Environ. Sci. Technol. 2005, 39, 1736-1740
Structure of the Toxaphene Compound 2,5-endo,6exo,8,9,9,10,10-Octachlorobornene-2: A Temperature-Dependent Formation of Two Rotamers H A R U N P A R L A R , * ,†,‡ JU ¨ RGEN BURHENNE,§ MEHMET COELHAN,‡ AND WALTER VETTER| Department of Chemical-Technical Analysis and Chemical Food Technology, Technical University of Munich, Weihenstephaner Steig 23, D-85354 Freising-Weihenstephan, Germany, Research Center Weihenstephan for Brewing and Food Quality, Alte Akademie 3, D-Freising-Weihenstephan, Germany, Department of Internal Medicine VI, Clinical Pharmacology and Pharmacoepidemiology, University of Heidelberg, Im Neuenheimer Feld 410, D-69120 Heidelberg, Germany, and Institute of Food Chemistry, University of Hohenheim, Garbenstrasse 28, D-70599 Stuttgart, Germany
The irradiation of 2,2,3-exo,5-endo,6-exo,8,9,9,10,10decachlorobornane in n-hexane at 254 nm leads to a spontaneous Cl2 elimination as the major reaction pathway. This results finally in the main product 2,5-endo,6-exo,8,9,9,10,10-octachlorobornene-2, of which the structure could be elucidated with the help of X-ray, 1H and 13C NMR, IR, and MS. Temperature-dependent 1H NMR spectroscopic investigations have shown that the -CHCl2 groups located at C1 and C7 are able to rotate slowly under normal circumstances. If such measurements, however, are exerted at low temperatures (-10 to -60 °C), so can be seen that two rotamers are formed due to the hindrance of the free rotation about the bonds C1-C10, C7-C8, and C7C9, which for the first time could be revealed for a toxaphene compound. Furthermore, as all 1H NMR chlorobornane spectra known so far show only sharp and clear signals, it can be assumed that chlorobornane compounds as main toxaphene components have fixed bonds, which requires to indicate chlorine atoms within the tentacles such as “a”, “b”, and “c” for characterizing their correct position. Those fixed tentacles are probably the reason that many toxaphene congeners remain very stable in environmental compartments, and particularly the biotic and abiotic transformation may strongly be hindered by the inflexibility of the tentacles.
Introduction Toxaphene, a technical mixture of more than 200 polychlorinated C10-terpenes, belongs to those organochlorine * Corresponding author phone: +49-(0)8161-71-3283; fax: +49(0)8161-71-4418; e-mail:
[email protected]. † Technical University of Munich. ‡ Research Center Weihenstephan for Brewing and Food Quality. § University of Heidelberg. | University of Hohenheim. 1736
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insecticides that are found worldwide because of their persistency and spacious transportability in different ecosystems (1-6). Therein, many of these substances remain rather stable under specific environmental conditions, whereas others decay relative easily. This is ascribable to microbial and photolytic effects, where the predominating mechanism is a reductive dechlorination or dehydrochlorination and an oxidative decay, which may occur as well (7-10). Especially the top predating species of the Northern Hemisphere are affected due to effective bioaccumulation (11-18). Toxaphene is mutagenic when applied to the Ames test and is above all very toxic to fish. To date, there is little known about the mode of action and the toxicity of some single components (19-23). Toxaphene is distinguished in terms of its dispersion tendency. Relatively high amounts of residues were found in different samples taken at areas far away from the region of application. This shows the high transportability of this substance, and the most important way of transportation is obviously via air (24, 25). The toxaphene content in the atmosphere at areas of Great Lakes in Canada and the United States has been determined frequently, which has shown that these regions are strongly affected (26-28). The production of toxaphene occurs via camphene, which in turn is the result of a catalytic isomerization of R-pinene. A permanent supply of chlorine for 8 h to a solution containing carbon tetrachloride and 20% camphene is leading under UV irradiation to a final solution, which contains 6769% of chlorinated terpenes. Since 1973, work has been carried out to isolate and to identify single components from technical toxaphene (29-34). Until now, more than 60 compounds have been identified and elucidated in their structures. The main products are chlorobornanes, followed by chlorocamphenes and chlorobornenes (35, 36). In past years, there has been a controversial discussion about whether rotating toxaphene isomers can occur due to hindered rotation about the bonds C1-C10, C7-C8, and C7-C9. This has also led to the nomenclature “a”, “b”, and “c” of the valences located at C1, C8, and C9 in order to express the exact structure (37). For answering correctly this question, it was necessary to show with the help of an example the existence of two or more rotamers derived from the same compound. In this work, spectroscopic data of the toxaphene compound 2,5-endo,6-exo,8,9,9,10,10-octachlorobornene-2 (Figure 1, compound 2) were used to show that two rotamers occur at low temperatures.
Experimental Section Materials. The toxaphene compound 2,2,3-exo,5-endo,6exo,8,9,9,10,10-decachlorobornane (Figure 1, compound 1) was isolated in our laboratory by excessive chlorination of 2-exo,10-dichlorobornane (30). All solvents used were of analytical grade. Petroleum ether (DAB quality, bp 60-80 °C) was purified by distillation over a glass column (1 m in length). The purity was controlled by HRGC/ECD. Silica gel (70-230 mesh) was obtained from Merck, Germany. Irradiation Experiments. Five portions weighing 146 mg each were prepared out of 730 mg of 1 and dissolved in 80 mL of n-hexane. The solutions have been freed from oxygen by passing through a gentle stream of nitrogen for 10 min prior to irradiation. In total, 16 quartz tubes (5 mm i.d., 35 cm in length) each containing 5 mL of the sample were placed circularly in a distance of 2 cm around the light source (Hg low-pressure lamp, Type Vycor 250 mA; Graentzel, Germany) and irradiated for 3 h at λmax ) 254 nm. 10.1021/es040075t CCC: $30.25
2005 American Chemical Society Published on Web 02/01/2005
FIGURE 1. Photoinduced reaction of 2,2,3-exo,5-endo,6-exo,8,9,9,10,10-decachlorobornane (1) to 2,5-endo,6-exo,8,9,9,10,10-octachlorobornene-2 (2) and compounds 3 and 4.
FIGURE 2. HRGC/EI-MS total ion chromatogram of the irradiated solution after 180 min. Retention times: Rt (2) ) 30.2, Rt (3) ) 34.9, Rt (4) ) 36.3. Isolation of Photoproducts. The course of photoreaction was monitored by analyzing one of the samples every 45 min by HRGC/EI-MS. After 180 min, 1 decayed into traces (Figure 1). Thereafter, all 16 samples were unified, and n-hexane was evaporated using a rotary evaporator. For the separation of the three photoproducts (2-4), a silica gel glass column (1 m in length, 3 cm i.d., petroleum ether flow rate: 30 mL min-1) was used, and the initial run of 2000 mL was discarded. Compound 3 eluted first, of which the elution started at 2250 mL rotation volume and lasted to above 2900 mL. It partially coeluted with 2, and the elution of this substance started at 2400 mL and finished at 3000 mL. Thereafter, 4 followed, which eluted completely at 3700 mL. As a result, yields of 30 mg were achieved for 3 and 4 with purities above 95%, as well as 50 mg for 2 with 99% purity (Figure 2). High-Resolution Gas Chromatography-Mass Spectroscopy (HRGC-MS/EI). A Hewlett-Packard (series 5890) gas chromatograph was used in the following mode: splitless (0.5 min) to split at 230 °C; injection volume: 1-2 µL; HP Ultra column: 25 m in length and 0.2 mm i.d.; 0.33 µm; helium gas flow rate: 1 mL min-1; temperature program: GC oven initially set at 140 °C, held for 3 min, programmed at a rate of 4 °C/min to 250 °C, held for 20 min; MS: HP 5988 A; interface: 280 °C; EI ion source: 200 °C with 70 eV. 1H and 13C NMR Spectroscopy. A Bruker AC-400 spectrometer has been applied with tetramethylsilane (TMS) as standard and deuterated chloroform (CDCl3) as the solvent. Both chemicals were purchased from Aldrich, Germany. All chemical shifts were referenced to the solvent peak and recalculated with respect to TMS, δ (1H) ) 7.24 ppm for CHCl3.
X-ray Spectroscopy. An Enray-Nonivs CAD4 V 5.0 was used; data: XCAD4PC; SHELXS-93, graphic: SCHAKAL 92: λ ) 0.71073 nm Å Mo KR. Spectroscopic Data of 1. EI-MS (m/z): 407, 359, 263, 243, 241, 229, 227, 195, 161; FTIR (cm-1): 3100-2900, 1470-1440, 1390-1370; 1H NMR (ppm) (400 MHz): 3.21 (d), 4.44 (d), 5.00 (d), 5.15 (d), 5.34 (s), 5.38 (d), 6.74 (s), 7.05 (d); 13C NMR: 41.9, 57.2, 63.8, 65.3, 66.5, 67.9, 69.7, 71.9, 73.5, 97.3; X-ray, P1: a ) 8.191, b ) 14.153, c ) 15.130, R ) 109.93°, β ) 93.50°, γ ) 94.06°, z ) 4, typ: triclin, V ) 1638.0, Mo KR; X-ray, ω-scan: (3.20 + 0.35 + 0)°, length: 2.44 < 0 < 24, 97°, -9 e h e 0, -16 e k e 16, -17 e l e 17, t ) 30 s, no. of reflections: 6162. Spectroscopic Data of 2. EI-MS (m/z): 325, 289, 277, 243, 241, 209, 207, 173; FTIR (cm-1): 3100-2900, 1597, 14701440, 1390-1370; 1H NMR (ppm) (400 MHz): details in Table 2; 13C NMR: 40.0, 55.2, 63.0, 66.2, 67.8, 67.9, 72.0, 73.1, 131.2, 132.0; X-ray, P21/c: a ) 13.890, b ) 7.640, c ) 13.980, R ) 90°, β ) 98.32°, γ ) 90°, z ) 4, typ: monoclin, V ) 1469.3, Mo KR; X-ray, ω-scan: (1.50 + 0.35 + 0)°, length: 1.48 < 0 < 24.97°, -16 e h e 10, -5 e k e 9, -16 e l e 12, t ) 30 s, no. of reflections: 3469. HRGC-FTIR Spectroscopy. A Hewlett-Packard HRGCFTIR (5890-5965) for generating the spectra has been applied. The FTIR system used the same column as in HRGC, and the experiments were performed under the same conditions.
Results and Discussion Three compounds (2-4) were formed upon irradiation of 1 at 254 nm in n-hexane and then separated in pure form by middle-pressure liquid column chromatography. Their structures were then elucidated with certainty by spectroscopic methods (1H and 13C NMR, IR, MS) and X-ray structure analysis (Figure 5). Compound 2 was the result of an intramolecular Cl2 elimination, whereupon a double bond is formed between C2 and C3. The formation of the photodechlorinated products 3 and 4, however, is an intermolecular reaction, where at the first step a chlorine radical is eliminated and a hydrogen atom is spontaneously abstracted from the solution. The dehalogenation takes place at the C2-atom, which is having a geminal C-Cl2 group. In the irradiated solution, 3 and 4 occur in a molar ratio of ca. 1:1 (Figure 2); therefore, the radicals formed in the first step can capture a hydrogen radical in the endo-position as well as in the exo-position from the n-hexane solution. The EI-MS spectrum of 2 does not show M+, M - Cl+, and M - HCl+ fragments. At first, a group of peaks is shown with a six-chlorine cluster and a relative intensity of 70% at a mass of 325. The base peak (100%) lies at mass 277 with five chlorine atoms. This fragment results from a retro-DielsAlder splitting, which is typical for unsaturated six-ring VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. 1H NMR spectra of compound 2,5-endo,6-exo,8,9,9,10,10-octachlorobornene-2 (2) in CDCl3 solution, taken at different temperatures. systems. Thereafter, fragments follow at the masses 241 (13%), 207 (12%), and 173 (8%), deriving from a fully aromatized, substituted system. Intensive fragments are further registered at the masses 193 (36%) and 159 (42%), which can be assigned to chlorinated dihydrotropylium cations. Rather typical as well is the fragment CHCl2+, observed at the mass 83 and at the intensity of 87%. The FTIR spectrum of 2 shows above 3000 cm-1 a C-C-H absorption (double bond between carbon atoms) and below it shows C-H valencies, jutting out to 2900 cm-1. Typical is the band at 1597 cm-1, which unambiguously relates to a C-C group. The location of this band is typical for a monochlorine-substituted double bond within the ring system. Absorptions between 1470 and 1440 cm-1 as well as 1390 and 1370 cm-1 derive from C-H bonds. The signals shown in the discoupled 13C NMR spectra (in total 10 signals) can be definitely assigned to the single carbon atoms of the bornene system. Furthermore, the spectra, which are not discoupled, show as well the structure of the bornene systems (40.0, triplet; 55.2, doublet; 63.0, doublet; 66.2, doublet; 67.8, doublet; 67.9, doublet; 72.0, doublet; 73.1, doublet). In the spectra not discoupled are also the olefinic carbon atoms clearly recognizable, which have been registered at 132.0 ppm as a singlet and at 131.2 ppm as a doublet. The 1H NMR spectra of 2 at 23 °C depicts, besides of sharp olefinic H3 protons, unresolved groups of peaks, indicating that the bonds between C1-C10, C7-C8, and C7C9 are able to rotate still freely although impeded. It can already be recognized at these temperatures that this solution 1738
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contains rotamers, occurring in a molar ratio of 3:5 (Figure 3, Table 1). To weaken the rotation barrier, the 1H NMR spectra have been recorded at temperatures of 30 and 45 °C (Figure 3, Table 1). Looking at these spectra, it is ascertainable that the peaks become slightly sharper and that the chemical shifts of the single peak groups lie in the middle of those peaks being registered at 23 °C. Therefore, it can be concluded that it must be a single compound, where the protons are definitely assignable. If 1H NMR spectra are recorded at deep temperatures, from -10 to -30 °C and at -60 °C, then the rotation becomes more blocked and all protons from both rotamers are clearly being registered and give sharp signals. It is also worth mentioning that both rotamers do not show the 4Jcoupling between the protons located at the atoms C8 and C9, which are usually found for this class of substances. The olefinic protons localized at C3 of both rotamers are sharply registered (δ ) 6.43 ppm) at higher temperatures. At deeper temperatures, however, this signal splits into two groups of peaks, registered at δ ) 6.45 and 6.47 ppm, which can be ascribed to both rotamers (Table 1, Figure 3). All other protons emitted further signals at -10 °C, and their assessment is not easy. Looking at the chemical shifts, some of these protons are assignable with some restrictions. From -10 °C onward, the existence of both rotamers 2x and 2y is clearly recognizable, and some signals can be determined without problems. It is particularly worth mentioning that
TABLE 1. 1H NMR Chemical Shifts and Coupling Constants According to Temperature of 2,5-endo,6-exo,8,9,9,10,10-Octachlorobornene-2 (2) and Its Rotamers 2x and 2ya temperature (°C) Hb H3
H4
H5
Cc
45
30
-10
23
2 6.43 (d) 2x 6.43 (d); JH3H4 ) 3.99 2y 6.43 (d); JH3H4 ) 3.99 2 3.72 (w) 2x 3.78 (w) 2y 3.45 (w)
6.43 (d); JH3H4 ) 3.99 6.44 (d); JH3H4 ) 3.99 6.43 (d); JH3H4 ) 3.99 6.44 (d); JH3H4 ) 3.99
-30 6.47 (d); JH3H4 ) 3.99 6.45 (d); JH3H4 ) 3.99
-60 6.47 (d); JH3H4 ) 3.99 6.45 (d); JH3H4 ) 3.99
3.80 (w) 3.65 (w)
3.81 (dd); JH4H3 ) 3.99 3.81 (dd); JH4H3 ) 3.99 3.81 (dd); JH4H3 ) 3.99 6.63 (dd); JH3H4 ) 3.99, 3.63 (dd); JH4H3 ) 3.99, 3.63 (dd); JH4H5 ) 2.85 JH4H5 ) 2.85 JH4H5 ) 2.85
2 4.80 (w) 2x 4.72 (w)
4.69 (w)
2y
4.82 (w)
4.80 (w)
4.67 (dd); JH5H4 ) 2.85, 4.67 (dd); JH5H4 ) 2.85, 4.67 (dd); JH5H4 ) 2.85, JH5H6 ) 3.00 JH5H6 ) 3.00 JH6H5 ) 3.00 4.82 (dd); JH5H4 ) 2.85, 4.82 (dd); JH5H4 ) 2.85, 4.82 (dd); JH5H4 ) 2.85, JH5H6 ) 3.00 JH5H6 ) 3.00 JH5H6 ) 2.00
4.35 (w) 4.50 (w)
4.35 (w)
4.28 (w) 4.49 (d); JH4H5 ) 3.00
4.78 (hw) 4.49 (d); JH6H5 ) 3.00
4.28 (d); JH6H5 ) 3.0 4.49 (d); JH6H5 )3.00
4.65 (w) 4.87 (w)
4.65 (w) 4.57
4.60 (d); JH8H8 ) 11.4 4.53 (d); JH8H8 ) 11.4
4.60 (d); JH8H8 ) 11.4 4.53 (d); JH8H8 ) 11.4
4.60 (d); JH8H8 ) 11.4 4.53 (d); JH8H8 ) 11.4
4.35 (w) 4.70 (w)
4.38 (w) 4.75 (w)
4.24 (w) 4.52; JH8H8 ) 11.4
4.24 (w) 4.52; JH8H8 ) 11.4
4.24 (d); JH8H8 ) 11.4 4.52; JH8H8 ) 11.4
6.35 (w) 6.25 (w)
6.38 (w) 6.28 (w)
6.38 (s) 6.27 (s)
6.38 (s) 6.27 (s)
6.38 (s) 6.27 (s)
5.95 6.05 (w)
5.35 (w) 6.05 (w)
5.86 (s) 6.03 (s)
5.86 (s) 6.03 (s)
5.86 (s) 6.03 (s)
H6
2 2x 2y H8 2 2x 2y H8′ 2 2x 2y H9 2 2x 2y H10 2 2x 2y
4.34 (w)
4.72 (w)
4.78 (w)
6.32 (w)
6.04 (w)
a Presented in the following order: chemical shift δ in ppm (peak characteristic); coupling constant J in Hz. At wide and small peaks, δ and J are uncertain. s ) singlet, d ) doublet, dd ) double doublet, w ) wide, hw ) half wide. b Hydrogen atom. c Compound.
FIGURE 4. Suggested structures of two rotamers from 2,5-endo,6exo,8,9,9,10,10-octachlorobornene-2 (2) on the basis of 1H NMR spectroscopic data.
TABLE 2. Chemical Shift Differences of Some Protons of the Rotamers 2x and 2y protons
∆δ values (Hz)
protons
∆δ values (Hz)
H32x-H32y H42x-H42y H52x-H52y H62x-H62y
+8.55 +79.80 -74.10 -45.71
H82x-H82y H8′2x-H8′2y H92x-H92y H102x-H102y
-114.0 -62.70 +37.05 -68.40
the H6 protons and one of the 2x isomer’s H8 protons start first to emit sharp signals at -60 °C. Above this temperature, the signals are broad and not resolvable, so that the coupling constants belonging to them cannot be measured. By taking all findings from the temperature-dependent 1H NMR measurements into consideration as well as the spacious need of the chlorine atoms within the bornane system, the structures of the two rotamers as to Figure 4 can be suggested. The structure of the rotamer 2x is almost similar with the one being derived according to the X-ray measurement. Provided that the bond between C7-C8 is more flexible than those between C1-C10 and C7-C9 and that it reaches first the rotation barrier at -60 °C, it can be assumed that the two rotamers are only formed when the bond between
FIGURE 5. Structure of compound 2,5-endo,6-exo,8,9,9,10,10-octachlorobornene-2 (2) obtained from X-ray measurements. C7-C8 is fixed at two different positions where the voluminous Cl atoms are as far away from each other as possible. The correctness of the structures of the rotamers is significantly supported by 1H NMR spectroscopy. Looking at the chemical shifts of some single protons, then significant differences in the δ values can be recognized, which reinforces the suggested structures of 2x and 2y (Table 2). In the structure of 2x, the group C8-Cl is located at position 8c, but in the isomer 2y, it is located at 8b. Due to this change of position, the protons H4 and H9 receive a positive ∆δ effect, VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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whereas the protons H5, H6, and both H8 receive a negative ∆δ effect. These differences can well be explained with the help of a simple 3D model by looking on the proximity or distance of these protons to the C8-Cl group (Figure 5). In conclusion, the results of this work clearly demonstrate that the free rotating bonds between C7-C8, C7-C9, and C1-C10 depict broad peaks in the 1H NMR spectrum. When the free rotation is strongly hindered or even blocked, then the 1H NMR signals become sharp. Because of the fact that at all 1H NMR chlorobornane spectra known so far, only sharp and clear signals have been registered, it is appropriate to assume that chlorobornane compounds have fixed bonds. Therefore, the necessity exists to indicate the chlorine atoms within the tentacles of such systems such as “a”, “b”, and “c”.
Literature Cited (1) Parlar, H. Analysis of toxaphene. Int. J. Environ. Anal. Chem. 1985, 20 (1-2), 141-158. (2) Saleh, M. A. Toxaphene: chemistry, biochemistry and environmental fate. Rev. Environ. Contam. Toxicol. 1991, 118, 1-85. (3) Swackhamer, D. L.; Hites, R. A. Occurrence and bioaccumulation of organochlorine compounds in fishes from Siskiwit Lake, Isle Royale, Lake Superior. Environ. Sci. Toxicol. 1988, 22, 543-548. (4) Casida, J. E.; Holmstead, R. L.; Khalifa, S.; Knox, J. R.; Ohsawa, T.; Palmer, K. J.; Wong, R. Y. Toxaphene insecticide: a complex biodegradable mixture Science 1974, 183 (4124), 520-521. (5) Musial, C. J.; Uthe, J. F. Widespread occurrence of the pesticide toxaphene in Canadian East Coast marine fish. Int. J. Environ. Anal. Chem. 1983, 14 (2), 117-126. (6) Parlar, H.; Reil, G.; Angerhoefer, D.; Coelhan, M. Structureactivity relationship model for toxaphene congeners. Fresenius Environ. Bull. 2001, 10 (2), 122-130. (7) Fingerling, G.; Coelhan, M.; Parlar, H. Investigations on the stability of seventeen single toxaphene components in the presence of UV-light and in an anaerobic soil environment. Fresenius Environ. Bull. 1998, 7 (7A-8A), 525-331. (8) Fingerling, G.; Maurer, M.; Coelhan, M.; Parlar, H. Photolysis of the toxaphene component 2,2,3-exo,5,5,8,9,9,10,10-decachlorobornane. Fresenius Environ. Bull. 1998, 7 (9-10), 610-617. (9) Luckas, B.; Vetter, W.; Fischer, P.; Heidemann, G.; Ploetz, J. Characteristic chlorinated hydrocarbon patterns in the blubber of seals from different marine regions. Chemosphere 1990, 21 (1-2), 13-19. (10) Ruppe, S.; Neumann, A.; Vetter, W. Anaerobic transformation of compounds of technical toxaphene. I. Regiospecific reaction of chlorobornanes with geminal chlorine atoms. Environ. Toxicol. Chem. 2003, 22 (11), 2614-2621. (11) Kosubova, P.; Grabic, R.; Holoubek, I. (HR)GC-MS/MS analysis of toxaphene congeners in various matrices from the Czech environment. Fresenius Environ. Bull. 2003, 12 (11), 1303-1308. (12) Gouteux, B.; Lebeuf, M.; Muir, D. C. G.; Gagne´, J.-P. Levels and temporal trends of toxaphene congeners in Beluga whales (Delphinapterus leucas) from the St. Lawrence Estuary, Canada. Environ. Sci. Technol. 2003, 37, 4603-4609. (13) Weber, K.; Goerke, H. Persistent organic pollutants (POPs) in antarctic fish: levels, patterns, changes. Chemosphere 2003, 53 (6), 667-678. (14) El Nemr, A.; Abd-Allah, A. M. A. Organochlorine contamination in some marketable fish in Egypt. Chemosphere 2004, 54 (10), 1401-1406. (15) Muir, D. C. G.; Ford, C. A.; Grift, N. P.; Stewart, R. E. A.; Bidleman, T. F. Organochlorine contaminants in narwhal (Monodon monoceros) from the Canadian Arctic. Environ. Pollut. 1992, 75 (3), 307-316. (16) Skopp, S.; Oehme, M.; Fuerst, P. Enantiomer ratios, patterns and levels of toxaphene congeners in human milk from Germany. J. Environ. Monit. 2002, 4 (3), 389-394. (17) Oehme, M. Toxaphene, the underestimated pesticide? Chemosphere 2000, 41 (4), 459. (18) de Boer, J.; Oehme, M.; Smith, K.; Wells, D. E. Toxaphene in standard solutions and cleaned biota extractssresults of the first QUASIMEME interlaboratory studies. Chemosphere 2000, 41 (4), 493-497.
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Received for review July 7, 2004. Revised manuscript received October 20, 2004. Accepted October 20, 2004. ES040075T
