Screening of Pentachlorophenol-Contaminated Wood by

Harald A. Beck, Zoltan Bozo´ ki,† and Reinhard Niessner* ... capable of on-site operation. .... operation of this acoustic system can be understood...
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Anal. Chem. 2000, 72, 2171-2176

Screening of Pentachlorophenol-Contaminated Wood by Thermodesorption Sampling and Photoacoustic Detection Harald A. Beck, Zoltan Bozo´ki,† and Reinhard Niessner*

Institute of Hydrochemistry, Technical University of Munich, Marchioninistr. 17, D-81377 Munich, Germany

The still-remaining high amounts of pentachlorophenol (PCP), used as wood preservative, in buildings and in waste wood are a potential risk for humans and the environment. To ensure a fast and selective measurement of PCP, a screening tool was developed, which is not only sensitive, but, unlike conventional methods, it requires no added chemicals, is simple, cost-effective, mobile, and capable of on-site operation. The instrument combines light-induced thermodesorption sampling followed by an external cavity diode laser based photoacoustic detector. Measurements on wood samples proved that the system can determine PCP to as low as a concentration of 10 µg/ cm2 within minutes without destruction of the sample. The system was calibrated with reference to the concentration of PCP impregnated on the wood surface. It is demonstrated that measurements are not influenced by moisture content of the wood samples. As a fungicide, Pentachlorophenol (PCP) (CAS 87-88-5) was used widely for wood preservation between the 1950s and 1980s. In many cases it was used frivolously, in high concentrations. Nowadays, due to its persistence, PCP is ubiquitously spread within the environment. Highly PCP-impregnated wood, which is still in use, can lead to high indoor pollution, being a severe risk for human health.1 PCP is enriched easily in adipose tissue of humans and animals after incorporation by dermal contact, respiration, or ingestion.2 Therefore, PCP is placed on the US Environmental Protection Agency priority pollutant list. Although the use of PCP in most of the industrial countries has been restricted or banned since the 1980s,3 there is still a problem existing due to the large amount of waste wood treated with PCP. The combustion of these woods has to be controlled because of the fact that PCP is a precursor for highly toxic substances, such as PCDDs and PCDFs.4 As a consequence, there is a strong need for a screening tool which ensures a fast and quantitative determination of PCP in * Corresponding author (tel.) 0049-89-7095-7981; (fax) 0049-89-70957999; (e-mail) [email protected] † On leave from the Research Group on Laser Physics of the Hungarian Academy of Sciences, Szeged. (1) Henneboele, H.; Parlar, H. Chemosphere 1990, 21, 919-922. (2) Tanjore, S.; Viraraghavan, T. Int. J. Environ. Stud. 1994, 45, 155-164. (3) Wild S. R.; Harrad, S. J.; Jones, K. C. Chemosphere 1992, 24, 833. (4) Altwicker, E. R.; Konduri, R. K.; Milligam, M. S. Chemosphere 1990, 20, 1935-1944. 10.1021/ac9912496 CCC: $19.00 Published on Web 03/31/2000

© 2000 American Chemical Society

wood. Moreover, to use it at different locations such as buildings, demolition sites, or recycling plants, a mobile instrument is preferable. The determination of PCP in wood is conventionally done by using expendable and time-consuming wet-chemical procedures. Usually the wood is homogenized before PCP is extracted from the matrix with organic solvents or supercritical fluid extraction and detected by gas chromatography (GC) coupled with mass selective (MS) or electron capture detectors (ECD).5-10 These technical laboratory procedures are indeed very sensitive, but they are time-consuming, expensive, and can only be handled by trained workers in well-equipped analytical laboratories. Recently, various alternative methods for the determination of PCP in wood were proposed. With many of them, such as the Curie-point pyrolyzer,5 FT-IR spectroscopy,6 or the photon activation analysis,7 on-site use is not possible. Detection of PCP with mobile GC-MS, GC-ECD, or ion mobility spectrometry8 still requires the homogenization of the wood sample, being both a time-consuming and destructive procedure. Other alternative methods, e.g., laser-induced plasma-spectroscopy and X-ray fluorescence spectroscopy, had already proven to be capable of detecting different elements in wood.8,11 As a consequence, the semiquantitative measurement of several inorganic wood preservatives is possible with these methods, but at present they are not suitable for the detection of either chlorine and other halogens or specific structures of single substances. The immunochemical analysis enables a selective determination of PCP, but it requires a time-consuming sampling and extraction procedure.12 In the work presented here, we introduce a new method for the fast and quantitative measurement of PCP concentration of surfacecontaminated wood samples, which is selective and sensitive enough to be incorporated into a screening instrument for use at wood recycling plants. (5) Horn, W.; Marutzky, R. Fresenius’ J. Anal. Chem. 1994, 348, 832-835. (6) Bresner, A.; Gilbert, R.; Tetreault, P.; Lepine, L.; Archambault, J.-F. Anal. Chem. 1995, 67, 442-446. (7) Barelt, G.; Buge, H. G.; Go ¨rner, W.; Win, T. Fresenius’ J. Anal. Chem. 1998, 360, 433-434. (8) Schro ¨der, W.; Matz, G.; Ku ¨ bler, J. Field Anal. Chem. and Technol. 1998, 2, 287-297. (9) Fries, G. F.; Paustenbach, D. J.; Mather, D. B.; Luksemburg, W. J. Environ. Sci. Technol. 1999, 33, 1165-1170. (10) Leblanc, Y. G.; Gilbert, R.; Hubert, J. Anal. Chem. 1999, 71, 78-85. (11) Lo ¨be, K.; Lucht, H. LaborPraxis 1997, 11, 82-87. (12) Wuske, T.; Fittkau, I.; Mahn, J.; Polzius, R.; Manns, A. Anal. Chim. Acta 1998, 359, 321-328.

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It is based on a diode laser light source and a photoacoustic detection system supplemented with a thermodesorption head. In thermodesorption sampling, the desorption rate of molecules is increased via increasing the surface temperature. Heating of the sample surface results in the evaporation of low-volatility compounds, such as PCP. Thermodesorption sampling in combination with GC-MS detection is already used in an instrument for the determination of PCP in wood.8 In that instrument thermodesorption sampling is based on direct heating of the sample by using a heated branding iron; therefore it requires contact with the contaminated material. Moreover, the use of high vacuum within the MS line further complicates the setup of the system. In our work the thermodesorption head is based on lightinduced heating of the sample surface, to avoid contact of the heating system with the contaminated material. In addition, our system is operated at atmospheric pressure. To heat the sample surface with high efficiency, an infrared enhanced halogen lamp is used. Temperature increase is caused by the light absorption of the sample matrix. The evaporated compounds are constantly sucked into the measuring device. The PCP contamination is then measured by use of the photoacoustic (PA) principle as follows. The wavelength of an intensity modulated external cavity diode laser light source is tuned to a NIR absorption band of PCP. The light is partially absorbed by the portion of PCP molecules, which were transferred from the wood surface into the PA cell after thermodesorption. The absorbed energy is converted first into heat through nonradiative relaxation and then to pressure fluctuations through thermal expansion. The laser light is modulated at an acoustical frequency which coincides with the resonance frequency of the PA cell; thereby efficient amplification of the acoustic signal is achieved. This amplified pressure wave is measured with a microphone attached to the cell. In a PA measurement, the amplitude of the microphone signal is proportional to the optical absorption coefficient of the gas and thus to the concentration of the measured component. Diode laser based photoacoustic gas detection was already successfully applied for the detection of substances such as soot aerosol,13 benzene and toluene,14 methane,15 ammonia,16 and water vapor.17 However, to the best of the authors’ knowledge, this is the first time photoacoustic spectroscopy has been coupled with thermodesorption sampling. EXPERIMENTAL SECTION The Thermodesorption Head. The half open thermodesorption head (see also Figure 1 and 2) contains a powerful halogen lamp (HLX 64635, Osram, Munich). A spherical back-reflecting mirror focuses its radiation into a spot, on the wood surface, with a diameter of 50 mm. This radiation heats the surface of the wood sample and transfers its PCP contamination into the gas phase. The reflecting mirror has a special high-reflection coating for the infrared wavelength range, to increase the effectiveness of the (13) Petzold, A.; Niessner, R. Appl. Phys. Lett. 1995, 66, 1285-1287. (14) Beenen, A.; Niessner, R. Analyst (Cambridge, U.K.) 1998, 123, 543-545. (15) Scha¨fer, S.; Mashni, M.; Sneider, J.; Miklo´s, A.; Hess, P.; Pitz, H.; Pleban, K.-U.; Ebert, V. Appl. Phys. B. 1998, 66, 511-516. (16) Fehe´r, M.; Jiang, Y.; Maier, J. P.; Miklo´s, A. Appl. Opt. 1994, 33, 16551658. (17) Bozo´ki, Z.; Sneider, J.; Gingl, Z.; Mohacsi, A.; Szakall, M.; Bor, Z.; Szabo, G. Meas. Sci. Technol. 1999, 10, 999-1003.

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Figure 1. Schematic view of the thermodesorption head (TDH), containing the halogen lamp (HL) protected with the quartz glass plate (QG) and the photoacoustic cell (PA cell), containing the central resonator (CR) and the acoustic filters (AF). The electret microphone (EM) is mounted at the end of steel tube ST’. HB is the heating blanket, and W are the windows through which the laser beam (LB) enters the cell. Steel tube CT is connecting the sampling head and the PA cell.

heating. To avoid deposition of the desorbed analyte on the heating lamp, and on the focusing mirror, a 1-mm-thick quartz glass separates the lamp from the desorption chamber. The desorption head is connected to the PA cell through a short (≈30 mm) 1/4 inch stainless steel tube. To avoid PCP condensation, the connecting tube is heated too. The Photoacoustic Cell. The PA cell (Figure 1) consists of a central resonator with an inner diameter of 8 mm and a length of 40 mm. Two identical “buffer” volumes are attached to each end of the central resonator, with diameters of 22 mm and lengths of 20 mm. These are followed with cavities having alternately small and large cross-sectional areas with a length of 20 mm each. The operation of this acoustic system can be understood as follows. The strongest acoustic resonance of this cell occurs at a frequency (f)

f)

c 2L

(1)

where c is the sound speed and L is the length of the central resonator. This is a longitudinal resonance, i.e., the generated acoustic standing wave varies along the symmetry axis of the central resonator. In our case, the resonance frequency is about 4 kHz at room temperature. However, due to the temperature dependence of the sound speed, the resonance frequency can shift considerably at elevated temperatures. The strength (S) of the PA signal at this frequency can be expressed as18

S)

(γ - 1)PQβ Afπ2

(2)

where γ ) 1.4 is the ratio of isobaric and isochoric specific heats for air, P is the modulated light power at the resonance frequency, Q is the quality factor of the longitudinal resonance, β is the optical absorption coefficient of the gas at the laser wavelength, and A is the cross sectional area of the resonator. By reducing A, S can (18) Rosencwaig, A. Photoacoustics and Photoacoustic Spectroscopy; WileyInterscience: New York, 1980.

Figure 2. Schematic view of the entire apparatus, containing the sampling head, the photoacoustic cell, the external cavity diode laser, and the pumping device.

be increased as long as Q remains independent from A. For central resonators with diameters down to ≈10 mm, the quality factor indeed is relatively independent from the cross-sectional area (Q ≈ 20-60). For diameters less than 10 mm, Q starts to decrease drastically, which finally results in a decrease of the PA signal. In addition, for thin resonators, an interfering background PA signal, generated by the portion of laser radiation absorbed on the wall of the resonator, occurs. The 8-mm diameter applied here is a result of a compromise between these different effects. The buffer volumes at the ends of the resonator act as acoustical barriers, preventing outside noise from entering into the central resonator as well as hindering the photoacoustically generated acoustic energy from escaping from it. This barrier effect is mainly caused by reflection of acoustic energy at the ends of the resonator. For a joint of two tubes with different crosssectional areas (A1 and A2), the acoustic reflection coefficient (R) can be written as:19

R)

|

|

A1 - A 2 A1 + A 2

2

(3)

Thus it can be seen that, for tubes with sufficiently different cross sectional areas, the reflection coefficient is close to 1 in both directions. Moreover, an additional acoustical filtering effect, due to destructive interference, occurs, as the length of the buffer volumes (and also the rest of the cavities) is set to a quarter of the wavelength at the resonance frequency. Therefore the cavities, having alternating small and large cross-sectional areas, serve as acoustic filters prevent outside noise from entering the PA cell through its openings. Also these acoustic filter elements make possible operation of the cell in the flow-through mode with a flow rate as high as ≈1 L/min. Because of the very low vapor pressure of PCP at room temperature (9 ×10-5 hPa), the use of a high-temperature (220 (19) Morse, P.; Ingard, K. Theoretical Acoustics; McGraw-Hill: New York, 1968.

°C) PA cell was needful. To ensure good temperature stability of the cell, a fast and efficient heating is required. Moreover, condensation of PCP on the cell’s windows should be prevented, otherwise a loss in the analyte quantity, and an uncontrollable increase in the background PA signal, might occur. Therefore, the cell body was made from a single piece of metal with a cylindrical shape, with its mantle surrounded with a heating blanket. The windows are fitted in the inner body of the cell. For the temperature measurement, a Ni/CrNi thermocouple is attached to the outer surface of the cell body. The measuring microphone (Knowles EK 3029) is attached to the middle of the central resonator, where the pressure distribution has its maximum. However, to avoid heating above its maximum operational temperature of 50 °C, the microphone is separated from the body of the cell with an 200-mm-long 1/8 inch stainless steel tube. Although, the use of longitudinal resonators is quite widespread in photoacoustics,15-17 unlike the cell used here, they are mostly unsuitable for high-temperature operation. Apparatus. The thermodesorption assisted photoacoustic PCP detector was integrated into the complete measuring system as shown in Figure 2. The gaseous analyte from the sampling head is sucked through the PA cell with a vacuum pump. The gas flow is regulated with a needle valve and a rotameter. After the cell, a column with active charcoal is used for removal of the PCP out of the gas stream in order to prevent pollution of the laboratory air. As a light source, a Littman type external cavity diode laser system20 is used (Sacher Lasertechnik, Marburg, Germany). The laser cavity is formed by an antireflection coated diode laser, a grazing incidence grating, and a tuning mirror. Wavelength tuning is accomplished by changing the mirror angle either with a micrometer screw (coarse tuning) or a piezo translator (fine tuning). The advantage of external cavity diode lasers compared with conventional diode lasers lays in the wide tuning range, the (20) Labachelerie, M.; Latrasse, C.; Kemssu P.; Cerez, P. J. Phys. III 1992, 2, 1557.

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single-mode operation, and the high stability. Application of external cavity diode lasers in PA gas detection is detailed in the literature.17 Further electronic components were installed in the system, as follows. The temperature of the PA cell was controlled by a homemade active temperature controller, which kept the cell temperature with a stability of (1 °C. A laser driver (Sacher Lasertechnik LVE200) is used for temperature stabilization, current control, and piezo wavelength tuning of the diode laser. For the PA measurements, the current of the diode laser is modulated through the analogous modulation input of the laser driver with a square-wave signal from the function generator (Voltcraft FG1617). The modulation frequency of the laser is set to the resonance frequency of the PA cell, and the laser beam is directed through the PA resonator. The microphone signal from the PA cell is amplified with a homemade microphone preamplifier (amplification, 1000) before it is recorded as a function of time with a lock-in amplifier (Stanford Research SR850). The lock-in integration time is set to 1 s. The recorded time-dependent signals are processed with a personal computer. Optical and Acoustical Optimization of the System. After heating the PA cell to the operational temperature of 220 °C, the resonance frequency of the cell was determined by tuning the laser to a water vapor absorption line. The laser modulation frequency was then scanned in the range of the frequency calculated from eq 1. The frequency corresponding to the maximum PA signal was selected as the resonance frequency of the PA cell. The resonance frequency was measured in both flowthrough mode (with a flow rate of 1 L/min) and stopped-flow mode. The optimum wavelength for PCP measurement was determined in the NIR range by using an FT-IR spectrograph (FT 1600 Series, Perkin-Elmer, Norwalk, CA) and a heatable gas cuvette. The absorption was scanned in a range between 7800 and 4000 cm-1 with a resolution of 4 cm-1. Next within the selected absorption band, the high-resolution spectra of water vapor was scanned with the tunable diode laser by using the PA cell, to find a water vapor interference-free operational wavelength area. The wavelength of the laser was measured with a wavemeter (WA 2500, Burleigh, Puchheim, Germany), and the laser power was measured with a power meter (Nova PD 300 IR, OPHIR, Jerusalem, Israel). Photoacoustic Measurement Procedure. Measurements were made on commercial spruce wood samples. To estimate the influence of the water content of the samples on the PA signal, half of the samples were dried in a heating chamber at 80 °C for as many as 8 days. For the photoacoustic measurements, different solutions of PCP in toluene (2-10 mg/mL) were prepared. From these solutions, 50 µL was spotted homogeneously on the sample surfaces within a circle of 50 mm in diameter. After the solvent was evaporated the sample was put underneath the sampling head. An air gap of 1 mm between the wood surface and the sampling head was left, to ensure the constant gas flow and to avoid pressure fluctuations. A constant flow rate of 1 L/min was adjusted. In parallel with switching on the heating lamp, the lock-in measurement was started. The heating lasted for 60 s with a power of 30 W (6 V/5 A), and the PA signal was recorded for 200 s. 2174

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Validation of the Thermodesorption Efficiency. GC/MS analysis was applied as a reference method to validate the thermodesorption sampling efficiency. The thermodesorption head was connected with a sample collector specially constructed for this use. The collector contained a fiberglass filter (22-mm diameter) and a polyurethane (PU) foam (23-mm diameter and a height of 50 mm) for absorbing PCP from the gas phase. The PU foam cylinders were subsequently cleaned with 1 L of toluene, 1 L of ethyl acetate, and finally with 1 L of cyclohexane each time for 24 h in a Soxhlet extractor. After cleaning, the PU foams were dried under vacuum. The fiberglass filters were conditioned by heating to 400 °C for 5 h. PU foams and filters were both kept over silicagel in a desiccator in the dark until the sampling. A standard solution of PCP in toluene (100 mg/L) was prepared. The validation of the thermodesorption sampling was tested by desorbing 0, 2, 4, 10, and 20 µg of PCP applied to the surface within a spot of 50 mm in diameter. The sampling procedure was identical to the one used in the PA measurements. After sampling, the foams and the filters were spiked with a solution of 13C6-PCP (100 mg/L). The foams were spiked with 100 µL and the filters with 50 µL of the solution. The PCP was extracted from the filters by adding 0.1 mL of sulfuric acid and 20 mL of toluene under ultrasonic agitation for 15 min. The PU foams were refluxed with 250 mL of toluene in a Soxhlet extractor for 3 h. After reducing the sample volume to 20 mL, the PCP was derivatized to the acetate by adding 2 mL of a solution of 0.5 M potassium carbonate and 2 mL of freshly distilled acetic anhydride. The organic phase was separated and dried with sodium sulfate. Before measuring, the sample volume was reduced to the standard volume of 1 mL, from which an aliquot of 1 µL was injected into the GC/MS. For GC/MS analysis, an HP 5980 Series II gas chromatograph combined with a high-resolution mass selective detector (VG Autospec) was used. The GC injector had a temperature of 270 °C. The following GC temperature program was used. It started at 100 °C, was held there for one minute, and then was heated to 280 °C with a heating rate of 20 °C/min. The program ended 10 min after reaching the final temperature. The samples were measured in the SIM mode with a resolution of about 10.000. The molecules were ionized by electron impact at 70 eV and a trap current of 600 µA. The analyte was detected at 265.8441 and 267.8411 amu. The internal standard (13C6-PCP) was detected at 271.8642 and 273.8612 amu. Chemicals. Except for the self-prepared standard solution of PCP, no other chemicals were needed for the photoacoustic measurements. All of the following chemicals were used only for the GC/MS validation of the thermodesorption. The solvents were all supplied by Merck, Darmstadt, Germany. Ethyl acetate, cyclohexane, toluene (analytical grade purity) as well as anhydrous sodium sulfate, p.a., were used as received. For the derivatization, acetic anhydride, p.a., was freshly distilled. From the potassium carbonate, p.a., a 0.5 M solution was prepared with aqua dest. The sulfuric acid 95-98% was diluted to a 0.1 M solution. PCP, p.a., was dissolved in toluene. Helium of 5.6 purity used as the GC carrier gas was supplied by Messer-Griesheim, Krefeld, Germany. 13C6-Pentachlorphenol, 99% (Cambridge Isotope Laboratories, Inc., Andover, USA), used as internal standard was dissolved in toluene, p.a.

RESULTS AND DISCUSSION Properties of the Apparatus. The PA cell was found to be heatable to as high as 250 °C. For the measurements the cell temperature was held at 220 °C. The measurement of the acoustic noise as a function of gas-flow rate showed a constant noise level of ≈400 nV to as high as a flow rate of 1.2 L/min. Above this flow rate, the noise increased rapidly. Therefore, a flow rate of 1 L/min was used for all the measurements. At 220 °C the resonance frequency of the PA cell was found to be 4800 Hz in flow-through mode. To verify the thermalization of the gas stream sucked into the cell, the resonance frequency was also measured in the stopped-flow mode, in which the temperature of the gas is equal to the cell temperature. Any difference between the resonance frequencies in stopped-flow and flow-through mode would indicate a nonperfect thermalization because of the temperature dependence of the resonance frequency (see the photoacoustic cell section). The resonance frequency was found to be the same as in both modes, proving a perfect thermalization of the gas. The wavelength tuning range of the laser was spanned from 1410 to 1490 nm. The modulated laser power was about 2 mW at this wavelength. The absorption band of PCP measured by FTIR was found at 1.44 µm (6930 cm-1) with a width of ∆ν ) 28 cm-1. By fine scanning of this region, strong water vapor absorption lines within the absorption band of PCP were found. A single region from 6929 to 6931 cm-1 was revealed, which is free from water vapor absorption lines. Within this area, PCP can be measured without water vapor interference. Signal Processing. Figure 3 demonstrates a typical timedependent PA signal for 500 µg of PCP applied to a dried (3a) and an undried (3b) wood sample. For comparison purposes, the PA signals in case of untreated wood samples are also shown in this figure. The PA signals are increasing initially after the heating is started. The maximum value in the case of the treated samples is reached at approximately 20 s, before the signal decays to the background noise. In the case of the untreated wood, the signal is relatively stable throughout the heating period. After switching off the heating lamp, the signal also decays to the background noise level. Drying the wood samples resulted in an 8% loss of weight as well as a change of color to a darker shade. Despite these changes, the signal profiles remain very similar both for the treated and untreated samples. This indicates that the measurement is not influenced by the water content of the wood. To measure the total amount of PCP desorbed from the wood surface, the PA signal was averaged over a period of the first 50 s. Within this time, most of the PCP is desorbed from the sample surface. Calibration of the PCP Detector. As the optical absorption coefficient of PCP in the gas phase at the laser wavelength is not known, the use of an independent method to estimate the absolute amount of PCP, which is transferred into the PA cell, was required. This was achieved by using GC/MS analysis as a reference method. The validation yielded a 40% ( 5% (n ) 5) sampling efficiency for all concentrations tested. Different amounts of PCP between 0 and 500 µg were used for the calibration of the thermodesorption assisted photoacoustic detector. In Figure 4 the averaged PA signals (S) for dried and undried wood samples are shown as a function of the mass of

Figure 3. Time-dependent PA signals for untreated (0 µg/cm2) and treated (25 µg/cm2 PCP) wood samples. For comparison purposes dried (a) and undried (b) wood samples are presented.

Figure 4. Calibration line for the PCP detection system. A linear relationship (R2 ) 0.98) exists between PA signal and applied PCP contamination of wood samples (n ) 36).

PCP (m) spiked onto the wood surface. Each point represents an average of 3 measurements. A calibration line (n ) 36) was fitted with

S ) 0.706m + 0.49

(4)

where S and m have the units of microvolts and milligrams, respectively. From the standard deviation of the background signal (sB ) 0.056 µV (n ) 6)), the detection limit was calculated to be 202 µg of PCP within the heated area. Taking into account the size of the heated area (20 cm2), an estimated 5-mm thickness of the contaminated surface layer in which PCP is distributed8 and a wood density of 0.5 g/cm3, this detection limit can be converted Analytical Chemistry, Vol. 72, No. 9, May 1, 2000

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into a concentration of 43 µg/g (ppm) of PCP relative to the mass of impregnated wood. CONCLUSION A universally applicable thermodesorption-assisted photoacoustic PCP detector was developed. It is based on a thermodesorption head and a PA detector. The thermodesorption head contains a halogen lamp, which transfers PCP from the wood surface into the gas phase, where it is sucked into the PA cell. A 40% absolute collection efficiency for the thermodesorption was determined by using GC/MS analysis. The cell was especially designed to work at high temperatures (to 220 °C). The PA signal is generated with the help of an external cavity diode laser tunable to a water-free absorption line of PCP at 6930 cm-1. The detector was calibrated with wood samples impregnated with PCP solution. Measurements were performed by switching on the halogen lamp and averaging the time-dependent PA signal. By proper wavelength selection, water vapor interference was avoided. In addition, the moisture content of the wood was found not to influence the measurement. Time-averaged PA signal was used as a measure of the amount of PCP. The calibration of the system showed a detection limit of 43 µg/g (ppm) of PCP on wood. This sensitivity is high enough to work as a screening tool

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as its detection limit is far below the range of common contaminations. The developed method is able to detect PCP in treated wood in a nondestructive way. The apparatus is simple and easy to handle. The system works at normal pressure without using any protective gas, vacuum, or extraction solvents. It has potential to be developed into an automatic screening system. Compared with standard analytical equipment it has a much lower price and a minimum operational cost. The compact setup of the system seems to be appropriate to be transformed into a transportable instrument for on-site operation. Routine measurements at a wood recycling plant have been started now. ACKNOWLEDGMENT The authors would like to thank the Bavarian Institute of Waste Research for their cooperation, the Bavarian State Ministry for State Development, and Environmental Affairs for financial support.

Received for review November 1, 1999. Accepted February 15, 2000. AC9912496