Facile On-Site Detection of Substituted Aromatic Pollutants in Water

ARTICLE pubs.acs.org/est

Facile On-Site Detection of Substituted Aromatic Pollutants in Water Using Thin Layer Chromatography Combined with Surface-Enhanced Raman Spectroscopy Dawei Li,§,† Lulu Qu,§,† Wenlei Zhai,† Jinqun Xue,† John S. Fossey,†,‡ and Yitao Long†,* †

Shanghai Key Laboratory of Functional Materials Chemistry & Department of Chemistry, East China University of Science and Technology, Shanghai, 200237, P. R. China ‡ School of Chemistry, University of Birmingham, Edgbaston, Birmingham, B15 2TT, U.K.

bS Supporting Information ABSTRACT: A novel facile method for on-site detection of substituted aromatic pollutants in water using thin layer chromatography (TLC) combined with surface-enhanced Raman spectroscopy (SERS) was explored. Various substituted aromatics in polluted water were separated by a convenient TLC protocol and then detected using a portable Raman spectrometer with the prepared silver colloids serving as SERS-active substrates. The effects of operating conditions on detection efficacy were evaluated, and the application of TLC
SERS to on-site detection of artificial and real-life samples of aromatics/polluted water was systematically investigated. It was shown that commercially available Si 60-F254 TLC plates were suitable for separation and displayed low SERS background and good separation efficiency, 2 mM silver colloids, 20 mM NaCl (working as aggregating agent), 40 mW laser power, and 50 s intergration time were appropriate for the detection regime. Furthermore, qualitative and quantitative detection of most of substituted aromatic pollutants was found to be readily accomplished using the developed TLC
SERS technique, which compared well with GC
MS in terms of identification ability and detection accuracy, and a limit of detection (LOD) less than 0.2 ppm (even at ppb level for some analytes) could be achieved under optimal conditions. The results reveal that the presented convenient method could be used for the effective separation and detection of the substituted aromatic pollutants of water on site, thus reducing possible influences of sample transportation and contamination while shortening the overall analysis time for emergency and routine monitoring of the substituted aromatics/polluted water.

’ INTRODUCTION Aromatic compounds are widely used in the chemical industry, and they are, however, also well-known as ubiquitous pollutants that threaten ecosystems and display significant toxicity to humans.1
3 Water pollution accidents involving release of aromatic compounds may be caused by anthropogenic activities or natural disaster, and in such an event rapid profiling and quantification of pollutants are extremely valuable in determining the correct rapid response action or treatment. Therefore, it is necessary to develop effective, convenient, and expedient methods to detect aromatic compounds at the site of the pollution incident. Up to now, various detection methods have been developed, such as high-performance liquid chromatography (HPLC) with UV or fluorescence spectroscopy detector (FLD), gas chromatography
mass spectrometry (GC
MS), and electrochemical sensors.4,5 However, HPLC-UV (and HPLC-FLD) is inconvenient to carry out on-site, GC
MS is expensive and hard to perform well on the compounds with low volatility although portable apparatuses have been developed. Electrochemical detection on-site has also been reported but is limited by the specificity to individual aromatic compounds.5 Surface-enhanced Raman spectroscopy (SERS) is a highly specific and sensitive technique;6 it can provide vibrational spectroscopic fingerprints from chemical and biological materials and provide molecular-level identification of extremely small samples.7,8 This makes SERS a useful detection technique widely r 2011 American Chemical Society

applied, for instance in biological analysis, art identification, and hazardous material detection.9
13 Indeed, a number of studies have successfully demonstrated the use of SERS to detect organic compounds, and there is a general acceptance that SERS has evolved to the stage where it can be used as a quantitative analytical technique.14 Furthermore, SERS has also been successfully employed in the detection of aromatic compounds by preparing alkanethiol-modified SERS substrates that create an environment to attract aromatics and hence enhance the Raman spectrum.15
17 Real-life samples, however, often contain two or more constituents thus rendering detection of each component challenging. Accordingly, separation techniques, such as column liquid chromatography (LC), 18 thin layer chromatography (TLC),19
26 ion-pair chromatography (IC),27 capillary electrophoresis (CE),28 and electrostatic separation (ES),29 have been combined with SERS as potential strategies to address separation. Among them, coupling TLC and SERS is very promising because it lessens the requirement of advanced apparatuses.26 TLC
SERS has been successfully applied to the separation and detection of various analytes. The detection of subfemtogram quantities of carotenoids on TLC plates was achieved by Received: December 10, 2010 Accepted: March 27, 2011 Revised: March 8, 2011 Published: April 12, 2011 4046

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Figure 1. Schematic illustration of TLC
SERS for on-site detection of substituted aromatic pollutants in wastewater. Two insert images show the SEM characterization of the blank TLC plate and the silver nanoparticles deposited TLC plate, respectively.

associating Fourier Transform (FT) Raman microspectroscopy with SERS, where the possibility of enhancing the sensitivity of this technique by micro-FT SERS at silver colloid-activated chromatographic spots was investigated.19 Further research of TLC
SERS was conducted on the analysis of medicinal herbs, and the result showed that this method could be used to analyze pharmaceuticals with high sensitivity.22 Recently, the usefulness of TLC
SERS for analyzing mixtures of red dyes was investigated systematically.26 In that investigation, TLC
SERS was used for separation and identification and demonstrated that significantly less materials and less sophisticated equipment was needed compared to HPLC, the applicability of this approach to the analysis of dyes in textile samples was demonstrated. The TLC
SERS technique was also studied for the detection of other more biomolecules, pharmaceuticals and dyestuff.23
25 Besides, evaluation of experimental conditions influencing SERS detection of substances separated by TLC was systematically investigated,20,21 which showed that, with proper selection of the excitation laser, metal colloid concentration, and type of the TLC media, excellent analyses could be accomplished without disturbing the process of chromatographic separation. TLC
SERS was systematically studied as a routine analytical method by Winefordner and Sutherland who have performed many experiments with the SERS technique.30
32 In their studies, commercially available TLC plates were evaluated as substrates for analysis by SERS. Signal versus concentration curves were found to be linear across at least 2 orders of magnitude with limits of detection in the low- to sub-nanogram range and with care the detection reproducibility could be improved by a further ∼10%. To the best of our knowledge, studies of the TLC
SERS technique have been performed in a laboratory environment, but on-site detection of aromatic pollutants in environmental water samples has yet to be reported. The development of portable Raman spectrometers makes it highly feasible to apply TLC
SERS to on-site detection, and indeed mobile Raman spectroscopy has been reported recently for the on-site identification of several classes of compounds such as pigments and minerals.33 Therefore, the objective of the present research is to explore a facile on-site detection method of aromatic pollutants in water using TLC
SERS based on a small portable Raman spectrometer. SERS-active silver colloids were prepared according to an established method34 and their characterization was also conducted. SERS analysis of sample spots on TLC plates and the on-site detection of aromatic pollutants in water samples using the approach of TLC
SERS were investigated, which showed that

the developed TLC
SERS technique could be a promising alternative method for the emergency and routine monitoring of the substituted aromatics/polluted water.

’ EXPERIMENTAL SECTION Materials. Silver nitrate (99%), sodium citrate (99%), and sodium chloride (>99%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). All other reagents of analytical grade used were obtained from Aladdin-Reagent Co., Ltd. (Shanghai, China). All solutions were prepared by 18 MΩ 3 cm deionized water obtained with a Mili-Q System (Billerica, MA, USA). Various eluents were prepared by mixing n-hexane and ethyl acetate in different proportions to elicit the best separation of pollutant mixture on TLC plates. Four different types of TLC plates manufactured by Merck Inc. (Germany) were used. Silica gel 60 plate (diameter (Ø): 200 nm, layer thickness (LT): 0.2 mm), Silica gel 60-F254 plate (Ø: 200 nm, LT: 0.2 mm), aluminum oxide 60 plate (Ø: 200 nm, LT: 0.2 mm), and aluminum oxide 60-F254 plate (Ø: 200 nm, LT: 0.2 mm) are all with glass back plate. The plate containing fluorescing additive, F254, was used for easy spot visualization. Apparatus. UV
vis spectra were recorded by a USB2000þ spectrometer (Ocean Optics Inc., U.S.A), and the scanning electron microscope (SEM) images of the prepared silver colloidal particles (S1) were acquired by a field-emission scanning electron microscope (Ultra 55, Carl Zeiss Ltd., Germany). Raman spectra were recorded at a small portable Raman spectrometer (BWS415, B&W Tek Inc., U.S.A) with an excitation wavelength of 785 nm, a resolution of 5 cm
1 and a beam diameter of 10 μm. A 1.5 m bifurcated fiber probe and a thermoelectric cooled detector equipped with the Raman spectrometer provided facile on-site SERS detection and high detection sensitivity. Detection Method. The TLC
SERS method (shown in Figure 1) was designed with on-site detection of aromatic pollutants in water in mind. First, 10 μL of mixed sample was spotted on the bottom of a TLC plate and chromatography was performed in a developing chamber with the optimal eluent chosen through screening various conditions. After that, the separated spots were located under illumination with a hand-held UV lamp with 254/365 nm wavelength (Beijing CBIO Ltd., China), to each spot 10 μL of a solution of the prepared silver colloids was added. Finally, SERS spectra for each separated spot were recorded using a small portable Raman spectrometer 4047

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’ RESULTS AND DISCUSSION Characterization and Evaluation of SERS-Active Silver Colloids Substrates. Qualities of SERS-active silver colloid

substrates are closely related with the performance of SERS detection.34 Accordingly, UV
vis spectroscopy and SEM were employed to characterize the dispersibility and morphology of the prepared silver colloidal particles. From the UV
vis spectrum (Figure S2 of the Supporting Information), an absorption maximum at 430 nm, and a full width at half-maximum (fwhm) about 95 nm can also be found. These values correspond to relatively monodispersed colloids with the diameter of silver nanoparticles (AgNPs) in the range of 50
60 nm.35 Representative SEM images of the AgNPs (Figure S3 of the Supporting Information) reveal that AgNPs have a spherical shape with a narrow size distribution and their dimensions are estimated at approximately 60 nm. This is in agreement with the results obtained in the UV
vis spectrum analysis and indicates that the prepared silver colloids may have an excellent SERS activity since significant improvements in Raman enhancement could be obtained by use of colloids with appropriate diameters (40
80 nm for 785 nm excitation).36 This can also be confirmed by the comparison of SERS spectrum and normal Raman spectrum of benzidine solution, an example analyte (Figure S4 of the Supporting Information). To further estimate SERS activity of silver colloids quantitatively, the SERS spectrum of p-aminothiophenol (p-ATP) solution dropped on the TLC plate and normal Raman spectrum of bulk p-ATP were recorded and compared, the corresponding enhancement factor (EF) was calculated (Figure S5 of the Supporting Information) to be about 1.5  105. These results imply that the prepared silver colloids could work effectively for SERS detection on TLC plates. SERS Analysis of Sample Spots on TLC Plate: Effect of the TLC Plate Material. When examining the Raman spectra of sample spots on TLC plates, the background Raman scattering originating from the blank stationary phase must be first determined.20,21 Therefore, before coupling of TLC with SERS for pollutant detection, it is necessary to investigate the effects of different TLC plate materials on the SERS spectrum of the analyte. The investigation result (Figure S6 of the Supporting Information) indicates that TLC plates are generally weak Raman scatterers that give rise to little background interference. Thus, it is conceivable that use of TLC plates with fluorescent indicators (Silica 60-F254) would be applicable to the presented detection method considering their lower cost, efficient separation for aromatic compounds and convenience of straightforward visualization under a UV lamp without the requirement of staining for visualization.19 SERS Analysis of Sample Spots on TLC Plate: Effect of the Aggregating Agent. An aggregating agent is routinely added into SERS-active colloids to achieve a degree of enhancement of SERS spectrum signals.37 Consequently, the effects of a wide range of aggregating agents on SERS response were investigated over the course of this study. From the result (Figure S7 of the Supporting Information), it can be observed that the different aggregating agents result in different signal enhancement and with NaCl giving the most intense bands. These effects may be a result of the difference in size and charge of the aggregating agents employed, which leads to the different sized aggregates

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and different spacing.37 It was also found that the intensity of the SERS signal rises and falls as NaCl concentration increases from 1 mM to 40 mM. This could arise from the effect of moderate addition of the aggregating agent promoting aggregation between AgNPs giving rise to large surface plasmon resonances, which bring about the surface enhancement. Whereas, adding increasing amounts of aggregating agent, the AgNP’s aggregation becomes larger, until they eventually fall out of solution, consequently a decline in the SERS activity of the silver colloid was observed.37 In this work, 20 mM was found to be the preferable concentration for NaCl aggregating agent. SERS Analysis of Sample Spots on TLC Plate: Effect of the Colloidal Silver Concentration. The concentration of silver colloids plays an important role in the development of the enhancement of Raman spectrum signals,38 so SERS spectra of 100 ppm benzidine sampled on TLC plates at seven different concentrations of silver colloids were recorded. As depicted in Figure S8 of the Supporting Information, SERS signal intensity of the benzidine sample shows an obvious increase with colloidal silver concentration over 0.4 mM ∼2 mM range, whereas a decrease is observed beyond 2 mM. On the one hand, this may be due to the increasing concentration of colloidal silver resulting in an accretion of monolayer coverage, which is considered to be the ideal scenario to produce intense SERS signals, however, if this concentration increases above the level required for monolayer formation, multilayer accumulation results and the SERS signal will be reduced accordingly. On the other hand, colloid concentration increases may lead to more particle
particle interactions that encourage more AgNPs in the colloid to be aggregated, and appropriate aggregation causes a boost of the SERS intensity, but excessive aggregation reduces the SERS activity of the silver colloids. Hence, 2 mM was chosen for the concentration of the silver colloids to carry out the next experiments. SERS Analysis of Sample Spots on TLC Plate: Effect of Laser Power and Integration Time. In addition to the factors discussed above, operating parameters of laser power and integration time used in the detection can influence the detected SERS signal intensity. The optimization of laser power and integration time was performed for the SERS detection of the sample on TLC plates. From the results (Figure S9 of the Supporting Information), it can be seen that a satisfactory signal intensity can be obtained under the conditions of 40 mW laser power and 50 s integration time which are appropriate for the TLC
SERS detection in the following tests of this work. Combination of TLC with SERS for Simultaneous On-Site Detection of Aromatic Pollutants in Water. Combination of TLC with SERS for simultaneous on-site qualitative detection was investigated for artificially produced model samples of polluted water containing a number of substituted aromatic compounds as samples of well-known priority pollutants. The components in the water sample were separated by a silica 60F254 TLC plate (eluent of 3:1 v/v n-hexane/ethyl acetate). The separated spots were then located under UV illumination and were each analyzed by SERS under the optimal condition of 2 mM silver colloids with 20 mM NaCl, 40 mW laser power, and 50 s integration time. SERS spectra for the four separated spots on the TLC plate are shown in parts a
d of Figure 2. It can be seen that the four SERS spectra are different from each other, which means that four different pollutants are detected. Comparing these SERS spectra with the SERS spectrum library obtained previously (Figure S10 of the Supporting Information), good matches are found between parts a
d of Figure 2 and parts f, c, d, 4048

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Figure 2. SERS spectra obtained from (a
d) the spots on the TLC plate after separation and (e) the spot of water sample without TLC separation. Laser power: 40 mW; integration time: 50 s; analyte concentration in the sample: 50 ppm, 50 ppm, 10 ppm, and 10 ppm for chlorobenzene, aniline, benzidine, and pyrocatechol, respectively.

and a respectively of Figure S10 of the Supporting Information, which correspond to the SERS spectra of chlorobenzene, benzidine, aniline, and pyrocatechol, respectively. It is important to note that the SERS spectrum for the polluted water sample recorded prior to TLC separation (shown in part e of Figure 2) does not match any SERS spectrum in the library and is difficult to deconvoluted, as such it is difficult to identify which pollutant is contained in the sample from the SERS spectrum without applying the TLC protocol developed here. This TLC
SERS technique was utilized for the detection of differently substituted aromatics (Figures S10 and S11 of the Supporting Information). The results demonstrate that the aromatic pollutants with different substiuents such as amino, hydroxyl, carboxyl, or halogens groups are ready detected, whereas unsubstituted (and alkyl substituted) aromatic compounds, like benzene, naphthalene, biphenyl, and ethylbenzene, cannot be detected. Possibly owing to that fact the former are much more eaily absorbed on the surface of AgNPs thus readily permitting SERS analysis than the later. Because many aromatic pollutants fall into the detectable class and they are easily visualized by TLC under UV illumination, it is likely that the presented technique will be useful in the detection of a large number of substituted aromatic pollutants. In addition to qualitative detection, the quantitative detection of substituted aromatic pollutants was also investigated. Four solutions of aniline, benzidine, pyrocatechol, and chlorobenzene with different concentrations were prepared with the same eluent used above. Aliquots of 10 μL of each solution were placed on a TLC plate and their SERS spectra were recorded after a short period of drying time under the optimized conditions. Parts A
D of

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Figure 3 illustrate the SERS spectra of aniline, benzidine, chlorobenzene, and pyrocatechol at concentrations ranging from 0.01 ppm to 600 ppm, respectively. The intensity of representative peaks of each analyte was measured, and the concentrationdependent SERS intensities at two representative peaks of aniline, benzidine, chlorobenzene and pyrocatechol are shown in parts E
H of Figure 3, respectively. It can be seen that, as the concentrations increase, the signal enhancements reach a plateau after 200 ppm for aniline, 100 ppm for benzidine, 200 ppm for chlorobenzene, and 100 ppm for pyrocatechol. Perhaps coverage of the AgNPs is complete resulting in full monolayer coverage for those compounds at the plateauing concentrations.37 Furthermore, it can also be observed that linearity satisfactory for quantitative analysis is obtained for all four tested analytes, and this is confirmed by correlation coefficients and error measurements on the repeated acquisition in this range (Table S12 of the Supporting Information). The accuracy of this quantitative analysis is good for the proposed application, namely preliminary monitoring of polluted water. The limit of detection (LOD) was estimated (Table S12 of the Supporting Information) and it is clear that the LOD of TLC
SERS was about 0.1 ppm, 0.008 ppm, 0.2 ppm, and 0.05 ppm for aniline (at 1597 cm
1), benzidine (at 1193 cm
1), chlorobenzene (at 808 cm
1), and pyrocatechol (at 1223 cm
1), respectively. This difference in LOD between different compounds may be due to their different substituent groups, which can cause different absorption of compounds on AgNPs leading to the apparent different SERS activities of those compounds. Therefore, different LOD for different compounds are observed. However, the result still implies that, using this TLC
SERS technique, the LOD less than 0.2 ppm (even at ppb level for some substituted aromatics) can be achieved directly for polluted water samples through equal proportion extraction. This value may be further lowered to meet the demand of detection at much lower concentration if a simple preconcentration procedure such as on-site solid phase extraction is applied. Thus, the presented method does match the requirement of preliminary quantitative analysis of the substituted aromatics/polluted water, not only for high concentrations but also for relatively lower concentrations. To validate this method, a detection with TLC
SERS was conducted on a standard mixture solution containing three known concentration of compounds (15 ppm aniline, 5 ppm benzidine and 10 ppm pyrocatechol), and it demonstrates that the difference between detected concentrations and control values is not more than 10% (Figure S13 of the Supporting Information). This infers that credible detection results can be obtained using this TLC
SERS technique. Facile On-Site Detection of Real Sample. To further test the performance of the presented method, TLC
SERS was employed in the on-site detection of substituted aromatic pollutants in the real wastewater provided by Modern Dyestuffs & Pigments Co., Ltd. in Ningbo, China (part A of Figure 4). The sample was separated by TLC under the conditions described earlier, then SERS spectrum of each separated spot was recorded with addition of the prepared silver colloids (part B of Figure 4). Matching the detection results with the SERS spectrum library and calibration curve library (Figures S10 and S14 of the Supporting Information), p-toluidine, p-nitroaniline, and lentine can be identified in the water sample and the corresponding concentrations were determined to be 91 ppm, 173 ppm, and 274 ppm respectively (part D of Figure 4). As a control the assayed samples were also analyzed using GC
MS (Shimadzu QP-2010, Japan), 4049

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Figure 3. SERS spectra of (A) aniline, (B) benzidine, (C) chlorobenzene, and (D) pyrocatechol at different concentrations; plots of intensities versus concentrations for (E) aniline at 989 cm
1 and 1597 cm
1, (F) benzidine at 977 cm
1 and 1193 cm
1, (G) chlorobenzene at 405 cm
1 and 808 cm
1, (H) pyrocatechol at 497 cm
1 and 1223 cm
1. Laser power, 40 mW; integration time, 50 s. Error bars represent the mean value of three replicate samples and corresponding standard deviation.

Figure 4. Detection of real wastewater sample. (A) On-site detection system of TLC
SERS. (B) SERS spectra of the detected pollutants: (a) p-toluidine, (b) p-nitroaniline, (c) lentine. (C) GC
MS spectrum of the real sample. (D) Comparison of the quantitative detection results between TLC
SERS and GC
MS. Error bars represent the mean value of three replicate samples and corresponding standard deviation.

which showed that p-toluidine, p-nitroaniline, and lentine were also detected in the sample with concentrations of 99 ppm, 197 ppm, and 291 ppm, respectively (parts C and D of Figure 4). Comparison of the results obtained from the two methods indicates that the detection performance of our simple TLC
SERS system is similar to that of more sophisticated GC
MS technique for the detection of substituted aromatic pollutants, detection concentration deviation is less than 15% between the two techniques. These tests were also performed on the two real wastewater samples provided by Guangcheng Chemical Co. Ltd. and Smart Pharmaceutical Co., Ltd. in Ningbo, China (Figure S15 of the Supporting Information), similar results were obtained for most substituted aromatic pollutants though toluene detected by GC
MS cannot be probed with TLC
SERS. Results presented herein demonstrate that the developed TLC
SERS technique is an excellent candidate for simultaneously detecting various substituted aromatic pollutants of water both qualitatively and quantitatively, and its analytical ability can be favorably compared to that of classic techniques such as GC
MS. More importantly, TLC
SERS is preferable for on-site detection of pollutants reducing the influence of sample contamination resulting from transportation and handling procedures and shortening the detection time, thus providing a valuable tool for both the emergency monitoring of pollution 4050

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’ ASSOCIATED CONTENT

bS

Supporting Information. Preparation of SERS-active silver colloids (S1); the UV
vis absorption spectrum of the prepared silver colloids (S2); the representative SEM image of AgNPs in the silver colloids (S3); normal Raman and SERS spectrum of benzidine dropped on the TLC plate (S4); calculation of enhancement factor (S5); effect of the TLC plate material. (S6); effect of the aggregating agent (S7); effect of the colloidal silver concentration (S8); effect of laser power and integration time (S9); SERS spectrum library of a number of substituted aromatic compounds (S10); SERS spectra of nonsubstituted (and alkyl substituted) aromatic compounds (S11); analytical data obtained from linear regression for analysis of aromatic compounds using SERS spectra (S12); detection of a standard mixture solution using TLC
SERS (S13); calibration curve library of intensity against to concentration for a number of substituted aromatic compounds (S14); detection of two real wastewater samples (S15). This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected], Tel/Fax: þ86-21-6425 2339. Author Contributions §

These authors contributed equally to this study.

’ ACKNOWLEDGMENT This research was supported by the National High-Tech Research and Development Program 863 of China (Project No: 2008AA06A406), National Nature Science Foundation of China (Project No: 21007015) and Shanghai Postdoctoral Sustentation Fund of China (Project No: 09R21411700). The Program supports YTL for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning. Y. T.L. and J.S.F. thank the CAtalysis and Sensing for our Environment (CASE) network, and the CASE09 workshop at ECUST that initiated interaction helping completion of this manuscript. J.S.F. thanks the University of Birmingham and AWM ERDF II for support and ECUST for a visiting Professorship. ’ REFERENCES (1) Knopp, D.; Seifert, M.; V€a€an€anen, V.; Niessner, R. Determination of polycylclic aromatic hydrocarbons in contaminated water and soil samples by immunological and chromatographic methods. Environ. Sci. Technol. 2000, 34, 2035–2041. (2) Kusie, H.; Rasulev, B.; Leszczynska, D.; Leszczynski, J.; Koprivanac, N. Prediction of rate constants for radical degradation of aromatic pollutants in water matrix: A QSAR study. Chemosphere 2009, 75, 1128–1134. (3) Liu, H.; Zhao, H. M.; Quan, X.; Zhang, Y. B.; Chen, S. Formation of chlorinated intermediate from bisphenol A in surface saline water under simulated solar light irradiation. Environ. Sci. Technol. 2009, 43, 7712–7717. (4) Koeber, R.; Bayona, J. M.; Niessner, R. Determination of benzo[a]pyrene diones in air particulate matter with liquid chromatography mass spectrometry. Environ. Sci. Technol. 1999, 33, 1522–1558.

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Environmental Science & Technology

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dx.doi.org/10.1021/es104155r |Environ. Sci. Technol. 2011, 45, 4046–4052