Extraction of trace nitrophenols in environmental water samples using boronate affinity sorbent

Analytica Chimica Acta xxx (2015) 1e10

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Extraction of trace nitrophenols in environmental water samples using boronate affinity sorbent Yong Zhang, Meng Mei, Xiaojia Huang*, Dongxing Yuan State Key Laboratory of Marine Environmental Science, Key Laboratory of the Ministry of Education for Coastal and Wetland Ecosystem, College of the Environment and Ecology, Xiamen University, Xiamen 361005, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


A new boronate affinity sorbent (BAS) was prepared.
The BAS was used as the extractive medium of stir cake sorptive extraction (SCSE).
The BeN coordination favored the extraction of nitrophenols pollutants.
Method of sensitive monitoring of nitrophenols in water samples was developed.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 July 2015 Received in revised form 5 October 2015 Accepted 7 October 2015 Available online xxx

In this research, the applicability of a new sorbent based on boronate affinity material is demonstrated. For this purpose, six strong polar nitrophenols were selected as models which are difficult to be extracted in neutral form (only based on hydrophobic interactions). The extracted nitrophenols were separated and determined by high-performance liquid chromatography with diode array detection. The sorbent was synthesized by in situ copolymerization of 3-acrylamidophenylboronic acid and divinylbenzene using dimethyl sulfoxide and azobisisobutyronitrile as porogen solvent and initiator, respectively. The effect of the preparation parameters in the polymerization mixture on extraction performance was investigated in detail. The size and morphology of the sorbent have been characterized via different techniques such as infrared spectroscopy, elemental analysis, scanning electron microscopy and mercury intrusion porosimetry. The important parameters influencing the extraction efficiency were studied and optimized thoroughly. Under the optimum extraction conditions, the limits of detection (S/N ¼ 3) and limits of quantification (S/N ¼ 10) for the target nitrophenols were 0.097e0.28 and 0.32e0.92 mg/L, respectively. The precision of the proposed method was evaluated in terms of intra- and inter-assay variability calculated as RSD, and it was found that the RSDs were all below 9%. Finally, the developed method was successfully applied for environmental water samples such as wastewater, tap, lake and river water. The recoveries varied within the range of 71.2e115% with RSD below 11% in all cases. The results well demonstrate that the new boronate affinity sorbent can extract nitrophenols effectively through multiinteractions including boronenitrogen coordination, hydrogen-bond and hydrophobic interactions between sorbent and analytes. © 2015 Elsevier B.V. All rights reserved.

Keywords: Boronate affinity sorbent Stir cake sorptive extraction Monolith Nitrophenols Water samples

* Corresponding author. P. O. Box 1009, Xiamen University, Xiamen 361005, China. E-mail address: [email protected] (X. Huang). http://dx.doi.org/10.1016/j.aca.2015.10.004 0003-2670/© 2015 Elsevier B.V. All rights reserved.

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1. Introduction Sample preparation plays a key role in the overall analytical process, since it can extract and concentrate target analytes. At the same time, suitable sample preparation can overcome matrix interferences and improve the detectability of the analytical technique. Typical sample pretreatment includes solvent-based and sorbent-based extraction [1]. Sorbent-based extraction is more popular than solvent-based approach because less toxic solvent is used. So far, all kinds of sorbent-based extraction formats such as solid-phase extraction (SPE) [2], solid-phase microextraction (SPME) [3], multiple monolithic fiber solid-phase microextraction (MMF-SPME) [4], stir bar sorptive extraction (SBSE) [5] and stir cake sorptive extraction (SCSE) [6] have been reported. The extractive medium is the core in sorbent-based extraction because it decides the extraction targets and performance. Until now, a great variety of sorbents based on functional materials has been emerged. Surfacemodified silicas (C8, C18) [2], macroporous polymeric materials [7], graphene-based materials [8], carbon nanotubes [9], polymeric ionic liquid (PIL) [10] and metal-organic frameworks (MOFs) [11] et al. have been widely used to extract all kinds of compounds from complicated matrices. Even so, there is still an urgent need to develop new sorbents. Boronate affinity materials (BAMs) derived from boronate affinity chromatography (BAC) are another popular and interesting separation media. The BAMs permit the specific isolation and separation of cis-diol containing compounds, such as glycoproteins, nucleosides and saccharides [12,13]. The main principle is based on reversible covalent complex formation/dissociation between boronic acids and cis-diols in an alkaline/acidic aqueous solution. Based on the unique property, BAMs have also been used as sorbents in sample preparation. So far, several kinds of boronate affinity sorbents (BASs) such as boronate affinity magnetite [14], boronate affinity-based molecularly imprinted polymers [15], boronic acid-functionalized filter paper [16] and boronate affinity monoliths [13,17] have been reported. These BASs could selectively capture and separate brassinosteroids, catechol, carbonhydrates, nucleosides and glycopeptides [14e17]. These is another outstanding feature of BASs that boronic-acid ligand can form BeN coordination effectively. Based on this feature, Liu et al. prepared a new boronate monolith which showed strong BeN coordination between a regular boronic-acid ligand and a diamine [13]. In our previous work, a BAS was prepared and used to extract ultra-trace sulfonamides in environmental water samples [18]. The results indicated that the BeN coordination between sorbent and sulfonamides played an important role in the effect extraction. Nitrophenols are generated by a number of polluting processes, including those in industries such as dyestuffs, petroleum, pesticide, paper pharmaceutical [19]. Nitrophenols possess significant toxicity which exhibit mutagenic, cyto- and phyto-toxic effects and are regarded as priority pollutants [20]. Therefore, they have been included in the list of priority pollutants of the US Environmental Protection Agency (EPA) and other organization [21,22]. At the same time, the maximum limits (ML) for some nitrophenols have been set. For example, the EPA and the Brazilian Environmental Council (BEC) have set the ML values for p-nitrophenol in drinking water, the corresponding values are 60 mg/L and 100 mg/L, respectively [21,22]. Therefore, it is important and necessary to develop an efficient and sensitive approach for the monitoring of nitrophenols in environmental water samples. Due to the high separation efficiency, chromatographic methods such as gas chromatography (GC) and HPLC are frequently used to analyze nitrophenols [23e25]. Compared with GC, HPLC is more simple and convenient to separate nitrophenols. Before HPLC analysis, sample preparation step is necessary because the concentration of NPs compounds in

real samples are quite low. At the same time, the matrices for some water samples (such as wastewater) are complicated, the matrix interferences can be reduced with suitable sample preparation. So far, some sample preparation techniques such as liquideliquid extraction (LLE) [26], liquideliquid microextraction (LLME) [27], solid-phase extraction (SPE) [2], solid-phase microextraction (SPME) [28], single-drop microextraction (SDME) [29], and stir bar sorptive extraction (SBSE) [30] have been developed to extract nitrophenols from waters. However, there are various shortcomings for these techniques. For example, LLE is labor-intensive, and it consumes much organic solvents. The extraction capacity of LLME is limited because low extraction solvent is used. SPE requires large volumes of toxic solvent, and the process is complicated and time consuming. The shortcomings of SDME include instability and volatility of the extraction solvent. For SBSE, long extraction times are needed. Therefore, for the monitoring of trace nitrophenols in waters, developing new sample preparation with simplicity, high extraction performance and environmental friendliness is highly desired. It is well known that nitrophenols belong to strongly polar compounds, which can produce intense interactions with water molecules in aqueous solutions. Hence, effective extraction of nitrophenols is challenging and interesting. In this work, to enrich the types of BASs and expand the application field of BAC, and develop simple, sensitive, low-cost and environmentally friendly analytical method for the monitoring of nitrophenols, a new BAS based on poly (3-acrylamidophenylboronic-co-divinylbenzene) monolith poly (APB-co-DB) was prepared and used as extractive medium of SCSE. Six nitrophenols including 2-nitrophenol, 4nitrophenol, 2,4-dinitrophenol, 5-methyl-2-nitrophenol, 5methoxy-2-nitrophenol and 4-tertbutyl-2-nitrophenol were selected as model analytes to evaluate the extraction performance of the new sorbent. After the optimization of preparation and extraction parameters of APBDB/SCSE, a simple and sensitive methodology combining SCSE and liquid desorption (LD), followed by high performance liquid chromatography with diode array detection (APBDB/SCSE-LD-HPLC/DAD) for the direct analysis of trace nitrophenols in environmental water samples was developed. 2. Experimental 2.1. Chemicals and reagents 3-Acrylamidophenylboronic (APB) (97%) and divinylbenzene (DB) (80%) were supplied by Soochiral Chemical Co. (Shuzhou, China) and Alfa Aesar (Tianjin, China), respectively; Azobisisobutyronitrile (AIBN) (97%, re-crystallized before use), dimethyl sulfoxide (96%) (DMSO) and trifluoroacetic acid (TFA) were purchased from Shanghai Chemical Co. (China); HPLC-grade acetonitrile (ACN) and methanol were purchased from Tedia (Fairfield, USA); Water used throughout the study was purified using a Milli-Q Reference water-purification system (Merck Millipore, Germany). 2-Nitrophenol (2-NP) (98%), 4-nitrophenol (4-NP) (97%), 2,4dinitrophenol (2,4-DNP) (98%), 5-methyl-2-nitrophenol (5-M-2NP) (97%), 5-methoxy-2-nitrophenol (5-MO-2-NP) (97%) and 4tertbutyl-2-nitrophenol (4-TB-2-NP) (98%) were supplied by Alfa Aesar Ltd. (Tianjin, China). The chemical properties of these analytes is shown in Table S1. Water samples were collected from Xiamen city and filtrated through 0.45 mm membranes. All samples were stored at 4  C before use. Individual stock solutions of nitrophenols were prepared at a concentration of 10 mg/L by dissolving methanol and renewed monthly. The standard mixtures of nitrophenols were prepared by dissolving 2 mg of each compound in methanol in 100 mL volumetric flask. The stock solutions were stored at 4  C and diluted with ultrapure water to give the required

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concentration. 2.2. Instruments HPLC analyses were carried out on a LC chromatographic system (Shimadzu, Japan) equipped with a binary pump (LC-20AB) and a diode array detector (SPD-M20A). Sample injection was carried out using a RE3725i manual sample injector with a 20 mL loop (Rheodyne, Cotati, CA, USA), all experiments were performed at room temperature. The morphologies of monolithic materials were examined by a Model XL30 scanning electron microscopy (SEM) instrument (Philips, Eindhoven, The Netherlands). The pore size distribution of the monolith was measured on mercury intrusion porosimeter (MIP) Model PoreMaster-60 (Quantachrome Instruments, Florida, USA). Elemental analysis (EA) was carried out on PerkinElmer (Shelton, CT, USA) Model PE 2400. FT-IR was performed on an Avatar-360 FT-IR instrument (Thermo Nicolet, Madison, WI, USA). 2.3. Chromatographic conditions The separation of nitrophenols was performed on a Phenomenex C18 column (5 mm particle size, 250 mm  4.6 mm i.d.). Optimum separation was obtained with a binary mobile phase composed of ultrapure water (solvent A) and ACN (solvent B). The gradient elution program was as follows: 0e10 min ¼ 50% B, 10e12 min ¼ 50% Be20% B and kept to 15 min, 15e19 min ¼ 20% Be90% B and kept to 25 min, 25e27 min ¼ 90% Be50% B and kept to 30 min. The detector wavelength was set at 270 nm for 2-NP and 4TB-2-NP, 300 nm for 4-NP and 5-MO-2-NP, 342 nm for other nitrophenols. The flow rate was 1 mL/min, and injection volume was 20 mL. 2.4. Preparation of APBDB/SCSE The preparation of APBDB/SCSE is quite simple and convenient. Three steps were involved in the whole procedure. In the first step, the poly (APB-co-DB) monolithic cake was synthesized. In the all polymerization reaction, AIBN (1% (w/w) of the total monomer amount) and DMSO were used as polymerization initiator and porogen solvent, respectively. To optimize the preparation parameters, different percentage of monomer and porogen concentration in polymerization solution was tested (Table 1). The monomer mixture and porogen were mixed ultrasonically into a homogenous solution. Then the reactant solution was purged with nitrogen for 3 min. Subsequently, the reactant mixture was poured into a syringe cartridge (1.2 cm i.d.), one side of which was blocked by the plug of syringe. After that, the cartridge was sealed with septa and kept at 70  C for 24 h. After polymerization, the monolithic cake

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was pushed out slowly by a handspike of syringe. The cake was Soxhlet-extracted with methanol for 24 h to remove the residue monomers, porogen, uncross-linked polymers and initiator. Finally, the monolith was dried in air for 0.5 h to obtain the final monolithic cake (1.2 cm in diameter and 0.3 cm in thickness). The polymerization sketch is depicted in Fig. 1. In the second step, extraction holder of SCSE was constructed according to our previous study [31]. Briefly, 5 mL syringe cartridge was used as holder (0.6 cm height), the one side of the cartridge was cut into indention so that the monolithic cake could contact with sample as much as possible. On the other side, the cartridge was pierced by a lab-made glass protected iron wire (1 mm in diameter, the length was the same as the outer diameter of cartridge) to allow the magnetic stirring of the device. In order to let the sample flow through the stir cake system, 6 small holes (1 mm in diameter) were drilled around the cartridge. After the construction of extraction holder, the monolithic cake was inserted into the holder gently to get the final APBDB/SCSE. Fig. S1 shows the 3D schematic diagram of the holder, the photographs of holder and the final prototype of APBDB/SCSE. Before the APBDB/SCSE was used to extract nitrophenols, it was conditioned in two consecutive steps of 30 min by immersion in methanol and ultrapure water, respectively.

2.5. APBDB/SCSE procedure Stirring extraction and stirring liquid desorption (LD) modes were used in this work. The 100 mL aqueous samples were stirred 2.5 h by APBDB/SCSE at 400 rpm at room temperature. After extraction, the APBDB/SCSE was removed and immersed in 3 mL desorption solvent, stirring for 0.5 h to release the extracted analytes. Subsequently, the APBDB/SCSE was removed and the stripping solvent was used for HPLC analysis directly. The used APBDB/ SCSE was placed into 3 mL of methanol for 30 min for cleaning and then dipped in ultrapure water for 30 min before the next use.

2.6. Preparation of environmental water samples Wastewater was collected from wastewater treatment plant in Xiamen. Tap, lake and river water samples were collected from our lab, Furong lake and Jiulong river, respectively. Wastewater, tap, lake and river water samples were collected in 2.5 L amber glass bottles and stored in the dark at 4  C until analysis. All the samples were vacuum-filtered through a 0.45 mm nylon filter to remove suspended matter. The pH values of sample solutions were adjusted to 5 by 0.1 mol/L HCl, and ionic strength was adjusted to 20% (w/v) by addition of NaCl. After that, APBDB/SCSE procedure was used to extract nitrophenols from the above-mentioned water samples.

Table 1 Extraction performance of different APBDV/SCSE for nitrophenols. NO

1 2 3 4 5 6 7 8 9 10 11

Monomer mixture

Polymerization mixture

Peak area

APB (%, w/w)

DB (%, w/w)

Monomer mixture (%, w/w)

Porogen solvent (%, w/w)

4-NP

2,4-DNP

2-NP

40 40 40 40 40 40 25 30 35 45 50

60 60 60 60 60 60 75 70 65 55 50

35 40 45 50 55 60 40 40 40 40 40

65 60 55 50 45 40 60 60 60 60 60

1.69 2.25 1.63 2.12 1.65 1.54 2.68 2.19 1.79 1.45 1.45

3.75 5.26 4.01 4.91 4.26 3.98 5.76 5.15 4.42 3.87 4.01

3.26 4.23 3.30 3.99 3.35 3.22 4.69 4.11 3.52 2.70 2.79

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Fig. 1. The reaction sketch of poly (APB-co-DB).

2.7. Method validation The LOD and LOQ values of each analyte were considered as the concentration giving a signal to noise ratio of 3 and 10, respectively. The calibration curves were made by fortified with the analytes at each of eight concentrations from 0.5 to 200 mg/L. The spiked samples were performed with complete APBDB/SCSE procedure. The calibration curves were calculated using the linear least squares regression analyses of the peak area to concentration ratios. To evaluate the intra-day precision of proposed method, three replicates samples with 100 mg/L spiking concentration were extracted and analyzed within one day. The inter-day precision of the method was assessed at a l00 mg/L spiking concentration during a period of four consecutive days.

3. Results and discussion 3.1. Preparation and characterization of APBDB/SCSE Typically, the extraction performance and longevity of sorbent based on porous monolith will be affected by the content of monomer, cross-linker and porogen in polymerization solution. Therefore, to obtain the best preparation conditions of APBDB/ SCSE, 2-NP, 4-NP and 2,4-DNP were selected as analytes to investigate the optimal proportion of monomer and porogen concentration in polymerization solution (Table 1). The data clearly show that the extraction performance of APBDB/SCSE for the three analytes is affected strongly by the content of APB, DB and porogen in polymerization. Although the best extraction performance could be achieved in APBDB/SCSE-7, the monolithic cake was crisp and the longevity was very limited. Comprehensively considering extraction capacity, extraction speed and useful longevity of APBDB/SCSE, the optimal conditions for the preparation of monolithic cake were the proportion of APB kept 40% in the monomer mixture, the ratio of monomer mixture to porogen was 40/60 (%, w/w) (APBDB/SCSE2). Under the optimal preparation conditions, satisfactory preparation reproducibility was achieved, the RSD (n ¼ 4) of extraction efficiencies for 2-NP, 4-NP and 2,4-DNP were 6.19%, 5.58% and 4.07%, respectively. The poly (APB-co-DB) monolith prepared at optimal conditions was characterized with EA, IR, MIP and SEM. EA results demonstrated that its carbon, hydrogen and nitrogen contents were 62.8%, 6.98% and 5.62% (w/w), respectively, indicating that APB and DB polymerized successfully. The FT-IR spectrum (Fig. 2a) further confirmed the polymerization of APB and DB. As can be seen from the spectrum, the strong absorption peaks at 2921 and 2851 cm1 belong to CH3 and CH2 groups. The sharp peaks at 1668 and 1540 cm1 are the absorption of amide groups The adsorption observed at 1598, 1489 and 1345 cm1 indicates the existence of phenyl groups. The band at 1427 cm1 is the vibration of BeO bond. Fig. 2b displays the SEM image of the poly (APB-co-DB) monolith (10,000  magnification). The homogeneous pore size and interconnected skeletons of the materials can be clearly observed. Fig. 2c

is the pore size distribution plot of poly (APB-DB) monolith. It can be seen that most of the pore sizes are around 320 nm and the pore size distribution is narrow, which will be favorable for mass transfer during extraction applications. The result is well in accordance with the SEM. The total specific surface area of the monolith was calculated from BrunauereEmmetteTeller plot. Result showed the total surface area was 39.8 m2/g. The relatively large surface area ensures the extraction performance of APBDB/SCSE towards nitrophenols because more adsorptive sites can be contacted by analytes. 3.2. Optimization of the extraction parameters In order to achieve the best extraction efficiency of the new APBDB/SCSE for polar nitrophenols, several parameters, including desorption solvents, salt concentration, organic phase content and extraction time, were investigated. 3.2.1. Desorption solvent Considering there is BeN coordination between sorbent and nitrophenols, addition of suitable acid in desorption solvent can disrupt the coordination interaction and favor the desorption of analytes from sorbent. At present study, binary solvent consisted of methanol/0.5% TFA aqueous was selected as desorption solvent. Results indicated that the extraction performance reached maximum for all studied nitrophenols when water content was 5% (v/v). Therefore, methanol/0.5% TFA aqueous (95/5, v/v) was chosen as the desorption solvent. 3.2.2. pH value Sample pH value is a significant parameter affecting the extraction efficiency because the existing form of sorbent and analytes are affected by sample pH value. Effect of pH of the sample matrix on the extraction efficiency of nitrophenols was evaluated in the pH range of 2e11. As shown in Fig. 4 an obvious improvement in extraction efficiencies was observed at pH 2e5. The extraction performance decreased when pH values increased continuously. This changed trend may be explained as follows: At low values, the nitrogen atoms of nitrophenols were protonized and made against the BeN coordination. Hence, only pep interactions contributed to the extraction because the molecules of NPs were ionic forms. With the increase of pH values, BeN coordination was formed gradually because the de-protonized procedure happened on nitrophenols. At the same time, hydrophobic and hydrogen-bond interactions between sorbent and analytes increased. Therefore, APBDB/SCSE enhanced the extraction performance for nitrophenols with the increase of pH values. However, when the pH values increased continuously, the BeN interactions were minified because of the dissociation of boronic acid groups in the monolith, resulting in the decrease of extraction performance. The results well demonstrate that BeN coordination between the BAS and nitrophenols plays a key role in the extraction. According to above results, setting the pH value of matrix at 5 was recommended for the extraction of

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Fig. 2. The FT-IR spectrum (a), SEM image (10,000  magnification) (b) and pore size distribution plot (c) of poly (APB-co-DB).

Fig. 3. The effect of desorption solvent on extraction efficiency. Conditions: extraction and desorption time were both 1 h; no salt was added in the sample and the pH values of sample matrix were not adjusted. The spiked concentration was 100 mg/L for each analyte. Symbols: 4-NP; 2,4-DNP; 2-NP; 5-M-2-NP; 5-MO-2-NP; 4-TB-2-NP.

nitrophenols in water matrix with APBDB/SCSE. 3.2.3. Ionic strength Ionic strength can increase or decrease the extraction efficiency, depending on the analytes, the type of sorbent and ionic strength [32]. At present work, the effect of ionic strength in the matrix was investigated by addition of NaCl from 0 to 25% (w/v) (Fig. 5). The results showed that the extraction performance of APBDB/SCSE for nitrophenols increased when suitable NaCl was added. Under the salt-out effect and the electrostatic interaction between polar molecules and salt ions in sample solution, 20% NaCl showed the maximum extraction efficiency for all analytes except for 4-TB-2-

NP (the optimal content of NaCl was 5%). For experimental convenience, addition of 20% NaCl was used in the following study. 3.2.4. Extraction time and desorption time In order to further assess the ability of the new BAS to extract nitrophenols, the extraction time profiles were investigated by increasing the extracting time from 0.5 h to 3 h. As shown in Fig. 6a, the extraction performance increased quickly when the extraction time increased from 0.5 to 2.5 h. The sharp slopes of the profiles indicated that the APBDB/SCSE possessed remarkable extraction capacity towards these analytes. At the same time, the extraction equilibrium was reached after 2.5 h. Consequently, the extraction

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Fig. 4. The effect of pH value of sample matrix on extraction efficiency. Conditions: using methanol/0.5% TFA aqueous (95/5, v/v) as desorption solvent; the sample pH values were adjusted by 0.1 mol/L HCl or 0.1 mol/L NaOH. The other conditions and symbols are the same as in Fig. 3.

Fig. 5. The effect of salt concentration in sample matrix on extraction efficiency. Conditions: pH value of sample matrix was adjusted to 5. The other conditions and symbols are the same as in Fig. 4.

time of 2.5 h was selected for further studies. Desorption time was investigated ranging from 0.5 h to 2.5 h when the extraction time was kept at 2.5 h. The results showed that the nitrophenols could be eluted from the sorbent completely in 1 h when the extraction time was 2.5 h (Fig. 6b). Although the whole extraction/desorption procedure is relative long (3.5 h), for routine analysis, a series of APBDB/SCSEs can be used to pre-treat different water samples simultaneously, since the preparation reproducibility of APBDB/SCSE is satisfactory. On the basis of the above discussion, the optimal conditions of APBDB/SCSE for nitrophenols are concluded as follows: methanol/ 0.5% TFA aqueous (95/5, v/v) was selected as desorption solvent; the pH value of matrix was 5; ionic strength was adjusted by addition of 20% (w/v) NaCl; extraction and desorption time were

2.5 h and 1 h, respectively. Under the optimized extraction conditions, the APBDB/SCSE exhibited satisfactory extraction performance to nitrophenols. Fig. 7b shows the chromatograms of target analytes after extraction. Compared with Fig. 7a (direct injection of spiked sample without extraction), it can be seen that all the nitrophenols are obviously enriched after treatment with APBDB/ SCSE. The expected results well demonstrate that the new sorbent can effectively enrich polar nitrophenols through multiinteractions such as BeN coordination, hydrogen-bond and pep interactions between sorbent and analytes. At the same time, under the optimal extraction conditions, the APBDB/SCSE possessed excellent longevity. It could be reused more than 150 times without losing the extraction efficiency. At present study, the selective enrichment of APBDB/SCSE for

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Fig. 6. The effect of extraction (a) and desorption (b) time on extraction efficiency. Conditions: methanol/0.5% TFA aqueous (95/5, v/v) was selected as desorption solvent; the pH value of matrix was 5; ionic strength was adjusted by addition of 20% (w/v) NaCl. The other conditions and symbols are the same as in Fig. 4.

Fig. 7. HPLC chromatograms of six nitrophenols. (a) Direct injection of spiked water sample; (b) Spiked water sample with each analyte at 100 mg/L and treated with APBDB/SCSE. Conditions: methanol/0.5% TFA aqueous (95/5, v/v) was selected as desorption solvent; the pH value of matrix was 5; ionic strength was adjusted by addition of 20% (w/v) NaCl; extraction and desorption time were 2.5 h and 1 h, respectively. The spiked concentration was 100 mg/L for each analyte.

nitrophenols and other compounds was also investigated. Phenol and m-methylphenol were used as model analytes. The data in Table S1 shows that under the optimal extraction conditions of nitrophenols, there is no obvious change of extraction performance of nitrophenols when the initial concentration of phenol and mmethylphenol increases from 10 to 500 mg/L. At the same time, APBDB/SCSE shows slight enrichment for phenol and m-methylphenol even if their initial concentration is 50 times higher than nitrophenols. The results well indicate that under the optimal extraction conditions of nitrophenols, APBDB/SCSE possesses extraction selectivity. 3.3. Validation of APBDB/SCSE-LD-HPLC/DAD for nitrophenols Ultrapure water spiked with different concentration of nitrophenols was taken for evaluating the proposed method. The data of linear dynamic range, correlation coefficients, limits of detection (LODs) (S/N ¼ 3), limits of quantification (LOQs) (S/N ¼ 10) and method reproducibility for target analytes under the optimal conditions are listed in Table 2. The linear dynamic ranges for 2,4-DNP, 5-M-2-NP and 4-TB-2-NP were 0.5e200 mg/L, and 1e200 mg/L for 4NP, 2-DNP, and 5-MO-2-NP. The all linear dynamic ranges possess good linearity (r > 0.99). The LODs and LOQs are in the range of

0.097e0.28 and 0.32e0.92 mg/L, respectively. The LOD values are low than the ML values regulated by EPA and BEC. At the same time, excellent method reproducibility was achieved in terms of intraand inter-day assay variability, indicating by the relative standard deviations (RSD) both of <9%. These results demonstrate that the proposed method has good reproducibility and high sensitivity for the detection of nitrophenols. 3.4. Real samples analysis The proposed APBDB/SCSE-LD-HPLC/DAD method was further used to determine nitrophenols in wastewater, tap, lake and river water samples. The results in Table 3 showed that low concentration of 2,4-DNP and 4-TB-2-NP was detected in river water, in wastewater sample, low content of 2-NP, 4-NP and 2,4-DNP were found. No other nitrophenols was detected in the all water samples. To further evaluate the feasibility of the proposed method, extraction recoveries were assessed by spiking different standard solutions (10 mg/L and 100 mg/L, respectively). The results showed that the recoveries of the all target analytes from the all samples were in the range from 71.2 to 115% with the RSDs less than 11%, indicating that the matrix did not interfere with the quantification of six nitrophenols under the optimized conditions, and the

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Table 2 Linear dynamic range, correlation coefficients, LODs and LOQs, inter-day and intra-day precisions achieved for six nitrophenols. Compound

Linear rangea (mg/L)

r

LODb (mg/L)

LOQc (mg/L)

Intra-day assay variability (RSD, %, n ¼ 3)

Inter-day assay variability (RSD, %, n ¼ 4)

4-NP 2,4-DNP 2-NP 5-M-2-NP 5-MO-2-NP 4-TB-2-NP

1e200 0.5e200 1e200 0.5e200 1e200 0.5e200

0.9997 0.9997 0.9997 0.9999 0.9995 0.9980

0.24 0.097 0.28 0.15 0.28 0.11

0.79 0.32 0.92 0.50 0.92 0.36

3.31 3.77 4.20 8.65 5.03 4.31

2.17 3.83 3.40 8.01 3.11 5.86

a b c

Spiked level includes 0.5, 1, 5, 10, 20, 50, 100, and 200 mg/L, respectively. S/N ¼ 3. S/N ¼ 10.

Table 3 Results of determination and recoveries of real water samples spiked with six nitrophenols. Samples

Spiked (mg/L)

Detected (mg/L)/recovery (%RSD, n ¼ 3) 4-NP

2,4-DNP

2-NP

5-M-2-NP

5-MO-2-NP

4-TB-2-NP

Tap water

0 10 100 0 10 100 0 10 100 0 10 100

ND* 11 7.86 ND 10.5 105 ND 7.31 85.3 2.65 9.77 90.8

ND 11.5 96.3 ND 11.1 112 1.12 12.4 106 3.02 86.6 93.2

ND 11.5 88.5 ND 10.4 86.6 ND 10.8 91.6 2.81 93.8 93

ND 10.7 88.4 ND 9.80 92 ND 10.3 95.5 ND 8.02 91

ND 10.9 91.8 ND 10.8 100 ND 10.8 99 ND 7.73 89.1

ND 104 93 ND 11.3 107 2.45 12.3 95.4 ND 9.36 90.8

Lake water

River water

Waste water

110 (1) 78.6 (2.7) 105 (5.8) 105 (6.5) 73.1 (1.8) 85.3 (6.4) 71.2 (6.8) 88.2 (6.5)

115 (3.5) 96.3 (4.6) 111 (2.1) 112 (3.2) 113 (5.8) 104 (3.2) 83.6 (10.2) 90.2 (6.3)

115 (5.6) 88.5 (3.2) 104 (4.9) 86.6 (5.4) 108 (5.1) 91.6 (2.2) 91 (5.2) 90.2 (2.3)

107 (4.1) 88.4 (5.2) 98 (1.2) 92 (3.3) 103 (4.7) 95.5 (2.9) 80.2 (8.9) 91 (9.2)

109 (4.5) 91.8 (4) 108 (3.8) 100 (3.9) 108 (4.8) 99 (3.5) 77.3 (2.6) 89.1 (5.3)

104 (5.6) 93.5(5.1) 113 (0.27) 107 (2.7) 98.5 (2.1) 93 (9.8) 93.6 (4.9) 90.8 (1.6)

ND: not detected.

proposed method was suitable for the monitoring of trace nitrophenols in water samples. 3.5. Comparison with other methods To illustrate the advantages of the new BAS as a novel extraction material, the comparative study of proposed method with other reported analytical methodologies for 2-NP, 4-NP and 2,4-DNP in environmental water samples is performed and the results are presented in Table 4. It can be seen from the comparison, lower LODs can be obtained in the present method than other methods

such as SBSE-HPLC/UV [30], MSPE-HPLC-UV [33], UAEM-HPLC-UV [34], MS-USAEME-UHPLC/UV [35], SPME-HPLC/UV [28], MIPSPME-HPLC/UV [36], DLLME-MSC/UV [39], and are comparable to those reported in LLME-HPLC/UV [27], MIPSCE-HPLC/UV [38] and MMF/AMIIED-SPME-HPLC/DAD [43]. Typically, higher sensitivity can be achieved when high sensitivity detectors such as flame ionization detection (FID) and mass spectrum (MS) are used. However, the LODs for 2-NP, 4-NP and 2,4-DNP achieved in proposed method are lower than that obtained with HS-SPME-GC/FID [40], HS-SPME-GC/MS [41], SPME-GC/MS [42] and SPE-GC/MS [2]. At the same time, the spiked recoveries achieved in the present

Table 4 Comparison of the limits of detection (mg/L) and recoveries of present method with other methods for nitrophenols detection. Methods

2-NP

4-NP

2,4-DNP

Recoveries (%)

Ref.

SBSE-HPLC/UV MSPEa-HPLC-UV UAEMb-HPLC-UV MS-USAEMEc-UHPLC/UV SPME-HPLC/UV MIP-SPME-HPLC/UV SPME-HPLC/UV LLME-HPLC/UV MIPSCEd-HPLC/UV DLLME-MSCe/UV HS-SPME-GC/FID HS-SPME-GC/MS SPME-GC/MS SPE-GC/MS MMF/AMIIED-SPME-HPLC/DAD BAM-SCSE-HPLC/DAD

0.14 0.4 1 / 0.67 / 1.6 0.26e0.58 0.3e0.5 40 7.5 0.38 / 0.30 0.25 0.28

1.18 0.3 0.25 0.6 0.25 0.33 3.6

30 0.4 0.5 3 0.65 / 4.1

10 / 0.75 10 1 0.13 0.24

/ / 1.6 / / 0.096 0.097

90.7e115.6 84e109 92e115 88e101 23.5e65.6 98e103 / 90.6e97.3 95.3e105.7 83e108 / 111e118 86.4e89.2 60e110 82.6e116 71.2e115

[30] [33] [34] [35] [28] [36] [37] [27] [38] [39] [40] [41] [42] [2] [43] Present work

a b c d e

MSPE-magnetic solid phase extraction. UAEM-Ultrasound-assisted emulsification microextraction. MS-USAEME-manual shaking-enhanced, ultrasound-assisted emulsification microextraction. MIPSCE-molecularly imprinted spin column extraction. LLME-MSC-dispersive liquideliquid microextraction multisyringe chromatography.

Please cite this article in press as: Y. Zhang, et al., Extraction of trace nitrophenols in environmental water samples using boronate affinity sorbent, Analytica Chimica Acta (2015), http://dx.doi.org/10.1016/j.aca.2015.10.004

Y. Zhang et al. / Analytica Chimica Acta xxx (2015) 1e10

method is far better than that obtained in SPME-HPLC/UV [28], and at the same level as that got in other works [27,30,33e36,38e43]. The comparison well indicates that the developed method is sensitive and practicable. 4. Conclusions In summary, a new boronate affinity sorbent was successfully prepared using 3-acrylamidophenylboronic acid and divinylbenzene as precursors. The extraction performance of the new sorbent was well demonstrated by the effective extraction of strong polar nitrophenols. Because the existence of BeN coordination between sorbent and analytes, the new sorbent exhibited high extraction performance for nitrophenols pollutants. Under optimized conditions, the developed method of APBDB/SCSE-LD-HPLC/DAD can be used to determine trace NPs in water samples effectively and exhibited some advantages such as simplicity, sensitivity, good reproducibility and environmental friendliness. Additional, our work enriches the types of BAS and supplies a new sample preparation for the analysis of nitrophenols or other nitrogencontaining pollutants. Acknowledgments The work described in this article was the supported by National Natural Science Foundation of China (grant: 21377105, 21577111); Fundamental Research Funds for the Central Universities (grant: 20720140510, 201412G014); Natural Science Foundation of Guangdong Province of China (grant: 2015A030313006).

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