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Chemiluminescence Determination of Chlorinated Volatile Organic Compounds by Conversion on Nanometer TiO2 Guo hong Liu,† Yong fa Zhu,† Xin rong Zhang,*,† and Bo qing Xu‡
Department of Chemistry, Tsinghua University, 100084, Beijing, P.R. China, and State Key Lab of C1 Chemistry & Technology, Tsinghua University, 100084, Beijing, P.R. China
A novel method based on conversion of chlorinated volatile organic compounds (CVOCs) to chlorine using a new type of column packed with nanometer TiO2 coupled with chemiluminescence (CL) has been developed for determination of them in workplace air. CVOCs are converted to chlorine by nanometer TiO2 at 220 °C. The Cl2 that is produced is selectively enriched on the column and subsequently released from the column at 600 °C. The Cl2 that is released is determined using a postcolumn CL detector. The CL intensity was linear with CCl4 in the range of 0.1-380 ppm, and the detection limit was 40 ppb (S/N ) 3). Higher sensitivity could be acquired by using a larger volume of enrichment. A similar procedure could be used for the determination of other CVOCs. CL intensities of CH2Cl2, CHCl3, and CCl4 at the same concentration increased in the order CH2Cl2 < CHCl3 < CCl4. The method has been successfully applied to the determination of CCl4 in workplace air, where 0.15-150 ppm CCl4 would be detected. The possible mechanism for the long lifetime of the column packed with nanometer TiO2 was tested using Raman spectrometer, X-ray powder diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. The results showed that the column packed with nanometer TiO2 could be operated in the reversible mode for determination of CVOCs under the present conditions. The method would be potentially applied to the analysis of other chlorinated compounds in environment, such as persistent organic pollutants.
humans and extremely persistent in the environment.3,4 As a consequence, much effort has been expended in developing methods to degrade these contaminants and to detect them at trace levels. The direct determination of trace CVOCs at extremely low concentrations is limited not only by insufficient sensitivity, but also by matrix interference. For this reason, the preliminary separation and enrichment of trace CVOCs are important aspects. Such procedures not only lead to an increase in CVOCs concentration, but also provide the additional advantage of elimination of matrix effects to a great extent. The most widely used techniques for separation and enrichment of trace elements include liquid-liquid extraction,5 solidphase extraction,6-8 coprecipitation,9 ion-exchange,10 resin chelation,11 and supercritical fluid extraction.12 Solid-phase extraction has become increasingly popular in comparison with the more traditional liquid-liquid extraction methods. But the method is mainly used to extract analytes from liquid-phase samples. Most methods for enrichment of volatile organic compounds are adsorbent-13 and cryogenic-trapping.14 However, the selectivity and extraction efficiency of the techniques still need to be further improved. Therefore, it is necessary to develop suitable methods to meet the need for enrichment of volatile organic compounds as well as separation of the analytes of interest from the matrix. During the past decade, nanoparticle research has become quite popular in various fields of chemistry.15 Nanomaterials are clusters of atoms or molecules of metals and oxide, ranging in
Chlorinated volatile organic compounds (CVOCs), such as carbon tetrachloride, chloroform, and dichloromethane have been widely used as lubricants, heat-transfer fluids, plasticizers, transformer fluids, and cleaning solvents because of their relative inertness and ability to dissolve organic compounds.1,2 Increasing amounts of CVOCs have been released into the environment as a result of leaking underground storage tanks and improper disposal practices so that the environment has become contaminated. CVOCs are suspected of being toxic and carcinogenic to
(3) Kaufman, J. J.; Koski, W. S.; Roszak, S.; Balasubramanian, K. Chem. Phys. 1996, 204, 233-237. (4) Gilani, A. H.; Janbaz, K. H.; Shah, B. H. Pharmacol. Res. 1998, 37, 31-35. (5) Peng, S. X.; Henson, C.; Strojnowski, M. J.; Golebiowski, A.; Klopfenstein, S. R. Anal. Chem. 2000, 72, 261-266. (6) Ferrer, I.; Furlong, E. T. Anal. Chem. 2002, 74, 1275-1280. (7) Vassileva, E.; Hadjiivanov, K. Fresenius’ J. Anal. Chem. 1997, 357, 881885. (8) Arena, M. P.; Porter, M. D.; Fritz, J. S. Anal. Chem. 2002, 74, 185-190. (9) Zui, O. V.; Birks, J. W. Anal. Chem. 2000, 72, 1699-1703. (10) Breadmore, M. C.; Macka, M.; Avdalovic, N.; Haddad, P. R. Anal. Chem. 2001, 73, 820-828. (11) Jain, V. K.; Sait, S. S.; Shrivastav, P.; Agrawal, Y. K. Sep. Sci. Technol. 1998, 33, 1803-1818. (12) Brewer, W. E.; Galipo, R. C.; Sellers, K. W.; Morgan, S. L. Anal. Chem. 2001, 73, 2371-2376. (13) Helmig, D.; Greenber, J. P. J. Chromatogr., A 1994, 677, 123-132. (14) Gorgenyi, M.; Dewulf, J.; Van Langenhove, H. Chromatographia 2000, 51, 461-466.
* Corresponding author. Phone: +86-10-6278-7678. Fax: +86-10-6277-0327. E-mail:
[email protected]. † Department of Chemistry. ‡ State Key Lab of C1 Chemistry & Technology. (1) Lara, J.; Tysoe, W. T. Langmuir 1998, 14, 307-312. (2) Kotvis, P. V.; Tysoe, W. T. Tribol. T. 1998, 41, 117-123. 10.1021/ac025882u CCC: $22.00 Published on Web 11/15/2002
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size from 1 nm to almost 100 nm, falling between single atoms or molecules and bulk materials. There has been a continued increase in the number of research investigations in recent years on the potential chemical applications of these novel materials, because nanometer materials display novel and often enhanced properties as compared to traditional materials.16 One of their properties is that most of the atoms are on the surface of the nanoparticle. The surface atoms are unsaturated, and can therefore bind with other atoms. Consequently, nanometer material can adsorb chemical substances and possess a high chemical activity. For example, nanometer TiO2,7,17 CeO218, and ZrO219 were used as solid sorbents of solid-phase extraction for the preconcentration of trace metal ions. Some nanometer metal oxides, such as nanometer magnesium oxide (MgO), calcium oxide (CaO), lanthanum oxide (La2O3), and cerium oxide (CeO2), were studied as sorbents for destructive adsorption of some chlorinated organic compounds,20-23 but they suffered from the disadvantage of a limited lifetime due to nonreversible reaction of chlorinated compounds with nanosized metal oxides. Our preliminary study showed that nanometer TiO2 could be used for the destructive conversion of volatile chlorinated organic compounds to chlorine, and the latter species could be sensitively detected by a chemiluminescence (CL) detector with luminol as the reagent. The result also showed that the process could be carried out in two steps. The first step was operated at ∼220 °C for chlorine adsorption on the column, and the second step was operated at 600 °C for chlorine desorption from the column. Therefore, the column could be worked under reversible operating conditions, so as to facilitate the application of the device to determine the CVOCs from polluted air. According to the ISO standard,24 the range of concentrations of airborne vaporous chlorinated hydrocarbons in workplace air is ∼0.15-150 ppm. Thus, the aim of this work is to develop a novel method based on conversion of CVOCs to Cl2 using this type of column packed with nanometer TiO2 coupled with CL for determination of them in workplace air. EXPERIMENTAL SECTION Reagents and Materials. A 1 × 10-2 mol/L luminol stock solution was prepared by dissolving 0.4454 g of luminol in 250 mL of 0.l mol/L NaOH. The stock solution was diluted with 0.l mol/L NaOH or appropriate solutions, as needed. All other chemicals were of analytical reagent grade, and doubly distilled water was used for the preparation of solution. Synthesis of Nanometer TiO2. The procedure for synthesis of TiO2 nanoparticles was as follows: 2 mL TiCl4 was slowly added (15) Chandross, E. A.; Miller, R. D. Chem. Rev. 1999, 99, 1641-1642. (16) Henglein, A. Chem. Rev. 1989, 89, 1861-1873. (17) Liang, P.; Qin, Y. C.; Hu, B.; Li, C. X.; Peng, T. Y.; Jiang, Z. C. Fresenius’ J. Anal. Chem. 2000, 368, 638-640. (18) Vassileva, E.; Varimezova, B.; Hadjiivanov, K. Anal. Chim. Acta 1996, 336, 141-150. (19) Vassileva, E.; Furuta, N. Fresenius’ J. Anal. Chem. 2001, 370, 52-59. (20) Hooker, P. D.; Klabunde, K. J. Environ. Sci. Technol. 1994, 28, 12431247. (21) Weckhuysen, B. M.; Mestl, G.; Rosynek, M. P.; Krawietz, T. R.; Haw, J. F.; Lunsford, J. H. J. Phys. Chem. B 1998, 102, 3773-3778. (22) Weckhuysen, B. M.; Rosynek, M. P.; Lunsford, J. H. Phys. Chem. Chem. Phys. 1999, 1, 3157-3162. (23) Koper, O. B.; Wovchko, E. A.; Glass, J. A.; Yates, J. T.; Klabunde, K. J. Langmuir 1995, 11, 2054-2059. (24) ISO 9486: 1991. 1991.
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Figure 1. Schematic diagram of the enrichment apparatus based on the nanometer TiO2 with CL determination: D, detector; C, reaction cell containing 2 mL of luminol solution; R, reactor based on nanometer materials; V, three-way valve.
into 20 mL of ethanol solution at room temperature. A light yellow solution was obtained and gelatinized for 3 days to form a solgel. After it was dried naturally, it was calcined in air. The temperature was raised to 800 °C at 7 °C/min and maintained for 2 h. Then the TiO2 powder was cooled to room temperature in air, which took ∼1 h. The synthesized TiO2 powder is rutile in structure and ∼80 nm in size. Apparatus. The system employed in this work is shown in Figure 1. The system consists of an enrichment column, a digital programmable temperature controller of the sensor, and an optical detector. The enrichment column filled with nanometer titanium dioxide was made of quartz glass. The column temperature was controlled by a digital temperature controller. The carrier gas was composed of 78% N2, 21% O2 and 1% Ar (by volume). The water concentration in the mixture was below 15 ppm, since the carrier gas was treated with silica gel (self-indicator), a molecular sieve, and active carbon. The releases from nanometer TiO2 were introduced into the reaction cell containing 2 mLof the luminol solution, producing CL. The CL signal was detected and recorded using a computerized ultraweak luminescence analyzer (type BPCL manufactured at the Institute of Biophysics, Academia Sinica, Beijing, China). The reaction cell was located directly facing the window of the CR-105 photomultiplier tube (Hammamatsu, Tokyo, Japan). Date acquisition and treatment were performed with BPCL software running under Windows 98. Column Preparation. A 280-mg portion of nanometer TiO2 was introduced into a quartz microcolumn [50 mm × 4 mm i.d. (see Figure 1)] plugged with a small portion of glass wool at both ends. One end of this column was connected to the gas samples; the other end was connected to the optical detector. Before use, the column was conditioned by programming the temperature from room temperature to 620 °C at a rate of 20 °C/min to avoid the influence of previous absorbates. Procedures. After the column was heated to 220 °C, gas samples were introduced into the column at a flow rate of 40 mL/ min for 15 min, and the waste gas was vented. Then the column was heated to 600 °C at a rate of 200 °C/min. This stream containing the releases from the column was merged with luminol at reaction cell, producing CL emission. The concentration of the sample was quantified by the CL intensity. RESULTS AND DISCUSSION Chemiluminescence of the Destructive Products of CCl4 by Nanometer TiO2. The CL reaction of oxidized luminol with
chlorine is well-known.25 Our preliminary experiment has shown that the CCl4 could be decomposed while passing it through a column packed with nanometer TiO2 at about 220 °C. No CL emission could be produced when a portion of CCl4 vapor was injected into the column at room temperature, which indicated that the CCl4 was not decomposed to chlorine by the nanometer TiO2 packed in the column at this operating condition, since it is well-known that CCl4 cannot produce CL emission directly with a luminol reagent in an alkaline medium. When the temperature was increased to 600 °C, CL emission was produced, possibly because decomposed products from CCl4, such as chlorine, could produce strong CL emission. The CL reaction showed fast dynamic characteristics. The CL signal could be traced at ∼1-2 s. The CL intensities could be increased by the injection of CCl4 vapor at higher concentrations. Enrichment of Decomposed Products of CCl4 on TiO2. Our preliminary experiment has shown that the destruction products could be adsorbed onto the column at relatively low temperature. For instance, when we decreased the operating temperature to ∼220 °C, we found that the decomposed products of CCl4 were adsorbed onto the surface of the nanoparticles in the column. By increasing the temperature to 600 °C, however, the adsorbed products were released from the column and introduced into the CL detector. CL emission was then produced. Temperature was a key factor for the enrichment of volatile organic chloride. The effect of temperature on enrichment was examined in the range of 25-282 °C. The results showed that the CL response increased with increasing temperature within the tested temperature range of 25-220 °C. The maximum was obtained at 220 °C. Therefore, 220 °C was chosen as the optimal temperature of enrichment. The temperature was also a critical condition affecting desorption. The CL intensity increased with increasing temperature in the range of 440-620 °C. The possible reason was that chlorine adsorbed on nanometer TiO2 was released faster at higher temperature, but above 620 °C would cause the sinter. Moreover, the CL intensity was relatively stable over the range of 580-620 °C. Therefore, 600 °C was selected as optimum for the future studies. These procedures allowed us to enrich the destructive products of CCl4 on the nanometer titanium dioxide column, and to release chlorine from the column subsequently. Therefore, the column was regenerated. To investigate the efficiency of enrichment of decomposed products of CCl4, different volumes of CCl4 of the same concentration passed through the quartz column at 220 °C, respectively. The releases from the quartz column by heating to 600 °C were detected using the CL method. As shown in Figure 2, the results illustrated that CL intensity increased with the increase of volume, further indicating that decomposed products of CCl4 were enriched on TiO2 packed in the quartz column. Considering the aspect of time consumption, 600 mL was chosen for the further experiments. Of course, higher sensitivity could be acquired by using a larger volume of enrichment. Effect of Flow Rate of Enrichment and Desorption. The flow rate of enrichment played an important role in destructive enrichment of CCl4, because the retention of substances and the extent of destruction depended upon the flow rate of enrichment. (25) Ishimaru, N.; Lin, J. M.; Yamada, M. Anal. Commun. 1998, 35, 67-69.
Figure 2. Effect of enrichment on CL intensity: CCl4, 80 ppm; luminol, 2 × 10-4 M; NaOH, 0.1 M.
To evaluate the effect of flow rate of enrichment, the flow rates were adjusted to a range of 20-160 mL/min. The CL intensity decreased with increasing flow rate of enrichment in the tested range. Although higher CL intensity was obtained at lower flow rates of enrichment over the range of 20-50 mL/min, the lower flow rate meant consuming more time. Considering CL intensity and time, a flow rate of 40 mL/min was used for all further investigations. The effect of flow rate of desorption was investigated in the range of 10-80 mL/min. The CL intensity increased with increasing flow rate of desorption in the tested range. The results also showed that column pressure increased with increasing flow rate of desorption in the tested range. Considering CL intensity and column pressure, a flow rate of 40 mL/min was used as the flow rate of desorption for subsequent investigations. Conditions of Chemiluminescence Detection. Because luminol CL light was produced in an alkaline medium, the effect of NaOH concentration was examined. NaOH concentration of 0.01 M was chosen for the subsequent studies. As the luminescence reagent, luminol concentration also affected the CL intensity. Therefore, the effect of the luminol concentration was investigated. The results showed that the optimal concentration of luminol was 1 × 10-3 M. Analytical Characteristics. Under the above optimal conditions, the linear response range for CCl4 was measured. The calibration of CL emission intensity versus CCl4 concentration was linear in the range from 0.1 to 380 ppm. The regression equation was I ) 0.0105C + 0.0493 (r2 ) 0.9964), where the concentration (C) was measured in ppm. The detection limit concentration for CCl4 was 40 ppb (signal/noise was 3). It was well-known that high sensitivity of detection was related to high catalytic activity. To study the destructive efficiency of CCl4, GC/MS (HP 6890 GC-5973 MS) spectra were used. A series of GC/MS spectra were obtained at different temperatures. Two peaks were observed in the chromatograms by the injection of CCl4 at room temperature. Mass spectrograms showed that the peaks were attributed to air and CCl4, which indicated that no destruction and adsorption of CCl4 happened at room temperature. When we injected CCl4 into the column at 220 °C, however, the peak attributed to CCl4 was disappeared, which indicated that CCl4 was decomposed completely. The results showed that the quartz column had a high catalytic activity for CCl4. Analytical Chemistry, Vol. 74, No. 24, December 15, 2002
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Figure 3. CL of 40 ppm CH2Cl2, CHCl3, and CCl4: luminol, 1 × 10-3 M; NaOH, 0.01 M.
To investigate the effect of the determination of other CVOCs, CL intensities of CH2Cl2, CHCl3, and CCl4 of the same concentration under the above optimal conditions were contrasted with each other. As shown in Figure 3, the sensitivity increased in the order CH2Cl2 < CHCl3 < CCl4. The possible reason for this trend was the increase in the number of available chorine atoms associated with each molecule. Interference Study. To evaluate the selectivity of the column, the foreign species were investigated by analyzing a standard gas sample of 10 ppm CCl4 to which increasing amounts of potential interfering species were added. Potential interfering species were measured in air, including CO2, CO, NO2, NO, SO2, and H2O, except O2 and N2, since the carrier gas was air the major components of which were O2 and N2. The limit of tolerance with respect to 10 ppm CCl4 for interferences at the 5% level were 30 000 ppm CO2, 10 000 ppm CO, 500 ppm NO2 and NO, 400 ppm SO2, and 21 000 ppm H2O. Lifetime of the Column. According to the literature, the destructive adsorption of CCl4 on metal oxides was performed in the absence of oxidant. The metal oxides were transformed to MxCly (such as BaCl2) by the injection of CCl4 into the column under the operation conditions. To regenerate the column, however, a complicated procedure had to be applied that added ammonium carbonate to the dissolved MxCly20-23 so as to transform the MxCly to the metal oxides again. This result indicated that the reactor used for CL detection of CCl4 was irreversible. To evaluate the lifetime of nanometer TiO2, CL for detection of 5 ppm CCl4 was repeated every day before a new 3-month study using the same material. The results showed that CL intensity of the analysis remains constant, which demonstrated the long lifetime of the column packed with nanometer TiO2. To investigate the possible mechanism for the long lifetime of the column packed with nanometer TiO2, a Raman spectrometer and X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were employed to examine if the TiCl4 species had also been formed from the reaction of TiO2 and CCl4 during the operating conditions of the CL detection of CCl4 in the present study. In the present work, Raman spectrometry (RM1000, Renishaw) was applied to detect the species on nanometer TiO2 produced by CCl4 decomposition. The Raman spectrum of nanometer TiO2 before destructive enrichment was dominated by bands 143, 250, 6282 Analytical Chemistry, Vol. 74, No. 24, December 15, 2002
Figure 4. TEM photos of TiO2: (a) before destructive enrichment; (b) after a 60-h reaction, including a destructive enrichment and desorption reaction. CCl4, 80 ppm.
450, and 612 cm-1, which were close to those reported in the literature.26 After introducing different concentrations of CCl4 into the column at 220 °C, no significant changes were observed on the Raman bands. Although we knew that the characteristic Raman bands27 for TiCl4 were situated at 128, 298, and 409 cm-1, we did not detect the bands under the present conditions. The Raman bands also did not change after the column was heated to 600 °C. This result would imply that no TiCl4 had been formed after the procedure of enrichment. To confirm our result that no TiCl4 had been formed after the procedure of enrichment, XRD (Bruker D8 Advance diffractometer) was applied in the present study to detect structure changes of TiO2 nanoparticles, because we supposed that, once TiCl4 had been formed, the significant difference of XRD patterns between TiO2 and TiCl4 would be observed. However, no significant differences were observed between the XRD patterns before and after the operation of enrichment. This result was correlated to the result by using Raman spectrometry, no detectable TiCl4 has been formed from TiO2 in the column. TEM (Hitachi H-800) operated at high temperature was used to investigate the stability of the TiO2 nanoparticles. As shown in Figure 4, no significant size changes of TiO2 were observed before enrichment and after 60 h reaction, including the enrichment and desorption reaction. To investigate whether TiCl4 was formed on nanometer TiO2, XPS (Perkin-Elmer (PHI) model 5300 ESCA), a relatively sensitive technique in comparison with the techniques of Raman and XRD, was subsequently used for the detection of TiCl4 on the surface of TiO2 nanoparticles. Figure 5 showed the acquired Ti 2p, O 1s, and Cl 2p spectra. The results showed that no chlorine atoms could be detected before destructive adsorption. Additionally, no chlorine atoms could be detected after destructive adsorption of CCl4 in the range from 0 ppm to10 000 ppm, which indicated that no TiCl4 was formed after destructive adsorption of CCl4 below 10 000 ppm. After destructive adsorption of CCl4 increasing to 15 000 ppm at 220 °C, however, the Cl 2p spectrum was detected, (26) Cheng, H. M.; Ma, J. M.; Zhao, Z. G.; Qi, L. M. Chem. Mater. 1995, 7, 663-671. (27) Gbureck, A.; Kiefer, W.; Schneider, M. E.; Werner, H. Vib. Spectrosc. 1998, 17, 105-115.
Figure 5. XPS spectra: (a) Ti 2p; (b) Cl 2p; (c) O 1s. CCl4, 15 000 ppm at 220 °C for 10 h.
although the concentration of the Cl species (represent by TiCl428) was extremely low in comparison with the Ti and O species. These results demonstrated that only a very low concentration of TiCl4 had been formed on the nanometer TiO2 at an extremely high concentration of CCl4. For atmospheric air samples, however, the formation of TiCl4 would be negligible, since the concentration of CCl4 in workplace air is far less than 10 000 ppm. The results above of Raman spectroscopy, XRD, transmission electron microscopy (TEM), and XPS showed that the quartz column was reversible for detection of CCl4 in workplace air under the present conditions. Possible Mechanism of Present CL Reaction. When the releases desorbed from the quartz column at 600 °C passed through the mixture solution of amylum and potassium iodide (KI), the mixture solution turned blue. The result showed that the releases might contain chlorine gas. To confirm that the release was chlorine, IC was applied to the identification of the decomposed products in the present study. A 0.01M NaOH solution was used for the collection of the decomposed products during desorption. The collected solution was then injected on the anion exchange column and chromatographed. Two peaks (28) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corp.: Eden Prairie, MN, 1992.
appeared on the chromatogram. The peaks were identified according to their retention times. ClO- and Cl- were 1.07 min and 6.0 min, respectively, by the determination of the retention times from the standard solutions of ClO- and Cl-. The results showed that the ClO- and Cl- anions were the only species in the collected solutions, which came from the dissolving chlorine in NaOH solutions. The ClO- and Cl- were not detected in the blank solution, since no CCl4 had been injected into the column. This experiment further proved that in addition to CO2, chlorine was one of the major species decomposed from CCl4. Although according to our deduction, COCl2 would also be produced in the present experiment, we did not observe the COCl2 species by using FT-IR because of the poor sensitivity of the technique. Further experiment was still needed to prove the conclusion. Therefore, we could propose a possible reaction mechanism of CL detection for CCl4. When nanometer TiO2 is thermally activated at 220 °C in atmospheric air, the injected CCl4 molecules are decomposed into CO2 and Cl2 by means of catalytic oxidation by the TiO2. Chlorine molecules are adsorbed onto the nanometer TiO2 surface. When the temperature is raised to 600 °C, chlorine molecules can be desorbed from nanometer TiO2. Then chlorine Analytical Chemistry, Vol. 74, No. 24, December 15, 2002
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can react with the basic luminol solution to produce CL emission. The possible CL reaction would be as follows:
the destructive and selective enrichment of trace CVOCs at 220 °C and the effective desorption of chlorine at 600 °C. Then, chlorine could react with the basic luminol solution to produce CL emission. The quartz column used could be operated in the reversible mode. The method could be potentially applied to the analysis of chlorinated compounds in the environment, such as persistent organic pollutants (POPs). ACKNOWLEDGMENT We gratefully acknowledged financial support of the work by the National Natural Science Foundation of China (20075014), National Science and Technology Committee of China (GN-994), and the Doctoral Research Foundation of Chinese Education Ministry (2000000303).
CONCLUSIONS In this paper, we proposed a novel method for determination of trace CVOCs in workplace air. This method could be used for
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Received for review June 24, 2002. Accepted October 2, 2002. AC025882U
