Journal of Hazardous Materials 280 (2014) 588–594
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Detection of nitrobenzene compounds in surface water by ion mobility spectrometry coupled with molecularly imprinted polymers Wei Lu a , Haiyang Li b , Zihui Meng a,∗ , Xixi Liang b , Min Xue a,∗ , Qiuhong Wang a , Xiao Dong a,∗ a b
School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing 100081, China Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
h i g h l i g h t s
g r a p h i c a l
• Ion mobility spectrometry (IMS) was
Ion mobility spectrometry (IMS) was explored for the selective detection of nitrobenzene compounds in surface water. And molecularly imprinted polymers (MIPs) were evaluated for the sample pretreatment. The MIP-IMS system promised excellent potential for the monitoring of nitrobenzene in real samples.
used to detect nitrobenzene compounds in surface water with high selectivity. • The transformation energy of product ion formation reaction was calculated. • IMS was used as an efficient detection system in molecularly imprinted polymer separation. • The short response time, high selectivity of MIP-IMS system showed its potential in the monitoring of nitrobenzene compounds in surface water.
a r t i c l e
i n f o
Article history: Received 6 May 2014 Received in revised form 13 August 2014 Accepted 24 August 2014 Available online 29 August 2014 Keywords: Ion mobility spectrometry Molecular imprinting Nitrobenzene compounds Surface water
a b s t r a c t Ion mobility spectrometry (IMS) was explored in the selective detection of nitrobenzene compounds in industrial waste water and surface water, and the selectivity was theoretically elucidated with the transformation energy in the product ion formation reaction. A linear detection range of 0.5–50 ppm and a limit of detection (LOD) of 0.1 ppm were found for 2,4,6-trinitrotoluene (TNT). With the IMS as the detection system of molecularly imprinted polymer (MIP) separation technique, the MIP-IMS system was proved to be excellent method to detect trace amount of nitrobenzene compounds in surface water, in which more than 87% of nitrobenzene compounds could be adsorbed on MIPs with 90–105% of recovery.
1. Introduction Nitrobenzene compounds, such as 2,4,6-trinitrotoluene (TNT) and 2,4-dinitrotoluene (2,4-DNT), are usually found in the water and soil near ammunition plants [1], and exist for years. Rapid
∗ Corresponding authors. Tel.: +86 10 68913065; fax: +86 10 68913065. E-mail address: m [email protected] (Z. Meng). http://dx.doi.org/10.1016/j.jhazmat.2014.08.041 0304-3894/© 2014 Elsevier B.V. All rights reserved.
a b s t r a c t
© 2014 Elsevier B.V. All rights reserved.
detection methods of nitrobenzene compounds are in urgent demands for human and environment due to their long-term carcinogenic effects. Most of the conventional detection methods of nitrobenzene compounds, such as LC/MS [2,3], spectrophotometry [4,5] and fluorescence [6], require exorbitant auxiliary equipment, which means the real-time on-site detection would be unavailable and the detection cost could be undesirable. As a high sensitive and field-portable analytical instrument, ion mobility spectrometry (IMS) has been proved to be a powerful
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Fig. 1. The schematic diagram of IMS.
technique for the on-site detection of trace amount of chemical warfare agents (CWAs) [7,8], explosives [9,10], narcotic drugs [11,12], and other hazardous chemicals [13,14]. Since 1971, the use of IMS analyzers has been investigated in the detection of nitrobenzene compounds particularly [15,16]. However, it is still very difficult to simultaneously analyze the constituents of a mixture with only IMS because of the mutual interferences from competitive ionization [17]. In order to improve the sensitivity and selectivity of IMS, it has been combined with other analytical technologies, such as GC [18,19], LC [20,21], and MS [22,23]. Since 2009, as a sample pretreatment technology for IMS, molecularly imprinted polymers (MIPs) have received considerable attention [24]. Due to the high selectivity of MIPs and the low limit of detection (LOD) of IMS, the MIP–IMS system has been proved as a powerful separation, pre-concentration and detection method for metronidazole drugs even in human serum, which inspired us to utilize this technique to monitor nitrobenzene compounds in surface water. Actually, as an enzyme mimic which can recognize the template molecule via the specific cavities, MIPs have been used as a convenient and versatile sample pretreatment method to analyze drugs [25,26], antibiotics [27,28], pesticides [29,30], explosives [31,32], etc. For the nitrobenzene compounds detection, taking a novel TNT detection method based on a surface imprinting strategy for example, the visual detection was obtained by the charge-transfer between acrylamide/acrylic acid and TNT [33]. The similar color response of TNT solution with different amount of 3-aminopropyltriethoxysilane (APTS) was also obtained by the imprinted silica nanotubes walls [34]. Moreover, by improving the structure of the TNT-imprinted nanowires which was grown in alumina membrane pores [35] or the single-hole hollow polymer microspheres [36], the sensitivity and adsorption were also intensively enhanced. And MIP–IMS systems have also been used successfully in the detection of testosterone [37], cocaethylene [38,39] and CWAs [8] in real samples. Our aim is to develop an MIP–IMS system for the on-site detection of nitrobenzene compounds in surface water. Based on the self-made IMS’s performance and the current understanding on the reaction process of IMS [40,41], the transformation energy was calculated with accurate algorithms to explain the selectivity. With the MIP–IMS, we investigated the LOD and the linear range of detection of TNT and 2,4-DNT, in which the mass spectra data were used to identify the IMS peaks. The outstanding adsorption and recovery proved that the MIP had a great potential in the sample pretreatment of IMS analysis.
2. Experimental 2.1. Materials Acrylic amide (AM), TNT and 2,4-DNT were purchased from Kermel Reagents Development Center; ethylene glycol dimethacrylate (EDMA) was purchased from TCI (Japan); azobisisoheptonitrile (ABVN) was purchased from Sigma (USA); acetonitrile was purchased from Aladdin. All of above chemicals were of analytical grade. Methanol and acetic acid were of chromatographic grade and were both purchased from Yu Wang Industrial Co. LTD. Deionized water was produced by Milli-Q Advantage. Industrial waste water and surface water were all donated by Qing Yang Chemical Plant in China. 2.2. Preparation of MIPs MIPs were prepared by precipitation polymerization. For a typical approach, 0.5 mmol TNT and 3 mmol AM were added into a 50 mL glass tube with 40 mL acetonitrile. The mixture was stored at 4 ◦ C overnight, followed by the addition of 3 mmol EDMA and 0.18 mmol ABVN. Then the pre-polymerization mixture was sonicated for 20 min and polymerized at 4 ◦ C under 365 nm UV irradiation for 15 h. The polymers were collected by centrifugation, methanol/acetic acid (7/3, v/v) washing overnight, and four rounds of washing with methanol (15 mL) and deionized water (15 mL), respectively. Similarly, the MIPs for 2,4-DNT were prepared with 2,4-DNT as template, and the corresponding non-imprinted polymers (NIPs) were prepared without the template. 2.3. Operation of IMS The schematic diagram of a self-made IMS with photo emissive ionization sources (63 Ni) in negative ion mode was shown in Fig. 1. The ion gate was Bradbury–Nielson type with an injection pulse of 200 s, and the product ions were collected by a Faraday plate. The length of ionization, reaction and drift region are 14, 24, and 62 mm, respectively. For the IMS, purified air was used as carrier gas at a flow rate of 460 mL/min, and the drift gas at a flow rate of 500 mL/min. Meanwhile, the injection port temperature was set at 180 ◦ C to ensure the complete evaporation of samples. The cell temperature was set at 100 ◦ C to avoid the interferences from the impurities in the pipeline. In addition, manual injection was used for the sample introduction, 2 L sample were dropped on a Teflon coated fiber slide, and
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W. Lu et al. / Journal of Hazardous Materials 280 (2014) 588–594 Table 1 The assignment of ion peak in MS.
Fig. 2. Ion mobility spectrum of TNT and 2,4-DNT.
then the slide was put on the solvent volatilization instrument at 80 ◦ C. Each of IMS spectrum was the average of 5 individual spectra. 3. Results and discussion 3.1. Detection of TNT and 2,4-DNT As shown in Fig. 2, IMS signals of TNT, 2,4-DNT standards and the reagent ions peak (RIP) which was caused by the hydrated ions were all clearly obtained. Even though IMS is suitable for the separation of subtle chemical structure, in the separation of TNT and 2,4-DNT, the conditions were very crucial, in which, the electrons from 63 Ni in the reaction region could attack the oxygen in carrier gas through three-body collisions to form O2 − (H2 O)n . However, under ambient pressure the electrons with low energy couldn’t impact molecular ions into their smaller fragment ions to form multiple-signals. The mechanism could be proposed as the following M + O 2 + e− → O 2 − + M −
(1)
−
M + H2 O + O2 ↔ O2 H2 O + M −
−
M + H2 O + O2 H2 O ↔ O2 (H2 O)2 + M
(2) (3)
where the M could be O2 , H2 O and other neutral molecule. With IMS, we detected TNT and 2,4-DNT in both industrial waste water and surface water. Due to the mobility of IMS, it is not only a simple detector, it can also be used to separate molecule. The
m/z
Ion
227 226 183 182 181
[TNT][TNT-H][2,4-DNT + H][2,6-DNT][2,4-DNT-H]-
selective detection results of TNT and 2,4-DNT in real samples by IMS and the identification by MS were studied (Fig. 3). Since IMS was very sensitive, both TNT and 2,4-DNT waste water were diluted for 100 times. It was shown that IMS detected the nitrobenzene compounds selectively in both industrial waste water and surface water without interference from impurities, and the information of MS was listed in Table 1. In order to explain the outstanding selectivity of IMS, we calculated the transformation energy of the product ion formation reaction. The initial produced ion of explosive was [MO2 − (H2 O)n ]* , where the M was TNT or 2,4-DNT. They could be transformed into [TNT-H]− and [2,4-DNT-H]− (Eqs 4) when O2 − ion captured a proton from the explosive molecule [42]. M + O2 − (H2 O)n → [MO2 − (H2 O)n ]∗ → [M-H]− (H2 O)n−1 + HO2 (4) When the carrier gas was clean and stable, the composition and properties of the reaction ions could keep well intact. Therefore, in order to simplify the calculation, we supposed that the reaction system was studied without water (n = 0). The transition states were first targeted at the B3LYP/6-31G(d) level, followed by IRC calculations to get structures close to the reactants and products. Then highly accurate thermochemical data could be obtained via the Complete Basis Set method CBS-QB3 [43,44] calculations. All theoretical studies in this work were carried out using Gaussian 09 Revision A.1. Transition structures in Equation 4 obtained at the B3LYP/6-31G(d) level were shown in Fig. 4. The activation energy and reaction heat of TNT were 228.96 kJ/mol and 7.43 kJ/mol respectively, and −3.64 kJ/mol and 24.47 kJ/mol for 2,4-DNT. The electron deficient aromatic ring with electron withdrawing nitro group was easy to deprotonate. Due to the small equilibrium constant, the product concentration, or the response of impurities, was
Fig. 3. IMS (a) and MS (b) spectra of wastewater and surface water.
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Fig. 4. Transition structures of TNT·O2 – (a) and 2,4-DNT·O2 − (b) during the deprotonation of TNT and 2,4-DNT with O2 − .
Fig. 5. SEM images of MIPs for TNT (a), 2,4-DNT (b) and NIPs (c).
negligible, compared to that of TNT and 2,4-DNT. Therefore, the IMS could be used in the selective analysis of nitrobenzene compounds. 3.2. Solid phase extraction of nitrobenzene compounds with MIPs The SEM images of MIPs were shown in Fig. 5. In IMS, methanol could reduce sample pretreatment time and offer a high sensitivity with stable detection environment. The different ratios of methanol/water had been optimized for MIPs adsorption. Mixtures, containing 1 ppm TNT and 1 mg/mL MIPs in methanol/water with different ratios, were incubated in 1.5 mL centrifuge tubes for 30 min. The adsorption of nitrobenzene compounds and the imprinting effect, which was defined as the adsorption ratio of MIPs over NIPs, were shown in Fig. 6. As shown in Fig. 6, although the adsorption of nitrobenzene compounds on both MIPs and NIPs was decreased with the increasing of methanol, the imprinting effect was increased at the same time. The imprinting effect of TNT (9.62) was superior in methanol due to the negligible absorption of TNT on NIPs in methanol. Thus, the addition of 10% methanol (volume ratio) into the water samples could improve the selectivity when MIPs were used to extract nitrobenzene compounds from surface water and be beneficial to disperse of the polymers. The IMS analysis results of nitrobenzene compounds in such solvent were shown in Table 2. Table 2 Analysis of nitrobenzene compounds in methanol/water (1/9, v/v) with IMS. Parameter
TNT
DNT
Linear range (ppm) Calibration equation Correlation coefficient Limit of detection (ppm) Relative standard deviation, % RSD (n = 5)
0.5–50 Y = 7.459X + 31.48 0.9931 0.1
0.1–10 Y = 22.18X + 32.74 0.9917 0.05
6.80
5.14
*Y = IMS response (arb.u.); X = concentration of TNT (ppm).
It was found that the LOD (S/N = 3) of nitrobenzene compounds relied on the solvent composition, evaporation time and temperature. If the evaporation temperature was set as 80 ◦ C, the corresponding concentration of TNT and DNT with a measured signal intensity which was three times of the noise, should be the LOD. The noise was estimated as the average difference intensity between the measured maximum and minimum of baseline. The LOD could be as low as 0.1 ppm for TNT and 0.05 ppm for 2,4-DNT. The linear range was three orders of magnitude. Additionally, the adsorption equilibrium could be achieved within 30 min with absorption of higher than 80% with all concentrations investigated. During the recovery test, polymers were eluted by methanol (1 mL × 3) until TNT residue couldn’t be detected with IMS. Subsequently, the eluent was collected and concentrated to 100 L. After the addition of 900 L of water, it’s analyzed with IMS and the recovery could reach to 95%. The similar results could be obtained for 2,4-DNT (Fig. 7). When TNT concentration was lower than 6 ppm, the adsorption of MIPs was from 102.83 to 95.47%, and the recovery was from 109.47 to 98.98%. In the MIPs for 2,4-DNT, the adsorption and recovery were 91.8 to 100.2% and 101.0 to 103.2%, respectively, with concentration below 2 ppm. To investigate the selectivity of MIPs for TNT, adsorption of TNT analogues (2,6-DNT, RDX and m-dinitrobenzene) were studied in methanol/water (9/1, v/v). The adsorption of TNT, RDX, m-dinitrobenzene and 2,6-DNT on the MIP were 40.27, 1.00%, -1, 0.69, and 0.50%, respectively. The high selectivity was derived from the “memory” effect of MIPs generated during the imprinting, and both template-shaped cavities and –* interactions between TNT and AM were the basis for it. To validate the performance of MIP–IMS system, nitrobenzene compounds in pure water spiked with 0.5 ppm 2,4-DNT and 2 ppm TNT were determined. MIPs for TNT and 2,4-DNT were added into water samples to extract nitrobenzene compounds, respectively, as shown in Tables 3 and 4. As the MIPs for TNT was used, the adsorption and recovery of TNT were varied between 88.57 and 103.98% while the adsorption of 2,4-DNT was 60%. Subsequently,
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Fig. 6. Adsorption of TNT (a) and 2,4-DNT (b) and the imprinting effect (c) of MIPs on TNT.
Fig. 7. The adsorption (a) and recovery (c) of TNT on the MIPs, and the adsorption (b) and recovery (d) of 2,4-DNT on the MIPs.
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Table 3 Adsorption and recovery of TNT on the MIPs for TNT. Added TNT/ppm
Average of TNT found/ppm
Adsorption of TNT/%
0.00 1.00 2.00 3.00 4.00
1.91 2.87 3.32 5.50 6.73
103.98 100.56 99.72 96.88 88.57
± ± ± ± ±
Recovery of TNT/%
11.91 10.53 6.61 2.91 1.03
105.55 99.05 97.37 90.27 91.89
± ± ± ± ±
14.70 6.71 9.13 5.79 3.10
n = 5. Table 4 Adsorption and recovery of 2,4-DNT on the MIPs for 2,4-DNT. Added 2,4-DNT ppm
Average of 2,4-DNT found/ppm
Adsorption of 2,4-DNT/%
0.00 0.50 1.00 1.50 2.00
1.00 1.46 1.76 2.51 3.02
103.77 105.74 95.82 93.17 97.30
± ± ± ± ±
Recovery of 2,4-DNT/%
6.38 8.95 4.20 3.87 6.61
103.76 99.33 100.38 98.71 100.66
± ± ± ± ±
3.47 5.03 10.22 3.10 14.42
n = 5.
Table 5 Adsorption and recovery of nitro-compounds on the mixture of MIPs for TNT and 2,4-DNT. Added 2,4-DNT/ppm
Average of 2,4-DNT found/ppm
TNT added/ppm
Average of TNT found/ppm
Adsorption of 2,4-DNT/%
0.00 0.50 1.00 1.50 2.00
0.58 1.23 1.35 1.51 2.19
0.00 1.00 2.00 3.00 4.00
2.31 3.33 3.93 4.66 5.26
97.95 101.61 93.50 89.81 87.00
± ± ± ± ±
13.27 3.32 4.50 6.95 2.50
Adsorption of TNT/% 99.35 102.61 103.81 96.37 100.08
± ± ± ± ±
4.77 0.75 2.83 8.16 3.47
Recovery of 2,4-DNT/% 117.04 103.58 79.96 91.64 84.55
± ± ± ± ±
26.54 10.30 12.18 10.98 9.17
Recovery of TNT/% 81.88 96.85 53.81 48.07 101.52
± ± ± ± ±
20.41 15.75 4.49 5.01 18.73
n = 5.
if both MIPs were used simultaneously, both TNT and 2,4-DNT in water samples could be removed completely (Table 5). 4. Conclusions The MIP–IMS system was employed to analyze and extract TNT and 2,4-DNT from the industrial waste water and surface water. With an external connection gas generator [45,46], the detection results could be easily obtained at the water source. So the homemade IMS could be a feasible on-site detector for nitrobenzene compounds at ppm level within seconds, and would not be interfered by the other substances. The IMS is also a desirable technology to evaluate the performance of MIPs. Hopefully, these MIPs may also be used to adsorb TNT from waste water for the purification purpose in the future. Furthermore, the MIP for TNT showed high selectivity and could be used to extract TNT from complicated samples before IMS detection, thus demonstrating its great potential as in solid phase extraction matrix. Therefore, this MIP–IMS system methodology could provide a helpful tool to screen of surface waters, which were suspected to contaminate with nitrobenzene compounds. Acknowledgements This study was supported by the National Science Foundation of China (No. 20775007, 21375009) and China Scholarship Council. We also greatly thank Xin Wang, Xinzhen Wang, Bangyu Ju, Wendong Chen from Dalian Institute of Chemical Physics, CAS, for their instruction of the home-made IMS in this research. References [1] Z. Meng, Q. Zhang, M. Xue, D. Wang, A. Wang, Removal of 2,4,6-trinitrotoluene from pink water using molecularly imprinted absorbent, Propellants Explos. Pyrotech. 37 (2012) 100–106.
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Detection of nitrobenzene compounds in surface water by ion mobility spectrometry coupled with molecularly imprinted polymers Wei Lu a , Haiyang Li b , Zihui Meng a,∗ , Xixi Liang b , Min Xue a,∗ , Qiuhong Wang a , Xiao Dong a,∗ a b
School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing 100081, China Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
h i g h l i g h t s
g r a p h i c a l
• Ion mobility spectrometry (IMS) was
Ion mobility spectrometry (IMS) was explored for the selective detection of nitrobenzene compounds in surface water. And molecularly imprinted polymers (MIPs) were evaluated for the sample pretreatment. The MIP-IMS system promised excellent potential for the monitoring of nitrobenzene in real samples.
used to detect nitrobenzene compounds in surface water with high selectivity. • The transformation energy of product ion formation reaction was calculated. • IMS was used as an efficient detection system in molecularly imprinted polymer separation. • The short response time, high selectivity of MIP-IMS system showed its potential in the monitoring of nitrobenzene compounds in surface water.
a r t i c l e
i n f o
Article history: Received 6 May 2014 Received in revised form 13 August 2014 Accepted 24 August 2014 Available online 29 August 2014 Keywords: Ion mobility spectrometry Molecular imprinting Nitrobenzene compounds Surface water
a b s t r a c t Ion mobility spectrometry (IMS) was explored in the selective detection of nitrobenzene compounds in industrial waste water and surface water, and the selectivity was theoretically elucidated with the transformation energy in the product ion formation reaction. A linear detection range of 0.5–50 ppm and a limit of detection (LOD) of 0.1 ppm were found for 2,4,6-trinitrotoluene (TNT). With the IMS as the detection system of molecularly imprinted polymer (MIP) separation technique, the MIP-IMS system was proved to be excellent method to detect trace amount of nitrobenzene compounds in surface water, in which more than 87% of nitrobenzene compounds could be adsorbed on MIPs with 90–105% of recovery.
1. Introduction Nitrobenzene compounds, such as 2,4,6-trinitrotoluene (TNT) and 2,4-dinitrotoluene (2,4-DNT), are usually found in the water and soil near ammunition plants [1], and exist for years. Rapid
∗ Corresponding authors. Tel.: +86 10 68913065; fax: +86 10 68913065. E-mail address: m [email protected] (Z. Meng). http://dx.doi.org/10.1016/j.jhazmat.2014.08.041 0304-3894/© 2014 Elsevier B.V. All rights reserved.
a b s t r a c t
© 2014 Elsevier B.V. All rights reserved.
detection methods of nitrobenzene compounds are in urgent demands for human and environment due to their long-term carcinogenic effects. Most of the conventional detection methods of nitrobenzene compounds, such as LC/MS [2,3], spectrophotometry [4,5] and fluorescence [6], require exorbitant auxiliary equipment, which means the real-time on-site detection would be unavailable and the detection cost could be undesirable. As a high sensitive and field-portable analytical instrument, ion mobility spectrometry (IMS) has been proved to be a powerful
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589
Fig. 1. The schematic diagram of IMS.
technique for the on-site detection of trace amount of chemical warfare agents (CWAs) [7,8], explosives [9,10], narcotic drugs [11,12], and other hazardous chemicals [13,14]. Since 1971, the use of IMS analyzers has been investigated in the detection of nitrobenzene compounds particularly [15,16]. However, it is still very difficult to simultaneously analyze the constituents of a mixture with only IMS because of the mutual interferences from competitive ionization [17]. In order to improve the sensitivity and selectivity of IMS, it has been combined with other analytical technologies, such as GC [18,19], LC [20,21], and MS [22,23]. Since 2009, as a sample pretreatment technology for IMS, molecularly imprinted polymers (MIPs) have received considerable attention [24]. Due to the high selectivity of MIPs and the low limit of detection (LOD) of IMS, the MIP–IMS system has been proved as a powerful separation, pre-concentration and detection method for metronidazole drugs even in human serum, which inspired us to utilize this technique to monitor nitrobenzene compounds in surface water. Actually, as an enzyme mimic which can recognize the template molecule via the specific cavities, MIPs have been used as a convenient and versatile sample pretreatment method to analyze drugs [25,26], antibiotics [27,28], pesticides [29,30], explosives [31,32], etc. For the nitrobenzene compounds detection, taking a novel TNT detection method based on a surface imprinting strategy for example, the visual detection was obtained by the charge-transfer between acrylamide/acrylic acid and TNT [33]. The similar color response of TNT solution with different amount of 3-aminopropyltriethoxysilane (APTS) was also obtained by the imprinted silica nanotubes walls [34]. Moreover, by improving the structure of the TNT-imprinted nanowires which was grown in alumina membrane pores [35] or the single-hole hollow polymer microspheres [36], the sensitivity and adsorption were also intensively enhanced. And MIP–IMS systems have also been used successfully in the detection of testosterone [37], cocaethylene [38,39] and CWAs [8] in real samples. Our aim is to develop an MIP–IMS system for the on-site detection of nitrobenzene compounds in surface water. Based on the self-made IMS’s performance and the current understanding on the reaction process of IMS [40,41], the transformation energy was calculated with accurate algorithms to explain the selectivity. With the MIP–IMS, we investigated the LOD and the linear range of detection of TNT and 2,4-DNT, in which the mass spectra data were used to identify the IMS peaks. The outstanding adsorption and recovery proved that the MIP had a great potential in the sample pretreatment of IMS analysis.
2. Experimental 2.1. Materials Acrylic amide (AM), TNT and 2,4-DNT were purchased from Kermel Reagents Development Center; ethylene glycol dimethacrylate (EDMA) was purchased from TCI (Japan); azobisisoheptonitrile (ABVN) was purchased from Sigma (USA); acetonitrile was purchased from Aladdin. All of above chemicals were of analytical grade. Methanol and acetic acid were of chromatographic grade and were both purchased from Yu Wang Industrial Co. LTD. Deionized water was produced by Milli-Q Advantage. Industrial waste water and surface water were all donated by Qing Yang Chemical Plant in China. 2.2. Preparation of MIPs MIPs were prepared by precipitation polymerization. For a typical approach, 0.5 mmol TNT and 3 mmol AM were added into a 50 mL glass tube with 40 mL acetonitrile. The mixture was stored at 4 ◦ C overnight, followed by the addition of 3 mmol EDMA and 0.18 mmol ABVN. Then the pre-polymerization mixture was sonicated for 20 min and polymerized at 4 ◦ C under 365 nm UV irradiation for 15 h. The polymers were collected by centrifugation, methanol/acetic acid (7/3, v/v) washing overnight, and four rounds of washing with methanol (15 mL) and deionized water (15 mL), respectively. Similarly, the MIPs for 2,4-DNT were prepared with 2,4-DNT as template, and the corresponding non-imprinted polymers (NIPs) were prepared without the template. 2.3. Operation of IMS The schematic diagram of a self-made IMS with photo emissive ionization sources (63 Ni) in negative ion mode was shown in Fig. 1. The ion gate was Bradbury–Nielson type with an injection pulse of 200 s, and the product ions were collected by a Faraday plate. The length of ionization, reaction and drift region are 14, 24, and 62 mm, respectively. For the IMS, purified air was used as carrier gas at a flow rate of 460 mL/min, and the drift gas at a flow rate of 500 mL/min. Meanwhile, the injection port temperature was set at 180 ◦ C to ensure the complete evaporation of samples. The cell temperature was set at 100 ◦ C to avoid the interferences from the impurities in the pipeline. In addition, manual injection was used for the sample introduction, 2 L sample were dropped on a Teflon coated fiber slide, and
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W. Lu et al. / Journal of Hazardous Materials 280 (2014) 588–594 Table 1 The assignment of ion peak in MS.
Fig. 2. Ion mobility spectrum of TNT and 2,4-DNT.
then the slide was put on the solvent volatilization instrument at 80 ◦ C. Each of IMS spectrum was the average of 5 individual spectra. 3. Results and discussion 3.1. Detection of TNT and 2,4-DNT As shown in Fig. 2, IMS signals of TNT, 2,4-DNT standards and the reagent ions peak (RIP) which was caused by the hydrated ions were all clearly obtained. Even though IMS is suitable for the separation of subtle chemical structure, in the separation of TNT and 2,4-DNT, the conditions were very crucial, in which, the electrons from 63 Ni in the reaction region could attack the oxygen in carrier gas through three-body collisions to form O2 − (H2 O)n . However, under ambient pressure the electrons with low energy couldn’t impact molecular ions into their smaller fragment ions to form multiple-signals. The mechanism could be proposed as the following M + O 2 + e− → O 2 − + M −
(1)
−
M + H2 O + O2 ↔ O2 H2 O + M −
−
M + H2 O + O2 H2 O ↔ O2 (H2 O)2 + M
(2) (3)
where the M could be O2 , H2 O and other neutral molecule. With IMS, we detected TNT and 2,4-DNT in both industrial waste water and surface water. Due to the mobility of IMS, it is not only a simple detector, it can also be used to separate molecule. The
m/z
Ion
227 226 183 182 181
[TNT][TNT-H][2,4-DNT + H][2,6-DNT][2,4-DNT-H]-
selective detection results of TNT and 2,4-DNT in real samples by IMS and the identification by MS were studied (Fig. 3). Since IMS was very sensitive, both TNT and 2,4-DNT waste water were diluted for 100 times. It was shown that IMS detected the nitrobenzene compounds selectively in both industrial waste water and surface water without interference from impurities, and the information of MS was listed in Table 1. In order to explain the outstanding selectivity of IMS, we calculated the transformation energy of the product ion formation reaction. The initial produced ion of explosive was [MO2 − (H2 O)n ]* , where the M was TNT or 2,4-DNT. They could be transformed into [TNT-H]− and [2,4-DNT-H]− (Eqs 4) when O2 − ion captured a proton from the explosive molecule [42]. M + O2 − (H2 O)n → [MO2 − (H2 O)n ]∗ → [M-H]− (H2 O)n−1 + HO2 (4) When the carrier gas was clean and stable, the composition and properties of the reaction ions could keep well intact. Therefore, in order to simplify the calculation, we supposed that the reaction system was studied without water (n = 0). The transition states were first targeted at the B3LYP/6-31G(d) level, followed by IRC calculations to get structures close to the reactants and products. Then highly accurate thermochemical data could be obtained via the Complete Basis Set method CBS-QB3 [43,44] calculations. All theoretical studies in this work were carried out using Gaussian 09 Revision A.1. Transition structures in Equation 4 obtained at the B3LYP/6-31G(d) level were shown in Fig. 4. The activation energy and reaction heat of TNT were 228.96 kJ/mol and 7.43 kJ/mol respectively, and −3.64 kJ/mol and 24.47 kJ/mol for 2,4-DNT. The electron deficient aromatic ring with electron withdrawing nitro group was easy to deprotonate. Due to the small equilibrium constant, the product concentration, or the response of impurities, was
Fig. 3. IMS (a) and MS (b) spectra of wastewater and surface water.
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Fig. 4. Transition structures of TNT·O2 – (a) and 2,4-DNT·O2 − (b) during the deprotonation of TNT and 2,4-DNT with O2 − .
Fig. 5. SEM images of MIPs for TNT (a), 2,4-DNT (b) and NIPs (c).
negligible, compared to that of TNT and 2,4-DNT. Therefore, the IMS could be used in the selective analysis of nitrobenzene compounds. 3.2. Solid phase extraction of nitrobenzene compounds with MIPs The SEM images of MIPs were shown in Fig. 5. In IMS, methanol could reduce sample pretreatment time and offer a high sensitivity with stable detection environment. The different ratios of methanol/water had been optimized for MIPs adsorption. Mixtures, containing 1 ppm TNT and 1 mg/mL MIPs in methanol/water with different ratios, were incubated in 1.5 mL centrifuge tubes for 30 min. The adsorption of nitrobenzene compounds and the imprinting effect, which was defined as the adsorption ratio of MIPs over NIPs, were shown in Fig. 6. As shown in Fig. 6, although the adsorption of nitrobenzene compounds on both MIPs and NIPs was decreased with the increasing of methanol, the imprinting effect was increased at the same time. The imprinting effect of TNT (9.62) was superior in methanol due to the negligible absorption of TNT on NIPs in methanol. Thus, the addition of 10% methanol (volume ratio) into the water samples could improve the selectivity when MIPs were used to extract nitrobenzene compounds from surface water and be beneficial to disperse of the polymers. The IMS analysis results of nitrobenzene compounds in such solvent were shown in Table 2. Table 2 Analysis of nitrobenzene compounds in methanol/water (1/9, v/v) with IMS. Parameter
TNT
DNT
Linear range (ppm) Calibration equation Correlation coefficient Limit of detection (ppm) Relative standard deviation, % RSD (n = 5)
0.5–50 Y = 7.459X + 31.48 0.9931 0.1
0.1–10 Y = 22.18X + 32.74 0.9917 0.05
6.80
5.14
*Y = IMS response (arb.u.); X = concentration of TNT (ppm).
It was found that the LOD (S/N = 3) of nitrobenzene compounds relied on the solvent composition, evaporation time and temperature. If the evaporation temperature was set as 80 ◦ C, the corresponding concentration of TNT and DNT with a measured signal intensity which was three times of the noise, should be the LOD. The noise was estimated as the average difference intensity between the measured maximum and minimum of baseline. The LOD could be as low as 0.1 ppm for TNT and 0.05 ppm for 2,4-DNT. The linear range was three orders of magnitude. Additionally, the adsorption equilibrium could be achieved within 30 min with absorption of higher than 80% with all concentrations investigated. During the recovery test, polymers were eluted by methanol (1 mL × 3) until TNT residue couldn’t be detected with IMS. Subsequently, the eluent was collected and concentrated to 100 L. After the addition of 900 L of water, it’s analyzed with IMS and the recovery could reach to 95%. The similar results could be obtained for 2,4-DNT (Fig. 7). When TNT concentration was lower than 6 ppm, the adsorption of MIPs was from 102.83 to 95.47%, and the recovery was from 109.47 to 98.98%. In the MIPs for 2,4-DNT, the adsorption and recovery were 91.8 to 100.2% and 101.0 to 103.2%, respectively, with concentration below 2 ppm. To investigate the selectivity of MIPs for TNT, adsorption of TNT analogues (2,6-DNT, RDX and m-dinitrobenzene) were studied in methanol/water (9/1, v/v). The adsorption of TNT, RDX, m-dinitrobenzene and 2,6-DNT on the MIP were 40.27, 1.00%, -1, 0.69, and 0.50%, respectively. The high selectivity was derived from the “memory” effect of MIPs generated during the imprinting, and both template-shaped cavities and –* interactions between TNT and AM were the basis for it. To validate the performance of MIP–IMS system, nitrobenzene compounds in pure water spiked with 0.5 ppm 2,4-DNT and 2 ppm TNT were determined. MIPs for TNT and 2,4-DNT were added into water samples to extract nitrobenzene compounds, respectively, as shown in Tables 3 and 4. As the MIPs for TNT was used, the adsorption and recovery of TNT were varied between 88.57 and 103.98% while the adsorption of 2,4-DNT was 60%. Subsequently,
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Fig. 6. Adsorption of TNT (a) and 2,4-DNT (b) and the imprinting effect (c) of MIPs on TNT.
Fig. 7. The adsorption (a) and recovery (c) of TNT on the MIPs, and the adsorption (b) and recovery (d) of 2,4-DNT on the MIPs.
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Table 3 Adsorption and recovery of TNT on the MIPs for TNT. Added TNT/ppm
Average of TNT found/ppm
Adsorption of TNT/%
0.00 1.00 2.00 3.00 4.00
1.91 2.87 3.32 5.50 6.73
103.98 100.56 99.72 96.88 88.57
± ± ± ± ±
Recovery of TNT/%
11.91 10.53 6.61 2.91 1.03
105.55 99.05 97.37 90.27 91.89
± ± ± ± ±
14.70 6.71 9.13 5.79 3.10
n = 5. Table 4 Adsorption and recovery of 2,4-DNT on the MIPs for 2,4-DNT. Added 2,4-DNT ppm
Average of 2,4-DNT found/ppm
Adsorption of 2,4-DNT/%
0.00 0.50 1.00 1.50 2.00
1.00 1.46 1.76 2.51 3.02
103.77 105.74 95.82 93.17 97.30
± ± ± ± ±
Recovery of 2,4-DNT/%
6.38 8.95 4.20 3.87 6.61
103.76 99.33 100.38 98.71 100.66
± ± ± ± ±
3.47 5.03 10.22 3.10 14.42
n = 5.
Table 5 Adsorption and recovery of nitro-compounds on the mixture of MIPs for TNT and 2,4-DNT. Added 2,4-DNT/ppm
Average of 2,4-DNT found/ppm
TNT added/ppm
Average of TNT found/ppm
Adsorption of 2,4-DNT/%
0.00 0.50 1.00 1.50 2.00
0.58 1.23 1.35 1.51 2.19
0.00 1.00 2.00 3.00 4.00
2.31 3.33 3.93 4.66 5.26
97.95 101.61 93.50 89.81 87.00
± ± ± ± ±
13.27 3.32 4.50 6.95 2.50
Adsorption of TNT/% 99.35 102.61 103.81 96.37 100.08
± ± ± ± ±
4.77 0.75 2.83 8.16 3.47
Recovery of 2,4-DNT/% 117.04 103.58 79.96 91.64 84.55
± ± ± ± ±
26.54 10.30 12.18 10.98 9.17
Recovery of TNT/% 81.88 96.85 53.81 48.07 101.52
± ± ± ± ±
20.41 15.75 4.49 5.01 18.73
n = 5.
if both MIPs were used simultaneously, both TNT and 2,4-DNT in water samples could be removed completely (Table 5). 4. Conclusions The MIP–IMS system was employed to analyze and extract TNT and 2,4-DNT from the industrial waste water and surface water. With an external connection gas generator [45,46], the detection results could be easily obtained at the water source. So the homemade IMS could be a feasible on-site detector for nitrobenzene compounds at ppm level within seconds, and would not be interfered by the other substances. The IMS is also a desirable technology to evaluate the performance of MIPs. Hopefully, these MIPs may also be used to adsorb TNT from waste water for the purification purpose in the future. Furthermore, the MIP for TNT showed high selectivity and could be used to extract TNT from complicated samples before IMS detection, thus demonstrating its great potential as in solid phase extraction matrix. Therefore, this MIP–IMS system methodology could provide a helpful tool to screen of surface waters, which were suspected to contaminate with nitrobenzene compounds. Acknowledgements This study was supported by the National Science Foundation of China (No. 20775007, 21375009) and China Scholarship Council. We also greatly thank Xin Wang, Xinzhen Wang, Bangyu Ju, Wendong Chen from Dalian Institute of Chemical Physics, CAS, for their instruction of the home-made IMS in this research. References [1] Z. Meng, Q. Zhang, M. Xue, D. Wang, A. Wang, Removal of 2,4,6-trinitrotoluene from pink water using molecularly imprinted absorbent, Propellants Explos. Pyrotech. 37 (2012) 100–106.
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