IR Spectroscopic Method for ... - ACS Publications

There are three major steps involved in the purge-and-trap ATR/IR technique: (1) removal of analytes from the soil matrix to the SPME-ATR/IR cell, (2)...
0 downloads 0 Views 92KB Size
Download PDF
Anal. Chem. 1999, 71, 4690-4696

Purge-and-Trap ATR/IR Spectroscopic Method for the Detection of Semivolatile Aromatic Compounds in Soils Jyisy Yang* and Jhy-Woei Her

Department of Chemistry, Chung-Yuan Christian University, Chung-Li, Taiwan

A rapid method for determination of semivolatile compounds in contaminated soil samples was developed by coupling solid-phase microextraction (SPME) with attenuated total reflectance (ATR) Fourier transform infrared (FT-IR) spectroscopy. A trapezoidal internal reflection element was mounted horizontally in a flow cell with the inlet port connected to a temperature-controlled glass extraction chamber. Soil samples were placed inside the glass tube and heated to the desired temperature. Vaporized semivolatile compounds were carried by a stream of nitrogen gas to the ATR/IR flow cell. To increase the trapping efficiency, the ATR crystal was coated with a hydrophobic polyisobutylene polymer that acted as the SPME phase. The method proved to be very sensitive in the detection of semivolatile compounds in soils. The relationship between various parameters affecting chemical quantitation, such as the film thickness, gas flow rate and water contents, was also studied. Three different compounds, 1-chloronaphthalene, nitrobenezene, and 2-chlorotoluene, were used to investigate the feasibility of this method in the analysis of organic compounds in sand and soil. Results indicated a linear relationship between concentration and IR signals can be obtained for the three analytes. The detection limit of this method was in the range of 200-300 ppb. Analysis of organic pollutants in environmental soil samples is an important task with respect to the protection of the environment. Conventionally, organic contaminants in solid samples are examined by Soxhlet extraction followed by separation and identification. To reduce the use of organic solvents and to increase the speed of analysis, several methods have been proposed such as supercritical fluid extraction (SFE),1,2 accelerated solvent extraction (ASE),3 subcritical fluid extraction,4,5 and * To whom correspondence should be addressed: (fax) +886-34563160; (e-mail) [email protected]. (1) Bøwadt, S.; Hawthorne, S. B. J. Chromatogr., A 1995, 703, 549-571. (2) Deuster, R.; Lubahn, N.; Friedrich C.; Kleibo ¨hmer, W. J. Chromatogr., A 1997, 785, 227-238. (3) Richter, B. E.; Jones, B. A.; Ezzell, J. L.; Poter, N. L.; Avdalovic, N.; Phol, C. Anal. Chem. 1996, 68, 1033-1039. (4) Hawthorne, S. B.; Yang, Y.; Miller, D. J. Anal. Chem. 1994, 66, 29122920. (5) Hawthorne, S. B.; Grabanski, C. B.; Hageman, K. J.; Miller, D. J. J. Chromatogr., A 1998, 814, 151-160.

4690 Analytical Chemistry, Vol. 71, No. 20, October 15, 1999

headspace solid-phase microextraction (HS-SPME).6-8 Separation and identification methods such as gas chromatography/mass spectrometry are typically used to examine the extracts. Unlike extraction/chromatographic methods, attenuated total reflectance (ATR)/IR spectroscopy9 provides a direct method for detecting organic species in samples of varying physical composition. ATR is most suitable for handling aqueous solutions because the evanescent wave penetrates into the adjoining medium for a short distance. The depth of penetration, dp, of the evanescent wave is calculated according to following equation:

dp ) λ/{2πn1[sin 2θ - (n2/n1)2]1/2}

(1)

where θ is the angle of incident, λ is the wavelength of the incident radiation, n1 is the refractive index of the internal reflection element (IRE), and n2 is the refractive index of the sample material. In the mid-infrared range, the dp varies from 0.1 to approximately 1 µm. ATR spectroscopy has been used in numerous other applications including detection and monitoring liquids,10-14 gases,15,16 and biological samples.17-20 To increase the sensitivity of the ATR/IR method of detecting volatile organic compounds, the principle of SPME was applied.21,22 The SPMEATR/IR methodology has been applied extensively to aqueous (6) Zhang, Z.; Pawliszyn, J. Anal. Chem. 1993, 65, 1843-1852. (7) Zhang, Z.; Pawliszyn, J. Anal. Chem. 1995, 67, 34-43. (8) Santos, F. J.; Sarrio´n, M. N.; Galceran, M. T. J. Chromatogr., A 1997, 771, 181-189. (9) Harrick, N. J. Internal Reflection Spectroscopy; Wiley: New York, 1967. (10) Messica, A.; Greestein, A.; Katzir, A. Appl. Opt. 1996, 35, 2274-2284. (11) Simhony, S.; Katzir, A.; Kosower, E. M. Anal. Chem. 1988, 60, 1908-1910. (12) Gobel, R.; Krska, R.; Kellner, R.; Kastner, J.; Lambercht, A.; Tacke, M.; Katzir, A. Appl. Spectrosc. 1995, 49, 1174-1177. (13) Paul, P. H.; Kychakoff, G. Appl. Phys. Lett. 1987, 51, 12-14. (14) Newby, K.; Reichert, W. M.; Andrade, J. D.; Benner, R. E. Appl. Opt. 1984, 23, 1812-1815. (15) Messica, A.; Greenstein, A.; Katzir, A. Schiessel, U.; Tacke, M. Opt. Lett. 1994, 19, 1167-1169. (16) Giulianli, J. F.; Wohltjen, H.; Jarvis, N. L. Opt. Lett. 1983, 8, 54-56. (17) Vo-Dinh, T.; Tromberg, , B.J.; Griffin, G. D.; Ambrose, K. R.; Sepaniak, M. J.; Gardenhire, E. M. Appl. Spectrosc. 1987, 41, 735-738. (18) Simhi, R.; Gotshal, , Y.; Bunimovich, D.; Sela, E.-A.; Katzir, A. Appl. Opt. 1996, 35, 3421-3425. (19) Heise, H. M.; Marbach, R.; Janatsch, G.; Kruse-Jarres, J. D. Anal. Chem. 1989, 61, 2009-2015. (20) Belardi, R. P.; Pawliszyn, J. Water Pollut. Res. J. Can. 1989, 24, 179-184. (21) Louch, D.; Motlagh, S.; Pawliszyn, J. Anal. Chem. 1992, 64, 1187-1199. (22) Janatsch, , G.; Kruse-Jarres, J. D.; Marbach, R.; Heise, H. M. Anal. Chem. 1989, 61, 2016-2023. 10.1021/ac990180z CCC: $18.00

© 1999 American Chemical Society Published on Web 09/15/1999

solutions.23-27 However, the application of ATR spectroscopy to the detection of organic species in soils by physical contact between soil particles and the surface of the IRE is difficult to achieve. To eliminate the long analysis time and tedious extraction procedure of extraction/chromatography methods and to extend ATR method to solid samples, a new method based on the combination of SPME and ATR/IR spectroscopy is proposed for analysis of contaminated soil samples. In this method, thermal energy is used to assist desorption and evaporation of the analytes from the solid matrix, and a stream of gas is used to carry the vaporized analytes from the sample container into the SPME-ATR/ IR flow cell. If the analytes rapidly adsorb to the SPME phase and they remain immobilized, an IR signal with sufficient signalto-noise ratio can measured and the quantity of the analyte determined. METHOD DEVELOPMENT There are three major steps involved in the purge-and-trap ATR/IR technique: (1) removal of analytes from the soil matrix to the SPME-ATR/IR cell, (2) adsorption of the vaporized analytes onto the SPME phase, and (3) removal of analytes from the SPME phase. In the first step, the temperature of the soil matrix and the flow rate of the purge gas are major factors that influence the efficiency in removing the analytes out of the soil matrix. The sample temperature affects the rate at which analytes evaporate from the solid matrix, and the flow rate of the purge gas affects the efficiency of removal of the vaporized analytes from the sample matrix to the ATR/IR cell. By keeping the heating temperature constant, the rate at which molecules are removed from soils should be related to both the volatility of the analyte and the purging efficiency. Soil has a small void volume. Therefore, the efficiency of removing vaporized compounds should be high. The number of molecules that are removed from the solid matrix is affected mainly by the ease of analyte vaporization. By keeping the heating temperature constant, the quantity of analyte carried to the sensing device at time t is related to following equation:

F1(t) ) D1m1(t) exp(-f(t/V1)) ) D1m1(t)

(2)

where F1(t) is a function of the total number of molecules of analyte removed at time t, D1 is a proportional constant, f is the gas flow rate, V1 is the void volume of solid matrix, and m1(t) is the amount of analyte remained in the soil at time t. Under the condition that the void volume, V1, is much smaller than ft, F1(t) can be further simplified and equal to D1m1(t). Assuming that the behavior of analyte desorption from soil is an exponential function, m1(t) can be derived on the basis of the vaporization efficiency as in following equation:

m1(t) ) mo [exp(-k′ft)] ) mo [exp(-ht)]

(3)

where mo is the number of molecules in the original sample, k′ is (23) Blair, D. S.; Burgess, L. W.; Brodsky, A. M. Anal. Chem. 1997, 69, 22382246. (24) Ertan-Lamontagne, M. C.; Lowry, S. R.; Seitz, W. R.; Tomellini, S. A. Appl. Spectrosc. 1995, 49, 1170-1173. (25) Gobel R.; Krska, R.; Kellner, R.; Seitz, R. W.; Tomellini, S. A. Appl. Spectrosc. 1994, 48, 678-683.

related to the ease of analytes in escaping out of the soil matrix, and h ) k′f. Therefore, eq 2 can be converted to eq 4 by substitute m1(t) by eq 3, as shown in following equation (extraction function):

F1(t) ) D1mo [exp(-ht)]

(4)

In the second step, analyte that is carried to the IR cell is adsorbed onto the SPME phase. Both temperature and flow rate of purge gas influence the adsorption efficiency at this point. Because the thermal energy in the sample vessel is carried to the IR cell by the purge gas, the temperature of the IR cell increases as more heat is applied. The temperature increase in the ATR cell can reduce the adsorption efficiency. The flow rate also influences adsorption efficiency, as there is less time for analytes to adsorb onto the SPME phase when a high flow rate is used. Therefore, if the heating temperature is kept constant, the quantity of analyte that can be effectively adsorbed onto the SPME phase can be expressed by the following equation (adsorption function):

F2(t) ) m2(D2/f)

(5)

where m2 is the quantity of analyte carried to the IR cell at time t, and D2 is a constant related to the affinity between the SPME phase, and f is flow rate. After the analyte is purged from the soil sample and adsorbed onto the polymer phase, a portion is removed from the flow cell by the gas stream. This behavior can be modeled by an exponential function by the following equation (removing function):

F3(t1) ) D3m3 [exp(-k′′ft1/V2)] ) D3m3 [exp(-gt1)]

(6)

F3(t1) is a function of the number of adsorbed molecules that have been removed after elapsed time t1, where D3 is a proportional constant, m3 is the number of molecules that are adsorbed onto the hydrophobic film at any time t, k′′ is a constant related to the affinity of analyte to the hydrophobic film, V2 is the “effective” volume related to both cell volume and film thickness, and g is the removing constant equal to k′′f/V2. Taking into consideration the above equations, the number of analyte molecules in the polymer film detected at time t can be integrated as in the following equation (details of this integration can be found in ref 27):

F(t) ) D [exp(-ht) - exp(-gt)]/(g - h)

(7)

where D is a constant equal to D1D2D3mo/f. The result of F(t) still needs to be normalized because the area of an exponential equation should be a constant. Therefore, F(t) in eq 7 is normalized by the integration of both the extraction function (eq 4) and the removing function (eq 6). Integration of eq 4 from 0 to infinity yields a value of 1/h and the same integration of eq 6 a value of 1/g. This equation indicates that parameters such as (26) Heo, J.; Rodrigues, M.; Saggese, S.; Sigel, G. H., Jr. Appl. Opt. 1991, 30, 3944-3951. (27) Yang, J.; Her, J.-W. Anal. Chem. 1999, 71, 1773-1779.

Analytical Chemistry, Vol. 71, No. 20, October 15, 1999

4691

Figure 2. Infrared spectra of 1-CN adsorbed on the surface of an uncoated (upper trace) and PIB-coated (lower trace) ZnSe IRE.

Figure 1. Schematic diagram of the purge-and-trap ATR/IR assembly.

volume of the film and gas flow rate should be held constant in order to quantitate analytes by this ATR method. As long as the IR measurements are made over a specific period and the heating temperature remains constant, IR absorbance can be used to determine the concentration of the analyte in the original samples. EXPERIMENTAL SECTION Apparatus. The diagram of the extraction chamber and ATR flow cell is shown in Figure 1. The extraction chamber consisted of a glass tube (40 × 2 cm) wrapped with heating tape. A temperature controller obtained from a local supplier was used to adjust the heat of the chamber. The exit of the extraction chamber was connected to the input port of ATR flow cell by copper tubing (1.5-mm i.d.) The flow cell was constructed of glass plate (40 × 60 × 5 mm) and the ATR crystal was arranged horizontally to simplify optical alignment. A 45° trapezoidal (55 × 20 × 2 mm) zinc selenide (ZnSe) internal reflection element was purchased from International Crystal Laboratory (Garfield, NJ). A silicone spacer 1.2 mm thick was used to form the internal volume (400 µL) of the ATR detection cell. Because silicone spacer may adsorb organic compounds, Teflon (1 mm) was also used as spacer to examine the adsorption effect of silicone. The IR signals of 1-chloronaphthalene at 971 cm-1 using the method proposed in this work were 867.4 ( 43.8 and 798.2 ( 53.6 mAU for Teflon and silicone spacers, respectively. While the two spacers produced similar signals, silicone was used as it provided a better seal. The flow cell was placed in the sample compartment of a Magna 550 FT-IR spectrometer (Nicolet Instrument Co., Madison, WI) equipped with a medium-range mercury-cadmium-telluride (1 mm2) detector. Spectra were acquired in 4-cm-1 resolution. The gas purge was regulated with a flowmeter equipped with a controller (Cole-Parmer Instrument Co., Niles, IL). Reagents. Reagent grade 1-chloronaphthalene (1-CN), nitrobenzene, and 2-chlorotoluene (2-CT), obtained from R.D.H. (Seelze, Germany), were used as model semivolatile organic compounds. Nitrogen gas (99.9%) was obtained from a local supplier. Polyisobutylene (PIB) was obtained from Acros Organics (Geel, Belgium) and served as the hydrophobic SPME film. Sand 4692 Analytical Chemistry, Vol. 71, No. 20, October 15, 1999

and soil were used as solid matrixes. The sand represented a weakly adsorbing solid matrix. It was obtained from the local riverside, cleaned with deionized water, and dried in an oven at 120 °C for 24 h before use. Soil was also used as a solid matrix and was obtained as a gift from the department of soil science, National Chung-Hsien University (Taichung, Taiwan). The soil (clay loam) consisted of 24.7% sand, 36.1% slit, and 39.2% clay and was used without any pretreatment. To ensure the cleanliness of the sand and soil, the samples were tested by the proposed SPMEATR/IR method. No contaminant absorption bands were detected in the spectral region of interest. Organic probe compounds were dissolved in diethyl ether to form 1 wt %/vol solutions. A 100-µL aliquot of the test solution was added to the solid matrix and vigorously mixed in a sealed volumetric flask. Test samples were air-dried for 30 min to remove the organic solvent. ATR Coating Procedure. Polyisobutylene was dissolved in toluene (1wt %/vol) and used to coat the ZnSe IREs. The film thickness was controlled by the amount of polymer solution placed on the ZnSe surface. Assuming a PIB film density of 0.9 g/mL, the calculated film thickness of a 100-µL coating of a 1 wt %/vol PIB on a ZnSe crystal (55 × 20 mm) was 2.0 µm. A relative standard deviation of 6.3% was calculated (for six runs), based on the -CH2 bending mode of PIB at 1400 cm-1. RESULTS AND DISCUSSION Trapping Efficiency of SPME. Trapping efficiency plays an important role in the detection of organic compounds for this purge-and-trap ATR/IR spectroscopic technique. Trapping efficiency was assessed by measuring semivolatile compounds recovered from a sand matrix. Bare and PIB-coated (2.0 µm thick) ZnSe IREs were used to trap 1-CN (1 mg) from 80 g of sand. The chamber was heated to 130 °C and purged with nitrogen at 50 mL/min for 10 min before IR spectra were acquired. Spectra were obtained by signal averaging 100 scans at 4-cm-1 resolution. A spectrum of 1-CN trapped in the flow cell containing a bare ZnSe IRE is shown in Figure 2. A 971-cm-1 band intensity of approximately 5 mAU was measured. This low signal intensity indicates partial condensation of the analyte on the surface of the ATR crystal. The procedure was repeated, and 1-CN was trapped on a PIB thin film. The 971-cm-1 band intensity increased to 225 mAU with no spectral interference from the PIB coating. This

Figure 3. Plots of the peak height of 1-CN from samples preheated 5 min and purged at 50 mL/min with nitrogen (2) and no preheating (9).

Figure 4. Optimization of PIB film thickness. Plots of the peak height of 1-CN from samples preheated 5 min and purged with nitrogen at 100 mL/min. PIB film thickness: (2) 0.16, (9) 2.0, and ([) 4.0 µm.

reveals that the PIB thin film contributed significantly to the retention of 1-CN purged from the sand matrix. Adsorption/Time Profiles for Different Extraction Procedures. In practice, several minutes are required before the sample in the reaction chamber reaches the set temperature. The purge must be applied at the appropriate time in order maximize transfer of the analyte to the ATR/IR flow cell. Two different methods in heating and purging were studied: (1) application of the nitrogen purge immediately after the heater was turned on and (2) application of the nitrogen purge after the sample was preheated for 5 min. In the first method, nitrogen starts to flow into the extraction chamber when the sample is still cold. As a result, the analytes vaporize slowly and are partially swept into the ATR flow cell. After the sample reaches the set temperature, the rate of vaporization increases rapidly. Therefore, large IR signals should be observed after the desired temperature is reached. In the second method, the analyte is vaporized before the nitrogen purge is applied. All analytes are vaporized before being swept into the ATR cell. If the affinity of 1-CN for the PIB film is high, both methods should produce the same results. However, if the analyte is not strongly adsorbed, the IR signal intensity will drop with time as the analyte is swept out of the ATR flow cell. The results obtained with and without chamber preheating are shown in Figure 3 along with the theoretical curve fits from eq 7. The temperature of the sample chamber and transfer line was set at 130 °C. When the sample was heated prior to the purge, a more intense 971-cm-1 absorption band (13.1% greater) was measured. In the absence of preheating, the 971-cm-1 band intensity increased at a much slower rate. The fitted results indicate that the time profile of the preheated data was more closely matched to the theoretical equation. Equation 7 does not provide a good fit of the time profile data from unheated sample. Assuming the analyte experiences two heating temperatures (one lower than the set temperature and one at the set temperature), the observed time profile without preheating can be fitted with eq 7 very well using two sets of parameters as the fitting results shown in Figure 3. This indicates that a portion of the analyte was removed from the sample chamber in the process of heating the sample to the set temperature. SPME Film Thickness. Gobel et al.25 showed that the thickness of of SPME phase affects the length of time required

for analytes to diffuse down within penetration depth of the evanescent wave. Therefore, the thicker the SPME film, the longer the time to obtain stronger IR signals. Once the analyte penetrates into the thicker SPME phase, the analyte becomes trapped, consequently a higher retention efficiency is achieved. To obtain the optimal film thickness for the SPME-ATR/IR technique in our optical system, three PIB film thicknesses were examined, 0.16, 2.0, and 4.0 µm. Assuming the refractive index of 2.4 for ZnSe at 1000 cm-1 and a PIB refractive index of 1.4, the dp of IR radiation at 1000 cm-1 is 1.66 µm at an angle of incidence of 45°. A thickness of 0.16 µm is much less than the dp at 1000 cm-1, 2.0; µm is close to the dp and 4.0 µm is thicker than the dp. To eliminate the influence of the solid matrix on the analysis, 1-CN (1 mg) was placed in a small-diameter glass tube (5-mm i.d.) within the extraction chamber. The sample was preheated to 130°C for 5 min and then purged with nitrogen at 100 mL/min. The thicker the polymer film, the greater the measured 971-cm-1 band intensity (Figure 4). For thinner films (0.16 and 2.0 µm), a maximum in the curve occurred. This reveals that the nitrogen flow used to carry the analyte to the ATR cell can also sweep it from the cell. In attempting to fit the data with eq 7, a large deviation was observed for the 0.16-µm PIB coating (see Figure 4). The fitted values of g and h are tabulated in Table 1. g was inversely proportional to the SPME volume, and the coefficient h was independent of SPME volume. Therefore, the thicker the SPME phase, the smaller the value of g, while h remains unchanged (see Table 1). The value of D was apparently related to film thickness too. Comparing the IR signals in Figure 4, a film thickness close to the dp provides a faster response time and the maximum signals were also close to the thicker SPME phase. Therefore, a 2-µm film thickness was selected as the thickness of the SPME phase for the following experiments. Purge Flow Rate. According to eq 7, both coefficients g and h are proportional to the flow rate of the purge gas. Solid samples have small void volumes. Therefore, a low flow of purge gas should effectively sweep the vaporized analytes from the sample chamber. Increasing the flow rate of the purge will only slightly increase the extraction efficiency (eq 4) but will greatly decrease the adsorption efficiency (eq 5) and retaining efficiency (eq 6) of the SPME phase. The effect of flow rate on the detection of organic Analytical Chemistry, Vol. 71, No. 20, October 15, 1999

4693

Table 1. Calculated Coefficients from Eq 7 for Different Sample Conditions conditions 1 mg of 1-CN/80 g of sand, 0% water, 100 mL/min gas flow rate, 130 °C heating temp PIB film thickness, µm 0.16 2 4 1 mg of 1-CN/80 g of sand, 0% water, 2-µm SPME film, 130 °C heating temp purge flow rate, mL/min 50 100 200 1 mg of 1-CN/20 g of sand, 10% water, 2-µm SPME film, 50 mL/min gas flow rate heating temp, °C 60 80

D

g

2674 1.112 4725 0.377 7366 0.109

h

0.0770 0.0700 0.0770

8428 0.0780 0.0770 3723 0.1540 0.1520 1093 0.2230 0.8670

5853 0.1060 0.1030 5127 0.2160 0.2140

Figure 5. Optimization of purge flow rate. Plots of the peak height of 1-CN from samples preheated 5 min and adsorbed on a PIB-coated (2.0 µm) IRE. Nitrogen flow rate: (9) 50, ([) 100, and (2) 200 mL/ min. Single-sided standard deviations are shown for clarity.

compounds was investigated by testing three nitrogen flow rates: 50, 100, and 200 mL/min. An 80-g sand sample containing 1 mg of 1-CN was used, and the sample chamber was preheated and then purged with nitrogen. More intense IR signals were obtained using a lower flow rate of purge gas (Figure 5). The total dead volume in the detection system is relative small compared to the flow rate used. Therefore, the efficiency of analyte removal from the sample chamber should be similar for the flow rates used in these experiments. On the negative side, the high gas flow leads to a shorter adsorption time and increased rate of removal or desorption of the analyte from the SPME phase. Therefore, a more intense IR signal at a lower flow rate is expected. The maximum peak height was very similar to data obtained without the sand matrix (see Figure 4). This indicates the nitrogen is effective in removing analyte from the solid sample. Equation 7 was used to fit the experimental data (see Figure 5). The coefficients g and h were both proportional to flow rate. Both values of g and h agreed with the theoretical equations. The high level of agreement between the theoretical expression and the experimental data was observed for curves at low flow (50 and 4694

Analytical Chemistry, Vol. 71, No. 20, October 15, 1999

100 mL/min) but a large deviation was observed at a flow rate of 200 mL/min. General Consideration in Detection of Water Content Samples. Soil samples usually contain a small amount of water that may affect the detected signals. To study the role of water in this detection method, both sand and soil were used as solid matrix. Different amounts of water were added to 80-g samples of sand containing 1 mg of 1-CN. To prevent a large evaporation of water molecules out of the solid matrix, the heating temperature was decreased to 60 °C. Also, to prevent any condensation of the water molecules or analytes on the transfer line, the temperature of the transfer line was kept at 120 °C. Results indicate that an IR signal was only observed when sand samples contained less than 2 wt % water. This behavior can be caused by condensation of water molecules on the surface of the PIB film and preventing the 1-CN from adsorbing onto the SPME phase. When the water content was lower than 2 wt %, some areas on the PIB film were not blocked by water molecules and were free to adsorb the analytes. Therefore, the 1-CN adsorbs and an IR signal was observed. While a measurable signal was recorded, large standard deviations were observed in these experiments. To reduce the condensation of water on the SPME phase, the ATR/IR setup was modified. The PIB-coated IRE, spacer, and base plate were inverted. To extend this method to examination of larger water content samples, the amounts of sand and soil were reduced to 20 and 10 g, respectively. When the amount of water is similar to 80 g of sand, the percentage of water in the sample is increased in smaller sample sizes. If water is present in the solid sample, preheating may lead to high back pressure in the sample vessel. Therefore, samples were not preheated in this study. Dry samples were heated to 130°C to increase the vaporization of the organic analytes while still maintaining the integrity of the PIB thin film. It has been reported that water present in the sample matrix can actually increase the extraction efficiency and lower the temperature required for analyte extraction.8,30 Therefore, two lower temperatures were selected to study the effect water has on extracting analytes from a sand matrix. The temperature of the transfer line was set at 120 °C to prevent condensation of the analyte or water. Each sample contained 1 mg of 1-CN in 20 g of sand and 10 wt % water. The flow rate of the purging gas was set at 50 mL/min, and a 2-µm-thick PIB film was used. Samples heated to 80 °C produced high signal intensities with maximums near T ) 7 min. In contrast, the plot of the 791-cm-1 band intensity measured at 60 °C was only half the intensity measured at 80 °C. The maximum was reached after approximately 15 min. Since the transfer line was held at 120 °C, the temperature of the ATR/IR cell was the same for both experiments. Therefore, the increase in signal intensity observed at 80 °C was a result of increased vaporization of analyte from the extraction chamber. These curves were also fit with eq 7 (see Figure 6) and tabulated in Table 1. The analyte adsorption profile was smoother at 60 °C than at 80 °C. The coefficients g and h increased significantly when the temperature was increased from 60 to 80 °C. Therefore, the (28) Schwarzenbach, R. P.; Gschwend, P. M.; Imboden, D. M. Environmental Organic Chemistry; John Wiley & Sons: New York, 1992. (29) Boublik, T.; Fried V.; Hala, E. The Vapor Pressure of Pure Substances, Elsevier Science Publishers: Amsterdam, The Netherlands, 1984. (30) Fromberg, A.; Nilsson, T.; Larsen, B. R.; Montanarella, L.; Facchetti, S.; Madsen, J. O. J. Chromatogr., A 1996, 746, 71.

Figure 6. Plots of the peak height of 1 mg of 1-CN extracted from 20 g of wet sand (10 wt % water). Samples were heated to 60 ([) and 80 °C (9). Purge flow rate was 50 mL/min, and the PIB film thickness was 2 µm.

Figure 7. Plots of the peak height of 1 mg of 1-CN extracted from 20 g of wet sand. Samples were heated to 60 °C with a purging flow rate of 50 mL/min, and the PIB film thickness was 2 µm. The water content was 0 ([), 5 (9), 10 (2), and 15 wt % (b).

heating temperature should be controlled precisely to obtain quantitative data. Effect of Water Content on Analyte Detection. To study the effect of water content on the detection of semivolatile organics, 5-15 wt % water was added to 20 g of sand samples containing 1 mg of 1-CN. The samples were heated at 60 °C. Plots of the IR signals as a function of time are shown in Figure 7. Because of large standard deviations in these curves, only the mean values were plotted. Based on triplicate runs, the average relative standard deviations were 15.4, 22.5, 26.5, and 23.2 for 0, 5, 10, and 15% water content samples, respectively. Basically, the higher the water content, the higher the 971-cm-1 band intensity. Santos et al.8 increased extraction efficiencies of three chlorobenzenes in soils by adding a large amount of water to the soil. Higher analyte signals were observed when the water content was less than 50 wt %. Similar observations were reported by Fromberg et al.30 The adsorption of organic compounds in soil is believed to be stronger than in sand due to the complex nature of soil. The particle size of a sand sample is also larger than that of the soil, and thus soils have much greater surface area. The retention of

Figure 8. Plots of the peak height of 1 mg of 1-CN from 10 g of wet soil. Samples were heated to 60 °C with a purging flow rate of 50 mL/min and 2-µm PIB film thickness. The water content was 0 ([), 5 (9), 10 (2) and 15 wt % (b).

water in soils is also greater than sand. Therefore, the sample matrix was switched from sand to soil and the experiment repeated. To reduce the clogging of nitrogen flow, the soil sample was further reduced to 10 g. The results are shown in Figure 8. The adsorption time profiles were similar to those of the sand samples. However, several differences in the detection of 1-CN in soil and sand were apparent. First, the standard deviations of detected signals of analytes in soils were smaller than in sand. Second, 1-CN was more strongly adsorbed to soil than sand, based on the plots of signal intensity of the dry samples in Figures 7 and 8. Third, water molecules can effectively release the analytes out of the adsorption sites of the soil. Therefore, the higher the water content, the faster the analyte was desorbed and swept into the ATR/IR detection cell for soil samples. For example, at 15 wt % water content in soil, the analyte signal begins to rapidly increase after 3 min as compared to 5 min for 10 wt % water content samples. Fourth, three different stages of analyte removal from the extraction chamber may exist in extraction of analytes from the soil samples. For example, a rapidly increase of signal was observed at 10 min detection time for 5 or 10 wt % water content soil samples as can be seen in Figure 8. This may indicate that analytes interact with the soil matrix differently. Therefore, weakly adsorbed analytes were partially removed by the purge gas before and after reaching the set temperature of the extraction chamber. The strongly bound analytes were released after heating the sample for a certain time. This reveals that the concentration of the analytes can affect the extraction behavior. But, because the IR signals of soil samples did not deviate strongly from eq 7, a linear relationship between the concentration of analytes and their IR signals can be obtained, as will demonstrated in following paragraphs. Effect of Analyte Volatility on Detection. Volatility is an important factor in either vaporization or desorption of the analyte from the sample matrix. Meanwhile, the removal of adsorbed analytes from the SPME phase is also affected by the volatility of the compounds. For example, highly volatile compounds are easily vaporized and extracted from the solid matrix but more difficult to retain in the SPME phase. Therefore, the temperature used to examine different compound volatilities should be varied. To Analytical Chemistry, Vol. 71, No. 20, October 15, 1999

4695

Figure 9. Plots of the peak height of 50 ppm NB from 10 g of wet soil (10 wt % water). Samples were heated to 40 (9), 60 ([), and 80 °C (2). The purge flow rate was 50 mL/min, and the PIB film thickness was 2 µm.

Figure 10. Plots of the peak height of 50 ppm 2-CT from 10 g of wet soil (10 wt % water). Samples were heated to 40 ([) and 60 °C (9). The purge flow rate was 50 mL/min, and the PIB film thickness was 2 µm. Samples were also heated to 80 °C but analytical signals were not observable.

examine the role of volatility of analytes in detection, two other compounds, NB, and 2-CT, were studied. The vapor pressures of 1-CN, NB and 2-CT at 25 °C are 0.017, 0.12, and 3.55 Torr, respectively.28,29 Their boiling points are 259.3, 210, and 158.9 °C for 1-CN, NB, and 2-CT, respectively. To reduce the amount of water evaporated from solid samples, the heating temperature is limited to 100 °C. Therefore, three different temperatures, 40, 60, and 80 °C were used to extract NB and 2-CT from 10-g soil samples. Heating at 80 °C resulted in a reduction of the analyte detected (Figure 9 for detection of NB) and not observed with 2-CT (Figure 10). This indicates a difference in compound volatility and that the heating temperature should be optimized for individual compounds. In general, a heating temperature at 60

4696

Analytical Chemistry, Vol. 71, No. 20, October 15, 1999

°C is applicable to detect analytes with a vapor pressure lower than 0.12 Torr. For analytes with vapor pressures higher than 0.12 Torr, a heating temperature lower than 60 °C is suggested to reduce the removal of adsorbed analytes from the SPME phase. Detection Limits and Linearity. The detection limit and linearity of the data were investigated. A purge of 50 mL/min and 2.0-µm-thick PIB film were used. Both sand and soil were used as the solid matrix. Sand samples were heated to 130 °C for dry sand samples and to 60 °C if they contained water. IR signals were coadded for 2 min (at 8 to 10 min) at 4-cm-1 resolution. An R2 of 0.9989 (concentration vs peak height at 971 cm-1) was calculated for 1-CN (500 ppb to 10 ppm concentration range) in 80 g of dry sand. The calculated detection limit of 1-CN based on 3 times the peak-to-peak noise is 156 ppb. If the sample contained water, the amount of soil was reduced to 10 g. The IR signal was acquired for 18-20 min at 4-cm-1 resolution for 1-CN and NB. For 2-CT, the IR signals were coadded for 6-8 min and a heating temperature of 40 °C was used. Based on the standard curve of 1-CN in soil containing 10 wt % water, R2 was 0.9832 in the concentration range below 100 ppm. The calculated detection limit based on 3 times the peak-to-peak noise was 1.69 ppm. For relatively high volatility compounds, such as NB, the R2 was 0.9971 (peak height at 850 cm-1). Based on 3 times the peak-to-peak noise of the spectra, a detection limit around 1.61 ppm was obtained. A detection limit of 12.3 ppm 2-CB (peak height at 746 cm-1) was obtained at a temperature of 40 °C. CONCLUSION A combination of ATR/IR spectrometry with solid-phase microextraction can be used effectively to detect semivolatile compounds in solid samples. The solid sample is heated. Semivolatile compounds are vaporized and readily removed by a stream of nitrogen. Analytes adsorb to a thin film of polyisobutylene coated on an ATR crystal. Experimental parameters, including flow rate, film thickness, heating temperature, and water content, can affect the IR signal significantly. However, the analytical system can be optimized through proper selection of the parameters. A low detection limit for 1-CN or NB can be obtained with high linearity in standard curves. Due to the removal of analytes by the purge gas, the detection of more volatile compounds in a solid matrix is less sensitive by this method. ACKNOWLEDGMENT The authors thank the National Science Council of the Republic of China for financial support of this work under Contract NSC862113-M-033-008. The authors also thank Dr. Ken Ishida for his help in the preparation of the manuscript.

Received for review February 16, 1999. Accepted August 3, 1999. AC990180Z