Fabrication and evaluation of a fluorophilic adsorbent for multiple monolithic fiber solid-phase microextraction of fluorobenzenes

Journal of Chromatography A, 1492 (2017) 12–18

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Fabrication and evaluation of a fluorophilic adsorbent for multiple monolithic fiber solid-phase microextraction of fluorobenzenes Yanmei Huang, Xiaojia Huang ∗ 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

a r t i c l e

i n f o

Article history: Received 13 December 2016 Received in revised form 1 March 2017 Accepted 2 March 2017 Available online 3 March 2017 Keywords: Fluorinated adsorbent Multiple monolithic fiber solid-phase microextraction Fluorobenzenes Fluorous interaction

a b s t r a c t A new type of highly fluorinated monolith (HFM) was fabricated and used as adsorbent of multiple monolithic fiber solid-phase microextraction (MMF-SPME). To prepare the HFM, a fluorinated monomer, 2,2,3,3,4,4,5,5,6,6,7,7-dodecafluoroheptyl acrylate was in situ copolymerized with dual crosslinkers (divinylbenzene and ethylenedimethacrylate). The fabrication parameters including the content of monomer and porogenic solvent in the polymerization mixture were optimized to obtain expected extraction performance and life span. The physicochemical properties of the HFM were systematically investigated with elemental analysis, infrared spectroscopy, scanning electron microscopy and mercury intrusion porosimetry. The effective extraction of six fluorobenzenes was selected as a paradigm to demonstrate the fluorophilic characteristic of HFM/MMF-SPME. At the same time, a convenient and effective method for the determination of trace fluorobenzenes in environmental water samples was developed by coupling HFM/MMF-SPME with high performance liquid chromatography/diode array detection (HFM/MMF-SPME-HPLC/DAD). Results indicated that the limits of detection (S/N = 3) for targeted compounds were in the range of 1.09–5.88 ␮g/L. The intra-day and inter-day precision (relative standard deviations, n = 4, %) at two spiked concentrations were 4.2–10.6% and 6.1–10.8%, respectively. Finally, the developed method was successfully applied to the analysis of fluorobenzenes in spiked real water samples with satisfactory recoveries and repeatability. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Sample preparation is a key step in the whole analytical procedure. Suitable sample preparation can remove interfering compounds and enrich trace targeted analytes. So far, all kinds of sample preparation techniques have been developed for various samples. Generally, sample pretreatment procedures can be divided into solvent-based and adsorbent-based extraction [1,2]. In compare with solvent-based extraction, adsorbent-based extraction is more interesting because it can circumvent the disadvantages of consuming large amounts of toxic solvents. Based on the advantages of adsorbent-based extraction, various extraction formats such as conventional solid-phase extraction (SPE) [3], solid-phase microextraction (SPME) [4], stir bar sorptive extraction (SBSE) [5], and current magnetic SPE (MSPE) [6], stir cake sorptive extraction (SCSE) [7] and multiple monolithic fiber solid-phase

∗ Corresponding author at: P. O. Box 1009, Xiamen University, Xiamen, 361005, China. E-mail address: [email protected] (X. Huang). http://dx.doi.org/10.1016/j.chroma.2017.03.001 0021-9673/© 2017 Elsevier B.V. All rights reserved.

microextraction (MMF-SPME) [8] have been developed. No matter what kinds of adsorbent-based extraction techniques, extractive media are the core because it decides the extraction targets and efficiency. To extract targeted compounds effectively, a great variety of functional materials have been prepared and used as adsorbents in various extraction formats. Silica-based carbonaceous (C2, C8, C18) [9], polymeric resins [10], graphene-based materials [11], boronate affinity materials (BAMs) [12], carbon nanotubes [13] and molecularly imprinted polymers (MIPs) [14] et al. have been fabricated and widely applied to extract various compounds from complicated matrices such as biological and food samples. Although many adsorbents have been reported and some of them are commercially available, there remains wide room for developing new adsorbents. Highly fluorinated materials (HFMS) are another interesting separation media because it can produce special fluorous-fluorous interactions with fluorine-rich compounds (namely, fluorophilicity). Based on the particular property, HFMs have been used as fluorous stationary phase in HPLC [15,16] and capillary electrochromatography (CEC) [17] for the separation of fluorochemicals. According to the fluorophilicity, HFMS were also used to selective extraction fluorinated compounds from nonfluorous species.

Y. Huang, X. Huang / J. Chromatogr. A 1492 (2017) 12–18

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Table 1 Extraction performance of different HFM/MMF for FBs. HFM/MMF

1 2 3 4 5 6 7

Polymerization mixture

Peak area

Monomer (%, w/w)

Monomer mixture Cross-linkers (%,w/w)

Monomer mixture (%,w/w)

Porogenic solvent (%,w/w)

FB

1,2-DFB

1,2,4-TFB

1,2,4,5-TEFB

PFB

HFB

30 35 40 45 30 30 30

70 65 60 55 70 70 70

55 55 55 55 65 60 55

45 45 45 45 35 40 45

31264 29383 25089 32744 25191 30370 31264

36626 35669 30101 40011 31645 34927 36626

29632 30127 19601 35557 27646 30422 29632

26491 27176 18682 30116 22695 25891 26491

18028 17287 10885 22734 17760 17073 18028

12954 10025 5629 13139 10410 5968 12954

Note: the matrix of FBs was ultrapure water.

Fig. 1. The reaction sketch of HFM.

This specific extraction was termed fluorous solid-phase extraction (FSPE). So far, FSPE has been used to specific enrich fluorous-derivatized phosphopeptides [18], fluorous-tagged glycans [19,20], perfluorinated compounds [21–24]. Undoubtedly, HFMS (F-adsorbents) are the core of FSPM. Fluorous reverse phase silica gel (FRPSS) and fluorous-functionalized magnetic microsphere (FFMM) are the main F-adsorbents in FSPE. FRPSS was prepared through silylating silica with fluorinated silylated reagent such as heptadecafluorodecyldimethylsilyl chloride [25]. Generally, FRPSS was employed as the adsorbent of SPE to selective enrichment of fluorous compounds. However, multiply reaction steps should be involved in the fabrication of FRPSS. At the same time, SPE format requires large volumes of toxic solvent, and the process is complex and time consuming. FFMM was combined with magnetic SPE (MSPE) to specifically extract fluorous compounds [20–23]. The MSPE process is quick and simple. However, the preparation procedure of FFMM is tedious and the FFMMs can become agglomeration easily. Therefore, to expand the application of FSPE, developing new F-adsorbents and combining suitable extraction format is highly desired. Fluorobenzenes (FBs) are a class of fluorinated benzene compounds, they are useful in many fields such as industry and pharmacy [26]. However, the FBs can enter into environmental waters through all kinds of ways such as industrial wastewater and pharmaceutical wastewater. FBs belong to toxicants and their adverse effects have been demonstrated [27,28]. Therefore, it is necessary to develop sensitive method to monitor FBs in environmental waters. Nevertheless, to the best of our knowledge, there is no related method for the monitoring of FBs in waters.

Because of the low concentrations of FBs and the complexity of the matrices in real waters, it is necessary to perform sample pretreatment before chromatographic analysis. In this work, according to the properties of FBs and the principle of fluorophilicity, a new highly fluorinated monolith (HFM) was simply fabricated based on the in-situ polymerization of monolithic material. The HFM was utilized as the adsorbent of MMF-SPME, which was developed in our group [29]. Compared with conventional SPME fiber, there are several outstanding properties of MMF-SPME, including simple preparation, high extraction capacity, fast mass-transfer and varied chemical properties. Six FBs including fluorobenzene (FB), 1,2-difluorobenzene (1,2-DFB), 1,2,4-trifluorobenzene (1,2,4-TFB), 1,2,4,5-tetrafluorobenzene (1,2,4,5-TEFB), pentafluorobenzene (PFB) and hexafluorobenzene (HFB) were selected as targeted analytes to evaluate the extraction performance of the new HFM/MMF-SPME. After the optimization of fabrication and extraction parameters of HFM/MMF-SPME, a methodology combining HFM/MMF-SPME and liquid desorption (LD), followed by high performance liquid chromatography with diode array detection (HFM/MMF-SPME-LD-HPLC/DAD) for the determination of trace FBs in environmental waters including tap, lake and river water samples was developed. 2. Experimental 2.1. Chemicals and materials Fluorous functional monomer of 2,2,3,3,4,4,5,5,6,6,7,7dodecafluoroheptyl acrylate (DFHA, ≥95%) was purchased from

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Y. Huang, X. Huang / J. Chromatogr. A 1492 (2017) 12–18

Fig 2. The FT-IR spectrum (a) and SEM image (50000× magnification) (b) of HFM.

Fig. 3. HPLC chromatograms of six targeted FBs. (a) Direct injection of spiked water sample; (b) Enrichment with PDMS-SPME; (c) Enrichment with CW/DVB-SPME; (d) Enrichment with HFM/MMF-SPME. Conditions: the mixture of ACN/TFA (99/1, v/v) was selected as desorption solvent; extraction and desorption time were 50 min and 20 min, respectively; ionic strength was adjusted by addition of 15.0% (w/v) NaCl and the pH value of sample matrix was not adjusted. The spiked concentration was 300.0 ␮g/L for each analyte.

Sigma-Aldrich (St. Louis, MO, USA). Ethylenedimethacrylate (EDMA, 97%) and divinylbenzene (DB, 80%) were supplied by Alfa Aesar Ltd. (Tianjin, China). 1-Propanol (97%), 1,4-butanediol (98%) (distilled before use), azobis(isobutyronitrile) (AIBN) (97%), acetic acid (AC), and trifluoroacetic acid (TFA) were supplied by Xilong Chemical Co. (Guangzhou, China). HPLC-grade acetonitrile (ACN) and methanol were acquired from Tedia (Fairfield, USA); Water used throughout the study was purified with a Milli-Q Reference water-purification system (Merck Millipore, Germany). Fused-silica capillary with polyimide coating (500 ␮m i.d.) was got from Ruifeng Instrumental Co. (Hebei, China). FB (99%), DFB (98%), TFB (98%), TEFB (99%), PFB (98%), HFB (99%), dichlorobenzene (DCB, 99%) and dibromobenzene (DBB, 98%) were obtained from Alfa Aesar Ltd. (Tianjin, China). Table S1shows the chemical properties of these FBs compounds. Tap water was collected from our lab. Lake and river water samples were gathered from Xiamen city and Zhangzhou city, respectively. Targeted FBs

were weighed accurately and dissolved in methanol respectively to obtain the individual standard stock solutions (10.0 mg/L). Working solutions (containing a standard mixture for each FB) were prepared at a concentration of 100.0 ␮g/L to validate the method. All samples were preserved in a refrigerator at −4 ◦ C before use and all the standard solutions were renewed weekly. 2.2. Instruments and methods All analysis of FBs was carried out using a HPLC chromatographic system (Shimadzu, Japan) equipped with a binary pump (LC-20AB) and a diode array detector (SPD-M20A). Sample injection was performed using a RE3725i auto sample injector with a 20 ␮L loop (Rheodyne, Cotati, CA, USA), all experiments were performed at room temperature. HPLC column used for the separation of FBs was Kromasil C18 (250 mm × 4.6 mm i.d., 5 ␮m particle size). The mobile phase was consisted of ultrapure water 50% (v/v) and ACN

Y. Huang, X. Huang / J. Chromatogr. A 1492 (2017) 12–18

50% (v/v) and isocratic elution was used. The flow rate, injection volume and detector wavelength were 1.0 mL/min, 20 ␮L and 202 nm, respectively. The microscopic morphology of HFM was obtained by scanning electron microscopy (Philips, Eindhoven, The Netherlands). FT-IR spectrum was obtained on an Avatar-360 FT-IR instrument (Thermo Nicolet, Madison, WI, USA). Elemental analysis (EA) was carried out on PerkinElmer (Shelton, CT, USA) Model PE 2400. A Model PoreMaster-60 (Quantachrome Instruments, Florida, USA) was used to measure the pore size distribution of HFM by mercury intrusion porosimetry (MIP).The specific surface area by nitrogen adsorption/desorption measurement was performed with Quadrasorb SI surface area analyzer (Quantachrome, Boynton Beach, USA). 2.3. Fabrication of HFM/MMF-SPME Two simple steps were involved in the fabrication of HFM/MMFSPME. In the first step, thin fibers were prepared by in situ polymerization. In this work, AIBN (2% (w/w) of the total amount) and the mixture of 1-propanol and 1,4-butanediol (60/40, w/w) were utilized as initiator and porogenic solvent, respectively. Crosslinker was consisted of DB and EDMA (w/w = 2/1). To optimize the ratios of composition in polymerization solution, different amount of monomer and porogenic solvent concentration was investigated (Table 1). Typically, 4 mg AIBN, 33 mg DFHA, 51 mg DB and 26 mg EDMA were put into a 5 mL beaker. To it, 54 mg 1-propanol and 36 mg 1,4-butanediol were added. After being mixed, high purity nitrogen was introduced into the mixture to remove the residue oxygen. Afterwards, the mixture was degassed by ultrasonication for 5 min, and transparent solution was resulted. Subsequently, the polymerization solution was injected into a glass capillary (0.5 mm in diameter and 10 cm in length), and using silicon rubbers to seal both ends of capillary. The polymerization reaction was performed in an oven at 70 ◦ C for 12 h. After that, 2 cm length of glass capillary was removed with a grindstone. The obtained thin fiber was white and elastic, and the dimension was 2 cm in length and 0.5 mm in diameter. In the second step, four thin monolithic fibers were carefully bound to form a HFM/MMF-SPME. To remove the residual DFHA, DB/EDMA and porogenic solvent in the HFM, the monolithic parts of HFM/MMF were dipped in methanol for 24 h. Before use, the HFM/MMF was activated in turn with methanol and ultrapure water for both 20 min, respectively. The reaction sketch of HFM is showed in Fig. 1. Fig. S1a and S1b show the photos of prepared single monolithic fiber and final HFM/MMF, respectively.

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addition of NaCl (15%, w/v) to adjust the ionic strength. Subsequently, the HFM/MMF-SPME procedure was applied to extract targeted FBs from the water samples. 3. Results and discussion 3.1. Fabrication of HFM/MMF-SPME Extraction efficiency and longevity are two key factors to evaluate the performance of adsorbents. For monolithic adsorbents, the extraction performance and life span are affected by the composition of polymerization solution. Therefore, to obtain the optimal fabrication conditions of HFM, the proportions of DFHA, DB/EDMA (w/w = 1/2) and porogenic solvent in the polymerization solution were investigated systematically. It can be seen from the data showed in Table 1 that the contents of DFHA, DB/EDMA (w/w = 1/2) and porogenic solvent in the polymerization solution affect the extraction performance of HFM obviously. Although the highest extraction performance could be obtained when high proportion of DFHA was used (HFM/MMF-SPME-4), the monolithic fibers were fragile and resulted in poor longevity. After weighting the extraction efficiency and useful life span, the optimal proportions for the fabrication of HFM/MMF-SPME were the proportion of functional monomer of DFHA was kept at 30% (w/w) in the monomer mixture, while the ratio of monomer mixture to porogenic solvent was 55/45 (%, w/w) (Table 1, HFM/MMF-SPME-7). Under the best preparation parameters, the HFM/MMF-SPME possessed satisfactory life span, it could be reused to extract FBs in real samples more than 100 times. No fracture of monolithic fibers and no loss of extraction efficiency were observed. 3.2. Characterization of HFM

In present study, stirring extraction and LD modes were used to pretreat the water samples. Twenty millilitres of sample solution (the pH value was not adjusted and the ionic strength was adjusted by the addition of 15% (w/v) NaCl) was added into a 25 mL vial. The monolithic parts of HFM/MMF-SPME were immersed into the sample solution to extract the FBs for 50 min at 500 rpm. After the extraction, the fibers were removed and desorbed with 400 ␮L desorption solvent (ACN/TFA, v/v = 99/1) in a 0.4 mL vial insert by stirring for 20 min to release adsorptive FBs. The stripping solvent was used for HPLC/DAD analysis directly. To avoid analyte carryover, the used HFM/MMF was reconditioned in 2.0 mL of ACN for 15 min and then dipped in ultrapure water for 15 min.

In this study, EA, FT-IR, SEM and MIP were used to characterize the fluorinated monolith prepared under the optimal conditions. The carbon and hydrogen contents achieved from EA results were 52.9% and 6.70%, respectively. Fig. 2a shows the FT-IR spectrum of HFM. The peak signal at 2985 cm −1 implied the presence of CH3 and CH2 groups in the adsorbent. The strong peak at 1732 cm −1 could be assigned to C O groups of EDMA. The existence of phenyl groups could be proved by the peaks of 1665, 1454 and 1388 cm −1 . The peaks of 1199 and 1141 cm −1 belonged to the vibration of C F bonds. The characteristic peaks showed in Fig. 2a obviously demonstrated that the polymerization reaction occurred successfully. Fig. S2 and Fig. 2b display the SEM images of the HFM at 50× and 50000× magnification, respectively. The Fig. S2 showed that the HFM was integrated and the surface was smooth. It could be seen from Fig. 2b that the fluorinated monolith presented a cauliflowerlike globular structure. At the same time, homogeneous pore size could be observed. Fig. S3 shows the pore size distribution plot of HFM achieved with MIP. It could be seen from the figure that most of the pore sizes were around 750 nm and the pore size distribution (PSD) was narrow. The narrow PSD and near micrometer-sized pores favor mass transfer during extraction application [30]. Based on the nitrogen adsorption/desorption measurement of dry bulk HFM, the total surface area of HFM was calculated as 42.3 m2 /g. The expected area indicates that there are abundant sorptive sites which can produce interactions with targeted FBs.

2.5. Pretreatment of environmental water samples

3.3. Optimization of HFM/MMF-SPME

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 filtered through a 0.45 ␮m nylon filter to remove suspended matter. The pH value of sample solutions was not adjusted,

To maximize the extraction performance of HFM/MMF-SPME for targeted FBs, some key variables, including desorption solvent, extraction and desorption time, pH value and ionic strength in sample matrix were investigated in detail. The peak areas of the tar-

2.4. HFM/MMF-SPME procedure

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Y. Huang, X. Huang / J. Chromatogr. A 1492 (2017) 12–18

Table 2 Analytical performance of the proposed method for six FBs. Compounds

FB 1,2-DFB 1,2,4-TFB 1,2,4,5-TEFB PFB HFB a b c

Calibration curvesa (␮g/L)

10.0–750 10.0–750 15.0–750 15.0–750 15.0–750 20.0–750

r2

0.9916 0.9967 0.9949 0.9966 0.9959 0.9929

LODb (␮g/L)

1.29 1.09 1.46 2.08 3.21 5.88

LOQc (␮g/L)

4.26 3.61 4.81 6.87 10.6 19.4

Fiber-to-fiber reproducibility (n = 4)

5.1 5.6 4.8 5.0 7.0 4.8

Intra-day assay variability (RSD, %, n = 4)

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

Spiked concentration (␮g/L)

Spiked concentration (␮g/L)

30.0

500

30.0

500

10.6 10.4 5.8 9.2 9.0 10.2

6.6 5.4 4.8 4.4 5.8 4.2

9.8 9.4 10.8 6.1 9.9 7.4

6.1 9.4 10.5 9.8 7.6 9.5

Spiked level includes 10.0, 15.0, 20.0, 50.0, 100, 200, 400, 500 and 750 ␮g/L, respectively. S/N = 3. S/N = 10.

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

Spiked

Detected (␮g/L)/Recovery(%RSD, n = 3)

(␮g/L)

FB

1,2-DFB

1,2,4-TFB

1,2,4,5-TEFB

PFB

HFB

Tap water

0 30.0 200 750

ND 24.0 80.2 (2.3) 164 81.9 (9.7) 634 84.6 (4.8)

ND 26.3 87.7 (9.8) 196 98.2 (8.8) 652 86.9 (2.7)

ND 31.6 106 (1.1) 213 107 (8.3) 600 80.1 (1.5)

ND 26.0 86.8 (4.4) 197 98.7 (8.6) 643 85.8 (2.4)

ND 34.0 113 (3.6) 202 101 (7.9) 665 88.7 (4.2)

ND 25.7 85.7 (8.1) 197 98.3 (5.3) 652 86.9 (2.7)

Lake water

0 30.0 200 750

ND 24.1 80.4 (1.0) 162 80.8 (8.7) 691 92.2 (8.3)

ND 26.2 87.2 (1.5) 182 91.1 (9.8) 691 92.1 (11.4)

ND 27.8 92.8 (4.8) 200 100 (9.3) 654 87.3 (9.5)

ND 28.8 96.1 (9.1) 182 91.2 (8.5) 656 87.4 (9.1)

ND 26.0 86.5 (7.7) 188 94.1 (8.6) 674 89.9 (9.9)

ND 24.8 82.6 (2.9) 208 104 (8.1) 707 94.3 (7.7)

River water

0 30.0 200 750

ND 24.7 82.5 (9.4) 162 81.0 (7.4) 693 92.5 (7.2)

ND 25.0 83.4 (1.9) 167 83.7 (10.6) 659 87.9 (7.7)

ND 26.6 88.7 (5.3) 190 95.1 (9.5) 651 86.7 (6.5)

ND 24.9 83.0 (3.3) 177 88.5 (8.9) 668 89.0 (5.6)

ND 33.5 112 (6.4) 175 87.7 (7.9) 666 88.8 (8.5)

ND 30.1 101 (8.3) 195 97.7 (5.8) 676 90.1 (6.9)

geted FBs were used to evaluate the extraction performance under different conditions and all the data were repeated in triplicate. Considering there are fluorous-fluorous interactions between adsorbent and FBs, addition of suitable acid can disrupt the F-F interactions and favor the desorption of FBs from HFM [31]. Therefore, three solvent including pure ACN, ACN/AC (99/1, v/v) and ACN/TFA (99/1, v/v) were selected as desorption solvent to elute the sorptive FBs from HFM (Fig. S4a). It could be seen from the results that the desorption effect with ACN/AC (99/1, v/v) and ACN/TFA (99/1, v/v) were better than with pure ACN. At the same time, the highest extraction performance could be achieved when ACN/TFA (99/1, v/v) was used as desorption solvent. The reason may be that the TFA could also produce the F-F interactions with FBs. Thus, TFA could compete with HFM to interact with FBs, which results in a better desoprtion. Subsequently, the effect of the amount of TFA in desorption solvent on extraction performance was check. The amount of TFA in desorption solvent was varied from 0 to 2.0% (v/v) with a 0.5% interval (Fig.S4b). Results indicated that best extraction performance could be achieved when 1.0% (v/v) TFA was used in desorption solvent. The above-mentioned results well indicate that F-F interactions between HFM and FBs are involved in the extraction. According to the experimental results, the mixture of ACN/TFA (99/1, v/v) was selected as the optimal desorption solvent. As conventional SPME, MMF-SPME is an equilibrium-based technique, the extraction efficiency obviously relates to extraction time. In this study, the extraction time profiles were investigated by varying the extracting time from 20 min to 60 min (Fig.S5a). It could be seen that the extraction performance of the targeted FBs enhanced quickly from 20 to 50 min and the extraction equilibrium was achieved when extraction time was 50 min. Hereby,

the extraction time of 50 min was chosen in the further studies. Fig.S5b shows the effect of desorption time on extraction performance when extraction time was kept at 50 min. Results indicated that 20 min was enough to elute the adsorptive FBs from HFM/MMF and no obvious carryover was found. Based on the results, 50 min and 20 min were used as the optimal extraction and desorption time, respectively, in our following research. The effect of sample pH value on the extraction performance was investigated from 3.0 to 10.0 (Fig.S6). Results demonstrated that there was no obvious change of extraction performance in the whole pH range. The results tally with our expectation because there is no any ionizable group in the adsorbent and targeted FBs. FBs are medium polar compounds because the log KO/W values for these targeted FBs are in the range of 2.27–2.55 (Table S1). Therefore, the ionic strength in sample matrix will affect the extraction performance through salting-out or salting-in effects [32]. In this work, the ionic strength in sample matrix was adjusted by addition of different amount of NaCl. Fig.S7 shows the changed profiles of extraction performance when the content of NaCl was changed from 0 to 25% (w/v). Results clearly showed that the salting-out played dominant effect when the concentration of NaCl was 15% (w/v). Therefore, when HFM/MMF-SPME is used to extract FBs in water samples, 15% (w/v) NaCl is recommended to be added. Under the optimized extraction conditions, the developed HFM/MMF showed satisfactory extraction performance for targeted FBs. Fig. 3 shows the HPLC chromatograms of direct injection of spiked water sample (a), enrichment with PDMS-SPME (b), CW/DVB-SPME (c) and developed HFM/MMF-SPME (d). It could be seen that the HFM/MMF-SPME enriched the targeted FBs expectedly, the peak heights for all FBs enhanced obviously after the

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extraction (Fig. 3d). Compared with the extraction performance obtained on commercial SPME fibers based on PDMS (100 ␮m) (Fig. 3b) and CW/DVB (65 ␮m) (Fig. 3c) under the same extraction conditions, the home-made HFM/MMF exhibited higher extraction performance for FBs than the selected commercial SPME fibers. This may due to the special F-F interactions between HFM/MMF and FBs. At the same time, HFB was selected as tested analyte to measure the extraction capacity of HFM/MMF (the detailed procedure is depicted in Supporting Information), the result showed that the extraction capacity was 1153 ␮g/g (RSD = 3.08%, n = 4), which demonstrated that the new developed HFM/MMF possessed expected extraction capability for FBs. The above-mentioned results well demonstrate that the proposed HFM/MMF-SPME can effectively extract FBs through multiply interactions including F-F, ␲-␲ and hydrophobic interactions.

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3.6. Application of the HFM/MMF-SPME-HPLC/DAD to real environmental water samples To evaluate practical applicability of the proposed method, three real waters including tap, lake and river waters were extracted and analyzed. The results showed in Table 3 indicated that no targeted FBs was detected in the three water samples. To check the effect of sample matrix on the analytical performance of HFM/MMFSPME-HPLC/DAD method, the recovery study was investigated by spiking 30.0, 200 and 750 ␮g/L individual targeted compounds in real waters. As listed in Table 3, the recoveries of the all targeted FBs were in the range from 80.2–112% with the RSDs less than 10%. The satisfactory recoveries and precision well indicate that the matrices of water samples did not interfere the quantification of six FBs under the optimal conditions. It also evidence that the HFM/MMFSPME-HPLC/DAD method is practically feasible for the monitoring of FBs in complex water samples. 4. Conclusions and future work

3.4. The investigation of method selectivity To investigate the selective enrichment of developed HFM/MMF-SPME for FBs, DCB and DBB were used as interfering analytes. DCB and DBB were added in the concentration range of 200–600 ␮g/L to a 20 mL solution containing 100 ␮g/L DFB. It could be seen from the results showed in Fig.S8 that there was no obvious change in extraction performance for DFB when the concentrations of DCB and DBB increased from 200 ␮g/L to 600 ␮g/L. The result well evidence that HFM/MMF-SPME can extract FBs selectively, and also further demonstrate that the F-F interactions play key role in the extraction.

3.5. Method validation To validate the developed method of HFM/MMF-SPMEHPLC/DAD, a series of parameters including linearity of the calibration curves, coefficients of determination, limit of detection (LOD), limit of quantification (LOQ) and precision were investigated. 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 constructed by spiking FBs to the ultrapure water over the range of 10.0–750 ␮g/L. The entire HFM/MMF-SPME procedure was applied to spiked samples. The calibration curves were obtained by plotting the mean peak area versus sample concentration in duplicate. To evaluate the intraday assay variability of proposed method, four replicates samples with 30.0 ␮g/L and 500 ␮g/L spiking concentration were extracted and analyzed within one day. The inter-day assay variability of the method was assessed at 30.0 ␮g/L and 500 ␮g/L spiked concentration during a period of four consecutive days. The detailed results are showed in Table 2. It could be seen from the data that the linear ranges were 10.0–750 ␮g/L for FB and DFB, 15.0–750 ␮g/L for TFB, TEFB and PFB. For HFB, the linear range was 30.0–750 ␮g/L. At the same time, the all standard curves of analytes possessed good coefficients of determination (r2 > 0.99). The LOD and LOQ values achieved by proposed method were in the range of 1.09–5.88 ␮g/L and 3.61–19.4 ␮g/L, respectively. Furthermore, good method precision was achieved according to the intra-day, inter-day assay variability and fiberto-fiber reproducibility, indicating by the all RSDs less than 11%. The results showed in Table 2 well evidence that the method of HFM/MMF-SPME-HPLC/DAD has satisfactory sensitivity and precision for the detection of FBs in water samples.

In conclusion, according to the principle of in-situ polymerization of monolithic materials, a HFM was conveniently fabricated and employed as the adsorbent of MMF-SPME. The HFM/MMF-SPME possessed fluorophilicity and displayed satisfactory selectively extract FBs through fluorous-fluorous interactions. After optimization of preparation and extraction parameters, a convenient analytical method for the monitoring of FBs in environmental water samples was developed firstly by the combination of HFM/MMF-SPME and HPLC/DAD. The proposed method exhibited some advantages such as simplicity, quickness, satisfactory sensitivity and environmental friendliness. Considering the volatility of FBs, a headspace SPME (HS-SPME) format for the extract of FBs with HFM/MMF-SPME will be constructed in our future study. At the same time, according to the fluorophilicity of HFM, the prepared HFM/MMF-SPME will be used to extract other fluorinated compounds such as perfluorinated persistent organic pollutants form complicated samples. Acknowledgements The work described in this article was the supported by National Natural Science Foundation of China (grant: 21377105, 21577111); National Key Research and Development Program of China (grant: 2016YFC0502904); Fundamental Research Funds for the Central Universities (grant: 20720140510); Natural Science Foundation of Fujian Province of China (grant: 2015J01061). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chroma.2017.03. 001. References [1] Z.Z. Huang, H.K. Lee, Materials-based approaches to minimizing solvent usage in analytical sample preparation, Trends Anal. Chem. 39 (2012) 228–244. [2] M. Locatelli, F. Sciascia, R. Cifelli, L. Malatesta, P. Bruni, F. Croce, Analytical methods for the endocrine disruptor compounds determination in environmental water samples, J. Chromatogr. A 1434 (2016) 1–18. [3] L. Ramos, Critical overview of selected contemporary sample preparation techniques, J. Chromatogr. A 1221 (2012) 84–98. [4] C.L. Arthur, J. Pawliszyn, Solid phase microextraction with thermal desorption using fused silica optical fibers, Anal. Chem. 62 (1990) 2145–2148. [5] E. Baltussen, P. Sandra, F. David, C. Cramers, J. Microcol, Automated sorptive extraction-thermal desorption-gas chromatography-mass spectrometry analysis: determination of phenols in water samples, J. MicrocolumnSep. 11 (1999) 471–474.

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