Environ. Sci. Technol. 2002, 36, 5014-5020
Investigation of the Volatile Organic Substances that Cause the Characteristic Odor of Pentachlorophenol Treated Wood Utility Poles M Y R I A M F O R T I N , † R O L A N D G I L B E R T , * ,‡ A N D R EÄ B E S N E R , ‡ J E A N - F R A N C¸ O I S L A B R E C Q U E , ‡ A N D JOSEPH HUBERT§ Institut de Recherche d’Hydro-Que´bec, 1800, Boulevard Lionel-Boulet, Varennes, Que´bec, Canada J3X 1S1, and De´partement de Chimie, Universite´ de Montre´al, P.O. Box 6128, Montre´al, Que´bec, Canada H3C 3J7
The nature of the volatile organic compounds that could be at the origin of the characteristic odor of treated wood utility poles was investigated by the study of compositional changes in the chromatographic profiles of freshpentachlorophenol (PCP) solvent samples and weathered samples collected from an in-service red pine pole. Over 99 peaks were identified in the chromatogram of the fresh solvent from which a large portion of the C3-, C4-, C5-, C6alkylbenzene isomers and early eluting n-alkanes was missing from the analysis of weathered samples. Three domains in the chromatographic profile (volatile, semivolatile, and nonvolatile components) were confirmed by assessing the headspace of fresh-PCP solvent samples using direct syringe sampling and solid-phase microextraction. A first level of field validation was achieved using an emission cell for measuring substances emanating from sapwood specimens at different temperatures. The average latent heat of vaporization (∆Hvap) of the PCP-solvent components was estimated at 99.9 kJ/mol from these results. Finally, the analysis of airborne substances at a treating plant and a utility pole storage site confirmed that the C4-, C5-, and C6alkylbenzene isomers could contribute to the characteristic odor perceived by humans.
Introduction Most of the 2.4 million poles used on Hydro-Que´bec’s distribution system are protected against rot through the impregnation of a solution of pentachlorophenol (PCP) in a petroleum based solvent. The odors associated with substance emissions are sometimes strong enough in the summer near the utility’s poles storage sites to disturb nearby residents. Very little is known about the solvent’s volatile compounds (VOCs) that are embedded in the wood cells during treatment or how the molecules are subsequently * Corresponding author fax: (450)652-8424; e-mail:
[email protected]. † Present address: CEGEP de Valleyfield, 169, rue Champlain, Salaberry-de-Valleyfield, Que´bec, Canada J6T 1X6. ‡ Institut de Recherche d’Hydro-Que ´ bec. § Universite ´ de Montre´al. 5014
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released into the environment. The PCP solvent used is a mid-distillate fraction of petroleum to which a hydrotreating process could be applied to give a more pronounced aliphatic character, thereby creating a more environmentally acceptable product (1). Although the identification of mid-distillate components has been the subject of various studies (2, 3), to date very little research has allowed olfactory nuisances to be linked to the volatile compounds that originate from products used as solvents in industrial processes such as in the wood preservative industry. The only study of which the authors are aware is that conducted by Hermia et al. (4) on the determination of VOCs found in paint shops. In this study, some of the BTEX (collective name for benzene, toluene, ethylbenzene, and p-, m-, and o-xylenes) and C3- to C5alkylbenzenes present in the petroleum cuts were found in the ambient air. Such molecules are expected to be found in substantial amounts in the mid-distillates used by the wood preservative industry. In another field of interest, a link has been established between the odorous products of fresh mushrooms and the presence of ethylbenzene, ethenylbenzene, xylene, and propenylbenzene (5). Other authors concluded that the olfactory perception threshold increases as the molecular weight of the alkylbenzenes decreases (6). The same conclusion was drawn by measuring the odor threshold in relation to the carbon chain length of a homologous series of alkylbenzenes (7). Various techniques have been used for collecting the airborne VOCs in ambient air, including direct headspace sampling (8), headspace solidphase microextraction (SPME) (9, 10), and headspace sampling by solvent or solid trapping (11). These techniques are generally coupled with capillary gas chromatography/ mass spectrometry (GC/MS) to separate and identify the species. In the present study, the volatile organic substances that cause the characteristic odor of the PCP-treated wood utility poles were investigated by measuring the compositional changes in the chromatographic profiles of fresh-PCP solvent samples and weathered samples collected from an in-service red pine pole. Three domains in the chromatographic profile of the PCP-solvent components (volatile, semivolatile and nonvolatile fractions) were confirmed by assessing the headspace of fresh-PCP solvent samples using direct syringe sampling and solid-phase microextraction (SPME). A first level of field validation of the molecules that could be found in the headspace of treated wood was then obtained by measuring substance emissions from the surface of sapwood specimens collected from a PCP-treated pole. The tests, which were carried out in an emission cell at different temperatures, were used to estimate the average latent heat of vaporization of the PCP solvent components. A final validation was obtained by using SPME fibers to collect the airborne substances in the ambient air of a treating plant and a utility pole storage site both located in the province of Que´bec.
Experimental Section Apparatus. The gas chromatographic analyses were performed in constant flow column mode using an Agilent 5890 chromatograph (Agilent Technologies) equipped with an automatic injector, model 7673, and an on-column injection port. The separation was performed with a nonpolar HP1MS 30-m × 0.25-mm-i.d. × 0.25-µm column (Agilent Technologies). The column was connected to an Agilent 5973 mass selective detector at 70 eV ionization energy in the electron impact mode. The instrument interface was maintained at 300 °C and a mass range, m/z ) 50-300, in a 0.18-s 10.1021/es020508x CCC: $22.00
2002 American Chemical Society Published on Web 11/02/2002
cycle, was scanned in total ion count mode (TIC). The HP mass spectral library was used off-line to identify the resolved components in the chromatograms. An acceptable balance between peak resolution and run time was achieved under an He carrier gas flow rate of 1 mL/min with the following oven conditions: 55 °C for 3 min, then ramped at 20 °C/min to 275 °C with a final hold for 2 min. The temperature of the on-column injector was set at 280 °C for the injection of the liquid samples and desorption of the SPME fibers. Direct Headspace Syringe Sampling of PCP Solvent Samples. The concentrated nature of the analytes assessed made it possible to sample directly the volatile compounds in the headspace of PCP-solvent samples. Aliquots of 5 mL of the PCP solvent were placed in a series of 20-mL glass vials (headspace vials from Supelco). The vials were then sealed by crimping an open-center aluminum cap containing a Teflon-faced butyl molded septum. After a 2-h stabilization at 20 °C in a conventional water bath, a 500-µL sample lock syringe (Supelco) was used to collect aliquots from the gas phase of each vial for GC/MS analysis. SPME Headspace Sampling of PCP Solvent Samples. The SPME extractions of the volatile compounds of the PCP solvent were performed using 100-µm poly(dimethylsiloxane) solid-phase microextraction fibers (Supelco). For these tests, 5-µL aliquots of the PCP solvent were placed in a series of 20-mL glass vials and then, the vials were stabilized for 2 h at 20 °C. The fibers were exposed for 15 min to the headspace of the stabilized vials for extraction and then 3 min in the injection port for desorption. For the tests at a treating plant and a utility pole storage site the fibers were used with an extraction time of 1 min for the samples collected in the vicinity of the pole piles and 15 min for the samples taken in the upstream and downstream winds (both with a desorption time of 1 min). Headspace Sampling of Wood Specimens. The emission cell used for sampling the headspace of thermostabilized wood specimens was adapted from a design proposed in prestandard ENV 1250-1 prepared by the European Committee for Standardization (12). The borosilicate glass cell assembled in our laboratory, which is shown in Figure 1, was installed in a 37.5-m3 stainless steel environmental chamber. Five sapwood specimens (letter K in Figure 1) of 5-cm × 5-cm × 1-cm were then inserted into the slots of the cell holder (letter J in Figure 1). The initial temperature of the chamber was set at 20 °C with a 45% relative humidity. The test specimens were first stabilized at this temperature for 2 h, after which sampling of the headspace was done by removing the air inlet stopcock (letter F in Figure 1). A laminar air flow was then created in the cell by actuating the pump (letter I in Figure 1) for 12 min (500 mL/min for a volume of 6 L) to trap in the solvent all the species contained in the headspace of the wood specimens. Following the sampling, the cell was closed, and the temperature of the chamber was raised to 25 °C. A second sampling of the headspace volume was carried out after 2 h at this temperature and so forth until 40 °C was reached in 5 °C increments. This range of temperatures was chosen to match those observed in the summer around pole piles. After each headspace sampling, the solvent (MeOH:CH2Cl2 3:1) contained in the trap (letter G in Figure 1) was transferred to a 10-mL volumetric flask, the volume of which was adjusted at the graduation line with fresh solvent. 0.5-µL aliquots were collected from the flask for GC/MS analysis. Prior to using the cell, a test done at 20 °C in absence of sapwood specimen showed no detectable peak under our chromatographic conditions. Chemical Products. The PCP solvent investigated in this study is a petroleum mid-distillate manufactured by Shell Canada under the label SOLVENT PCP, Shell code #645-900 (72.2% (v/v) aromatics, 1.5% (v/v) olefins, and 25.3% (v/v)
FIGURE 1. Emission cell for sampling the headspace of thermostabilized sapwood specimens - adapted from ref 12. saturates with a bp of 205-366 °C). The sample kindly supplied by a treating plant meets the Type A requirements of CSA Standard O80.201-97 (13) and American WoodPreservers’ Association Standard P9-98 (14). The He (99.999% pure) for the GC was supplied by Prodair, while MeOH and CH2Cl2 used for wood extraction and solvent trapping were provided by Fisher Scientific (Optima grade). The reference standards used for retention time identification are listed in Table 1 together with the suppliers, catalog numbers, and percent purities. Wood Samples. The weathered oil samples were obtained from a 5-year in-service PCP-treated red pine pole installed on Hydro-Que´bec’s distribution system. Details on the ultrasonic procedure used to extract the PCP solvent from the core samples collected from this pole are given elsewhere (see ref 15). The sapwood specimens used in the emission cell were collected at the groundline of a PCP-treated red pine pole after four months of weathering exposure. Only the side of the specimen corresponding to the perimeter of the pole was exposed to the air of the cell by wrapping all the other surfaces with an aluminum foil.
Results and Discussion Determination of the Volatile Fraction of the PCP Solvent Components. Figure 2 shows a typical TIC chromatogram recorded from the injection of a sample obtained by diluting the PCP solvent in a MeOH:CH2Cl2 3:1 mixture. The components identified by comparing the GC retention times with reference standards and literature values and the mass spectrum recorded at the maximum of each peak with those from the MS library are listed in Table 2. As seen in this table, the peaks detected in the time range of 3.5-7.5 min (Figure 2a) are mostly associated with C3-, C4-alkylbenzene isomers, with the exception of peaks 7, 22, 26, and 30, which were identified as the first members of the n-alkanes and the C5alkylbenzenes. It is interesting to note that the highly volatile VOL. 36, NO. 23, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Reference Standards Used to Identify Retention Times composition
supplier, catalog number, purity
mixture of n-decane, n-docosane, n-dodecane, n-eicosane, n-hexacosane, n-hexadecane, n-octacosane, n-octadecane, n-tetracosane and n-tetradecane mixture of benzene, ethylbenzene, m-xylene, methyl tert-butyl ether, naphthalene, o-xylene, p-xylene, toluene, 1,2,4-trimethylbenzene, and 1,3,5-trimethylbenzene propylbenzene 1,2,3,4-tetrahydronaphthalene 2-ethylnaphthalene 1,3-dimethylnaphthalene amylbenzene 1,4-diethylbenzene hexylbenzene 2,3-dihydro-1H-indene azulene 2,3,5-trimethylnaphthalene mixture of acenaphthene, acenaphthylene, anthracene, benzo (A) anthracene, benzo (A) pyrene, benzo (B) fluoranthene, benzo (G, H, I) perylene, benzo (K) fluoranthene, chrysene, dibenz (A, H) anthracene, fluoranthene, fluorene, indeno (1,2,3-CD) pyrene, naphthalene, phenanthrene and pyrene
FIGURE 2. Peaks eluted in the time range of a) 3.5 to 7.5 min, b) 7.5 to 10.0 min, and c) 10.0 to 16.0 min of the TIC chromatogram of the PCP solvent components - Peak numbering as reported in Table 2. BTEX components (4.069 min < tR < 4.749 min) were not detected from this sample. In the time range of 7.5-10.0 min (Figure 2b), most of the peaks are associated with the C5and C6-alkylbenzenes, with the exception of peaks 36, 47, 55, 5016
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UST Modified Diesel Range Organics, Supelco, 48166 UST Modified Gasoline Range Organics, Supelco, 48167
Aldrich Chemical Co., P5,240-7, 98% Aldrich Chemical Co., 42-932-5, 99% Fluka Chemie AG, 047520, 99.5% Fluka Chemie AG, 040780, 96% Aldrich Chemical Co., 11,317-4, 99% Aldrich Chemical Co., D9,100-4, 96% Aldrich Chemical Co., P2,570-1, 97% Aldrich Chemical Co., I-180-4, 95% Aldrich Chemical Co., A9,720-3, 99% Aldrich Chemical Co., T7-740-2, 98% EPA 610 Polynuclear Aromatic Hydrocarbons Mix, Supelco, 48743
60, 61, 62, 63, and 64, which were found to correspond to the n-alkanes or the first members of the C7-alkylbenzenes. The comparisons of the mass spectra and the retention times of reference standards are both indicative of the presence of two-ring molecules such as alkyl-branched indenes and naphthalenes. In the time range of 10.0-16.0 min (Figure 2c), the peak identification becomes more uncertain because of the gradual loss of resolution. Nevertheless, the comparisons of the mass spectra and the retention times of the reference standards both pointed the C7-, C8-, C9-, and C10alkylbenzenes, C16 to C25, n-alkanes and some compounds containing sulfur atoms. According to our field observations, at least three to four years of weathering exposure is required to reduce to a negligible level the odors emanating from newly treated red pine poles. A first insight into the modifications in the compositional profile of the PCP solvent components was then obtained by analyzing the solvent extracted from the sapwood of a 5-year in-service PCP-treated pole. The TIC chromatogram is shown in Figure 3a with in overlay the signal recorded for the fresh-PCP solvent (same chromatogram as for Figure 2). The amount of compounds normally found in the time range of 3.5 to 10.0 min is significantly reduced in the chromatogram of the weathered sample. Besides, there is a major peak at tR ) 11.329 min that corresponds to PCP (99% spectral match with a tR for the reference standard at 11.347 min) coextracted with the petroleum solvent from the wood cells. Based on the peak identification given in Table 2, it is possible to attribute the C3- and C4-alkylbenzenes to the volatile components (almost complete loss of these peaks in the region of 3.5 to 7.5 min), the C5- and C6alkylbenzenes, and the early eluting n-alkanes to the semivolatile components (partial loss of these peaks in the range of 7.5 to 10.0 min) and the remaining compounds to the nonvolatile components (no obvious loss of these peaks in the range of 10.0 to 16 min). Such weathering changes in the compositional profile of the alkylbenzenes were previously reported by Wang et al. (16) for crude oil samples. Using a laboratory evaporative technique for preparing the weathered samples, these authors have shown that the loss rate is significantly correlated with the molecular weight and boiling points of alkylbenzene compounds. To further validate the chromatographic domain relevant to the PCP solvent components, two techniques were used
TABLE 2. Typical Compounds Identified in the TIC Chromatogram of a Fresh-PCP Solvent Sample peak no.
tR
1 2 3 4 5 6 7 8 9 10 11 12 13
5.079 5.424 5.509 5.599 5.704 5.867 6.030 6.058 6.158 6.198 6.265 6.436 6.463
14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
6.500 6.527 6.552 6.610 6.707 6.722 6.777 6.963 7.024 7.073 7.104 7.250 7.296 7.335 7.378
tR of reference standard
tR calcd from lit. data (ref 16)
4.069-4.483 4.505-4.749 5.048 5.412 5.617
5.503 5.588 5.718
5.883 6.039 6.035 6.145 6.175 6.263 6.426 6.443 6.490
29 30 31 32 33 34 35 36 37 38 39 40 41
7.424 7.460 7.506 7.579 7.631 7.668 7.744 7.863 7.884 7.948 8.030 8.079 8.158
42 43 44
8.265 8.463 8.497
45 46 47 48 49 50 51 52
8.515 8.579 8.613 8.674 8.851 8.939 9.030 9.171
53 54 55 56 57 58 59 60
9.195 9.256 9.305 9.344 9.369 9.408 9.472 9.509
61 62
9.527 9.564
63
9.902
6.476 6.521 6.551 6.604 6.700 6.718 6.779 6.957 7.066 7.105 7.294 7.390 7.420 7.421 7.451 7.506 7.587
7.865 7.938 8.074
146 142 160
C0-, C1-, C2-b C2-b C3-b C3-b C3-b C3-b C3-b C3-b n-C10 C4-b C4-b C4-b C3-b C4-b C4-b C4-b C4-b C4-b C4-b C4-b C4-b C4-b C4-b C5-b n-C11 C4-b C4-b C4-b C5-b C4-b C4-b C5-b C4-b C5-b C5-b C4-b C5-b C5-b C5-b n-C12 C5-b C5-b C5-b C6-b C5-b C6-b C5-b C5-b C6-b
160 142 184 160 160 160 154 156
C6-b C5-b n-C13 C6-b C6-b C6-b C6-b C6-b
160 156 198 156 156 160 156 174
C6-b C6-b n-C14 C6-b C6-b C6-b C6-b C7-b
174 174
C7-b C7-b
170
C7-b
120 120 120 120 120 120 142 134 134 134 118 134 134 134 134 134 134 134 134 134 134 148 156 134 134 132 148 132 134 132 148 148 128 146 146 146 170 146 146 146 160 146
8.249 8.463
compds identified or possible isomer (% match)a
MW
8.514
9.165 9.179 9.301 9.342
9.515 9.744
benzene, toluene, ethylbenzene m-,p-,o-xylene isopropylbenzene (87%) propylbenzene (86%) 1-ethyl-3-methylbenzene (94%) 1,3,5-trimethylbenzene (94%) 1-ethyl-2-methylbenzene (95%) 1,2,4-trimethylbenzene (95%) decane (87%) sec-butylbenzene (91%) m-cymene (91%) p-cymene 95%) 2,3-dihydro-1H-indene (93%) 1,3-diethylbenzene (97%) 1-methyl-3-propylbenzene (94%) 1,4-diethylbenzene 1-methyl-4-propylbenzene (90%) 1-ethyl-3,5-dimethylbenzene (91%) 1,2-diethylbenzene (90%) 1-methyl-2-propylbenzene (94%) 1-ethyl-2,4-dimethylbenzene (91%) 2-ethyl-1,4-dimethylbenzene (90%) 4-ethyl-1,2-dimethylbenzene (60%) 1-methyl-4-(1-methylpropylbenzene (87%) undecane (91%) 1,2,4,5-tetramethylbenzene (94%) 1,2,3,5-tetramethylbenzene (91%) 1-butenylbenzene (91%) 2,4-diethyl-1-methylbenzene (91%) 2,3-dihydro-1-methyl-1H-indene (95%) 1,2,3,4-tetramethylbenzene (94%) amylbenzene 1,2,3,4-tetrahydronaphthalene (96%) 1-methyl-4-(1-methylpropyl)-benzene (87%) diethylmethylbenzene (76%) naphthalene (95%) 2,3-dihydro-4,7-dimethyl-1H-indene (95%) 2,3-dihydro-1,6-dimethyl-1H-indene (97%) 2,3-dihydro-1,6-dimethyl-1H-indene (95%) dodecane (94%) 1,2,3,4-tetrahydro-2-methylnaphthalene (97%) 1,2,3,4-tetrahydo-1-methylnaphthalene (96%) 2-ethyl-2,3-dihydro-1H-indene (90%) 2,3-dihydro-1,1,5-trimethyl-1H-indene (94%) 2,3-dihydro-5,6-dimethyl-1H-indene (95%) hexylbenzene 1,2,3,4-tetrahydro-5-methylnaphthalene (95%) 2-methylnaphthalene (93%) 1,2,3,4-tetrahydro-1,5-dimethylnaphthalene (87%) azulene 1,2,3,4-tetrahydro-5,7-dimethylnaphthalene (94%) 1-methylnaphthalene (91%) tridecane (92%) 1,2,3,4-tetrahydro-2,7-dimethylnaphthalene (93%) 1,2,3,4-tetrahydro-1,4-dimethylnaphthalene (93%) 5-ethyl-1,2,3,4-tetrahydronaphthalene (94%) biphenyl (90%) 2-ethylnaphthalene (83%) 1-ethylnaphthalene 1,2,3,4-tetrahydro-1,4-dimethylnaphthalene (94%) 2,6-dimethylnaphthalene (98%) tetradecane (96%) 1,3-dimethylnaphthalene (97%) 2,6-dimethylnaphthalene (97%) 1,2,3,4-tetrahydro-6,7-dimethylnaphthalene (91%) 2,3-dimethylnaphthalene (98%) 1,2,3,4-tetrahydro-2,5,8-trimethylnaphthalene (93%) acenaphthylene 1,2,3,4-tetrahydro-1,5,7-trimethylnaphthalene (90%) 1,2,3,4-tetrahydro-1,1,6-trimethylnaphthalene (83%) acenaphthene 2-(1-methylethyl)-naphthalene (95%)
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TABLE 2 (Continued) peak no.
tR
64 65 66 67 68 69 70 71
9.948 10.006 10.036 10.073 10.165 10.192 10.277 10.366
72 73 74 75 76 77 78 79 80
10.567 10.796 10.878 10.924 11.015 11.058 11.091 11.146 11.210
81
11.494
82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99
11.701 11.747 11.905 12.003 12.116 12.143 12.222 12.320 12.436 12.726 12.823 12.854 12.915 13.198 13.652 14.088 14.546 15.055
a
tR of reference standard
tR calcd from lit. data (ref 17)
10.276 10.389 10.554
11.347 11.510 11.571 11.676
12.698
13.630 14.524
compds identified or possible isomer (% match)a
MW 212 170 170 170 170 170 170 170
n-C15 C7-b C7-b C7-b C7-b C7-b C7-b C7-b
226 184 254 184 184 180 182 240 268
n-C16 C8-b
178 254 194 198 198 192 192 268 208 212 282 206 206 206 296 310 324 338 352
C8-b C8-b C8-b C8-b n-C17 C8-b C8-b n-C18 C9-b C9-b C9-b n-C19 C8-b
n-C20 C10-b C10-b C10-b n-C21 n-C22 n-C23 n-C24 n-C25
pentadecane (93%)
trimethylnaphthalene isomers (93-97%) 2,3,5-trimethylnaphthalene trimethylnaphthalene isomer (93%) fluorene hexadecane (97%) (4,5,5-trimethyl-1,3-cyclopentadien-1-yl)benzene (93%) 7,9-dimethylhexadecane (89%) 1-methyl-7-(1-methylethyl)-naphthalene (90%) 1,2,3,4-tetramethylnaphthalene (94%) 2-methyl-9H-fluorene (96%) 4,4′-dimethylbiphenyl (92%) heptadecane (95%) 2,6,10,14-tetramethylpentadecane (94%) pentachlorophenol phenanthrene (96%) anthracene octadecane (96%) 2,3-dimethyl-9H-fluorene (96%) 3-methyldibenzothiophene (96%) 3-methyldibenzothiophene (95%) 1-methylphenanthrene (94%) 1-methylanthracene (96%) nonadecane (98%) 1,2,5,6-tetramethylacenaphthylene (93%) 2,8-dimethyldibenzo(B,D)thiophene (89%) eicosane (99%) 2,5-dimethylphenanthrene (98%) 9,10-dimethylanthracene (93%) 3,6-dimethylphenanthrene (91%) heneicosane (97%) docosane (91%) tricosane (87%) tetracosane (87%) pentacosane (83%)
Alkylbenzene isomers identied as C1-b, C2-b, C-3b, etc.
for sampling these substances directly from the headspace of fresh solvent samples. In a first series of tests, a syringe was used to collect the species in the headspace of 20-mL vials containing 5 mL of PCP solvent. A TIC chromatogram recorded from the injection of a 5-µL aliquot of the headspace of a vial is shown in Figure 3b. As indicated in this figure, most of the peaks are eluted in a time range of 3.7 to 10.0 min with a major contribution seen in the domain previously attributed to the volatile components of the PCP solvent. It is interesting to note that contrary to what was found for the solution of the PCP-solvent in the MeOH-CH2Cl2 mixture (see Figure 2), it is now possible to detect some BTEX components (more or less resolved peaks eluted between 4.0 and 5.0 min in the chromatogram). These compounds, which are known to have their fingerprints easily modified by sample treatment processing (16), were possibly lost during the preparation and storage of the previous solution. In a second series of tests, an SPME fiber was used to extract the volatile compounds in the headspace of 20-mL vials containing 5 µL of PCP solvent. A typical TIC chromatogram is shown in Figure 3c. Although the SPME fiber was not found to be more advantageous than direct syringe sampling for collecting the BTEX in the headspace of the vials, a larger amount of molecules belonging to the semivolatile fraction of the PCP solvent components appeared in the chromatograms. Such differences arising from the sampling technique were also observed by James and Stack (17) while determining 5018
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the VOCs in petrochemical wastewater samples. The molecules belonging to the semivolatile fraction probably show a greater affinity for the poly(dimethylsiloxane) coating than the lower boiling components. No such segregation is observed when a syringe is used to collect the headspace of fresh-PCP solvent samples (see Figure 3b). Headspace Species Assessment of Sapwood Specimens. In this study, a first insight into the pole field emanations was obtained by measuring the VOC emissions of sapwood specimens collected from a pole after 4 months of weathering exposure. A typical TIC chromatogram of the compounds found in the ambient air of the emission cell is shown in Figure 4. As seen in this figure, most of the peaks appear in the chromatographic region initially attributed to the volatile and semivolatile fractions of the PCP-solvent components. The presence in this chromatogram of C3-, C4-, C5-, and C6alkylbenzenes was confirmed by the mass spectral analysis, giving then a strong indication that these molecules could contribute to the characteristic odor of the poles. The BTEX components were shown to be present in the ambient air only in negligible amounts while for the PCP molecules, they were not detected under the temperature range investigated in these tests. On the other hand, the variation in the amounts of the PCP-solvent fraction components with the temperature is shown in Figure 5. When plotting these graphs, the integrated signal at a given T was added to the signal of the next 5 °C-incremented value to account for the fact that the
FIGURE 5. Temperature effect on the integrated TIC chromatographic signals of the species collected in the headspace of the sapwood specimens.
TABLE 3. Variation in the Amount of Substances Found in the Headspace of the Emission Cell with Temperature
FIGURE 3. TIC chromatograms of a) a weathered sample extracted from a wood specimen and a fresh PCP solvent sample, b) a sample collected with a syringe in the headspace of a PCP solvent sample, and c) a sample collected with a SPME fiber in the headspace of a PCP solvent sample.
FIGURE 4. TIC chromatogram of the species collected in the emission cell after a thermostabilization at 25 °C of the sapwood specimens. same specimens were used in the cell to cover the full range of temperatures (20 °C to 40 °C). These results show an increase of the signal with T whatever the type of components of the PCP solvent considered (volatile, semivolatile, and nonvolatile). There is also an increase in the ratio of the amounts of volatile to semivolatile fraction components (ratio of 0.58 at 20 °C and of 0.68 at 40 °C). As for the building product VOC emissions (18), compound emissions from sapwood specimens appear to be governed by two mechanisms: (1) the diffusion kinetics of the embedded components within the wood cells to the wood surface and (2) their vaporization from the wood surface to the ambient air.
temp (°C)
concen (µg/m3/m2)
20 25 30 35 40
10.0 ( 0.8 26.4 ( 0.9 50 ( 1 87 ( 1 144 ( 2
The small molecules found in large amounts in the volatile fraction are then expected to diffuse toward the wood surface more easily than the larger ones with a net effect that their proportion will increase with the temperature as indicated by our results. The integrated TIC signals of standard solutions of PCP-solvent in MeOH/CH2Cl2 were used to estimate the cumulative amount of the volatile compounds found in the headspace of the emission cell. As seen in Table 3, the amount of PCP-solvent components in the ambient air increases rapidly with an increase in temperature. The latent heat of vaporization of the PCP solvent (∆Hvap) was estimated from these results using the Clausius-Clapeyron relationship between pressure (P) and temperature (T) for a vapor treated as an ideal gas: d(lnP)/d(1/T) ) -∆Hvap/R. An average value of 99.9 kJ/mol is obtained compared to a value of 98.2 kJ/ mol for PCP introduced into the wood cells by a liquefied petroleum gas (19). Field Ambient Air Sample Assessment. The PCP-solvent components previously identified as potentially contributing to the characteristic odor of treated wood poles were confirmed by sampling with SPME fibers the airborne substances found in a treating plant and a utility pole storage site. These extractions were carried out when the temperature was varying from 20 to 25 °C. Typical TIC chromatograms obtained for samples collected in the vicinity of a pole pile after 5 min of pole weathering exposure (treating plant samples) and 9 months of pole weathering exposure (utility storage site samples) are shown in Figure 6a,b. All the chromatograms obtained from the treating plant samples (Figure 6a) showed only trace amounts of BTEX, definitely confirming the elimination of these components during the posttreatment thermal conditioning (application in the treating cylinder of a steaming at 105 °C for 4 h followed by a final vacuum for 3 h). The chromatograms obtained from the utility storage site samples (Figure 6b) contained mostly peaks associated with C4-, C5-, and C6-alkylbenzenes, while those at the treating plant also contained C3 isomers. However, the TIC signal relevant to these substances is significantly reduced when sampling is carried out at longer VOL. 36, NO. 23, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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In summary, the C4-, C5-, and C6-alkylbenzene isomers were identified as the species that could be found in the ambient air of utility pole storage sites. The posttreatment thermal conditioning as applied by the industry could possibly be refined to reduce the content of these molecules in the impregnated wood cells. The exact contribution of a specific class of identified molecules to the odors perceived by humans remains to be substantiated by applying a conventional GC/sniffing technique.
Acknowledgments The authors thank G. Be´langer from Hydro-Que´bec’s research institute (IREQ) for providing the initial funding for the project. They are grateful to Que´bec’s fonds pour la Formation des chercheurs et l’aide a` la recherche (FCAR) for a postgraduate scholarship awarded to M.F. A special word of thanks goes to P. Lachapelle from Hydro-Que´bec’s Distribution for the funding provided for the preparation of the manuscript.
Literature Cited
FIGURE 6. TIC chromatograms of airborne substances extracted in a) the vicinity of a pole pile at a treating plant after 5 min of weathering exposure, b) the vicinity of a pole pile at a utility storage site after 9 months of weathering exposure, and c) downstream winds about 300 m from the pole piles at the utility pole storage site. time of weathering exposure (Figure 6a versus Figure 6b). On the other hand, it is interesting to note that a substantial amount of PCP was detected in both types of samples, contrary to what was seen for the sapwood specimens in the emission cell. The presence of the biocide in the ambient air is consistent with previous observations made by Waite et al. (20) on air samples collected adjacent to a utility pole storage site. Additional peaks are shown at longer retention times (tR > 10 min; not evident under the TIC scale used for Figure 6a) that were also detected in the upstream and downstream wind samples collected at the utility site. The MS library search is indicative of some oxidation products, residuals in the poly(dimethylsiloxane) fiber material or solidphase column degradation material. Finally, a TIC chromatogram obtained from a sample extracted in the downstream winds of the utility site is shown in Figure 6c. The presence of C4-, C5-, and C6-alkylbenzenes was confirmed in these samples, while they were found to be totally absent from the upstream wind samples. Even if an important dilution effect is seen considering that the SPME fiber was exposed for 15 min rather than for 1 min as in Figure 6b, these results give a strong indication that such species could be transported to residential areas.
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Received for review January 9, 2002. Revised manuscript received June 19, 2002. Accepted September 17, 2002. ES020508X
