of Detection of the BTEX Compound8 Wing Raman Spectroscopy with and wlthout SPME
chemical
benzene toluene ethylbenzene
Ramail band, cm-1
o-xylene
992" 1003" 967 1007" 1222"
m-xylene
1W
pxylene
1094 827 1208"
LOD, pg/mL with SPME without SPMEb
(RSD, %Ic
464 1830 18 781 1830 3940 1890 45 278 8611 2140
3.4 (8.9) 2.5 (7.5) 12.7 (4.5) 2.0 (4.7) 4.0 (5.0) 1.4 (2.8) 33.0 (4.5) 5.7 (7.0) 2.7 (3.3)
0 Most intense band in OV-1 window. * In acetone except for 0- and +xylene in methanol (all bands) and m-xylene (1094 an-') in benzene. Averages and relative standard deviations from triplicate measurements.
?5Q
9hO
I ioa
lioo
R a m a n Shift (cm-1) Flgure 2. Raman spectra of OV-1 following 30 rnin extractions of 20 ppm solutions of (A) benzene, toluene, and ethylbenzene at 2 cm-i resolution, (B) benzene, toluene, and ethylbenzene at 8 cm-i resolution, (C) e,m, and pxylenes at 8 cm-i resolution, and (D) blank. Raman band key: B, benzene; T, toluene; E, ethylbenzene; 0, @xylene; M, mxylene; P, pxylene; V, poly(dimethylsi1oxane).
solution containing 20 ppm each of benzene, toluene, and ethylbenzene. Although several Raman bands can be seen for each compound, the most intense bands in Figure 2B are associated with the ringvibrations.'6 The characteristic symmetric ring breathing band of benzene is observable as a shoulder at 992 cm-l, and the similar trigonal ring vibrations of toluene and ethylbenzene are observable as a single band at 1005 cm-l. A higher resolution Raman spectrum (2 cm-l) in the region of the ring breathing vibrations is shown in Figure 2A and illustrates that the benzene band can be resolved from the ethylbenzene and toluene bands with a corresponding loss of ca. 10 in the S/N. Although toluene has no unique bands in this window (toluene and ethylbenzene could not be resolved from one another under these concentration conditions, even at 2-cm-l resolution), it should be noted from Figure 2B that a unique Raman band of ethylbenzene can be observed at 967 cm-l. Using a CCD camera as a detector, it may be possible to use this ethylbenzene band to subtract the ethylbenzenecomponent of the Raman signal at 1005 cm-1 so that the toluene Raman signal can be isolated. Figure 2C shows a Raman spectrum of the OV-1 bead following a 30-min extraction of a solution containiig 20 ppm each of the three xylene isomers. Although several Raman bands can be seen for each of the three isomers, the most intense Raman bands in this window correspond to the characteristic aromatic trigonal ring breathing vibrations of the compounds. A comparison of the spectra shown in Figure 2B and C reveal that the xylene isomers have Raman bands in this window which do not overlap those of benzene, ethylbenzene, or toluene. Table 1 summarizes the useful Raman bands for each of the BTEX compounds. (16)Dollish, F. R; Fateley, W. G.; Bentley,F. F.CharacteristicRaman Frequencies of Organic Compounds,John Wiley and Sons: New York, 1974; pp 162179.
It should be noted that the spectra shown in Figure 2 are from preliminary experiments and are merely illustrative of the capability that SPME and Raman spectroscopy have for speciation. It should be pointed out, however, that many factors will govern the ability of this method to distinguish between different compounds in mixtures. For example, spectral selectivity could be adversely affected by concentration (through band broadening) and the co-extraction of compounds which give rise to interfering Raman bands. In addition, if a visible laser is used for the Raman experiment, fluorescence of co-extracted compounds could effectively mask the Raman signals of the analytes. However, it is not expected that band broadening due to concentration will significantly affect the ability of Raman spectroscopy to speciate in this application because the BTEX compounds have low solubilities in water. Likewise, the interferences due to the coextracted compounds may be minimized though the judicial selection of the stationary phase and/or the utilization of a red/ near-infrared laser for the Raman scattering (for fluorescence interference). For cases where significant band overlap of the analytes and/or spectral interferences occur (and assuming that higher spectral resolution is not practical), algorithmsexist which can be used to mathematically extract quantitative information about individual components from a spectrum of a mixture.5 If a specific BTEX analyte is to be determined and it has been established that no spectral interference exists, the monochromator can be set to its analytical band and the intensity obtained directly. However, if a survey spectrum over the entire spectral window is to be acquired (e.g., to establish the presence of an analyte or an interferant), then the monochromator should be scanned slowly in order to obtain the best S/N. The double monochromator employed in these experiments possesses a minimum scan speed of 1nm/min (ca. 38 cm-l/min) and requires 14.5 min to acquire a spectrum over the spectral range of 7501300 cm-l. Of course, for the duration of the Raman scan, it is essential that the concentration of the analytes in the extraction bead does not chahge signMcantly. Although it has been shown that the analytes evaporate from the extraction medium on exposure to air at ambient conditions, analyte evaporation can be greatly accelerated in this experiment because of the heating of the extraction bead by the laser.13 Figure 3 shows plots of the Raman signals of the ring breathing vibrations (the most intense bands) from 30-ppm concentrations of the BTEX compounds Analytical Chemistry, Vol. 67, No. 3, February 1, 1995
603
0
0
0
A
A
A
A
A
A
0
A
A
D
10 -
01
0
-0
10
20
20
10
45
30
8
LLI
d
30
* 0
40
Thne (minutea)
Time (minutes) Figure 3. Raman scattering intensity as a function of time when the extraction bead is exposed to air following extraction. Solutions are 30 ppm each of (0)benzene, (A) toluene, (0)ethylbenzene, (*) @xylene, (D) mxylene, and (A)pxylene.
Figure 4. Raman scattering intensity as a function of time when the extraction bead is covered with water following extraction. Solutions are 30 ppm each of (0) benzene, (A) toluene, (0) ethylbenzene, (*) o-xylene, (D) mxylene, and (A)pxylene.
Table 2. Selected Physical Data for BTEX Compounds
chemical benzene toluene ethylbenzene (I
Kowu
bp, "C
135 490 1413
80.1 110.6 136.2
chemical o-xylene m-xylene +xylene
Kown
bp, "C
589 1585 1413
144.4 139.1 138.4
KO, values from ref 13.
1:: 8 2 0
versus time when the extraction bead is exposed to air for the duration of the data acquisition. Zero on the time axis for the plots shown in Figure 3 refers to the end of the extraction periods (i.e., before the bead can be positioned in the Raman spectrometer and data acquired). It can easily be seen in Figure 3 that the Raman signals for all compounds decrease with time because of evaporative loss. In fact, the signal due to benzene is reduced to the level of the baseline noise after 40 min. In order to minimize the evaporative loss of the BTEX compounds from the extraction beads, about 1 mL of distilled water was added to the bead (which was held in a test tube) for the duration of the Raman spectral acquisition. Water works well in this application because it is a poor Raman scatterer. In addition, reverse partitioning of the analytes (Le., from the bead into the water) would be expected to be very smallbased on their octanol-water partition constants (K, values) shown in Table 2. Figure 4 shows plots of the Raman signals of the ring breathing vibrations versus time when the extraction bead is held in 1 mL of distilled water and shows that analyte loss over 50 min is minimal. As in the plots of Figure 3, zero on the time axis for the plots of Figure 4 refers to the end of the extraction periods. The time required for each of the BTEX compounds to reach an equilibrium between the aqueous phase and the stationary phase was determined by shaking the bead in a standard 100ppm solution of each compound for increasing periods of time. It was found that the extraction times for the BTEX compounds did not vary significantly with concentration as long as the bead was shaken with the solutions. Absorption-time profiles for each of the BTEX compounds (Le., the amount of each analyte absorbed by the solid phase as a function of time) were constructed by plotting the intensity of the strongest Raman band versus time. The resulting profiles are shown in Figure 5 and indicate that the equilibrium times for the BTEX compounds using a 55pL bead 604 Analytical Chemistry, Vol. 67, No. 3, February 1, 7995
10 0
0
10
20
30
40
50
60
Time (millute#) Figure 5. Absorption-time profiles for the BTEX compounds (100 ppm each) obtained by plotting the Raman intensities of the ring breathing vibrations versus time: (0) benzene, (A) toluene, (0) ethylbenzene, (') @xylene, (D) mxylene, and (A)pxylene.
of OV-1 are between 16 (for benzene) and 30 min (for the others). These equilibration times are longer than those found by Louch et al. on the application of SPME to the extraction of BTEX compounds from water." However, these extraction times are consistent with their work when the increased bead volume and different solid-phasegeometry (a coating on a glass fiber in their work versus a solid cylinder in this work) are taken into account. Calibration curves for the BTEX compounds can be seen in Figures 6 and 7. Figure 6 shows calibration curves for the most intense Raman bands (the ring breathing vibrations), and Figure 7 shows calibration curves for other useful Raman bands. The upper limit of the concentration for each calibration curve was selected to avoid saturating the water with the chemical (ca. 50100 ppm less than the solubility limit in water). The linear dynamic ranges of calibration are from the LODs to the upper concentrations examined in this study (100-400 ppm) and span 1-2 orders of magnitude. The correlation coefficients (2) for the calibration curves were in the range of 0.9924-0.9997. The LODs for the various BTEX compounds using the analytically useful bands are shown in Table 1. The most intense Raman bands yield LODs in the 1-4ppm range. For comparison purposes, the LODs for the BTEX compounds in solvents which (17) Louch. D.;Motlagh, S.; Pawliszyn, J. Anal. Chem. 1992,64, 1187-1199.
120
1
/ I
-mh@pm) Figure 6. Calibration curves for the BTEX compounds using the most intense Raman bands: (0) benzene, (A) toluene, (0) ethylbenzene, (*) @xylene,).( mxylene, and (A)pxylene. Table 1 lists the analytical bands. F
60
-
1 5 0
1
w
.
1::!
p; 0
20
40
60
-e)
80
100
120
Figure 7. Calibration curves for the BTEX compounds using other analytically useful Raman bands: (0) ethylbenzene,).( mxylene, and (A)pxylene. Table 1 lists the analytical bands.
are optically transparent in the regions of analyte scattering are also shown in Table 1. The BTEX compounds could not be directly detected in water (i.e., without SPME) because, with the exception of benzene, their LODs would far exceed their solubility limits. A comparison of the limits of detection of the BTEX compounds with and without SPME shows a nearly uniform improvement of 2-3 orders of magnitude. The degree of improvement depends upon the magnitude of the Kow (from eq 1). In fact, the LODs obtained by using the SPME step can be qualitatively predicted by dividing the solvent LOD by the octanol-water partition constant. The relative standard deviations (RSDs) of the Raman measurements using SPME were determined by extracting triplicate water/BTEX solutions at a BTEX component concentration of 7 ppm for each compound and are shown in Table 1. As found in similar studies on the use of SPME with GC, the RSDs are governed mainly by analyte evaporation. Specifically, some analyte evaporation occurs over the &min length of time required to remove the extraction bead from the flask, position it in the Raman spectrometer, and record a spectrum of the analyte band. As expected, the RSDs of the more volatile analytes are larger. However, in addition to analyte evaporation, the reproducibilities of the measurementsin this experiment are also affected by optical
noise. For example, a comparison of the RSDs of the measurements for pxylene using the two vibrational bands shown in Table 1reveal that the measurement at 1208 cm-l has better reproducibility than the measurement at 827 cm-l. The poorer RSD using the band at 827 cm-l arises because this Raman band is located on the shoulder of an OV-1 band (as shown in Figure 2). Because the Raman signals are additive, the measurement of the signal at 827 cm-1 has a larger noise component associated with it (Le., from laser flicker noise and shot noise) than the band at 1208 cm-l. Nevertheless, the RSDs of these measurements are in the range of 3-9% and compare favorably to those obtained by SPME/ GC. Finally, this new method was tested and evaluated on realworld samples. In order to simulate contaminated waters, both naturally occurring well water and river water were spiked with the BTEX chemicals to have final concentrations of 20 ppm. Following triplicate 30-min extractionswith the SPME beads, the BTEX Raman signals of the naturally occurring water samples were compared to the Raman signals of laboratory prepared standards. The Raman signal intensities of the spiked BTEX water samples were 95-97% as intense as those of the laboratory standards and showed no fluorescencedue to naturally occurring materials (e.g., fulvic or humic acids). The reproducibilities of the triplicate measurements yielded RSDs of 3-4%. Of course, these results are only from preliminary experiments, and a more detailed study on the application of this new method to real water samples is currently underway in this laboratory. CONCLUSIONS Spontaneous Raman spectroscopy has been shown to be an excellent in situ detection means for BTEX compounds by SPME. It is selective, reasonably fast (
