Detection of volatile organic compounds using ...

Analytica Chimica Acta 488 (2003) 15–23

Detection of volatile organic compounds using surface enhanced Raman spectroscopy substrates mounted on a thermoelectric cooler P.A. Mosier-Boss∗ , S.H. Lieberman SPAWAR Systems Center San Diego, Code 236, San Diego, CA 92152, USA Received 8 April 2003; received in revised form 26 May 2003; accepted 26 May 2003

Abstract Preliminary results for a volatile organic compound (VOC) sensor based on surface enhanced Raman spectroscopy (SERS) are described. The sensor is comprised of a SERS substrate mounted on a thermoelectric cooler (TEC). The SERS substrate is chemically modified with a thiol coating that prevents oxidation of the roughened silver surface and attracts the analyte of interest to the SERS surface. Using this sensor, detection of chlorinated solvents, aromatic compounds, and methyl t-butyl ether (MTBE) is demonstrated. Published by Elsevier B.V. Keywords: Volatile organic compound; Raman spectroscopy; Thermoelectric cooler

1. Introduction Because of its sensitivity and specificity, surface enhanced Raman spectroscopy (SERS) is a very attractive technique to detect and identify contaminants of environmental concern. SERS spectra can be obtained remotely over optical fibers in real time and there have been significant advances in the development of inexpensive, portable Raman spectrometers, charge-coupled device (CCD) detectors, and near IR diode lasers [1–5]. The SERS substrates can be chemically modified by reacting the silver/gold surfaces with thiols to form compact self-assembled monolayers (SAMs) [6]. The thiol coating protects the SERS surface from degradation thereby extending its lifetime. There are commercially available a wide ∗ Corresponding author. E-mail address: [email protected] (P.A. Mosier-Boss).

0003-2670/03/$ – see front matter. Published by Elsevier B.V. doi:10.1016/S0003-2670(03)00676-7

range of aliphatic and aromatic thiols with functional groups such as amines, carboxylates, sulfonates, etc. Consequently, thiol coatings can be chosen that have an affinity for an analyte. The coating attracts the analyte to the siver/gold surface allowing its detection/identification by SERS. Use of thiol-modified SERS substrates has been used to detect aromatic compounds [7], chlorinated solvents [8], anions [9,10], alkali metal ions [11]; and heavy metal ions such as lead, cadmium, and copper [12]. Depending on the analyte and coating, detection limits in the ppb–ppm concentration range have been reported. Besides protecting the SERS surface and attracting analytes of interest, the thiol coating has a characteristic SERS spectrum that can be used for calibration purposes. Thiol-coated SERS substrates have been coupled with ultramicroelectrode (UME) technology [13] and gas chromatography [14]. In the former [13], a Au/Au disc UME sensor was treated with 4-chlorothiophenol.

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The coating attracted the analyte (methylene chloride) to the UME sensor, which was polarized at 8 V. Using these sensors it was possible to obtain electrochemical data and SERS spectra simultaneously. The spectral data were used to identify the electroactive species. The GC-SERS detection cell was an aluminum block that could be heated or cooled [14]. Using a 1-propanethiol coating, a detection limit of 50 ng benzene was achieved. It was also possible to spectrally resolve components that eluted off the column simultaneously. For site characterization and long-term monitoring purposes, it is desirable to obtain data remotely, rapidly, and in situ. The ideal field-deployable sensor would be able to detect contaminants reversibly, with little or no sample preparation, and with no interferences. The SERS technique meets many of these criteria. In this communication, the feasibility of detecting volatile organic compounds (VOCs) by SERS is examined. Specifically, thiol-coated SERS substrates are mounted on a thermoelectric cooler (TEC). Using this sensor, detection of VOCs, such as chlorinated solvents, methyl t-butyl ether (MTBE), and aromatic compounds is demonstrated.

2. Experimental 2.1. Reagents

500 mW s−1 were used. For each scan, the potential was held at 1.3 s at the positive limit and 30 s at the negative limit. After electrochemical roughening, the silver electrode was rinsed with water (HPLC grade, Aldrich) and then ethanol (HPLC grade, Aldrich). The electrode was immersed in a dilute thiol solution in ethanol and allowed to react for approximately 24 h to form a SAM. Before use, the substrates were rinsed thoroughly with ethanol and stored in water between uses. 2.3. SERS measurements SERS measurements were made using the Raman Solution 785 system (Detection Limit). The Raman Solution 785 system has a f number of 2; a fixed position, 1200 grooves mm−1 grating; and a TE cooled Kodak 0400 CCD. A fiber optic sampling probe operating at 785 nm (InPhotonics, Model RPS785-12-10) was used to deliver the laser excitation to the sample and transfer the Raman emissions to the spectrometer. The excitation source was a tunable, continuous-wave (CW) laser diode (Spectra Diode Laser, SDL-8630) operating at 785 nm. A tunable optical isolator (Optics for Research, Model IO-7-NIR) was used to prevent backscatter of the laser beam into the laser cavity. The 785 nm laser line was focused into the silica/silica clad, 100 ␮m, excitation fiber using a 5× microscope objective lens.

Thiophenol (TP), 1-propanethiol, and pentafluorothiophenol (PFTP) were purchased from Aldrich and were used as received. Benzene (Fluka) was used as received. Toluene, perchloroethylene (PCE), trichloroethylene (TCE), MTBE, and chloroform were obtained from Aldrich and used as received. Ethanol (HPLC grade, Aldrich) was dried by refluxing over magnesium turnings, distilling, and collecting the middle fraction.

2.4. Manipulation of spectral data

2.2. Preparation of SERS substrates

3.1. The TEC-SERS system

The 6 mm diameter silver disk electrode was electrochemically roughened in a 0.1 M KCl solution using a PAR 173 potentiostat under computer control. To roughen silver, 25 successive oxidation-reduction cycles (ORCs) from −300 to 1200 mV versus the Ag/AgCl reference electrode at a sweep rate of

Fig. 1 shows a schematic of the TEC-SERS system. The housing of the SERS cell was constructed out of Delrin. The SERS substrate was a thiol-coated, 6 mm diameter silver disk that was mounted on a three-stage TEC (Melcor, 3CP055065-127-71-31) capable of a
Tmax of 96 ◦ C. An o-ring around the silver

All manipulations of the spectral data were done using GRAMS/AI7 (ThermoGalactic). This software package can be used to subtract spectra interactively and integrate peak areas. 3. Results and discussion

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3-stage TEC SERS substrate

insulation sample injection port

thermistor

optical windows

purge line

sample line

N2

N2

outlet

mass flowmeter

3-stage TEC

SERS cell

thermistor bubbler sample chamber

silicone tubing

Fig. 1. Schematic of the laboratory TEC-SERS system to detect VOCs.

disk provided a leak-tight seal. Foam insulation was used to fill the voids between the Delrin housing and the TEC. A 100 k thermistor (YSI Incorporated, 440H) in close proximity to the SERS substrate (Fig. 1) was used to monitor the temperature of the TEC. To transport heat away from the hot side of the TEC, the TEC-SERS assembly was mounted on an extruded fin heat sink (Melcor, EXT-201) coupled to a fan (Melcor, FAN-101-LP). A temperature controller (Melcor, MTCA-6040) maintained the temperature of the TEC. The sample chamber, Fig. 1, houses a hollow fiber (i.d. = 0.64 mm, o.d. = 1.19 mm), silicone membrane (Dow Corning Silastic 508-003). Such membranes have been used to extract VOCs prior to analysis by capillary gas chromatography [15] and mass spectrometry [16]. The silicone membrane is impermeable to water. Either aqueous or vapor samples can be introduced inside the sample chamber. Ni-

trogen gas flows through the hollow membrane. The outside part of the membrane is in contact with the sample. Nonpolar compounds adsorb to the nonpolar surface of the membrane. These compounds then partition inside the membrane body and begin to diffuse through the bulk of the polymer under the force of the concentration gradient. Once the compounds reach the inside and desorb, the nitrogen carrier gas transports the VOC vapors to the SERS cell for detection/identification. Between samples, the SERS cell is purged with nitrogen gas. The sample chamber is used to evaluate the temperature response of the coated SERS substrate and the VOC. To quantify the amount of VOC adsorbed on the SERS substrate, VOC samples are injected directly into the nitrogen gas stream through the septum in the injection port spliced into the purge line, Fig. 1. The outlet of the TEC-SERS cell is connected to a 0–50 ml min−1 mass flowmeter (Aalborg model GFM171).

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3.2. Detection of chlorinated solvents using TEC-SERS Chlorinated solvents, the primary component of dense nonaqueous phase liquids (DNAPLs), have been used for decades in cleaning and degreasing operations resulting in groundwater contamination at sites nationwide. Accurate delineation of contaminant source zones is essential in choosing an appropriate remediation scheme and in evaluating the effectiveness of the remediation scheme in removing the source zone. Source zone delineation and evaluation of the effectiveness of remediation schemes in the removal of chlorinated solvents usually has relied on conventional soil and water sampling followed by laboratory analysis using GC-MS. There are a number of difficulties associated with this approach. Most of these difficulties result from the nature of DNAPL transport in the vadose and saturated zones [17–19]. As DNAPLs sink through permeable soils, small quantities of free product are left behind in widely dispersed micro-globules in the pore spaces of the soil matrix. In the saturated zone, the distribution of these micro-globules is very heterogeneous. It is precisely this heterogeneous distribution in the real world that has made it so difficult to delineate these source zones. This is because, to date, delineation of these subsurface contaminants usually requires trial-and-error placement of a significant number of monitoring wells and extensive sample collection efforts. It is easy to see that if measurements are made at widely spaced intervals (e.g. several feet to tens of feet apart) the likelihood of locating micro-globules is very remote. Depending upon soil type, the use of conventional split spoon sampling below the water table may be questionable. Unconsolidated sands and silty soils tend to flow in the saturated zone resulting in poor retention of the sample in the split spoon. In turn, this may lead to significant uncertainties in contaminant distributions and mass estimates. Even when sampling is not problematic the sample density that can be achieved using conventional methods is so sparse that it may not be possible to accurately delineate contaminant source zones or localize thin layers of contamination. There is a need for a technology that can be used to delineate chlorinated solvent source zones. Gillespie et al. [20] has developed a halogen specific detector (XSD) comprised of

a miniaturized gas chromatography detector that exhibits a specific response to chlorinated species with approximately 5000:1 selectivity relative to petroleum hydrocarbons. The XSD is deployed inside a cone penetrometer (CPT) probe and is behind a membrane interface probe (MIP) that samples the soil formation for VOCs. While the XSD responds to chlorinated solvents, it cannot differentiate the chlorinated solvents. Such information would be useful to indicate whether or not transformation of chlorinated solvents is occurring, determine that the transformation is biological in origin, and indicate whether the transformation is aerobic or anaerobic. This information is also useful in determining the optimum remediation scheme. Because chlorinated solvents are polyatomic molecules that exhibit characteristic Raman spectra, it is believed that a field-deployable SERS-based sensor would be useful in determining the chemical composition of DNAPL micro-globules. Measurements of chlorinated solvents using the TEC-SERS system, Fig. 1, were conducted to validate this notion. Fig. 2 shows the SERS spectrum of the TP coating on a silver substrate. This is an aromatic coating and is a strong Raman scatterer. Figs. 3–5 summarize the TEC-SERS data that were obtained using this coating in the presence of TCE, PCE, and chloroform. The temperature response data was obtained by injecting a 0.5 ml aliquot of neat solvent into the 150 ml sample chamber and varying the temperature of the TEC. The solvent vapors diffuse through the silicone tubing and the nitrogen carrier gas transports the solvent vapors to the TEC-SERS module. Fig. 3b shows the normal Raman spectrum obtained for TCE. Fig. 3a

35000 intensity

18

29000 23000 17000 11000 5000 250

750

1250 wavenumber (cm-1)

1750

Fig. 2. SERS spectrum of Ag/TP. Spectra obtained using 785 nm excitation at 62.8 mW power and a 60 s acquisition time.

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19

2000

1000 (a) (a)

(b)

295.3

644.8

411.5

(b)

850.2

312.7

773.4

799.2 479.2

1213.4

1245.5

250

800

500

1350

wavenumber (cm-1) Fig. 3. Normal Raman and SERS spectra of the Ag/TP-TCE system. Spectra obtained using 785 nm excitation at 62.8 mW power and a 60 s acquisition time. (a) Spectra of Ag/TP exposed to TCE as a function of temperature (bottom to top: 25, 20, 15, 10, and 5 ◦ C). Gas flow rate varied between 9 and 12 ml min−1 . The spectral features of the coating have been subtracted out. (b) Normal Raman spectrum of neat TCE.

shows the difference spectra obtained for Ag/TP and TCE as a function of temperature. At 25 ◦ C, weak TCE peaks at 644.8 and 411.5 cm−1 are observed. As the temperature is lowered, the intensity of these TCE peaks increases and additional TCE peaks appear in the difference spectra. At 15 ◦ C, TCE peaks at 312.7, 479.2, and 850.2 cm−1 appear, Fig. 3a. At 5 ◦ C, the TCE peak at 1245.5 cm−1 appears. Shifts in the TP peaks are observed when TCE is present in the gas stream. These shifts could be indicative of interactions between the coating and TCE. Earlier, Angel [21] demonstrated that carbon tetrachloride could be concentrated on a diamond substrate mounted on a cold finger and detected by normal Raman spectroscopy. However, TCE could not be detected using this approach because much lower temperatures were required to freeze TCE on the diamond substrate. As shown by the results summarized in Fig. 3, this is not true when a coated SERS substrate is used that has an affinity for chlorinated solvents. Similar results were obtained for chloroform and PCE. Fig. 4a shows the difference spectra obtained for Ag/TP and CHCl3 as a function of temperature. At 25 ◦ C, weak chloroform peaks at 394.4 and 681.3 cm−1 are observed in the difference spectrum.

peak area

250

681.3

394.4

750

1000

1250

wavenumber (cm -1) 25000 20000 15000 10000 5000 0

(c)

0

400

800

1200

1600

ng of CHCl 3

Fig. 4. Normal Raman and SERS spectra of the Ag/TP-chloroform system. Spectra obtained using 785 nm excitation at 62.8 mW power and a 60 s acquisition time. (a) Spectra of Ag/TP exposed to chloroform as a function of temperature (bottom to top: 25, 20, 15, 10, and 5 ◦ C). Gas flow rate varied between 10 and 11 ml min−1 . The spectral features of the coating have been subtracted out. (b) Normal Raman spectrum of neat chloroform. (c) Chloroform concentration response at T = −10 ◦ C and gas flow rates of 16.2 ml min−1 (top) and 5.5 ml min−1 (bottom).

At 15 ◦ C, additional chloroform peaks at 295.5 and 773.4 cm−1 appear in the difference spectrum. At 5 ◦ C, the 1213.4 cm−1 chloroform peak appears. The concentration response of the 681.3 cm−1 peak at two different gas flow rates is shown in Fig. 4c. In these experiments, chloroform samples were injected directly into the nitrogen gas stream through the sample injection port, Fig. 1. The chloroform peak area varies linearly with concentration. The limit of detection (LOD) was determined using the limit calculation method [22–24] which states that LOD = 3σ/m where σ is the uncertainty in the y-intercept and m is the slope of the line. For a gas flow rate of 16.2 ml min−1 , the LOD = 158 ng chloroform and for a gas flow of 5.5 ml min−1 , the LOD is 106 ng. Lowering the gas flow rate results in lower LODs. The LODs obtained for chloroform using the Ag/TP substrates are higher

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(a)

7000

472.9

(b)

peak area

380

480

580

680

780

wavenumber (cm -1) 35000 (c) 28000 21000 14000 7000 0 -1.0 2.0 5.0 8.0 11.0 14.0 17.0 time (min)

Fig. 5. Normal Raman and SERS spectra of the Ag/TP-PCE system. Spectra obtained using 785 nm excitation at 62.8 mW power and a 60 s acquisition time. (a) Spectra of Ag/TP exposed to PCE at 0 ◦ C as a function of time (bottom to top: times 0, 0.5, and 11 min). Gas flow rate varied between 9.5 and 10.0 ml min−1 . (b) Normal Raman spectrum of neat PCE. (c) Plot of PCE peak area as a function of time.

than the LOD obtained by Carron and Kennedy [14] for benzene using the GC-SERS sensor. However, chloroform is a poorer Raman scatterer than benzene so it is expected that the LOD of chloroform would be higher. In addition, Carron et al. used a SERS substrate coated with 1-propanethiol to detect aromatic compounds. Compared to the TP coating used in this investigation, 1-propanethiol is a poorer Raman scatterer. The SERS signal of 1-propanethiol would be weaker than that of TP which will affect the LOD. However, the peaks due to 1-propanethiol significantly overlap with the peaks due to the chlorinated solvents. Comparing Figs. 2 and 3b, it can be seen that there is minimal overlap between TCE and TP. Consequently, not only does the coating need to exhibit an affinity for the analyte, but it must also have a SERS spectrum that minimally interferes with the Raman spectrum of the analyte.

Fig. 5a shows SERS spectra in the presence of PCE as a function of time. In this experiment, a sample of PCE was injected into the sample chamber, Fig. 1. Nitrogen gas flowed through the silicone membrane and transported PCE vapors to the TEC-SERS cell. As shown in Fig. 5a, the PCE peak increases with time while, simultaneously the 447.6 and 711.5 cm−1 TP peaks decrease in intensity. The decrease in the TP peaks is attributed to absorption of the excitation energy by the PCE condensed onto the SERS substrate. Fig. 5c shows a plot of the area of the 472.9 cm−1 PCE peak as a function of time. With time, PCE continues to accumulate on the SERS substrate indicating that longer sampling times will result in lower LODs. Comparing the spectral data summarized in Figs. 3–5, it can be seen that the spectral features (i.e. peak positions as well as intensity and shape) of the chlorinated solvent peaks in the SERS difference spectra are similar to those in the Raman spectra of the neat solvents. This facilitates speciation. The results summarized in Figs. 3–5 indicate that the magnitude of the measured VOC response is dependent upon the chemical natures of the coating and the VOC, the temperature of the SERS substrate, and the gas flow rate. Also, the same SERS substrate was used to generate the data shown in Figs. 3–5 over a one-month period. During that one-month period, the substrate had been housed in the Delrin SERS cell, Fig. 1. Little deterioration in the SERS signal of the coating was observed indicating that the coating protects the SERS surface from degradation and that these substrates can be used for long-term monitoring purposes. 3.3. Detection of aromatic compounds and MTBE using TEC-SERS Along with chlorinated solvents, aromatic compounds (benzene, toluene, ethylbenzene, and xylenes) are common contaminants in hazardous waste sites. Monitoring of aromatic compounds at these sites also relies on collecting soil and water samples which are sent away for analysis by GC-MS. The problems with this approach have been discussed vide infra. As with the chlorinated solvents, there is a need for a field-deployable technology to detect aromatic compounds in situ. Earlier Carron and Kennedy [14] showed that aromatic compounds could be detected by SERS as these

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compounds eluted off a GC column. Their detector was a SERS substrate mounted on an aluminum block. Temperature studies of the GC-SERS detector were performed by cooling the aluminum block that housed the SERS substrate prior to the chromatographic measurement or heating the block with heat tape. Results in this effort are similar to the ones obtained by Carron and Kennedy. Using a Ag/1-propanethiol SERS substrate, benzene could be detected at 15 ◦ C, in agreement with the earlier results of Carron and Kennedy. A silver SERS substrate coated with PFTP was used to detect toluene. Results are summarized in Fig. 6.

intensity

160000

120000

80000

40000 200

700

1200

1700

wavenumber (cm -1 )

(a)

9 ˚C 15 ˚C neat toluene 700 (b)

900

1100

1300

1500

wavenumber (cm-1)

Fig. 6. TEC-SERS results obtained for toluene. Spectra obtained using 785 nm excitation at 62.8 mW power and a 60 s acquisition time. (a) SERS spectrum of the PFTP coating. (b) A 0.5 ml sample of toluene was injected into the sample chamber. The top spectra are SERS spectra of the coated substrate exposed to toluene at 9 and 15 ◦ C. Spectral features of the coating have been subtracted. Bottom spectrum is the normal Raman spectrum of neat toluene. At 15 ◦ C, toluene does not condense onto the thiol-coated SERS substrate. The toluene peaks in the SERS spectrum obtained at 9 ◦ C correspond to that for neat toluene in the normal Raman spectrum.

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Fig. 6a shows the SERS spectrum of PFTP. Difference spectra as a function of temperature are shown in Fig. 6b. No toluene peaks are observed in the difference spectrum obtained at 15 ◦ C. However, toluene peaks are observed at 9 ◦ C. It was observed that the toluene and benzene peaks in the SERS spectra corresponded with those in the normal Raman spectra of the neat solvents. Although the TEC-SERS detector gave comparable results as the GC-SERS detector for aromatic compounds, the advantages of the TEC-SERS detector are better control of the temperature and the ability to go to temperatures below 0 ◦ C. A more recent contaminant of environmental concern is MTBE. MTBE is a polar, semivolatile organic compound that was used as an octane booster in gasoline in the 1970s and as an oxygenate additive to gasoline in the 1990s. Although the use of fuel oxygenates, combined with newer cars using more advanced pollution control equipment, has resulted in significant improvements in air quality, there are reports of MTBE contamination in drinking water supplies nationwide [25]. In 1996, seven wells supplying 50% of the water for the city of Santa Monica, California were removed from service because of MTBE at concentrations as high as 600 ␮g l−1 . Since Santa Monica closed its wells, the state of California has identified 10,000 MTBE-contaminated groundwater sites. Furthermore, 49 states have found MTBE in groundwater and 21 of these have had to shut down at least one of their wells due to MTBE [25]. The main sources of localized MTBE contamination of groundwater supplies are leaking underground storage tanks and pipelines, spills, contaminated sites, and MTBE manufacturing and storage sites [25]. MTBE is highly soluble and very mobile in groundwater and is not readily biodegradable. Conventional monitoring well networks currently installed at fuel leak sites are generally insufficient to properly locate and define the extent of MTBE plumes. MTBE plumes can be long, narrow, and erratic (meandering). Movement of MTBE plumes, as with other dissolved contaminants, is primarily controlled by groundwater flow lines [26]. These flow lines can be dramatically affected by discontinuities and can drop vertically in certain parts of the groundwater basins, such as recharge zones, cascade zones, and near pumping wells. In addition, the plumes can appear as discontinuous slugs particularly for those releases that occurred during the use of

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2000

(a)

737.6

(b)

859.9 921.8

396.6 403

1432.7

}

533.6

1133.71331.6

250

750

1250

1750

peak area

wavenumber (cm-1) 7400 (c)

7000 6600 6200 500

1200

1900

2600

3300

4000

ng MTBE Fig. 7. Normal Raman and SERS spectra of the Ag/TP-MTBE system. Spectra obtained using 785 nm excitation at 62.8 mW power and a 60 s acquisition time. (a) Spectra of Ag/TP exposed to MTBE as a function of temperature (bottom to top: 15, 10, 5, and 0 ◦ C). Gas flow rate varied between 10 and 12 ml min−1 . The spectral features of the coating have been subtracted out. (b) Normal Raman spectrum of MTBE. (c) MTBE concentration response at T = 0 ◦ C and gas flow rate of 12 ml min−1 .

MTBE as a wintertime oxygenate. As with the chlorinated solvents and aromatic compounds, there is a need for a technology that can detect MTBE in situ. Fig. 7b shows the normal Raman spectrum of MTBE. The 1-propanethiol coating could not be used to detect MTBE as the peak due to the C-S stretching mode of the gauche conformer significantly overlaps with the primary MTBE peak at at 737.6 cm−1 . No such overlap occurs with MTBE and TP. Fig. 7a shows difference spectra for Ag/TP and MTBE as a function of temperature. At 25 ◦ C, MTBE did not condense onto the SERS substrate. At 15 ◦ C, the MTBE peak at 737.6 cm−1 appears in the difference spectrum. At 10 ◦ C, additional MTBE peaks at 533.6, 859.9, 921.8, the multiplet at 1138.7–1331.6, and 1432.7 cm−1 appear in the difference spectrum. At

5 ◦ C, the doublet at 396.6 and 403 cm−1 appears. The linear concentration response is shown in Fig. 6c and the LOD is 501 ng MTBE. Lower LODs can be achieved by either lowering the gas flow rate or by using longer sampling times.

4. Conclusions In this effort, a thiol-coated SERS substrate was mounted on a TEC. The purpose of the coating was to protect the SERS substrate from degradation, to provide an internal calibration standard, and to attract contaminants of interest onto the SERS surface. Using this TEC-SERS sensor, the detection of VOCs such as aromatic solvents, chlorinated hydrocarbons,

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and MTBE was demonstrated. It was shown that the magnitude of the measured VOC response is dependent upon the chemical natures of the coating and the VOC, the temperature of the SERS substrate, and the gas flow rate. The VOC peaks in the SERS spectra directly correspond to the peaks observed in the normal Raman spectra of the neat VOC, which facilitates speciation. These preliminary results indicate that the TECSERS technology shows promise as a field-deployable sensor. Currently the TEC-SERS sensor is being miniaturized for deployment inside a CPT probe. The TEC-SERS module will be coupled to a membrane interface as was done with the XSD probe developed by Gillespie et al. [20]. Conceptually, as the probe is pushed, nitrogen gas flows across the heated membrane extracting VOC vapors from the surrounding soil/groundwater. The VOC vapors are transported to the SERS sensor module located inside the probe. A TEC condenses the vapors onto the SERS substrate. The vapors are then identified/quantified by their characteristic SERS response.

Acknowledgements This work was supported by the Navy’s Y0817 Pollution Abatement Ashore Technology/Validation Program. References [1] R.L. McCreery, M. Fleischmann, P. Hendra, Anal. Chem. 55 (1983) 146. [2] C.D. Allred, R.L. McCreery, Appl. Spectrosc. 44 (1990) 1229. [3] S.M. Angel, M.R. Myrick, Anal. Chem. 61 (1989) 1648. [4] T.F. Cooney, H.T. Skinner, S.M. Angel, Appl. Spectrosc. 49 (1995) 1846. [5] J.B. Cooper, P.E. Flecher, S. Albin, T.M. Vess, W.T. Welch, Appl. Spectrosc. 49 (1995) 1692.

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[6] M.A. Bryabt, J.E. Pemberton, J. Am. Chem. Soc. 113 (1991) 3629. [7] K. Carron, L. Pietersen, M. Lewis, Environ. Sci. Technol. 26 (1992) 1950. [8] K. Carron, K. Mullen, Anal. Chem. 66 (1994) 478. [9] P.A. Mosier-Boss, S.H. Lieberman, Appl. Spectrosc. 54 (2000) 1126. [10] P.A. Mosier-Boss, S.H. Lieberman, Appl. Spectrosc. 55 (2001) 1327. [11] J.B. Heynes, L.M. Sears, R.C. Corcoran, K.T. Carron, Anal. Chem. 66 (1994) 1572. [12] L.G. Crane, D. Wang, L.M. Sears, B. Heynes, K. Carron, Anal. Chem. 67 (1995) 360. [13] P.A. Mosier-Boss, S.H. Lieberman, J. Electroanal. Chem. 460 (1999) 105. [14] K.T. Carron, B.J. Kennedy, Anal. Chem. 67 (1995) 3353. [15] M.J. Yang, S. Harms, Y.Z. Luo, J. Pawliszyn, Anal. Chem. 66 (1994) 1339. [16] V. Lopez-Avila, J. Benedicto, H. Prest, S. Bauer, Am. Lab. (1999) 32. [17] F. Schwille, Dense Chlorinated Solvents in Porous and Fracture Media, Lewis Publishers, Inc., Chelsea, Michigan, 1988 (J.F. Pankow, Trans.). [18] J.F. Pankow, J.A. Cherry, Dense Chlorinated Solvents and Other DNAPLs in Groundwater: History, Behavior, and Remediation, Waterloo Press, 1996. [19] S. Feenstra, J.A. Cherry, B.L. Parker, Conceptual models for the behavior of dense non-aqueous phase liquids (DNAPLs) in the subsurface, in: Dense Chlorinated Solvents and Other DNAPLs in Groundwater: History, Behavior, and Remediation, Waterloo Press, 1996. [20] G.D. Gillispie, S.H. Lieberman, R. St. Germain, S. Adamek, Haloprobe: Direct Push Tool with Highly Specific Chlorinated Response, SERDP/ESTCP Partners in Environmental Technology Technical Symposium and Workshop, Washington D.C., December 2002. [21] S. Michael Angel, University of South Carolina, personal communication. [22] I.S. Krull, M.E. Swartz, LC·GC 15 (1997) 534. [23] I.S. Krull, M.E. Swartz, LC·GC 15 (1997) 842. [24] I.S. Krull, M.E. Swartz, LC·GC 16 (1998) 464. [25] R. Johnson, J. Pankow, D. Bender, C. Price, J. Zogorski, Environ. Sci. Technol. 34 (2000) 2A. [26] American Petroleum Institute, Strategies for Characterizing Subsurface Releases of Gasoline Containing MTBE, API Publishing Services, Washington D.C., Publication Number 4699, 2000.